Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session J1: Invited Session: Buckley Prize Session
Sponsoring Units: DCMP GMAGChair: Allan MacDonald, Univesity of Texas at Austin
Room: Ballroom I
Tuesday, March 19, 2013 2:30PM - 3:06PM |
J1.00001: Oliver E. Buckley Condensed Matter Prize Lecture: Transfer of spin momentum between magnets: its genesis and prospect Invited Speaker: John Slonczewski Consider two nanoscopic monodomain magnets connected by a spacer that is composed of a non-magnetic metal or a tunnel barrier. Any externally applied electric current flowing through these three layers contributes tiny pseudo-torques to both magnetic moments ($J. S.$ 1989). Such a weak spin-transfer torque (STT) may counteract and overcome a comparably small torque caused by viscous dissipation (\textit{L. Berger }1996; $J. S.$ 1996). Any initial motion (e. g. excited by ambient temperature) of one moment (or both), may grow in amplitude and culminate in steady precession or a transient switch to a new direction of static equilibrium. In a memory element, the STT effect writes 0 or 1 in a magnetic-tunnel junction. Indeed, world-wide developments of memory arrays and radio-frequency oscillators utilizing current-driven STT today enjoy a nine-digit dollar commitment. But the fact that transfer of each half-unit of spin momentum $h$/4$\pi $ through a barrier requires the transfer of at least one unit of electric charge limits its efficiency. Arguably, STT should also arise from the flow of external heat, in either direction, between an insulating magnet, of ferrite or garnet (e. g. YIG) composition, and a metallic spacer ($J. S.$ 2010). Whenever s-d exchange annihilates a hot magnon at the insulator/metal-spacer interface, it transfers one unit $h$/2$\pi $ of spin momentum to the spacer. Conduction electrons within the spacer will transport this spin momentum to the second magnet without requiring an electric current. Such a \textit{thermagnonic }method, modestly powered by a Joule-effect heater, can substantially increase the efficiency of STT. Support for this prediction comes from (1) an estimate of the sd-exchange coefficient from data on spin relaxation in magnetically dilute (Cu,Ag,Au):Mn alloys; (2) a DFT computation (\textit{J. Xiao et al} 2010); and (3) most persuasively, data from spin pumping driven across a YIG/Au interface by ferromagnetic resonance (\textit{B. Heinrich} \textit{et al} 2011; \textit{C. Burrowes} \textit{et al} 2012). [Preview Abstract] |
Tuesday, March 19, 2013 3:06PM - 3:42PM |
J1.00002: Oliver E. Buckley Condensed Matter Prize Lecture: S-d Exchange, Spin Accumulation, And The Roots Of Spintronics Invited Speaker: Luc Berger The success of spintronics in metals such as nickel, cobalt, Ni-Fe and Ni-Co is based on the existence of high-mobility spin-up 4s electrons at the Fermi level, which carry most of the current. The spin-up Fermi level is located above the top of the 3d band. This basic fact, first recognized by Mott in 1936, was confirmed by the Hall-effect measurements of Pugh et al. (1950-1965), and by data of deviation from Matthiessen's rule by Campbell, Fert and Jaoul (1967-1977). In order to explain giant magnetoresistance and the existence of the spin-transfer torque, an interaction is needed which couples 4s conduction electrons to magnetic 3d electrons. This is the s-d exchange interaction, introduced by Vonsovskii in 1946 and Zener in 1951. Theories of Gilbert damping, based on s-d exchange, were soon developed (Turov (1955), Mitchell (1957)). But a serious problem was caused by the existence of a momentum gap between spin-up and spin-down Fermi surfaces, which prevents spin switching from happening at low T. The problem can be solved if local defects exist which act as extra sources of momentum. One such source is spin-flip scattering (Turov (1961), Heinrich, Freitova and Kambersky (1967)). A second one is the presence of an interface (Slonczewski (1996), Berger (1996)). Spin accumulation is another concept of importance to spintronics. It represents an imbalance between spin-up and spin-down Fermi levels. Introduced by Aronov in 1976, it was developed by Johnson and Silsbee (1985-1993) and by Valet and Fert (1993). It is the hidden agent through which the current ``pumps'' energy into many spintronics devices. In semiconductor lasers, the same role is played by the difference between conduction-band and valence-band Fermi levels. A momentum gap problem also exists in lasers made of indirect-gap semiconductors, and it is solved similarly. [Preview Abstract] |
Tuesday, March 19, 2013 3:42PM - 4:18PM |
J1.00003: From point contacts to spin-transfer torque Invited Speaker: Maxim Tsoi Point contacts - nanoscale electrical contacts between conductors - have been around for decades and proved to be unique experimental tools for studying the electronic transport properties of metals. Following the theoretical prediction of spin-transfer torque (STT) by John Slonczewski [1] and Luc Berger [2], point contacts were instrumental for the first experimental demonstration of STT in spin-valve multilayers [3], thanks to extremely high current densities routinely produced in such contacts. In this talk I will briefly review the point-contact technique and its contributions to the field of current-induced control over magnetic nanostructures. \\[4pt] [1] J. C. Slonczewski, J. Magn. Magn. Mater. 159, L1 (1996).\\[0pt] [2] L. Berger, Phys. Rev. B 54, 9353 (1996).\\[0pt] [3] M. Tsoi et al., Phys. Rev. Lett. 80, 4281 (1998). [Preview Abstract] |
Tuesday, March 19, 2013 4:18PM - 4:54PM |
J1.00004: to be determined Invited Speaker: Daniel Ralph |
Tuesday, March 19, 2013 4:54PM - 5:30PM |
J1.00005: The Spin Torque Lego - from spin torque nano-devices to advanced computing architectures Invited Speaker: Julie Grollier Spin transfer torque (STT), predicted in 1996 [1], and first observed around 2000, brought spintronic devices to the realm of active elements. A whole class of new devices, based on the combined effects of STT for writing and Giant Magneto-Resistance or Tunnel Magneto-Resistance for reading has emerged. The second generation of MRAMs, based on spin torque writing : the STT-RAM, is under industrial development and should be out on the market in three years. But spin torque devices are not limited to binary memories. We will rapidly present how the spin torque effect also allows to implement non-linear nano-oscillators, spin-wave emitters, controlled stochastic devices and microwave nano-detectors. What is extremely interesting is that all these functionalities can be obtained using the same materials, the exact same stack, simply by changing the device geometry and its bias conditions. So these different devices can be seen as Lego bricks, each brick with its own functionality. During this talk, I will show how spin torque can be engineered to build new bricks, such as the Spintronic Memristor, an artificial magnetic nano-synapse. I will then give hints on how to assemble these bricks in order to build novel types of computing architectures, with a special focus on neuromorphic circuits. \\[4pt] [1] J. C. Slonczewski, J. Magn. Magn. Mater. 159, 1 (1996) \& L. Berger, Phys. Rev. B 54, 9353 (1996) [Preview Abstract] |
Session J2: Invited Session: Topological States and Plasmonics in Graphene
Sponsoring Units: DCMPChair: Qian Niu, University of Texas at Austin
Room: Ballroom II
Tuesday, March 19, 2013 2:30PM - 3:06PM |
J2.00001: Topological insulator gap in graphene with heavy adatoms Invited Speaker: Ruqian Wu It is important to search an effective approach to expand the spin-orbit coupling gap of graphene for the realization of the two-dimensional topological insulator (TI) state. We found that heavy In or Tl adatoms may dramatically enhance the gap to detectable values of order 7 or 20 meV, large enough for the realization of quantum spin Hall effect in experimental conditions. However, In and Tl atoms may easily coalesce on graphene due to their weak binding energies and shallow segregation barriers. We proposed a new way to produce a two-dimensional spin-orbit coupling gap using the impurity bands that are mediated through graphene. First principles calculations predict that the gaps generated by osmium and iridium exceed 200 meV over a broad range of adatom coverage The position of the Fermi level can be manipulated by using external electric field and co-adsorbates. The mechanism at work is expected to be rather general and may open the door to designing new TI phases in many materials. [Preview Abstract] |
Tuesday, March 19, 2013 3:06PM - 3:42PM |
J2.00002: Engineering topological states in graphene systems Invited Speaker: Zhenhua Qiao In this talk, I will introduce our recent progress on engineering various topological states in graphene systems. The presentation includes two parts: (i) We show that in monolayer graphene, Rashba spin-orbit coupling (SOC) together with Zeeman field can open a nontrivial bulk gap to host the quantum anomalous-Hall effect [1]. We further show that this can be realized via doping magnetic metal atoms on graphene [2,3]. In Bernal stacking bilayer graphene, an interlayer potential difference breaks the inversion symmetry and opens a bulk gap to support the quantum valley-Hall effect. We find that Rashba SOC can induce a topological phase transition from the quantum valley-Hall effect to a Z2 topological insulator [4]. When the Zeeman field is further considered, a rich variety of topological phases emerge. (ii) When the mass term (e.g., sublattice potential in monolayer graphene, or interlayer potential difference in bilayer graphene) varies spatially, topologically protected 1D kink states arise along zero lines. We demonstrate that such 1D kink state exhibits zero bend resistance for arbitrary turns in its propagating path [5]. We further point out that similar kink states can be tailored in graphene nanoroads in boron nitride sheets [6]. When the kink current experiences a crossing junction composed of four zero lines, we find the splitting of the 1D kink state at the bifurcation point obeys an explicit law of current partition [7].\\[4pt] References:\\[0pt] [1] Z.H. Qiao \textit{et al.}, Phys. Rev. B 82, 161414(R) (2010)\\[0pt] [2] J. Ding \textit{et al.}, Phys. Rev. B 84, 195444 (2011)\\[0pt] [3] H. Jiang \textit{et al.}, Phys. Rev. Lett. 109, 116803 (2012)\\[0pt] [4] Z.H. Qiao \textit{et al.}, Phys. Rev. Lett. 107, 256801 (2011)\\[0pt] [5] Z.H. Qiao \textit{et al.}, Nano Letters 11, 3453 (2011)\\[0pt] [6] J. Jung \textit{et al.}, Nano Letters 12, 2936 (2012)\\[0pt] [7] Z.H. Qiao \textit{et al.}, to be submitted. [Preview Abstract] |
Tuesday, March 19, 2013 3:42PM - 4:18PM |
J2.00003: Giant Rashba effect and spin polarization of Dirac fermions in graphene Invited Speaker: Oliver Rader Graphene in spintronics has so far meant a material with low spin-orbit coupling which could be used as high-performance spin current leads. If the spin-orbit interaction could be enhanced by an external effect, the material could serve also as an active element in a spintronics device such as the Das-Datta spin field effect transistors. We show that by intercalation of Au under graphene grown on Ni(111), a Rashba-type spin-orbit splitting of $\sim$ 100 meV can be created in a wide energy range while the Dirac cone is preserved and becomes slightly p-doped. We discuss different superstructures of Au under the graphene which are observed in the experiment. Ab initio calculations indicate that a sharp graphene-Au interface at the equilibrium distance accounts for only $\sim$ 10meV spin-orbit splitting and enhancement can occur due to Au atoms in the hollow position that get closer to graphene while preserving the sublattice symmetry. For the system graphene/Ir(111) we observe a large splitting of the Dirac cone as well. The large lattice mismatch of this system allows us to investigate properties of the pseudospin that are related to the structure of minigaps that occur at the zone boundary of the superstructure. We also report on the giant Rashba splitting of an Ir(111) surface state which persists underneath the graphene. Finally, we re-investigate with p(1 $\times$ 1) graphene/Ni(111) and Co(0001) typical examples where the sublattice symmetry breaking by the substrate is believed to lead to a large band gap at the Dirac point. We show that this is not the case and the Dirac point of graphene stays instead intact, and we discuss implications of this finding. [Preview Abstract] |
Tuesday, March 19, 2013 4:18PM - 4:54PM |
J2.00004: Infrared nano-imaging and nano-spectroscopy of graphene plasmons Invited Speaker: Zhe Fei Graphene plasmons, which are collective oscillations of Dirac fermions in graphene, are of broad interests in both fundamental research and technological applications. In this talk, we present first nano-imaging and nano-spectroscopy studies of graphene plasmons using scattering-type scanning near-field optical microscope --a unique technique allowing efficient excitation and high-resolution imaging of graphene plasmons. With this technique, we were able to show that common graphene/SiO2/Si back-gated structure support propagating surface plasmons in the infrared frequencies. The observed plasmons are highly confined surface modes with a wavelength around 200nm that are conveniently tunable by the back gate voltages [Nature 487, 82--85 (2012)]. In addition, we performed nano-spectroscopy of graphene over a broad range of mid-infrared frequencies. Our spectroscopy results provide evidence of strong coupling between graphene plasmons and SiO2 optical phonons [Nano Lett. 11(11), 4701-4705 (2011)]. Finally, we were able to map and characterize various types of line defects inside CVD graphene film by exploring real space patterns of propagating surface plasmons. These line defects, including cracks, wrinkles, and even grain boundaries, trigger distinct plasmonic features due to plasmon interference. Further modeling and analysis unveiled unique electronic properties associated with these line defects. [Preview Abstract] |
Tuesday, March 19, 2013 4:54PM - 5:30PM |
J2.00005: Quantum Anomalous Hall Effects and Topological Phase Transitions in Silicene Invited Speaker: Motohiko Ezawa Silicene is a monolayer of silicon atoms forming a two-dimensional honeycomb lattice, which is experimentally manufactured this year. The low energy theory is described by Dirac electrons, but they are massive due to a relatively large spin-orbit interaction. I will explain the following properties of silicene: 1) The band structure is controllable by applying an electric field [1]. Silicene undergoes a phase transition from a topological insulator to a band insulator by applying external electric field [1]. 2) The topological phase transition can be detected experimentally by way of diamagnetism [7]. 3) There is a novel circular dichroism and spinvalley selection rules by way of photon absorption [6]. 4) Silicene shows a quantum anomalous Hall effects when ferromagnet is attached onto silicone [3]. 5) Silicene shows a photo-induced quantum Hall effects when we apply strong laser onto silicene [8]. 6) Single Dirac cone state emerges when we apply photo-irradiation and electric field, where the gap is open at the K point and closed at the K' point [8].\\[4pt] [1] M. Ezawa, New J. Phys. 14, 033003 (2012).\\[0pt] [2] M. Ezawa, J. Phys. Jpn. 81, 064705 (2012). \\[0pt] [3] M. Ezawa, Phys. Rev. Lett. 109, 055502 (2012)\\[0pt] [4] M. Ezawa, Europhysics Letters 98, 67001 (2012).\\[0pt] [5] M. Ezawa, J. Phys. Soc. Jpn. 81, 104713 (2012).\\[0pt] [6] M. Ezawa, Phys. Rev. B 86, 161407(R) (2012).\\[0pt] [7] M. Ezawa, cond-mat/arXiv:1205.6541 (to be published in EPJB).\\[0pt] [8] M. Ezawa, cond-mat/arXiv:1207.6694.\\[0pt] [9] M. Ezawa, cond-mat/arXiv: 1209.2580. [Preview Abstract] |
Session J3: Invited Session: Colloidal Carbon Nanotubes
Sponsoring Units: DCMP GSNPChair: Erik K. Hobbie, North Dakota State University
Room: Ballroom III
Tuesday, March 19, 2013 2:30PM - 3:06PM |
J3.00001: Soft Materials Approaches to Carbon Nanotubes: from Gels to Composites Invited Speaker: Mohammad Islam Carbon nanotubes combine low density with exceptional mechanical, electrical and optical properties. Unfortunately, these nanoscale properties have not been retained in bulk structures. I will describe surface modification assisted self-assembly of single wall carbon nanotube into macroscopic nanotube networks - hydrogels and aerogels. The nanotube networks are ultra-lightweight, electrically conducting and thermally insulating. The shapes and sizes of these nanotube networks are readily tunable and is a tremendous strength of our fabrication method. The interesting properties and structure of these nanotube networks make them suitable for diverse applications. For example, we have used these networks as scaffolds to enhance elastic modulus of polymers by 36,000{\%}. The porous nanotube networks also show high capacitance, and can be impregnated with catalysts nanoparticles at high loading, which can then be simultaneously used as electrodes and catalysts supports in electrochemical cells. A weakness of the nanotube networks is their fragility -- but we have recently developed a method to transform these inelastic networks into superelastic materials by coating them with between one and five layers of graphene nanoplates. [Preview Abstract] |
Tuesday, March 19, 2013 3:06PM - 3:42PM |
J3.00002: Connectedness percolation of carbon nanotube dispersions: impact of interactions, polydispersity and external fields Invited Speaker: Paul van der Schoot There is considerable industrial interest in novel flexible, transparent electrodes for electro-optical applications, in part because of dwindling natural reserves of indium, a component of transparent electrodes used, e.g., in LCD display technology. For this purpose, frantic research is currently being conducted worldwide into polymeric composites containing electrically conducting inorganic and metallic nanowires, carbon nanotubes, grafite flakes, graphene and so on. One of the objectives is to get as high as possible a conduction for as low as possible a nanoparticle loading but progress is slow. Unclear is why, e.g., carbon nanotubes dispersed in plastic matrix materials can have such widely diverging electrical percolation thresholds, even when their mean physical dimensions and other characteristics seem very similar. In an effort to shed light on this, we apply continuous space connectedness percolation theory to collections of anisometric particles with arbitrary polydispersity in length, width and levels of conduction between them. We find that the percolation threshold is extremely sensitive to even quite modest degrees of polydispersity and of alignment induced in the processing of the fluid composites before they set and become the final solid product. We find that the way polydispersity influences the percolation threshold depends on whether or not the length and width distributions are coupled or not. Finally, we provide an explanation why composites with graphene filler seems to have a larger percolation threshold than those with carbon nanotubes. [Preview Abstract] |
Tuesday, March 19, 2013 3:42PM - 4:18PM |
J3.00003: Fluid Phases of Carbon Nanotubes and Graphene Invited Speaker: Matteo Pasquali Nanoscale carbon---including Carbon Nanotubes (CNTs) as well as graphene, i.e., graphite in its single layered form---has remarkable electrical, thermal, and mechanical properties, more so than previously known polymer molecules or colloidal particles. Realizing these properties in applications requires understanding and controlling the behavior of fluid phases of CNTs and graphene. Biological and environmental applications are likely to require dilute phases of CNTs and graphene; material processing, e.g., production of coatings and fibers, will require more concentrated phases. Fluid processing is one of the most important frontiers of applied research in CNTs and graphene. Nano-carbon fluids are almost considered an oxymoron because dispersing or dissolving CNTs and graphene into fluid phases is exceedingly difficult. This talk reviews advances in understanding and controlling fluid phases of CNTs and graphene, with specific focus on single-object properties and true solutions. The dynamics of individual CNTs can be studied by fluorescence microscopy, revealing that their translational and rotational motion and bending stiffness can be described well by the semiflexible chain model. Even at low concentrations (few parts per million), CNTs form complex fluid phases with intriguing properties. In strong acids, CNTs as well as graphene dissolve spontaneously. At low concentration, these fluids can be used for making transparent, conducting films and coatings. In crowded environments, CNTs reptate like stiff polymers. At sufficiently high concentrations, CNTs and graphene form liquid crystals that can be spun into well-aligned, macroscopic fibers. Like in polymeric systems, the properties of macroscopic CNT materials depend on the length (molecular weight) of the constituent CNT macromolecules. [Preview Abstract] |
Tuesday, March 19, 2013 4:18PM - 4:54PM |
J3.00004: Field-assisted assembly and orientational order of colloidal ellipsoids Invited Speaker: Michael Solomon Colloidal particles with anisotropy in shape and interactions can potentially be assembled into colloidal crystals with unusual structure and symmetry. Field-assisted assembly is likewise a means to produce structures that are otherwise difficult to achieve by equilibrium self-assembly. Here we show, by means of confocal microscopy direct visualization, how controlled application of electric fields can improve general prospects for assembly of any anisotropic colloid. By studying the model case of ellipsoidal colloidal rods, we find that applied fields can be designed which produce liquid crystal phases of colloids in a simple, versatile manner. By directly visualizing the assembled particles in three dimensions we learn that the quality of orientational order achieved is comparable to that of materials such as liquid crystalline polymers. We understand the results in terms of the underlying electrokinetics of the system as well as connect the observed field-induced orientational order to the equilibrium isotropic-nematic transition predicted for rods with prolate spheroidal shape. Specifically, the applied field generates a force that is balanced by a gradient in osmotic pressure generated by the density dependence of the rod suspension. If the field strength is sufficiently large, the resultant osmotic pressure produces a phase transition. We discuss how the required field conditions for assembly can be tailored based on the shape and size of the anisotropic building block. [Preview Abstract] |
Tuesday, March 19, 2013 4:54PM - 5:30PM |
J3.00005: DNA-wrapped Carbon nanotubes as a model rod-like colloid system Invited Speaker: Ming Zheng Single-wall carbon nanotubes (SWCNTs) exhibit many fascinating physical behavior as the result of their quasi one-dimensional crystalline structures. SWCNTs can be dispersed into rod-like colloid particles by a few small molecules and polymers, among which the most effect one is single-stranded DNA (\textbf{\textit{Nature Materials}} 2, 338, 2003). The structure of a DNA-SWCNT hybrid is controlled by both the sequence of the wrapping DNA, and the atomic configuration, or chirality, of the SWCNT (\textbf{\textit{Science}} 302, 1545, 2003). This has been exploited by us to purify single-chirality SWCNTs from synthetic mixtures via liquid chromatography (\textbf{\textit{Nature}} 460, 250, 2009; \textbf{\textit{JACS}} 133, 12998, 2011). DNA-SWCNTs have well-defined surface structures, tunable aspect-ratios, and ultra-small diameters. These attributes provide unique advantages to the DNA-SWCNT colloid system in probing inter-particle interactions in crowded and high salt environment (\textbf{\textit{ACS Nano}} 5, 8258, 2011). In this talk, I will present some recent observations we made on DNA-SWCNT clustering that shed new light on the Hofmeister effect. [Preview Abstract] |
Session J4: Invited Session: Physics Challenges in Biophysics
Sponsoring Units: FIAPChair: Cha-Mei Tang, Creatv Micro Tech Inc
Room: Ballroom IV
Tuesday, March 19, 2013 2:30PM - 3:06PM |
J4.00001: Phase contrast imaging with conventional x-ray sources at acceptable dose levels and exposure times Invited Speaker: Alessandro Olivo X-ray Phase Contrast imaging (XPCi) generates image contrast from interference and refraction effects (instead of x-ray attenuation), which leads to enhanced visibility of all details and to the detection of features classically considered ``x-ray invisible.'' XPCi thus has great potential in a wide range of applications, from the earlier diagnosis of lesions in medical imaging to the detection of faint blemishes in non-destructive testing. However, XPCi seemed to require a high level of (at least spatial) coherence, which restricted its use to synchrotron facilities. Microfocal sources can be used but, due to low emitted flux, result in exposure times too long (hours) for most practical applications. Other attempts were based on aperturing/collimating the focal spot of a conventional source to create sufficient spatial coherence, again limiting the source output and resulting in excessive exposure times and/or delivered dose. This talk will present a method, based on appropriately designed x-ray masks, which works with unapertured and uncollimated conventional sources, at acceptable exposure times and delivered doses. It will describe how the method works, explain how quantitative features can be extracted from the images, and provide examples of application in various fields. [Preview Abstract] |
Tuesday, March 19, 2013 3:06PM - 3:42PM |
J4.00002: Diagnostic Applications and Methods to Isolate Circulating Tumor Cells (CTCs) from Blood Invited Speaker: Cha-Mei Tang Each year a million new cancer cases are diagnosed in the United States. Ninety percent of the deaths will be the result of metastasis, not from the primary tumor. Tissue biopsy is a universally accepted tool for cancer diagnosis and determination of treatment. The procedure varies, but is invasive, costly, and can be fatal, and for these reasons is seldom repeated after initial diagnosis. Monitoring of treatment response and for possible relapse is usually done by CT or MRI scan, both of which are expensive and require the tumor to change size perceptibly. Further, cancer can mutate or develop resistance to therapeutics and require modification of the treatment regimen. The initial tissue biopsy often cannot reflect the disease as it progresses, requiring new biopsy samples to determine a change of treatment. All carcinomas, about 80{\%} of all cancer, shed tumor cells into the circulation, most often at the later stages when treatment is more critical. These circulating tumor cells (CTCs) are the cause of metastasis, and can be isolated from patient blood to serve as ``liquid biopsy''. These CTCs contain a valuable trove of information that help both patient and clinician understand disease status. In addition to counting the number of CTCs (known to be a prognostic indicator of survival), CTCs can provide biomarker information such as protein expressions and gene mutations, amplifications, and translocations. This information can be used to determine treatment. During treatment, the number of intact and apoptotic CTCs can be measured on a repeated basis to measure the patient's response to treatment and disease progression. Following treatment, liquid biopsy can be repeated at regular intervals to watch for relapse. Methods to isolate CTCs can be grouped into three categories: i) immunocapture based on surface markers of CTCs, ii) size exclusion based on CTC size, typically larger than blood cells, and iii) negative selection utilizing red blood cell lysis, white blood cell depletion or FICOLL. Various implementations of the CTC isolation methods will be presented. [Preview Abstract] |
Tuesday, March 19, 2013 3:42PM - 4:18PM |
J4.00003: A Retinal Prosthetic Strategy with the Capacity to Restore Normal or Near-Normal Vision Invited Speaker: Sheila Nirenberg A pressing problem in neuroscience is determining the neural code. We know that neurons send their signals in the form of trains of action potentials, but we don't know what the code is, that is, we don't know what the unit of information is. Is it the number of spikes per unit time? Is it the individual spike or some pattern of spikes? Getting a clear answer to this affects a great deal of work in neuroscience, both basic and applied. For basic research, it tells us what quantity we need for building models of neural computations (i.e., what spike train features we need). For applied research, it tells us what quantity we need to effectively transmit information from one brain area to another via brain-machine-interfaces or prosthetic devices. Here we describe a strategy for finding neural codes and use it to develop a powerful new kind of prosthetic device for treating blindness. [Preview Abstract] |
Tuesday, March 19, 2013 4:18PM - 4:54PM |
J4.00004: Smart Prosthetics Invited Speaker: Stuart Harshbarger |
Tuesday, March 19, 2013 4:54PM - 5:30PM |
J4.00005: Artifical Pancreas Invited Speaker: Jiangfeng Fei In 2006, JDRF launched the Artificial Pancreas Project (APP) to accelerate the development of a commercially-viable artificial pancreas system to closely mimic the biological function of the pancreas individuals with insulin-dependent diabetes, particularly type 1 diabetes. By automating detection of blood sugar levels and delivery of insulin in response to those levels, an artificial pancreas has the potential to transform the lives of people with type 1 diabetes. The 6-step APP development pathway serves as JDRF's APP strategic funding plan and defines the priorities of product research and development. Each step in the plan represents incremental advances in automation beginning with devices that shut off insulin delivery to prevent episodes of low blood sugar and progressing ultimately to a fully automated ``closed loop'' system that maintains blood glucose at a target level without the need to bolus for meals or adjust for exercise. [Preview Abstract] |
Session J5: Focus Session: Computational Discovery and Design of New Materials: Graphene
Sponsoring Units: DMP DCOMPChair: Richard Hennig, Cornell University
Room: 301
Tuesday, March 19, 2013 2:30PM - 2:42PM |
J5.00001: Electronic structure and lattice matching in graphene/h-BN stacked thin films Yuki Sakai, Susumu Saito, Marvin Cohen In this work, we study the electronic structure and possibility of lattice matching of thin films composed of graphene and hexagonal boron nitride (h-BN) within the framework of the density functional theory. Since graphene and h-BN have different in-plane lattice constants intrinsically, we first study relative stabilities of commensurate thin films with lattice matching and incommensurate thin films without lattice matching by comparing total energies in order to clarify the stable geometries of graphene/h-BN thin films. As a result, we find some stacking patterns where commensurate thin films are more stable than incommensurate thin films. We also find that the energy gain due to interlayer interaction depends on the number of layers in thin films. In addition, we report electronic properties of these thin film systems. Some commensurate thin films are found to possess finite band gaps, while induced band gaps should be almost canceled out in incommensurate phases. We also discuss the electric field effect on the electronic properties of graphene/h-BN thin films. [Preview Abstract] |
Tuesday, March 19, 2013 2:42PM - 2:54PM |
J5.00002: Accessing the Strong-Coupling Regime in Graphene on hBN Substrate Andrey Shytov, Justin Song, Leonid Levitov Recent experiments [1,2] report on an insulating behavior at charge neutrality in single-layer graphene on hBN substrate. Ref.[1] attributed this behavior to weak localization due to residual short-range disorder. However, in Ref.[2] a much stronger insulating behavior was observed at a larger separation between graphene and the gates, in the regime when interactions are largely unscreened. This suggests that interactions play a decisive role in the observed phenomena, ruling out the weak localization scenario. We propose an alternative mechanism in which a gap opens up due to a combined effect of sublattice modulation in hBN [3] and electron-electron interactions. We argue that sublattice modulation in hBN amplifies the effective fine structure constant enhancing electron-electron interactions. In this regime, a weak gap induced by sublattice modulation can be strongly enhanced by interactions, giving rise to near-spontaneous excitonic order. \\[4pt] [1] L.A.Ponomarenko et al., Nature Physics {\bf 7}, 958 (2011) \\[0pt] [2] F.Amet et al, arXiv:1209.6364 \\[0pt] [3] M.Kindermann, B.Uchoa, and D.Miller, arXiv:1205.3194 [Preview Abstract] |
Tuesday, March 19, 2013 2:54PM - 3:06PM |
J5.00003: Observation of a Massive Dirac Spectrum in Monolayer Graphene on Boron Nitride Benjamin Hunt, Javier D. Sanchez-Yamagishi, Andrea F. Young, T. Taniguchi, K. Watanabe, Pablo Jarillo-Herrero, Raymond Ashoori Graphene on hexagonal boron nitride (hBN) has emerged as the new standard for high-mobility graphene devices. However, the role of the hBN substrate in modifying the electronic properties of the graphene has only recently been investigated, with particular attention paid to the effects of the Moire produced by the interplay between the graphene and hBN lattices. Here we show that the hBN substrate can have a dramatic effect on the electronic structure of monolayer graphene, leading to the formation of superlattice Dirac points (SLDPs) and an insulating state at charge neutrality in zero magnetic field. The SLDPs imply that the insulator is related to the presence of a long-wavelength Moire. In samples which show the zero-field insulator, we also observe incompressible features associated with fractional quantum Hall (FQH) states at filling fractions $\nu$=$\pm$5/3. Their absence in previous measurements has been attributed to the presence of low-energy valley excitations; in our measurement we find the strength of the $\nu$=5/3 gap is comparable to that of all other observed FQH states. Taken together, these observations imply that for small twist angles between the graphene and hBN substrates, the appropriate low-energy theory describing monolayer graphene features a mass. [Preview Abstract] |
Tuesday, March 19, 2013 3:06PM - 3:18PM |
J5.00004: Ideal strength and phonon instability of atomically-thin materials under strain Eric B. Isaacs, Chris A. Marianetti Recent \textit{ab initio} calculations suggest that the ideal strength of graphene is limited by a finite-wavevector phonon instability [1]. In order to understand the origin and generality of phonon instabilities in two-dimensional crystals, we investigate the ideal strength of other monolayer materials including boron nitride (BN) and molybdenum disulfide (MoS$_2$) with density functional theory calculations. We find a soft phonon mode at the K-point of the Brillouin zone leading to mechanical failure for both BN and MoS$_2$ under biaxial tensile strain, which suggests that Fermi surface nesting cannot be a universal explanation for this type of phonon instability in monolayer materials since BN and MoS$_2$ have substantial band gaps. While BN distorts similarly to graphene upon mechanical failure, MoS$_2$ undergoes a more complex phase transformation with both in- and out-of-plane atomic displacements. We discuss general features of phonon instabilities in monolayer materials under strain and make connection to results from nanoindentation experiments when available. [1] C. A. Marianetti and H. G. Yevick, Phys. Rev. Lett. \textbf{105}, 245502 (2010). [Preview Abstract] |
Tuesday, March 19, 2013 3:18PM - 3:30PM |
J5.00005: First-Principles Study of Multilayer Lithium and Graphene Compounds for Battery Applications Alper Buldum, Gulcin Tetiker Recently, graphene and graphene based materials have attracted great interest for energy storage applications such as rechargable Li-ion batteries and supercapacitors. However, recent experiments showed that these materials may have different electrochemical mechanism compared to graphite. Porosity and presence of single or few layers of graphene play important roles in carbon based anode materials in lithium ion batteries. In this work, a study of different multilayer lithium-graphene compounds is performed using first-principles density-functional theory(DFT). Relaxed structures are determined, adsorption energies, density of states and charge density are calculated. Possible multilayer structures for energy storage are discussed. [Preview Abstract] |
Tuesday, March 19, 2013 3:30PM - 3:42PM |
J5.00006: Self-assembly mechanisms and even/odd disparity of short atomic chains on graphene V. Ongun Ozcelik, Salim Ciraci Self-assembly mechanisms of carbon chains on boron nitride and short BN chains on graphene are investigated using first-principles plane wave calculations. Once a C$_2$ nucleates on h-BN, the insertion of each additional carbon at its close proximity causes a short segment of carbon atomic chain to grow by one atom at a time in a quaint way: The existing chain leaves its initial position and subsequently is attached from its bottom end to the top of the carbon ad-atom. The electronic, magnetic and structural properties of these chains depend on the number of carbon atoms in the chain, such that they exhibit an even-odd disparity. An individual carbon chain can also modify the electronic structure with localized states in the wide band gap of h-BN. As a reverse situation, the growth of short BN atomic chains on graphene is also examined. These results reveal the interesting self-assembly behavior short atomic chains. Furthermore, we find that these atomic chains enhance the chemical activity of h-BN and graphene sheets by creating active sites and can act as pillars between two and multiple sheets of these honeycomb structures leaving wider spacing between them to achieve high capacity storage of specific molecules.\\[4pt] [1] V.O. Ozcelik and S. Ciraci, Phys. Rev. B 86 155421 (2012). [Preview Abstract] |
Tuesday, March 19, 2013 3:42PM - 3:54PM |
J5.00007: Ab initio Study of the Interactions between Single Vacancies in Graphene Mahmoud Hammouri, Igor Vasiliev Graphene is a promising material for nanoelectronic and spintronic applications. The introduction of point defects such as vacancies can turn graphene into a magnetic material. We present a first-principles computational study of the interactions between single vacancies in graphene. The total energies, lattice deformation energies, and spin magnetic moments of the interacting vacancies are calculated using the SIESTA density functional electronic structure code combined with the generalized gradient approximation for the exchange correlation functional. We discuss the variation of the total energy and the total magnetic moment of defective graphene as a function of the separation distance between vacancies. Our calculations show that the magnetic moment of graphene disappears when the vacancies are located within a certain distance from each other. [Preview Abstract] |
Tuesday, March 19, 2013 3:54PM - 4:06PM |
J5.00008: Ab Initio Study of the Interactions between Dopant Atoms and Vacancies in Graphene Tarek Tawalbeh, Igor Vasiliev We apply a first-principles computational method based on density functional theory to study the interaction of substitutional boron and nitrogen atoms with single vacancies in graphene. Our calculations are carried out using a pseudopotential technique combined with the generalized gradient approximation for the exchange-correlation functional implemented in the SIESTA electronic structure package. The equilibrium geometries, total energies, electronic structures, magnetization, and spin-polarized densities of states of doped and defective graphene sheets are examined as a function of the separation distance between dopant atoms and vacancies. Our study shows the presence of attractive interaction between dopant atoms and vacancies. Furthermore, we found that boron dopants enhance the magnetism of graphene sheets containing single vacancies, whereas nitrogen dopants reduce it. The influence of dopant site location on both the interaction energy and magnetization is discussed. [Preview Abstract] |
Tuesday, March 19, 2013 4:06PM - 4:18PM |
J5.00009: Doping of graphene nanomeshes by ion-chelation Ahmed Maarouf, Razvan Nistor, Ali Afzali, Marcelo Kuroda, Dennis Newns, Glenn Martyna Graphene nanomeshes (GNM's) are formed by the creation of a superlattice of pores in graphene. Depending upon the pore shape, size, superlattice constant and symmetry, GNM's can be semimetallic, or semiconducting with a fractional eV band gap, allowing them to be fruitfully employed in applications that pristine graphene cannot. In this work, first principles calculations are used to study the doping of semiconducting GNM's using a chemically motivated approach. It is shown that {\it ion-chelation} leads to a stable doping of the GNM's, and that it occurs within a rigid band doping picture. Such chelated or ``crown'' GNM structures are thus stable, high mobility semiconducting materials which can serve as building blocks for novel graphene-based nanoelectronics applications. [Preview Abstract] |
Tuesday, March 19, 2013 4:18PM - 4:30PM |
J5.00010: Magnetism of single and divacancies in graphene nanoflakes Silvia Fernandez-Sabido, M.E. Cifuentes-Quintal, Carlos Ramos, Romeo de Coss Vacancy-induced magnetism in graphene is a recent topic of great scientific relevance because of the potential applications of spin-polarized graphene-based systems. In this work, we have studied by means of DFT calculations, the structural, electronic and magnetic properties of single and divacancies in hexagonal zigzag graphene nanoflakes $C_{6n^2}H_{6n} (n=2,\dots,7)$. We have found that, when a single carbon atom is removed, the structure undergoes a magnetic 5-9 reconstruction where the interatomic distances depend on the nanoflake size. The charge density distribution suggests that there is not a complete bond reconstruction in the vacancy zone, however, the existence of a partial bond is sufficient to conclude that only two electrons remain unpaired, resulting in the $2\mu_B$ spin moment. The spin moment is equally distributed over the localized-$sp^2$ and delocalized-$p_z$ orbitals. For a carbon-divacancy, we have varied the distance between the vacancies and we have found magnetic structural reconstructions (9-4-9, 5-9-9-5, 5-9-6-5) which have not been reported for graphene layer with magnetic moments between $2$ and $4\mu_B$; although the most stable is the nonmagnetic 5-8-5 reconstruction. [Preview Abstract] |
Tuesday, March 19, 2013 4:30PM - 4:42PM |
J5.00011: Scattering by periodic defect lines in graphene J.N.B. Rodrigues, N.M.R. Peres, J.M.B. Lopes dos Santos Recently, Tsen et al. [1] demonstrated how one can probe the electric properties of a single grain boundary in graphene. Following this remarkable possibility, we study, from a theoretical point of view, the electronic transport across periodic defect lines in graphene. In the continuum low-energy limit, such defects act as infinitesimally thin stripes separating two regions where the Dirac Hamiltonian governs the low-energy phenomena. The behaviour of these systems is determined by the boundary condition imposed by the defect on the massless Dirac fermions. We demonstrate how this low-energy boundary condition can be computed from the tight-binding model of the defect line. We illustrate this procedure by considering a simple zigzag oriented defect line solely composed by pentagons: the {\it pentagon-only} defect line. The recently observed $zz(558)$ defect line [2], as well as the $zz(5757)$ defect line will also be considered [3].\\[4pt] [1] A. W. Tsen et al., Science 336, 1143 (2012).\\[0pt] [2] J. Lahiri et al., Nature Nanotechnology 5, 326 (2010).\\[0pt] [3] J. N. B. Rodrigues et al., arXiv:1208.0822 (2012). [Preview Abstract] |
Tuesday, March 19, 2013 4:42PM - 4:54PM |
J5.00012: First-Principles Investigation of Mn and Co Doped Trilayer Graphene Xuan Luo First-principles calculations were performed through ABINIT to investigate trilayer graphene for spintronics materials. We studied two stacking orders for trilayer graphene: Bernal (ABA) and rhombohedra (ABC) by using interstitial and substitution transition metal Mn and Co doped trilayer graphene. We found that the ABC stacking order exhibits larger band gap than that of ABA, the Co doped ABC trilayer graphene possesses a band gap and is ferromagnetic. This results show that interstitial Co doped ABC stacking trilayer graphene has potential applications in spintronics. [Preview Abstract] |
Tuesday, March 19, 2013 4:54PM - 5:06PM |
J5.00013: Oxygen plasma etching of graphene: A first-principles dynamical inspection of the reaction mechanisms and related activation barriers Kenichi Koizumi, Mauro Boero, Yasuteru Shigeta, Atsushi Oshiyama Oxygen plasma etching is a crucial step in the fabrication of electronic circuits and has recently received a renovated interest in view of the realization of carbon-based nanodevices. In an attempt at unraveling the atomic-scale details and to provide guidelines for the control of the etching processes mechanisms, we inspected the possible reaction pathways via reactive first principles simulations. These processes involve breaking and formation of several chemical bonds and are characterized by different free-energy barriers. Free-energy sampling techniques (metadynamics and blue moon), used to enhance the standard Car-Parrinello molecular dynamics, provide us a detailed microscopic picture of the etching of graphene surfaces and a comprehensive scenario of the activation barriers involved in the various steps. [Preview Abstract] |
Tuesday, March 19, 2013 5:06PM - 5:18PM |
J5.00014: Simulating Lattice Image of Suspended Graphene Taken by Helium Ion Microscopy Yoshiyuki Miyamoto, Hong Zhang, Angel Rubio Atomic scale image in nano-scale helps us to characterize property of graphene, and performance of high-resolution transmission electron microscopy (HRTEM) is significant, so far. While a tool without pre-treatment of samples is demanded in practice. Helium ion microscopy (HIM), firstly reported by Word {\it et. al.} in 2006, was applied for monitoring graphene in device structure (Lumme, {\it et. al.}, 2009 ). Motivated by recent HIM explorations, we examined the possibility of taking lattice image of suspended graphene by HIM. The intensity of secondary emitted electron is recorded as a profile of scanned He$^+$-beam in HIM measurement. We mimicked this situation by performing electron-ion dynamics based on the first-principles simulation within the time-dependent density functional theory. He$^+$ ion collision on single graphene sheet at several impact points were simulated and we found that the amount of secondary emitted electron from graphene reflected the valence charge distribution of the graphene sheet. Therefore HIM using atomically thin He-beam should be able to provide the lattice image, and we propose that an experiment generating ultra-thin He$^+$ ion beam (Rezeq {\it et. al.}, 2006) should be combined with HIM technique. [Preview Abstract] |
Tuesday, March 19, 2013 5:18PM - 5:30PM |
J5.00015: Electronic structures and transport properties of silicene on Ag surface Yun-Peng Wang, Hai-Ping Cheng It has been predicted from first-principle that ``silicene'', a two-dimensional buckled honeycomb structure of silicon, is thermally stable and has a graphene-like band structure. In experiments, epitaxial silicene were observed to form at hexagonal Ag(111) and $\mathrm{ZrB_2(0001)}$ surfaces. However, electronic structure and transport properties related to silicene have not been thoroughly studied. In this work, we have studied band structures of silicene on top of Ag surface using density-functional theory. The effective band structure mapped onto $1\times1$ unit cell of monolayer silicene on Ag(111) surface could be compared directly with Angle-Resolved Photoemission Spectra (ARPES). We have also studied electronic transport property across monolayer and bilayer silicene sheets using the Non-Equilibrium Green's Function (NEGF) method. The transmission curve shows a maximum at Fermi energy for the monolayer silicene case, but shows a minimum for the bilayer silicene case, which can be explained by their band structures. [Preview Abstract] |
Session J6: Growth, Structure, Properties, and Defects
Sponsoring Units: DMPChair: Paola Barbara, Georgetown University
Room: 302
Tuesday, March 19, 2013 2:30PM - 2:42PM |
J6.00001: Temperature Dependence of the Raman Spectra of CVD-grown Monolayer MoS$_2$ A. Glen Birdwell, Frank J. Crowne, Terrance P. O'Regan, Pankaj B. Shah, Madan Dubey, Sina Najmaei, Zheng Liu, Pulickel M. Ajayan, Jun Lou, Rusen Yan, Huili Grace Xing We investigated the temperature dependence of the E$^{1}_{\mathrm{2g}}$ and A$_{\mathrm{1g}}$ peaks in the Raman spectra of monolayer MoS$_{2}$ grown by chemical vapor deposition (CVD) on Si/SiO$_{2}$ substrates. Micro-Raman spectroscopy was carried out using the 532 nm laser excitation over the temperature range from 30 to 175 $^{\circ}$C. The extracted values of the temperature coefficient of these modes are $\chi =$ -0.013 cm$^{-1}$/$^{\circ}$C and $\chi =$ -0.016 cm$^{-1}$/$^{\circ}$C, respectively. The obtained results may shed light on the anomalous behavior of these modes observed in laser power dependent studies of monolayer MoS$_{2}$. [Preview Abstract] |
Tuesday, March 19, 2013 2:42PM - 2:54PM |
J6.00002: Grains and grian boundaries in highly-crystalline monolayer molybdenum disulfide Arend M. van der Zande, Pinshane Y. Huang, Daniel A. Chenet, Yumeng You, Timothy C. Berkelbach, Gwan-Hyoung Lee, David R. Reichman, David A. Muller, Tony F. Heinz, James C. Hone Recent progress in large-area chemical vapor deposition (CVD) synthesis of monolayer molybdenum disulfide, a new two-dimensional direct-bandgap semiconductor, is paving the way for applications in atomically thin electronics. Little is known, however, about the microstructure of this material.~ Here we have refined CVD synthesis to grow highly crystalline islands of monolayer molybdenum disulfide up to 120 micrometers in size with optical and electrical properties comparable or superior to exfoliated samples. Using transmission electron microscopy, we correlate lattice orientation, edge morphology, and crystallinity with island shape to demonstrate that triangular islands are single crystals.~The crystals merge to form faceted tilt and mirror boundaries that are stitched together by lines of 8- and 4- membered rings.~ Density functional theory reveals localized mid-gap states arising from these 8-4 defects. The knowledge gained about grain structure enables systematic studies of the optical, mechanical, and electronic properties of grain boundaries. [Preview Abstract] |
Tuesday, March 19, 2013 2:54PM - 3:06PM |
J6.00003: Synthesis Single Layer Transition Metal Dichalcogenides with Chemical Vapor Deposition Yi-Hsien Li, Han Wang, Lili Yu, Wenjing Fang, Tomas Palacios, Lain-Jong Li, Jing Kong Recently, monolayers of layered transition metal dichalcogenides (LTMD), such as MX2 (M$=$Mo, W and X$=$S, Se), have been reported to exhibit significant spin-valley coupling and optoelectronic performances because of the unique structural symmetry and band structures. Monolayers in this class of materials offered a burgeoning field in fundamental physics, energy harvesting, electronics and optoelectronics. However, most studies to date are hindered with great challenges on the synthesis and transfer of high quality LTMD monolayers. Hence, a feasible synthetic process to overcome the challenges is essential. Here, we demonstrate the growth of high-quality MS2 (M$=$Mo, W) monolayers using ambient-pressure-chemical-vapor-deposition (APCVD) with the seeding of aromatic molecules. Electronic transport and optical performances of the as-grown MS2 monolayers are comparable to those of exfoliated MS2 monolayers. The growth of MS2 monolayer is achieved on various surfaces. Growth mechanism on the novel synthetic process is investigated. Understanding and better control of seeds for the novel growth on the class of materials may stimulate the progress in the emerging filed. [Preview Abstract] |
Tuesday, March 19, 2013 3:06PM - 3:18PM |
J6.00004: Oxidation of atomically thin MoS$_2$ on SiO$_2$ Mahito Yamamoto, William Cullen, Theodore Einstein, Michael Fuhrer Surface oxidation of MoS$_{2}$ markedly affects its electronic, optical, and tribological properties. However, oxidative reactivity of atomically thin MoS$_{2}$ has yet to be addressed. Here, we investigate oxidation of atomic layers of MoS$_{2}$ using atomic force microscopy and Raman spectroscopy. MoS$_{2}$ is mechanically exfoliated onto SiO$_{2}$ and oxidized in Ar/O$_{2}$ or Ar/O$_{3}$ (ozone) at 100-450 $^{\circ}$C. MoS$_{2}$ is much more reactive to O$_{2}$ than an analogous atomic membrane of graphene and monolayer MoS$_{2}$ is completely etched very rapidly upon O$_{2}$ treatment above 300 $^{\circ}$C. Thicker MoS$_{2}$ (\textgreater\ 15 nm) transforms into MoO$_{3}$ after oxidation at 400 $^{\circ}$C, which is confirmed by a Raman peak at 820 cm$^{-1}$. However, few-layer MoS$_{2}$ oxidized below 400 $^{\circ}$C exhibits no MoO$_{3}$ Raman mode but etch pits are formed, similar to graphene. We find atomic layers of MoS$_{2}$ shows larger reactivity to O$_{3}$ than to O$_{2}$ and monolayer MoS$_{2}$ transforms chemically upon O$_{3}$ treatment even below 100 $^{\circ}$C. Work supported by the U. of Maryland NSF-MRSEC under Grant No. DMR 05-20741. [Preview Abstract] |
Tuesday, March 19, 2013 3:18PM - 3:30PM |
J6.00005: Comprehensive Studies of NbS$_{2}$ and the Affects of Nb Doping in the Layered Systems of WS$_{2}$ and MoS$_{2}$ Brian Cooper, Mauricio Terrones, Tom Mallouk, Camden Henderson, Nina I. Kovtyukhova Research on layered dichalcogenides (compounds of the form MX$_{2}$, with M as a metal and X as any member of group 16 in the periodic table) has picked up momentum due to a sympathetic reverberation created in response to the enormously prodigious research in graphene. Although much progress in graphene research has been made, there are still many hurdles to clear, and prudence has made requisite parallel courses in research. Layered dichalcogenides exhibit similar features to graphene; namely they are relatively easy to exfoliate, and have hexagonal symmetries, but unlike graphene, these compounds represent a spanning set of the materials under investigation in various scientific branches ($e.g.$ superconductors, semiconductors, topological insulators, \textit{etc.}). We have taken many approaches to the synthesis, manipulation, and device design of these materials. In our attempts to better understand the role of doping Nb into the MoS$_{2}$ and WS$_{2}$ systems, we serendipitously realized the merits, which previously lay quiescent, of studying NbS$_{2}$ itself. A metallic dichalcogenide, NbS$_{2}$ exhibits both charge density wave and superconducting phase transitions below respective appropriate temperatures. Studying mono, bi, and tri-layer geometries have afforded us the opportunities to probe not only the details of quantum confinement effects in the NbS$_{2}$ system, but also how these effects percolate through and affect the various properties of other dichalcogenidal systems. [Preview Abstract] |
Tuesday, March 19, 2013 3:30PM - 3:42PM |
J6.00006: Effects of strain on band structure and effective masses in MoS$_2$ Hartwin Peelaers, Chris G. Van de Walle Molybdenum disulfide (MoS$_2$) is a layered semiconductor that shows great promise for devices such as field-effect transistors. It has an important advantage compared to graphene, namely that it has a band gap. However, a lot of crucial information about the band structure and electronic properties of this material is still lacking, hampering interpretation of experiments and preventing accurate device modeling. Here we use hybrid density functional theory to calculate key materials parameters such as band gaps and effective masses, as well as to investigate effects of strain. We show how strain allows engineering the nature (direct vs. indirect) and size of the band gap and the magnitude of effective masses. In addition, insight into the fundamental physics is provided by considering the transition between the bulk and the monolayer as a function of tensile uniaxial stress. [Preview Abstract] |
Tuesday, March 19, 2013 3:42PM - 3:54PM |
J6.00007: Two-Dimensional Semiconductors From Theory to Experiments: MoS$_{2}$ and MoSe$_{2}$ Can Ataca, Jeffrey C. Grossman, Sefaattin Tongay, J. Zhou, K. Lo, Junqiao Wu After the synthesis of graphene, single layer transition metal dichalcogenides have been shown to possess superior optical properties than those of graphene. Until now, both theorists and experimentalists have mainly focused on the properties of single-layer MoS2. In this work, the first synthesis of single and few layers of MoSe2 are shown experimentally and are complemented by stability analysis through phonon and electronic structure calculations using density functional theory (DFT). The DFT calculations include van der Waals and spin-orbit interactions which are shown to play an important role in the geometric structure, electronic, magnetic and vibrational properties. Single-layer MoSe2 is measured and calculated as a direct band gap material, having band gap values suitable for solar cells and optical devices. Dimensionality effects predicted by DFT calculations such as variation of the energy band structures and Raman active vibrational modes are confirmed by experiments. Optical and electronic properties of single and few layers MoSe2 can be tuned by varying the temperature, number of layers and applying pressure to the samples. Single layer MoS2 and MoSe2 possess a number of properties that make them highly promising materials for future nanoscale applications. [Preview Abstract] |
Tuesday, March 19, 2013 3:54PM - 4:06PM |
J6.00008: ABSTRACT WITHDRAWN |
Tuesday, March 19, 2013 4:06PM - 4:18PM |
J6.00009: Electronic Structure of Defect-Single-Layer MoS2 Takat Rawal, Duy Le, Talat S. Rahman We present density functional theory based investigations of the electronic structural properties of a single-layer molybdenum disulfide (MoS$_{\mathrm{2}})$ with a sulfur vacancy row. We show that the vacancy row introduces a defect state in the band-gap of MoS$_{\mathrm{2}}$. This state is unoccupied and localized at the vacancy row. We also present detailed analysis of the density of states and charge density of the system. The defect state reduces the band gap of the system to 0.5 eV from 1.8 eV that we obtain for the clean single-layer. In particular, we find Kohn-Sham wave functions that are confined to the defective site are responsible for this particular energy band. We discuss the possible applications of this effect on other physical properties of the system. We also extend the calculations to the case of a MoS$_{\mathrm{2}}$ on Cu(111) for which experimental observations [1] suggest the presence of structures of the type under discussion here. [1] D. Kim, D. Sun, W. Lu, Z. Cheng, Y. Zhu, D. Le, T. S. Rahman, and L. Bartels, Langmuir 27, 11650 (2011). [Preview Abstract] |
Tuesday, March 19, 2013 4:18PM - 4:30PM |
J6.00010: The Incorporation of C in the Crystal Lattice of Metals; Its Role on the Structure and Properties of these New Materials Called Covetics Lourdes Salamanca-Riba, Romaine Isaacs, David Forrest, Azzam Mansour, Andrew Herzing, Melburne LeMieux, Jason Shugart Nanocarbon has been successfully incorporated in molten metals and metal alloys using a new method of manufacturing in which the molten metal (or metal alloy) acts as ionizing medium causing nanocarbon structures to form in-situ. C in concentrations up to $\sim$10{\%} weight was incorporated in Ag, Al and Cu. The bonding between the carbon and the metal is very strong and persists after re-melting and resolidification. These materials, called ``covetics,'' show improved properties over those of the host metal. For example, the thermal conductivity of Cu covetic is higher than pure Cu. The electrical conductivity of Al covetic is higher than for pure Al. The yield strength of Al and Cu covetics is higher than the pure metals. We have used X-ray diffraction, X-ray photoelectron spectroscopy, and Raman spectroscopy to investigate the incorporation of C in the metal. Scanning and transmission electron microscopy (TEM) were also employed along with energy dispersive X-ray spectroscopy and electron energy loss spectroscopy (EELS). The nanocarbons in the covetics are in the form of, graphene nanoribbons, and amorphous nanocarbon, and all are bonded to the metal. The C-K edge in the EELS, and the Raman spectra from these samples show signals characteristic of graphitic sp2 bonding. [Preview Abstract] |
Tuesday, March 19, 2013 4:30PM - 4:42PM |
J6.00011: Tuning Valley Magnetic Moment in Bilayer MoS2 via Symmetry Control Sanfeng Wu, Jason Ross, Gui-Bin Liu, Grant Aivazian, Aaron Jones, Zaiyao Fei, Wenguang Zhu, Di Xiao, Wang Yao, David Cobden, Xiaodong Xu Monolayer MoS2 provides the opportunity to explore the coupled spin-valley physics arising from broken inversion symmetry. Although inversion symmetry is present in pristine bilayer MoS2, it can be broken by applying a perpendicular electric field. It offers the remarkable possibility of switching on/off and continuously tuning the physical properties of the Dirac valleys, such as valley magnetic moment and Berry curvature, by reversible electrical control. In this work, we employ polarization-resolved photoluminescence (PL) to investigate this effect using bilayer and monolayer MoS2 field effect transistors. We find that in bilayer MoS2 the circularly polarized PL can be continuously tuned from -15{\%} to 15{\%} as a function of gate voltage, whereas in structurally non-centrosymmetric monolayer MoS2 the PL polarization is gate-independent. The observations are well explained as resulting from the continuous variation of orbital magnetic moments between positive and negative values via symmetry control. [Preview Abstract] |
Tuesday, March 19, 2013 4:42PM - 4:54PM |
J6.00012: Electrical tunability and optical control of valley and spin in WSe2 Aaron Jones, Hongyi Yu, Nirmal Ghimire, Bo Zhao, Sanfeng Wu, Grant Aivazian, Jason Ross, Guibin Liu, Jiaqiang Yan, David Mandrus, Wang Yao, Di Xiao, Xiaodong Xu Monolayer group VI transition metal dichalcogenides have enormous potential for use in nano- and optoelectronic applications due to their reduced dimensionality and direct bandgap in the visible wavelength range. Their hexagonal structure is graphene-like, but with strong spin-orbit coupling effects. The interesting coupled spin-valley physics has been investigated both theoretically and experimentally based on the single particle picture. Here we investigate the physical properties of valley excitons in monolayer field effect transistor devices via photoluminescence measurements. By tuning the chemical potential to control exciton species, we are able to investigate the optical selection rules, photo-excitation energy dependence, and temperature dependence of individual excitons. These studies reveal the fine structures of valley excitons due to the electron-electron interactions, electron-phonon interactions, and coupled spin-valley degrees of freedom, which are important for the potential application of valleytronics/spintronics based on monolayer semiconductors. [Preview Abstract] |
Tuesday, March 19, 2013 4:54PM - 5:06PM |
J6.00013: Electronic Properties of Few-layer Black Phosphorus Likai Li, Yijun Yu, G.J. Ye, X.H. Chen, Yuanbo Zhang Black phosphorus is a layered allotropy of phosphorus that closely resembles graphite. In a single atomic layer, phosphorus atoms are covalently bonded into a puckered honey comb structure. All five valence electrons are localized, so unlike graphene monolayer black phosphorus is a semiconductor with a band gap of $\sim$ 2 eV. In a bulk crystal the interlayer coupling reduces the band gap to $\sim$ 0.3 eV. Using mechanical exfoliation method, we have successfully fabricated few layer black phosphorus field effect transistors. Our samples exhibit bipolar behavior with on-off ratio up to 10$^6$, and a low off-state current. Electronic mobilities up to $\sim$ 1000 cm$^2$V$^{-1}$s$^{-1}$ are currently achieved, with possibilities for further improvement. Such characteristics make black phosphorus a potential candidate for future nanoelectronic applications. [Preview Abstract] |
Tuesday, March 19, 2013 5:06PM - 5:18PM |
J6.00014: Bilayer Silicene: a first principles investigation Renato Borges Pontes, Jos\'e Eduardo Padilha, Aldaberto Fazzio, Ant\^onio J.R. da Silva By performing ab-initio total energy calculations we study the structural and electronic properties of a silicene bilayer. We show that the lowest energy configuration, contrary to the Bernal stacking of graphene, is when two silicene sheets are placed exactly one on top of the other. In this configuration, there is an energy gain if the system loses its $\pi$ cloud to create extra ($\sigma$-like) chemical bonds between the two layers. To minimize the total energy and the forces that arise due to these new connections made between the layers, the system increases the lattice constant, becoming planar and, consequently, loosing its buckled structure. Moreover, the bilayer of silicene on this planar configuration is a metal and it is insensitive to the presence of an applied external electrical field, a behaviour different from the single layer. We also discuss the role played by the unoccupied 3d-orbital of the silicon in the formation of this new structure. Theoretical STM calculations show excellent agreement with experimental images of silicene bilayers. [Preview Abstract] |
Tuesday, March 19, 2013 5:18PM - 5:30PM |
J6.00015: ABSTRACT WITHDRAWN |
Session J7: Focus Session: Graphene Devices VI
Sponsoring Units: DMPChair: Roman Gorbachev, University of Manchester
Room: 303
Tuesday, March 19, 2013 2:30PM - 2:42PM |
J7.00001: Effects of optical and surface polar phonons on the optical conductivity of doped graphene Vasili Perebeinos, Benedikt Scharf, Jaroslav Fabian, Phaedon Avouris During the past decade, graphene has attracted immense interest, mainly due to its excellent transport and optical properties, which make it an attractive candidate for possible applications in nanoscale electronics and optoelectronics. Using the Kubo linear response formalism, we study the effects of intrinsic graphene optical and surface polar phonons (SPPs) on the optical conductivity of doped graphene. We find that inelastic electron-phonon scattering contributes significantly to the phonon-assisted absorption in the optical gap. At room temperature, this midgap absorption can be as large as about 20-25{\%} of the universal ac conductivity for graphene on polar substrates (such as Al$_{\mathrm{2}}$O$_{\mathrm{3}}$ or HfO$_{\mathrm{2}})$ due to strong electron-SPP coupling. The midgap absorption, moreover, strongly depends on the substrates and doping levels used. We predict that with increasing temperature, the midgap absorption increases, while the Drude weight decreases. These predictions can serve as an experimental signature for the role of SPPs on transport and optical properties of graphene, which have important implications for the performance of graphene-based electronic devices and broadband modulators. [Preview Abstract] |
Tuesday, March 19, 2013 2:42PM - 2:54PM |
J7.00002: Electrically detected spin resonance in epitaxial graphene Ramesh Mani, John Hankinson, Claire Berger, Walter de Heer Graphene is an appealing material for electron-spin quantum computing (QC) and spintronics, due to the expected weak spin-orbit interaction, and the scarcity of nuclear spin in natural carbon. Due to QC and spintronics, the microwave control and electrical detection of spin have become topics of interest, now in graphene nanostructures, where the small number of spins limits the utility of traditional spin resonance. Here, we report results of an experimental study examining the microwave response of epitaxial graphene.[1] The results suggest the possibility of resistive detection of spin resonance, and they provide a measurement of the g-factor and the spin relaxation time in this novel system.\\ \\ \noindent [1] R. G. Mani, J. Hankinson, C. Berger, and W. de Heer, Nature Comm. 3, 996 (2012). [Preview Abstract] |
Tuesday, March 19, 2013 2:54PM - 3:06PM |
J7.00003: Growth, fabrication, and applications of graphene nanostructures Guangyu Zhang Recently, a broad category of research focused on graphene nanostructures, including graphene nanosheets and nanoribbons. These graphene nanostructures have unique properties related to their sizes, shapes, and edge configurations and might be used as building blocks for various miniaturized graphene-based devices. Large-are growth and scaled-up fabrication of high-quality graphene nanostructures are challenging. In this talk, I will introduce our recent progress on grow large area nanographene directly on substrates and scaled-up fabrication of graphene nanoribbons with controlled width and edge configurations (zigzag edges). Electronic and optical spectroscopy studies on the zigzag-edged graphene nanoribbons yield the experimentally observed metallic edge states and electron-phonon coupling effect. Nanographene-based piezoresistive strain sensors and resistive randomly accessed memories will also be introduced. [Preview Abstract] |
Tuesday, March 19, 2013 3:06PM - 3:42PM |
J7.00004: Toward Graphene-Based Microwave Photon Counter Invited Speaker: Kin Chung Fong Graphene is a material with remarkable electronic properties. However, the thermal properties of this two-dimensional Dirac Fermions, that determine the characteristics of photo detectors, plasmonic devices, and bolometers, are less explored. Here, we present our measurement of specific heat capacity, Wiedemann-Franz (WF), and electron-phonon (e-ph) thermal conductance from 0.3 to 100 K using the novel single layer graphene bolometer [1]. These measurements suggest that graphene-based devices can generate substantial advances in the areas of ultra-sensitive bolometry, calorimetry, microwave, and terahertz single photon detection for applications in areas such as observational astronomy, quantum information and measurement. The physics of the e-ph coupling and the possible violation of Wiedemann-Franz Law near the charge neutrality point in single layer graphene will be discussed.\\[4pt] This work is a collaboration with Emma Wollman, Harish Ravi, and K. C. Schwab of Caltech. This work has been supported by the FCRP Center on Functional Engineering Nano Architectonics (FENA) and U.S. NSF Contract No. (DMR-0804567).\\[4pt] [1] K.C. Fong and K.C. Schwab, Phys. Rev. X 2, 031006 (2012) [Preview Abstract] |
Tuesday, March 19, 2013 3:42PM - 3:54PM |
J7.00005: Photovoltaic response time in dual-gated bilayer graphene M.-H. Kim, J. Yan, R.J. Suess, T. Murphy, M.S. Fuhrer, H.D. Drew The intrinsic thermal response timescale of bilayer graphene is sub nanosecond, due to cooling of hot electrons mediated by acoustic phonon emission. We compare the response times of the photovoltaic and bolometric response as a function of temperature and dual-gate voltages in a gapped bilayer graphene device using a pulse coincidence technique at 1.5 $\mu$m. We find that the photovoltaic and bolometric response time are identical and vary from 100 ps to 10 ps for temperatures from 3 K to 100 K. This result shows that the near IR photovoltaic response of bilayer graphene is thermal over this temperature range. This work was supported by IARPA, the ONR MURI program, and the NSF (grants DMR-0804976 and DMR-1105224), and in part by the NSF MRSEC (grant DMR-0520471). [Preview Abstract] |
Tuesday, March 19, 2013 3:54PM - 4:06PM |
J7.00006: Graphene-Superconductor hybrid device as Bolometer Heli Vora, Naomi Mizuno, Piranavan Kumaravadivel, Bent Nielsen, Xu Du Low electronic heat capacity and small achievable volume has made graphene a promising candidate for a fast and sensitive bolometric detector. In our device scheme, in addition to low electron-phonon coupling we can further limit out-diffusion of hot electrons from graphene into the leads by placing tunnel-type superconducting contacts on graphene and preventing tunneling through the oxide barrier into the superconducting gap. We fabricate NbN contacts on graphene with a sandwich layer of titanium oxide tunnel barrier. Due to high dielectric constant of titanium oxide, our design allows device impedance matching with the antenna circuitry at THz frequencies, necessary to achieve practical device efficiency. We present our measurements of bolometric characteristics of thermal conductance, Noise equivalent Power (NEP) and responsivity for such a device. [Preview Abstract] |
Tuesday, March 19, 2013 4:06PM - 4:18PM |
J7.00007: Temporal characterization of hot-electron thermoelectric effect in monolayer graphene devices Ryan J. Suess, Xinghan Cai, Andrei Sushkov, Greg Jenkins, M.-H. Kim, Jun Yan, H. Dennis Drew, Thomas E. Murphy, Michael S. Fuhrer Graphene's unique electronic and optical properties have made it an attractive candidate material for photonics applications such as broadband optical detection. We report the temporal response of a monolayer graphene device with dissimilar metal electrodes in which optically induced hot-electrons are detected via a thermoelectric voltage induced between the electrodes. Measurements are carried out with a pulsed laser system (60 fs pulse width) at the telecom wavelength of 1.5 $\mu$m using an asynchronous optical sampling pulse coincidence technique. Graphene's weak electron-phonon coupling and our compact device geometry (comparable to the thermal diffusion length) result in a fast 10 - 20 ps non-linear thermal response that is nearly independent of temperature over the measured range of 15 - 150 K. Sensitivity of the devices response to optical power will also be discussed. These results are a follow-on to other talks reported by our group at this conference in which the fabrication, operating principal, and broad wavelength (THz to near IR) response of the graphene-based hot-electron bolometer are described. [Preview Abstract] |
Tuesday, March 19, 2013 4:18PM - 4:30PM |
J7.00008: Sensitive bolometry using hot-electron thermoelectric effect in graphene devices Xinghan Cai, Ryan J. Suess, Andrei Sushkov, Greg Jenkins, M.-H. Kim, Jun Yan, H. Dennis Drew, Thomas E. Murphy, Michael S. Fuhrer Due to the weak electron-phonon coupling and strong electron-electron interaction in graphene, the hot-electron thermoelectric effect provides a highly sensitive detection mechanism for heat absorbed in the electronic system, either by radiation or Joule heating. We have fabricated graphene devices using mechanically exfoliated single layer graphene contacted by two dissimilar metal electrodes (chromium and gold) in order to generate an asymmetry in the device and a net thermoelectric response to heating. We measure the thermoelectric response to Joule heating by an AC 2$^{nd}$ harmonic method, and compare to the thermoelectric response due to optical excitation in the near infrared and at THz frequencies. We find a sensitivity exceeding 100 V/W at room temperature. We also demonstrate that the sensitivity can be significantly enhanced by patterning the graphene sheet into nanoribbon arrays. The transport measurements indicate that graphene is a promising candidate for sensitive broadband photo detectors at room temperature. Related work by our group showing that ultra-broadband detection of light can be realized in such devices will be presented in other talks at this meeting. [Preview Abstract] |
Tuesday, March 19, 2013 4:30PM - 4:42PM |
J7.00009: Large-scale 2D Electronics based on Single-layer MoS2 Han Wang, Lili Yu, Yi-Hsien Lee, Wenjing Fang, Allen Hsu, Patrick Herring, Matthew Chin, Madan Dubey, Lain-Jong Li, Jing Kong, Tomas Palacios 2D nanoelectronics based on MoS2 and other transition metal dichalcogenides (TMD) materials are attractive as high-mobility options in the emerging field of large-area low-cost electronics that is currently dominated by low-mobility amorphous silicon and organic semiconductors. Single-layer MoS2 can also complement graphene to build flexible digital and mixed-signal circuits, overcoming its lack of bandgap while still sharing many of graphene's excellent mechanical and thermal properties. This paper addresses several key challenges in the development of 2D nanoelectronics on MoS2 and TMD materials in general. First, large-area single-layer MoS2 material is grown by chemical vapor deposition (CVD) that makes the wafer-scale fabrication of MoS2 devices and circuits possible for the first time. Second, the top-gated transistors, fabricated for the first time on single-layer MoS2 grown by CVD, show multiple state-of-the-art characteristics, such as high mobility, ultra-high on/off current ratio, record current density and current saturation. Finally, key circuit building blocks for digital and analog electronics such as inverter, NAND gate, memory and ring oscillator are demonstrated for the first time. [Preview Abstract] |
Tuesday, March 19, 2013 4:42PM - 4:54PM |
J7.00010: Band-like transport in high mobility single-layer MoS$_{2}$ FETs Deep Jariwala, Vinod Sangwan, James Johns, Dattatray Late, Ken Everaerts, Julian McMorrow, Lincoln Lauhon, Vinayak Dravid, Tobin Marks, Mark Hersam The recent realization of monolayered MoS$_{2}$ as a direct band gap two-dimensional semiconductor in contrast to zero gap graphene, has attracted significant attention for digital electronic applications. In most measurements to date, single-layer MoS$_{2}$ field-effect transistors (FETs) have shown low field-effect mobility values that have been explained by Mott variable range hopping (VRH) transport. In contrast, here we report variable temperature measurements on high mobility (greater than 50 cm$^{2}$/V.s at room temperature) single-layer MoS$_{2}$ FETs that show band-like transport with monotonic increase in mobility with decreasing temperature suggesting phonon quenching at low temperatures as also observed for graphene. The magnitude of the drain current remains constant across the range of temperatures (5.7 - 298 K), while the threshold voltage displays a positive shift. In this presentation we emphasize on high quality single-layer MoS$_{2}$ FETs with band-like transport and the highest reported field-effect mobility values (120 cm$^{2}$/V.s at 5.7 K) in devices without encapsulation in a high-$\kappa $ dielectric. [Preview Abstract] |
Tuesday, March 19, 2013 4:54PM - 5:06PM |
J7.00011: Wafer-scalable high-performance CVD graphene devices and analog circuits Li Tao, Jongho Lee, Huifeng Li, Richard Piner, Rodney Ruoff, Deji Akinwande Graphene field effect transistors (GFETs) will serve as an essential component for functional modules like amplifier and frequency doublers in analog circuits. The performance of these modules is directly related to the mobility of charge carriers in GFETs, which per this study has been greatly improved. Low-field electrostatic measurements show field mobility values up to 12k cm$^{\mathrm{2}}$/Vs at ambient conditions with our newly developed scalable CVD graphene. For both hole and electron transport, fabricated GFETs offer substantial amplification for small and large signals at quasi-static frequencies limited only by external capacitances at high-frequencies. GFETs biased at the peak transconductance point featured high small-signal gain with eventual output power compression similar to conventional transistor amplifiers. GFETs operating around the Dirac voltage afforded positive conversion gain for the first time, to our knowledge, in experimental graphene frequency doublers. This work suggests a realistic prospect for high performance linear and non-linear analog circuits based on the unique electron-hole symmetry and fast transport now accessible in wafer-scalable CVD graphene. *Support from NSF CAREER award (ECCS-1150034) and the W. M. Keck Foundation are appreicated. [Preview Abstract] |
Tuesday, March 19, 2013 5:06PM - 5:18PM |
J7.00012: Graphene Field-Effect Transistors with Gigahertz-Frequency Power Gain on Flexible Substrates Nicholas Petrone, Inanc Meric, Kenneth Shepard, James Hone The development of flexible electronics operating at radio-frequencies (RF) requires materials which combine excellent electronic performance and the ability to withstand high levels of strain. Graphene's unique electronic and mechanical properties make it a promising material for the fabrication of field-effect transistors (FETs) which require both high flexibility and high operating frequencies. Furthermore, large-area films of graphene which display excellent electronic properties, crucial for the commercial realization of graphene-based devices, can be synthesized facilely by chemical vapor deposition (CVD). We utilize CVD graphene to fabricate graphene FETs (GFETs) on flexible substrates. Our GFETs demonstrate unity-current-gain frequencies, $f_{T}$, and unity-power-gain frequencies, $f_{max}$, up to 10.7 and 3.7 GHz, respectively, with strain limits of 1.75{\%}. These devices represent the only reported technology to achieve gigahertz-frequency power gain at strain levels above 0.5{\%}. As such, they demonstrate the potential for CVD graphene to enable a broad range of flexible electronic technologies which require both high-flexibility and RF operation. [Preview Abstract] |
Tuesday, March 19, 2013 5:18PM - 5:30PM |
J7.00013: Optimization of Ferroelectric Polymer$\backslash $Graphene Films for Transparent and Flexible Electronics Orhan Kahya, Jing Wu, Guang-Xin Ni, Chee-Tat Toh, Sang-Hoon Bae, Jong-Hyun Ahn, Barbaros Oezyilmaz Nonvolatile, electrostatic doping of graphene-based devices with ferroelectric polymers such as Poly (vinylidene fluoride-trifluoroethylene) are promising for realizing ultra-fast, flexible memory devices, nanogenerators and actuators. More recently, the same approach has been shown to provide an alternative route in enabling graphene based transparent electrodes for touch screen applications. Here, we report a systematic study of optimizing the ferroelectric polymer-graphene heterostructure as a function of thickness, various copolymer blends and coating techniques. Optimized films show outstanding mechanical properties, low sheet resistance ($\sim$ 100$\Omega $/sq) and optical transparency levels as high as 96{\%}. [Preview Abstract] |
Session J8: Focus Session: Graphene: Raman, Strain, Thermal
Sponsoring Units: DMPChair: Jun Yan , University of Massachusetts at Amhearst
Room: 307
Tuesday, March 19, 2013 2:30PM - 2:42PM |
J8.00001: Identifying Few-Layer Graphene with Raman Spectroscopy David Tran, Nathaniel Gillgren, Kevin Myhro, Yongjin Lee, Jairo Velasco Jr., Lei Jing, Marc Bockrath, Jeanie Lau Few-layer graphene (FLG) exists in various crystallographic stacking sequences, which can strongly influence the material's electronic properties. We characterize stacking order in FLG using the distinctive features of the Raman 2D-mode's full-width at half-maximum (FWHM), relative peak size, and shape. Raman imaging allows us to visualize directly the spatial distribution of bilayer graphene, Bernal (ABA) trilayer graphene, and rhombohedral (ABC) trilayer graphene. [Preview Abstract] |
Tuesday, March 19, 2013 2:42PM - 2:54PM |
J8.00002: Optical Separation of Mechanical Strain from Charge Doping in Graphene Sunmin Ryu, Ji Eun Lee, Gwanghyun Ahn Graphene, due to its superior stretchability, exhibits rich structural deformation behaviors and its strain-engineering has proven useful in modifying its electronic and magnetic properties. Despite the strain-sensitivity of the Raman G and 2D modes, the optical characterization of the native strain in graphene on silica substrates has been hampered by excess charges interfering with both modes. Here we show that the effects of strain and charges can be optically separated from each other by correlation analysis of the two modes, enabling simple quantification of both. Graphene with in-plane strain randomly occurring between -0.2{\%} and 0.4{\%} undergoes modest compression (-0.3{\%}) and significant hole doping upon thermal treatments. This study suggests that substrate-mediated mechanical strain is a ubiquitous phenomenon in two-dimensional materials. The proposed analysis will be of great use in characterizing graphene-based materials and devices. [Preview Abstract] |
Tuesday, March 19, 2013 2:54PM - 3:06PM |
J8.00003: Folded Optical Phonons in Twisted Bilayer Graphene: Raman Signature of Graphene Superlattices Yanan Wang, Zhihua Su, Wei Wu, Sirui Xing, Xiaoxiang Lu, Xinghua Lu, Shin-shem Pei, Francisco Robles-Hernandez, Viktor Hadjiev, Jiming Bao In contrast to Bernal-stacked graphene exfoliated from HOPG, twisted bilayer graphene are widely observed in the samples prepared by silicon sublimation of SiC or chemical vapor deposition (CVD). However, many of its basic properties still remain unrevealed. In this work, hexagon-shaped bilayer graphene islands synthesized by CVD method were systematically studied using Raman spectroscopy. A series of folded phonons were observed in the range from 1375 cm$^{-1}$ to 1525 cm$^{-1}$. The frequency of folded phonon modes doesn't shift with laser excitation energy, but it is highly dependent on the rotational angle between two layers. In general, the frequency of folded phonon decreases with the increase of rotation angle. This rotation dependence can be qualitatively explained by the folding of phonon dispersion curve of single layer graphene into the reduced Brillouin zone of bilayer superlattice. The obseravtion of folded phonon is an important indication of superlattice band structure. [Preview Abstract] |
Tuesday, March 19, 2013 3:06PM - 3:42PM |
J8.00004: Large Physisorption Strain in Graphene Grown by Chemical Vapor Deposition on Copper Substrates Invited Speaker: Rui He Single layer graphene grown by chemical vapor deposition (CVD) on Cu substrates is subject to non-uniform physisorption strain that is dependent on the orientation of the Cu surface. The blue-shift and broadening of Raman bands of graphene on the Cu single crystal (111) surface reveal that the graphene layer is under compressive strain. This interpretation is consistent with Moire patterns seen in scanning tunneling microscopy. Graphene grown on the Cu (100) surface is subject to a highly non-uniform strain due to the mismatch between the graphene honeycomb lattice and the square lattice at this Cu surface. Molecular Dynamics simulations are in excellent agreement with experiment, predicting compressive strain on the order of 0.5 percent in graphene/Cu(111). In graphene/Cu(100) the simulated physisorption strain patterns show linear superstructures spaced about 1 nm apart and a highly non-uniform bond length distribution which leads to both compressive and tensile strains. CVD graphene grown on polycrystalline Cu foil is also studied for comparison. The strain in graphene is even more non-uniform for growth on Cu foil. However, this strain is greatly reduced after the graphene layer is removed and transferred onto a SiO2 substrate. Physisorption strain is thus revealed to be a major factor in the growth of CVD graphene on transition metals. [Preview Abstract] |
Tuesday, March 19, 2013 3:42PM - 3:54PM |
J8.00005: 2D line enhancement by quantum interference in graphene superlattice Zhihua Su, Yanan Wang, Wei Wu, Sirui Xing, Xiaoxiang Lu, Xinghua Lu, Shin-shem Pei, Francisco Robles-Hernandez, Viktor G. Hadjiev, Jiming Bao Raman scattering is used to study twisted bilayer graphene synthesized by chemical vapor deposition (CVD) method with rotation angle determined by relative edge misalignment. Degenerate Dirac band of twisted bilayer graphene is revealed by enhanced intensity of 2D line. This Raman signature is systematically studied and found to be correlated with G-line resonance and laser excitation energy. 2D enhancement only happens when the laser excitation energy is smaller than G-line resonance energy, while enhancement ratio increases as laser excitation energy decreases. The anomalous enhancement of 2D intensity is ascribed to the constructive quantum interference between two Raman paths enabled by degenerate Dirac cone. [Preview Abstract] |
Tuesday, March 19, 2013 3:54PM - 4:06PM |
J8.00006: Wrinkling instability in nanoparticle-supported graphene: implications for strain engineering William Cullen, Mahito Yamamoto, Olivier Pierre-Louis, Jia Huang, Michael Fuhrer, Theodore Einstein We have carried out a systematic study of the wrinkling instability of graphene membranes supported on SiO$_{2}$ substrates with randomly placed silica nanoparticles. At small nanoparticle density, monolayer graphene adheres to the substrate and is highly conformal over the nanoparticles. With increasing nanoparticle density, and decreasing nanoparticle separation to $\sim$100 nm, graphene's elastic response dominates substrate adhesion, and elastic stretching energy is reduced by the formation of wrinkles which connect protrusions. Above a critical nanoparticle density, the wrinkles form a percolating network through the sample. As the graphene membrane is made thicker, delamination from the substrate is observed. Since the wrinkling instability acts to remove inhomogeneous in-plane elastic strains through out-of-plane buckling, our results can be used to place limits on the possible in-plane strain magnitudes that may be created in graphene to realized strain-engineered electronic structures.\footnote{M. Yamamoto et al., ``Princess and the Pea at the nanoscale: Wrinkling and unbinding of graphene on nanoparticles,'' arXiv:1201.5667 (2012).} [Preview Abstract] |
Tuesday, March 19, 2013 4:06PM - 4:18PM |
J8.00007: Graphene slides over the substrate when you pull it: Direct measurement, theory, and frictional forces Alexander Kitt, Zenan Qi, Harold Park, Anna Swan, Bennett Goldberg Using graphene sealed cylindrical microchambers we characterize, for the first time, the sliding of graphene over a thermal oxide substrate. High spatially resolved Raman spectra recorded as external pressure is applied to the microchamber shows that as the graphene is pressed into the hole it drags some of the previously substrate-supported graphene with it. The well-understood strain response of the Raman G-band allows us to measure strains of less than .01{\%}, corresponding to 1nm stretching over a micron, with 500 nm lateral resolution as pressure is applied to the system. Our results are compared to both atomistic and continuum models, with interesting new conclusions, of the system in order to quantify the sliding for mono, bi, and tri layer graphenes over holes of radii between 1.25 and 5 $\mu$m with applied pressures between 0 and 100 psi. [Preview Abstract] |
Tuesday, March 19, 2013 4:18PM - 4:54PM |
J8.00008: Raman spectroscopy of graphene under strain Invited Speaker: Hyeonsik Cheong Raman spectroscopy is one of the most widely used characterization tools in graphene research. It has been used to estimate the number of layers, carrier concentration, edge types, thermal conductivity, etc. The effect of strain on the Raman spectrum of graphene is of particular interest, because the modification of the lattice due to strain is directly reflected on the Raman spectrum. Under uniaxial strain, the G and 2D bands of the Raman spectrum of single-layer graphene split due to the breaking of symmetry. From the polarization dependence of the split bands, one can determine the crystallographic orientation of the graphene sample. Furthermore, the splitting of the 2D band depends on the direction of the applied strain with respect to the crystallographic orientation, which provides critical information on the dominant scattering process responsible for this band. Bilayer graphene under uniaxial strain exhibits similar effects on the G and 2D bands, although the effect on the 2D band is rather complicated as each of the 4 components comprising the 2D band splits under strain. One can also estimate the thermal expansion coefficient and Young's modulus using Raman measurements under strain. Recent experimental results will be presented. [Preview Abstract] |
Tuesday, March 19, 2013 4:54PM - 5:06PM |
J8.00009: Experimental Measurement of Thermal Expansion Coefficient of few layer graphene Lei Jing, Wenzhong Bao, Hoon Cho, Fenglin Wang, Chun Ning(Jeanie) Lau In contrast to most materials, graphene has negative thermal expansion coefficient (TEC), which has important implications on device applications. We experimentally measure the TEC of single- and few-layer graphene by suspending them across predefined trenches, and monitoring their sagging arc lengths during cooling via in situ SEM imaging. Latest experimental data will be discussed and compared to theoretical models. [Preview Abstract] |
Tuesday, March 19, 2013 5:06PM - 5:18PM |
J8.00010: Lengthy logarithms and graphene's Debye-Waller factor B.C. Regan, Brian Shevitski, William A. Hubbard, E.R. White, Ben Dawson, M.S. Lodge, Masa Ishigami, Matthew Mecklenburg In an infinite, two-dimensional crystal, long wavelength thermal phonons create a divergence in the mean-square displacement $u_p^2$ of atoms from their ideal lattice positions, which has led some to infer that the existence of graphene might depend on the stabilizing influence of ripples in the third dimension. Using the Debye model to approximate graphene's phonon band structure, we calculate $u_p^2$ and the resulting Debye-Waller suppression of high order peaks in graphene's electron diffraction pattern. We find that at room temperature in a 10~$\mu$m sample $\sqrt{u_p^2}$ is less than 5\% of the carbon-carbon bond length, well below the Lindemann melting threshold. Our TEM measurements of the Debye-Waller factor in suspended, exfoliated graphene agree with the calculation. Finite size effects are sufficient to explain graphene's evident stability at room temperature. Surprisingly, in the case of graphene even $6\times 10^{23}$ carbon atoms, representing a sheet 126~m on a side, are not enough to approximate an infinitely large crystal. [Preview Abstract] |
Tuesday, March 19, 2013 5:18PM - 5:30PM |
J8.00011: In-Situ Thermal Mapping of Graphene via TEM Measurement of the Debye-Waller Factor William A. Hubbard, Matthew Mecklenburg, B.C. Regan Thermal motion of the constituent atoms attenuates high-order peaks in a crystal's electron diffraction pattern. Using TEM we measure this attenuation, parameterized by a Debye-Waller factor, in single-layer cleaved graphene that is Joule-heated \emph{in situ}. We find that the Debye-Waller factor, as probed with selected area electron diffraction, provides a reliable measure of the local temperature and thus allows for quantitative thermal mapping on the nanoscale. [Preview Abstract] |
Session J9: Invited Session: Computational Physics at the Bleeding Edge: To Exascale and Beyond
Sponsoring Units: DCOMPChair: Timothy Germann, Los Alamos National Laboratory
Room: 308
Tuesday, March 19, 2013 2:30PM - 3:06PM |
J9.00001: Intricacies of modern supercomputing illustrated with recent advances in simulations of strongly correlated electron systems Invited Speaker: Thomas C. Schulthess The continued thousand-fold improvement in sustained application performance per decade on modern supercomputers keeps opening new opportunities for scientific simulations. But supercomputers have become very complex machines, built with thousands or tens of thousands of complex nodes consisting of multiple CPU cores or, most recently, a combination of CPU and GPU processors. Efficient simulations on such high-end computing systems require tailored algorithms that optimally map numerical methods to particular architectures. These intricacies will be illustrated with simulations of strongly correlated electron systems, where the development of quantum cluster methods, Monte Carlo techniques, as well as their optimal implementation by means of algorithms with improved data locality and high arithmetic density have gone hand in hand with evolving computer architectures. The present work would not have been possible without continued access to computing resources at the National Center for Computational Science of Oak Ridge National Laboratory, which is funded by the Facilities Division of the Office of Advanced Scientific Computing Research, and the Swiss National Supercomputing Center (CSCS) that is funded by ETH Zurich. [Preview Abstract] |
Tuesday, March 19, 2013 3:06PM - 3:42PM |
J9.00002: Seeking a sustainable approach for computational science Invited Speaker: Robert Harrison Many are now questioning whether our current approaches to developing software for science and engineering are sustainable. In particular, can we deliver to society and the nation the full benefits expected from high-performance simulation at the peta and exascales? Or is innovative science being stifled by the increasing complexities of all aspects of our problem space (rapidly changing hardware, software, multidisciplinary physics, etc.)? Focusing on applications in chemistry and materials science, and motivated by the co-design of exascale hardware and software, I will discuss many of these issues including how chemistry has already been forced to adopt solutions that differ quite sharply to those in the mainstream, and how these solutions position us well for the technology transitions now under way. Radical changes in how we compute, going all the way back to the underlying numerical representation and algorithms used for the simulation, also promise great enhancements to both developer productivity and the accuracy of simulations. [Preview Abstract] |
Tuesday, March 19, 2013 3:42PM - 4:18PM |
J9.00003: Nicholas Metropolis Award for Outstanding Doctoral Thesis Work in Computational Physics Lecture: The Janus computer, a new window into spin-glass physics Invited Speaker: David Yllanes Spin glasses are a longstanding model for the sluggish dynamics that appears at the glass transition. They enjoy a privileged status in this context, as they provide the simplest model system both for theoretical and experimental studies of glassy dynamics. However, in spite of forty years of intensive investigation, spin glasses still pose a formidable challenge to theoretical, computational and experimental physics. The main difficulty lies in their incredibly slow dynamics. A recent breakthrough has been made possible by our custom-built computer, Janus, designed and built in a collaboration formed by five universities in Spain and Italy. By employing a purpose-driven architecture, capable of fully exploiting the parallelization possibilities intrinsic to these simulations, Janus outperforms conventional computers by several orders of magnitude. After a brief introduction to spin glasses, the talk will focus on the new physics unearthed by Janus. In particular, we recall our numerical study of the nonequilibrium dynamics of the Edwards-Anderson Ising Spin Glass, for a time that spans eleven orders of magnitude, thus approaching the experimentally relevant scale (i.e. seconds). We have also studied the equilibrium properties of the spin-glass phase, with an emphasis on the quantitative matching between non-equilibrium and equilibrium correlation functions, through a time-length dictionary. Last but not least, we have clarified the existence of a glass transition in the presence of a magnetic field for a finite-range spin glass (the so-called de Almeida-Thouless line). We will finally mention some of the currently ongoing work of the collaboration, such as the characterization of the non-equilibrium dynamics in a magnetic field and the existence of a statics-dynamics dictionary in these conditions. [Preview Abstract] |
Tuesday, March 19, 2013 4:18PM - 4:54PM |
J9.00004: Programming for 1.6 Millon cores: Early experiences with IBM's BG/Q SMP architecture Invited Speaker: James Glosli With the stall in clock cycle improvements a decade ago, the drive for computational performance has continues along a path of increasing core counts on a processor. The multi-core evolution has been expressed in both a symmetric multi processor (SMP) architecture and cpu/GPU architecture. Debates rage in the high performance computing (HPC) community which architecture best serves HPC. In this talk I will not attempt to resolve that debate but perhaps fuel it. I will discuss the experience of exploiting Sequoia, a 98304 node IBM Blue Gene/Q SMP at Lawrence Livermore National Laboratory. The advantages and challenges of leveraging the computational power BG/Q will be detailed through the discussion of two applications. The first application is a Molecular Dynamics code called ddcMD. This is a code developed over the last decade at LLNL and ported to BG/Q. The second application is a cardiac modeling code called Cardioid. This is a code that was recently designed and developed at LLNL to exploit the fine scale parallelism of BG/Q's SMP architecture. Through the lenses of these efforts I'll illustrate the need to rethink how we express and implement our computational approaches. [Preview Abstract] |
Tuesday, March 19, 2013 4:54PM - 5:30PM |
J9.00005: Overcoming Communication Latency Barriers in Massively Parallel Molecular Dynamics Simulations on Anton Invited Speaker: Ron Dror Strong scaling of scientific applications on parallel architectures is increasingly limited by communication latency. This talk will describe the techniques used to reduce latency and mitigate its effects on performance in Anton, a massively parallel special-purpose machine that accelerates molecular dynamics (MD) simulations by orders of magnitude compared with the previous state of the art. Achieving this speedup required both specialized hardware mechanisms and a restructuring of the application software to reduce network latency, sender and receiver overhead, and synchronization costs. Key elements of Anton's approach, in addition to tightly integrated communication hardware, include formulating data transfer in terms of counted remote writes and leveraging fine-grained communication. Anton delivers end-to-end inter-node latency significantly lower than any other large-scale parallel machine, and the total critical-path communication time for an Anton MD simulation is less than 3{\%} that of the next-fastest MD platform. [Preview Abstract] |
Session J10: Invited Session: Fostering Collaborations in Minority-Serving Institutions
Sponsoring Units: COMChair: Joseph Barranco, San Francisco State University
Room: 309
Tuesday, March 19, 2013 2:30PM - 3:06PM |
J10.00001: Pipeline and Research Collaborations with MSIs Invited Speaker: Charles Weatherford I am a Physics faculty member at an HBCU (Historically Black College/University). I am currently the chairperson of physics at Florida A\&M University and have occupied this position for seventeen of the last twenty-three years. I am very supportive of MSIs in general but my experience has been at an HBCU and thus my statements are mostly directed at interactions with HBCUs. My remarks are directed towards facilitating pipeline and research collaborations between MSIs (minority-serving institutions) and majority research universities, government scientific laboratories and agencies, and industry (majority collaborating institutions--MCIs). I will make some generally applicable statements about what I consider are prerequisites for the MSIs and the MCIs needed to establish effective collaborations. I will then make several distinctions which are specific to HBCUs. [Preview Abstract] |
Tuesday, March 19, 2013 3:06PM - 3:42PM |
J10.00002: The Center for Gravitational Wave Astronomy at UTB Invited Speaker: Mario Diaz In this talk I will succinctly describe the first ten years of research and educational activity at the Center for Gravitational Wave Astronomy at The University of Texas at Brownsville as a potential model for fostering collaborations between minority serving institutions and major research institutions in the USA. [Preview Abstract] |
Tuesday, March 19, 2013 3:42PM - 4:18PM |
J10.00003: Successful strategies for building thriving undergraduate physics programs at minority serving institutions Invited Speaker: Quinton Williams After having been pulled back from the brink of academic program deletion, Jackson State University (Jackson, Mississippi) is now the only HBCU (Historically Black College and University) listed as a top producer of B.S. degrees earned by African Americans in both fields of physics and geoscience. Very pragmatic, strategic actions were taken to enhance the undergraduate degree program which resulted in it becoming one of the most productive academic units at the university. Successful strategies will be shared for growing the enrollment of physics majors, building productive research/educational programs, and improving the academic performance of underprepared students. Despite myriad challenges faced by programs at minority serving institutions in a highly competitive 21$^{\mathrm{st}}$ century higher education system, it is still possible for undergraduate physics programs to transition from surviving to thriving. [Preview Abstract] |
Tuesday, March 19, 2013 4:18PM - 4:54PM |
J10.00004: A Perspective on the Intersection of Institutional Identity and Collaborative Research: Toward More Effective Partnering With Historically Black Colleges and Universities (HBCUs) Invited Speaker: John Harkless Science departments at historically black colleges and universities (HBCUs) play important roles in providing quality education in a distinctive environment. ~The presenter is an HBCU alumnus who earned his doctorate from a primarily majority institution (PMI) and has had experience as both PMI and HBCU faculty. This experience frames and informs the observations shared in this presentation about the unique challenges and opportunities across an array of HBCU departments. Resources available, demographics impacted, current challenges, and the value of the institution to students, external partners, and the community-at-large will be discussed, with a focus on development of dialogue on the cultural and collaborative competencies necessary to working across institutional types. [Preview Abstract] |
Session J11: Dillon Medal Symposium
Sponsoring Units: DPOLYChair: Arun Yethiraj, University of Wisconsin
Room: 310
Tuesday, March 19, 2013 2:30PM - 3:06PM |
J11.00001: John H. Dillon Medal Lecture: Molecular Heterogeneity in Block Copolymer Self-Assembly Invited Speaker: Mahesh Mahanthappa Narrow molecular weight dispersity in block copolymers has long been considered necessary for well-defined, periodic structure formation, by analogy to various crystallization processes. Consequently, much attention has focused on narrow dispersity copolymers derived from controlled and ``living'' polymerization techniques. However, these methods restrict the palette of functional monomers amenable to block copolymerization, thus constraining the physical and chemical properties of the resulting materials. New polymer syntheses enable access to a ``Pandora's Box'' of block copolymers with unusual chemical functionalities and useful physical properties, at the expense of introducing significant segmental dispersities into the resulting copolymers. The development and use of these functional materials requires basic understanding of the physical implications of continuous segmental dispersity on block copolymer phase behavior. Our work aims to understand the physical principles underlying polydisperse ABA-type triblock copolymer self-assembly, in order to transform segmental dispersity into a predictable and useful tool for manipulating block copolymer morphology. We have systematically demonstrated that mid-segment dispersity in ABA triblock copolymers does not preclude the formation of classical, structurally periodic, microphase separated morphologies. Mid-segment dispersity instead shifts the locations of the composition-dependent phase windows, dilates the microdomains, and unexpectedly stabilizes the microphase separated ABA triblock copolymer melts. Studies of three different polydisperse copolymer systems have provided general insights into the consequences of chain length heterogeneity on block copolymer self-assembly. [Preview Abstract] |
Tuesday, March 19, 2013 3:06PM - 3:18PM |
J11.00002: Sphere Forming SISO Tetrablock Terpolymers Frank Bates, Jingwen Zhang, Sangwoo Lee A series of poly(styrene-$b$-isoprene-$b$-styrene-$b$-ethylene oxide) (SISO) tetrablock terpolymers was synthesized using anionic polymerization methods and investigated for melt morphology by synchrotron small-angle x-ray scattering (SAXS), electron microscopy and dynamic mechanical spectroscopy. Thermodynamic incompatibility between the I and O blocks, relative to that characterizing S and I and S and O leads to the formation of spherical domains containing an O core surrounded by a S rich shell embedded in a matrix of mixed S and I. Varying the composition and relative length of the internal and terminal S blocks resulted in five distinct states of ordering, including two cubic (Im3m and Pm3n symmetry), a hexagonal (p6/mm), a tetragonal (P4$_{\mathrm{2}}$/mnm) and a quasicrystalline (dodecagonal) phase. These results demonstrate the concept of decoupling domain geometry from domain packing in multiblock polymers. [Preview Abstract] |
Tuesday, March 19, 2013 3:18PM - 3:30PM |
J11.00003: What do we understand about equilibrium in block polymer micelles? Timothy Lodge The factors that dictate the choice of micellar morphology - sphere, worm, or vesicle - are generally well known. In contrast, the pathways by which micelles reach equilibrium (either in terms of morphology or aggregation number), are not. Even as simple a question as ``How does the critical micelle concentration depend on chain length?'' does not have a clear answer. The current situation will be summarized, in light of recent experimental results. [Preview Abstract] |
Tuesday, March 19, 2013 3:30PM - 3:42PM |
J11.00004: Melt and Solid-State Structures of Polydisperse Polyolefin Block Copolymers Richard Register, Sheng Li Recent developments in coordinative chain transfer polymerization have enabled the synthesis of ethylene-co-octene block copolymers, where the blocks are either crystallizable (an ethylene-co-octene random copolymer block with low octene content) or amorphous (analogous block with high octene content). With a suitable choice of catalyst type(s) and reactor train configuration, accessible chain architectures include diblock, where each block ideally has the most-probable distribution of chain lengths, and multiblock, where both the individual blocks and the number of blocks per chain follow the most-probable distribution. With a sufficiently large interblock octene differential, block copolymers of both architectures, containing roughly equal masses of the two types of block, self-assemble in the melt into well-ordered lamellar structures, despite the large polydispersity. Interblock mixing, induced by the modest Flory interaction parameter and the broad distribution of block lengths, yields an enormous domain spacing (\textgreater\ 100 nm) despite the relatively low average block molecular weights (\textless\ 50 kg/mol). Extensive interblock mixing also allows the polyethylene crystals to grow freely and nearly isotropically across the domain interfaces, while preserving the domain structure present in the melt; in the solid state, the optical and x-ray contrasts between dissimilar domains are greatly enhanced due to their different levels of crystallinity. (Work conducted in collaboration with Jeffrey Weinhold, Philip Hustad, and Brian Landes of Dow Chemical Core R{\&}D.) [Preview Abstract] |
Tuesday, March 19, 2013 3:42PM - 3:54PM |
J11.00005: Effects of molecular architecture and degree of hydration on the structure and properties of electrostatically self-assembled block copolymers Matthew Tirrell, Daniel Krogstad, Nathaniel Lynd, Jason Spruell, Soo-Hyung Choi, Craig Hawker, Edward Kramer Mixtures of water-soluble block copolymers, with one neutral block and the other blocks(s) either positively or negatively charged, are known to form micelles in water with micellar cross that are formed from polyelectrolyte complexes. At sufficiently high total polymer concentrations, such micellar suspensions undergo a disorder-order transition to a bcc structure. This typically occurs between 10 to 15{\%} polymer concentration. In this work, we present new data comparing the behavior of diblock copolymers and triblock copolymers. Initial results suggest that very similar final structures are formed in the two systems, when the diblock is just one of two symmetrical halves of the triblock. Kinetically, diblocks assemble much more rapidly and exhibit different rheological properties from triblocks. We are also investigating the structures formed and properties developed when these systems are dehydrated to less than 50{\%} water content. [Preview Abstract] |
Tuesday, March 19, 2013 3:54PM - 4:06PM |
J11.00006: Probing Nanoparticle Correlations in Filled Elastomers during Tensile Deformation by SAXS Edward J. Kramer, Arthur K. Scholz, Alexander Hexemer, Huan Zhang, Costantino Creton The 2D SAXS pattern from an unstrained 20 wt{\%} nanosilica-filled and crosslinked siloxane elastomer is isotropic and monotonically decreasing with scattering vector q, revealing a fractal aggregate structure of primary silica particles about 10 nm in radius. Under tensile strain along z, the invariant of the SAXS pattern, corrected for the change in sample thickness, is constant, demonstrating the absence of nanovoiding but the pattern itself shows a ``2 bar'' enhancement of intensity along z at q* = 2$\pi$/$<$z$>$. The distance $<$z$>$ and peak intensity Ip of the 2 bar pattern increase roughly linearly with extension ratio $\lambda$ until $\lambda$ $\sim$ 3 with $<$z$>$ saturating and Ip decreasing at higher $\lambda$s. Reverse Monte Carlo simulations of particle redistribution suggest that the silica aggregates separate into short rafts with compliant polymer in between along z; the extension ratio from $<$z$>$ of the nearly particle free polymer regions nearly matches $\lambda$ until $\lambda$ $\sim$ 3. For $\lambda$ $>$ 3 the rafts begin to break up, providing a partial explanation for the strong Mullins effect above $\lambda = 3$ for this filled elastomer. [Preview Abstract] |
Tuesday, March 19, 2013 4:06PM - 4:18PM |
J11.00007: Do thermally activated transitions influence the deformation of polymer glasses? Mark Ediger, Hau-Nan Lee, Benjamin Bending The availability of large scale computer simulations and new experiments allows fundamental questions about the influence of temperature on polymer glass deformation to be addressed from a microscopic perspective. Some recent simulations indicate that the total mobility induced during polymer glass deformation is a function of strain but independent of the strain rate. This result suggests that thermally activated transitions are not important during deformation which would be inconsistent with many models. We find that the integrated molecular mobility in polystyrene and PMMA glasses during deformation is roughly independent of strain rate. However, the relaxation time distribution narrows with increasing strain rate, indicating that thermally activated processes do play a role. [Preview Abstract] |
Tuesday, March 19, 2013 4:18PM - 4:30PM |
J11.00008: Self-Assembly of Gemini Surfactants Arun Yethiraj, Jagannath Mondal, Mahesh Mahanthappa The self-assembly behavior of Gemini (dimeric or twin-tail) dicarboxylate disodium surfactants is studied using molecular dynamics simulations. This gemini architecture, in which two single tailed surfactants are joined through a flexible hydrophobic linker, has been shown to exhibit concentration-dependent aqueous self-assembly into lyotropic phases including hexagonal, gyroid, and lamellar morphologies. Our simulations reproduce the experimentally observed phases at similar amphiphile concentrations in water, including the unusual ability of these surfactants to form gyroid phases over unprecedentedly large amphiphile concentration windows. We demonstrate quanitative agreement between the predicted and experimentally observed domain spacings of these nanostructured materials. Through careful conformation analyses of the surfactant molecules, we show that the gyroid phase is electrostatically stabilized related to the lamellar phase. By starting with a lamellar phase, we show that decreasing the charge on the surfactant headgroups by carboxylate protonation or use of a bulkier tetramethyl ammonium counterion in place of sodium drives the formation of a gyroid phase. [Preview Abstract] |
Tuesday, March 19, 2013 4:30PM - 4:42PM |
J11.00009: Exotic nanoparticles with block copolymer design and solution construction with kinetic control Darrin Pochan Kinetic pathways and temporal stabilities of different micelles and nanoscale aggregates have been used to construct exotic nanoparticles in solution. Due to low chain exchange dynamics between block copolymeric micelles and solvent, global thermodynamic equilibrium is extremely difficult, if not impossible, to achieve in block copolymer assembly. However, by taking advantage of this slow kinetic behavior of polymeric micelles in solution, one can purposely produce multicompartment nanoparticles and mulitgeometry nanoparticles by forcing different block copolymers to reside in the same nanoscale structure through kinetic processing. While kinetically trapped in common nanostructures, local phase separation can occur producing compartments. This compartmentalization can be used within common micelle geometries to make complex spheres and cylinders or can be used to make new nanostructures such as multigeometry aggregates (e.g. hybrid cylinder-sphere aggregates, disk-cylinder nanoparticles). [Preview Abstract] |
Tuesday, March 19, 2013 4:42PM - 4:54PM |
J11.00010: Polymer Solar Cell Device Characteristics Are Independent of Vertical Phase Separation in Active Layers Yueh-Lin Loo Preferential segregation of organic semiconductor constituents in multicomponent thin-film active layers has long been speculated to affect the characteristics of bulk-heterojunction polymer solar cells. Using soft-contact lamination and delamination schemes -- with which we have been able to remove compositionally well characterized polymer thin films, flip them over so as to reverse their composition profiles, and then transfer them onto existing device platforms -- we showed unambiguously that the device performance of P3HT:PCBM solar cells are independent of the interfacial segregation characteristics of the active layers. Temperature-dependent single-carrier diode measurements of the organic semiconductor constituents suggest that the origin of this invariance stems from the fact that P3HT comprises a high density of mid-gap states. Hole carriers in these mid-gap states can in turn recombine with electrons at the electron-collecting interface, effectively promoting electron transfer from the cathode to the active layer. [Preview Abstract] |
Tuesday, March 19, 2013 4:54PM - 5:06PM |
J11.00011: Dynamics of Magnetic Field Alignment of Block Copolymers by In-Situ SAXS Chinedum Osuji, Manesh Gopinadhan, Pawel Majewski The use of external fields to direct block copolymer self-assembly is well documented. Magnetic fields offer particular promise due to their space-pervasive nature and the ability to produce arbitrary alignments over truly macroscopic length scales in appropriate systems. We present here the results of in-situ SAXS studies performed using a custom superconducting magnet integrated with lab-scale x-ray scattering instruments. We consider the case of side-chain liquid crystalline diblock copolymers ordering under high magnetic fields. Despite the coincidence of the block copolymer order-disorder transition (ODT) and the LC clearing temperature in these weakly segregated materials, there is no measurable effect of the field on the ODT of the system, up to 6 T. This is in line with estimates based simply on the magnitudes of the relevant energy scales - the free energy of field interaction and the enthalpy of the isotropic-LC transition. We show that the alignment of the system is critically limited by the viscosity of the mesophase such that alignment can only be advanced by residence in a small temperature window near the ODT. This residence produces a weakly aligned system which thereafter transitions to a strongly aligned state on cooling even in the absence of the field. [Preview Abstract] |
Tuesday, March 19, 2013 5:06PM - 5:18PM |
J11.00012: Anion Transport in Hydrated Block Copolymers Nitash Balsara, Guillaume Sudre, Sebnem Inceoglu Polystyrene-block-polychloromethylstyrene (PS-b-PCMS) copolymers, were synthesized by nitroxide-mediated controlled radical polymerization. Separate aliquots of the PS-b-PCMS samples were quarternized to transform the PCMS block. This resulted in block copolymers with ionizable blocks. We refer to ion-containing block copolymers synthesized from the same precursor as matched pairs: SAM (containing trimethylammonium chloride) and SIM (containing n-butylimidazolium chloride). Self-assembly in these copolymers resulted in the formation of lamellar phases regardless of composition, chemical formula of the bound ion, and chain length. Chloride ion conductivity and water uptake measurements on one of the matched pairs led to similar results. The chloride ions in this matched pair were replaced by hydroxide ions and the changes in conductivity due to this are reported. [Preview Abstract] |
Session J12: Topological Insulators: Magnetic Transport and Weak Localization
Sponsoring Units: DCMPChair: Hadar Steinberg, Massachusetts Institute of Technology
Room: 314
Tuesday, March 19, 2013 2:30PM - 2:42PM |
J12.00001: ABSTRACT WITHDRAWN |
Tuesday, March 19, 2013 2:42PM - 2:54PM |
J12.00002: Transport Signatures of the Quantum Anomalous Hall Effect in 3D Topological Insulators Brian Dellabetta, Taylor Hughes, Matthew Gilbert The unique physics of spin-orbit coupled topological insulators (TIs) exposed to magnetic moments leads to a quantized conductance known as the quantum anomalous Hall effect (QAHE)\footnote{R. Yu et al., \emph{Science} {\bf 329}, 5987 (2010).}. While magnetic disorder has been experimentally shown to open a gap in the surface states of TIs\footnote{Y. L. Chen et al., \emph{Science} {\bf 329}, 659 (2010).}, no clear transport signatures of the QAHE have been observed in 3-dimensional TIs. We perform 3D real space calculations using the Non-Equilibrium Green's Function Formalism to show that topological insulators in proximity to arrays of ferromagnets offer a unique environment in which to study this phenomenon. We show that ferromagnetic domain walls on patterned surfaces manifest chiral surface modes with quantized currents that can be altered by changing the configuration of the magnetic arrays. We compare topologically trivial and nontrivial models to show a qualitative difference in the induced transport flow based on ferromagnet orientation, and propose a variety of experimental configurations which yield transport signatures of the QAHE. [Preview Abstract] |
Tuesday, March 19, 2013 2:54PM - 3:06PM |
J12.00003: Singular spin response of topological insulators in ac magnetic fields Haiming Deng, Lukas Zhao, Inna Korzhovska, Zhiyi Chen, Lia Krusin-Elbaum Orbital magnetic susceptibility in weak magnetic fields has several contributions whose physical origin is not simple in contrast to the clear Landau diamagnetism of free electrons. Experimentally, anomalous magnetism has been observed in graphite, and bismuth and Bi-Sb alloys, both of which are narrow gap semimetals. Here we report an observation of a singular response in ac magnetic susceptibility -- a suppression of diamagnetism at low magnetic fields that appears ubiquitous in all topological insulators (Sb$_2$Te$_3$, Bi$_2$Se$_3$, PbBi$_2$Se$_4$) we have studied. We observe two distinct contributions to this effect: a broader one that typically disappears around 40-50~K and is likely related to edges, and a divergent-like one (in the $H \rightarrow 0$ limit) that is robust up to room temperature and is likely related to the bulk. The frequency dependence and the dependence on the Fermi level of these effects will be discussed in the context of separation of orbital and spin effects. [Preview Abstract] |
Tuesday, March 19, 2013 3:06PM - 3:18PM |
J12.00004: Megnetoresistance in thin Bi2Te3 layers contacted by Indium (In) superconducting electrodes Zhuo Wang, Ramesh Mani Topological Insulators (TIs) are materials that insulate in the bulk but conduct electricity on their surfaces, which topologically protected by time-reversal symmetry. Transport measurements of topological insulators in the proximity of a superconductor are theoretically predicted to be a significant method to detect Majorana Fermions. Our experiment studied the interplay between superconductivity and TI surface states below the critical temperature of a type-I superconductor. Here, we used the four terminal lock-in technique to study the transport properties of Bi2Te3 specimens contacted by Indium superconducting electrodes, while sweeping perpendicular magnetic field, at T \textless\ 4.2 K. The results indicate a sharp suppression of the longitudinal resistance at weak magnetic fields, below the critical temperature of Indium. What's more exciting is that the interaction between superconductivity and TI surface states induces resistance enhancement at T $\le $ 2.8K, well below the critical temperature of Indium, in the absence of a magnetic field. [Preview Abstract] |
Tuesday, March 19, 2013 3:18PM - 3:30PM |
J12.00005: ABSTRACT WITHDRAWN |
Tuesday, March 19, 2013 3:30PM - 3:42PM |
J12.00006: Magneto-optical Studies of Bi$_2$Te$_3$ Flakes Li-Chun Tung, Wenlong Yu, Zhigang Jiang, Dmitry Smirnov Magneto-transmittance spectroscopy is used to probe the magnetic-field induced excitations in topological insulator-Bi$_2$Te$_3$. Bi$_2$Te$_3$ single crystals are repeatedly exfoliated on scotch tape untill the sample flakes are sufficiently thin and become permeable in the infrared frequency range. The sample with the underlying tape is placed in a 4K cryostat and the magneto-optical properties of Bi$_2$Te$_3$ are investigated by a broadband Fourier-Transform infrared spectrometer (Bruker 66) using light-pipe optics. The magneto-transmittance data of the sample on the tape and the bare tape up to 35T are collected and analyzed as a stacked multilayer. The average conductivity of the sample flakes at different magnetic fields is evaluated and several magnetic-field dependent features are revealed. These features coincide with the cyclotron resonance energy of the bulk band electrons and potentially linked to the surface state electrons. Implications of these results will be discussed in the presentation. [Preview Abstract] |
Tuesday, March 19, 2013 3:42PM - 3:54PM |
J12.00007: Spin-orbit scattering in quantum diffusion of massive Dirac fermions Wenyu Shan We theoretically study the effects of spin-orbit scattering on weak (anti-)localization in two-dimensional massive Dirac systems. We clarify that weak anti-localization and localization of a single massive Dirac cone come from the diffusion of a singlet Cooperon in the massless limit and one of triplet Cooperons in the large-mass limit, respectively. Spin-orbit scattering behaves like random magnetic scattering to the triplet Cooperon, and suppresses the weak localization in the large-mass regime, different from in conventional systems where spin-orbit scattering leads to a crossover from weak localization to antilocalization. This behavior suggests an experiment to detect the weak localization of bulk subbands in topological insulator thin films, in which an enhancement of ``weak anti-localization'' is expected after doping heavy-element impurities. Finally, we compare the conventional electron and Dirac fermion systems in the quantum diffusion transport under ordinary, spin-orbit, and magnetic scattering. [Preview Abstract] |
Tuesday, March 19, 2013 3:54PM - 4:06PM |
J12.00008: Transport properties of SnPbTe topological crystalline insulator films Badih A. Assaf, Ferhat Katmis, Peng Wei, Jagadeesh S. Moodera, Don Heiman A new phase of topological insulators, the topological crystalline insulator, has been recently predicted to arise in band-inverted Sn-Pb chalcogenides, where the topological surface states are protected by crystal symmetry instead of spin-orbit coupling [1]. We grew epitaxial thin films of SnTe and Sn$_{1-x}$Pb$_{x}$Te by MBE and sputtering on (100) and (111) surfaces of BaF$_{2}$ and Si. We report on magnetotransport measurements on SnTe films having hole densities ranging between 10$^{20}$ and 10$^{21}$ cm$^{-3}$ and mobilities up to 200 cm$^{2}$/Vs. Weak antilocalization is observed in all films, allowing us to study the behavior of the phase coherence length versus temperature in an attempt to shed light on the dimensionality of the transport as a function of the Fermi level. Some evidence of 2D transport is found in low carrier density films. [1]T. H. Hsieh et al. \textit{Nature Communication} \textbf{3,} 982 (2012). [Preview Abstract] |
Tuesday, March 19, 2013 4:06PM - 4:18PM |
J12.00009: ABSTRACT WITHDRAWN |
Tuesday, March 19, 2013 4:18PM - 4:30PM |
J12.00010: Kondo resistance minimum in topological insulators Jie Wang, Dimitrie Culcer We present a theory of the Kondo resistance minimum applicable to topological insulators (TI) and spin-orbit coupled semiconductors in the high-temperature limit, defined as T \textgreater T\textunderscore K, the Kondo temperature. We derive the T-matrix for a general strongly spin-orbit coupled system, including the many-body Kondo scattering terms. The physics is qualitatively different from the well-known case of metals due to the interplay of impurity degrees of freedom with the spin-orbit induced spin-momentum locking of the conduction electrons. TI have a single Fermi surface, while in spin-orbit coupled semiconductors scattering between the two spin-split Fermi surfaces must be taken into account. We determine the resistance minimum and Kondo temperature, and comment briefly on Kondo screening and Kondo singlet formation in the presence of strong spin-orbit coupling. [Preview Abstract] |
Tuesday, March 19, 2013 4:30PM - 4:42PM |
J12.00011: Competing weak localization and weak antilocalization in ultrathin topological insulators Murong Lang, Liang He, Xufeng Kou, Pramey Upadhyaya, Yabin Fan, Hao Chu, Nai-Chang Yeh, Kang Wang We demonstrate the evidences of a surface gap opening in (Bi$_{\mathrm{0.57}}$Sb$_{\mathrm{0.43}})_{\mathrm{2}}$Te$_{\mathrm{3}}$ samples for film thickness below 6 quintuple layers, through magnetotransport and scanning tunneling spectroscopy measurements. By tuning Fermi level position relative to the gap, the striking crossover between weak antilocalization and weak localization is observed in nonmagnetic 4 and 5 QL films at low field region, a characteristic feature of quantum interferences competition, possibly owing to the change of net Berry phase. Furthermore, when the Fermi level is swept into the surface gap, the overall unitary behaviors are revealed at higher magnetic field, which are in contrast to the pure WAL signals obtained in thicker films. Besides, the surface bandgap of ultrathin film is also determined by low temperature STS measurements. Our findings show an exotic phenomenon characterizing the gapped TI surface states and point to the future realization of quantum spin Hall effect and dissipationless TI-based applications. [Preview Abstract] |
Tuesday, March 19, 2013 4:42PM - 4:54PM |
J12.00012: Weak anti-localization in ultrathin Sb(111) films S. Cairns, N. McGlohon, C. Robison, J. Keay, C.K. Gaspe, K.S. Wickramasinghe, T.D. Mishima, M.B. Santos, S.Q. Murphy We report the first studies of localization in ultrathin Sb films. Sb is a topological semi-metal with a negative bandgap of 180meV, however it is anticipated that in ultra-thin films, quantum confinement will open the bulk gap, such that transport is dominated by the topological surface states. We have studied the magneto-transport of ~nominally 4.5nm thick films of Sb(111) grown via molecular beam epitaxy at a temperature of 300C on nearly lattice matched epilayers. The longitudinal resistance shows positive magneto-resistance, well described by the standard weak anti-localization (WAL) theory of Hikami, Larkin and Nagaoka. The WAL response is consistent with that of a single conducting channel with a phase breaking length of $\sim$200nm at 1.8K. Scanning electron microscopy shows that the Sb growth proceeded by a Volmer-Weber (islanding) process resulting in disordered films. More recent growths performed at lower temperature have yielded significantly less resistive, smoother and thinner films for which transport measurements are ongoing. [Preview Abstract] |
Tuesday, March 19, 2013 4:54PM - 5:06PM |
J12.00013: Weak-Localization-Like Magnetoresistance on a Topological Insulator - Ferromagnetic Insulator Interface Qi Yang, Merav Dolev, Li Zhang, Jinfeng Zhao, Min Liu, Subhash Risbud, Alexander Palevski, Aharon Kapitulnik In this talk, we will present measurements on the interface between a topological insulator (TI) and a ferromagnetic insulator (FI). The results provide a likely indication for gap opening in the TI surface states by its proximity to the FI. While above the Curie temperature ($T_C$) of the FI we observed weak-antilocalization-like positive magnetoresistance as ubiquitously in TIs, below $T_C$ an unusual weak-localization-like negative magnetoresistance was seen at low magnetic fields, which has never been observed in TIs without any magnetic doping. Such proximity introduced gap-opening and resulting massive Dirac fermions will hopefully lead to realization of many intriguing phenomena such as the quantum anomalous Hall effect and the inverse spin-galvanic effect. [Preview Abstract] |
Tuesday, March 19, 2013 5:06PM - 5:18PM |
J12.00014: Weak Localization and Antilocalization in Topological Insulator Thin Films with Coherent Bulk-Surface Coupling Ion Garate, Leonid Glazman We evaluate quantum corrections to conductivity in an electrically gated thin film of a three-dimensional (3D) topological insulator (TI). We derive approximate analytical expressions for the low-field magnetoresistance as a function of bulk doping and bulk-surface tunneling rate. Our results reveal parameter regimes for both weak localization and weak antilocalization, and include diffusive Weyl semimetals as a special case. [Preview Abstract] |
Tuesday, March 19, 2013 5:18PM - 5:30PM |
J12.00015: Coherent Topological Transport on the Surface of Bi$_2$Se$_3$ Dohun Kim, Paul Syers, Nicholas P. Butch, Johnpierre Paglione, Michael S. Fuhrer We report weak anti-localization (WAL) measurements in gate-tuned, bulk insulating Bi$_2$Se$_3$ thin crystals with thicknesses varying between 5 and 17 nm. The gate-voltage dependent WAL behavior shows systematic variation as a function of crystal thickness. For the thickest samples, we observe two decoupled surfaces exhibiting perfect WAL as expected for the symplectic class. As the films are made thinner, we observe a gate-voltage tuned crossover from two decoupled surfaces to a single coherently coupled 2D system exhibiting WAL. The observed crossover is governed by competition between the phase coherence time and inter-surface tunneling time associated with the hybridization gap. In contrast to classical transport in which the signature of the hybridization gap appears only in the ultrathin limit ($\le $ 3nm), phase coherent transport is extraordinarily sensitive to sub-meV coupling between top and bottom topological surfaces, and the surfaces of a TI film may be coherently coupled even for thicknesses as large as 12 nm. On further thinning, the WAL behavior is suppressed altogether at small carrier density, which we associate with the opening of a sizable gap on order the Fermi energy and destruction of topological protection. [Preview Abstract] |
Session J13: Focus Session: Topological Materials - Thin Films
Sponsoring Units: DMPChair: Dennis Drew, University of Maryland
Room: 315
Tuesday, March 19, 2013 2:30PM - 2:42PM |
J13.00001: Topological limit of ultrathin quasi-freestanding Bi$_2$Te$_3$ films grown on Si(111) Yang Liu, Huan-hua Wang, Guang Bian, Mark Bissen, Zhan Zhang, Tom Miller, Hawoong Hong, Tai-chang Chiang A fundamental issue for ultrathin topological films is the thickness limit below which the topological surface states become impacted by interfacial interactions. We show that for Bi$_{2}$Te$_{3}$ grown on Si(111) this limit is four quintuple layers (QLs) based on angle-resolved photoemission measurements, using optimized photon energies and polarizations, of the Dirac cone warping and interaction-induced gap as a function of film thickness. The results are close to theoretical predictions for free-standing films, despite the expected strong bonding of the film with the reactive Si(111) substrate. In-situ surface X-ray scattering (SXS) study shows that a buffer layer exist on the Si(111) surface, which effectively saturates all the Si(111) dangling bonds. These interfacial properties, revealed only by diffractions from deeply penetrating X-rays, are critical in understanding the topological surface states in ultrathin films, where electronic coupling is strongly enhanced. Our SXS measurement also yields new information regarding the internal structures of these topological thin films, including layer stacking, QL-by-QL growth, relaxations, etc. [Preview Abstract] |
Tuesday, March 19, 2013 2:42PM - 2:54PM |
J13.00002: Transport and Capacitance Measurements of Bi2Se3 Devices Valla Fatemi, Hadar Steinberg, Ferhat Katmis, Benjamin M. Hunt, Lucas Orona, Jagadeesh S. Moodera, Pablo Jarillo-Herrero We report electronic transport and capacitance measurements on Bi2Se3 thin-film and exfoliated devices. Strong modulation of the charge carrier density is achieved via the electric field effect with a local top-gate electrode utilizing either high-k dielectric insulators or transferred hexagonal boron nitride. The understanding of ambipolarity due to the electric field effects in these systems is addressed by comparing the modulation of the quantum capacitance and resistance in different devices, accompanied by a model. Additionally, we report capacitance and resistance measurements on these devices at high magnetic fields. [Preview Abstract] |
Tuesday, March 19, 2013 2:54PM - 3:06PM |
J13.00003: Photo-galvanic effect in Bi$_{2}$Se$_{3}$ thin films with ionic liquid gating Yu Pan, Anthony Richardella, Joon Sue Lee, Thomas Flanagan, Nitin Samarth A key challenge in three dimensional (3D) topological insulators (TIs) is to reveal the helical spin-polarized surface states via electrical transport measurements. A recent study [Nature Nanotech. {\bf 7}, 96 (2012)] showed that circularly polarized light can be used to generate and control photocurrents in the 3D TI Bi$_2$Se$_3$, even at photon energies that are well above the bulk band gap. Symmetry considerations suggest that this ``photo-galvanic effect'' arises purely from photo-currents induced in the surface Dirac states. To gain insights into this phenomenon, we have carried out systematic measurements of the photo-galvanic effect in electrically gated MBE-grown Bi$_2$Se$_3$ thin films of varying thickness. By using an ionic liquid as an optically transparent gate, we map out the behavior of the photo-galvanic effect as a function of Fermi energy over a temperature range 5 K $\leq T \leq$ 300 K. [Preview Abstract] |
Tuesday, March 19, 2013 3:06PM - 3:42PM |
J13.00004: Topological insulator engineering of Bi$_{2}$Se$_{3}$ through molecular beam epitaxy Invited Speaker: Seongshik Oh Despite numerous reports proving the presence of the surface states on various topological insulator (TI) materials, all existing TI materials suffer from the bulk conductance problem at various levels. Therefore, achieving a truly insulating bulk state without degrading the surface state in their transport properties is one of the most important tasks of the TI materials research. In this talk, I will present how we address this problem by utilizing various molecular beam epitaxy (MBE) schemes with focus on Bi$_{2}$Se$_{3}$ family of materials. Considering that the bulk conductance problem originates mostly from the selenium vacancies in Bi$_{2}$Se$_{3}$, the typical MBE growth condition characterized by low growth temperature and high selenium vapor pressure is ideal for solving this bulk conductance problem. Moreover, thin films have another advantage of naturally reduced bulk effect due to the enhanced surface-to-bulk ratio. These intrinsic advantages of MBE-grown TI thin films recently led to a number of new findings. High quality Bi$_{2}$Se$_{3}$ thin films did show the expected dominant surface transport characters with negligible bulk conductance. However, the strong tendency toward downward band bending in undoped Bi$_{2}$Se$_{3}$ introduces trivial surface transport channels in addition to the topological surface states, leading to complications in the interpretations of transport results. Furthermore, even if reducing the thickness of TI samples helps reveal the surface transport channels by reducing the bulk contribution, it does not really solve the bulk conductance problem because regardless of how small it may be, the bulk state still remains metallic, shorting the top and bottom surfaces. According to the Mott-criterion of metal-insulator transition, in order to implement a truly insulating bulk state in the current generation TI materials, it is necessary to suppress the defect density below $\sim$ 10$^{14}$ cm$^{-3}$, which might be fundamentally impossible considering the weak Van der Waals bonding character of these materials. However, we have found that it is possible to overcome this limit and achieve a bulk-insulating topological insulator with fully decoupled surface states in thin film TIs. [Preview Abstract] |
Tuesday, March 19, 2013 3:42PM - 3:54PM |
J13.00005: Thin films of topological crystalline insulators in IV-VI semiconductors Junwei Liu, Timothy H. Hsieh, Wenhui Duan, Jagadeesh Moodera, Liang Fu Topological crystalline insulators (TCI) are new topological states of matter protected by crystalline symmetry of solids. The first example of TCI has been recently predicted and subsequently observed in the SnTe class of IV-VI semiconductors. In this work, we show that thin films of TCI realize a two-dimensional Dirac fermion system with a tunable band gap and host the quantum spin Hall state in an extended thickness range. We propose a ferromagnet-TCI device to measure the spin-dependent transport through helical edge states. [Preview Abstract] |
Tuesday, March 19, 2013 3:54PM - 4:06PM |
J13.00006: What Limits Mobility and Carrier Concentration in Epitaxial Topological Insulator Films? Ferhat Katmis, Valla Fatemi, Hadar Steinberg, Peng Wei, Pablo Jarillo-Herrero, Jagadeesh Moodera In order to investigate the predicted exotic behavior of topological insulators (TIs) epitaxial films with near ideal electronic properties are essential. Obtaining high quality TI films requires careful control of not only growth parameters but also a good understanding of the dynamics of film formation. We have developed methods to obtain consistently high mobility and low carrier density by carefully controlling the nucleation and growth process of Bi2Se3~epitaxial films. Such MBE grown epitaxial films have been well characterized by different diffraction based techniques and electrical transport to obtain a correlation between structural and electrical properties. This has allowed us to see their systematic dependence. For example, in thin films, carrier density in low 10$^{12}$/cm$^2$ range with bulk mobilities higher than 3000 cm$^2$/V-s are routinely seen which nicely compares very well with structural data.~Acknowledgements: NSF grant DMR 1207469 and NSF DMR 08-19762 (CMSE -- Initiative 2). [Preview Abstract] |
Tuesday, March 19, 2013 4:06PM - 4:18PM |
J13.00007: Pairing of the spirals on epitaxially grown Bi2Se3 on Si(111) Yuxuan Chen, Christopher Mann, Chih-Kang Shih Bi$_{2}$Se$_{3}$ is a 3D topological insulator that exhibits backscattering suppression and helical Dirac-like Quasiparticles, making it an ideal candidate for topological physics research. Molecular beam epitaxy (MBE) can control the Se stoichiometry and vacancy density by controlling the Se overpressure during growth, thereby producing bulk insulators, allowing access to the novel physics promised by these systems. We have prepared Bi$_{2}$Se$_{3}$ thin films on Si(111) substrates by MBE. Atomic force microscopy and scanning tunneling microscopy topographies of these films often show large (100 to 500nm in diameter) triangular wedding-cake-shaped islands with spirals on top. More interestingly, the spirals often come in pairs of a clockwise and a counter-clockwise spiral. The high density of spiral pairs suggests that, such a surface structure is thermodynamically more favorable during the MBE. Our ongoing study of the very early stages of the MBE growth is unveiling more information of the spiral pairs. The knowledge of this growth mode will help us improve the sample quality. [Preview Abstract] |
Tuesday, March 19, 2013 4:18PM - 4:30PM |
J13.00008: Substrate-Independent Vapor-Solid Growth of Bi2Se3 Nanostructures Jerome T. Mlack, Atikur Rahman, Gary L. Johns, Ken J.T. Livi, Nina Markovic We describe a synthesis technique and low-temperature transport measurements of nanostructures of high-purity of topological insulator Bi2Se3. Our growth method is a catalyst-free atmospheric pressure vapor-solid growth, with the use of hydrogen as a carrier gas. It yields abundant amounts of a variety of nanostructures: nanowires, ribbons, platelets, and flakes of different sizes and shapes. Materials analysis shows highly ordered structures of bismuth selenide in all cases. The nanostructures can be used for electronic and optical applications including flexible ones: we show growth results on glass, silicon and flexible mica substrates. Low-temperature measurements of as-grown nanostructures indicate weak-antilocalization and tunable carrier density in all samples. With doping, the transport properties of the samples can be altered to exhibit superconductivity. [Preview Abstract] |
Tuesday, March 19, 2013 4:30PM - 4:42PM |
J13.00009: Tunable topological electronic structures in Sb(111) bilayers: A first-principles study Feng-Chuan Chuang, Chia-Hsiu Hsu, Chia-Yu Chen, Zhi-Quan Huang, Vidvuds Ozolins, Hsin Lin, Arun Bansil Electronic structure and band topology of a single Sb(111) bilayer in the buckled honeycomb configuration are investigated using first-principles calculations with the inclusion of spin-orbit coupling. While a trivial band insulator is predicted for the free-standing thin film, a band inversion at the Brillouin zone center can be induced by tensile strain, resulting in a topological insulator with a nontrivial topological invariant $Z_2=1$. Our study points at the possibility of realizing the quantum spin Hall state for an Sb(111) single bilayer on a suitable substrate. Moreover, the presence of buckling provides an advantage in controlling the band gap through an out-of-plane external electric field, which breaks the inversion symmetry and lifts the spin degeneracy. A topological phase transition driven by gating is demonstrated, and six spin-polarized Dirac cones are found at the critical point. With a tunable gap and reversible spin polarization, Sb thin films are promising candidates for spintronic applications. [Preview Abstract] |
Tuesday, March 19, 2013 4:42PM - 4:54PM |
J13.00010: Topological surface states of Sb thin films adsorbed with impurities Chih-Kai Yang, Chi-Hsuan Lee An antimony film is known to exhibit topological surface states depending on the thickness of the film. If the thickness of the film is reduced to as low as four bilayers, for example, Dirac cones disappear as a result of quantum tunneling. We use density functional calculation to investigate the electronic structure of the four-bilayer Sb film and find that adsorptions of non-magnetic impurity atoms of hydrogen, copper, or zinc on the film actually facilitate the formation of Dirac cones that preserve time-reversal symmetry. But magnetic atoms such as iron and manganese do just the opposite. The results suggest the counterintuitive concept of achieving topological conduction by doping nonmagnetic foreign atoms on thin films of topological insulators. [Preview Abstract] |
Tuesday, March 19, 2013 4:54PM - 5:06PM |
J13.00011: Medium Energy Ion Scattering investigation of In diffusion in In$_2$Se$_3$/Bi$_2$Se$_3$ H.D. Lee, C. Xu, S. Shubeita, M. Brahlek, N. Koirala, S. Oh, T. Gustafsson In$_2$Se$_3$, a band insulator, and Bi$_2$Se$_3$, a three-dimensional topological insulator, have inherently good chemical and structural compatibility. This suggests possible promising applications of In$_2$Se$_3$/Bi$_2$Se$_3$ devices as tunnel barriers and gate dielectrics. Recently, it has been shown that the similar (Bi$_{\mathrm{1-x}}$In$_{\mathrm{x}})_2$Se$_3$ thin system undergoes a transition from topological insulator to band insulator as a function of In concentration [1]. It is therefore important to understand the extent of In diffusion in In$_2$Se$_3$/Bi$_2$Se$_3$ and its consequences for the transport properties. We have grown In$_2$Se$_3$/Bi$_2$Se$_3$ thin films on sapphire by Molecular Beam Epitaxy at three different temperatures. Medium Energy Ion Scattering measurements of those films showed that the higher growth temperature resulted in more In diffusion while our transport measurements showed that the Bi$_2$Se$_3$ mobility increases as the growth temperature decreases. We found that the trend of the mobility change of In$_2$Se$_3$/Bi$_2$Se$_3$ depending on the diffusion of In is similar with the trend of the mobility of (Bi$_{\mathrm{1-x}}$In$_{\mathrm{x}})_2$Se$_3$ as a function of In concentration [1].\\[4pt] [1] M. Brahlek, et al, Phys. Rev. Lett. 109, 186403 (2012) [Preview Abstract] |
Tuesday, March 19, 2013 5:06PM - 5:18PM |
J13.00012: First-principles exploration of high-energy facets of bismuth chalcogenide nanocrystals Oleg V. Yazyev, Naunidh Virk Binary bismuth chalcogenides Bi$_2$Se3, Bi$_2$Te$_3$, and derived materials are currently considered as the reference topological insulators (TIs) due to their simple surface-state band structures and relatively large bulk band gaps. Nanostructures of TIs are of particular interest as a large surface-to-volume ratio enhances the contribution of surfaces states. So far, the vast majority of research efforts have focused on the low-energy (111) surfaces which correspond to weak planes in the layered crystal structures. Low-dimensional nanostructures such as nanowires and nanoparticles will inevitably involve higher energy facets. We perform a systematic ab initio investigation of the high-energy surfaces of bismuth chalcogenide TIs characterized by different crystallographic orientations as well as surface reconstructions and stoichiometries. We find several stable surfaces which exist under varying thermodynamic equilibrium conditions. Surface orientation and stoichiometry are found to dramatically affect band dispersion and spin polarization of the topological surface-state charge carriers. [Preview Abstract] |
Tuesday, March 19, 2013 5:18PM - 5:30PM |
J13.00013: Vapor-Liquid-Solid Synthesis of Bi$_2$Te$_3$ Nanowires via Metalorganic Chemical Vapor Deposition L.D. Alegria, J.R. Petta Bi$_2$Te$_3$ is a topological insulator and high figure-of-merit thermoelectric material. In the context of thermoelectrics, the synthesis of ultra-thin nanowires could enable more efficient energy conversion technologies due to quantum confinement. We describe a route for the synthesis of Bi$_2$Te$_3$ nanowires using low-pressure metalorganic chemical vapor deposition (MOCVD). The combination of metalorganic precursors trimethyl bismuth and diisopropyl telluride allow a low 350$^\circ$C growth temperature that is conducive to nanowire formation. The nanowires form by VLS growth from gold nanoparticles deposited on a growth substrate. Structural and chemical characterizations of the growth products are presented, indicating that the nanowires are high quality, single crystals of Bi$_2$Te$_3$. [Preview Abstract] |
Session J14: Magnetic Devices and Techniques
Sponsoring Units: GMAGChair: Mohammad Fashami, The Virginia Commonwealth University
Room: 316
Tuesday, March 19, 2013 2:30PM - 2:42PM |
J14.00001: Implications of stochastic magnetization dynamics on reliability of dipole coupled nanomagnetic logic Mohammad Salehi Fashami, Jayasimha Atulasimha, Supriyo Bandyopadhyay Straintronic nanomagnetic logic (SML), where Boolean computation is elicited from dipole coupled multiferroic nanomagnets switched with electrically generated strain, has emerged as an extremely energy-efficient computing paradigm. We have studied the reliability of such logic circuits by computing the gate error rates in the presence of thermal noise by simulating switching trajectories with the stochastic Landau-Lifshitz-Gilbert (LLG) equation. In addition, we examine the lower bound of energy dissipation as a function of switching error and explain how the out-of-plane excursion of the magnetization vector leads to excess energy dissipation over this bound for a given switching error. This analysis is performed to understand the connection between reliability and energy dissipation for a single switch and then extended to larger nanomagnetic logic circuits to assess the viability of dipole coupled SML. [Preview Abstract] |
Tuesday, March 19, 2013 2:42PM - 2:54PM |
J14.00002: Experimental realization of straintronic nanomagnetic logic using strain-induced magnetization switching in magnetostrictive nanomagnets elastically coupled to PMN-PT Noel D'Souza, Mohammad Salehi-Fashami, Supriyo Bandyopadhyay, Jayasimha Atulasimha Single-domain magnetostrictive Ni nanomagnets are grown on a bulk \textless 001\textgreater\ PMN-PT substrate and their domain switching is studied through Magnetic Force Microscopy (MFM) and Scanning Electron Microscopy with Polarization Analysis (SEMPA) techniques. By applying a voltage across the length of the PMN-PT substrate ($d_{33}$ coupling), a mechanical strain is applied along the nanomagnet's easy axis of magnetization resulting in domain switching and is investigated for several scenarios. First, the magnetization switching of single, isolated nanomagnets of various sizes is observed. This is followed by studying the dipole interactions through anti-ferromagnetic (AF) and ferromagnetic (F) coupling. The accurate, unidirectional propagation of the magnetization state is also investigated through an array of three AF-coupled nanomagnets. Finally, NAND logic operation using these nanomagnets is explored. Since SEMPA analysis involves no alteration of a sample's magnetic state, unlike in MFM imaging, we also analyze these scenarios using this technique at NIST, Gaithersburg. [Preview Abstract] |
Tuesday, March 19, 2013 2:54PM - 3:06PM |
J14.00003: ABSTRACT WITHDRAWN |
Tuesday, March 19, 2013 3:06PM - 3:18PM |
J14.00004: Single Circuit Parallel Computing with Phonons through Magneto-acoustics Sophia Sklan, Jeffrey Grossman Phononic computing \textendash{} the use of (typically thermal) vibrations for information processing \textendash{} is a nascent technology; its capabilities are still being discovered. We analyze an alternative form of phononic computing inspired by optical, rather than electronic, computing. Using the acoustic Faraday effect, we design a phonon gyrator and thereby a means of performing computation through the manipulation of polarization in transverse phonon currents. Moreover, we establish that our gyrators act as generalized transistors and can construct digital logic gates. Exploiting the wave nature of phonons and the similarity of our logic gates, we demonstrate parallel computation within a single circuit, an effect presently unique to phonons. Finally, a generic method of designing these parallel circuits is introduced and used to analyze the feasibility of magneto-acoustic materials in realizing these circuits. [Preview Abstract] |
Tuesday, March 19, 2013 3:18PM - 3:30PM |
J14.00005: Giant magnetoimpedance effect of Co-based magnetic ribbon as a chemical sensing probe Alejandro Ruiz, Jagannath Devkota, Pritish Mukherjee, Hariharan Srikanth, Manh-Huong Phan The giant magnetoimpedance (GMI) effect consists of a large change in the AC impedance of a soft ferromagnetic conductor subject to an external dc magnetic field that forms the basis for developing a new generation of magnetic sensors. Since the impedance of a soft ferromagnetic material is a function of the skin depth at radio frequency region, the GMI effect of the material can be modified via changes in the resistivity and permeability even at a fixed frequency. This effect arises due to the change in the magnetic anisotropy, material geometry, or electrochemical changes. In the present study, we demonstrate the GMI-based detection of various concentrations of corrosive chemicals using an amorphous Co-based ribbon. Under corrosive fluids, the magnetic permeability and hence the GMI effect of the ribbon changes due to the surface modification of the ribbon. We have found that the GMI ratio decreases with time, reaches a minimum value at a certain time, and then remains almost constant with time. The change in the GMI ratio and the time to achieve a stable value depends on the corrosive strength of the used chemical. These results show promise in developing a new class of chemical sensor using the GMI technology. [Preview Abstract] |
Tuesday, March 19, 2013 3:30PM - 3:42PM |
J14.00006: All-thin-film multiferroic heterostructured cantilevers in linear and nonlinear dynamic regimes Tiberiu-Dan Onuta, Yi Wang, Samuel E. Lofland, Christian J. Long, Ichiro Takeuchi We report on fabrication and characterization of all-thin-film multiferroic magnetoelectric (ME) cantilever devices and their different modes of operation in both linear and nonlinear dynamic regimes. The devices are built on micro-electromechanical system (MEMS) platforms that involve stress-engineered designs based on \textit{silicon oxide/nitride/oxide (ONO)} stacks. The ME layers consist of a magnetostrictive Fe$_{0.7}$Ga$_{0.3}$ thin film and a Pb(Zr$_{0.52}$Ti$_{0.48}$)O$_{3}$ piezoelectric thin film. The resonant frequency was found to display DC magnetic field dependence indicative of the interplay between the anisotropy and Zeeman energies. In the magnetically-driven mode, the harvested peak power at 1 \textit{Oe} is 0.7\textit{ mW/cm}$^{3}$ (RMS) at the resonant frequency (\textit{3.8 kHz}) and the quality factor also displays strong dependence on the DC magnetic bias. In certain conditions, the multiferroic devices show nonlinear behaviors important to logic implementation and parametric amplification. [Preview Abstract] |
Tuesday, March 19, 2013 3:42PM - 3:54PM |
J14.00007: Reliability of Signal Propagation in Magnetostatically Coupled Arrays of Magnetic Nanoelements Reinier van Mourik, Li Gao, Brian Hughes, Charles Rettner, Bert Koopmans, Stuart Parkin Nanomagnetic logic (NML) has promise as a low-power, non-volatile, and radiation resistant alternative to CMOS-based computational devices. Lines of magnetostatically coupled magnetic nano-elements (NEs) propagate information, and the intersections between lines form logic gates. We present simulations and experiments exploring the reliability of signal propagation in NML devices composed of lines of nominally rectangular permalloy NEs, typically 90$\times$60 nm$^2$ in size. An external magnetic field sets the magnetic state of an input bit and also resets each of the NEs' magnetizations along their hard axis direction. As the field is reduced to zero the input state propagates along the line of NEs as they successively relax into one of two equilibrium states. The state of the NEs is probed by (i) a magnetic tunnel junction sensing device integrated with the output NE and (ii) magnetic force microscopy imaging. We conclude that signal propagation is inherently unreliable both through variations in fabrication of the NEs and due to the innate lack of directionality of the flow of information. We demonstrate an alternative clocking method where a domain wall passing underneath an NML device clocks each NE sequentially, thereby increasing the success of signal propagation. [Preview Abstract] |
Tuesday, March 19, 2013 3:54PM - 4:06PM |
J14.00008: Dynamic state switching in nonlinear multiferroic cantilevers Yi Wang, Tiberiu-Dan Onuta, Christian J. Long, Samuel E. Lofland, Ichiro Takeuchi We demonstrate read-write-read-erase cyclical mechanical-memory properties of all-thin-film multiferroic heterostructured Pb(Zr$_{0.52}$Ti$_{0.48})$O$_{3}$ / Fe$_{0.7}$Ga$_{0.3}$ cantilevers when a high enough voltage around the resonant frequency of the device is applied on the Pb(Zr$_{0.52}$Ti$_{0.48})$O$_{3}$ piezo-film. The device state switching process occurs due to the presence of a hysteresis loop in the piezo-film frequency response, which comes from the nonlinear behavior of the cantilever. The reference frequency at which the strain-mediated Fe$_{0.7}$Ga$_{0.3}$ based multiferroic device switches can also be tuned by applying a DC magnetic field bias that contributes to the increase of the cantilever effective stiffness. The switching dynamics is mapped in the phase space of the device measured transfer function characteristic for such high piezo-film voltage excitation, providing additional information on the dynamical stability of the devices. [Preview Abstract] |
Tuesday, March 19, 2013 4:06PM - 4:18PM |
J14.00009: Verification of modified Jiles-Atherton model for determination of hysteresis behavior of materials with two ferromagnetic phases Neelam Prabhu Gaunkar, Cajetan Nlebedim, David Jiles Robust theoretical models of hysteresis are important for describing the properties of ferromagnetic materials. Of the available hysteresis models, the J-A model is widely studied. Efforts have been made to modify and extend the applicability of this model and to improve its accuracy in accounting for different conditions that affect the magnetic state of ferromagnetic materials, such as stress. Recently, the J-A model has been extended to describe the ferromagnetic hysteresis in two-phase magnetic materials. Modeling hysteresis of multi-phase ferromagnetic materials is crucial especially due to the need to develop high performance composite magnetic structures. In this study, the extension of the J-A to accommodate materials with two ferromagnetic phases is experimentally verified. The approach to extracting of the J-A model parameters including saturation magnetization (M$_{\mathrm{s}})$, domain coupling factor ($\alpha )$, domain density (a), reversibility (c) and pinning coefficient (k) in two-phase materials will be presented. [Preview Abstract] |
Tuesday, March 19, 2013 4:18PM - 4:30PM |
J14.00010: Phase encoding technique for super-resolution NV magnetometry Keigo Arai, Chinmay Belthangady, Huiliang Zhang, Stephen DeVience, Ronald Walsworth We report recent progress towards improving the spatial resolution of nitrogen-vacancy-center-based magnetometers by use of phase encoding techniques which are widely used in conventional magnetic resonance imaging. Since the electronic spin state of nitrogen-vacancy (NV)centers is initialized and read out optically, the resolution of current NV magnetometers is limited by optical diffraction. By applying magnetic field gradients, spatial information can be imparted to the phase of NV electron spin precession, and the resolution is inversely proportional to the magnitude of the field gradient. We will discuss methods to make magnetic field gradients of 1 T/cm which can be switched at a rate of 1 MHz in order to achieve 100 nm resolution along two spatial directions. [Preview Abstract] |
Tuesday, March 19, 2013 4:30PM - 4:42PM |
J14.00011: Solid State Hanle Magnetometry Christopher Wolfe, Vidya Bhallamudi, Vivek Amin, Dominic Labanowski, Andrew Berger, Helena Reichlova, David Stroud, Jairo Sinova, Chris Hammel The development of spatially resolved imaging of strongly varying vector magnetic fields is a fundamental challenge that would have scientific and technological implications in fields ranging from materials characterization to the study of magnetic particles in scanned probe techniques and tracking of biological tags. We have extended magnetometry based on the Hanle effect\footnote{A. Kastler, Nucl. Instrum. Methods \textbf{110}, 259 (1973).} to the characterization of vector fields in solid state systems. Local Hanle curves were measured in a GaAs membrane at various positions around a NdFeB micro-magnetic particle using spin-photoluminescence. The spatially varying vector magnetic fields from the micro-magnet cause calculable changes to the shape of the Hanle curve, and by fitting these curves we can extract information about all three components of the field of the micromagnet and infer its properties. I will also discuss the possibility of an all electrical device which could be more easily and broadly utilized. [Preview Abstract] |
Tuesday, March 19, 2013 4:42PM - 4:54PM |
J14.00012: Faraday rotation echo spectroscopy of phase transitions Shaowen Chen, Renbao Liu Faraday rotation is widely used to study magnetic dynamics. We designed a scheme of Faraday rotation echo spectroscopy (FRES) that can be used to study spin noise dynamics in transparent materials by measuring the fluctuation of Faraday rotation angle. The FRES suppresses the static part of the noise and reveal the quantum fluctuations at relatively high temperature, which shares the same idea of the spin echo technique in nuclear magnetic resonance (NMR). We tested our theory on a rare-earth compound LiHoF$_{4}$. The quantum fluctuations obtained by FRES give an enhanced feature at the phase boundary. The FRES can be straightforwardly generalized to more complicated configurations that correspond to more complex dynamical decoupling sequences in NMR and electron spin resonance, which may give us more extensive information on the structural and dynamical properties of magnetic materials. [Preview Abstract] |
Tuesday, March 19, 2013 4:54PM - 5:06PM |
J14.00013: Reading a Magnetic Non-Erasable Magnetic Memory Alan Edelstein, Greg Fischer, Jonathan Petrie, Robert Burke Two major disadvantages of current magnetic memory are that it can be erased by inadvertently applying a magnetic field and the superparamagnetic limit is beginning to make it difficult to increase the density of magnetic recording without further limiting the already too short storage lifetime of seven years. The superparmagnetic limit can be expressed as the requirement to store information for 10 years requires that $KV/k_{B} T > 50$, where $K$ is the crystalline anisotropy, $V$ is the volume of the bit $k_{B}$ is the Boltzman constant and $T$ is the absolute temperature. Alternative methods of information storage, at present, do not have the density of magnetic memory and generally do not store information indefinitely. We have demonstrated a method for reading media in a new magnetic non-erasable memory technology based on regions of high and low magnetic permeability. We have been able to use magnetic tunnel junctions and a probe field to read 10 micron wide lines of a soft magnetic material, permalloy, with a signal to noise ratio of 45 db. [Preview Abstract] |
Tuesday, March 19, 2013 5:06PM - 5:18PM |
J14.00014: Sensing RF and microwave energy with fiber Bragg grating heating via soft ferromagnetic glass-coated microwires M.H. Phan, J. Devkota, H. Srikanth, P. Colosimo, A. Chen The fiber Bragg grating (FBG) is the basis of numerous sensors. For the most part, strain and temperature are the primary environmental parameters that can be detected with FBGs. Other variables can be measured by using a probe design that converts the desired variable to a strain or temperature change. For example, an FBG bonded to the wall of a vacuum chamber might be used to measure pressure if the wall strain vs. pressure calibration were known. We present results from a new type of microwave energy sensor that relies on Joule heating of a soft ferromagnetic glass-coated microwire to change the temperature of an FBG. The microwire absorbs microwave energy and heats up thus raising the temperature of the FBG. Compared to a similar sensor that uses gold to absorb electromagnetic radiation, the microwire yields a sensor with greater sensitivity (10 times at $f=$3.25 GHz) relative to the perturbation of the microwave field. With this newly developed sensor, the best sensitivity to electromagnetic radiation corresponds to AC electric fields that have root mean square (RMS) amplitude of approximately 36 V/m. It is physically very small, can be deployed as a distributed sensor, and often only minimally perturbs the field being measured. [Preview Abstract] |
Tuesday, March 19, 2013 5:18PM - 5:30PM |
J14.00015: Magnetic Field Assisted Assembly: Breaking the 50$\mu $m Barrier Vijay Kasisomayajula, David Cunningham, Anthony Fiory, N.M. Ravindra Magnetic Field Assisted Assembly is used to facilitate heterogeneous device assembly on various substrates. The aim of this work is to illustrate techniques that help assemble devices of dimensions less than 50$\mu $m in any direction onto Silicon/GaAs wafers. Novel methods are developed to produce highly localized magnetic fields using microfabricated solenoids and preconditioned devices whose motion is controlled with nanometer precision. The efficiency of this directed assembly is discussed and comparison is made with existing directed and self assembly techniques. [Preview Abstract] |
Session J15: Focus Session: Quantum Spin Liquid Theory
Sponsoring Units: GMAG DMPChair: Todari Senthil, Massachusetts Institute of Technology
Room: 317
Tuesday, March 19, 2013 2:30PM - 2:42PM |
J15.00001: $Z_2$-vortex lattice in the ground state of the triangular Kitaev-Heisenberg model Maria Daghofer, Ioannis Rousochatzakis, Ulrich K. Roessler, Jeroen van den Brink Investigating the classical Kitaev-Heisenberg Hamiltonian on a triangular lattice, we establish the presence of an incommensurate non-coplanar magnetic phase, which is identified as a lattice of $Z_2$ vortices. The vortices, topological point defects in the SO(3) order parameter of the nearby Heisenberg antiferromagnet, are not thermally excited but due to the spin-orbit coupling and arise at temperature $T\to 0$. This $Z_2$-vortex lattice is stable in a parameter regime relevant to iridates. We show that in the other, strongly anisotropic, limit a robust nematic phase emerges. [Preview Abstract] |
Tuesday, March 19, 2013 2:42PM - 2:54PM |
J15.00002: Dislocations in the Kitaev honeycomb model Olga Petrova, Oleg Tchernyshyov We study the effects of introducing dislocations into the Kitaev honeycomb model [1]. In the gapped phase, dislocations are $Z_2$ ``twist defects'' associated with the transmutation of electric and magnetic excitations, studied previously in the context of $Z_N$ rotor models [2,3]. We show that each dislocation hosts one unpaired Majorana mode. As a consequence, twist defects have the statistics of Ising anyons. Because dislocations are confined, an additional phase is accumulated due to the change in system's energy during the braiding process. This means that the result of braiding can only be defined up to a phase. Therefore, twists are said to have projective non-Abelian statistics. \\[4pt][1] Alexei Kitaev, Annals of Physics \textbf{321}, 2 (2006) \\[0pt][2] Hector Bombin, Phys. Rev. Lett. \textbf{105}, 030403 (2010) \\[0pt][3] Yi-Zhuang You and Xiao-Gang Wen, Phys. Rev. B \textbf{86}, 161107 (2012) [Preview Abstract] |
Tuesday, March 19, 2013 2:54PM - 3:06PM |
J15.00003: Quantum Phase Transition in Heisenberg-Kitaev Model Robert Schaffer, Subhro Bhattacharjee, Yong Baek Kim We explore the nature of the quantum phase transition between a magnetically ordered state with collinear spin pattern and a gapless $Z_2$ spin liquid in the Heisenberg-Kitaev model. We construct a slave particle mean field theory for the Heisenberg-Kitaev model in terms of complex fermionic spinons. It is shown that this theory, formulated in the appropriate basis, is capable of describing the Kitaev spin liquid as well as the transition between the gapless $Z_2$ spin liquid and the so-called stripy antiferromagnet. Within our mean field theory, we find a discontinuous transition from the $Z_2$ spin liquid to the stripy antiferromagnet. We argue that subtle spinon confinement effects, associated with the instability of gapped $U(1)$ spin liquid in two spatial dimensions, play an important role at this transition. The possibility of an exotic continuous transition is briefly addressed. [Preview Abstract] |
Tuesday, March 19, 2013 3:06PM - 3:18PM |
J15.00004: Changing topology by knotting in the three-dimensional Toric Code Andrej Mesaros, Yong Baek Kim, Ying Ran A novel way to study the ground state degeneracy (GSD) of topological matter is through lattice dislocations: When a second copy of a lattice model is introduced through translation by half a lattice constant ($|\vec{b}|=a/2$), then a lattice dislocation with Burgers vector $\vec{b}$ locally smoothly connects the two model copies. Such dislocations are ``genon'' defects, effectively changing the topology of lattice. In three dimensions (3d), dislocations are closed loops that can be linked and knotted, leading to complex three dimensional manifolds on which the topological theory is defined. We give an analytical construction, supported by exact numerical calculations, for the dependence of GSD on dislocations of such a doubled version of the exactly solvable Kitaev's Toric Code (having $Z_2$ topological order) in both 2d and 3d. Surprisingly, we find that GSD of the 3d model depends only on the total number of dislocation loops, no matter their linking or knotting. The analytical proof is extended to $Z_n$ generalizations of the model. Additionally, we consider the phase in which dislocations become dynamical through proliferation of double dislocations (2$\vec{b}$) in 2d: the resulting gauge theory is non-Abelian, in the special case of $Z_2$ Toric Code it is $D_4$. [Preview Abstract] |
Tuesday, March 19, 2013 3:18PM - 3:30PM |
J15.00005: Kibble-Zurek Scaling and String-Net Coarsening in Topologically Ordered Systems Vedika Khemani, Anushya Chandran, F.J. Burnell, S.L. Sondhi We consider the non-equilibrium dynamics of topologically ordered systems, such as spin liquids, driven across a continuous phase transition into proximate phases with no, or reduced, topological order. This dynamics exhibits scaling in the spirit of Kibble and Zurek but now without the presence of symmetry breaking and a local order parameter. The non-equilibrium dynamics near the critical point is universal in a particular scaling limit. The late stages of the process are seen to exhibit slow, quantum coarsening dynamics for the extended string-nets characterizing the topological phase, a potentially interesting signature of topological order. Certain gapped degrees of freedom that could potentially destroy coarsening are, at worst, dangerously irrelevant in the scaling limit. We also note a time dependent amplification of the energy splitting between topologically degenerate states on closed manifolds. We illustrate these phenomena in the context of particular phase transitions out of the abelian $Z_2$ topologically ordered phase of the toric code, and the non-abelian $SU(2)_k$ ordered phases of the relevant Levin-Wen models. [Preview Abstract] |
Tuesday, March 19, 2013 3:30PM - 3:42PM |
J15.00006: A classification of symmetry enriched topological phases with exactly solvable models Ying Ran, Andrej Mesaros Recently a new class of quantum phases of matter: symmetry protected topological phases, such as topological insulators, attracted much attention. In presence of interactions, group cohomology provides their classification. These phases are only short-range entangled, while phases with long-range entangled topological order (having topological ground state degeneracy and/or anyons in the bulk) in presence of global symmetries are much less understood. We present a classification of bosonic gapped quantum phases with or without long-range entanglement, in the presence or absence of on-site global symmetries. In 2+1 dimensions, the quantum phases with global symmetry group $SG$, and with topological order described by finite gauge group $GG$, are classified by the cohomology group $H^3(SG\times GG, U(1))$. We present an exactly solvable local bosonic model for each class. When global symmetry is absent our models are described by Dijkgraaf-Witten discrete gauge theories. When topological order is absent, they become models for symmetry protected topological phases. When both global symmetry and topological order are present, the models describe symmetry enriched topological phases. Our classification includes, but goes beyond the projective symmetry group classification. [Preview Abstract] |
Tuesday, March 19, 2013 3:42PM - 3:54PM |
J15.00007: Classifying fractionalization: symmetry classification of gapped Z2 spin liquids in two dimensions Andrew Essin, Michael Hermele Quantum number fractionalization is a remarkable property of topologically ordered states of matter, such as the fractional quantum Hall liquids, and certain quantum spin liquid states. For a given type of topological order, there are generally many ways to fractionalize the quantum numbers of a given symmetry. Not all distinct fractionalizations will necessarily correspond to distinct phases of matter, however. In this work, we establish a formalism for characterizing fractionalization in gapped, two-dimensional Z2 spin liquids, which leads immediately to a classification of these topologically ordered phases. [Preview Abstract] |
Tuesday, March 19, 2013 3:54PM - 4:06PM |
J15.00008: Realization of symmetry classes for gapped $Z_{2}$ spin liquids in simple models Hao Song, Michael Hermele Recently it has been proposed that gapped $Z_{2}$ spin liquids in two dimensions can be partially classified by the distinct types of fractional quantum numbers carried by the $Z_{2}$ charge and flux excitations. On the square lattice with space group and time reversal symmetry, there are about $2^{19}$ symmetry classes. It is an open question which of these classes can be realized in simple models and, more fundamentally, whether all of these classes can actually be realized. We will present results on a class of exactly solvable models addressing these issues. [Preview Abstract] |
Tuesday, March 19, 2013 4:06PM - 4:18PM |
J15.00009: Chern-Simons theory for frustrated quantum magnets Krishna Kumar, Eduardo Fradkin We study the problem of frustrated quantum magnets by mapping models with Heisenberg spins, which are hard-core bosons, onto a problem of fermions coupled to a Chern-Simons gauge field [1]. Similar methods have been used successfully in the case of unfrustrated systems like the square lattice [2]. However, in the case of frustrated systems there always exists some arbitrariness in defining the problem. At the mean-field level these issues can be over looked but the effects of fluctuations, which are generally strong in these systems, are expected to alter the mean-field physics [3-4]. We discuss the difficulties involved in setting up this problem on a triangular or kagome lattice and some approaches to tackle these issues. We study the effects of fluctuations in these systems and the possibility of spin-liquid type phases.\\[4pt] [1] E. Fradkin, Phys. Rev. Lett. 63, 322-325 (1989)\\[0pt] [2] A. Lopez, A. G. Rojo, and E. Fradkin, Phys. Rev. B 49, 15139 (1994)\\[0pt] [3] G. Misguich, Th. Jolicoeur, and S. M. Girvin Phys. Rev. Lett. 87, 097203 (2001)\\[0pt] [4] Kun Yang, L. K. Warman and S. M. Girvin, Phys. Rev. Lett. 70, 2641 (1993) [Preview Abstract] |
Tuesday, March 19, 2013 4:18PM - 4:54PM |
J15.00010: Identifying Topological Quantum Spin Liquid in Physical Realistic Models Invited Speaker: Hong-Chen Jiang Quantum spin liquids (QSLs) are elusive magnets without magnetism, resisting symmetry breaking even at zero temperature due to strong quantum fluctuations and geometric frustration. The simplest QSLs known theoretically are characterized by topological order, i.e., topological quantum spin liquid, and support fractionalized excitations. However, there is no practical way to directly determine the topological nature of states, such as QSLs. We propose a practical and extremely simple approach, i.e., cylinder construction, to numerically calculate the topological entanglement entropy (TEE), and thereby identify topological order of the state [H. C. Jiang, Z. Wang, and L. Balents, arXiv:1205.4289]. We have successfully applied this approach to a variety of lattice models and S$=$1/2 Kagome Heisenberg model. By extracting an accurate TEE, we identify a quantum spin liquid with topological order for the first time in physically realistic SU(2)-invariant lattice model. We emphasize that the TEE provides positive, ``smoking gun'' evidence for a topological quantum spin liquid, and excludes any topologically trivial states, including the valence bound solid state. Besides the Kagome Heisenberg model, based on large-scale accurate density-matrix renormalization group studies of numerous long cylinders with circumferences up to 14 lattice spacings, our results [H. C. Jiang, H. Yao, and L. Balents, Physical Review B 86, 024424(2012)], through a combination of the absence of magnetic or VBS order, nonzero spin singlet and triplet gaps, as well as a finite TEE extremely close to ln(2), provide compelling evidence that the two-dimensional ground state of the square J$_{1}$-J$_{2}$ Heisenberg model is a topological quantum spin liquid. [Preview Abstract] |
Tuesday, March 19, 2013 4:54PM - 5:06PM |
J15.00011: Finite size scaling of entanglement entropy at the Anderson transition with interactions An Zhao We study the entanglement entropy(EE) of disordered one-dimensional spinless fermions with attractive interactions. With intensive numerical calculation of the EE using the density matrix renormalization group method, we find clear signatures of the transition between the localized and delocalized phase. In the delocalized phase, the fluctuations of the EE becomes minimum and independent of the system size. Meanwhile the EE's logarithmic scaling behavior is found to recover to that of a clean system. We present a general scheme of finite size scaling of the EE at the critical regime of the Anderson transition, from which we extract the critical parameters of the transition with good accuracy, including the critical exponent, critical point and a power-law divergent localization length. [Preview Abstract] |
Tuesday, March 19, 2013 5:06PM - 5:18PM |
J15.00012: Three dimensional symmetry protected topological phase and algebraic spin liquid Cenke Xu It is well-known that one dimensional spin chains are described by O(3) nonlinear sigma models with a topological $\Theta-$term, and $\Theta = 2\pi S$. A pin-1/2 chain (described by $\Theta = \pi$) must be either gapless or degenerate, while a spin-1 chain (described by $\Theta = 2\pi$) is a symmetry protected topological phase, namely its bulk is gapped and nondegenerate, while its boundary is a free spin-1/2 with two fold degeneracy. We prove that these phenomena also occur in arbitrary odd dimensions. For example, in three dimensional space, we construct a series of SU(N) antiferromagnet models, whose low energy field theories are nonlinear sigma models with a 3+1d $\Theta-$term. We will also prove that when $\Theta = \pi$, the disordered phase of this system cannot be gapped and nondegenerate, namely it can be an algebraic liquid phase. When $\Theta = 2\pi$, the system is a three dimensional symmetry protected topological phase, whose 2+1d boundary must be either gapless or degenerate. [Preview Abstract] |
Tuesday, March 19, 2013 5:18PM - 5:30PM |
J15.00013: Continuous phase transition between N\'eel and spin liquid states with topological order Yang Qi, Zhengcheng Gu It is well known that on square lattice N\'eel and valence bond solid states are connected by a continuous phase transition, and the critical theory consists fractionalized spinons and an emergent U(1) gauge field. Motivated by recent numerical works revealing N\'eel and gapped spin liquid states in $J_1$-$J_2$ model on square lattice, we study other phases that can be obtained after destroying the N\'eel order. We show that by condensing fields that carry both electric charge and magnetic flux of the emergent gauge field, one can obtain spin liquid phases with topological order and no lattice symmetry breaking. [Preview Abstract] |
Session J16: Focus Session: Molecular Nanomagnets/Devices
Sponsoring Units: GMAG DMPChair: Stefano Carretta, University of Parma
Room: 318
Tuesday, March 19, 2013 2:30PM - 2:42PM |
J16.00001: Modification of Molecular Spin Crossover in Ultra-Thin Films Daniel Dougherty, Alex Pronschinske, Yifeng Chen, Arrigo Calzolari, Geoff Lewis, David Shultz, Marco Buongiorno-Nardelli Iron (II) spin crossover compounds exhibit a strong connection between molecular spin state and electronic structure that make them exciting candidates for highly tunable materials for spintronic applications. The spin crossover phenomenon is often extremely sensitive to crystal packing effects that may be modified in device environments compared to bulk materials. We report evidence for dramatic modification of spin crossover in bilayer films of Fe[(H$_{\mathrm{2}}$Bpz$_{\mathrm{2}})_{\mathrm{2}}$bpy] on Au(111) compared to bulk behavior. Scanning Tunneling Microscopy, spectroscopy, and local conductance mapping show spin-state coexistence in bilayer films of Fe[(H$_{\mathrm{2}}$Bpz$_{\mathrm{2}})_{\mathrm{2}}$bpy] on Au(111) that is independent of temperature between 130 K and 300 K due to the unique packing constraints of the bilayer film that promote deviations from bulk behavior. Local density of states measured for different spin states show that high-spin molecules have a smaller transport gap than low-spin molecules in agreement with density functional theory calculations. In addition, aggregation of spin states into ``like-spin'' domains is observed. [Preview Abstract] |
Tuesday, March 19, 2013 2:42PM - 2:54PM |
J16.00002: Electronic and transport properties of Fe-based spin crossover complexes from first principles Yifeng Chen, Marco Buongiorno Nardelli Using calculations from first principles, we studied the electronic and transport properties of the Fe(II) spin crossover (SCO) compound Fe[H$_2$B(pz)$_2$]$_2$(bpy). The magnetic transition has been imposed by constrained magnetization calculations and the computed electronic structure agrees with available experimental data. The unique bilayer configuration achievable by vacuum evaporation on Au(111) in experiments, is modeled by a $\pi$-stacking dimer structure that is used for the interpretation of STM and transport data. Our results explain the meandering spinodal decomposition of the spin domains of the bilayer films and the conductive properties of the system. In particular, we found the high-spin configuration to be more conductive than the low-spin case, in agreement with experimental measurements of corresponding currents through disordered thin films. The spin-switchable electronic transport properties of this kind of Fe(II) SCO compound systems provide viable proofs for future switchable molecular spintronic devices and applications. [Preview Abstract] |
Tuesday, March 19, 2013 2:54PM - 3:06PM |
J16.00003: Complex Materials for Molecular Spintronics Applications: Cobalt Bis(dioxolene) Valence Tautomers, from Molecules to Polymers Marco Buongiorno Nardelli, Arrigo Calzolari, Yifeng Chen, Daniel Dougherty, David Shultz Using first principles calculations we predict a complex multifunctional behavior in cobalt bis(dioxolene) valence tautomeric compounds. Molecular spin-state switching is shown to dramatically alter electronic properties and corresponding transport properties. This spin state dependence has been demonstrated for technologically-relevant coordination polymers of valence tautomers as well as for novel conjugated polymers with valence tautomeric functionalization. As a result these materials are proposed as promising candidates for spintronic devices that can couple magnetic bistability with novel electrical and spin conduction properties. Our findings pave the way to the fundamental understanding and future design of active multifunctional organic materials for spintronics applications. [Preview Abstract] |
Tuesday, March 19, 2013 3:06PM - 3:42PM |
J16.00004: Electronic read-out of a single nuclear spin using a molecular spin transistor Invited Speaker: Franck Balestro Thanks to recent advances of nanofabrication techniques, molecular electronics devices can address today the ultimate probing of electronic transport flowing through a single molecule. Not only this electronic current can show signatures of the molecular quantum levels but it can also detect the magnetic state of the molecule. As a consequence, an entirely novel research field called \textit{molecular spintronics} in which quantum magnetism of molecular systems can be interfaced to nanoelectronics is now emerging. One of the recent challenges of this field was to probe by this current, not the only spin state of an electron, but the state of a single nuclear spin. Such an achievement was experimentally unimaginable a few years ago. Indeed, the magnetic signal carried by a single nuclear spin is a thousand times less than that of a single electron spin ... Using a Single Molecular Magnet (TbPc2) as a molecular spin transistor in a three terminals configuration, the experiment consists in measuring the current changes when ones sweep the external magnetic field applied to the molecule. When the magnetic spin of the molecule changes its quantum state, a change of current is recorded. Because of the well-defined relationship that exists between the electron spin and nuclear spin carried by the nuclei of the Terbium atom, it is possible to perform the electronic read-out of the electronic spin state which, in turn give information on the state of a single nuclear spin. Application of this effect for quantum information manipulation and storage can be envisioned, as the observation of energy level lifetimes on the order of tens of seconds opens the way to coherent manipulations of a single nuclear spin.\\[4pt] Reference:\\[0pt] ``Electronic read-out of a single nuclear spin using a molecular spin transistor,'' R. Vincent, S. Klyatskaya, M. Ruben, W. Wernsdorfer, F. Balestro, Nature, Vol. 488, p.357, (2012). [Preview Abstract] |
Tuesday, March 19, 2013 3:42PM - 3:54PM |
J16.00005: Quantum Dot Spin Valves Controlled by Single Molecule Magnets Fatemeh Rostamzadeh Renani, George Kirczenow We explore theoretically for the first time the properties of a new class of spintronic nano-devices in which the electrical resistance of a non-magnetic quantum dot contacted by non-magnetic electrodes is controlled by transition metal-based single molecule nanomagnets (SMMs) bound to the dot. Although the SMMs do not lie directly in the current path in these devices, we show that the relative orientation of their magnetic moments can strongly influence on the electric current passing through the device. If the magnetic moment of one of the SMMs is reversed by the application of a magnetic field, we predict a large change in the resistance of the dot, i.e., a strong spin valve effect. The mechanism is resonant conduction via molecular orbitals extending over the entire system. The spin valve is activated by a gate that tunes the transport resonances through the Fermi energy. Detailed results will be presented for the case of Mn$_{12}$ SMMs bound to a gold quantum dot. [Preview Abstract] |
Tuesday, March 19, 2013 3:54PM - 4:06PM |
J16.00006: The effect of current-induced spin switching in the presence of quantum tunneling of magnetization Maciej Misiorny, J\'{o}zef Barna\'{s} Knowledge of transport properties of individual large-spin ($S> 1/2$) atoms/molecules exhibiting magnetic anisotropy is of key importance from the point of view of information processing technologies [1]. The ultimate aim is to incorporate such objects as functional elements of spintronic devices, with the objective of employing spin-polarized currents to control the magnetic state of the system. In particular, for an atom/molecule with the predominant \emph{`easy-axis' uniaxial} magnetic anisotropy this allows for switching the system's spin between two metastable states [2,3]. However, the \emph{uniaxial} component of magnetic anisotropy, underlying the magnetic bistability, is frequently accompanied by the \emph{transverse} one, whose presence manifests, e.g., as quantum tunneling of magnetization (QTM). Here, we show that not only does QTM induce an effective energy barrier for the spin switching, but also its effect on the transport reveals as an additional signal in transport characteristics. Furthermore, we propose how to experimentally investigate QTM by means of the STM inelastic transport spectroscopy. [1] M. Mannini et al., Nature Mater. 8, 194 (2009); [2] M. Misiorny and J. Barna\'{s}, Phys. Rev. B 75, 134425 (2007); [3] S. Loth et al., Nature Phys. 6, 340 (2010). [Preview Abstract] |
Tuesday, March 19, 2013 4:06PM - 4:18PM |
J16.00007: Vibrational properties of single-molecule magnet Fe4 Michael Warnock, Kyungwha Park, Yoh Yamamoto A single-molecule magnet (SMM) Fe$_4$ consists of four Fe ions interacting through O anions via antiferromagnetic superexchange coupling, with the total ground-state spin of $S=5$. The SMM Fe$_4$ has a magnetic anisotropy energy of $16$ K, and its ground-state spin multiplet is well separated from the first excited spin multiplet. A recent experimental effort demonstrated that SMMs Fe$_4$ can be deposited on various substrates with magnetic cores intact and that individual Fe$_4$ molecules can be bridged between electrodes. SMMs Fe$_4$ deposited on substrates or in contact with electrodes revealed interesting magnetic and transport properties. Electronic and spin degrees of freedom of SMM Fe$_4$ may be coupled to vibrational degrees of freedom. Such coupling can affect various properties of SMM Fe$_4$. Here we present our calculation of vibrational spectra (Raman and infrared) of SMM Fe$_4$ using density-functional theory (DFT) within simple harmonic oscillator approximation. We identify normal modes and compare our calculated result with available experimental data. [Preview Abstract] |
Tuesday, March 19, 2013 4:18PM - 4:30PM |
J16.00008: Cotunneling signatures of spin-electric coupling in frustrated triangular single-molecule magnets Javier Nossa, Carlo Canali The ground state (GS) of frustrated (antiferromagnetic) triangular single-molecule magnets is characterized by two total-spin $S =$ 1$/$2 doublets with opposite chirality. According to a group theory analysis [M. Trif \textit{et al.}, Phys. Rev. Lett. \textbf{101}, 217201 (2008)] an external electric field can efficiently couple these two chiral spin states, even when the spin-orbit interaction (SOI) is absent. The strength of this coupling, $d$, is determined by an off-diagonal matrix element of the dipole operator, which can be calculated by \textit{ab-initio} methods [M. F. Islam \textit{et al.}, Phys. Rev. B \textbf{82}, 155446 (2010)]. In this work we propose that Coulomb-blockade transport experiments in the cotunneling regime can provide a direct way to determine the spin-electric coupling strength. Indeed, an electric field generates a $d$-dependent splitting of the GS manifold, which can be detected in the inelastic cotunneling conductance. Our theoretical analysis is supported by master-equation calculations of quantum transport in the cotunneling regime. We employ a Hubbard-model approach to elucidate the relationship between the Hubbard parameters $t$ and $U$, and the spin-electric coupling constant $d.$ This allows us to predict the regime in which the coupling constant $d$ can be extracted from experiment. [Preview Abstract] |
Tuesday, March 19, 2013 4:30PM - 4:42PM |
J16.00009: ABSTRACT WITHDRAWN |
Tuesday, March 19, 2013 4:42PM - 4:54PM |
J16.00010: Spin moment distributions in Cr-based antiferromagnetic rings Cr7M (M$=$Ni and Cd) studied by $^{53}$Cr NMR Yuji Furukawa, Cecilia Casadei, Lorenzo Bordonali, Ferdiando Borsa, Grihore Timco, Richard Winpenny Recent progress in synthesizing molecular magnets offers the opportunity to investigate magnetic properties of the system composed of small number of magnetically coupled spins. In this study, we have investigated magnetic properties of Cr-based antiferromagnetic (AF) ring Cr7M (M$=$Ni and Cd)). The ancestor of Cr7M is a well-known AF ring Cr8 with a spin single S$=$0 ground state due to AF interaction (J $\sim$ 16K) between nearest neighbor Cr$^{3+}$ (s$=$3/2) spins. A substitution of one of eight Cr$^{3+}$ ions with Ni$^{2+}$ (s$=$1) or Cd$^{2+}$ (s$=$0) leads to destroy the coherence of spin singlet ground state in Cr8. As a result, the Cr7M has a magnetic ground state with total spin S$_{\mathrm{T}}=$1/2 and S$_{\mathrm{T}}=$3/2 for Cr7Ni and Cr7Cd, respectively. In the magnetic ground state, local spin moments will appear on each Cr$^{3+}$ ion. In order to investigate the details of spin moments distributions on Cr ions in the systems, we have carried out $^{53}$Cr-NMR measurements in Cr7M in its magnetic ground state at low temperature. Based on the $^{53}$Cr-NMR results, we will discuss differences in distributions of the spin moments in Cr7M systems in its magnetic ground state. [Preview Abstract] |
Tuesday, March 19, 2013 4:54PM - 5:06PM |
J16.00011: Spin dynamics in atomically assembled antiferromagnets Sebastian Loth Antiferromagnetic materials possess ordered magnetic states that have vanishing magnetization. We used a low-temperature scanning tunneling microscope to construct few-atom antiferromagnets. Even-numbered arrays of antiferromagnetically coupled atoms were found to have no net spin. Their shapes can be defined precisely by atom manipulation avoiding uncompensated magnetic moments at the nanoparticle's edge. We use such spin-compensated atomic arrays to study the intrinsic dynamics of nanoscale antiferromagnets [1]. For chains of more than four atoms we observe two Neel-ordered ground states and frequent switching between them. The spontaneous switching rates depend strongly on the number of coupled atoms and we observed magnetic tunneling of the Neel vector for the smallest structures. In arrays with ten or more atoms the residence time in each state can exceed many hours but current-induced switching proceeds at nanosecond speed. These properties enable a model demonstration of dense magnetic data storage that uses antiferromagnets as memory elements. [1] S. Loth, S. Baumann, C. P. Lutz, D. M. Eigler and A. J. Heinrich, Science 335, 196 (2012). [Preview Abstract] |
Tuesday, March 19, 2013 5:06PM - 5:18PM |
J16.00012: Spin dynamics of molecular nanomagnets unraveled at atomic scale by four-dimensional inelastic neutron scattering Paolo Santini, Michael Baker, Tatiana Guidi, Stefano Carretta, Jacques Ollivier, Hannu Mutka, Hans Guedel, Grigore Timco, Eric McInnes, Giuseppe Amoretti, Richard Winpenny Molecular nanomagnets (MNMs) have been test-beds for addressing several elusive but important phenomena in quantum dynamics, but to this point it has been impossible to determine the spin dynamics directly. We show that recently-developed inelastic-neutron-scattering instrumentation, yielding the cross-section in vast portions of reciprocal space, enables two-spin dynamical correlation functions of MNMs to be directly determined without assuming an underlying model Hamiltonian. We use the Cr$_8$ antiferromagnetic ring as a benchmark to demonstrate the potential of this approach which allows us, for example, to examine how a quantum fluctuation propagates along the ring or to test the degree of validity of the Neel-vector-tunneling framework [1]. This result opens remarkable perspectives in the understanding of the quantum dynamics in several classes of MNMs. [1] M. Baker et al., Nature Physics in press (doi:10.1038/nphys2431) [Preview Abstract] |
Tuesday, March 19, 2013 5:18PM - 5:30PM |
J16.00013: Diamagnetic Exciton Properties in Asimetrical Quantum Dot Molecules Nelson Ricardo Fino Puerto, Hanz Ramirez, Angela Camacho Beltran The magnetic properties of nanostructures like quantum dots and rings are the subject of intense research. In particular, magnetic control of coupled quantum dots has become subject of interest. By using a first order perturbation approach, and within the effective mass approximation, we calculate magnetic field dependent electronic structures of confined excitons and trions in vertically coupled quantum dots. With these results we study the photoluminescence spectra of neutral and charged excitons in these structures that are coupled via magnetic field in the Faraday configuration (quantum dot molecules QDM). In this work study this spectra around three charge configurations: neutral exciton (X), positive trion (X$^{\mathrm{+}})$ and negative trion (X), where the charged can be distributed over any of the dots in the basis of the optically active excitons and tunneling electron through the interdot barrier. Also we study different different ratios between the dots, that allow the appearance of crossings and anticrossings in the behavior of the energy with respect to the magnetic field. [Preview Abstract] |
Session J17: Focus Session: Magnetic Oxide Nano- & Hetero-Structures
Sponsoring Units: DMP GMAGChair: Yayoi Takamura, UC Davis
Room: 319
Tuesday, March 19, 2013 2:30PM - 2:42PM |
J17.00001: Mesoscale spin domain formation and their correlations in quasi-1D La$_{0.67}$Sr$_{0.33}$MnO$_3$ nanowires Xiaoqian M. Chen, Nick Bronn, Nadya Mason, Peter Abbamonte, Jason Hoffman, Anand Bhattacharya Creating materials with nano-scale dimensions can introduce finite size and boundary effects, where the scale of the system boundaries near criticality becomes comparable to the correlation of competing orders in the material. To study these effects, we have fabricated arrays of quasi-1D nanowires from epitaxially grown La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ (LSMO) thin films on SrTiO$_{3}$(STO) substrates. Our studies with resonant soft x-ray scattering (RSXS) reveal a non-trivial magnetic domain formation along different momentum directions in these wires. In addition, a new magnetic order was observed below 110K, likely induced by the STO structural transition. Below the Curie temperature we also observed a series of magnetic superlattice reflections, indicating collective mesoscale ordering of the magnetic moments into a pattern with a spatial period of five wires. Our calculations using dielectric susceptibility and Ising model simulations provide us an interpretation for the mechanism of domain formation and their long-range interaction through dipole coupling. [Preview Abstract] |
Tuesday, March 19, 2013 2:42PM - 2:54PM |
J17.00002: Dynamic resistive switching controlled by local lateral gating in phase separated manganite wires Hangwen Guo, Joo Hyon Noh, Shuai Dong, Philip Rack, Zheng Gai, Xiaoshan Xu, Elbio Dagotto, Jian Shen, Thomas Z. Ward Behaviors such as high T$_{\mathrm{c}}$ superconductivity, colossal magnetoresistivity, and the metal-insulator transition, have been tied to inherent electronic phases coexisting in a single crystal material. Here we demonstrate a novel approach to induce resistive electric field effect transitions based on the modification of the inherent electronic domain structures in single crystal materials. A phase separated manganite system confined to a scale which isolates a few electronic domains is controlled using laterally gated which give repeatable resistive changes of up to 50{\%}. This technique also makes it possible to create multistate switching devices from a single confined transport channel. These findings provide an avenue to control inherent electronic phases in strongly correlated materials as a means of creating novel nano-electronic devices. Supported by the US DOE Office of Basic Energy Sciences, Materials Sciences and Engineering Division. [Preview Abstract] |
Tuesday, March 19, 2013 2:54PM - 3:06PM |
J17.00003: Magnetic structure of epitaxial self-assembled La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ nanoislands Jone Zabaleta, Sergio Valencia, Florian Kronast, Miriam Jaafar, Patricia Abellan, Cesar Moreno, Jaume Gazquez, Oscar Iglesias-Freire, Felip Sandiumenge, Teresa Puig, Agustina Asenjo, Narcis Mestres, Xavier Obradors The mixed-valence manganite La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ (LSMO), in nanoscale configuration, is a strong candidate for magnetic logic and sensor applications because of its Curie temperature (360 K) and high degree of spin polarization. In this work we unravel the magnetic structure of self-assembled ferromagnetic LSMO epitaxial nanoislands smaller than 200 nm in lateral size and less than 40 nm in height, grown using a bottom-up solution-based methodology. Magnetic force microscopy shows that LSMO islands stabilize either single domain, multidomain, or vortex state configurations, depending on their lateral size and aspect ratio. The vortex state of islands with different morphology and two distinct crystallographic orientations is further explored using spatially-resolved x-ray magnetic circular dichroism in photoemission electron microscopy measurements. The vortex evolution of individual islands is tracked in-situ by applying in-plane magnetic field. The magnetic structure study is complemented with crystal structure, strain state, and chemical composition studies. [Preview Abstract] |
Tuesday, March 19, 2013 3:06PM - 3:18PM |
J17.00004: Control with the switching behavior in exchange-coupled nanomagnets Erik Folven, Yayoi Takamura, Andreas Scholl, Andrew Doran, Anthony Young, Scott T. Retterer, Helen Gomonay, Thomas Tybell, Jostein Grepstad Control with the switching behavior of monodomain nanomagnets is key to a range of magnetic device technologies. We have recently demonstrated that shape-induced stabilization of antiferromagnetic (AFM) domains can be achieved in embedded $LaFeO_3$ thin film nanostructures.$^{1,2}$ This finding offers a pathway to influence the switching behavior of nanoscale thin film ferromagnets through exchange coupling across the interface between an antiferromagnet and a ferromagnet. Here, we show how the switching field for rectangular nanomagnets may be significantly reduced in $LaFeO_3$ (AFM)/$La_{0.7}Sr_{0.3}MnO_3$ (FM) heterostructures. Mediated by the interface exchange coupling, the engineered domains in the $LaFeO_3$ layer give rise to a uniaxial bias field acting on the magnetic moments in the $La_{0.7}Sr_{0.3}MnO_3$. By tailoring the AFM domain state, we can align this bias field perpendicular to the long axis of the magnetic element, effectively lowering the potential barrier between the two stable single domain states of the rectangular nanomagnet. The experimental data obtained with element specific x-ray spectromicroscopy is compared with a simple theoretical model. 1. Folven et al., Nano Letters 10, 4578 (2010) 2. Folven et al., Nano Letters 12, 2386 (2012) [Preview Abstract] |
Tuesday, March 19, 2013 3:18PM - 3:30PM |
J17.00005: Fabrication and study of CoF$_{2}$O$_{4}$ structures on Graphene substrates employing scanning probe microscopy techniques Irma Kuljanishvili, Marko Surtchev, John Cavin, Alexander Smetana, Saju Nattikadan Graphene materials are being investigated in recent years for verity of applications, including electric and optical devices and novel substrates. In this study we explore the route for assembling micro- and nanoscale architectures of magnetic complex oxide material directly on graphene surface using `direct write' parallel patterning techniques. Ferrimagnetic oxide CoFe$_{2}$O$_{4}$ (CFO) was prepared by sol-gel chemical route and used as `ink' for patterning structures. An array of CFO dots was fabricated using Dip Pen Nanolithography method at specific locations. Here we will discuss the surface properties of the formed dot structures of CoFe$_{2}$O$_{4}$ on graphene as compared to those formed on Si/SiO$_{2}$ substrate. Structures fabricated on each substrate with the same ambient conditions and thermal processing show different morphology and magnetic interactions when studied using AFM and MFM techniques. We will describe our findings and results acquired on individual CFO dots of different sizes. We will also show that graphene substrate is likely influencing the magnetic characteristics of CFO dots that are formed on its surface, although the role of graphene as a substrate for CFO dot formation should be further investigated. [Preview Abstract] |
Tuesday, March 19, 2013 3:30PM - 3:42PM |
J17.00006: Electrical manipulation of interface conduction in BiFeO3-CoFe2O4 columnar heterostructures Yi-Chun Chen, Ying-Hui Hsieh, Jia-Ming Liou, Chia-Ying Shen, Ying-Hao Chu Complex oxide interfaces emerge as one of the most exciting subjects in the condensed-matter field due to its unique physical properties and new possibilities for next-generation electronic devices. Recently, we found local conduction at the tubular interfaces of self-assembled BiFeO3 (BFO)-CoFe2O4 (CFO) heterostructures. In this study, to further investigate the electrical properties of the tubular oxide interface, conductive atomic force microscopy (CAFM) at different temperatures was performed to examine the sample. The origin of local conduction at the BFO-CFO vertical interface is identified as a result of the accumulation of oxygen vacancies. In addition, the interface conduction can be modulated with non-volatile and reversible behaviors via an external electric field. This memritor-like phenomenon can be understood owing to the movement of oxygen vacancies driven by the applied bias. The bias causes the oxygen vacancies either accumulate or deplete to the metal contact tip, which in turn affect the resistance at the tubular interface. Our results provide the control of the conduction at complex oxide interfaces and suggest the possibility for new devices based on complex oxide interfaces. [Preview Abstract] |
Tuesday, March 19, 2013 3:42PM - 3:54PM |
J17.00007: Self-Assembled Multiferroic Nanocomposites for Use in Magnetic Logic Architecture Ryan Comes, Mikhail Khokhlov, Hongxue Liu, Jiwei Lu, Stuart Wolf CoFe$_{\mathrm{2}}$O$_{\mathrm{4}}$ (CFO) offers unique properties as a magnetoelectric material due to its large magnetoelastic response when strained. Previous work has shown that when CFO is co-deposited with BiFeO$_{\mathrm{3}}$ (BFO) nanostructured phase segregation occurs, with CFO pillars forming in a BFO matrix. The CFO-BFO nanocomposite system has been proposed as a possible multiferroic logic or memory scheme.[1] We will discuss the patterning and growth of CFO-BFO composites using e-beam lithography and pulsed electron deposition.[2] Our results have demonstrated the ability to pattern the composites into square arrays of pillars with spacing as small as 100 nm. The magnetic properties of the patterned films have been characterized using magnetic force microscopy and are in good agreement with previous results from our group for unpatterned composites.[3] Cross-sectional TEM analysis of the films was used to quantify the strain in the CFO pillars and evaluate the elastic anisotropy. Piezoresponse force microscopy analysis and lithographic domain patterning of the BFO matrix is also presented. [1] S.A. Wolf, et al. Proc. IEEE 98 (2010). [2] R. Comes, et al. Nano Lett. 12 (2012). [3] R. Comes, et al. J. App. Phys. 111 (2012). [Preview Abstract] |
Tuesday, March 19, 2013 3:54PM - 4:06PM |
J17.00008: Magnetoelectric effects in oxide magnetic tunnel junctions with ferroelectric barriers Javier Tornos, Y.H. Liu, S.G.E. te Velthuis, M.R. Fitzsimmons, A. Rivera, R. Lopez Anton, G. Sanchez Santolino, M. Varela del Arco, N.M. Nemes, S.J. Pennycook, Z. Sefrioui, C. Leon Yebra, J. Santamaria Functional properties of magnetic tunnel junction can be enhanced by employing a ferroelectric material as the barrier layer. We report on La0.7Sr0.3MnO3(LSMO)/BaTiO3(BTO)/LSMO magnetic tunnel junctions(MTJ) with BTO ferroelectric tunnel barrier. Switching BTO ferroelectric polarization influences the tunneling magnetoresistance (TMR) achieving two different r resistance states for each magnetic state (parallel or antiparallel) of the magnetization of the electrodes . The voltage dependence of the differential conductance obtained from IV curves displays oscillations whose period depends on the BTO electric polarization. This unusual behavior could be related to the presence of an induced magnetic moment in BTO ferroelectric barrier detected by XMCD measurements. These results reveal that spin polarization, and its tunneling conductance can be electrically tuned through reversal of the ferroelectric polarization of the barrier. [Preview Abstract] |
Tuesday, March 19, 2013 4:06PM - 4:18PM |
J17.00009: Magnetic field effects on dielectrophoresis in manganites Daniel Grant, Galin Dragiev, Amlan Biswas Perovskite-type manganese oxides (manganites) are of interest for many of the different properties they possess, including colossal magnetoresistance (CMR) and ferroelectric behavior. With the application of an electric field, large resistance decreases have been noted near the insulator-to-metal transition temperature in samples of (La$_{\mathrm{1-y}}$Pr$_{\mathrm{y}})_{\mathrm{1-x}}$Ca$_{\mathrm{x}}$MnO$_{\mathrm{3}}$ (LPCMO). Two proposed models have emerged to explain the behavior, dielectric breakdown and dielectrophoresis, with experimental evidence showing some aspects of the dielectrophoresis model to be correct. However, neither model accounts for magnetic interactions among the ferromagnetic metallic regions and the effects of a magnetic field applied in conjunction with an electric field. We have performed measurements on LPCMO samples by varying the strength and orientation of the magnetic field and the applied voltage. Cross-shaped microstructures have been made on LPCMO samples to allow us to investigate the effects of sample size on dielectrophoresis. We will present resistance and magnetization data obtained on LPCMO samples at various magnetic field strengths, magnetic field orientations, and sample sizes to elucidate the effect of magnetic interactions on dielectrophoresis induced transport and magnetic properties. [Preview Abstract] |
Tuesday, March 19, 2013 4:18PM - 4:30PM |
J17.00010: The observation and control of electronic nematic phase in manganites by stripy domains Changcheng Ju During the past decades, novel electronic liquid crystal phases have been revealed in strongly correlated electronic systems, especially the electronic nematic phase in strontium ruthenate and superconductors. Transport measurements show strongly transport anisotropies in these otherwise isotropic electronic systems. In this work, we report 71$^{\circ}$ striped ferroelectric domains created in BiFeO3 can also epitaxially lock the perovskite manganites leading to the emerge of an electronic nematic phase. Firstly, La1-xSrxMnO3/BiFeO3 (LSMO/BFO) bilayer samples are deposited by PLD. The 71$^{\circ}$ periodic striped ferroelectric domains and coherent growth are demonstrated by PFM and X-ray rocking curve. X-ray reciprocal space mapping have been used to confirm the epitaxial relationships of the layers and in-plane lattice constants. Transport measurements reveal a nematic phase transition without high magnetic fields. By changing the thickness of BFO and LSMO layer respectively, we observed substantial anisotropic resistivities and a shift of transition temperature for nematic phase and M-I transition. Unlike the other electronic liquid crystals, magnetic fields perpendicular to the film can suppress the appearance of nematic phase. XMCD and NEXAFS at the Mn L2, 3 edge revealed an in-plane preferential occupation of orbitals and a broken rotational symmetry for Mn-O-Mn bonds at nematic phase. At last, we also demonstrate a nonvolatile electric-field control of anisotropic resistivity switching. [Preview Abstract] |
Tuesday, March 19, 2013 4:30PM - 4:42PM |
J17.00011: Characterization of interfacial charge accumulation in ferroelectric BaTiO$_3$/manganite interfaces using atomic-resolution annular bright field imaging and electron energy-loss spectroscopy Robert Klie, Qiao Qiao, Patrick Phillips, Hanghui Chen, Matthew Marshall, Fred Walker, Sohrab Ismail-Beigi, Charles Ahn Interfaces in functional oxides have been the focus of many studies due to potential emergence of novel phases. In this study, we will focus on ferroelectric/manganite, more specifically the LaSrMnO$_3$/BaTiO$_3$ interfaces in single-crystal thin films grown on SrTiO$_3$. Using atomic-resolution annular bright field (ABF) imaging, as well as atomic-column resolved electron energy-loss spectroscopy in the aberration-corrected, cold-field emission gun JEOL ARM200CF, we will demonstrate that the interfacial accumulation/depletion of charges, depending on the orientation of the ferroelectric polarization, can be directly quantified. We find that the interfacial accumulation of electron/holes is screen within three unit-cells of LaSrMnO$_3$. Moreover, using ABF imaging, we will shows that the distortions of the oxygen sublattice can be directly quantify, in both the BaTiO$_3$ layer, as well as the interfacial LaSrMnO$_3$. Our experimental results imaging and spectroscopy results will be complemented by first-principles density functional theory calculations. [Preview Abstract] |
Tuesday, March 19, 2013 4:42PM - 4:54PM |
J17.00012: Magnetic properties of La$_{0.7}$Sr$_{0.3}$MnO$_{3}$/BaTiO$_{3}$ interfaces Yaohua Liu, S.G.E. te Velthuis, J.W. Freeland, N.J. Tornos, C. Leon, J. Santamaria Interfaces between the ferromagnetic (FM) and ferroelectric (FE) oxides may host nanoscale multiferroic phases with strong magnetoelectric coupling, which can be potentially utilized for energy-efficient spintronics. In this work, we have investigated the magnetic properties of the interface between ferromagnetic La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ (LSMO) and ferroelectric BaTiO$_{3}$ (BTO) via X-ray resonant magnetic scattering (XRMS) and X-ray magnetic circular dichroism (XMCD) on a series of 10 nm LSMO / t BTO bilayers, with t = 1.2, 2.4 and 4.8 nm, respectively. Additionally, we have studied a LSMO/BTO/LSMO trilayer. Interestingly, we have observed magnetic dichroism from Ti ions between 30 K and 210 K, which closely tracks the Mn's dichroism during the magnetization reversal. In contrast, no Ti magnetization has been observed in a single-layer BTO film on a SrTiO$_{3}$ substrate. These results suggest that there are Ti$^{3+}$ ions that reside at the LSMO/BTO interfaces and the interfacial Mn and Ti moments are exchange coupled. [Preview Abstract] |
Tuesday, March 19, 2013 4:54PM - 5:06PM |
J17.00013: Magnetoelectric coupling at the EuO/BaTiO$_{3}$ interface Shi Cao, Pan Liu, Jinke Tang, Chung Wung Bark, Sangwoo Ryu, Chang Beom Eom, Peter Dowben, Alexei Gruverman Magnetization modulation by ferroelectric polarization pinning is reported for the ferromagnetic-ferroelectric EuO/BaTiO$_{3}$ (EuO/BTO) heterostructures. Away from T$_{c}$, the critical exponent $\beta $ indicates that the magnetization of EuO is consistent with mean field theory despite suggestions that EuO is a typical Heisenberg ferromagnetic semiconductor. The Heisenberg model is also inconsistent with the significant band dispersion seen in EuO thin films. The possible mechanisms include extrinsic doping and/or pinning of interface states at the EuO/BTO interface. The results are discussed in the context of data also obtained for La$_{0.67}$Sr$_{0.33}$MnO$_{3}$/BaTiO$_{3}$ heterostructures, where the critical exponent $\beta $ is also close to the predictions of mean field theory, suggesting a similarity in the importance of the magnetic interface with a ferroelectric and the possible importance of ferroelectric polarization reversal. [Preview Abstract] |
Tuesday, March 19, 2013 5:06PM - 5:18PM |
J17.00014: Coupled ferromagnetism and ferroelectricity in superlattices of non-ferroelectric antiferromagnetic manganites J.D. Burton, K. Rogdakis, J.W. Seo, Z. Viskadourakis, Y. Wang, L. Ah Qune, E. Choi, E. Tsymbal, J. Lee, C. Panagopoulos Complex oxide heterostructures present a promising avenue for the design of multifunctional properties which may find application in a variety of technological systems. In heterostructures composed of transition metal oxides the disruption introduced by an interface can affect the balance of the competing interactions among spins, charges and orbitals. This has led to the emergence of properties absent in the original building blocks of a heterostructure. We will report on the discovery of magnetically tunable ferroelectricity in artificial tri-layer superlattices consisting of non-ferroelectric and non-ferromagnetic components: NdMnO$_3$/SrMnO$_3$/LaMnO$_3$.[1] Ferroelectricity was observed below 40 K exhibiting strong tunability by superlattice periodicity. Furthermore, magnetoelectric coupling resulted in 150{\%} magnetic modulation of the polarization. First-principles calculations indicate that broken space inversion symmetry and mixed valency give rise to the observed behavior. This discovery highlights the importance of tri-layered systems for the engineering of emergent properties in oxide heterostructures. [1] K. Rogdakis et al, Nat Commun 3, 1064 (2012) [Preview Abstract] |
Tuesday, March 19, 2013 5:18PM - 5:30PM |
J17.00015: Anomalous exchange bias at collinear/noncollinear spin interface Tao Wu We report on the interfacial magnetic coupling in manganite bilayers of collinear ferromagnetic La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ and noncollinear multiferroic TbMnO$_{3}$. Exchange bias emerges at the Neel temperature of TbMnO$_{3}$ (about 41 K) due to the onset of long-range antiferromagnetic ordering in the Mn spin sublattice. Interestingly, an anomalous plateau of exchange bias emerges at the ordering temperature of Tb spins (about 10 K), and we ascribe this unique feature to the strong coupling between Tb and Mn spin sublattices in TbMnO$_{3}$, which in turn influence the magnetic coupling across the interface. On the other hand, the enhancement of coercivity in La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ shows monotonous temperature dependence. Our results illustrate a strong interfacial magnetic coupling at the La$_{0.7}$Sr$_{0.3}$MnO$_{3}$/TbMnO$_{3}$ interface, highlighting the roles of competing spin orders, magnetic frustration, and coupling between multiple spin sublattices in artificial collinear/noncollinear spin heterostructures. [Preview Abstract] |
Session J18: Two Dimensional Topological Insulators: Theory
Sponsoring Units: DCMPChair: Byounghak Lee, Texas State University
Room: 320
Tuesday, March 19, 2013 2:30PM - 2:42PM |
J18.00001: Correlated effects in topological phase transitions Hsiang-Hsuan Hung, Lei Wang, Zheng-Cheng Gu, Gregory A. Fiete Correlation effects in topological phases have been a central topic of interest, yet elusive in experiment. In this talk, we present the results of a numerical study beyond mean-field theory of a phase transition between a two-dimensional Z2 topological insulator phase and a trivial insulator that is driven by correlation effects. In addition to the Z2 invariant, we find that certain features of the single-particle Green's functions (simpler to compute than the full Z2 invariant) carry important information that are strongly indicative of a non-trivial Z2 topological character. We observe that the fluctuations originating from correlations tend to move the topological phase transition boundary to larger values of interactions. [Preview Abstract] |
Tuesday, March 19, 2013 2:42PM - 2:54PM |
J18.00002: Topological insulators of interacting bosons in two dimensions: Classification, effective field theory and microscopic construction Yuan-Ming Lu, Ashvin Vishwanath While topological insulators of non-interacting fermions have been extensively studied, we know very little about topological insulators of bosons, whose realization necessitates strong interaction. In this work we apply Chern-Simons effective theory to classify and characterize interacting bosonic topological insulators in two spatial dimensions. These topological phases have a unique ground state on any closed manifold and no fractional excitations: yet they feature gapless edge states which are often protected by a symmetry. Examples include a bosonic analog of chiral superconductors, bosonic integer quantum Hall states (with Hall conductance quantized to even integers) and bosonic analog of the quantum spin Hall state. We show that these topological phases can be constructed in various ways: such as in arrays of coupled one-dimensional quantum wires. Our formulation also naturally applies to topological insulators of two-dimensional interacting fermions. [Preview Abstract] |
Tuesday, March 19, 2013 2:54PM - 3:06PM |
J18.00003: Topological parity invariant in interacting two-dimensional systems from quantum Monte Carlo Thomas C. Lang, Victor Gurarie, Andrew M. Essin, Stefan Wessel We report results on calculating the parity invariant from Green's functions in quantum Monte Carlo simulations of strongly interacting systems. The topological invariant is used to study the trivial- to topological-insulator transitions in the Kane-Mele-Hubbard model with an explicit bond dimerization. We explore accessibility and behavior of this invariant within quantum Monte Carlo simulations. [Preview Abstract] |
Tuesday, March 19, 2013 3:06PM - 3:18PM |
J18.00004: Rotating spin density wave and inverse spin pumping in quantum spin Hall edges Qinglei Meng, Taylor Hughes, Smitha Vishveshwara We explore interaction effects in quantum spin Hall (QSH) edges in the presence of a finite bias voltage. Using bosonization techniques, we show that repulsive interactions give rise to a spin density wave phase in which the transverse magnetization shows spatial rotation. The effect of a finite bias voltage on this phase is to give the rotation a temporal variation. Using spin transfer torque methods, we show that the system can induce an inverse spin pumping effect in which the magnetic moment of a ferromagnet placed in its proximity can be made to rotate. We demonstrate that this device is equivalent to an electric inductor and in principle can also emit microwave radiation, thus providing a unique ways of probing QSH properties. [Preview Abstract] |
Tuesday, March 19, 2013 3:18PM - 3:30PM |
J18.00005: Theory of correlated topological insulators with broken axial spin symmetry Stephan Rachel, Johannes Reuther, Ronny Thomale The two-dimensional Hubbard model defined for topological band structures exhibiting a quantum spin Hall effect poses fundamental challenges in terms of phenomenological characterization and microscopic classification. We consider weak, moderate, and strong interactions and argue that the resulting phase diagrams depend on the microscopic details of the spin orbit interactions which give rise to the non-trivial topology. In particular, it turns out that there is a crucial difference between models with broken and with conserved axial spin symmetry. These results suggest that there is a general framework for correlated 2D topological insulators with broken axial spin symmetry. [Preview Abstract] |
Tuesday, March 19, 2013 3:30PM - 3:42PM |
J18.00006: ABSTRACT WITHDRAWN |
Tuesday, March 19, 2013 3:42PM - 3:54PM |
J18.00007: Band geometry of fractional topological insulators Rahul Roy Recent numerical simulations of flat band models with interactions which show clear evidence of fractionalized topological phases in the absence of a net magnetic field have generated a great deal of interest. We provide an explanation for these observations by showing that the physics of these systems is the same as that of conventional fractional quantum Hall phases in the lowest Landau level under certain ideal conditions which can be specified in terms of the Berry curvature and the Fubini study metric of the topological band. In particular, we show that when these ideal conditions hold, the density operators projected to the topological band obey the celebrated $W_{\infty}$ algebra. Our approach provides a quantitative way of testing the suitability of topological bands for hosting fractionalized phases. [Preview Abstract] |
Tuesday, March 19, 2013 3:54PM - 4:06PM |
J18.00008: An effective theory of two-dimensional fractional topological insulators Predrag Nikolic A generic spin-orbit coupling in 2D electron systems can be represented by an SU(2) gauge field with a non-trivial SU(2) flux. This makes it possible to stabilize novel non-Abelian incompressible quantum liquids by appropriate interactions (perhaps useful in quantum computing). We will discuss a generalization of the Chern-Simons Lagrangian to an arbitrary SU(N) symmetry group that describes such liquids. This effective field theory contains a Landau-Ginzburg part, which identifies the low energy fluctuations near any putative second-order quantum phase transition between conventional phases. Whenever an incompressible quantum liquid intervenes in such a phase transition, the fractional statistics of its quasiparticles is governed by the topological term of this theory and determined by the low energy dynamics. Commuting external gauge fields reduce the topological term to a Chern-Simons or BF form appropriate for fractional quantum (spin) Hall states, but the generic non-commuting gauge fields are expected to yield new classifiable topological orders without a quantum Hall analogue. We will discuss the possible non-Abelian fractional states in topological insulator quantum wells shaped by the Rashba spin-orbit coupling. [Preview Abstract] |
Tuesday, March 19, 2013 4:06PM - 4:18PM |
J18.00009: Exactly soluble lattice models for abelian topological phases Chien-Hung Lin, Michael Levin We construct exactly soluble bosonic lattice models that realize a large class of abelian topological phases. These models are a generalization of the ``string-net'' models of Ref. [1], but unlike the original construction, we find that our models can realize phases with broken time reversal symmetry. We analyze the braiding statistics of the quasiparticle excitations and show that they are described by nonchiral $U(1) \times U(1) \times \cdots \times U(1)$ Chern-Simons theories(i.e. equal numbers of left and right moving edge modes).\\[4pt] [1] M. Levin and X.-G. Wen, Phys. Rev. B 71, 045110 (2005) [Preview Abstract] |
Tuesday, March 19, 2013 4:18PM - 4:30PM |
J18.00010: Topological Phases in gapped edges of fractionalized systems Frank Pollmann, Johannes Motruk, Erez Berg, Ari Turner We present an extension of the classification scheme for topological phases in interacting one-dimensional fermionic systems to parafermionic chains. We find that the parafermions support both topological as well as symmetry broken phases in which the parafermions condense. In a series of recent works an experimental way of creating parafermions had been proposed: they can arise on the edge of a two-dimensional fractional topological insulator when coupled to superconducting and ferromagnetic domains. The low-energy edge degrees of freedom are described by a chain of coupled parafermions. As a concrete example of our classification we consider the $\nu=1/3$ fractional topological insulator for which we calculate the phase diagram and study the entanglement spectra. We furthermore discuss a concrete physical realization which allows us to tune between the different topological phases. [Preview Abstract] |
Tuesday, March 19, 2013 4:30PM - 4:42PM |
J18.00011: Quantum Geometry of the ``Fuzzy-Lattice'' Hubbard Model and the Fractional Chern Insulator Sagar Vijay, F.D.M. Haldane Recent studies of interacting particles on tight-binding lattices with broken time-reversal symmetry reveal ``zero-field'' fractional quantum Hall (FQH) phases (fractional Chern insulators, FCI). In a partially-filled Landau level, the non-commutative guiding-centers are the residual degrees of freedom, requiring a ``quantum geometry'' Hilbert-space description (a real-space Schr\"odinger description can only apply in the ``classical geometry'' of unprojected coordinates). The continuum description does not apply on a lattice, where we describe emergence of the FCI from a non-commutative quantum lattice geometry. We define a ``fuzzy lattice'' by projecting a one-particle bandstructure (with more than one orbital per unit cell) into a single band, and then renormalize the orbital on each site to unit weight. The resulting overcomplete basis of local states is analogous to a basis of more than one coherent state per flux quantum in a Landau level. The overlap matrix characterizes ``quantum geometry'' on the ``fuzzy lattice'', defining a ``quantum distance'' measure and Berry fluxes through elementary lattice triangles. We study quantum geometry at transitions between topologically-distinct instances of a fuzzy lattice, as well as $N$-body states with local Hubbard interactions. [Preview Abstract] |
Tuesday, March 19, 2013 4:42PM - 4:54PM |
J18.00012: Series of Abelian and Non-Abelian States in C$>$1 Fractional Chern Insulators Antoine Sterdyniak, C\'ecile Repellin, Bogdan Bernevig, Nicolas Regnault We report the observation of a new series of abelian and non-abelian topological states in fractional Chern insulators (FCI). The states appear at bosonic filling nu= k/(C+1) (k, C integers) in a wide variety of lattice models, in fractionally filled bands of Chern numbers C $\geq$ 1 subject to on-site Hubbard interactions. We show strong evidence that the $k=1$ series is abelian while the k $>$ 1 series is non-abelian. The energy spectrum at both ground-state filling and upon the addition of quasiholes shows a low-lying manifold of states whose total degeneracy and counting matches, at the appropriate size, that of the Fractional Quantum Hall (FQH) SU(C) (color) singlet k-clustered states (including Halperin, non-abelian spin singlet(NASS) states and their generalizations). The ground-state momenta are correctly predicted by the FQH to FCI lattice folding. However, the counting of FCI states also matches that of a spinless FQH series, preventing a clear identification just from the energy spectrum. The entanglement spectrum lends support to the identification of our states as SU(C) color-singlets but offers new anomalies in the counting for C $>$ 1, possibly related to dislocations that call for the development of new counting rules of these topological states. [Preview Abstract] |
Tuesday, March 19, 2013 4:54PM - 5:06PM |
J18.00013: Rydberg-Atom Quantum Simulation and Chern Number Characterization of a Topological Mott Insulator Alexandre Dauphin, Markus Mueller, Miguel-Angel Martin-Delgado In this talk we consider a system of spinless fermions with nearest and next-to-nearest neighbor repulsive Hubbard interactions on a honeycomb lattice within the mean-field treatment, and propose and analyze a realistic scheme for analog quantum simulation of this model with cold atoms in a two-dimensional hexagonal optical lattice. Besides a semi-metallic and a charge-density-wave ordered phase, the system exhibits a quantum anomalous Hall phase, which is generated dynamically, i.e. purely as a result of the repulsive fermionic interactions and in the absence of any external gauge fields. We establish the topological nature of this dynamically created Mott insulating phase by the numerical calculation of a Chern number, and study the possibility of coexistence of this phase with the other phases characterized by local order parameters. Based on the knowledge of the mean-field phase diagram, we then discuss in detail how the interacting Hamiltonian can be engineered effective ly by state-of-the-art experimental techniques for laser-dressing of cold fermionic ground-state atoms with electronically excited Rydberg states that exhibit strong dipolar interactions.\\[4pt] [1] A. Dauphin, M. Mueller, and M. A. Martin-Delgado, arXiv:1207.6373. Submitted to PRA and accepted on Sep 26, 2012. [Preview Abstract] |
Tuesday, March 19, 2013 5:06PM - 5:18PM |
J18.00014: Spin-orbit interactions in a helical Luttinger liquid with a Kondo impurity Erik Eriksson We study the transport properties of a helical Luttinger liquid with a Kondo impurity and spin-orbit interactions. Such a system, which may be realized at the edge of a quantum spin Hall insulator with a gate-induced electric field, provides a mechanism to electrically control the conductance. A Rashba spin-orbit interaction may even change the nature of the Kondo screening [Eriksson et al., Phys. Rev. B 86, 161103(R) (2012)]. Considering other types of spin-orbit interactions, together with an extended non-equilibrium analysis, we further improve the understanding of these phenomena. [Preview Abstract] |
Tuesday, March 19, 2013 5:18PM - 5:30PM |
J18.00015: Manipulating Majorana Fermions in Quantum Nanowires with Broken Inversion Symmetry Alejandro M. Lobos, Xiong-Jun Liu We study a Majorana-carrying quantum wire, driven into a trivial phase by breaking the spatial inversion symmetry with a tilted external magnetic field. Interestingly, we predict that a supercurrent applied in the proximate superconductor is able to restore the topological phase and therefore the Majorana end-states. Using Abelian bosonization, we further confirm this result in the presence of electron-electron interactions and show an insightful connection of this phenomenon to the physics of a one-dimensional doped Mott-insulator. The present results have important applications in e.g., realizing a supercurrent assisted braiding of Majorana fermions, which proves highly useful in topological quantum computation with realistic Majorana networks. [Preview Abstract] |
Session J19: Quantum Criticality in Lanthanide/Actinide & Related Systems - Experiment
Sponsoring Units: DCMPChair: Makariy Tanatar, Ames Laboratory
Room: 321
Tuesday, March 19, 2013 2:30PM - 2:42PM |
J19.00001: Quantum criticality of YbBiPt G.M. Schmiedeshoff, E.D. Mun, S.L. Bud'ko, C. Martin, H. Kim, M.A. Tanatar, R. Prozorov, J.-H. Park, T. Murphy, N. Dilley, P.C. Canfield YbBiPt is a stoichiometric heavy fermion compound with an enormous Sommerfeld coefficient and a magnetic ground state that can be suppressed by fields of about 4 kOe. We will present and discuss recent thermodynamic and transport measurements, and the evidence for field induced quantum criticality in this material. Work at Ames Laboratory was supported by the Department of Energy, Basic Energy Sciences under Contract No. DE-AC02-07CH11358. The National High Magnetic Field Laboratory was supported by the US National Science Foundation, the State of Florida and the US Department of Energy. Work at Occidental College was supported by the National Science Foundation under DMR-1006118. [Preview Abstract] |
Tuesday, March 19, 2013 2:42PM - 2:54PM |
J19.00002: ABSTRACT WITHDRAWN |
Tuesday, March 19, 2013 2:54PM - 3:06PM |
J19.00003: The High-Field Fermi Surface of YbRh$_2$Si$_2$ Aaron Sutton, Patrick M.C. Rourke, Valentin Taufour, Alix McCollam, Gerard Lapertot, Georg Knebel, Jacques Flouquet, Stephen R. Julian We report the culmination of our de Haas-van Alphen (dHvA) oscillation rotation studies on the heavy Fermion material YbRh$_2$Si$_2$. Past measurements included rotations in the a-b and a-c planes and resulted in the observation of a previously unobserved frequency attributed to the so-called J-sheet of the Fermi surface. While the purpose of these measurements was to determine whether or not the high field Fermi surface resembled a small or large Fermi surface, the measurements have highlighted the need for more advanced band structure calculations in order to determine its nature. In our latest measurements we completed our study by rotating from the (110) direction towards the c-axis. The experiment was successful in elucidating a new aspect of the Fermi surface, and though qualitative agreement with rudimentary band structure calculations was observed, the measurement has reinforced the need for a more comprehensive theoretical understanding of the material. [Preview Abstract] |
Tuesday, March 19, 2013 3:06PM - 3:18PM |
J19.00004: CePt$_2$In$_7$: Focused Ion Beam Sample Preparation for Quantum Oscillation Measurements under High Pressure Jakob Kanter, P. Moll, S. Friedemann, P. Alireza, M. Sutherland, S. Goh, F. Ronning, E.D. Bauer, B. Batlogg Electronic transport measurements under high pressures face several experimental challenges due to confined sample space and high forces acting on contacts and leads. As a result conventional preparation methods are often limited in the number of possible leads and usually do not allow for sample structuring. The Focused Ion Beam (FIB) enables sample contacting and structuring down to a sub-micrometre scale, making the measurement of several samples with complex shapes on a single anvil feasible. This talk will discuss Shubnikov-de Haas measurements of FIB prepared CePt$_2$In$_7$ samples under high pressures. CePt$_2$In$_7$ belongs to the Ce$_m$\emph{M}$_n$In$_{3m+2n}$ heavy fermion family. Compared to the Ce\emph{M}In$_5$ members of this group, the structure of CePt$_2$In$_7$ has a more pronounced two dimensional character, but also exhibits an antiferromagnetically ordered as well as a superconducting phase. We have studied the changes of the quasiparticle masses for the various orbits as function of pressure approaching the quantum critical point. [Preview Abstract] |
Tuesday, March 19, 2013 3:18PM - 3:30PM |
J19.00005: A THz spectroscopy study of the field-induced quantum phase transition in the heavy fermion antiferromagnet CeCu$_2$Ge$_2$ Grace Bosse, C.M. Morris, Y. Li, J. Eckstein, N.P. Armitage We report time domain THz spectroscopy data of a thin film of the heavy fermion compound CeCu$_2$Ge$_2$ in the presence of a magnetic field. It has been shown that it is possible to tune the antiferromagnetic long-range order of CeCu$_2$Ge$_2$ towards a quantum critical point using magnetic field as a tuning parameter. Measurements to obtain the frequency dependent complex conductivity as a function of temperature and field were taken down to temperatures below the onset of magnetic order and fields as high as 7 T. The effects of the quantum critical fluctuations on the frequency dependent scattering rate and mass renormalization, which are obtained using an extended Drude model analysis, will be discussed. [Preview Abstract] |
Tuesday, March 19, 2013 3:30PM - 3:42PM |
J19.00006: Quantum Criticality in high purity specimens of Ce$_{2}$Rh$_{3}$Ge$_{5}$ and Ce$_{2}$Pt$_{3}$Si$_{5}$ Eric D. Bauer, Ryan E. Baumbach, Xin Lu, Ross D. McDonald, Filip Ronning, Joe D. Thompson We report results for high purity specimens of the heavy fermion antiferromagnets Ce$_{2}$Rh$_{3}$Ge$_{5}$ and Ce$_{2}$Pt$_{3}$Si$_{5}$, which have similar ordering temperatures: T$_{N} =$ 5.5 K and 6.3 K, respectively, and belong to the same family of materials that includes the pressure-induced superconductor Ce$_{2}$Ni$_{3}$Ge$_{5}$. Our measurements show that the antiferromagnetic state is suppressed to zero temperature at similar magnetic fields (H$_{c} =$ 23 T and 36 T, respectively), suggesting comparable magnetic energy scales in these compounds. In contrast, while the pressure needed to access a quantum critical point (QCP) in Ce$_{2}$Rh$_{3}$Ge$_{5}$ is extremely low (P$_{c}$ $\sim$ 5 kbar), the N\'{e}el temperature for Ce$_{2}$Pt$_{3}$Si$_{5}$ is insensitive to pressures up to 15 kbar. This result implies that although these compounds are markedly similar, the mechanism that drives the QCP in Ce$_{2}$Rh$_{3}$Ge$_{5}$ is not present in Ce$_{2}$Pt$_{3}$Si$_{5}$. We discuss possible differences between these compounds and mechanisms for their quantum criticality with an emphasis on how the shape of the Fermi surface affects their physical properties. [Preview Abstract] |
Tuesday, March 19, 2013 3:42PM - 3:54PM |
J19.00007: Magnetic cluster glass formation in Ni-V close to the disordered ferromagnetic quantum phase transition Ruizhe Wang, Sara Ubaid-Kassis, Almut Schroeder, P.J. Baker, F.L. Pratt, S.J. Blundell, T. Lancaster, I. Franke, J.S. Moeller, Thomas Vojta The d-metal alloy Ni$_{1-x}$V$_{x}$ undergoes a quantum phase transition from a ferromagnetic ground state to a paramagnetic ground state as the vanadium concentration $x$ is increased. We present magnetization, ac-susceptibility and muon-spin relaxation data at several vanadium concentrations below and above the critical concentration $x_c \approx$ 11$\%$ where the onset of ferromagnetic order is suppressed. Below $x_c$, Ni$_{1-x}$V$_{x}$ is characterized as a strongly disordered ferromagnet since the muon data reveal a broad magnetic field distribution. Above $x_c$, the temperature dependence of the magnetic susceptibility is best described in terms of a magnetic quantum Griffiths phase. At the lowest temperatures, we identify a magnetic cluster glass phase which masks the actual ferromagnetic quantum critical point. We study how this cluster glass is formed (i) by lowering the temperature from the quantum Griffiths phase and (ii) by increasing the vanadium concentration starting from the disordered ferromagnet. The onset of the cluster glass phase is recognized by a change of the magnetic dynamics revealed through susceptibility and muon-spin relaxation measurements. [Preview Abstract] |
Tuesday, March 19, 2013 3:54PM - 4:06PM |
J19.00008: Anisotropic transport and magnetic properties, and magnetic-field tuned ground states of CeZn$_{11}$ H. Hodovanets, S.L. Bud'ko, M.G. Kim, D.K. Pratt, A. Kreyssig, A.I. Goldman, P.C. Canfield We have studied the electrical, magnetic, and thermal properties of single crystals of CeZn$_{11}$ by the means of magnetization, resistivity, heat capacity, and thermoelectric power. The compound exhibits an antiferromagnetic long-range order below 2.0 K. The zero-field temperature dependent resistivity of CeZn$_{11}$ is similar to that of other strongly correlated, Kondo lattice, compounds. $T_N$ is suppressed with the applied magnetic field and disappears for $H\sim$47.5 kOe ($H\|$[110]) and $H\sim$ 120 kOe ($H\|$[011]). Temperature-dependent resistivity for $H\|$[110] shows sub-linear behavior up to 2.5 K for $H$=45 kOe, followed by Fermi liquid behavior for limited range of temperatures ($T<$1.1 K) and fields (47.5 kOe$\leq H\leq$60 kOe). The $\it H-T$ phase diagrams for $H\|$[110] and $H\|$[011] will be discussed. [Preview Abstract] |
Tuesday, March 19, 2013 4:06PM - 4:18PM |
J19.00009: Electrical resistivity of CeZn$_{11}$ under pressure Valentin Taufour, Stella K. Kim, Halyna Hodovanets, Sergey L. Bud'ko, Paul C. Canfield In most Ce-based intermetallic compounds, the magnetic exchange is assumed to be due to the RKKY interaction. This interaction competes with the Kondo interaction, leading to the suppression of the magnetic order and the possibility of field and/or pressure induced quantum criticality. In order to study this competition in CeZn$_{11}$, a compound that orders antiferromagnetically below $T_N = 2$~K, we performed electrical resistivity measurements on a single crystal of CeZn$_{11}$ under pressure up to $5$~GPa in a Bridgman pressure cell modified to use a liquid pressure transmitting medium (1:1 mixture of n-pentane: iso-pentane). $T_N(p)$ slightly increases and approaches a broad maximum in the studied pressure range. At ambient pressure, the antiferromagnetic order is suppressed by a magnetic field along the [1,1,0] direction of the tetragonal crystal structure. The temperature versus magnetic field phase diagram at $5$~GPa will be compared to the one at ambient pressure. [Preview Abstract] |
Tuesday, March 19, 2013 4:18PM - 4:30PM |
J19.00010: Magnetic structure of R$_{2}$CoGa$_{8}$ (R $=$ Gd, Tb and Dy) and evolution of the magnetic structures along the series of intermetallic compounds with R $=$ Gd - Tm Carlos Giles, Jose Renato Madergan, Cris Adriano, Rafael Vescovi, Pascoal Pagliuso In this work we have determined the magnetic structure of R$_{2}$CoGa$_{8}$ (R $=$ Gd, Tb and Dy) intermetallic compounds using X-ray resonant magnetic scattering in order to study the evolution of the anisotropic magnetic properties along the series for R $=$ Gd-Tm. The three compounds have a commensurate antiferromagnetic structure with a magnetic propagation vector (1/2 1/2 1/2) with N\'{e}el temperatures of 21.0, 27.5 and 15.2 K for R $=$ Gd, Tb and Dy, respectively. The critical exponent $\beta$ obtained from the temperature dependence of the integrated intensity of the resonant magnetic peaks suggest a 3D magnetism for the three compounds. The energy line shapes at the L$_{2}$ and L$_{3}$ edges of the magnetic peaks for these compounds present a purely dipolar character as demonstrated by comparison to first principle calculations. Comparing the simulated and integrated intensities corrected for absorption, we conclude that the magnetic moment direction is in the \textit{ab}-plane for Gd$_{2}$CoGa$_{8}$ compound and parallel to the $c-$axis for the Tb$_{2}$CoGa$_{8}$ and Dy$_{2}$CoGa$_{8}$ compounds. This information is used to discuss the evolution of the magnetic structure of R$_{2}$CoGa$_{8}$ series for R $=$ Gd-Tm where both the direction of the ordered moment and the ordering temperature evolution along the series can be explained through the competition between the indirect Ruderman-Kittel- Kasuya-Yoshida exchange interaction and crystalline electric field effects. [Preview Abstract] |
Tuesday, March 19, 2013 4:30PM - 4:42PM |
J19.00011: Electronic structure and Fermi surface topology in PuIn3 and PuSn3 Cheng-Ching Wang, Matthew Jones, Jian-Xin Zhu The itinerant-to-localized crossover of the 5f electrons that occurs near plutonium in the actinide series is one of the most challenging issues in condensed matter physics, while the highest superconductivity across the whole f-electron systems emerges in PuCoGa5. These novel behaviors are indicative of strong electronic correlations effects. Electronic band structure calculations serve as the first step for better understanding of these correlation effects. The compounds PuIn3 and PuSn3 crystallize into cubic AuCu3-type structure and have an actinide-actinide distance far above the Hill limit, making the 5f-ligand hybridization the dominant mechanism for Pu 5f -electron delocalization. With their simple crystallographic structure and rich magnetic and electronic properties, these two compounds provide a particularly convenient and systematic way to study the delocalization-localization crossover of Pu 5f electrons. It is particularly encouraging that PuIn3 is the first Pu-based compound in which the de-Haas van-Alphen effect has been observed. In this talk, we present a systematic study of electronic structure calculations on PuIn3 and PuSn3 in the framework of density functional theory with the generalized gradient approximation. [Preview Abstract] |
Tuesday, March 19, 2013 4:42PM - 4:54PM |
J19.00012: Quantum Criticality in the strongly correlated 3d electron system YFe$_{2}$Al$_{10}$ Liusuo Wu, Keeseong Park, Monika Gamza, Moosung Kim, Meigan Aronson A remarkable behavior in quantum critical systems is the critical scaling near the quantum critical point (QCP), where Fermi liquid (FL) physics usually breaks down. This kind of behavior has been observed in many $f$ electron based heavy fermion (HF) systems. We have measured the magnetization and specific heat of the 3$d$-electron metal YFe$_{2}$Al$_{10}$. non-FL behavior with strong divergence in magnetic susceptibility ($\chi $ $\sim$ T$^{-\gamma }$, $\gamma =$1.4) and specific heat (C$_{M}$/T$\sim$ -log T) were observed, and this suggested YFe$_{2}$Al$_{10}$ may locate close to a ferromagnetic QCP. What attracts us most is the unusual scaling of magnetic susceptibility (d$\chi $/dT$=$B$^{-\gamma }\varphi $(T/B$^{\beta }))$ and specific heat ($\Delta $C$_{M}$/T$=\psi $(T/B$^{\beta }))$, which was observed over a range more than three decades in T/B$^{\beta }$. The overall scaling behaviors mapped well with the assumption that a FL phase was resumed as the system was tuned far from the QCP, where all the critical fluctuation was suppressed. Based on the scaling analysis, a possible form of the critical free energy will also be discussed. [Preview Abstract] |
Tuesday, March 19, 2013 4:54PM - 5:06PM |
J19.00013: Terahertz conductivity of MnSi thin films J. Steven Dodge, Laleh Mohtashemi, Amir Farahani, Eric Karhu, Theodore Monchesky We present measurements of the low-frequency optical conductivity of MnSi thin films, using time-domain terahertz spectroscopy. At low temperatures and low frequencies, we extract the DC resistivity, scattering life time and plasma frequency from a Drude fit. We obtain a value of $\omega_p\simeq1.0$ eV, which can be used to estimate the renormalization coefficient through comparison with band theory. At higher temperatures, deviations from Drude behavior are observed, suggesting a loss of quasi-particle coherence. In the region of low temperatures and high frequencies, we see evidence for a crossover to the anomalous power law dependence observed by Mena \textit{et al.}\footnotemark[1] As the temperature increases, the anomalous frequency dependence becomes more pronounced, and the plasma frequency inferred from a Drude fit decreases dramatically. Above T$\approx 50$ K, $\sigma_2(\omega)$ develops a negative slope that is inconsistent with both a Drude model and the anomalous power law observed earlier,\footnotemark[1] indicating a sharp pseudogap in the conductivity spectrum. \footnotetext[1]{F.P. Mena \textit{et al.} \textrm{Phys. Rev. B.} \textrm{\bf67}, 241101(R) (2003).} [Preview Abstract] |
Tuesday, March 19, 2013 5:06PM - 5:18PM |
J19.00014: Effect of Nd Substitution on PrOs$_4$Sb$_{12}$ Investigated by $\mu$SR Experiments P.-C. Ho, B. Somsanuk, D. E. MacLaughlin, M. B. Maple, L. Shu, O. O. Bernal, T. Yanagisawa The pseudo ternary system Pr$_{1-x}$Nd$_x$Os$_4$Sb$_{12}$ has been used as a model system to investigate the effect of ferromagnetism (FM) on the unconventional superconductivity (SC) and quantum critical behavior of PrOs$_4$Sb$_{12}$ [1]. SC in this system disappears near a critical concentration $x_{cr,1}\sim 0.58$ and FM appears above $x_{cr,2}\sim 0.33$ [1,2]. The new $\mu$SR measurements have been performed on samples with $x =$ 0.25, 0.75, and 1. For $x = 1$ and 0.75, the estimated frozen moments agree with the Nd$^{3+}$ CEF ground state moment. For $x = 0.25$, neither time reversal symmetry breaking nor evidence of freezing of Nd$^{3+}$ spins was observed in zero-field $\mu$SR measurements, the behavior of which is very different than what is observed for $x = 0.45-0.55$ [2]. In the SC state, an unexpected linear $T$ dependence of the Gaussian relaxation rate was also found in the transverse field $\mu$SR data for $x = 0.25$, which is different than the plateau in PrOs$_4$Sb$_{12}$ below 1.3K [3]. [1] Ho, et al., PRB 83, 024511 (2011).[2] Ho, et al., 2010 APS March Meeting, A38.00005 (2010). [3] MacLaughlin et al., PRL 105, 019701 (2010). [Preview Abstract] |
Session J20: Focus Session: Metamaterials - Nanowires and Plasmonic Enhancement
Sponsoring Units: DMPChair: Peter Nordlander, Rice University
Room: 322
Tuesday, March 19, 2013 2:30PM - 2:42PM |
J20.00001: Excited state dynamics of single metal and semiconductor nanowires studied by transient absorption microscopy Shun S. Lo, Hong Y. Shi, Todd A. Major, Nattasamon Petchsang, Libai Huang, Masaru K. Kuno, Gregory V. Hartland Transient absorption microscopy (TAM) is a relatively new technique that allows the study of single nanostructures with sub-picosecond time resolution. Here, we present results for CdTe and Au Nanowires (NW). For the first material, we find an interesting power dependence of the excited dynamics, suggesting that a trap-filling mechanism is responsible for the observed behaviour. Additionally, acoustic phonons were observed, which were well described using continuum elastic models.\footnote{S. S. Lo et al. ACS Nano, \textbf{6}, 5274 (2012)} Carrier diffusion along these NWs are also reported. In the case of Au NWs, the propagation of surface plasmon polaritons was investigated. The results are in agreement with previous studies performed with fluorescence based techniques.\footnote{B. Wild et al. ACS Nano, \textbf{6}, 472 (2012)}$^,$\footnote{A. Paul et al. ACS Nano, \textbf{6}, 8105 (2012)} Unlike fluorescence techniques, multiple measurements on the same nanostructures are possible with TAM allowing one-to-one comparisons under different excitation polarizations and environments. [Preview Abstract] |
Tuesday, March 19, 2013 2:42PM - 2:54PM |
J20.00002: Damping of Acoustic Vibrations of Single Suspended Gold Nanowires and Nanoplates in Air and Water Environments Todd Major, Aur\'elien Crut, Bo Gao, Mary Devadas, Shun Lo, Natalia Del Fatti, Fabrice Vall\'ee, Gregory Hartland The dynamics of metal nanoparticles are affected by intrinsic properties, such as crystal structure, and the viscosity and acoustic impedance of the environment. In order to separate the contribution of the environment from the dynamics of individual nanostructures, ultrafast transient absorption measurements are taken on gold nanowires suspended over trenches on the substrate in air and in water. Measurements taken in an air environment represent damping from intrinsic sources, whereas experiments in water represent a liquid environment with a well known viscosity. Quality factors of the acoustic modes from the gold nanowires were measured and match well with previous studies. The results are compared to Continuum Mechanics Calculations. The calculations show that the viscosity of water plays a minor impact on the damping of the acoustic modes. This study has been recently extended to nanoplates suspended over trenches, but the effect of viscosity has not been investigated for these materials yet. [Preview Abstract] |
Tuesday, March 19, 2013 2:54PM - 3:06PM |
J20.00003: ABSTRACT WITHDRAWN |
Tuesday, March 19, 2013 3:06PM - 3:18PM |
J20.00004: Advanced scanning transmission electron microscopy characterization of UV LED nanowires Patrick Phillips, Rajan Kumar, Santino Carnevale, Roberto Myers, Robert Klie The role of aberration-corrected scanning transmission electron microscopy (STEM) in materials characterization is examined in regards to Al(x)Ga(1-x)N nanowires. Wires were graded from x$=$0 to x$=$1 and then from x$=$1 to x$=$0 with a small active quantum disk region located between the two gradations. This configuration is the basis for previously reported UV light emitting diodes. However, to assist subsequent growth processes while striving for optimum efficiency, both structural and chemical characterization methods are necessary, which can be provided at sufficiently high resolutions by advanced STEM instruments. Specifically, structural characterization will focus on determining layer thicknesses and wire polarity, as well as visualizing any short-range ordering and/or stacking faults that may be present. STEM multislice image simulations will also be discussed. Chemically, both energy dispersive X-ray (EDX) and electron energy loss (EEL) spectroscopies will be discussed in various capacities, ranging from quantum well composition (EDX) to N K-edge fine structure of both GaN and AlN (EELS). [Preview Abstract] |
Tuesday, March 19, 2013 3:18PM - 3:30PM |
J20.00005: Electrochemically Grown, Composite Au/CdS/Au Nanowires: Structural and Optical Properties Todd Brintlinger, Rhonda Stroud, James Long, Stefanie Sherrill, Sang Bok Lee, Blake Simpkins We present growth, characterization, and optical response of solution-synthesized nanoplasmonic structures coupled with nonlinear dielectrics. Transmission electron microscopy indicates the templated electrochemical growth of 60-300 nm diameter, 200-5000 nm long composite Au-CdS-Au cylindrical nanostructures yield wurtzite CdS spanning small gaps between Au nanowires. The electrodeposited CdS exhibits an absorption band at $\sim$ 500 nm consistent with band edge absorption of crystalline CdS and broad defect band luminescence centered $\sim$ 625 nm. CdS exhibits sufficient quality to produce second harmonic generation stimulated with a pulsed, linearly polarized pump-light from a femtosecond Ti-sapphire laser. The effect of structure geometry and environment on optical response is investigated through variations in substrates, growth parameters and focused-ion-beam (FIB) shaping of nanostructures. [Preview Abstract] |
Tuesday, March 19, 2013 3:30PM - 3:42PM |
J20.00006: Synthesis and Characterization of Gold-Titanium Dioxide Nanoparticles Hailey Cramer, Ismat Shah Nanoparticles are of recent scientific interest due to their unique size-dependent optical, electrical, and catalytic properties. Gold nanoparticles specifically, have many potential applications, especially in optoelectronic devices due to their optical properties and plasmon resonance. The specific goals of this research are to synthesize Au/TiO$_{2}$ core-shell nanoparticles for their use in improving the overall efficiency of P3HT/PCBM polymer solar cells previously prepared in our lab. The standard sodium citrate reduction method was used to synthesize gold nanoparticles with an average diameter of 15 nm. Through changing the concentration of sodium citrate in solution we were able to tune the size of the nanoparticles, and therefore change their light-absorbing properties. The goals of this research are to cap the gold nanoparticles with TiO$_{2}$ through a sol-gel method. Characterization of the Au/TiO$_{2}$ particles will be performed using high resolution tunneling electron microscopy to determine the size of the nanoparticles and the thickness of the TiO$_{2}$ shell. In addition, ultraviolet-visual spectroscopy was used to determine the absorption of the particles, and dynamic light scattering was used to confirm the size distribution of the particles. The incorporation of Au/TiO$_{2}$ nanoparticles in P3HT/PCBM devices will be discussed. [Preview Abstract] |
Tuesday, March 19, 2013 3:42PM - 3:54PM |
J20.00007: New experimental evidences of Au-Cu$_{2}$S core-shell nanoparticles and atomic resolution imaging by aberration-corrected STEM Subarna Khanal, Gilberto Casillas, Nabraj Bhattarai, J. Jesus Velazquez-Salazar, Miguel Jose Yacaman Au-Cu$_{2}$S core-shell nanoparticles present different properties than their monometallic counterparts, opening a wide range of possibilities for different applications. Au-Cu$_{2}$S core-shell nanostructures have raised interest for their many applications in photoelectronic, sensing, catalysis and so on. Au and Au-Cu$_{2}$S core-shell nanoparticles were prepared by using a modified polyol method. First Au seeds were prepared by reducing HAuCl$_{4}$.xH$_{2}$O in ethylene glycol (EG) in the presence of poly(vinylpyrrolidone) (PVP) as a polymer surfactant. Then Cu$_{2}$S shells were overgrown on Au core seeds by reducing CuSO$_{4}$ in EG with PVP. The morphology and structural characteristics of Au and Au-Cu$_{2}$S nanostructures were studied in detail using scanning electron microcopy HITACHI S-5500 and high resolution transmission electron microscope (HRTEM), a resolution 0.19 nm. Moreover, the Cs-corrected scanning transmission electron microscopy (Cs-corrected STEM) technique allowed us to probe the structure at the atomic level of these nanoparticles revealing new structural information. We determined the structure of the four main polyhedral morphologies obtained in the synthesis: decahedral, icosahedral, triangular plates, and rods. [Preview Abstract] |
Tuesday, March 19, 2013 3:54PM - 4:06PM |
J20.00008: Mechanism for resonant energy transfer in plasmonic light-harvesting materials Scott Cushing, Jiangtian Li, Nianqiang Wu, Alan Bristow Localized surface plasmon resonance (LSPR) is a promising route to extending the light-harvesting of semiconductors into the visible and near infrared. Core-shell nanostructures are studied using transient absorption spectroscopy to explore the carrier dynamics and energy harvesting mechanism [1]. The metal core@Cu$_{\mathrm{2}}$O shell nanoparticles have a broad plasmon resonance centered at 650 nm. The amplitude of the spectral dependence of the transient absorption can be fit using three contributions: the semiconductor density of states, the LSPR, or the overlap integral between the two. The fitting procedure reveals the energy transfer mechanism in Au@Cu$_{\mathrm{2}}$O is dominated by a plasmon induced resonant energy transfer, while the energy transfer in Ag@Cu$_{\mathrm{2}}$O is a combination of resonant energy transfer and hot electron injection from the metal to semiconductor. The effects of core composition and shell thickness are studied with the aim of finding the best combination for a viable full solar spectrum, plasmon-enhanced photocatalyst. [1] S. K. Cushing, J. T. Li, F. K. Meng, T. R. Senty, S. Suri, M. J. Zhu, M. Li, A. D. Bristow, N. Q. Wu, J. Am. Chem. Soc. 134, 15033 (2012). [Preview Abstract] |
Tuesday, March 19, 2013 4:06PM - 4:18PM |
J20.00009: Effects of plasmonic environment on electric and magnetic dipole spontaneous emission Rabia Hussain, Yuri Barnakov, Natalia Noginova Luminescence of Eu ions was used to study effects of plasmonic environment on spontaneous emission of magnetic and electric dipoles in several nanostructured systems, including gold nanostrips, gold and silver nanomesh and thin films.~ Significant changes in polarization and radiation patterns were observed in the spectral range of plasmonic resonance. The effects were different for electric and magnetic dipole related transitions. The results are discussed in terms of coupling of emitters with radiative and plasmonic modes with account for losses. We also demonstrate the possibility to map the enhancement factors for magnetic and electric dipoles separately in near field optical studies. [Preview Abstract] |
Tuesday, March 19, 2013 4:18PM - 4:30PM |
J20.00010: Single particle optical investigation of gold-enhanced upconverted fluorescence emission Kory Green, Shuang Fang Lim Near IR excited upconverting nanoparticles (UCNPs) are ideal fluorescent contrast agents, leading to background free bioimaging. However, their fluorescent brightness is hampered by low quantum efficiency due to material limitations. We investigate the plasmonic coupling of 20 nm diameter core NaYF4: Yb, Er upconverting nanoparticles (UCNPs) coupled to both gold nanoparticles and coated in a gold nanoshell. The structures of the UCNPs composites have been verified by transmission electron microscopy (TEM), UV-Vis absorption and fluorescent emission. Spectroscopic studies such as single particle spectra and time resolved decay has been performed to investigate and fine tune the luminescent enhancement. In particular, we have performed time-resolved~spectroscopy between 400 nm and 1700 nm. The monitoring of all relevant energy level transitions, including intermediate levels, contribute to a complete understanding of~the mechanisms at work in plasmonicly enhanced, high-efficiency, small UNCPs. [Preview Abstract] |
Tuesday, March 19, 2013 4:30PM - 4:42PM |
J20.00011: Enhancement of single particle rare earth doped NaYF$_{4}$: Yb, Er emission with gold shell Shuang Lim, Ling Li, Hans Hallen We report on the enhancement in emission of NaYF$_{4}$: Yb, Er upconverting nanoparticles (UCNPs) coated with a gold nanoshell. We have synthesized a doped NaYF$_{4}$ core of 350 nm, with a sufficiently thick undoped NaYF$_{4}$ shell of 65 nm thickness to minimize contact with the gold surface plasmons, and effectively minimizing luminescence quenching. Absorption and fluorescence emission measurements of single NaYF$_{4}$ particles show enhanced absorption in the near infrared and a 1.5 times overall enhanced emission intensity. A relative increase in green/red emission was observed for both gold seed attachment and shell growth, of approximately 1.9 and 2.3 times respectively. Both Au seed attachment and shell growth has been shown to double the green/red emission ratio. The surface plasmon resonance of the UCNP core/Au shell composite is shown to be dependent on the gold shell thickness. Our experimental results are corroborated by finite element calculations. [Preview Abstract] |
Tuesday, March 19, 2013 4:42PM - 4:54PM |
J20.00012: Two-color Surface Plasmon Polariton Assisted Upconversion Luminescence in NaYF$_{4}$:Yb:Tm on Au Nanopillar Arrays Steve Smith, Robert Anderson, Amy Hor, Jon Fisher, Khadijeh Bayat, Mahdi Baroughi, QuocAhn Lu, P. Stanely May Spectroscopic imaging was used to study the surface plasmon polariton (SPP) enhanced upconversion luminescence of NaYF$_{4}$:Tm:Yb nanoparticles embedded in PMMA supported on Au nanopillar arrays. Spatially-resolved upconversion spectra show enhancement in both the visible and near-infrared region of the spectrum, clearly associated with the plasmonic resonances of an engineered periodic array of nanopillars. The array has a lattice resonance associated with the SPP near 980nm, at the peak absorption of the Yb$^{3+}$ ion, while the local surface plasmon resonance (LSPR) of the individual pillars is seen to enhance the near-infrared emission of Tm$^{3+}$ near 800nm. The combined effect results in a significantly higher enhancement of the near-infrared emission when compared to the visible upconversion lines of Tm$^{3+}$, consistent with the interpretation of sequential surface plasmon assisted absorption and emission at two separate and disparate energies. The presence of SPP and LSPR were confirmed by spectrally resolved reflectivity, and the mechanism for luminescence enhancement was investigated by time resolved measurements of the luminescence decay. Reflectivity measurements are compared to finite difference time domain simulations (FDTD). [Preview Abstract] |
Tuesday, March 19, 2013 4:54PM - 5:30PM |
J20.00013: Beller Lectureship: Surface Plasmon Laser Action Near the Surface Plasmon Frequency Invited Speaker: Rupert F. Oulton Lasers have recently been scaled in size beyond the diffraction limit of light by using electromagnetic surface excitations of metals. In this talk, I will discuss our approach to constructing surface plasmon (SP) lasers using semiconductor materials and outline potential applications that exploit the strong interaction of nanoscale light with matter. I will also present recent results on room temperature SPs lasers operating near the SP frequency by utilizing Zinc Oxide as a gain material combined with a Silver substrate. Surface plasmon lasers could be the most efficient and compact method of delivering optical energy to the nanoscale. There are two benefits: firstly, the efficiently generated (focused) coherent laser field can be extremely intense; and secondly, vacuum fluctuations within the laser cavity are considerably stronger than in free space. Consequently, SP lasers have the unique ability to drastically enhance both coherent and incoherent light-matter interactions bringing fundamentally new capabilities to bio-sensing, data storage, photolithography and optical communications. While there is a great deal of research to do on SP laser systems, this talk highlights the feasibility of nano-scale light sources and the potential of laser science at the nanoscale. [Preview Abstract] |
Session J21: Focus Session: Oxide Superlattices, Interfaces and Growth
Sponsoring Units: DMPChair: Hena Das, Cornell University
Room: 323
Tuesday, March 19, 2013 2:30PM - 2:42PM |
J21.00001: Oxygen Octahedral Rotations in BaTiO$_{3}$/CaTiO$_{3}$ Superlattices Margaret Cosgriff, Pice Chen, Nathaniel Corey, Xifan Wu, Apurva Mehta, Hiroo Tajiri, Ho Nyung Lee, Paul Evans Complex oxide superlattices have a wide range of electronic and magnetic properties, which are affected by the structure of the interfaces between different components of the superlattice. The magnitude, coherence, and electric field response of oxygen displacements in three different BaTiO$_{3}$/CaTiO$_{3}$ superlattice compositions are measured using x-ray diffraction. The displacements are greater in compositions with more consecutive CaTiO$_{3}$ layers. The pattern of layer-by-layer alternating displacements is coherent over less than two superlattice unit cells. The net in-phase rotation of the oxygen octahedra gives rise to an x-ray reflection at (3/2 1/2 1). Density functional theory calculations for a 2(BaTiO$_{3})$/4(CaTiO$_{3})$ composition predict a decrease in displacements of oxygen octahedra between barium and calcium layers when an electric field is applied, causing an intensity increase in this reflection. We found the intensity of this reflection for this composition increases by 1.6{\%} when a 12.5 V pulse is applied, a weaker response than the 11{\%} increase predicted. When a 20 V pulse is applied, the reflection intensity actually decreases by 3{\%}, indicating a more complicated response. [Preview Abstract] |
Tuesday, March 19, 2013 2:42PM - 2:54PM |
J21.00002: Coherent X-ray Diffraction from Striped Nanodomain in a PbTiO3/SrTiO3 Superlattice Qingteng Zhang, Pice Chen, Zhonghou Cai, Matthew Dawber, Sara Callori, Paul Evans Polarization striped domains in ferroelectric/dielectric superlattices reflect the coupling between the polarization and a lattice distortion in each component layer. We have used coherent x-ray diffraction to study the variation of the striped domain pattern in a PbTiO3/SrTiO3 ferroelectric/dielectric superlattice over lateral length scales of hundreds of nanometers to microns. A coherent beam of synchrotron x-rays with a photon energy of 10 keV was focused to a spot with a diameter of approximately 200 nm. The arrangement of domains produces a speckle pattern of intensity in reciprocal space that varies according to the detailed arrangement of domains within the focal spot. When the focal spot is moved across the sample, it is found that the intensity of the total diffuse scattering remains constant while the positions of speckles vary in reciprocal space. This provides additional spatial information about the speckles which leads to better understandings of the configurations of the striped domains in ferroelectric/dielectric superlattices. This work is supported by US DOE under Grant No. DE-FG02-10ER46147. [Preview Abstract] |
Tuesday, March 19, 2013 2:54PM - 3:06PM |
J21.00003: Component-Layer-Dependent Distortion of Striped Domains in PbTiO3/SrTiO3 Superlattices Pice Chen, Margaret Cosgriff, Qingteng Zhang, Sara Callori, Bernhard Adams, Eric Dufresne, Matthew Dawber, Paul Evans Weakly-coupled ferroelectric/dielectric superlattices show novel ferroelectric properties that are not accessible in compositionally uniform ferroelectrics. Nanoscale polarization striped domains are formed as a result of the minimization of the energy associated with depolarization fields. The dielectric layers are polarized, however, with a magnitude that is much smaller than in the ferroelectric layers. The unequal distribution of polarization has been predicted to induce layer-dependent dynamics of the polarization switching of striped domains. Here we experimentally test this prediction in a PbTiO3/SrTiO3 superlattice with time-resolved x-ray diffraction under electric fields up to 2.38 MV/cm. The intensities of x-ray reflections arising from striped domains decrease at a nanosecond timescale, as the polarization switching occurs. The relative magnitude of the intensity change depends on the indices of reflections. We compared the observed intensity changes of domain reflections with a kinematic x-ray simulation. The measurement agrees with a model in which the average polarization of striped domains in dielectric SrTiO3 layers is slightly increased under applied electric fields, and the ferroelectric PbTiO3 layers are unchanged. [Preview Abstract] |
Tuesday, March 19, 2013 3:06PM - 3:42PM |
J21.00004: Static and Dynamic Properties of Ferroelectric Nanodomains Invited Speaker: Pavlo Zubko The performance of ferroelectric devices is closely connected with the structure and dynamics of ferroelectric domains. In ultrathin ferroelectrics, very dense domain structures can arise naturally in response to the presence of a depolarizing field and are expected to exhibit unusual static and dynamic behavior. Superlattices composed of ultrathin ferroelectric blocks sandwiched between paraelectric layers offer an ideal system for investigating the structure and functional properties of such nanodomains. The electrostatic coupling between the ferroelectric layers can be controlled by modifying the thicknesses of the paraelectric layers and the domain structure can be tailored by exploiting Kittel's law. X-ray diffraction and transmission electron microscopy combined with electron energy loss spectroscopy were used to study the electrostatic interactions in PbTiO$_3$-SrTiO$_3$ superlattices, revealing highly inhomogeneous polarization and structural profiles that arise due to the presence of ferroelectric nanodomains. The superlattice geometry is also ideal for studying the response of the ultrathin ferroelectric layers to applied fields. Large uniform electric fields can be supported without significant leakage, while changes in the domain structure can be observed simultaneously using X-ray diffraction. The tiny, reversible displacements of the nanodomain walls were found to contribute to a large enhancement of the effective dielectric response that persists over a broad range of temperatures and exhibits low losses. The static and dynamic properties of nanodomains in PbTiO$_3$-SrTiO$_3$ superlattices will be discussed and compared with those of isolated thin films. [Preview Abstract] |
Tuesday, March 19, 2013 3:42PM - 3:54PM |
J21.00005: Photo-induced stabilization and enhancement of the ferroelectric polarization in Ba$_{0.1}$Sr$_{0.9}$TiO$_{3}$/La$_{0.7}$Ca(Sr)$_{0.3}$MnO$_{3}$ thin film heterostructures Y.M. Sheu, S.A. Trugman, L. Yan, Q.X. Jia, A.J. Taylor, R.P. Prasankumar We demonstrate that optically pumping carriers across the interface between ferroelectric Ba$_{0.1}$Sr$_{0.9}$TiO$_{3}$ and ferromagnetic La$_{0.7}$Ca(Sr)$_{0.3}$MnO$_{3}$ thin films can not only stabilize but also enhance (``write'') the remanent polarization, which breaks crystal inversion symmetry, generating an optical second-harmonic signal that we ``read.'' The new photo-induced (``written'') ferroelectric state remains stable at low temperatures for over one day after removing the laser pulse. By optically decoupling the energy of the internal electric field from the ferroelectric double potential wells, we show that the displacement of the Ti atom increases, leading to a larger, more stable polarization state that may be suitable for applications in data storage (using similar writing and reading processes) as well as energy storage (e.g., solar nanocapacitors). [Preview Abstract] |
Tuesday, March 19, 2013 3:54PM - 4:06PM |
J21.00006: Low dielectric loss in electric field-tunable Ba$_{x}$Sr$_{1-x}$TiO$_3$ thin films grown by hybrid molecular beam epitaxy Adam Kajdos, Evgeny Mikheev, Adam Hauser, Susanne Stemmer Ba$_{\mathrm{x}}$Sr$_{\mathrm{1-x}}$TiO$_3$ (BST) is an electric field-tunable dielectric that exhibits both low dielectric loss and high tunability, making it a system of particular interest for microwave device applications. In this presentation we report on the dielectric properties of paraelectric BST films (x $=$ 0.19 - 0.46) grown by hybrid molecular beam epitaxy (MBE) on epitaxial Pt bottom electrodes. Using the hybrid MBE technique to achieve unprecedented stoichiometry control and low defect densities, we demonstrate dielectric quality factors ($Q =$1/tan $\delta $, where tan $\delta $ is the dielectric loss tangent) exceeding 1000, an order of magnitude greater than any previously reported BST thin film. These low-loss films also exhibit high electric field tunability, with the relative tunability, $n$(E) $= \varepsilon $(0)/$\varepsilon $(E), i.e. the ratio of the dielectric permittivity under zero and positive applied field, respectively, exceeding $n =$ 5. The high quality of these BST films enables the investigation of intrinsic dielectric loss mechanisms, such as quasi-Debye loss. We will discuss the effect of point defect densities, stoichiometry and microstructure on the dielectric properties of these BST thin films. [Preview Abstract] |
Tuesday, March 19, 2013 4:06PM - 4:18PM |
J21.00007: Tunable dielectric properties of Barium Magnesium Niobate (BMN) doped Barium Strontium Titanate (BST) thin films by magnetron sputtering Fikadu Alema, Aaron Reinholz, Konstantin Pokhodnya We report on the tunable dielectric properties of Mg and Nb co-doped Ba$_{0.45}$Sr$_{0.55}$TiO$_{3}$ (BST) thin film prepared by the magnetron sputtering using BST target (pure and doped with BaMg$_{0.33}$Nb$_{0.67}$O$_{3}$ (BMN)) on Pt/TiO$_{2}$/SiO$_{2}$/Al$_{2}$O$_{3}$ 4'' wafers at 700 $^{\circ}$C under oxygen atmosphere. The electrical measurements are conducted on 2432 metal-ferroelectric-metal capacitors using Pt as the top and bottom electrode. The crystalline structure, microstructure, and surface morphology of the films are analyzed and correlated to the films dielectric properties. The BMN doped and undoped BST films have shown tunabilities of 48{\%} and 52{\%}; and leakage current densities of 2.2x10$^{-6}$ A/cm$^{2}$ and 3.7x10$^{-5}$ A/cm$^{2}$, respectively at 0.5 MV/cm bias field. The results indicate that the BMN doped film exhibits a lower leakage current with no significant decrease in tunability. Due to similar electronegativity and ionic radii, it was suggested that both Mg$^{2+}$ (accepter-type) and Nb$^{5+}$ (donor-type) dopants substitutTi$^{4+}$ ion in BST. The improvement in the film dielectric losses and leakage current with insignificant loss of tunability is attributed to the adversary effects of Mg$^{2+}$ and Nb$^{5+}$ in BST. [Preview Abstract] |
Tuesday, March 19, 2013 4:18PM - 4:30PM |
J21.00008: Atomic Layer-by-Layer Growth of Homo-epitaxial SrTiO$_{3}$ Films by Laser MBE Maryam Golalikhani, Qingyu Lei, Guozhen Liu, Ke Chen, Suilin Shi, Fuqiang Huang, Xiaoxing Xi A precise customization of oxide hetero-structures at the atomic layer level became possible with layer-by-layer growth of oxide thin films by laser MBE from separate oxide targets. In situ characterization during growth helps to optimize the composition of these superlattices. In this work we focused on the reflection high energy electron diffraction (RHEED) spot analysis for in situ growth control of stoichiometric SrTiO$_{3}$ thin films in an atomic layer-by-layer manner from separate SrO and TiO$_{2}$ targets. We have shown that both stoichiometry and full monolayer dose can be controlled using RHEED diffraction spot intensity oscillations. Observations of a single sharp peak in x-ray diffraction spectra confirm the same composition of the films as that of the stoichiometric SrTiO$_{3}$ substrate. We have successfully demonstrated that this new approach of laser MBE can achieve the same precise stoichiometry control as shown by reactive MBE. [Preview Abstract] |
Tuesday, March 19, 2013 4:30PM - 4:42PM |
J21.00009: In-situ x-ray diffraction studies of the epitaxial growth of BaTiO$_{3}$ on SrTiO$_{3}$ S.J. Callori, J. Sinsheimer, B. Bein, P.V. Chinta, A. Ashrafi, R. Headrick, M. Dawber When BaTiO$_{3}$ is grown on SrTiO$_{3}$ it is subject to a large epitaxial compressive strain, which means that it can actually be ferroelectric during growth. Therefore, screening provided by a bottom electrode is important in realizing fully strained BaTiO$_{3}$ thin films. To fully understand the role of strain, electrical boundary conditions and deposition technique in forming highly strained ferroelectric thin films, we grew thin films of BaTiO$_{3}$ on SrTiO$_{3}$, both with and without SrRuO$_{3}$ bottom electrodes, using both off axis RF magnetron sputtering and pulsed laser deposition, while the growth was monitored by in-situ x-ray diffraction at X21 at the National Synchrotron Light Source. Out-of-plane and in-plane lattice parameters and x-ray reflectivity were measured during growth, allowing changes in strain and tetragonality of the films to be correlated with changes in growth modes. The presence or absence of an electrode impacted the relaxation and tetragonality of the films differently for the two different growth techniques. Information gained by these synchrotron experiments provides important guidance for the growth of high quality ferroelectric thin films and superlattices. [Preview Abstract] |
Tuesday, March 19, 2013 4:42PM - 4:54PM |
J21.00010: Polar structure evolution of ultrathin BaTiO$_3$ films: in-situ LEED I-V Junsoo Shin, Von Braun Nascimento, Diogo Duarte dos Reis, Lina Chen, E. Ward Plummer, Jiandi Zhang Understanding the fundamental physics of ferroelectricity in ultrathin films is a key issue of a critical size, of which a strong debate has erupted over the existence for decades. Ferroelectricity has previously been observed experimentally down to a few unit cells, depending on a complex interplay of electrostatic depolarization energy, domain formation, and so on. Using in situ Low Energy Electron Diffraction (LEED) I-V, we have systematically examined the structure evolution of ultrathin fully strained BaTiO$_{3}$ films (1-12 ML) on Nb-doped SrTiO$_{3}$. Comparison of observed diffraction intensities for 10 ML films at 300 K with calculated intensities reveals a vertical displacement of the central Ti, corresponding to a single-domain upward polar state. To investigate the polar structure evolution of ultrathin films, we have calculated all R-factors between two sets of experimental curves from 1 ML to 12 ML with 10 ML polar structure curve as a reference. As a result, we demonstrate that 8-12 ML thick BaTiO$_{3}$ films have very similar polar structures, whereas thinner films (1-7 ML) have continuously evolved from uncorrelated to correlated polar structures. [Preview Abstract] |
Tuesday, March 19, 2013 4:54PM - 5:06PM |
J21.00011: Epitaxial growth of BaTiO$_{3}$ on Ge Patrick Ponath, Agham Posadas, Kurt Fredrickson, Alexander Kvit, Alex Demkov Germanium with its higher hole and electron mobility than silicon, in conjunction with a ferroelectric material like barium titanate (BTO) might be a potential candidate for a Ferroelectric RAM in the future. We report the epitaxial growth of BTO directly on a germanium (100) substrate. First, 0.5 monolayer of strontium metal is deposited on the cleaned Ge surface as a passivation layer at 600C. Molecular oxygen to a pressure of 5x10$^{-6}$ torr is then introduced and barium and titanium are alternately deposited on the substrate at the same temperature. The BTO film is crystalline as-deposited and remains so throughout the growth as monitored by in situ reflection high energy electron diffraction. X-ray diffraction measurements of BTO films show only substrate peaks and ($h$00) peaks of BTO, indicating an in-plane ferroelectric polarization. This is expected due to the thermal expansion mismatch of BTO and Ge. We will report on efforts to induce out of plane polarization of BTO films grown on Ge. We have also measured the valence band offset between BTO and Ge using x-ray photoelectron spectroscopy (XPS) and found it to be 2.6 eV, resulting in a zero conduction band offset. We compare this value to density functional calculations of the offset. [Preview Abstract] |
Tuesday, March 19, 2013 5:06PM - 5:18PM |
J21.00012: Theoretical study of growth of Pt on BaTiO$_{3}$ slabs Kurt Fredrickson, Agham Posadas, Catherine Dubourdieu, John Bruley, Alexander Demkov BaTiO$_{3}$ (BTO) is a well-known ferroelectric perovskite, which is tetragonal at room temperature. As BTO has potential as a component of a ferroelectric field-effect transistor (Matthews \textit{et al.}, Science \textbf{276}, 238 (1997)), development of metal electrodes is of crucial importance. We investigate Pt deposition on (001) BTO. The metal is deposited in ultra-high vacuum in the molecular beam epitaxy reactor and characterized using transmission electron microscopy and in situ photoemission. Using density functional theory we calculate the surface energies of Pt(001), (011), and (111), and BTO (001) and investigate the wetting conditions of Pt(001) and (011) on TiO$_{2}$-terminated BTO(001). We use transmission electron microscopy to examine the interfaces of epitaxially grown Pt on BTO and compare the sizes of the islands with the sizes predicted by theory. In addition, we examine the dependence of ferroelectricity on the sample thickness. Results of our photoemission experiments are compared with first principles spectra. [Preview Abstract] |
Tuesday, March 19, 2013 5:18PM - 5:30PM |
J21.00013: Dielectric properties of perovskite oxynitride epitaxial thin films Daichi Oka, Yasushi Hirose, Hideyuki Kamisaka, Tomoteru Fukumura, Tetsuya Hasegawa, Seiji Ito, Akira Morita, Hiroyuki Matsuzaki, Katsuyuki Fukutani, Satoshi Ishii, Kimikazu Sasa, Daiichiro Sekiba Perovskite oxynitrides with the formula \textit{AB}O$_{2}$N are expected to show unique electric properties hardly accessible by conventional oxides. For example, N-2$p$ orbitals tend to form a shallow band at the top of the oxygen-nature valence band. This enables us to develop narrow-bandgap ferroelectric materials with $d^{0}$ configuration, which is applicable to ferroelectrics-based photovoltaic cells. In this study, we fabricated (001)-oriented epitaxial thin films of SrTaO$_{2}$N by nitrogen-plasma assisted pulsed laser deposition on (Nb-doped) SrTiO$_{3}$ substrate. X-ray diffraction measurements revealed large lattice distortion (c/a of 1.015-1.03) due to compressive strain from substrate (mismatch of -3.2 {\%}), though it is partially relaxed. The films had yellow color with a bandgap of about 600 nm. Ferroelectric behavior was observed at room temperature by piezoresponse force microscopy. As far as we know, this is the first experimental observation for ferroelectricity in perovskite oxynitrides. First principles calculations suggested that the ferroelectricity originates from \textit{trans}-type nitrogen ordering, which can be driven by compressive strain. [Preview Abstract] |
Session J22: Optical Properties and Interactions in Quantum Dots and Wells
Sponsoring Units: DCMPChair: Michael Scheibner, University of California, Merced
Room: 324
Tuesday, March 19, 2013 2:30PM - 2:42PM |
J22.00001: Temperature dependence of highly homogeneous excitonic spectra of site-controlled pyramidal quantum dots Valentina Troncale, Emanuele Pelucchi, Alok Rudra, Eli Kapon Site-controlled pyramidal quantum dots grown by MOVPE on patterned GaAs substrates offer many advantages such as emission wavelength, heterostructure tailoring and higher symmetry for efficient photon entanglement. We address the temperature dependence of X, 2X, X-, X$+$ exciton linewidths, providing insight on exciton-phonon interaction in this system. The investigated structures consist of GaAs/AlGaAs pyramidal QDs, positioned on 5$\mu$m centers, characterized using non-resonant micro-photolumionescence at low temperatures. PL spectra of individual QDs are highly reproducible, showing transitions excitons with inhomogeneous brodening as low as 2 meV, caused by slight thickness/composition fluctuations. Interferometric T-dependent linewidth measurements of the four excitonic transitions revealed values at T$=$0 K smaller than previously reported but larger than the estimated exciton radiative lifetime. We conclude that even at T$=$0 K the exciton decoherence time in GaAs QDs is not completely governed by a radiative lifetime and discuss this effect. [Preview Abstract] |
Tuesday, March 19, 2013 2:42PM - 2:54PM |
J22.00002: Carrier Dynamics in Site-Controlled InGaN/GaN Quantum Dots Tyler Hill, Lei Zhang, Hui Deng, Chu-Hsiang Teng, Brandon Demory, Pei-Cheng Ku We investigate the individual micro-photoluminescence and time resolved photoluminescence properties of several hundred site-controlled InGaN/GaN quantum dots fabricated ``top down'' by plasma etching. The optical properties of semiconductor quantum dots can be very inhomogeneous due to small fluctuations in dot size, compositions, growth conditions, or doping levels. Controlled variation of growth conditions combined with the knowledge of experimental uncertainties in the semiconductor properties allows for a statistical analysis to obtain quantitative correlations between the optical properties of the quantum dots and the growth conditions or structural properties. We find that, with an indium fraction of 10-15\%, quantum dots with diameters smaller than 33 nm show markedly different carrier dynamics than those with a diameter larger than 60nm: 1) fluctuations in indium mole fraction or monolayer fluctuations in the InGaN layer have a more significant effect on photoluminescence than changing dot diameter; 2) non-radiative decay related to surface recombination is the dominant decay channel in the system; 3) Increasing surface to volume ratio helps suppress the internal quantum efficiency of multi-exciton states, leading to more strongly antibunched photon sources. [Preview Abstract] |
Tuesday, March 19, 2013 2:54PM - 3:06PM |
J22.00003: Narrow optical line width from site-controlled InGaAs quantum dots Lily Yang, Michael Yakes, Timothy Sweeney, Samuel Carter, Chulsoo Kim, Mijin Kim, Allan Bracker, Daniel Gammon The incorporation of self-assembled quantum dots (QDs) in systematically scalable quantum devices requires a method of nucleating dots with nanometer-scale spatial accuracy while preserving their narrow optical line width. We have developed a technique combining e-beam lithography, wet etching, and molecular beam epitaxial (MBE) growth to deterministically position InGaAs QDs with spectrometer limited photoluminescence line widths. Our technique takes advantage of the anisotropy in GaAs growth to evolve an etched pattern of holes and lines into faceted structures in which dots nucleate. Using this technique, we were able to grow a buffer layer of pure GaAs up to 90 nm in thickness between the processed surface and the dot nucleation surface, effectively separating the QDs from unavoidable residual defects and impurities on the patterned surface that broaden their optical line widths. Additionally, we demonstrate control over the number of dots nucleating per site, from single to a chain of several, by varying the dimensions of the original pattern. Our dots are grown in a Schottky diode structure. Their PL spectrum shows discrete charging transitions, with narrow linewidths near the spectrometer's resolution limit of 20 micro eV. [Preview Abstract] |
Tuesday, March 19, 2013 3:06PM - 3:18PM |
J22.00004: Eliminating the fine structure splitting of excitons in self-assembled InAs/GaAs quantum dots via combined stresses Lixin He, Jianping Wang, Ming Gong, G.-C. Guo Eliminating the fine structure splitting (FSS) of excitons in self-assembled quantum dots (QDs) is essential to the generation of high quality entangled photon pairs. We show by a extended two-level model that the FSS of excitons in a general self-assembled InGaAs/GaAs QD can be fully suppressed via combined stresses along the [110] and [010] directions. The results of the model Hamiltonian are confirmed by atomic empirical pseudopotential calculations. For all the QDs we calculated, the FSS can be tuned to be vanishingly small ($<$ 0.1 $\mu$eV), which is sufficient small for high quality entangled photon emission. [Preview Abstract] |
Tuesday, March 19, 2013 3:18PM - 3:30PM |
J22.00005: Rabi-Kondo correlated state in a laser-driven quantum dot Moshe Goldstein, Bjoern Sbierski, Markus Hanl, Andreas Weichselbaum, Hakan Tureci, Leonid Glazman, Jan von Delft, Atac Imamoglu Spin exchange between a single-electron charged quantum dot and itinerant electrons leads to the emergence of Kondo screening. When the quantum dot is driven resonantly by a weak laser light, the resulting emission spectrum serves as a direct probe of these correlations. In the opposite limit of vanishing exchange interaction and strong laser drive, the quantum dot exhibits coherent Rabi oscillations between the single-spin and optically excited states at the ``bare'' frequency $\Omega$. Here we show that the interplay between strong exchange and non-perturbative laser coupling leads to the formation of a new non-equilibrium quantum-correlated state, featuring a second screening cloud. We elucidate the signatures of that state in the spectrum of luminescence. The spectrum consists of a delta-function peak at the laser light frequency (the peak weight scales as $\Omega^{2/3}$) and a broad peak red-shifted by the renormalized Rabi frequency $\Omega^{*} \propto \Omega^{4/3}$. The shape of the broad peak carries detailed information about the spin screening cloud. [Preview Abstract] |
Tuesday, March 19, 2013 3:30PM - 3:42PM |
J22.00006: Adiabatic rapid passage in single InGaAs quantum dots: Towards a method of ``incoherent control'' Peter Brereton, Megan Stanley, Alexandra Graham, Barbara Van Hattem, Pierre Corfdir, Isobel Houghton, Yanwen Wu, Mark Hopkinson, Richard Phillips Adiabatic rapid passage (ARP) using frequency-swept optical pulses was shown to invert an InGaAs quantum dot from the ground state to the neutral exciton state [1,2]. As in atomic systems, ARP couples the confined electronic states of a quantum dot to a pulse that is chirped to sweep through resonance. If the sweep rate is slow with respect to the instantaneous Rabi frequency but faster than any decay rates, the dressed state of the system will adiabatically switch from one bare state to the other. Damping of the ARP inversion suggests confirmation of theoretical predictions of the effect of phonon-assisted dephasing [3]. ARP allows a train of chirped pulses to control the population state of a quantum dot without the need for locking the relative phase of the pulses. Each pulse pair will effectively drive the state vector through a \(2\pi\) rotation on the Bloch sphere, regardless of the relative phase. Initial work toward this method of `incoherent control' is presented, showing an enhancement of the photocurrent under excitation with two chirped pulses separated by greater than the electron tunneling time. [1] Y. Wu, et al, PRL 106, 067401 (2011). [2] C.-M. Simon, et al PRL, 106, 166801 (2011). [3] A. Debnath, et al PRB, 86, 161304 (2012). [Preview Abstract] |
Tuesday, March 19, 2013 3:42PM - 3:54PM |
J22.00007: Investigation of exciton states under two color optical excitation in quantum dot molecules Ramana Thota, Eric Stinaff, Allan Bracker, Dan Gammon It has been shown that vertically stacked InAs quantum dots may form quantum dot molecules (QDMs) where the tunneling of the carriers results in molecular wavefunction formation. These states are potentially useful for the preparation and manipulation of entangled spins, necessary components for quantum information processing. It has also been previously shown that certain charged exciton states can be created optically resulting in a straightforward method for optical spin initialization. We will present a study of optical charge state creation in vertically stacked In$_{\mathrm{x}}$Ga$_{\mathrm{1-x}}$As quantum dots grown by molecular beam epitaxy. This includes using a two color micro-photoluminescence experiment where we tune one laser through the states associated with the quantum dot (resonant excitation) and keep the other laser fixed with its excitation at the energy of the wetting layer (non-resonant excitation). This technique may result in a method for enhancement of various charged and neutral exciton states. In particular we have investigated the doubly charged exciton state, where the ground state is two spins in a know configuration, as well as biexciton enhancement, possibly useful for generating entangle photon pairs. [Preview Abstract] |
Tuesday, March 19, 2013 3:54PM - 4:06PM |
J22.00008: Coulomb interaction signatures in self-assembled lateral quantum dot molecules Xinran Zhou, Jihoon Lee, Gregory Salamo, Miquel Royo, Juan Climente, Matthew Doty Lateral quantum dot molecules (LQDMs) consist of at least two closely spaced InGaAs quantum dots arranged along axes perpendicular to the growth direction. Coherent interactions between neighboring QDs can lead to the formation of delocalized states with unique and useful properties. LQDMs provide an opportunity for independent control of both coupling and charge occupancy, and are thus of interest for prototype devices that use the QDs as bit registers. The experimental evidence for the existence of delocalized states and inter-dot tunneling in LQDMs, limited by the large center-to-center distance and weak tunneling strength, has been indirect. We use photoluminescence spectroscopy to investigate the ground state of single LQDMs. We apply a voltage along the growth direction that allows us to control the total charge occupancy of the quantum dot molecule. Using a combination of computational modeling and experimental analysis, we assign the observed discrete spectral lines to specific charge distributions. We explain the dynamic processes that lead to these charge configurations through electrical injection and optical generation. Our systemic analysis provides experimental evidence of inter-dot tunneling of electrons as predicted in previous theoretical work. [Preview Abstract] |
Tuesday, March 19, 2013 4:06PM - 4:18PM |
J22.00009: Study of optical and electronic properties of self-assembled InAs/GaAs quantum rings Gabriel Linares, Samar Alsolamy, Morgan Ware, Yuriy Mazur, Zhiming Wang, Jihoon Lee, Greg Salamo, Eric Stinaff, Lilia Meza-Montes We will present a theoretical study of the properties of self assembled InAs/GaAs quantum rings. These nanostructures are grown by metal droplet epitaxy and do not follow the traditional strain driven growth model. For certain growth conditions, two quantum dots are formed on the ring structure which then, in a sense, acts as a wetting ring. A `wetting layer' of 2D InAs is formed by migrating InAs material away from the initial In droplet. We have calculated the eigenstates of electrons and holes inside of the nanostructure using \textbf{k\textbullet p }theory, within the 1 and 4 bands approximation. We include effects such as the strain, the concentration of Indium and external electric field. The wave functions are then used to calculate optical properties and the energies of various exciton states as a function of the indium concentration and distribution. These results are compared with photoluminescence data on existing structures grown under different conditions. The energies of the various states along with the possibility of energy transfer between the dots will be explored. [Preview Abstract] |
Tuesday, March 19, 2013 4:18PM - 4:30PM |
J22.00010: Luminescence studies of pairs of quantum dots formed on quantum rings by droplet epitaxy Samar Alsolamy, Morgan Ware, Yuriy Mazur, Zhiming Wang, Jihoon Lee, Greg Salamo, G. Linares, Lilia Meza-Montes, Eric Stinaff The use of metal droplet epitaxy may provide a novel method of growing laterally coupled nanostructures. We will present optical studies of InAs/GaAs nanostructures which result in twin quantum dots (QD) formed on a quantum ring (QR). Previous studies have investigated the coupling between vertically grown quantum dot pairs. Here we have used photoluminescence (PL) and photoluminescence excitation (PLE) to examine the possibility of energy transfer and coupling between quantum dot pairs in a single InGaAs quantum ring grown by droplet epitaxy. Power dependent photoluminescence spectra reveals a few peaks at low power, which are identified with emission from the ground state of the individual dots. As the power is increased we observe multi-exciton and excited state emission. We then perform PLE, tuning the excitation laser energy continuously from the high energy ring emission down to the individual dot states. We have observed resonant PLE emission in the QD/QR structures both at high energy and when resonant with the indentified ground states of one of the QDs which may indicate energy transfer and/or coupling between the dots. [Preview Abstract] |
Tuesday, March 19, 2013 4:30PM - 4:42PM |
J22.00011: Coulomb Enhancement of Superfluorescence Bursts from the Fermi Edge in Highly-Excited Quantum Wells Ji-Hee Kim, Tim Noe, Stephen A. McGill, Yongrui Wang, Aleksander K. W\'ojcik, Alexey A. Belyanin, Junichiro Kono Superfluorescence (SF) is a many-body process in which an ensemble of excited dipoles spontaneously develops macroscopic coherence and abruptly decays by producing a burst of coherent radiation. We have recently reported the first observation of SF from semiconductor quantum wells in the presence of a strong perpendicular magnetic field [1]. Here, we report on results of our systematic magnetic field dependent studies of light emission from high-density electron-hole systems with gain. We observed SF pulses even at 0 Tesla when the excitation power is high and the temperature is low. The SF radiation at 0 Tesla shows a continuous band of emission in time-resolved photoluminescence images, i.e., the photon energy of the emitted light changes continuously with time. We interpret this phenomenon in terms of Coulomb enhancement of gain near the Fermi energy in a high-density electron-hole system. In addition, we demonstrate that the delay between the pump pulse and the SF pulses is tunable through the magnetic field and excitation pump power. Finally, the delay is longer for a lower-energy Landau level at a given magnetic field, i.e., the SF bursts proceed in a sequential manner from higher to lower Landau levels. \\[4pt] [1] Noe \textit{et al.}, Nature 8, 219 (2012) [Preview Abstract] |
Tuesday, March 19, 2013 4:42PM - 4:54PM |
J22.00012: Comparative Study on Intersubband Absorption in AlGaN/GaN and AlInN/GaN Heterostructures Grown on Low-Defect Substrates Colin Edmunds, Liang Tang, Jiayi Shao, Donghui Li, Geoff Gardner, Michael Manfra, Oana Malis, Andrew Grier, Zoran Ikonic, Paul Harrison, Dimitri Zakharov Intersubband (ISB) devices utilizing III-nitrides have attracted attention for near- and far-infrared optoelectronic applications. However, the lattice mismatch between GaN and commonly used substrates results in a high defect density that hinders the vertical transport required for these devices. Furthermore, most devices in the literature utilize AlGaN/GaN heterostructures for which there is no lattice-matched alloy composition. Due to this lattice mismatch, AlGaN is not ideal for the development of complex devices such as quantum cascade lasers that often require active-region thicknesses on the order of microns for efficient operation. Fortunately, exact lattice matching occurs in AlInN/GaN heterostructures at roughly 18{\%} In composition. To investigate the challenges of lattice-matched nitrides, we presents a comparative study of ISB absorption in high-quality AlGaN/GaN and near lattice-matched AlInN/GaN heterostructures grown by molecular-beam epitaxy on low-defect free-standing GaN substrates. Experimental measurements of transition energy, integrated absorbance and linewidth were compared to theoretical predictions that included many-body effects, interface roughness and calculations of the transition lifetime. [Preview Abstract] |
Tuesday, March 19, 2013 4:54PM - 5:06PM |
J22.00013: Coulomb correlation effects and density dependence of radiative recombination rates in polar AlGaN quantum wells Greg Rupper, Sergey Rudin, Francesco Bertazzi, Gregory Garrett, Michael Wraback AlGaN narrow quantum wells are important elements of deep-ultraviolet light emitting devices. The electron-hole radiative recombination rates are important characteristics of these nanostructures. In this work we evaluated their dependence on carrier density and lattice temperature and compared our theoretical results with the experimentally determined radiative lifetimes in the c-plane grown AlGaN quantum wells. The bands were determined in the k$\cdot$p approximation for a strained c-plane wurtzite quantum well and polarization fields were included in the model. In order to account for Coulomb correlations at relatively high densities of photo-excited electron-hole plasma and arbitrary temperature, we employed real-time Green's function formalism with self-energies evaluated in the self-consistent T-matrix approximation. The luminescence spectrum was obtained from the susceptibility by summing over scattering in-plane directions and polarization states. The recombination coefficient was obtained from the integrated photo-luminescence. The density dependence of the radiative recombination rate shows effects of strong screening of the polarization electric field at high photo-excitation density. [Preview Abstract] |
Tuesday, March 19, 2013 5:06PM - 5:18PM |
J22.00014: Second quantum state transitions in GaAs/AlGaAs Bragg MQW photonic crystal probed by Optical Reflectance and Electroreflectance Yuechao Chen, Z. Liu, M.L. Nakarmi, V.V. Chaldyshev Electroreflectance spectroscopy measurement provides sharp and derivative-like spectral features in the energy region of excitonic transitions, while suppressing uninteresting background effects due to electro-modulation. We employed both electroreflectance and optical reflectance spectroscopies to probe excitonic transitions in a GaAs/AlGaAs multiple quantum well (MQW) structure. The sample used in this experiment consists of 60 periods of quantum well structures with GaAs well layer (13 nm) and AlGaAs barrier layer (94 nm), grown by solid source molecular beam expitaxy on a semi-insulating GaAs substrate. We performed electroreflectance and optical reflectance measurements by tuning the incident angle to coincide the second state of the heavy hole exciton (e2-hh2) transitions and the Bragg resonance. We observed a significant enhancement of excitonic features at the (e2-hh2) exciton transitions around incident angle of 23 degree in both techniques, revealing the double resonance condition. In the temperature dependent measurement of electroreflectance under the double resonance condition, we observed redshift of the excitonic features with increasing temperature. We will also discuss the effect of polarization in the electroreflectance measurements. [Preview Abstract] |
Tuesday, March 19, 2013 5:18PM - 5:30PM |
J22.00015: Equispaced level in the quantum well calculated for seven semiconductor ternary alloys conduction band Arthur Ejere, Godfrey Akpojotor A model of equispaced-level conduction band in semiconductor quantum well (QW) nanostructures is derived. The procedure starts with the effective-mass Schrodinger equation, with the local conduction-band edge as the potential experienced by an electron in the QW. Then the effective-mass Schrodinger equation with linear harmonic potential is made to coincide with it . In this study, an attempt has been made to model some semiconductor ternary alloys (A$_{\mathrm{x}}$B$_{\mathrm{1-x}}$C) using this procedure, thereby adding to the varieties of QW nanostructures designs in existence. Two models are derived, one with a confining potential that may be realized by appropriate grading of the semiconductor alloy and the other with a non-confining potential where the electron effective-mass tends to zero as z tends to infinity [m($z\to \pm \infty )\to 0)$. This latter type of model is not realizable. [Preview Abstract] |
Session J23: Optical Excitations, Defects and Synthesis of Dielectrics
Sponsoring Units: DCMPChair: Javier Junquera, Universidad de Cantabria, Spain
Room: 325
Tuesday, March 19, 2013 2:30PM - 2:42PM |
J23.00001: Unconventional Transport of Spin Bipolarons on an Antiferromagnetic buckled hexagonal lattice of half-filled $d$-band Mn$^{2+}$ ions Verner Thorsmolle, Alexander Ignatov, Maria Pezzoli, Kristjan Haule, David Kolchmeyer, Alexander Lee, Jack Simonson, Meigan Aronson, Girsh Blumberg CaMn$_2$Sb$_2$ presents a magnetic system with a buckled hexagonal lattice of half-filled $d$-band Mn$^{2+}$ ions. AC resistivity and susceptibility exhibit non-monotonic temperature dependence at 85-210~K. Below 85~K it has an antiferromagnetic (AF) phase with an activation energy of 28~meV, and above 210~K a paramagnetic phase. Using Raman spectroscopy we find a mode at 32~meV which develops below the AF transition. We attribute this excitation to the activation energy associated with the motion of spin bipolarons. Here, hybridization between Sb and Mn results in extra electrons for the Mn 3$d$-shells. It is energetically favorable for these extra carriers to form spin-singlets. These spin-bipolarons cover two Mn sites with a binding energy of $\sim$80~meV and conduction proceed via photo-assisted hopping with an activation energy of $\sim$32~meV. This spin bipolaron model explains the spectroscopic features providing a self-consistent picture of this conductivity mechanism that also clarifies reported unusual temperature-dependent magnetic and transport data. [Preview Abstract] |
Tuesday, March 19, 2013 2:42PM - 2:54PM |
J23.00002: Infrared evidence for multiple structural transitions in single crystal Cu$_3$(SeO$_3)_2$Cl Kevin H. Miller, Helmuth Berger, David B. Tanner Infrared reflection and transmission over a broad temperature range (10-300 K) have been measured on the anisotropic single-crystal Cu$_3$(SeO$_3)_2$Cl. Two distinct space groups have previously been reported for Cu$_3$(SeO$_3)_2$Cl at 300 K (monoclinic C2/m and triclinic P1bar). Comparing the number of observed infrared active phonons with group theoretical predictions points towards the existence of the triclinic structure at 300 K; however, an impurity-rich monoclinic structure cannot be ruled out. New phonon modes are observed upon cooling below 90 K, and again upon cooling below 40 K. The latter temperature range corresponds to the onset of long range magnetic order in the material. The structural and magnetic properties of Cu$_3$(SeO$_3)_2$Cl will be discussed in terms of our infrared spectra, group theoretical predictions, and comparisons to related compounds. [Preview Abstract] |
Tuesday, March 19, 2013 2:54PM - 3:06PM |
J23.00003: Optical spectroscopy and Fermi Surface studies of the Rashba spin-splitting compound BiTeI Catalin Martin, K.H. Miller, S. Buvaev, A.F. Hebard, E.D. Mun, V. Zapf, H. Berger, D.B. Tanner We measured the temperature dependent optical reflectivity $R(\omega$) and Shubnikov-de Haas oscillations in samples of BiTeI with different carrier concentrations. The electronic excitation spectrum, although consistent with Rashba spin-splitting of the bulk electronic bands, reveals additional features: a low energy excitation band and a larger number of phonons than expected from crystal structure. Some of the vibrational modes have strongly asymmetric line-shape. The period of quantum oscillations scales remarkably well with the component of magnetic field along the crystallographic $c$-axis and is rapidly suppressed when the field is tilted from this axis. We discuss our results in connection with possible charge accumulation at the surface of BiTeI. [Preview Abstract] |
Tuesday, March 19, 2013 3:06PM - 3:18PM |
J23.00004: Enhancement of charge and spin orders in a photoexcited one-dimensional strongly correlated system Hantao Lu, Shigetoshi Sota, Hiroaki Matsueda, Janez Bonca, Takami Tohyama By using the time-dependent Lanczos method, the nonequilibrium pro- cess of the half-filled one-dimensional extended Hubbard model, driven by a transient laser pulse, is investigated. In the case of large on-site Coulomb interactions, there are two phases separated by a first order quantum phase transition, i.e., spin-density-wave (SDW) and charge-density-wave (CDW) phases, which are characterized by algebraic decay of spin cor- relations and a long-range (staggered) charge order, respectively. When the system is subjected to the irradiation of a laser pulse, from the SDW side near the phase boundary, with proper laser frequency and strength, a sustainable charge order enhancement can be realized while local spin correlations remain. Analogously, from the CDW side, the suppression of long-range charge order is accompanied with a local spin correlation enhancement. We analyze the conditions and investigate possible mecha- nisms of the emerging order enhancements. In off-resonance region, more extended recovery of spin correlations which may come from nonlinear effect is also observed. [Preview Abstract] |
Tuesday, March 19, 2013 3:18PM - 3:30PM |
J23.00005: Computational study of novel half metallic compounds Zhijian Wu, Jing Wang Half-metallic (HM) materials are metallic for one spin direction while at the same time semiconducting for the other spin direction [1]. The unique feature of HM material is that it has an integer spin magnetic moment. For a carefully selected material, the integer can be zero (compensated). Besides ferromagnetic (FM) parallel spin arrangements, ferrimagnetic or even antiferromagnetic (AFM) alignments are also possible. In particular, half-metallic antiferromagnet (HM-AFM) possesses no macroscopic magnetization, yet their carriers are fully spin-polarized. In this work, half metallic compounds have been predicted by using the first principles, such as NiMoO$_3$ [2].\\[4pt] [1] de Groot, R. A.; Mueller, F. M.; van Engen, P. G.; Buschow, K. H. J. Phys. Rev. Lett., \textbf{\textit{1983}}, \textbf{\textit{50}}\textit{, 2024}\\[0pt] [2] Wang, J., Wu, Z. J. Appl. Phys. Lett, \textbf{\textit{2012}}, \textbf{\textit{101}}\textit{, 042414} [Preview Abstract] |
Tuesday, March 19, 2013 3:30PM - 3:42PM |
J23.00006: A non-perturbative general expression for the conductance through a leaky chiral edge mode Kun Woo Kim, Alexandra Junck, Israel Klich, Gil Refael Chiral edge modes of topological insulators and Hall states exhibit non-trivial behavior of conductance in the presence of impurities or additional channels. We will present a simple formula for the conductance through a chiral edge mode coupled to a disordered bulk. For a given coupling matrix between the chiral mode and bulk modes, and a Green function matrix of bulk modes in real space, the renormalized Green function of the chiral mode is expressed in a closed form ratios of determinants. We will conclude with examples of how the formula could be applied to describe the behavior of a chiral mode coupled to different types of bulk systems. [Preview Abstract] |
Tuesday, March 19, 2013 3:42PM - 3:54PM |
J23.00007: Generalization of the Peierls phase for gauge-invariant Green functions Sylvia D. Swiecicki, J.E. Sipe Solids in time-varying fields can be characterized with the non-equilibrium Green function formalism. If the interaction is described through potentials, the identification of sum rules is necessary to remove unphysical divergences that can appear in low frequency response calculations. For isolated atoms divergences are avoided by moving to a gauge-invariant Hamiltonian with the Power-Zienau-Woolley transformation.\footnote{W. Healy, Non-relativistic quantum electrodynamics (1982)} For solids, a gauge-invariant Green function formalism was proposed by Levanda and Fleurov\footnote{M. Levanda, V. Fleurov, J. Phys: Cond. Matt. \textbf{6} (1994) 7889}; in the generalization of the Peierls phase they introduced they consider only straight lines in spacetime. We extend this work to arbitrary paths in spacetime and show that the results for isolated atoms can be derived as a special case. More general applications are considered. [Preview Abstract] |
Tuesday, March 19, 2013 3:54PM - 4:06PM |
J23.00008: Evolution of the Coherent State and the Electronic Structure in the Kondo Insulator SmB$_6$ Xiaohang Zhang, N.P. Butch, P. Syers, S. Ziemak, R.L. Greene, J. Paglione As an exemplary Kondo insulator, SmB$_6$ has been studied for several decades; however, direct evidence for the development of the Kondo coherent state and the evolution of the electronic structure in the material has not been obtained due to the rather complicated electronic and thermal transport behavior. Recently, these open questions attracted increasing attention as the emergence of a time-reversal invariant topological surface state in the Kondo insulator has been suggested [1]. Here, we use the quasiparticle tunneling spectroscopy technique to directly investigate the temperature dependence of the electronic states in SmB$_6$. As a signature of the Kondo screening effect in the material, a Fano-like resonance line shape is observed in the tunneling spectroscopy at temperatures below $\sim$ 100 K. We further demonstrate that inter-ion correlation has to be taken into account [2] in order to precisely describe the observed asymmetric tunneling conductance at low temperatures. Our quasiparticle tunneling spectroscopy results also provide important implications for the predicted nontrivial topology in the Kondo insulator.\\[4pt] [1] Dzero et al., PRL 104, 106408 (2010);\\[0pt] [2] Maltseva et al., PRL 103, 206402 (2009) [Preview Abstract] |
Tuesday, March 19, 2013 4:06PM - 4:18PM |
J23.00009: Intervalley scattering and localization behaviors of group-VI transition metal dichalcoginides Haizhou Lu, Wang Yao, Di Xiao, Shun-Qing Shen We study the quantum diffusive transport of multi-valley massive Dirac cones coupled by intervalley spin-orbit scattering. We show that the intervalley spin-orbit scattering and intravalley spin-conserved scattering can be distinguished from the quantum conductivity that corrects the semiclassical Drude conductivity, due to their distinct symmetries and localization trends. In immediate practice, it allows transport measurements to estimate the intervalley scattering rate in hole-doped monolayers of group-VI transition metal dichalcogenides (e.g., molybdenum dichalcogenides and tungsten dichalcogenides), an ideal class of materials for valleytronics applications. The results can be generalized to a large class of multi-valley massive Dirac systems where time-reversal symmetry demands opposite spins in opposite valleys. [Preview Abstract] |
Tuesday, March 19, 2013 4:18PM - 4:30PM |
J23.00010: The structural origin of energy band gap in ultraviolet borates. Zheshuai Lin, Ran He Borate crystals have been intensively studied for their broad significant application in nonlinear optics materials, fluorescent materials, and laser crystals, especially in the ultraviolet (UV) spectral region (photon energy larger than 6.2 eV). However, due to the structural complexity the mechanism determining the energy band gap in the UV borates still hides in clouds. In this work, the structural origins of the energy band gaps in UV borates are systematically studied by ab initio methods and modeling considerations. Through analyzing the electronic band structures, we find that the top of valence bands in UV borates are dominant from the orbitals on oxygen. These orbitals construct the non-bonding states which determine the energy band gaps and their magnitudes depend on the local environments around oxygen atoms. Accordingly, the UV borates are categorized into three structural types, and in each type the ideal energy band gaps by removing the non-bonding states are almost the same. Moreover, a modified Bond Valence Sum method is adopted to parameterize the local environment around oxygen atoms, and the good agreement between the calculated and experimental energy band gaps within the accuracy of 0.3 eV can be achieved in UV borates. [Preview Abstract] |
Tuesday, March 19, 2013 4:30PM - 4:42PM |
J23.00011: Electron Transport in Edge Metal-Insulator-Metal Tunnel Junctions Modulated by Underlying Ferroelectric Polarization Switching Kibog Park, Youngeun Jeon, Sungchul Jung, Han Byul Jin, Jae-Hyeon Go, Soon-Yong Kwon, Nam Kim The electron energy band profile in an Edge Metal-Insulator-Metal tunnel junction (EMIM) on a Insulator/Ferroelectric thin film was calculated by performing finite-element electrostatic modeling. It is found that the energy band profile in the EMIM junction alters significantly near the underlying Insulator/Ferroelectric layer depending on the polarization direction of ferroelectric layer. The energy band profile shows pinch-off when the interface bound charge at Insulator/Ferroelectric interface is negative while it shows a valley-like shape when the interface bound charge is positive. The change of the energy band profile depending on ferroelectric polarization was confirmed to result in a significant change of electron tunneling current by using WKB method. It is believed that this switching of electron tunneling resistance in the EMIM junction opens up a way to develop non-volatile ferroelectric memory devices using non-destructive read-out. [Preview Abstract] |
Tuesday, March 19, 2013 4:42PM - 4:54PM |
J23.00012: Material Designs and Combinational Growth Techniques to Enable Novel Multiferroic Devices Melanie Cole, Eric Ngo, Mathew Ivill, S. Gary Hirsch, Cliff Hubbard, Ryan Toonen, Wendy Sarney Voltage control of magnetism in magnetic/ferroelectric bilayers has been most recently demonstrated in ultrathin metallic magnetic films through an electric field induced spin polarized charge screening effect. Voltage-controlled magnetism in magnetic/ferroelectric multilayers would provide a unique opportunity for integrating voltage-tunable RF/microwave magnetic devices on integrated circuits. It has been theoretically predicted that the voltage-control of magnetism in ferromagnetic/ferroelectric heterostructures can be significantly enhanced by utilizing high-K dielectrics. The critical challenge is how to enhance the permittivity of the ferroelectric film while maintaining low loss and low leakage characteristics and accomplishing this in an affordable manner by employing industry standard processing methods and large area low cost substrates. In this work we demonstrate the achievement of high-k, low loss and low leakage BST films utilizing optimized sputtered SrTiO3 buffer layers combined with a MOSD grown Mg-doped Ba0.60Sr0.40TiO3 overgrowth film on affordable large area substrates. Results of this research serves to promote enhanced EM coupling to enable a new class of charge mediated integratable voltage control multiferroic devices exploiting the converse ME effect. [Preview Abstract] |
Tuesday, March 19, 2013 4:54PM - 5:06PM |
J23.00013: Synchroton Soft X-ray Absorption Studies of YbFe$_{\mathrm{1-x}}$Mn$_{\mathrm{x}}$O$_{3}$ (0.0 $\le$ x $\le$ 1.0) Perovskites P. Olalde-Velasco, W.L. Yang, C. Hernandez, E. Chavira, I. Rosales, A. Tejada, L. Huerta, J. Jimenez-Mier, E.E. Marinero This work aims to correlate the interplay between structure-bonding (O2p-TM3d) and magnetic properties (TM 3d) in YbFe$_{\mathrm{1-x}}$Mn$_{\mathrm{x}}$O$_{3}$ (0.0 $\le$ x $\le$ 1.0) perovskites which are synthesized by the solid state reaction method. We have investigated by XAS the O2p and the magnetic TM3d unoccupied states of YbFe$_{\mathrm{1-x}}$Mn$_{\mathrm{x}}$O$_{3}$ (0$\le $x$\le $1). We find that increasing Mn doping promotes the creation of new states at the O2p band, it also induces a shifts towards lower energies of the O K pre-edge (with reference to the O2p-TM3d hybridization) and changes the spectral distribution in the region of TM 4s, p -- O 2p- Yb 5d hybridization. These changes are most marked for x \textgreater 0.2. A correlated effect with Mn doping is observed in the Fe L$_{\mathrm{2,3}}$ spectra where again new electronic states and systematic changes are observed x \textgreater\ 0.2. This is in contrast with Mn L$_{2,3}$ spectra, where all the spectra are very similar except for x$=$0.2. Thus, we provide a comprehensive picture of the electronic structure evolution in the conduction band in these materials as a function of the Mn content. [Preview Abstract] |
Tuesday, March 19, 2013 5:06PM - 5:18PM |
J23.00014: Thermodynamic stability of radiogenic Ba in CsAlSi$_2$O$_6$ pollucite John Jaffe, Ren\'ee Van Ginhoven, Weilin Jiang Pollucite, a zeolite-like nanoporous aluminosilicate structure with nominal composition CsAlSi$_2$O$_6$, has been suggested as a nuclear waste storage form for fission-product radioactive isotopes of cesium, especially $^{137}$Cs. One factor affecting the long-term stability of this waste form is the valence change associated with the beta decay that converts Cs into barium. We have used first-principles density functional total energy calculations to evaluate the thermodynamic stability of pollucite with Ba replacing Cs at regular lattice sites with respect to the precipitation of Ba, Cs or their oxides. We included small clusters of substitutional Ba$_{\mathrm{Cs}}$ as well as localized complexes of Ba$_{\mathrm{Cs}}$ with compensating electron donor defects, specifically Cs vacancies and interstitial oxygen. We conclude that Cs-Ba pollucite is thermodynamically stable against precipitation of Cs or its oxide, but that partial precipitation of Ba or BaO may be thermodynamically favored under some conditions. Even this change may be kinetically limited, however. [Preview Abstract] |
Tuesday, March 19, 2013 5:18PM - 5:30PM |
J23.00015: Metal-induced gap states in ferroelectric capacitors and its relationship with complex band structures Javier Junquera, Pablo Aguado-Puente At metal-isulator interfaces, the metallic wave functions with an energy eigenvalue within the band gap decay exponentially inside the dielectric (metal-induced gap states, MIGS). These MIGS can be actually regarded as Bloch functions with an associated complex wave vector. Usually only real values of the wave vectors are discussed in text books, since infinite periodicity is assumed and, in that situation, wave functions growing exponentially in any direction would not be physically valid. However, localized wave functions with an exponential decay are indeed perfectly valid solution of the Schrodinger equation in the presence of defects, surfaces or interfaces. For this reason, properties of MIGS have been typically discussed in terms of the complex band structure of bulk materials. The probable dependence on the interface particulars has been rarely taken into account explicitly due to the difficulties to include them into the model or simulations. We aim to characterize from first-principles simulations the MIGS in realistic ferroelectric capacitors and their connection with the complex band structure of the ferroelectric material. We emphasize the influence of the real interface beyond the complex band structure of bulk materials. [Preview Abstract] |
Session J24: Quantum Many-Body Systems and Methods I
Sponsoring Units: DCOMPChair: Cyrus Umrigar, Cornell University
Room: 326
Tuesday, March 19, 2013 2:30PM - 2:42PM |
J24.00001: Density dependence of fixed-node errors in quantum Monte Carlo: spin-polarized systems and triplet correlations Adem Kulahlioglu, Kevin Rasch, Shuming Hu, Lubos Mitas We present an analysis focused on the fixed-node bias of trial wave functions for fully spin-polarized atomic systems. We benchmark the case of three electrons in the lowest state for a given symmetry which exhibits near-degeneracy effects similar to the in Be-like systems. The trial wave functions examined have been constructed at the HF level and at the pairing level in the form of a pfaffian. We find very significant fixed-node errors at the HF level, of the order of tens of percent. On the other hand, we observe that the pfaffian wave function correlated in the triplet pair channel enables us to get essentially exact results. We demonstrate that the key reason behind the large fixed-node erorr of the HF wavefunction is its artificial nodal domain topology. In addition, the fixed-node error is studied as a function of electron density by varying the atomic charge Z. We find that it scales linearly with Z what is very similar to our previous study on Be-like systems with similar dependence on density but pairing in the singlet channel. [Preview Abstract] |
Tuesday, March 19, 2013 2:42PM - 2:54PM |
J24.00002: Fixed-node errors in electronic structure quantum Monte Carlo: interplay of density and node nonlinearities Lubos Mitas, Kevin Rasch, Shuming Hu We analyze valence electronic structure quantum Monte Carlo (QMC) calculations of first- and second-row atom systems. It turns out that there are significant differences (twofold or more) between the valence fixed-node errors of the first- vs second-row atom systems for single-configuration trial wave functions. The differences are illustrated on a set of atoms, molecules and Si and C solids that are valence isoelectronic, have similar correlation energies, bond patterns, geometries, same ground states and symmetries. Our analysis shows that the root cause of these differences is the increase of electron density combined with the degree of the node nonlinearity. The findings have implications for QMC fixed-node biases in systems with many elements including transition metals and others, which fall under the same electronic structure pattern. The finding has implications for both for accuracy of fixed-node energies, efficiency in elimination of the fixed-node bias and also for pseudopotential construction for very heavy elements. It has potential implications also for other correlated wave function approaches. [Preview Abstract] |
Tuesday, March 19, 2013 2:54PM - 3:06PM |
J24.00003: Diffusion quantum Monte Carlo for atomic spin-orbit interactions Minyi Zhu, Shi Guo, Lubos Mitas We present a generalization of the quantum Monte Carlo methods (QMC) for dealing with the spin-orbit (SO) effects in heavy atom systems. For heavy elements, the spin-orbit interaction plays an important role in electronic structure calculation and becomes comparable to the exchange, correlations and other effects. We implement relativistic lj-dependent effective core potentials for valence-only calculations. Due to the spin-dependent Hamiltonian, the antisymmetric trial wave functions are constructed from two-component spinors in jj-coupling so that the states are labeled by its total angular momentum J. A new spin representation is proposed which is based on summation over all possible spin states without generating large fluctutations and the fixed-phase approximation is used to avoid the sign problem. Our approach is different from the recent idea based on rotating (sampling) the spinors according to the action of the spin-orbit operator. We demonstrate the approach on heavy atom and small molecular systems in both variational and diffusion Monte Carlo methods and we calculate both ground and excited states. The results show very good agreement with independent methods and experimental results within the accuracy of the used effective core potentials. [Preview Abstract] |
Tuesday, March 19, 2013 3:06PM - 3:18PM |
J24.00004: Wavefunction Monte Carlo for Transport in Open Quantum Systems James Gubernatis The wave function Monte Carlo method is a technique for solving the stochastic differential equation associated with the master equation (Lindblad equation) for transport in an open quantum system. For an anisotropic, spin 1/2, XXZ Heisenberg chain in an external magnetic field, whose ends interact with heat baths, we compute the heat transport through the chain as a function of chain length, temperature difference at the ends, and the anisotropy of the chain's exchange interaction from both a wavefunction Monte Carlo simulation and a deterministic solution of the master equation for the open system's density matrix. Having both solutions creates benchmarks for the more fundamental objective of studying the consequence of replacing a piecewise deterministic step, which is typically part of the wavefunction Monte Carlo method, with a stochastic step. This replacement affords the potential of simulating longer chain lengths. [Preview Abstract] |
Tuesday, March 19, 2013 3:18PM - 3:30PM |
J24.00005: Prospects for efficient QMC defect calculations: the energy density applied to Ge self-interstitials Jaron Krogel, Jeongnim Kim, David Ceperley Defect formation energies require expensive energy difference calculations between defective and bulk systems over a range of system sizes. At the point of convergence, subregions added to represent larger systems no longer contribute to the formation energy and therefore display similar local energetics. A recent formulation of the energy density for QMC is capable of identifying separate energetic contributions from each atom, enabling the identification of the bulk-like regions in a defect system that only add noise to the final result. The potential efficiency gains of this approach are explored in a realistic defect system, the germanium self-interstitial. Calculations involving up to 217 atoms at fixed volume show that the extent of the strain energy field depends strongly on the interstitial site. Bulk-like regions are largest for the hexagonal interstitial increasing the efficiency by a factor of 2-3. In contrast, the split structure interstitial has few bulk-like atoms and shows no speedup. Possible approaches to further improve the efficiency will be discussed. [Preview Abstract] |
Tuesday, March 19, 2013 3:30PM - 3:42PM |
J24.00006: Analytic time evolution, random phase approximation, and Green functions for matrix product states Jesse M. Kinder, Claire C. Ralph, Garnet Kin-Lic Chan Drawing on similarities in Hartree-Fock theory and the theory of matrix product states (MPS), we explore extensions to time evolution, response theory, and Green functions. We derive analytic equations of motion for MPS from the least action principle, which describe optimal evolution in the small time-step limit. We further show how linearized equations of motion yield a MPS random phase approximation, from which one obtains response functions and excitations. Finally, we describe site-based Green functions associated with MPS. Using the fluctuation-dissipation theorem, we analyze the correlations introduced by the random phase approximation relative to the ground state wave function. [Preview Abstract] |
Tuesday, March 19, 2013 3:42PM - 3:54PM |
J24.00007: Application of Multi-Orbital DMFT to the Dynamic Hubbard Model Christopher Polachic, Frank Marsiglio Using multi-orbital dynamical mean field theory we explore the relationship between site parameters, band filling and electron-hole asymmetry arising through the electronic dynamic Hubbard model. We evaluate the emergence of hole pairing which has previously been observed through exact diagonalization and two-site DMFT studies. [Preview Abstract] |
Tuesday, March 19, 2013 3:54PM - 4:06PM |
J24.00008: Thermo-Electric Transport Out-of-Equilibrium Prasenjit Dutt, Karyn Le Hur The manipulation of mesoscopic systems to engineer quantum circuits has become a crucial tool to test and explore novel phenomena which arise due to quantum coherence effects. Electronic transport through these systems under the combined influence of voltage biases and thermal gradients poses several open questions, the understanding of which has an immense scope for future applications. We provide an effective equilibrium description of the steady state dynamics of quantum impurity models far-from-equilibrium, which generalizes the theory presented in P.Dutt et al. (Annals of Physics, 326, 2963(2011)), to include thermal gradients. We study the interplay of strong voltage biases and large thermal gradients and its effect on the emergent Abrikosov-Suhl resonance. Taking the linear response limit, we compute the various thermo-electric coefficients of the system, such as the Peltier coefficient and thermal conductance, and verify the reciprocity relations of Onsager. [Preview Abstract] |
Tuesday, March 19, 2013 4:06PM - 4:18PM |
J24.00009: Thermalization threshold in models of 1D fermions Subroto Mukerjee, Ranjan Modak, Sriram Ramswamy The question of how isolated quantum systems thermalize is an interesting and open one. In this study we equate thermalization with non-integrability to try to answer this question. In particular, we study the effect of system size on the integrability of 1D systems of interacting fermions on a lattice. We find that for a finite-sized system, a non-zero value of an integrability breaking parameter is required to make an integrable system appear non-integrable. Using exact diagonalization and diagnostics such as energy level statistics and the Drude weight, we find that the threshold value of the integrability breaking parameter scales to zero as a power law with system size. We find the exponent to be the same for different models with its value depending on the random matrix ensemble describing the non-integrable system. We also study a simple analytical model of a non-integrable system with an integrable limit to better understand how a power law emerges. [Preview Abstract] |
Tuesday, March 19, 2013 4:18PM - 4:30PM |
J24.00010: Topological Entanglement Entropy with a Twist Benjamin Brown, Stephen Bartlett, Andrew Doherty, Sean Barrett Topologically ordered phases of matter offer an attractive approach to fault tolerant quantum computation. They give rise to exotic quasi-particle excitations known as anyons. Anyons have a degenerate Hilbert space associated to them, which can be used to encode quantum information over non-local degrees of freedom. Recently, it has been shown that twists, the end points of dislocations in the toric code model, and the quasi-particles available on the toric code have some features analogous to a different anyon model; the Ising anyon model. Characteristics of topologically ordered phases can be assessed by calculating the topological entanglement entropy of regions of the ground state of its Hamiltonian. Further to this, the data of its anyonic excitations can be calculated using the von Neumann entropy. We present analytic results showing that twists have the same topological data as Ising anyons using extensions of known topological entanglement entropy formulas. This extends further the analogy between twists and Ising anyons. [Preview Abstract] |
Tuesday, March 19, 2013 4:30PM - 4:42PM |
J24.00011: Nature of the Spin-Liquid Ground State of the S=1/2 Heisenberg Model on the Kagome Lattice Stefan Depenbrock, Ian McCulloch, Ulrich Schollwoeck We perform a density-matrix renormalization group (DMRG) study of the $S=\frac{1}{2}$ Heisenberg antiferromagnet on the kagome lattice to identify the conjectured spin liquid ground state. Exploiting SU(2) spin symmetry, which allows us to keep more than 16,000 DMRG states, we consider cylinders with circumferences up to 17 lattice spacings and find a spin liquid ground state with an estimated per site energy of $-0.4386(5)$, a spin gap of $0.13(1)$, very short-range decay in spin, dimer and chiral correlation functions and finite topological entanglement $\gamma$ consistent with $\gamma=\textrm{log}_2 2$, ruling out gapless, chiral or non-topological spin liquids. At the same time, DMRG results provide strong evidence for a gapped topological $Z_2$ spin liquid. [Preview Abstract] |
Tuesday, March 19, 2013 4:42PM - 4:54PM |
J24.00012: Geometrically Constructed Markov Chain Monte Carlo Study of Quantum Spin-phonon Complex Systems Hidemaro Suwa We have developed novel Monte Carlo methods for precisely calculating quantum spin-boson models and investigated the critical phenomena of the spin-Peierls systems. Three significant methods are presented. The first is a new optimization algorithm of the Markov chain transition kernel based on the geometric weight allocation. This algorithm, for the first time, satisfies the total balance generally without imposing the detailed balance and always minimizes the average rejection rate, being better than the Metropolis algorithm. The second is the extension of the worm (directed-loop) algorithm to non-conserved particles, which cannot be treated efficiently by the conventional methods. The third is the combination with the level spectroscopy. Proposing a new gap estimator, we are successful in eliminating the systematic error of the conventional moment method. Then we have elucidated the phase diagram and the universality class of the one-dimensional {\it XXZ} spin-Peierls system. The criticality is totally consistent with the $J_1-J_2$ model, an effective model in the antiadiabatic limit. Through this research, we have succeeded in investigating the critical phenomena of the effectively frustrated quantum spin system by the quantum Monte Carlo method without the negative sign. [Preview Abstract] |
Tuesday, March 19, 2013 4:54PM - 5:06PM |
J24.00013: A Study of the Uniqueness of the Density for Nonequilibrium Systems Selman Hershfield By the Hohenberg-Kohn theorem the density in equilibrium is a unique functional of the the single particle potential. To gain an understanding of whether this is true in a nonequilibrium system with a current flowing, the density is studied for several noninteracting models. Although noninteracting models are not as realistic as interacting ones, they do have the advantage that they can be solved exactly. For sufficiently high bias or chemical potential difference we find that the density is not a unique functional of the potential for some models in the finite spatial region we study numerically. In other models the density is a unique functional of the potential even for large bias. An algorithm will be presented for finding cases where degeneracies exist and a simple physical picture will be given to understand them. [Preview Abstract] |
Tuesday, March 19, 2013 5:06PM - 5:18PM |
J24.00014: Scaling of the Renyi entropy in 1D critical SU(N) spin chains Jonathan Demidio, Matthew S. Block, Ribhu K. Kaul Using quantum Monte Carlo techniques, we study an SU($N$) antiferromagnet with each spin transforming in the fundamental representation. The spin interaction simply permutes ``colors'' on neighboring sites. This permutation operator is of interest to ultra-cold atomic systems, since at low energies it is the dominant effective interaction of the SU($N$) symmetric Hubbard model with one atom per site. We calculate the entanglement entropy across a partition in the spin chain via the so-called ``replica trick,'' whereby the partition function is simulated on the modified topology of an n-sheeted Riemann surface. In the thermodynamic limit, quantum critical spin chains in 1D are described by 2D conformal field theories (CFTs). Thus, the scaling form of the entanglement entropy provides information about the underlying CFT. In particular we extract the central charge of the CFT, which depends only on the symmetries of the spin model and not its microscopic details. We find that the central charge is given by $c=N-1$, which is in agreement with previous theoretical predictions. We also find agreement in the scaling form of the entanglement entropy, which depends on the number of replicas in the Riemann surface. [Preview Abstract] |
Tuesday, March 19, 2013 5:18PM - 5:30PM |
J24.00015: Measuring Entanglement at a Quantum Critical Point with Numerical Linked Cluster Expansion Ann B. Kallin, Katharine Hyatt, Rajiv R. P. Singh, Roger G. Melko We develop a method to calculate the bipartite entanglement entropy of quantum lattice models in the thermodynamic limit, using a Numerical Linked Cluster Expansion (NLCE) involving only rectangular clusters. The NLCE is based on exact diagonalization of all N x M rectangular clusters at the interface between entangled subsystems A and B. We show that the method can be used to obtain the Renyi entanglement entropy of the two-dimensional transverse field Ising model, for arbitrary real Renyi index. Furthermore, extrapolating these results as a function of the order of the calculation, one can obtain subleading universal pieces of the entanglement entropy at a quantum critical point. These results are compared with series expansions, quantum Monte Carlo simulations and field theories, where available, and they demonstrate the utility of the NLCE in obtaining accurate results for the universal properties of this critical point for von Neumann and non-integer Renyi entropies. [Preview Abstract] |
Session J25: Focus Session: Explicitly correlated Methods and Quantum Few-Body Systems
Sponsoring Units: DCOMPChair: Ludwik Adamowicz, University of Arizona
Room: 327
Tuesday, March 19, 2013 2:30PM - 3:06PM |
J25.00001: Ultracold physics with 3, 4, or 5 atoms Invited Speaker: Chris Greene Recent studies will be reviewed [1-3], which utilize hyperspherical coordinates to treat few-body systems, concentrating on processes such as recombination, in which the initial state has 3 or more free particles in the continuum. Of particular interest are ultracold species with large two-body scattering lengths, for which universal behavior has been seen experimentally [4] that goes beyond the ordinary universality associated with the Efimov effect. The so-called three-body parameter, now understood to be universal for systems having van der Waals interactions, is readily interpreted using this theoretical framework, and predictions are made concerning A$+$A$+$B collisions as well as the homonuclear case A$+$A$+$A. Various aspects of the work presented have been carried out in collaboration with Jia Wang, Yujun Wang, Jose D'Incao, Javier von Stecher, and Brett Esry. \\[4pt] [1] J. Wang et al., Phys. Rev. Lett. \textbf{108}, 263001 (2012)\\[0pt] [2] J. Wang et al., Phys. Rev. A \textbf{84}, 052721 (2011)\\[0pt] [3] Y. Wang et al. arXiv:1207.6439 (2012).\\[0pt] [4] M. Berninger et al., Phys. Rev. Lett. \textbf{107}, 120401 (2011). [Preview Abstract] |
Tuesday, March 19, 2013 3:06PM - 3:18PM |
J25.00002: A continued fraction approach for calculating Auger electron sprectra Anamitra Mukherjee, Mona Berciu, George Sawatzky In 'core valence valence' Auger spectroscopy (AES), X-ray absorption leads to the appearance of a core hole, which then decays into two valence holes and an Auger electron. The Auger electron carries information about the spectrum of the two additional holes thus introduced in the system. This is straightforward to compute if the two holes move in an otherwise full band, but accurate results for partially filled bands are still missing. Here we present a novel approach to calculating few-body lattice Green's functions that allows us to obtain the AES spectrum for systems with both filled and open bands, such as CuO and NiO. For full bands, comparison against exact results allows us to propose efficient variational schemes, which can then be used to study partially filled bands. [Preview Abstract] |
Tuesday, March 19, 2013 3:18PM - 3:30PM |
J25.00003: Progress in calculating the PES of H$_3^+$ Michele Pavanello, Alexander Alijah, Ludwik Adamowicz The most accurate electronic structure calculations are performed using wave-function expansions in terms of basis functions explicitly dependent on the interelectron distances. In our recent work we use such basis functions to calculate a highly accurate potential energy surface (PES) for the H$_3^+$ ion. The functions are explicitly correlated Gaussians which include inter-electron distances in the exponent. Key to obtaining the high accuracy in the calculations has been the use of the analytical energy gradient determined with respect to the Gaussian exponential parameters in the minimization of the Rayleigh-Ritz variational energy functional. The effective elimination of linear dependencies between the basis functions, as well as the automatic adjustment of the positions of the Gaussian centers to the changing molecular geometry of the system, are key to the success of the computational procedure. After adiabatic and relativistic corrections are added to the PES and with an effective accounting of the non-adiabatic effects in the calculation of the rotational/vibrational states, the experimental H$_3^+$ rovibrational spectrum is reproduced at the 0.1~cm$^{-1}$ accuracy level up to 16,600~cm$^{-1}$ above the ground state. [Preview Abstract] |
Tuesday, March 19, 2013 3:30PM - 4:06PM |
J25.00004: Variational methods with all-particle explicitly correlated Gaussians Invited Speaker: Sergiy Bubin Accurate treatment of electron correlation in quantum systems of various nature remains an important challenge for modern theoretical and computational approaches. The variational method in conjunction with explicitly correlated Gaussian (ECG) basis sets is one of the most capable, accurate, and conceptually simple methods for calculating the ground, excited, and even scattering state properties of small quantum systems. I will review the basic theoretical foundations, recent advances, and the applications of the ECG method to Coulomb systems such as atoms, molecules, and systems containing positrons. I will also discuss some of the most important challenges that need to be overcome in order to extend the current range of applicability of the method. [Preview Abstract] |
Tuesday, March 19, 2013 4:06PM - 4:18PM |
J25.00005: Hylleraas coordinates in few-body atomic and molecular systems Z.-C. Yan, L.-M. Wang, H.-X. Qiao, G. W. F. Drake In this talk, we will present recent progress on the calculations of few-body Coulombic systems, such as atomic lithium and hydrogen molecular ions, using variational method in Hylleraas coordinates, including relativistic and quantum electrodynamic corrections. We will also discuss the applications of these calculations in the determination of nuclear charge radii and the proton-electron mass ratio. [Preview Abstract] |
Tuesday, March 19, 2013 4:18PM - 4:30PM |
J25.00006: Development of explicitly correlated congruent transformed Hamiltonian Mike Bayne, John Drogo, Arindam Chakraborty The central idea of the explicitly correlated congruent transformed Hamiltonian (CTH) method is the treatment of the Coulomb singularity in the Hamiltonian by performing congruent transformation using an explicitly correlated wave function. However, unlike the transcorrelated methods, the CTH is Hermitian and amenable to standard variational methods. The variational solution of the CTH was obtained using FCI and the comparison between the transformed and untransformed calculation will be discussed. We found that the CTH dramatically improves the convergence of the FCI expansion. The CTH can also be represented in the occupation number (ON) space, however this representation is approximate due to the finite size of the underlying basis. Analogous to the diagrammatic summation in MBPT, we have developed partial infinite order summation (PIOS) for improving the CTH calculation in ON space and analysis of the real space, ON and ON-PIOS calculation of CTH will be discussed. The CTH has been applied to a series of 10 electron systems and comparison of the results with other methods will be presented. Preliminary results on the excited state of water will be compared with R12-MP2 and MRCI methods. The size-consistency of the CTH method was numerically analyzed and will be discussed. [Preview Abstract] |
Tuesday, March 19, 2013 4:30PM - 4:42PM |
J25.00007: Stability of high and low spin states Hannes Raebiger, Shuhei Fukutomi, Hiroshi Yasuhara Octahedral CoL$_6$ complexes exhibit high or low spin states, depending on ligand L. We present an explicitly correlated first principles calculation of CoL$_6$ with five different ligands, and show that the total energy difference $\Delta E$ between the high and low spin states is variationally determined in an intricate interplay of the interelectron repulsion $V_{ee}$, internuclear repulsion $V_{nn}$, and electronuclear attraction $V_{ne}$. This is in stark contrast to ``ligand field theory'' [1,2], where $\Delta E$ is approximated as $\Delta E \approx \Delta V_{ee}$ in a first order perturbation theory. Moreover, we show that $\Delta V_{ee}$ exhibits the opposite trend to $\Delta E$ and is three or four orders of magnitude greater than $\Delta E$, which demonstrates the failure of ligand field theory both quantitatively and qualitatively. Correctly, the crossover of high and low spin states is a consequence of different Co--L bondings, ionic or covalent, which is found by an accurate treatment of Coulomb correlation between ligand $p$ and cobalt $d$ electrons in the present calculation. [1] J. H. Van Vleck, J. Chem Phys {\bf 3}, 807 (1935). [2] Y. Tanabe and S. Sugano, J. Phys. Soc. Jpn. {\bf 9}, 766 (1954). [Preview Abstract] |
Tuesday, March 19, 2013 4:42PM - 4:54PM |
J25.00008: Computation of Low-Energy Positronium-Hydrogen Collisions using the Kohn Variational Method Denton Woods, S.J. Ward, P. Van Reeth The Kohn variational method is an established method that can provide benchmark calculations for quantum few-body systems. We consider the four-body Coulomb process of positronium-hydrogen (Ps-H) scattering. We improve upon the numerics of prior accurate S- and P-wave Kohn variational calculations of Ps-H elastic scattering [1,2]. For instance, we use a procedure that removes Hylleraas-type terms that lead to linear dependence [3]. In addition to using the Kohn and inverse Kohn variational methods as previously used, we use the generalized and complex Kohn variational methods [4]. We are extending the calculations of Ps-H to include the D-wave.\\[4pt] [1] P. Van Reeth and J. W. Humberston, J. Phys. B \textbf{36}, 1923 (2003).\\[0pt] [2] P. Van Reeth and J. W. Humberston, Nucl. Instrum. Methods B \textbf{221}, 140 (2004).\\[0pt] [3] A. Todd, Ph.D. thesis, The University of Nottingham, (2007), \emph{unpublished}.\\[0pt] [4] J.N. Cooper, M. Plummer, and E.A.G. Armour, J. Phys. A \textbf{43}, 175302 (2010). [Preview Abstract] |
Tuesday, March 19, 2013 4:54PM - 5:06PM |
J25.00009: Influence of Angular and Spin-dependent Terms on Variational Energies of Lithium Gordon Drake, Zong-Chao Yan, Liming Wang Improved nonrelativistic energy bounds for the low-lying states of lithium are presented using the variational method in Hylleraas coordinates [1]. For example, the nonrelativistic energies for the infinite nuclear mass case are $-7.478\,060\,323\,910\,147(1)$ a.u. for $1s^22s\;^2{\rm S}$, $-7.354\,098\,421\,444\,37(1)$ a.u. for $1s^23s\;^2{\rm S}$, $-7.318\,530\,845\,998\,91(1)$ a.u. for $1s^24s\;^2{\rm S}$, $-7.410\,156\,532\,652\,4(1)$ a.u. for $1s^22p\;^2{\rm P}$, and $-7.335\,523\,543\,524\,688(3)$ a.u. for $1s^23d\;^2{\rm D}$. These results represent the most accurate nonrelativistic energies in the literature. The completeness of the angular momentum and spin configurations is investigated and examples presented for the 2P and 3D states to demonstrate the effect of different coupling schemes. In particular, the so-called second spin function (i.e.\ coupled to form an intermediate triplet state) is shown to have no effect on the final converged results, even for the expectation values of spin-dependent operators. This resolves a long-standing controversy concerning the completeness of the spin-coupling terms.\\ \mbox{}[1] L.M. Wang, Z.-C. Yan, H.X. Qiao, and G.W.F. Drake, Phys.\ Rev.\ A {\bf 85}, 052513 (2012). [Preview Abstract] |
Tuesday, March 19, 2013 5:06PM - 5:18PM |
J25.00010: Multi-determinant electron-nuclear quantum Monte Carlo method for ground state solution of molecular Hamiltonian Abhinanden Sambasivam, Jennifer Elward, Arindam Chakraborty The focus of this work is to obtain the ground state energy of the non-relativistic spin-independent molecular Hamiltonian without making the Born-Oppenheimer (BO) approximation. In addition to avoiding the BO approximation, this approach avoids imposing separable-rotor and harmonic oscillator approximations. The ground state solution is obtained variationally using multi-determinant variational Monte Carlo method where all nuclei and electrons in the molecule are treated quantum mechanically. The multi-determinant VMC provides the right framework for including explicit correlation in a multi-determinant expansion. This talk will discuss the construction of the basis functions and optimization of the variational coefficient. The electron-nuclear VMC method will be applied to H$_2$, He$_2$ and H$_2$O and comparison of the VMC results with other methods will be presented. The results from these calculations will provide the necessary benchmark values that are needed in development of other multicomponent method such as electron-nuclear DFT and electron-nuclear FCIQMC. [Preview Abstract] |
Tuesday, March 19, 2013 5:18PM - 5:30PM |
J25.00011: Low energy model estimation from detailed quantum Monte Carlo calculations for transition metal systems Lucas Wagner Systems of strongly correlated electrons have incredible potential for new devices and new quantum states. However, it is very challenging to a priori predict the quantum state of a system of correlated electrons. Detailed calculations using quantum Monte Carlo methods on the first principles Hamiltonian have in recent years shown to be quite reliable for some example transition metal oxide systems, such as FeO, ZnO, among others. These calculations, although they are accurate, have not provided much information in terms of the correct approximate low-energy model that should describe the systems in question. In this talk, I'll summarize the results of matching the two-body correlations from first principles quantum Monte Carlo on transition metal systems to models and discuss the implications for the commonly used models. [Preview Abstract] |
Session J26: Semiconductor Qubits - Spin Measurement and Noise
Sponsoring Units: GQIChair: Bill Coish, McGill University
Room: 328
Tuesday, March 19, 2013 2:30PM - 3:06PM |
J26.00001: Taming spin decoherence in silicon Invited Speaker: Stephen Lyon Electron spins in semiconductor hosts have been candidate qubits since the early days of experimental quantum computing research, but it was generally assumed that the solid state environment would limit coherence to times much shorter than that seen in isolated atoms or ions. The longest measured electron spin coherence, measured in isotopically enriched silicon, was of order 1 ms. However, over the last 8 or 10 years the measured electron spin coherence times have steadily increased as materials and experimental techniques have improved. Much of the decoherence observed in the early ensemble Electron Spin Resonance (ESR) experiments arose from interactions amongst the spins being measured. In the most highly enriched bulk silicon measured to date, the residual silicon isotopes with nuclear magnetic moments affect the coherence of electrons bound to phosphorus donors on about a 1 second time scale. The remaining decoherence is still dominated by interactions between the donor spins, even in very lightly doped Si. Other decoherence processes have been shown to be at least an order of magnitude weaker. Recent work suggested that longer spin coherence would be obtained in bismuth doped Si, where magnetic-field insensitive ``clock transitions'' occur in the GHz frequency range. Recent experiments are bearing out these suggestions. [Preview Abstract] |
Tuesday, March 19, 2013 3:06PM - 3:18PM |
J26.00002: Spin-bath autocorrelation functions directly from quantum theory Wayne Witzel, Kevin Young, Sankar Das Sarma Cluster expansion techniques have enabled accurate modeling of the effects of a bath of local spins on solid state spin qubits with proven predictive power. These calculations are performed in the context of specific echo decay experiments (Hahn echo, CPMG, etc.). Classical noise, on the other hand, is described by a single autocorrelation function (or spectral density, equivalently) that is applicable to any control-specific experiment. Such a description is very useful in searching for optimal controls to produce high fidelity quantum logic gates using well-studied techniques. We demonstrate a cluster expansion method for directly computing autocorrelation functions as expectation values in the quantum spin-bath setting and show that it is a sufficient description of the noise effects for certain regimes, particularly in the high fidelity regime of interest. We use this approach to study the theoretical impact of using optimized pulse sequences tailored to individual qubits in enriched silicon. [Preview Abstract] |
Tuesday, March 19, 2013 3:18PM - 3:30PM |
J26.00003: Error in a spin-SWAP gate due to hyperfine interaction in a double quantum dot Jo-Tzu Hung, {\L}ukasz Cywi{\'n}ski, Xuedong Hu We study the SWAP gate for two exchange-coupled electron spins under the influence of hyperfine (hf) interaction in a double quantum dot. A gate error develops during such a gate because hf interaction causes dephasing between any pair of two-spin states. We find that this gate error is initial-state-dependent. For example, an initial state in the $S_z=0$ subspace suffers only from $S-T_0$ dephasing, leading to smaller gate error than in the case of other initial states. We calculate the gate fidelity for typical initial states, and compare the resulting gate errors. We also analyze the effects of inhomogeneous broadening on the gate fidelity in the presence of a random Overhauser field. [Preview Abstract] |
Tuesday, March 19, 2013 3:30PM - 3:42PM |
J26.00004: Electron Spin Relaxation and Coherence Times in Si/SiGe Quantum Dots R.M. Jock, Jianhua He, A.M. Tyryshkin, S.A. Lyon, C.-H. Lee, S.-H. Huang, C.W. Liu Single electron spin states in Si/SiGe quantum dots have shown promise as qubits for quantum information processing. Recently, electron spins in gated Si/SiGe quantum dots have displayed relaxation (T$_{\mathrm{1}})$ and coherence (T$_{\mathrm{2}})$ times of 250 $\mu $s at 350mK. The experiments used conventional X-band (10 GHz) pulsed Electron Spin Resonance (pESR) on a large area (3.5 x 20 mm$^{\mathrm{2}})$, double gated, undoped Si/SiGe heterostructure, which was patterned with 2 x 10$^{\mathrm{8}}$ quantum dots using e-beam lithography. Dots with 150 nm radii and 700 nm period are induced in a natural Si quantum well by the gates. Smaller dots are expected to reduce the effects of nearly degenerate valley states and spin-orbit coupling on the electron spin coherence. However, the small number of spins makes signal recovery extremely challenging. We have implemented a broadband cryogenic HEMT low-noise-amplifier and a high-speed single-pole double-throw switch operating at liquid helium temperatures. The switch and preamp have improved our signal to noise by an order of magnitude, allowing for smaller samples and shorter measurement times. We will describe these improvements and the data they have enabled. [Preview Abstract] |
Tuesday, March 19, 2013 3:42PM - 3:54PM |
J26.00005: Quantum theory of dynamic nuclear polarization in quantum dots Sophia Economou, Edwin Barnes Nuclear spins play a major role in the dynamics of spin qubits in III-V semiconductor quantum dots. Although the hyperfine interaction between nuclear and electron (or hole) spins is typically viewed as the leading source of decoherence in these qubits, understanding how to experimentally control the nuclear spin polarization can not only ameliorate this problem, but in fact turn the nuclear spins into a valuable resource for quantum computing. Beyond extending decoherence times, control of this polarization can enable universal quantum computation as shown in singlet-triplet qubits and, in addition, offers the possibility of repurposing the nuclear spins into a robust quantum memory. In [1], we took a first step toward taking advantage of this resource by developing a general, fully quantum theory of non-unitary electron-nuclear spin dynamics with a periodic train of delta-function pulses as the external control driving the electron spin. Here, we extend this approach to other types of controls and further expand on the predictions and physical insights that emerge from the theory. [1] Edwin Barnes and Sophia E. Economou, Phys. Rev. Lett. 107, 047601 (2011) [Preview Abstract] |
Tuesday, March 19, 2013 3:54PM - 4:06PM |
J26.00006: Enhanced spin-flip transport in a quantum dot spin-valve with uniform hyperfine coupling Stefano Chesi, William A. Coish We study the transport current and nuclear spin polarization dynamics in a quantum dot spin-valve, for which a strong enhancement of the spin-flip electron tunneling rates can be realized in the limit of uniform hyperfine interaction. We extend the analogy of transport to superradiance, directly applicable to a spin valve with half-metal leads and a maximally polarized nuclear system, to the more general situation of ferromagnetic contacts and a nuclear system initially fully dephased and partially polarized, as naturally realized at finite bias under stationary conditions. An analytic treatment of the dynamics in terms of simple rate equations becomes possible for very fast/slow nuclear dephasing. We recover these limiting results, as well as analyze the crossover regime, from a general master equation for the nuclear dynamics. We also present strategies to approach the limit of uniform hyperfine interaction in realistic heterostructures. [Preview Abstract] |
Tuesday, March 19, 2013 4:06PM - 4:18PM |
J26.00007: Single Electron Spin Resonance in a Si-MOS Double Quantum Dot Xiaojie Hao, Ming Xiao, Hongwen Jiang, Rusko Ruskov, Charles Tahan Pauli spin blockade is used as a means to detect the flip of spins in a silicon metal-oxide-semiconductor (MOS) based double quantum dot. Microwave driven electron spin resonance (ESR) signals, with a linewidth as narrow as 1.5 G, has been observed only in a narrow range of magnetic fields. ESR spectroscopy in the magnetic field - microwave frequency plane shows an unexpected level anti-crossing, with an energy gap of about 50 MHz. The spectral line gives an estimation of the lower bound for inhomogeneous phase decoherence time $T_{2}^{*}$ of about a couple of hundred ns for individual spins in the nano-structured system with a Si/SiO2 interface. We explain the anti-crossing gap as due to spin-orbit mixing with higher states, which is also responsible for the narrow-window visibility of the ESR signal in Si based double quantum dots. [Preview Abstract] |
Tuesday, March 19, 2013 4:18PM - 4:30PM |
J26.00008: Theory of Spin Relaxation in Two-Electron Laterally Coupled GaAs and Si Quantum Dots Martin Raith, Peter Stano, Jaroslav Fabian We present quantitative results of the phonon-induced spin relaxation in two-electron lateral double quantum dots for a wide range of tuning parameters. Both spin-orbit coupling and hyperfine coupling are taken into account. Our analysis of GaAs [1] and silicon [2] based dots includes the variation of the electric field (detuning), the exchange coupling, and the magnetic field strength and orientation. The focus is on experimentally important regimes. We find that even in strong magnetic fields, the hyperfine coupling can dominate the relaxation rate of the unpolarized triplet in a detuned double dot. Where the spin-orbit coupling dominates, the rate is strongly anisotropic and its maxima and minima are generated by an in-plane magnetic field either parallel or perpendicular to the dots' alignment dependent on specifics, such as spectral (anti-)crossings (spin hot spots), or the detuning strength. For all regimes, we give qualitative explanations of our observations. We emphasize the differences between GaAs and Si based dots. By understanding the spin lifetimes ($T_1$), this work marks a crucial step toward the realization of two-electron semiconductor qubits for quantum information processing.\\[4pt] [1] M. Raith et. al., PRL 108, 246602 (2012)\\[0pt] [2] M. Raith et. al., arXiv:1206.6906 [Preview Abstract] |
Tuesday, March 19, 2013 4:30PM - 4:42PM |
J26.00009: Anomalous electron spin decoherence in an optically pumped quantum dot Xiaofeng Shi, L.J. Sham We study the nuclear-spin-fluctuation induced spin decoherence of an electron (SDE) in an optically pumped quantum dot. The SDE is computed in terms of the steady distribution of the nuclear field (SDNF) formed through the hyperfine interaction (HI) with two different nuclear species in the dot. A feedback loop between the optically driven electron spin and the nuclear spin ensemble determines the SDNF [W. Yang and L. J. Sham, Phy. Rev. B 85, 235319(2012)]. Different from that work and others reviewed therein, where a bilinear HI, $S_{\alpha}I_{\beta}$, between the electron (or hole) spin $\mathbf{S}$ and the nuclear spin $\mathbf{I}$ is used, we use an effective nonlinear interaction of the form $S_{\alpha}I_{\beta}I_{\gamma}$ derived from the Fermi-contact HI. Our feedback loop forms a multi-peak SDNF in which the SDE shows remarkable collapses and revivals in nanosecond time scale. Such an anomalous SDE results from a quantum interference effect of the electron Larmor precession in a multi-peak effective magnetic field. In the presence of a bilinear HI that suppresses the nuclear spin fluctuation, the non-Markovian SDE persists whenever there are finite Fermi contact interactions between two or more kinds of nuclei and the electron in the quantum dot. [Preview Abstract] |
Tuesday, March 19, 2013 4:42PM - 4:54PM |
J26.00010: Mechanisms for Electric Field Control of Single Spin Relaxation in Double Quantum Dots V. Srinivasa, K.C. Nowack, M. Shafiei, L.M.K. Vandersypen, J.M. Taylor We theoretically investigate electrically-tunable spin-flip transitions for a single electron confined within a double quantum dot. In the presence of spin-orbit and hyperfine interactions, the rate at which phonon-induced spin relaxation occurs depends non-monotonically on the detuning between the dots. We analyze this detuning dependence for both direct decay to the ground state and indirect decay via an intermediate excited state of the double dot. A description in terms of a simple toy model captures characteristic features of the relaxation rate recently measured for GaAs double quantum dots. Our results suggest that spin-orbit mediated relaxation via phonons serves as the dominant mechanism through which the electron spin-flip rate in these systems varies with detuning. [Preview Abstract] |
Tuesday, March 19, 2013 4:54PM - 5:06PM |
J26.00011: Enhanced hyperfine-induced spin dephasing in a magnetic field gradient Felix Beaudoin, William A. Coish Magnetic field gradients are important for single-site addressability and electric-dipole spin resonance of electrons in quantum dots or in donor impurities. We show that these advantages are offset by a potential reduction in coherence time. Although the magnetic field appears uniform to the electron, it provides a non-uniform field for the nuclear-spin bath. This leads to a finite bath correlation time, preventing the full recovery of electron-spin coherence. We apply our model to single electron spins in quantum dots and single donor impurities, singlet-triplet spin qubits, and consider both free-induction decay and spin-echo. This mechanism can dominate over known dephasing sources due to nuclear dipole-dipole interactions and hyperfine flip-flops. This result is especially important for systems requiring large magnetic field gradients, including spin qubits coupled to superconducting stripline resonators. [Preview Abstract] |
Tuesday, March 19, 2013 5:06PM - 5:18PM |
J26.00012: Spin Qubit Relaxation in a Moving Quantum Dot Peihao Huang, Xuedong Hu Long-range quantum communication for spin qubits is a significant open problem in the scale-up of spin qubit architectures. Among the many spin information transfer proposals, directly moving the electrons themselves is attractive because of its conceptual simplicity and its similarity to the conventional charge-coupled devices. Here we focus on electron spin decoherence when the quantum dot is in motion. Specifically, we study a spin decoherence mechanism for a moving but confined electron due to the spin-orbit interaction and an environmental random electric potential. We find that at the lowest order, the magnetic fluctuations experienced by the spin have only components transverse to the total magnetic field, so that the motion induced spin decoherence is a pure longitudinal relaxation channel. Our calculated spin relaxation time ranges from as fast as sub $\mu$s in GaAs to above ms in Si. Our results also clearly indicates how to reduce the decoherence effects of electron motion. [Preview Abstract] |
Session J27: Focus Session: Nano/Optomechanics I
Sponsoring Units: DAMOPChair: Mohammad Hafezi, University of Maryland
Room: 329
Tuesday, March 19, 2013 2:30PM - 3:06PM |
J27.00001: Nanomechanics and superconducting qubits for quantum information Invited Speaker: Andrew Cleland There has been tremendous progress in the capabilities of superconducting quantum circuits, both for fundamental quantum science as well as for applications in quantum information. Superconducting qubits are based on the Josephson junction, which provides the fundamental inductive nonlinearity that affords full quantum control of otherwise quite simple electrical circuits. I will outline how a superconducting qubit can be used to measure and control the quantum state of a nanomechanical system [1], completely control multi-photon states in superconducting resonators [2,3], factor the number 15 using a von Neumann-style computing architecture [4,5], and possibly allow the transfer of a GHz-frequency quantum state to an optical signal.\\[4pt] [1] A.D. O'Connell et al., ``Quantum ground state and single-phonon control of a mechanical resonator,'' Nature 464, 697-703 (2010)\\[0pt] [2] M. Hofheinz et al., ``Generation of Fock states in a superconducting quantum circuit,'' Nature 454, 310-314 (2008)\\[0pt] [3] M. Hofheinz et al., ``Synthesizing arbitrary quantum states in a superconducting resonator,'' Nature 459, 546-549 (2009)\\[0pt] [4] M. Mariantoni et al., ``Implementing the quantum von Neumann architecture with superconducting circuits,'' Science 334, 61 (2011) \\[0pt] [5] E. Lucero et al., ``Computing prime factors with a Josephson phase qubit quantum processor,'' Nature Physics 8, 719 (2012) [Preview Abstract] |
Tuesday, March 19, 2013 3:06PM - 3:18PM |
J27.00002: Observation of optical quantum measurement backaction on a mechanical resonator Thomas Purdy, Robert Peterson, Pen-Li Yu, Cindy Regal Quantum mechanics provides an inextricable link between measurement and backaction on the subsequent dynamics of a system. Here we continuously monitor the position of a membrane microresonator in a cavity optomechanical system. We observe a fluctuating backaction force on the resonator which rises with measurement strength in accordance with the minimum allowed by the Heisenberg position-momentum uncertainty limit. For our optically-based position measurement the backaction takes the form of a fluctuating radiation pressure due to optical shot noise. We demonstrate radiation pressure shot noise that is comparable to in magnitude to thermal fluctuations at frequencies near the mechanical resonance. Additionally, we observe temporal correlations between fluctuations in the radiation force and resonator position, which we interpret as a non-demolition measurement of the intracavity photon field fluctuations. We will also discuss possible methods to lower the technical noise floor in all measurement quadratures. [Preview Abstract] |
Tuesday, March 19, 2013 3:18PM - 3:30PM |
J27.00003: Quantum optics experiments with micromechanical oscillators Simon Groeblacher, Amir Safavi-Naeini, Jeff Hill, Jasper Chan, Oskar Painter Mechanical oscillators coupled to optical fields via the radiation pressure force have been of great interest lately as they allow for quantum experiments with macroscopic systems. Recent experiments have shown ground-state preparation and measurement of such resonators via sideband-resolved laser cooling. We will discuss our recent work that aims at achieving quantum control over nanoscale optomechanical crystal devices, both using strong coherent optical beams as well as single photons. [Preview Abstract] |
Tuesday, March 19, 2013 3:30PM - 3:42PM |
J27.00004: Optomechanical Coupling Between Membrane Modes Alexey B. Shkarin, Nathan E. Flowers-Jacobs, Scott W. Hoch, Christian Deutsch, Jakob Reichel, Jack G.E. Harris In an optomechanical device, radiation pressure couples optical power to mechanical motion. While typical experiments couple a single optical cavity to a single mechanical resonance, there has been increasing theoretical and experimental interest in multi-mode systems where there is coupling between multiple mechanical resonances and/or multiple optical cavity modes. We report on a device consisting of a dielectric SiN membrane located inside a high finesse fiber-cavity, where two nearly-degenerate mechanical modes couple to a single cavity mode. We observe that the original mechanical modes can experience a large coupling that is mediated by intracavity field. This causes the mechanical eigenmodes of the system to depend strongly on the radiation pressure and change from the original mechanical modes to a symmetric and antisymmetric combination of the original modes. The symmetric/antisymmetric modes are also known as ``dark'' and ``bright'' modes, as they have very different coupling to the cavity. In the quantum regime, this effective interaction between mechanical modes would open up the possibility of state transfer between multiple mechanical modes. [Preview Abstract] |
Tuesday, March 19, 2013 3:42PM - 3:54PM |
J27.00005: Gain-enhanced optical cooling in cavity optomechanics Li Ge, Sanli Faez, Florian Marquardt, Hakan Tureci We study the optical cooling of the mechanical motion of the resonator mirror in a cavity-optomechanical system that contains an optical gain medium. We find that the optical damping caused by radiation pressure force is vanishingly small if the active medium is pumped incoherently above its lasing threshold. In addition, we find that the spontaneous emission of the active medium always tends to increase the final effective temperature of the mechanical motion. In the presence of an additional seeding signal, i.e. a coherent drive of fixed frequency within the width of the gain curve however, we find that the cooling rate can be enhanced significantly with respect to that of a passive cavity. We attribute this effect to a reduced effective optical damping in the presence of incoherent pumping. [Preview Abstract] |
Tuesday, March 19, 2013 3:54PM - 4:06PM |
J27.00006: Novel cooling mechanisms in optomechanical systems Juan Restrepo, Ivan Favero, Cristiano Ciuti We present here our theoretical work on unconventional cooling mechanisms in optomechanical systems. In particular our classical and quantum theory of photothermal cooling [1] and our more recent work on cooling of a mechanical oscillator in cavity QED systems [2].\\[4pt] [1]J. Restrepo, J. Gabelli, C. Ciuti and I. Favero, Comptes Rendus Physique,12, 860-870 (2011). doi:10.1016/j.crhy.2011.02.005 (arXiv:1011.3911)\\[0pt] [2] J. Restrepo, I. Favero, C. Ciuti. in preparation. [Preview Abstract] |
Tuesday, March 19, 2013 4:06PM - 4:18PM |
J27.00007: Optical measurement of the thermal motion of a micromechanical resonator and its modal interaction by sideband actuation scheme Sungwan Cho, Myung Rae Cho, Sung Un Cho, Sang Goon Kim, Seung Bo Shim, Yun Daniel Park We present measurement of the thermal motion of a micromechanical resonator and excitation of flexural mode by sideband actuation. Doubly-clamped micromechanical resonators are fabricated from high-stress silicon nitride on SiO2/Si substrate and patterned with standard e-beam lithographic techniques. Optical measurement of resonant response of micromechanical resonator reveals its fundamental flexural mode of thermal motion at approximately 3.4 MHz ($f_{o})$ with quality factor up to 180,000 and higher modes at room temperature in moderate vacuum. With fundamental and higher flexural modes of thermal motion and sideband actuation scheme, we also observe amplitude increase in flexural mode of thermal motion with blue-detuned sideband pumping. [Preview Abstract] |
Tuesday, March 19, 2013 4:18PM - 4:30PM |
J27.00008: Optomechanics in a Fiber-Cavity Nathan E. Flowers-Jacobs, Scott W. Hoch, Alexey B. Shkarin, Jack C. Sankey, Anna Kashkanova, Andrew M. Jayich, Christian Deutsch, Jakob Reichel, Jack G.E. Harris In an optical displacement measurement, the quantum back-action is radiation pressure shot noise (RPSN), which is the Poissonian noise in the momentum transferred by reflecting photons. In an attempt to measure RPSN at room temperature, we have made an optomechanical device consisting of a fiber-based optical cavity containing a silicon nitiride membrane. In comparison with typical free-space cavities, the fiber-cavity's small mode size (10 micron waist, 60 micron length) allows the use of smaller, lighter membranes and increases the cavity-membrane linear coupling to 3 GHz/nm. This device is also intrinsically fiber-coupled and uses v-grooves for passive alignment; these improvements greatly simplify the use of optomechanical devices. Based on the parameters demonstrated by this device, we expect it to be able to detect RPSN at room temperature. The increased coupling in this system also makes it an excellent testbed for investigating optomechanical coupling between mechanical modes, and for demonstrating quadratic coupling between a single mechanical mode and the cavity. [Preview Abstract] |
Tuesday, March 19, 2013 4:30PM - 4:42PM |
J27.00009: Robust entanglement via optomechanical dark mode: adiabatic scheme Lin Tian, Ying-Dan Wang, Sumei Huang, Aashish Clerk Entanglement is a powerful resource for studying quantum effects in macroscopic objects and for quantum information processing. Here, we show that robust entanglement between cavity modes with distinct frequencies can be generated via a mechanical dark mode in an optomechanical quantum interface. Due to quantum interference, the effect of the mechanical noise is cancelled in a way that is similar to the electromagnetically induced transparency. We derive the entanglement in the strong coupling regime by solving the quantum Langevin equation using a perturbation theory approach. The entanglement in the adiabatic scheme is then compared with the entanglement in the stationary state scheme. Given the robust entanglement schemes and our previous schemes on quantum wave length conversion, the optomechanical interface hence forms an effective building block for a quantum network. [Preview Abstract] |
Tuesday, March 19, 2013 4:42PM - 4:54PM |
J27.00010: Development of a dispersive read-out technique for quantum measurements of nanomechanical resonators Francisco Rouxinol, Matthew LaHaye, Hugo Hao, Seung-Bo Shim Over the last decade, there has been an active effort to prepare and measure mechanical structures in the quantum regime for the purpose of sensing weak forces and for studying fundamental topics in quantum mechanics such as quantum measurement, entanglement and decoherence in new macroscopic limits. One promsing tool for such studies is the qubit-coupled mechanical resonator. In this work we discuss some of our first results towards the development of a nanoelectromechanical system that integrates a charge-type superconducting qubit as a detector to probe the number-states of a nanomechanical mode. In our system the qubit-coupled nanoresonator is embedded in a superconducting microwave resonator (SMR); the SMR then serves to perform spectroscopic measurements of the qubit to infer the number-state statistics of the nanoresonator in a manner analogous to dispersive measurement techniques used in circuit and cavity QED to probe the number-states of electromagnetic cavities. We will discuss the design and measurement of our latest generation devices and the prospects for achieving single-phonon measurement resolution with this system. [Preview Abstract] |
Tuesday, March 19, 2013 4:54PM - 5:06PM |
J27.00011: Fabricating Micro-Optomechanical Resonators Using High-Stress Si$_{3}$N$_{4}$ Brian Pepper, Petro Sonin, Dirk Bouwmeester Optomechanical systems have been highly researched as a platform for testing macroscopic quantum effects and quantum decoherence. However, the required optical and mechanical properties are difficult to achieve. Increasing the tensile stress of a device is known to correlate with higher mechanical frequency and quality factor. We discuss fabrication of monolithic optomechanical devices using dielectric mirrors and high-stress stoichiometric Si$_{3}$N$_{4}$. We also present preliminary data on their mechanical and optical properties. [Preview Abstract] |
Tuesday, March 19, 2013 5:06PM - 5:18PM |
J27.00012: Coherent optical wavelength conversion via cavity-optomechanics Jeff Hill, Amir Safavi-Naeini, Jasper Chan, Oskar Painter In this talk we theoretically propose and experimentally demonstrate coherent wavelength conversion of optical photons using photon-phonon translation in a cavity-optomechanical system. Our system is an engineered silicon optomechanical crystal nanocavity supporting a $4$~GHz localized phonon mode, optical signals in a $1.5$~MHz bandwidth are coherently converted over a $11.2$~THz frequency span between one cavity mode at wavelength $1460$~nm and a second cavity mode at $1545$~nm with a 93\% internal (2\% external) peak efficiency. The thermal and quantum limiting noise involved in the conversion process is also analyzed, and in terms of an equivalent photon number signal level are found to correspond to an internal noise level of only $6$ and $4 \times 10^{-3}$ quanta, respectively [1].\\[4pt] [1] J.~T.~Hill, A.~H.~Safavi-Naeini, J.~Chan, O.~Painter, arXiv:1206.0704 (2012). [Preview Abstract] |
Tuesday, March 19, 2013 5:18PM - 5:30PM |
J27.00013: Fast readout of carbon nanotube mechanical resonators Harold Meerwaldt, Vibhor Singh, Ben Schneider, Raymond Schouten, Herre van der Zant, Gary Steele We perform fast readout measurements of carbon nanotube mechanical resonators. Using an electronic mixing scheme, we can detect the amplitude of the mechanical motion with an intermediate frequency (IF) of 46 MHz and a timeconstant of 1 us, up to 5 orders of magnitude faster than before. Previous measurements suffered from a low bandwidth due to the combination of the high resistance of the carbon nanotube and a large stray capacitance. We have increased the bandwidth significantly by using a high-impedance, close-proximity HEMT amplifier. The increased bandwidth should allow us to observe the nanotube's thermal motion and its transient response, approaching the regime of real-time detection of the carbon nanotube's mechanical motion. [Preview Abstract] |
Session J28: Liquid Crystals I
Sponsoring Units: DCMPChair: Peter Collins, Swarthmore College
Room: 336
Tuesday, March 19, 2013 2:30PM - 2:42PM |
J28.00001: 2D Brownian motion of inclusions in low pressure environment on freely suspended liquid crystal film Zhiyuan Qi, Cheol Park, Joseph Maclennan, Matthew Glaser, Noel Clark The homogeneous freely suspended fluid SmA liquid crystal film of several nanometer thickness provides a very good system for studying 2D hydrodynamics. Using microscope and high-speed camera, we track the motion of inclusions of about 2-20$\mu$m in diameter that doing Brownian motion on the film. We report 2D Brownian motion experiment of drops in different air pressure environment. We found that at ambient pressure, the Hughes, Pailthorpe, and White (HPW) theory can perfectly predict the diffusion coefficient of those inclusions, while under low pressure when the mean free path of the air molecules is comparable with the size of inclusions, the HPW theory fails. We propose a model, based on freely diffused air molecules with Maxwell distribution, to explain the elevated diffusion coefficient in low pressure. [Preview Abstract] |
Tuesday, March 19, 2013 2:42PM - 2:54PM |
J28.00002: Between soap bubbles and vesicles: The dynamics of freely floating smectic bubbles Ralf Stannarius, Kathrin May, Kirsten Harth, Torsten Trittel The dynamics of droplets and bubbles, particularly on microscopic scales, are of considerable importance in biological, environmental, and technical contexts. We introduce freely floating bubbles of smectic liquid crystals and report their unique dynamic properties. Smectic bubbles can be used as simple models for dynamic studies of fluid membranes. In equilibrium, they form minimal surfaces like soap films. However, shape transformations of closed smectic membranes that change the surface area involve the formation and motion of molecular layer dislocations. These processes are slow compared to the capillary wave dynamics, therefore the effective surface tension is zero like in vesicles. Freely floating smectic bubbles are prepared from collapsing catenoid films and their dynamics is studied with optical high-speed imaging [1]. Experiments are performed under normal gravity and in microgravity during parabolic flights.\\[4pt] [1] K. May et al. {\em EPL} {\bf 100} 16003 (2012). [Preview Abstract] |
Tuesday, March 19, 2013 2:54PM - 3:06PM |
J28.00003: Critical Behavior of A Non-polar Smectic Liquid Crystal via Optical Birefringence Measurements Mehmet Can Cetinkaya, Selen Erkan, Sevtap Yildiz, Haluk Ozbek We present high sensitivity and high temperature resolution experimental data on the temperature dependence of the optical birefringence in the nematic and smectic A phases of nonpolar monolayer smectogen 4-butyloxyphenyl-4'-decyloxybenzoate liquid crystal by using a rotating-analyzer technique. We have used the birefringence data to probe the temperature behavior of the nematic order parameter S(T) in the vicinity of both the nematic-isotropic (N-I) and the nematic-smectic A (N-SmA) transitions. The critical behavior of S(T) at the N-I transition has been discussed in detail by comparing our results with the latest reports in literature and we have then concluded that the isotropic internal field assumption by Vuks model is adequate to extract the critical behavior of S(T) from the birefringence data [1-3]. We have tested the validity of the scaling relation $\lambda =$1-$\alpha $ between the critical exponent $\lambda $ describing the limiting behavior of the nematic order parameter and the specific heat capacity exponent $\alpha $. We have shown that the temperature derivative of the nematic order parameter S(T) near the N-SmA transition has the same power law behavior as the specific heat capacity [4,5]. [Preview Abstract] |
Tuesday, March 19, 2013 3:06PM - 3:18PM |
J28.00004: Statistical mechanics of bend flexoelectricity and the twist-bend phase in bent-core liquid crystals Shaikh Shamid, Subas Dhakal, Jonathan Selinger We develop a Landau theory for bend flexoelectricity in a liquid crystals of bent-core molecules. In the nematic phase of the model, the bend flexoelectric coefficient increases as we reduce the temperature, and it diverges at the nematic to polar phase transition. At this critical point, there is a second order transition from high-temperature uniform nematic phase to low-temperature nonuniform polar phase composed of twist-bend or splay-bend deformations. To test the predictions of Landau theory, we perform Monte Carlo simulations to find the behavior as a function of temperature, applied electric field and interaction parameters, and to determine the orientational distribution of the mesogenic molecules. [Preview Abstract] |
Tuesday, March 19, 2013 3:18PM - 3:30PM |
J28.00005: Imaging helical nano-filament and modulated smectic phases of bent shaped liquid crystals by cryo-TEM Cuiyu Zhang, Hans Sawade, Wolfgang Weissflog, Antal Jakli Recently we showed that cryo-TEM can be used to visualize smectic layers of thermotropic liquid crystals. Here we describe cryo-TEM studies of the nanofilaments (B4 phase) and the modulated smectic layers (B7 phase ) of various bent shaped liquid crystal compounds. In the B4 phase a periodic array of about 15 nm wide bands of parallel stripes, separated by a distance equal to the layer spacing, appear with a periodicity of about 120 nm corresponding to the half pitch of the nanofilaments. As cryo-TEM shows only layers that are parallel to the electron beam, these results indicate grains of straight layers twisted along the filament axis compose the nano-filaments. In the B7 phase cryo-TEM not only can visualize the smectic layers, but also the periodic modulation indicating defects with less dense molecular packing. In addition we observe a labyrinth structure with curvature radii in the 150 nm ranges. These results yield information complementary to freeze fracture TEM and X-ray observations. [Preview Abstract] |
Tuesday, March 19, 2013 3:30PM - 3:42PM |
J28.00006: Wide temperature range and hysteresis free blue phase liquid crystals doped with bent-core compound Jie Xiang, Oleg Twieg, Oleg Lavrentovich We explore an approach to widen the temperature range of the liquid crystalline blue phases based on mixtures of calamitic (rod-like) and bent-core mesogens. The calamitic component has a relatively low value of the bend elastic constant that is further reduced by adding the bent-core component. The mixtures exhibit the blue phase state in a wide temperature range, about 5$^{\circ}$C in the regime of heating and 40$^{\circ}$C (including the room temperature) upon cooling. We present a phenomenological model to illustrate the link between the temperature range of the blue phase and the bend elastic constant that is based on Kleman's model of double twist in liquid crystals. We also study the electro-optic properties of the mixtures. The electrooptic switching is reversible in the upper temperature range of the blue phase, but once the temperature decreases below a certain level, the electrooptic switching shows a hysteresis associated with phase separation of the components. The work was supported by NSF grant DMR 11212878. [Preview Abstract] |
Tuesday, March 19, 2013 3:42PM - 3:54PM |
J28.00007: Piezoelectric properties of polymers containing bent-shape liquid crystal molecules N. Diorio, M. Varga, A. Carif, J.E. Puskas, K. Fodor-Csorba, S. Sprunt, J.T. Gleeson, A. Jakli Recently, bent-core liquid crystal elastomers have shown to exhibit large values of flexoelectricity as many as 3 orders of magnitude larger than liquid crystal elastomers containing rod-shaped molecules. These unusual high responses are attributed to have piezoelectric origin. Motivated by this, in this study, two bent-core liquid crystals were used to make various types of materials; low molecular weight bent-core nematic fluid, side chain bent-core liquid crystal polymer, low molecular liquid crystal dispersed in a polyisobutylene-based thermoplastic elastomer, and side-chain bent-core elastomers. Liquid crystal elastomers combine elasticity and flexibility inherent to rubbers and the optical and electrical properties of liquid crystals, and are promising materials for applications such as electro-optics, flexible electronics and actuator technologies for biomedical applications. Most conventional liquid crystal elastomers have rod-shaped liquid crystal molecules chemically attached to a crosslinked polymer network. Converse piezoelectric responses were measured by a Mirau interferometer and the direct piezoelectric signals were studied by home-made device where the stress is provided by an audio speaker. The results will be analyzed in terms of ferroelectric clusters of the materials in the nematic phase and will be compared with other piezoelectric materials. [Preview Abstract] |
Tuesday, March 19, 2013 3:54PM - 4:06PM |
J28.00008: SAXS studies of short-range order in the nematic phase of reduced symmetry mesogens S. Chakraborty, N. Diorio, C. Zhang, R. Breckon, R. Twieg, J. Gleeson, A. Jakli, S. Sprunt Recently, we proposed a model based on persistent, nano-scale smectic-C-like domains (``smectic clusters'') to explain the features present in the small angle x-ray diffraction patterns from certain bent-core nematic liquid crystals (which do not possess an underlying smectic phase). We report on new results from a wider range of nematics formed by reduced-symmetry molecules -- including laterally-branched (``Y''-shaped) mesogens and ``H'' shaped dimers -- that also lack a low temperature smectic phase. We find that our model, extended to incorporate the notion of staggered molecular arrangements, is successful in reproducing the SAXS patterns and reveals variation in the temperature-dependence of cluster size among different systems. Supported by NSF DMR-0964765. [Preview Abstract] |
Tuesday, March 19, 2013 4:06PM - 4:18PM |
J28.00009: Local orientation and temperature effects of a liquid crystal in contact with a nanoparticle Jefferson Ward Taylor, Lynn K. Kurihara, Luz J. Martinez-Miranda We have studied the effects on the orientation of the liquid crystal in the immediate vicinity of a nanoparticle. We have observed a ``halo'' surrounding the nanoparticle, when studying the effects of the nanoparticle on the liquid crystal with the AFM. We believe this halo has an effect on the ordering of the liquid crystal in the immediate vicinity of the nanoparticle. We have also observed a short range order peak in the X-ray scattering signal, which is also associated with the effects on the liquid crystal in the immediate vicinity of the nanoparticle. The value of the coherence length of this peak is close to the value of the molecular spacing or very close of the liquid crystal in the X-ray scattering experiment for all nanocomposites studied. This coherence length does not change as a function of temperature, when the temperature is changed and goes through the SmA-nematic transition temperature. The peak and its coherence length persist into the nematic phase. [Preview Abstract] |
Tuesday, March 19, 2013 4:18PM - 4:30PM |
J28.00010: Electro-optical Characteristics of Carbon Nanotupe Doped Polar Smectic Liquid Crystal Ilknur Koseoglu, Mehmet Can Cetinkaya, Haluk Ozbek, Sevtap Yildiz We present the results of electro-optical characteristics of the liquid crystal octyl-cyanobiphenyl (8CB) doped with well-dispersed multiwall carbon nanotubes (MWCNT) under an AC driving voltage. 8CB-MWCNT composites were prepared by following the procedures in literature [1-4]. Polarized optical microscopy (POM) has been performed to check the homogenous dispersion of 8CB-MWCNT composite. We compare threshold voltages and switching behavior of pure 8CB and 8CB doped with MWCNTs which have surfaces of untreated and treated with carboxyl functional group. Threshold voltages have been determined from optical transmittance-driving voltage curves at various temperatures. While the pure 8CB switches from a bright state through some intensity oscillations to the dark state, a drastic change has been observed in the transmittance curves for 8CB-MWCNT composites, namely hysteretic behavior has been detected. For 8CB-MWCNT composites we have observed that the first cycle for the transmittance-voltage curves shows the highest amount of loop area, which gradually decreases through the following cycles, then reaching saturation. Notice that the number cycle at which the saturation is reached depend on temperature [5]. [Preview Abstract] |
Tuesday, March 19, 2013 4:30PM - 4:42PM |
J28.00011: Effect of quantum dots on the isotropic to nematic and nematic to smectic-A phase transitions in nano composites Parvathalu Kalakonda, Germano S. Iannacchione Modulated Differential Scanning Calorimetry (MDSC) is used to investigate the weakly first-order isotropic to nematic ($I$-$N$) and the continuous nemat to smectic-$A$ ($N$-Sm$A$) phase transitions of the liquid crystal octylcyanobiphenyl (8CB) doped with well-dispersed quantum dots (QdS) as a function of Qd concentrations. Thermal scans were performed for all samples having Qd (CdS) weight percent from $\phi_{w}$ = 0.3 to 3 wt$\% $ first on cooling and then heating under near-equilibrium conditions. The I-N transitions heat capacity peak first glows then decreases in magnitude with increasing $\phi_{w}$ leaving a maximum at $\phi_{w}$ = 0.3$\% $. The N-SmA heat capacity peak remains bulk-like for all samples. Both transitions temperatures shift lower monotonically by 3 K for $\phi_{w}$ = 0.3$\% $. The enthalpy of both transitions evolve in a nontrival way, generally decreasing with increasing $\phi_{w}$. These results are discussed in terms of the predominate disordering effects of the Qds. [Preview Abstract] |
Tuesday, March 19, 2013 4:42PM - 4:54PM |
J28.00012: Towards an optical nano-laboratory in a liquid crystal defect Paul Ackerman, Ivan Smalyukh, Jao van de Lagemaat Probing photonic effects due to nanoscale interactions between colloids such as quantum dots and rods and anisotropic plasmonic metal nanoparticles is of great interest for applications in third-generation solar cells, optical metamaterials, and nanoantennas. Liquid crystal (LC) structures and defects stabilized by chirality, confinement, and/or presence of colloidal microparticles can enable trapping and well-defined alignment of anisotropic semiconductor, plasmonic, and other nanoparticles with respect to the far-field director and each other. Minimization of the free energy due to LC defects provides a rich environment for precisely controlled experiments with individual and small groups of nanoparticles in the LC. This presentation will discuss characterization of trapping and alignment of various nanoparticles by LC defects and also photonic experiments performed on a single-particle level for metal and semiconductor quantum nanoparticles entrapped by these defects. This work was supported by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the US Department of Energy under Contract No. DE-AC36-08GO28308 with the National Renewable Energy Laboratory (J.v.d.L. and J.S.E.). [Preview Abstract] |
Tuesday, March 19, 2013 4:54PM - 5:06PM |
J28.00013: Interaction of discotics and nanoparticles Luz J. Martinez-Miranda, Eduardo A. Soto-Bustamante We mixed a discotic, and 5 nm nanoparticles of ZnO up to a percentage weight of 30 -- 35{\%}, by heating them together, past the isotropic transition temperature. At that point, we mixed them together, and allowed them to cool to room temperature. We then prepared a sample for Xray study, by taking a small amount of the crystallites formed and placing them in a glass slide. We prepared a sample of the pure discotic to compare to the mixture. We found that the addition of the nanoparticle results in an enhancement of the axis in the direction parallel to the glass slide, with an intensity approximately six times that of the discotic alone and a correlation length approximately 1.3 times better. The role of the nanoparticle isvery similar to the alignment role of a flat surface observed on discotics. [Preview Abstract] |
Tuesday, March 19, 2013 5:06PM - 5:18PM |
J28.00014: High resolution synchrotron X-ray studies of lyotropic liquid crystal phases of monolayer Zirconium Phosphate nanosheet Yue Shi, Yongqiang Shen, Noel Clark, Min Shuai, Zhengdong cheng Aqueous suspensions of monolayer zirconium phosphate nanosheets (ZrP-NS) form various lyotropic liquid crystal phases. An interesting stripe pattern can be observed in a range of nanosheet concentrations when the suspensions were confined between flat surfaces. The stripe patterns were stable while slow evaporation of the solvent and were well-preserved even when the suspensions dried out. A high resolution synchrotron X-ray study gives detailed investigations of ZrPNS lyotropic phases at different concentrations. [Preview Abstract] |
Session J29: Non-Equilibrium Statistical Mechanics
Sponsoring Units: GSNPChair: Michel Pleimling, Virginia Polytechnic Institute and State University
Room: 337
Tuesday, March 19, 2013 2:30PM - 2:42PM |
J29.00001: Three-dimensional Potts systems with magnetic friction Linjun Li, Michel Pleimling Using extensive Monte Carlo simulations we study the properties of the non-equilibrium phase transition encountered in driven three-dimensional Potts systems with magnetic friction. Our system consists of two three-dimensional blocks, coupled through boundary spins, that move along their boundaries with a constant relative velocity. Changing the number of states in the system from two (Ising case) to nine states, we find different scenarios for the surface behavior depending on whether the bulk transition is continuous or discontinuous. In order to fully assess the properties of this non-equilibrium phase transition, we vary systematically the strength of the coupling between the two blocks as well as the value of the relative velocity. For strong couplings between the blocks the phase transition is found to be strongly anisotropic. [Preview Abstract] |
Tuesday, March 19, 2013 2:42PM - 2:54PM |
J29.00002: Non-equilibrium steady states in two-temperature Ising models with Kawasaki dynamics Nick Borchers, Michel Pleimling, R.K.P. Zia From complex biological systems to a simple simmering pot, thermodynamic systems held out of equilibrium are exceedingly common in nature. Despite this, a general theory to describe these types of phenomena remains elusive. In this talk, we explore a simple modification of the venerable Ising model in hopes of shedding some light on these issues. In both one and two dimensions, systems attached to two distinct heat reservoirs exhibit many of the hallmarks of phase transition. When such systems settle into a non-equilibrium steady-state they exhibit numerous interesting phenomena, including an unexpected ``freezing by heating.'' There are striking and surprising similarities between the behavior of these systems in one and two dimensions, but also intriguing differences. These phenomena will be explored and possible approaches to understanding the behavior will be suggested. [Preview Abstract] |
Tuesday, March 19, 2013 2:54PM - 3:06PM |
J29.00003: Aging processes in systems with anomalous slow dynamics Nasrin Afzal, Michel Pleimling Recent studies of coarsening in disordered systems show a crossover from an initial, transient, power-law domain growth to a slower logarithmic growth. Due to the anomalous slow dynamics, numerical simulations are usually not able to fully enter the asymptotic regime when investigating the relaxation of these systems toward equilibrium. In order to gain some new insights into the non-equilibrium properties of systems with logarithmic growth, we study two simple driven systems, the one-dimensional ABC-model and a related domain model with simplified dynamics, where the asymptotic regime can be accessed. Studying two-times correlation and response functions, we focus on aging processes and dynamical scaling during logarithmic growth. [Preview Abstract] |
Tuesday, March 19, 2013 3:06PM - 3:18PM |
J29.00004: Random Fields at a Nonequilibrium Phase Transition Hatem Barghathi, Thomas Vojta We study nonequilibrium phase transitions in the presence of disorder that locally breaks the symmetry between two equivalent macroscopic states. In low-dimensional equilibrium systems, such random-field disorder is known to have dramatic effects: it prevents spontaneous symmetry breaking and completely destroys the phase transition. In contrast, we show that the phase transition of the one-dimensional generalized contact process persists in the presence of random-field disorder. The ultraslow dynamics in the symmetry-broken phase is described by a Sinai walk of the domain walls between two different absorbing states. We discuss the generality and limitations of our theory, and we illustrate our results by large-scale Monte Carlo simulations. [Preview Abstract] |
Tuesday, March 19, 2013 3:18PM - 3:30PM |
J29.00005: Monte-Carlo simulations of the clean and disordered contact process in three space dimensions Thomas Vojta The absorbing-state transition in the three-dimensional contact process with and without quenched randomness is investigated by means of Monte-Carlo simulations. In the clean case, a reweighting technique is combined with a careful extrapolation of the data to infinite time to determine with high accuracy the critical behavior in the three-dimensional directed percolation universality class. In the presence of quenched spatial disorder, our data demonstrate that the absorbing-state transition is governed by an unconventional infinite-randomness critical point featuring activated dynamical scaling. The critical behavior of this transition does not depend on the disorder strength, i.e., it is universal. Close to the disordered critical point, the dynamics is characterized by the nonuniversal power laws typical of a Griffiths phase. We compare our findings to the results of other numerical methods, and we relate them to a general classification of phase transitions in disordered systems based on the rare region dimensionality. This work has been supported in part by the NSF under grants no. DMR-0906566 and DMR-1205803. [Preview Abstract] |
Tuesday, March 19, 2013 3:30PM - 3:42PM |
J29.00006: Fluctuation Effects in the Pair Annihilation Process with Levy Dynamics Ingo Homrighausen, Anton Winkler, Erwin Frey Reaction diffusion models provide a plethora of intensively studied classical nonequilibrium many body systems. One example is the diffusion limited pair annihilation process $A+A\to 0$, where the reactants diffuse in space and annihilate on contact. Inspired by the fact that many phenomena observed in nature exhibit superdiffusive behavior, we investigate the pair annihilation process in the case where the particles perform superdiffusion, realized by Levy flights. As a consequence, the critical dimension depends continuously on the control parameter of the Levy flight distribution. This instance is used to study the density decay in the pair annihilation process close to the critical dimension by means of the non-perturbative renormalization group theory. Close to the critical dimension, long-range fluctuations cause the law of mass action to break down. One crucial consequence of these fluctuations is that the law of mass action is complemented by additional non-analytic correction terms above the critical dimension. An increasing number of those corrections accumulate and give an essential contribution as the critical dimension is approached. [Preview Abstract] |
Tuesday, March 19, 2013 3:42PM - 3:54PM |
J29.00007: Directed polymers in random media with short-range correlated disorder Vivien Lecomte, Elisabeth Agoritsas, Thierry Giamarchi One-dimensional boundary interfaces between different phases are described at macroscopic scales by a rough fluctuating line, whose geometrical properties are dictated by the disorder in the underlying medium, by the temperature of the environment, and by the elastic properties of the line. A widely used and successful model is the directed polymer in a random medium, pertaining to the Kardar-Parisi-Zhang (KPZ) universality class. Much is known for this continuous model when the disorder is uncorrelated, and it has allowed to understand the static and dynamical features of experimental systems ranging from magnetic interfaces to liquid crystals. We show that short-range correlations in the disorder at a scale $\xi>0$ modify the uncorrelated (i.e. zero $\xi$) picture in a non-obvious way. If the geometrical fluctuations are still described by the celebrated 2/3 KPZ exponent, characteristic amplitudes are however modified even at scales much larger than $\xi$, in a well-controlled and rather universal manner. Our results are also relevant to describe the slow (so called `creep') motion of interfaces in random media, and more formally (trough replicae) one-dimensional gases of bosons interacting with softened delta potential. [Preview Abstract] |
Tuesday, March 19, 2013 3:54PM - 4:06PM |
J29.00008: Novel phases in an accelerated exclusion process Jiajia Dong, Stefan Klumpp, Royce K.P. Zia We introduce a class of distance-dependent interactions in an accelerated exclusion process (AEP) inspired by the cooperative speed-up observed in transcribing RNA polymerases. In the simplest scenario, each particle hops to the neighboring site if vacant \emph{and} when joining a cluster of particles, triggers the frontmost particle to hop. Through both simulation and theoretical work, we discover that the steady state of AEP displays a discontinuous transition with periodic boundary condition. The system transitions from being homogeneous (with augmented currents) to phase-segregated. More surprisingly, the current-density relation in the phase-segregated state is simply $J=1-\rho$, indicating the particles (or holes) are moving at unit velocity despite the inclusion of long-range interactions. [Preview Abstract] |
Tuesday, March 19, 2013 4:06PM - 4:18PM |
J29.00009: ABSTRACT WITHDRAWN |
Tuesday, March 19, 2013 4:18PM - 4:30PM |
J29.00010: Intrinsically Localized Modes in the three-dimensional Quantal Fermi-Pasta-Ulam Lattice Derya Kanbur, Peter S. Riseborough Intrinsically Localized Modes (ILMs) are spatially localized oscillatory modes in homogeneous lattices, that are stabilized by anharmonic interactions. ILMs are frequently found in classical low-dimensional systems, where the frequency of the oscillations is a continuous variable. By contrast, due to the internal frequencies quantized,the quantum systems support a hierarch of excitations. The hierarchy of quantal excitations can be described in terms of a hierarchy of bound states of a multiple numbers of phonons. In one-dimension, the existence of the ILMs is ensured for any strength of the repulsive interactions by the divergent van-Hove singularities in the multi-phonon density of states. Inelastic neutron scattering measurements on NaI have revealed unexpected excitations which have been interpreted in terms of ILMs. Since the energies of the observed excitations are discrete, the experiments indicate that the ILMs have quantum character. Therefore, we search for low-energy quantized ILMs in a three-dimensional generalization of the Fermi-Pasta-Ulam lattice. We find that quantized ILMs may exist for values of the interaction strengths which exceeds a critical value. We examine the polarization-dependence, dispersion and the spatial characteristics of the lowest-energy ILMs. [Preview Abstract] |
Tuesday, March 19, 2013 4:30PM - 4:42PM |
J29.00011: Dynamics of Linked and Knotted Vortices Dustin Kleckner, Martin Scheeler, William T. M. Irvine Recently developed experimental methods have allowed us to generate topologically linked fluid vortices for the first time. The intrinsically geometric nature of vortex dynamics allows us to measure physical quantities, such as energy, by reconstructing the core centerline in three-dimensions using high-speed laser scanning tomography. This novel approach offers insights into the evolution of linked and knotted vortices up to and through changes in topology. [Preview Abstract] |
Tuesday, March 19, 2013 4:42PM - 4:54PM |
J29.00012: Thermally Activated Avalanches in Twinned Crystals Ido Regev, Xiangdong Ding, Turab Lookman In previous work it was shown that the power-spectrum of the energy release in a twinned crystal under deformation, exhibits a transition from a low-temperature power-law to a high temperature activated dynamics. In this work we provide a statistical mechanics explanation to this behavior based on the understanding that the origin of the power-law behavior stems from a pattern of vertical twins that forms at the onset of yield, and serves as pinning sites to the motion of the (horizontal) twins. The transition to activated behavior is explained by a master equation based on a ``trap model.'' [Preview Abstract] |
Tuesday, March 19, 2013 4:54PM - 5:06PM |
J29.00013: Exploring the scaling laws of crystal plasticity with a Phase Field Crystal model Georgios Tsekenis, Thomas Fehm, Pak Yuen Chan, Jonathan Uhl, Jonathan Dantzig, Nigel Goldenfeld, Karin Dahmen A wealth of experiments and simulations the last years has cemented the fact that crystalline materials deform in an intermittent way with slip-avalanches that are power-law distributed. Recently we showed that zero temperature discrete dislocation dynamics simulations predict mean field scaling exponents for both static and dynamic critical exponents. To model a wider range of experimental observations and predict the dependence on experimental parameters that are not captured by discrete dislocation dynamics we work with a Phase Field Crystal (PFC) model in two dimensions. The PFC model has the advantage that it does not require any ad hoc assumptions about the dislocation interaction or their creation and annihilation. It also models the dislocation dynamics at finite temperature. We extract the avalanche distributions and show that they scale according to the Mean Field Depinning universality class even though there is no quenched disorder. [Preview Abstract] |
Tuesday, March 19, 2013 5:06PM - 5:18PM |
J29.00014: Intermittency in brittle cracks: Model experiment in artificial rocks Jonathan Bares, Daniel Bonamy, Davy Dalmas, Lamine Hattali Continuum theory fails to account for disorder effect on the crack propagation in brittle heterogeneous materials: It can explain neither the crackling dynamics, nor the statistics of the macro-scale mechanical observables. In this context, some tools issued from out-of-equilibrium statistical physics that identifies crack propagation onset with a depinning transition appear promising, but lack for quantitative comparisons with experiments. We designed a model experiment set up based on a material with tunable micro-scale (ceramics of sintered polymer beads) in which tensile cracks is grown over a wide range of speeds. Crack length, mechanical energy and acoustic emission (AE) are monitored with good resolution (ms for the first two, $\mu$s for AE) during the experiments. These measures were used (i) to provide information on the nature of the acoustic energy emitted during a breaking event, (ii) to unravel the relation between material toughness and relative system size. We believe our experiment to find applications in mechanical engineering, by helping to understand the microstructural disorder effect on the toughness properties. In statistical physics, it provides a model system to study collective complex crackling dynamics. Finally, in geophysics it help to interpret AE signal used to monitor the damage in Earth crust. [Preview Abstract] |
Session J30: Focus Session: Continuum Descriptions of Discrete Materials
Sponsoring Units: GSNPChair: Kenneth Kamrin, Massachusetts Institute of Technology
Room: 338
Tuesday, March 19, 2013 2:30PM - 2:42PM |
J30.00001: Homogenized Mechanical Behavior of Cross-Linked Fiber Networks Embedded in Matrix Catalin Picu, Lijuan Zhang, Ali Shahsavari Most biological and some biomimetic materials are made from fiber networks embedded in an elastic medium. The mechanical behavior of these composites depends in interesting ways on the elasticity of the matrix. In this work we study this issue using both 2D and 3D models, with the goal of deriving expressions linking microstructural parameters and the composite elastic properties. We show that the strong interaction between network and matrix precludes the use of linear superposition of effects and that the effective moduli are a complex function of the constituent moduli. The internal distribution of stresses is also studied and discussed in relation with failure mechanisms. [Preview Abstract] |
Tuesday, March 19, 2013 2:42PM - 2:54PM |
J30.00002: Size Effects in the Mechanical Behavior of Sparsely Cross-Linked Fiber Networks Ali Shahsavari, Catalin Picu Random fiber networks are structural elements in many biological and man-made materials and the prediction of their mechanical properties is desirable in many applications. In this work we first address the problem of the scale of homogeneity of these discrete systems, i.e. the size of the model above which the elastic response is model size- independent. Further, using models large enough to eliminate the size effect, we determine a structure-property relation for networks with variable concentration of cross-links. [Preview Abstract] |
Tuesday, March 19, 2013 2:54PM - 3:06PM |
J30.00003: Role of Inhomogeneity in Mechanochemically Active Polymers Meredith Silberstein Mechanically-induced reactivity is a promising means for designing self sensing and autonomous materials. Mechanically sensitive chemical groups termed mechanophores can be covalently linked into polymers in order to trigger specific chemical reactions upon mechanical loading. The mechanophore reaction kinetics, as determined by ab initio steered molecular dynamics, are exponential in force. As such the mechanochemical behavior of a solid-state polymer is highly sensitive to stress carried by that polymer, including local spatial and temporal fluctuations. Previously we developed microstructurally-based continuum models for fluorescence response in spiropyran-linked rubbery (poly methacrylate) and glassy (poly methylmethacrylate) polymers. The homogenization scheme in each relied on assigning mean effective forces acting on the mechanophores. Here we explore the theoretical influence of nanoscale spatial force distributions and fast temporal force fluctuations on the mechanochromic response of these systems. The effect of each is found to be significant and highly dependent on the intrinsic polymer mechanical behavior. [Preview Abstract] |
Tuesday, March 19, 2013 3:06PM - 3:42PM |
J30.00004: Marginal Matter Invited Speaker: Martin Van Hecke All around us, things are falling apart. The foam on our cappuccinos appears solid, but gentle stirring irreversibly changes its shape. Skin, a biological fiber network, is firm when you pinch it, but soft under light touch. Sand mimics a solid when we walk on the beach but a liquid when we pour it out of our shoes. Crucially, a marginal point separates the rigid or jammed state from the mechanical vacuum (freely flowing) state - at their marginal points, soft materials are neither solid nor liquid. Here I will show how the marginal point gives birth to a third sector of soft matter physics: intrinsically nonlinear mechanics. I will illustrate this with shock waves in weakly compressed granular media, the nonlinear rheology of foams, and the nonlinear mechanics of weakly connected elastic networks. [Preview Abstract] |
Tuesday, March 19, 2013 3:42PM - 3:54PM |
J30.00005: Capturing nonlocal effects in 2D granular flows Ken Kamrin, Georg Koval There is an industrial need, and a scientific desire, to produce a continuum model that can predict the flow of dense granular matter in an arbitrary geometry. A viscoplastic continuum approach, developed over recent years, has shown some ability to approximate steady flow and stress profiles in multiple inhomogeneous flow environments. However, the model incorrectly represents phenomena observed in the slow, creeping flow regime. As normalized flow-rate decreases, granular stresses are observed to become largely rate-independent and a dominating length-scale emerges in the mechanics. This talk attempts to account for these effects, in the simplified case of 2D, using the notion of nonlocal fluidity, which has proven successful in treating nonlocal effects in emulsions. The idea is to augment the local granular fluidity law with a diffusive second-order term scaled by the particle size, which spreads flowing zones accordingly. Below the yield stress, the local contribution vanishes and the fluidity becomes rate-independent, as we require. We implement the modified law in multiple geometries and validate its flow and stress predictions in multiple geometries compared against discrete particle simulations. In so doing, we demonstrate that the nonlocal relation proposed is satisfied universally in a seemingly geometry-independent fashion. [Preview Abstract] |
Tuesday, March 19, 2013 3:54PM - 4:06PM |
J30.00006: Predicting dense granular flows: continuum modeling with a length-scale David Henann, Ken Kamrin Dense granular materials display a complicated set of flow properties, which differentiate them from ordinary fluids. In particular, slowly-flowing granular media form clear, experimentally-robust features; most notably, shear bands, which can have a variety of possible widths and which decay non-trivially into the surrounding quasi-rigid material. Despite the ubiquity of granular flows, no model has been developed that captures or predicts these complexities, posing an obstacle in industrial and geophysical applications. We present a three-dimensional constitutive model for well-developed, dense granular flows aimed at filling this need. The key ingredient of the theory is a grain-size-dependent nonlocal rheology -- inspired by efforts for emulsions -- in which flow at a point is affected by both the local stress as well as the flow in neighboring material. With a single new material parameter, we show that the model is able to quantitatively describe dense granular flows in an array of different geometries. Of particular importance, it is the first model to pass the stringent test of capturing all aspects of the highly-nontrivial flows observed in split-bottom cells -- a geometry that has resisted modeling efforts for nearly a decade. [Preview Abstract] |
Tuesday, March 19, 2013 4:06PM - 4:18PM |
J30.00007: Changes in fluctuation patterns of a granular hopper flow near jamming Michal Dichter, Shubha Tewari, Bulbul Chakraborty Jams in gravity-driven flows in a vertical hopper with rigid walls occur under extremely inhomogeneous conditions, distinct from what is observed in spatially homogeneous flows. In this work, we use event-driven simulations to study velocity fluctuations in a collisional, 2D gravity-driven flow near jamming. We find a heterogeneous spatial distribution of velocity autocorrelation relaxation times, with the spatial structure changing significantly as the flow approaches jamming. At high flow rates, the flow at the center has lower kinetic temperatures and longer autocorrelation times than at the boundary. Unexpectedly, however, this trend reverses itself as the flow rate slows, with fluctuations relaxing more slowly at the boundaries though the kinetic temperatures remain high in that region. We suggest that this behavior is an indication of the flow becoming glassy close to the boundaries as jamming is approached. [Preview Abstract] |
Tuesday, March 19, 2013 4:18PM - 4:30PM |
J30.00008: Interplay between packing and flow in the shear zone at the wall of a granular hopper flow Brenda Carballo-Ramirez, Maya Lewin-Berlin, Nalini Easwar, Narayanan Menon A granular medium flowing through a vertical channel has a flat velocity profile in the bulk with a shear zone at the wall. The size of the shear zone and the dependence on flow parameters is poorly understood. To address this issue we image the flow of spherical steel spheres under gravity in a vertical, straight-walled 2-dimensional hopper, where the flow velocity is controlled by a taper at the outlet. ~Our measurements focus on the role of microstructure in controlling the shear zone. ~We have found that the size of this zone is larger in bidisperse, disordered flow that in monodisperse, nearly-crystalline flows. We report the effect of packing as quantified by local dilation, as a function of flow rate for systems of both bidisperse and monodisperse grains. [Preview Abstract] |
Tuesday, March 19, 2013 4:30PM - 4:42PM |
J30.00009: Boundary layer model for intruder drag Stephan Koehler, Jonathan Goldsmith, Mingjiang Tao We propose a boundary layer model for drag on vertical intruders with uniform cross-sections in granular beds. The drag is the surface integral of the stress over a monolayer of particles, where the stress has a simple dependence on depth beneath the surface and angle of the surface normal relative to the direction of flow. This model is in good experimental agreement, accounts for the scale effect and the associated force focusing observed on edges of intruders. [Preview Abstract] |
Tuesday, March 19, 2013 4:42PM - 4:54PM |
J30.00010: Rheology and migration in colloidal and noncolloidal suspensions Jeffrey Morris Suspensions of solid particles in liquids provide a useful setting for development of continuum description of particle-laden fluids. These mixtures can be made density matched, so that the volume fraction is freely variable, and the rheology can be measured in standard rheometric apparatus. This work will describe the rheology of concentrated suspensions and its implications in continuum description of the bulk flow of the mixture; the development will focus on colloidal suspensions where Brownian motion is relevant, with the limit of strong shear taken to describe noncolloidal suspensions. The normal stress response of these suspensions will be shown to be critical to description of the migration of the particles, leading to strong concentration gradients. The normal stress differences as well as the isotropic normal stress of the particle phase, or nonequilibrium osmotic pressure, will be described and related to these migration phenomena. The implications of the normal stress differences in secondary flow generation will also be described. [Preview Abstract] |
Tuesday, March 19, 2013 4:54PM - 5:06PM |
J30.00011: How to predict polydisperse hard-sphere mixture behavior using maximally equivalent tridisperse systems Vitaliy Ogarko, Stefan Luding Polydisperse hard sphere mixtures have equilibrium properties which essentially depend on the number density and a reduced number $K$ of moments of the size distribution function. Such systems are equivalent to other systems with different size distributions if the $K$ moments are matched. In particular, a small number $s$ of components, such that $2s-1=K$ is sufficient to mimic systems with continuous size distributions. For most of the fluid phase $K=3$ moments ($s=2$ components) are enough to define an equivalent system, while in the glassy states one needs $K=5$ moments ($s=3$ components) to achieve good agreement between the polydisperse and its maximally-equivalent tridisperse system. With $K=5$ matched moments they are also close in number- and volume-fractions of rattlers. Finally, also the jamming density of maximally-equivalent jammed packings is very close, where the tiny differences can be explained by the distribution of rattlers. [Preview Abstract] |
Tuesday, March 19, 2013 5:06PM - 5:18PM |
J30.00012: Finite element modeling of the dynamic effective mass of granular media John Valenza, David Henann, Ken Kamrin, David Johnson Finite sized granular media have a frequency dependent, complex valued effective mass, characterized by several resonant features. In the vicinity of the corresponding frequencies the associated mass can be several times the static mass. This complicated behavior is due to mechanical interactions between neighboring grains. In contrast we investigate the viability of using a continuum approximation for the mechanical response to model the effective mass. We find that the granular medium is suitably represented by a linear elastic stress-strain relationship with viscous damping. The free parameters in the linear elastic model, the elastic modulus and poisson's ratio, are measured using conventional mechanical testing equipment, and a novel sensor which permits the measurement of lateral stress. Moreover, we characterize the frequency dependent displacement profile on the surface of the granular medium. In this talk we demonstrate that our continuum model is suitable for reproducing the frequency dependent effective mass, and the displacement profile at the resonant frequencies. [Preview Abstract] |
Tuesday, March 19, 2013 5:18PM - 5:30PM |
J30.00013: A terradynamics of legged locomotion on granular media Chen Li, Tingnan Zhang, Daniel Goldman The theories of aero and hydrodynamics form the bases for prediction of animal movement and device design in air and water, and allow computation of lift, drag, and thrust forces on wings and fins. While models of terrestrial legged locomotion have focused on interactions with solid ground, many legged animals (and increasingly robots) move on substrates such as sand, gravel, soil, mud, snow, grass, and leaf litter that flow in response to intrusion. However, locomotor-ground interaction models on such flowable ground are often unavailable. Here we develop a resistive force model that predicts forces on arbitrary-shaped legs and bodies moving freely in granular media in the vertical plane. Our resistive force measurements reveal a complex but generic dependence of stresses on an intruder on its depth, orientation, and movement direction in granular media of different particle size, density, friction, and compaction. Our resistive force model and a multi-body simulation predict a small legged robot's locomotion on granular media using various leg shapes and stride frequencies, and give insight into the effects of leg morphology and kinematics on movement on granular media. Our study is an initial but important step in creation of ``terradynamics'' of locomotion on flowable ground. [Preview Abstract] |
Session J31: Focus Session: Dynamics of Glassy Polymers Under Nanoscale Confinement: Friction and Adhesion
Sponsoring Units: DPOLYChair: Connie Roth, Emory University
Room: 339
Tuesday, March 19, 2013 2:30PM - 3:06PM |
J31.00001: DILLON MEDAL BREAK
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Tuesday, March 19, 2013 3:06PM - 3:18PM |
J31.00002: Viscous Friction of Polymer Brushes Aykut Erbas, Michael Rubinstein Polymer brushes are unique soft structures that can exhibit solid-like behaviors, i.e., if they are deformed by an external force, they can relax and take their original conformations when the external force is removed. Despite their solid-like character, tribological behavior of polymer brushes exhibits fluid-like properties: For instance, friction force exerted on two interdigitated brushes sheared in opposite directions goes to zero linearly as the shear velocity vanishes, i.e., no static friction occurs, which is a property observed mostly for fluidic friction. In this talk, we present our simulation result and scaling arguments on the friction of planar brush-on-brush systems. Our theoretical approach and simulation regimes encompass both linear and non-linear regimes. We show that individual brush ends move on well-defined average trajectories. The dissipation in the system can be related to these average trajectories for a wide range of shear velocities. [Preview Abstract] |
Tuesday, March 19, 2013 3:18PM - 3:30PM |
J31.00003: Stick-Slip Dynamics Using Velcro as Model System Lisa Mariani, Cara Esposito, Paul Angiolillo Described by Galileo and further developed phenomenologically by Amontons and Coulomb, friction remains to be poorly understood especially with respect to its transition from the static to the kinetic regimes. In particular, the dynamics and control thereof of systems exhibiting stick-slip motion continues to be an area of fascination. The dry sliding behavior of the hook-and-loop system evinced by common Velcro captures many of the hallmarks of stick-slip motion typically manifested in systems at very small and very large length scales in addition to satisfying some of the classical laws as put forth by Amontons and Coulomb. Specifically, the kinetic frictional force is independent of driving velocity over nearly three orders of magnitude. In stark contrast to classical behavior, both the maximum static and the kinetic frictional forces reveal a linear dependence on the ``area of contact'' or more appropriately, hook number. Moreover, the frictional force (static and kinetic) exhibits a power law dependence on load with an exponent of approximately 0.25 similar to behavior seen in AFM, the implication being non-constant coefficients of static and kinetic friction. Statistical analysis shows that the fluctuations of stick-slip events follow a power law behavior with an exponent of approximately 0.5. Interestingly, this relatively simple system demonstrates evidence of precursor events prior to the onset of motion and may provide insight to the nucleation and transition from static to kinetic friction. [Preview Abstract] |
Tuesday, March 19, 2013 3:30PM - 3:42PM |
J31.00004: ABSTRACT WITHDRAWN |
Tuesday, March 19, 2013 3:42PM - 3:54PM |
J31.00005: Structure and dynamics of hyperbranched polymers in bulk and under nanoscopic confinement S. H. Anastasiadis, K. Chrissopoulou, K. Karatasos, S. Fotiadou, C. Karageorgaki, I. Tanis, D. Tragoudaras, B. Frick The structure and dynamics of a hyperbranched polyesteramide (Hybrane S 1200) and its nanocomposites with natural montmorillonite (Na+-MMT) are investigated. In bulk, the behavior is probed by QENS with MD simulations employed for a deeper insight into the relevant relaxation processes. The energy-resolved elastically scattered intensity from the polymer relaxes with two steps, one below and one above the polymer Tg. The QENS spectra are consistent with the elastic measurements and can be correlated to the results emerging from the detailed description afforded by the atomistic simulations, which cover a broad time range and predict the existence of three relaxation processes. The nanocomposites are investigated by XRD, DSC and QENS. XRD reveals an intercalated nanocomposite structure. The polymer chains confined within the galleries show similarities in the dynamic behavior with that of the bulk polymer for temperatures below the bulk polymer Tg, whereas they exhibit frozen dynamics under confinement at temperatures higher than that. Sponsored by the Greek GSRT ($\Sigma\Upsilon$NEP$\Gamma$A$\Sigma$IA 09$\Sigma\Upsilon$N-42-580). [Preview Abstract] |
Tuesday, March 19, 2013 3:54PM - 4:06PM |
J31.00006: Contact Mechanics of Nanoparticles J.-M.Y. Carrillo, A.V. Dobrynin We perform molecular dynamics simulations of the detachment of nanoparticles from a substrate. The critical detachment force, $f$*, is obtained as a function of the nanoparticle radius, $R_{p}$, shear modulus, $G$, surface energy, $\gamma_{p}$, and work of adhesion, $W$. The magnitude of the detachment force is shown to increase from $\pi $\textit{WR}$_{p}$ to 2.2$\pi $\textit{WR}$_{p}$ with increasing nanoparticle shear modulus and nanoparticle size. This variation of the detachment force is a manifestation of a neck formation upon nanoparticle detachment. Using scaling analysis, we show that the magnitude of the detachment force is controlled by the balance of the nanoparticle elastic energy, surface energy of the neck, and nanoparticle adhesion energy to a substrate. It is a function of the dimensionless parameter $\delta \propto \gamma_{p} (GR_{p} )^{-1/3}W^{-2/3}$which is proportional to the ratio of the surface energy of a neck and the elastic energy of deformed nanoparticle. In the case of small values of the parameter $\delta $ \textless \textless 1, the critical detachment force approaches a critical Johnson, Kendall and Roberts force, $f\ast \approx 1.5\pi WR_{p} $, as is usually the case for strongly crosslinked large nanoparticles. However, in the opposite limit, corresponding to soft small nanoparticles, for which $\delta $\textgreater \textgreater 1, the critical detachment force, $f$*, scales as $f\ast \propto \gamma_{p}^{3/2} R_{p}^{1/2} G^{-1/2}$. Simulation data are described by a scaling function $f\ast \propto \gamma_{p}^{3/2} R_{p}^{1/2} G^{-1/2}\delta^{-1.89}$ . [Preview Abstract] |
Tuesday, March 19, 2013 4:06PM - 4:18PM |
J31.00007: Probing gradient of dynamics in confined polymers with nanoparticles Sivasurender Chandran, Nafisa Begam, Jaydeep Basu, Mrinmay Mukhopadhyay We report [1] the evidence of gradient in dynamics by probing the diffusion coefficient of polymer grafted nanoparticles (PGNP) in polymer thin films of different thickness (2.5 R$_{g}$ and 8R$_{g\, }$of the matrix). Using surface x-ray scattering, we observe a systematic vertical dispersion of PGNP from a pinned in substrate interface layer to the surface on thermal annealing. Even after annealing at high temperature (T\textgreater \textgreater T$_{g})$ and longer times, a fraction of PGNP pertain to stay at the substrate forming a stable interface layer. This hints about the low mobility of particles at the substrate interface and also emphasizes the presence of high viscous/gel-like interfacial layer. Real space microscopic images show the formation of lateral domains of the particles at air surface suggesting the higher surface mobility. In addition, it is also observed that the fraction of particles in the air surface is more in annealed thinner films compared to the thicker ones. Thus, we have correlated the observed lateral and vertical dispersion and its evolution with annealing, to the gradient in dynamics along the thickness of the thin films. \textit{[1] Sivasurender Chandran, J. K. Basu and M. K. Mukhopadhyay, in communication} [Preview Abstract] |
Tuesday, March 19, 2013 4:18PM - 4:30PM |
J31.00008: Confinement of conjugated polymers into soft nanoparticles: molecular dynamics simulations Sidath Wijesinghe, Dvora Perahia, Gary S. Grest The structure and dynamics of conjugated polymers confined into soft nanoparticles (SNPs) have been studies by molecular dynamic simulations. This new class of tunable luminescent SNPs exhibits an immense potential as bio-markers as well as targeted drug delivery agents where tethering specific groups to the surface particles offers a means to target specific applications. Of particular interest are SNPs that consist of non- crosslinked polymers, decorated with polar groups. These SNPs are potentially tunable through the dynamics of the polymer chains, whereas the polar entity serves as internal stabilizer and surface encore. Confinement of a polymer whose inherent conformation is extended impacts not only their dynamics and as a result their optical properties. Here we will present insight into the structure and dynamics of dialkyl poly \textit{para} phenylene ethynylene (PPE), decorated by a carboxylate groups, confined into a soft particle. The conformation and dynamics of polymer within SNP will be discussed and compared with that of the linear chain in solution. [Preview Abstract] |
Tuesday, March 19, 2013 4:30PM - 4:42PM |
J31.00009: Forces between nanoparticles grafted with rigid polymers: a pathway for tunable hybrids Sabina Maskey, Dvora Perahia, J. Matthew D. Lane, Gary S. Grest The forces between the nanoparticles hybrids that consist of para dialkyl phenyleneethynylenes (PPEs) grafted to a silica nanoparticle have been studied using molecular dynamic simulations. PPEs are rigid polymers whose conformation determines their degree of conjugation and their assembly mode which in turn affects the electro-optical response of the nanoparticle-polymer complexes. When confined to a nanoparticle surface, the PPE chains are fully extended but cluster as the quality of the solvents is reduced. Tuning the degree of clustering by tuning the solvent-polymer interaction is expected to direct the assembly of the particles. Results for the forces between two nanoparticles functionalized with rigid polymers as a function of solvent quality, velocities and distances will be presented. These simulations will provide for the first time insight to the interactions of the nanoparticles grafted with rigid polymer, which in turn, results in formation of tunable hybrids. [Preview Abstract] |
Tuesday, March 19, 2013 4:42PM - 4:54PM |
J31.00010: Polymer Film Surface Fluctuation Dynamics in the Limit of Very Dense Branching Mark Foster, Boxi Liu, Suresh Narayanan, David T. Wu The surface height fluctuations of melt films of densely branched comb polystyrenes of thicknesses greater than 55nm and at temperatures more than 23 C above the $T_{g,bulk}$ can be rationalized using the hydrodynamic continuum theory (HCT) known to describe melts of unentangled linear and cyclic chains. Film viscosities ($\eta_{XPCS})$ for three combs inferred from fits of the HCT to X-ray Photon Correlation Spectroscopy (XPCS) data are the same as bulk viscosities ($\eta_{bulk})$ measured with rheometry. For the comb most like a star polymer and the comb closest to showing bulk entanglement behavior, $\eta_{XPCS}$ is greater than $\eta_{bulk}$. However, the values of $\eta_{XPCS}-\eta_{bulk}$ are much smaller than those seen for less densely branched polystyrenes. We conjecture that the smaller magnitude of $\eta_{XPCS}-\eta_{bulk}$ for the densely grafted combs is due to a lack of interpenetration of the side chains when branching is very dense. While data of relaxation time versus $T$ for cyclic chains virtually collapse to a single curve when $T_{g,bulk}$ is accounted for, that is not the case for combs. $T_{g,bulk}$ and specific chain architecture both play important roles in determining the surface fluctuations. Acknowledgements: NSF CBET 0730692, CBET-0731319, DURIP W911NF-09-1-0122. [Preview Abstract] |
Tuesday, March 19, 2013 4:54PM - 5:06PM |
J31.00011: Axial and radial nanostructures in electrospun polymer fibers Israel Greenfeld, Andrea Camposeo, Francesco Tantussi, Stefano Pagliara, Francesco Fuso, Maria Allegrini, Dario Pisignano, Eyal Zussman The high tensional stresses during electrospinning of semidilute polymer solutions affect the dynamic conformation of the polymer network within the liquid jet, leaving a distinctive trace in the molecular structure after solidification. We investigated such effects in electrospun nanofibers made of conjugated polymers. Modeling the polymer network evolution during electrospinning showed that as the network stretches axially, it contracts towards the jet core. The model represents the semi-flexible conjugated polymer chains as flexible freely-jointed chains, whose joints are bonding defects. Using the conjugated polymer MEH-PPV dissolved in a mixture of THF and DMF solvents, and taking advantage of its unique photophysical characteristics, we investigated optically the variations in the density and orientation of the polymer macromolecules in electrospun nanofibers. In agreement with our model, we found higher density and axial orientation at the fiber core, while lower density and radial orientation closer to the fiber surface. The non-uniformity of the resulting molecular structure can be tuned and exploited in diverse optical and structural applications. [Preview Abstract] |
Tuesday, March 19, 2013 5:06PM - 5:18PM |
J31.00012: Correlation between the interfacial bond orientational order and the shift in T$_{g}$ upon confinement Simone Napolitano The two-order-parameter (TOP) model rationalizes the interfacial slower dynamics in terms of enrichment in bond orientational order (BOO), near the wall [1]. Recently, we verified that the dielectric strength, $\Delta\epsilon$, is a robust parameter for measuring the BOO, as $\Delta\epsilon$ = g$<$$\mu$$^{2}$$>$/k$_{B}$T, where $\mu$ is the dipole moment, and g accounts for the correlation among neighbor dipole moments. We obtained interfacial values of the dielectric strength, $\Delta\epsilon$$_{int}$, analyzing the thickness dependence of all the polymers for which $\Delta\epsilon$ was measured upon confinement in ultrathin films. Although for all the investigated systems $\Delta\epsilon$ decreases in proximity of a solid interface due to the reduction in $<$$\mu$$^{2}$$>$ upon adsorption [3], we identified a striking correlation between $\Delta\epsilon$$_{int}$ and the shift in T$_{g}$ upon confinement. Increases in T$_{g}$ were univocally correlated to nonzero positive values of $\Delta\epsilon$$_{int}$, implying a larger BOO near the wall, in line with the predictions of the TOP model. [1] Watanabe, Kawasaki, Tanaka, Nature Materials 2011, 10, 512 [2] Capponi, Napolitano, Wuebbenhorst, Nature Comm. 2012, accepted [3] Napolitano, Wuebbenhorst, Nature Comm. 2011, 2, 260 [Preview Abstract] |
Tuesday, March 19, 2013 5:18PM - 5:30PM |
J31.00013: Conformational Relaxation of Polystyrene at Substrate Interface Hirofumi Tsuruta, Yoshihisa Fujii, Hiroshi Morita, Keiji Tanaka The local conformation of polymer chains in a film at a substrate interface was examined by sum-frequency generation spectroscopy. When a polystyrene (PS) film was prepared on a quartz substrate by a spin-coating method, the chains were aligned in the interfacial plane of the substrate. A dissipative particle dynamics simulation revealed that a spinning torque induced the chain orientation during the film preparation process and the extent of the orientation was a function of the distance from the interface. This interfacial orientation of chains was not observed for a PS film prepared by a solvent-casting method. Interestingly, the local conformation of chains at the substrate interface was unchanged even at a temperature that was 80 K higher than the bulk glass transition temperature. This observation means that polymer chains at the substrate interface can be only partially relaxed under conditions where the bulk chains are fully relaxed. On the other hand, interfacial chains could be easily relaxed by solvent annealing. [Preview Abstract] |
Session J32: Focus Session: Assembly & Function of Biomimetic & Bioinspired Materials I
Sponsoring Units: DMP DPOLY DBIOChair: Jim de Yoreo, Lawrence Berkeley National Laboratory
Room: 340
Tuesday, March 19, 2013 2:30PM - 3:06PM |
J32.00001: DILLON MEDAL BREAK
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Tuesday, March 19, 2013 3:06PM - 3:18PM |
J32.00002: Designing ``catch and release'' systems by utilizing functionalized oscillating fins Yongting Ma, Amitabh Bhattacharya, Olga Kuksenok, Ximin He, Joanna Aizenberg, Anna C. Balazs Designing a biomimetic ``catch and release'' device for the selective removal of target species from the surrounding solution is critical for developing many useful sensors and sorters. Via computational modeling, we simulate an array of oscillating fins that are localized on the floor of a microchannel and immersed in a two-fluid stream. The fins reach the upper fluid when they are upright and are located entirely within the lower stream when they are tilted. We introduce specific adhesive interactions between the fins and particulates in the solution and determine conditions where the oscillating fins can selectively bind (``catch'') target nanoparticles within the upper fluid stream and then release these particles into the lower stream. Using our hybrid computational approach, which combines the lattice Boltzmann model for binary fluids and a Brownian dynamics model for the nanoparticles, we isolate systems parameters (e.g., frequency and amplitude of fins' oscillations) that lead to the efficient extraction of target species from the upper stream and placement into the lower fluid. Our findings provide fundamental insights into the system's complex dynamics, as well as a unique solution for detection, separation, and purification of multi-component mixtures. [Preview Abstract] |
Tuesday, March 19, 2013 3:18PM - 3:30PM |
J32.00003: The Study of Lipid-Based Nanodis as a Novel Carrier for Hydrophobic Cargo Ying Liu, Mu-Ping Nieh, Hyunsook Jang, Yike Huang, Yong Wang Monodispersed nanodiscs can be self-assembled in an aqueous mixture of 1,2-dipalmitoyl-sn-glycero- 3-phosphocholine (DPPC), 1,2-dihexanoyl-sn-glycero-3-phosphocholine (DHPC) and 1,2-dipalmitoyl-sn-glycero-3-phospho- (1'-rac-glycerol) (sodium salt)(DPPG) and 1,2-distearoyl-sn-glycero- 3-phosphoethanolamine-N-[methoxy(polyethylene glycol)-2000] (ammonium salt) (PEGylated DSPE). The stability of discs and the effect of polyethylene glycol (PEG), including molecular chain length and concentration, on the disc morphology are characterized by dynamic light scattering, negative staining transmission electron microscopy and small angle neutron scattering. Fluorescent Spectroscopy is used to study the loading capacity of a hydrophobic dye, Nile red entrapped in the nanodiscs. The exchanging of Nile red between discs will be correlated with the release of hydrophobic molecule. In-vitro studies indicate that the non-specific binding of these Nile-red loaded nanodiscs to the CCRF-CEM cells is greatly reduced upon the addition of PEGylated DSPE. The system has a potential application of delivering hydrophobic molecules. The incorporation of targeting molecules with the nanodiscs is also investigated. [Preview Abstract] |
Tuesday, March 19, 2013 3:30PM - 3:42PM |
J32.00004: Harnessing Fluid-Driven Vesicles to Pick Up and Drop Off Janus Particles Xin Yong, Isaac Salib, Emily Crabb, Nicholas Moellers, Gerald McFarlin, Olga Kuksenok, Anna Balazs Using dissipative particle dynamics (DPD) simulations, we model the interaction between nanoscopic lipid vesicles and Janus nanoparticles in the presence of an imposed flow. Both the vesicle and Janus nanoparticles are localized on a hydrophilic substrate and immersed in a hydrophilic solution. The fluid-driven vesicle successfully picks up Janus particles on the substrate and transports these particles as cargo along the surface. The vesicle can carry up to four particles as its payload. Hence, the vesicles can acts as nanoscopic ``vacuum cleaners'', collecting nanoscopic debris localized on the floors of the fluidic devices. Importantly, these studies reveal how an imposed flow can facilitate the incorporation of nanoparticles into nanoscale vesicles. With the introduction of a hydrophobic domain on the substrate, the vesicles can also robustly drop off and deposit the particles on the surface. The controlled pickup and delivery of nanoparticles via lipid vesicles can play an important step in the bottom-up assembly of these nanoparticles within small-scale fluidic devices. [Preview Abstract] |
Tuesday, March 19, 2013 3:42PM - 3:54PM |
J32.00005: Cob-Weaving Spiders Design Attachment Discs Differently for Locomotion and Prey Capture Vasav Sahni, Jared Harris, Todd Blackledge, Ali Dhinojwala Spiders' cobwebs ensnare both walking and flying prey. While the scaffolding silk can entangle flying insects, gumfoot silk threads pull walking prey off the ground and into the web. Therefore, scaffolding silk needs to withstand the impact of the prey, whereas gumfoot silk needs to easily detach from the substrate when contacted by prey. Here, we show that spiders accomplish these divergent demands by creating attachment discs of two distinct architectures using the same pyriform silk. A ``staple-pin'' architecture firmly attaches the scaffolding silk to the substrate and a previously unknown ``dendritic'' architecture weakly attaches the gumfoot silk to the substrate. Gumfoot discs adhere weakly, triggering a spring-loaded trap, while the strong adhesion of scaffolding discs compels the scaffolding threads to break instead of detaching. We describe the differences in adhesion for these two architectures using tape-peeling models and design synthetic attachments that reveal important design principles for controlled adhesion. [Preview Abstract] |
Tuesday, March 19, 2013 3:54PM - 4:06PM |
J32.00006: ABSTRACT WITHDRAWN |
Tuesday, March 19, 2013 4:06PM - 4:18PM |
J32.00007: Self-Tailoring of Amphiphilic Block Copolymer Assemblies by Osmotic Pressure Jinhye Bae, Ryan Hayward Compartmentalization is a crucial architectural principle employed by eukaryotic cells, and correspondingly, pathways to assemble multi-compartmental polymeric assemblies are of considerable research interest. We report a study of the self-generation of water-in-oil-in-water (w/o/w) double emulsions with inner droplet sizes of $\sim$ 2-3 micrometers due to the osmotic pressure provided by salts initially dissolved in the organic phase. We show that this process can explain previously mysterious examples of spontaneous emulsion formation, due to the presence of initiator salt impurities within copolymer samples. Further, we harness it to tailor the structures of multiple emulsions, which upon solvent evaporation can yield multi-vesicular structures or hierarchically structured porous films. Such osmotically-driven polymer assemblies may have potential applications in therapeutic, pharmaceutical, cosmetic, and separation technologies. [Preview Abstract] |
Tuesday, March 19, 2013 4:18PM - 4:30PM |
J32.00008: Effect of Intrinsic Twist on Length of Crystalline and Disordered Regions in Cellulose Microfibrils Abdolmadjid Nili, Oleg Shklyaev, Zhen Zhao, Linghao Zhong, Vincent Crespi Cellulose is the most abundant biological material in the world. It provides mechanical reinforcement for plant cell wall, and could potentially serve as renewable energy source for biofuel. Native cellulose forms a non-centrosymmetric chiral crystal due to lack of roto-inversion symmetry of constituent glucose chains. Chirality of cellulose crystal could result in an overall twist.~Competition between unwinding torsional/extensional and twisting energy terms leads to twist induced frustration along fibril's axis. The accumulated frustration could be the origin of periodic disordered regions observed in cellulose microfibrils. These regions could play significant role in properties of~cellulose bundles and ribbons as well as biological implications on plant cell walls.~ We propose a mechanical model based on Frenkel-Kontorova mechanism to investigate effects of radius dependent twist on crystalline size in cellulose microfibrils. Parameters of the model are adjusted according to all-atom molecular simulations. [Preview Abstract] |
Tuesday, March 19, 2013 4:30PM - 4:42PM |
J32.00009: The Effect of Small Molecule Additives on the Self-Assembly and Functionality of Protein-Polymer Diblock Copolymers Carla Thomas, Liza Xu, Bradley Olsen Self-assembly of globular protein-polymer block copolymers into well-defined nanostructures provides a route towards the manufacture of protein-based materials which maintains protein fold and function. The model material mCherry-b-poly(N-isopropyl acrylamide) forms self-assembled nanostructures from aqueous solutions via solvent evaporation. To improve retention of protein functionality when dehydrated, small molecules such as trehalose and glycerol are added in solution prior to solvent removal. With as little as 10 wt\% additive, improvements in retained functionality of 20-60\% are observed in the solid-state as compared to samples in which no additive is present. Higher additive levels (up to 50\%) continue to show improvement until approximately 100\% of the protein function is retained. These large gains are hypothesized to originate from the ability of the additives to replace hydrogen bonds normally fulfilled by water. The addition of trehalose in the bulk material also improves the thermal stability of the protein by 15-20 $^\circ$C, while glycerol decreases the thermal stability. Materials containing up to 50\% additives remain microphase separated, and, upon incorporation of additives, nanostructure domain spacing tends to increase, accompanied by order-order transitions. [Preview Abstract] |
Tuesday, March 19, 2013 4:42PM - 4:54PM |
J32.00010: Material Structure of a Graded Refractive Index Lens in Decapod Squid Jing Cai, Paul Heiney, Alison Sweeney Underwater vision with a camera-type eye that is simultaneously acute and sensitive requires a spherical lens with a graded distribution of refractive index. Squids have this type of lens, and our previous work has shown that its optical properties are likely achieved with radially variable densities of a single protein with multiple isoforms. Here we measure the spatial organization of this novel protein material in concentric layers of the lens and use these data to suggest possible mechanisms of self-assembly of the proteins into a graded refractive index structure. First, we performed small angle x-ray scattering (SAXS) to study how the protein is spatially organized. Then, molecular dynamic simulation allowed us to correlate structure to the possible dynamics of the system in different regions of the lens. The combination of simulation and SAXS data in this system revealed the likely protein-protein interactions, resulting material structure and its relationship to the observed and variable optical properties of this graded index system. We believe insights into the material properties of the squid lens system will inform the invention of self-assembling graded index devices. [Preview Abstract] |
Tuesday, March 19, 2013 4:54PM - 5:06PM |
J32.00011: Phase Transitions in Concentrated Solution Self-Assembly of Globular Protein-Polymer Block Copolymers Christopher Lam, Bradley Olsen The self-assembly of globular protein-polymer bioconjugate block copolymers to form biofunctional nanostructures presents potentially complex behavior due to the tertiary structures and specific interactions of protein blocks. To understand the thermodynamics of these systems, the phase behavior of the model globular protein-polymer block copolymer mCherry-$b$-PNIPAM (mChP) is investigated in concentrated aqueous solution as a function of both concentration and temperature. At low concentrations, mChP forms a homogeneous disordered phase at low temperature and macrophase separates into an ordered conjugate-rich phase and a solvent-rich phase at temperatures above the PNIPAM thermoresponsive transition temperature. mChP solutions undergo a lyotropic, low-temperature ODT and both lyotropic and thermotropic OOTs at high concentration. Similar to coil-coil block copolymers, both coil fraction and solvent selectivity have large effects on the morphologies formed---disordered micelles, hexagonally packed cylinders, lamellae, and perforated lamellae. The order-disorder transition concentration (ODTC) of mChP is minimized for symmetric conjugates, suggesting that repulsive solvent-mediated protein-polymer interactions provide a driving force for self-assembly. [Preview Abstract] |
Tuesday, March 19, 2013 5:06PM - 5:18PM |
J32.00012: Interfacial curvature effects in the self-assembly and responsiveness in polypeptide-based triblock copolymers Daniel Savin, Jacob Ray, Ashley Johnson, Jack Ly, Charles Easterling The self-assembly of amphiphilic block copolymers is dictated primarily by the balance between the hydrophobic core volume and the hydrophilic corona.~ In these studies, ABA and BAB triblock copolymers containing poly(lysine) (PK) and poly(propylene oxide) were synthesized and their solution properties studied using dynamic light scattering, circular dichroism spectroscopy and transmission electron microscopy.~ This talk will present some recent studies in solution morphology transitions that occur in these materials as a result of the helix-coil transition and associated charge-charge interactions. The solution properties and responsiveness of these novel materials will be discussed in terms of their ability to encapsulate and deliver cancer therapeutics. [Preview Abstract] |
Tuesday, March 19, 2013 5:18PM - 5:30PM |
J32.00013: Amphiphilic Spider Silk-Like Block Copolymers with Tunable Physical Properties and Morphology for Biomedical Applications Wenwen Huang, Sreevidhya Krishnaji, David Kaplan, Peggy Cebe Silk-based materials are important candidates for biomedical applications because of their excellent biocompatibility and biodegradability. To generate silk amphiphilic biopolymers with potential use in guided tissue repair and drug delivery, a novel family of spider silk-like block copolymers was synthesized by recombinant DNA technology. Block copolymer thermal properties, structural conformations, protein-water interactions, and self-assembly morphologies were studied with respect to well controlled protein amino acid sequences. A theoretical model was used to predict the heat capacity of the protein and protein-water complex. Using thermal analysis, two glass transitions were observed: Tg1 is related to conformational changes caused by bound water removal, while Tg2 ($>$Tg1) is the glass transition of dry protein. Real-time infrared spectroscopy and X-ray diffraction confirmed that different secondary structural changes occur during the two Tg relaxations. Using scanning electron microscopy, fibrillar networks and hollow vesicles are observed, depending on protein block copolymer sequence. This study provides a deeper understanding of the relationship between protein physical properties and amino acid sequence, with implications for design of other protein-based materials. [Preview Abstract] |
Session J33: Focus Session: Organic Electronics and Photonics - Photophysics and Charge Transfer
Sponsoring Units: DMPChair: Seth Darling, Argonne National Laboratory
Room: 341
Tuesday, March 19, 2013 2:30PM - 3:06PM |
J33.00001: DILLON MEDAL BREAK
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Tuesday, March 19, 2013 3:06PM - 3:18PM |
J33.00002: Investigation of Pyrene Excimer formation in various manufacturing processes and ionic structures Hyun-Sook Jang, Mu-Ping Nieh Electrospun pyrene (Py)/polystyrene/tetrabutylammonium hexafluorophosphate (TBAPF6) thin films can provide high-sensitivity and high-selectivity detection of nitro-aromatic explosives through fluorescence quenching of the Py excimers [1]. However, we have found that the formation of Py excimers in Py/PS/TBAPF6 thin films depends greatly on the manufacturing processes. Our results indicate that high solvent vapor pressure promotes the Py excimer fluorescence, while high temperature (around or greater than Tg of the PS) has an opposite effect in absence of solvent -- reducing the Py excimer fluorescence. Moreover, we have found that salts structure such as cation chain length, anion strength can significantly affect the formation of Py excimer both in solution and solid state, presumably due to self-aggregation of the salts and electrostatic interactions between ions and pyrene excimer. 13C-NMR and steady-state fluorescence result indicate that the salt induces peak shift to the downfield in the spectra and quenches the Py excimer intensity drastically.\\[4pt] [1] Wang, Y.; La, A.; Ding, Y.; Liu, Y.;Lei, Y. Advanced Functional Materials 2012, 22, 3547. [Preview Abstract] |
Tuesday, March 19, 2013 3:18PM - 3:30PM |
J33.00003: Photoisomerization dynamics of azobenzene materials for solar thermal fuels David A. Strubbe, Jeffrey C. Grossman A solar thermal fuel absorbs sunlight and stores the energy chemically via an induced structural change, which can later be reversed to release the energy as heat. Azobenzene molecules have a cis-trans photoisomerization with these properties, and hydrogen-bonding and packing via attachment to rigid template structures have shown promise in increasing the energy stored and the length of time it can be stored [A Kolpak et al, Nano Lett. 11, 3156-3162 (2011)]. Other important factors in determining the efficiency of a solar thermal fuel are the absorption cross-section and the quantum yield for photoisomerization, which must also be optimized for a successful material. We employ time-dependent density-functional theory (TDDFT) and the GW/Bethe-Salpeter formalism to calculate the optical absorption and dynamics in the excited-state to address these two factors. We use excited-state forces to map out potential-energy surfaces and follow the structural change after absorption for azobenzene-derived materials, to correlate the efficiency of photoisomerization with the functionalization and template. [Preview Abstract] |
Tuesday, March 19, 2013 3:30PM - 3:42PM |
J33.00004: Optical absorption in fluorenone-based push-pull molecules Eduardo Cruz-Silva, Paul J. Homnick, Paul M. Lahti, Vincent Meunier Push-pull organic molecules include both electron donor and acceptor substituents, which upon excitation induce a charge separation with potential uses in conductive polymers and light-harvesting materials for use in solar cells. In a recent work, a new set of such molecules using fluorenone as the electron-acceptor unit have been reported [1]. Here we present a comprehensive study of their electronic structure and and optical properties using time-dependent density functional theory (TDDFT) as implemented in the NWChem software suite [2]. The remarkable agreement between experimental and computed spectra among all test systems show that TDDFT can be readily used as a predictive tool for assessing and optimizing the optical properties on these systems. 1. P.J. Homnick and P.M. Lahti, Phys. Chem. Chem. Phys. 14, 11961-11968 (2012). 2. M. Valiev, E.J. Bylaska, N. Govind, K Kowalski, et al., Comput. Phys. Commun. 181, 1477 (2010). [Preview Abstract] |
Tuesday, March 19, 2013 3:42PM - 3:54PM |
J33.00005: Exciton-Plasmon Interaction Effects in Individual Carbon Nanotubes Igor Bondarev, Areg Meliksetyan We have recently developed a theory for the electrostatically controlled coupling between excitons and low-energy inter-band plasmons in individual semiconducting carbon nanotubes [1]. Here, we report on our studies towards the applications of this effect of both applied and fundamental interest. One practical application is the electromagnetic absorption/photoluminescence control for individual nanotubes [2]. Another, fundamental one, comes from the fact that the coupling of the excitons to the same inter-band plasmon resonance results in their entanglement, a pre-requisite for strong quantum correlations/quantum phase transitions in many-particle systems [3]. Our coupled exciton-plasmon excitation is a quasi-1D Bose system and could possibly be Bose-condensed in an individual carbon nanotube under appropriately created external conditions --- despite the mathematical statements [4] of the BEC impossibility in ideal 1D and 2D quantum systems and previously reported evidence [5] for no free-exciton BEC in carbon nanotubes.\\[4pt] [1] I.V.Bondarev, et al, PRB80, 085407 (2009).\\[0pt] [2] I.V.Bondarev, PRB85, 035448 (2012).\\[0pt] [3] J.Anders, PRA77, 062102 (2008).\\[0pt] [4] R.K.Pathria, P.D.Beale, Statistical Mechanics (Elsevier, 2011).\\[0pt] [5] Y.Murakami, J.Kono, PRL102, 037401 (2009). [Preview Abstract] |
Tuesday, March 19, 2013 3:54PM - 4:06PM |
J33.00006: Quantitative analysis of valence photoemission spectra and quasiparticle excitations at chromophore-semiconductor interfaces Christopher Patrick, Feliciano Giustino Understanding electron energetics at interfaces between solids and molecules is a key challenge in many areas of nanotechnology research. Here we develop a quantitative theory of quasiparticle excitations at these interfaces and apply it to the prototypical dye-sensitized solar cell interface of N3 dye molecules adsorbed on the anatase TiO$_2$ (101) surface.\footnote{C. E. Patrick and F. Giustino, Phys. Rev. Lett. 109, 116801 (2012)} Our approach combines density-functional calculations on large interface models, bulk GW calculations,\footnote{C. E. Patrick and F. Giustino, J. Phys. Condens. Matter 24, 202201 (2012)} image charge renormalization, thermal broadening and configurational disorder to obtain a quasiparticle spectrum in good agreement with experimental photoemission data. Our calculations clarify the atomistic origin of the chromophore peak at low binding energy, and illustrate the dual role played by the TiO$_2$ substrate in screening the quasiparticle states of the N3 molecule through both long-range image-charge effects and direct charge transfer via the covalently-bonded anchor groups. [Preview Abstract] |
Tuesday, March 19, 2013 4:06PM - 4:18PM |
J33.00007: Two-dimensional Fourier transform spectroscopy of primary excitations, in conjugated polymers Kenan Gundogdu, Cong Mai, Andrew Barrette, Robert Younts, Terry McAfee, Harald Ade Conjugated polymers have tremendous potential for use in cheap, flexible, lightweight, energy efficient opto-electronic applications, Despite years of work, critical fundamental aspects about their optical and electronic properties are still poorly understood. Photo absorption in pure semi-conducting polymer thin films eventually results in both free charges and bound excitons with varying branching ratios. However the identification of the nature of early excitations and charge generation is an unresolved problem. There has been no direct observation of initial excitons or free electron-hole pairs, and competing views persist. Here we use 2D Fourier transform spectroscopy methods to separate the spectral signatures of various processes in the photoabsorption process in a homopolymer and show that initial excitation results in an intrachain electronic coherence that persists more than 200 fs. As these coherences evolve they collapse to transient population states i.e excitons, polarons and bipolarons. [Preview Abstract] |
Tuesday, March 19, 2013 4:18PM - 4:30PM |
J33.00008: Revealing photoinduced charge transfer mechanism across $\pi $-conjugated heterojunctions Yongwoo Shin, Xi Lin The adapted Su-Schrieffer-Heeger (aSSH) model is extended to the $\pi $-conjugated bulk heterojunction system. The New aSSH Hamiltonian incorporated interchain $\pi $-$\pi $ stacking and dynamic electron-phonon coupling effects. Excellent agreements are found between the computed photoadsorption and photoinduced adsorption spectra and their corresponding experimental measurements. It is found that excitons generated in the bulk poly-(p-phenylene vinylene) (PPV) phase must overcome an energy barrier of 0.23 eV to reach heterojunction interface. These interfacial excitons show clear charge separations, with their electron states leaning towards the interface. Therefore, electron transfers from the D$_1^{\mathrm{\ast }}$ state of PPV to the t$_{\mathrm{1u}}^{\mathrm{\ast }}$ state of C$_{60}$ follow non-adiabatic mechanisms, which are accelerated by the 0.97 eV energy drop, close vicinity of the D$_1^{\mathrm{\ast}}$ state to the C$_{60}$ phase, and suppressed inversion symmetry that doubles the number of electron-accepting states. After non-adiabatic electron transfers, the hole D$_1$ states are screened by the optical phonons in PPV, forming self-localized hole polarons and moving further away from heterojunction interface. [Preview Abstract] |
Tuesday, March 19, 2013 4:30PM - 4:42PM |
J33.00009: Charge transfer excitations in water-soluble sulfonated zinc-phthalocyanine (ZnPcS) donor molecules coupled to C$_{60}$ Rajendra Zope, Luis Basurto, Marco Olguin, Tunna Baruah We present a study of charge transfer (CT) excited states for a recently synthesized group of water-soluble sulfonated zinc-phthalocyanine (ZnPcS) donor molecules coupled to C$_{60}$. The ZnPcS donors (ZnPcS2, ZnPcS3, and ZnPcS4) are promising materials for achieving solar cell device production with the photoactive area prepared from aqueous solution. Experimentally, decreasing the number of sulfonate substituent groups for ZnPc increased the photocurrent and lowered the open circuit voltage V$_{OC}$. Measurements show that the V$_{OC}$ is largest for ZnPc-S4/C$_{60}$ and lowest for ZnPc-S3/C$_{60}$. The degree of sulfonation and the measured device V$_{OC}$ does not result in the expected pattern of values based on donor-acceptor HOMO/LUMO energy differences. Variations in film morphology may account for the unexpected pattern of V$_{OC}$ values. Our charge transfer excited state calculations show that the lowest CT excitation energy among the group of ZnPcS/C60 donor-acceptor pairs corresponds to the disulfonated ZnPc/C60 system. The largest CT excited state energies belong to the tetrasulfonated ZnPc/C60 complex. We also examine the effect of geometrical orientation on the CT energies for the ZnPcS donor-acceptor pairs. [Preview Abstract] |
Tuesday, March 19, 2013 4:42PM - 4:54PM |
J33.00010: Packing effects in charge transfer dynamics in organic molecular heterojunctions consisting of TFB and F8BT Mikiya Fujii, Koichi Yamashita Organic semiconductors have been widely investigated for photovoltaic and light emitting devices. Especially, further improvements for more efficient organic solar cells (OSCs) are desired. Thus, we explored computationally possibilities to make OSCs more efficient by adjusting the packing of molecular heterojunctions. We analyzed a molecular heterojunction that consists of poly(9,9-dioctylfluorene-co-N-(4-butylphenyl)diphenylenediamine) (TFB) and poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT). Geometrical optimization of TFB(monomer)/F8BT(monomer) complex was carried out with DFT-D/B3LYP/6-31G*. Excited states were also calculated with CIS/6-31G*. To analyze packing effects, we rotated TFB around a principal axis. Then, charge transfer dynamics is analyzed with a quantum master equation (QME) approach in each packing From the excited states calculations, it is clarified that the packing strongly affects the energy level of the charge transfer state only. This packing dependency arises from a packing dependency of the exciton binding energy that is Coulomb interaction between an electron localized to F8BT and a hole localized to TFB. From the QME approach, it is confirmed that qualitative different electronic relaxation dynamics occurs in each different packing. [Preview Abstract] |
Tuesday, March 19, 2013 4:54PM - 5:06PM |
J33.00011: Charge-Transfer Complexation Mechanism of Poly (4-Vinyl Pyridine)/[6,6] - Phenyl-C$_{61}$-Butyric Acid Methyl Ester in DMF Solution He Cheng, Guangmin Wei, Charles Han The mechanism of charge-transfer complexation in electron-donor(D)/electron-acceptor(A) active layer was studied for a pseudo-binary blend model system, poly(4-vinyl pyridine) /[6,6]-phenyl-C$_{61}$-butyric acid methyl ester in DMF. The time evolution of the system can be characterized by four distinct stages, i.e., induction, complexation, aggregation and precipitation, respectively. In the induction stage, the conformation of P4VP remained unchanged, while the UV-vis showed that the charge-transfer complexation had almost accomplished. In the complexation stage, each P4VP chains complexed with about 3 PCBM molecules at [4VP]/[PCBM]$=$57:1, and shrinked in size with almost no change in UV-vis spectrum. In the subsequent aggregation stage, P4VP/PCBM complexes aggregated with each other to form spherical aggregates with again unchanged UV-vis signals. FA model can be used to explain this mechanism. In the final precipitation stage, huge P4VP/PCBM agglomerate began to phase out. The almost unchanged UV-vis spectrum after the induction stage indicated that the electronic transition from ground to excited state is not necessarily to be influenced by any inter- or intra-polymer structural transition. [Preview Abstract] |
Tuesday, March 19, 2013 5:06PM - 5:18PM |
J33.00012: Probing charge transfer complex states in organic solar cells using photocurrent spectroscopy Dhanashree Moghe, Danish Adil, Catherine Kanimozhi, Gitesh Dutta, Satish Patil, Suchismita Guha Diketopyrrolopyrrole (DPP) containing copolymers-fullerene blends have gained a lot of interest in organic optoelectronics with a great potential in organic photovoltaics (OPVs). The interfacial charge transfer complex (CTC) states formed in donor-acceptor blended OPVs play a major role in the overall efficiency of the device. We investigate the spectral photocurrent characteristics of five DPP based copolymers; two of them being benzothiadiazole and carbazole -based statistical copolymers of DPP. These systems provide a wide range of bandgap energies ranging from $\sim$ 1.4 to 1.7 eV. We use Fourier transform photocurrent spectroscopy (FTPS) and monochromatic photocurrent (PC) to identify the CTC states in these DPP copolymer -fullerene blends. The stability of the CTC state is found to be dependent on the band gap energy difference between the donor copolymer and the acceptor. We support our inferences from theoretical results obtained using density-functional theory (DFT) and time-dependent DFT for two DPP based copolymers The theoretical calculations reveal a higher contribution of the CTC states to the lowest excited state in the phenyl-based DPP monomer, which has a larger bandgap energy compared to the thiophene-based DPP system, in the presence of a fullerene molecule. [Preview Abstract] |
Session J34: Focus Session: Charged Colloids with Short-Range Attractions II
Sponsoring Units: DPOLY DCMP DBIOChair: Frank Schreiber, Institut fuer Angewandte Physik, Universitaet Tuebingen
Room: 342
Tuesday, March 19, 2013 2:30PM - 3:06PM |
J34.00001: DILLON MEDAL BREAK
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Tuesday, March 19, 2013 3:06PM - 3:42PM |
J34.00002: Concentrated dispersions of therapeutic proteins Invited Speaker: Thomas Truskett In this talk, recent experiments characterizing highly concentrated dispersions of therapeutic proteins, which are of interest for at-home treatment of disease via subcutaneous injection, are discussed. In particular, evidence for protein nanocluster formation in these systems is explored. The roles of dispersion composition, pH, and experimental pathway are elucidated for several protein systems. Observed correlations between nanocluster properties, solution viscosity, and protein stability/activity, as well as prospective theoretical explanations for these behaviors, are highlighted. [Preview Abstract] |
Tuesday, March 19, 2013 3:42PM - 3:54PM |
J34.00003: Transition from monomeric phase to dynamic cluster phase in lysozyme protein solutions Yun Liu, Peter Falus, Lionel Porcar, Emiliano Fratini, Wei-Ren Chen, Antonio Faraone, Kunlun Hong, Piero Baglioni Intermediate range order (IRO) has been recently observed in lysozyme solution that is caused by a combination of a short-range attraction and long-range repulsion. At very high concentration, there is observed cluster formation in lysozyme solutions that is one type of IRO structures. Here, we investigate the temperature effect on the dynamic cluster formation and identify the transition concentration from a monomeric protein phase to a cluster phase. The normalized short-time self-diffusion coefficient is not affected by changing attraction strength at the concentration of about 10{\%} mass fraction, indicating that the system is still dominated by monomeric protein phase. However, at high concentrations, the average self-diffusion coefficient is sensitive to the change of short-range attraction strength, which is interpreted due to the growth of the size of dynamic clusters in solution. The transition concentration from dominating monomeric phase to dynamic cluster phase is estimated to be around 14 {\%} mass fraction. [Preview Abstract] |
Tuesday, March 19, 2013 3:54PM - 4:06PM |
J34.00004: Langevin Dynamics Simulation of DNA Condensation Induced by Nanoparticles in Confinement Guo-Jun Liao, Yeng-Long Chen We study nanoparticle-induced DNA condensation in a confined suspension of dilute DNA molecules and ideal nanoparticles (NPs) with Langevin dynamics simulation. DNA condensation has been observed in a solution of dilute DNA molecules (persistence length $P \approx 50$ nm) and high concentration of electrostatically neutral NPs (diameter $d \approx$ 5 to 35 nm) in recent experimental measurements. It is believed that NPs entropically induce an attraction between DNA segments. For NPs much smaller than $P$, a DNA molecule can be considered as a chain of connected rods, and the NP-induced depletion attraction between DNA segments can be regarded as rod-rod attraction. Thus, the strength of the depletion attraction is proportional to the number of persistence length in a DNA chain, $N=L/P$, the depletion volume $NP^2d$, and the NP density $\rho$, where $L$ is the DNA contour length. In slit confinement, DNA conformation changes are much different from in an unconfined environment. The height of the slit relative to the NPs size ($H/d$) strongly influences the DNA conformation. For $H/d \approx 1$, DNA size decreases monotonically as $\rho$ increases, while non-monotonic dependence happens for $H/d \approx 5$, due to the competition between DNA-DNA, DNA-NP, and NP-wall interactions. [Preview Abstract] |
Tuesday, March 19, 2013 4:06PM - 4:18PM |
J34.00005: Small-Angle Neutron Scattering and Neutron Spin Echo Characterization of Monoclonal Antibody Self-Associations at High Concentrations Eric Yearley, Isidro (Dan) Zarraga, Paul (Doug) Godfrin, Tatiana Perevozchikova, Norman Wagner, Yun Liu Concentrated therapeutic protein formulations offer numerous delivery and stability challenges. In particular, it has been found that several therapeutic proteins exhibit a large increase in viscosity as a function of concentration that may be dependent on the protein-protein interactions. Small-Angle Neutron Scattering (SANS) and Neutron Spin Echo (NSE) investigations have been performed to probe the protein-protein interactions and diffusive properties of highly concentrated MAbs. The SANS data demonstrate that the inter-particle interactions for a highly viscous MAb at high concentrations (MAb1) are highly attractive, anisotropic and change significantly with concentration while the viscosity and interactions do not differ considerably for MAb2. The NSE results furthermore indicate that MAb1 and MAb2 have strong concentration dependencies of dynamics at high Q that are correlated to the translational motion of the proteins. Finally, it has also been revealed that the individual MAb1 proteins form small clusters at high concentrations in contrast to the MAb2 proteins, which are well-dispersed. It is proposed that the formation of these clusters is the primary cause of the dramatic increase in viscosity of MAb1 in crowded or concentrated environments. [Preview Abstract] |
Tuesday, March 19, 2013 4:18PM - 4:30PM |
J34.00006: The Structural Properties and Stability of Monoclonal Antibodies at Freezing Conditions Tatiana Perevozchikova, Isidro Zarraga, Thomas Scherer, Norman Wagner, Yun Liu Monoclonal Antibodies (MAb) have become a crucial therapeutic agent in a number of anti-cancer treatments. Due to the inherent unstable nature of proteins in an aqueous formulation, a freeze-drying method has been developed to maintain long-term stability of biotherapeutics. The microstructural changes in Mabs during freezing, however, remain not fully described, and it was proposed that the formed morphology of freeze drying samples could affect the final product quality after reconstitution. Furthermore, it is well known that proteins tend to aggregate during the freezing process if a careful processing procedure is not formulated. Small Angle Neutron Scattering (SANS) is a powerful tool to investigate the structural properties and interactions of Mabs during various stages of lyophilization in situ. Here we present the SANS results of freeze-thaw studies on two MAbs at several different freezing temperatures. While the chosen proteins share a significant sequence homology, their freezing properties are found to be strikingly distinctive. We also show the effect of excipients, concentration and quenching speed on the final morphology of the frozen samples. These findings provide critical information for more effective lyophilization schemes for therapeutic proteins, as well as increase our understanding on structural properties of proteins under cryogenic conditions. [Preview Abstract] |
Tuesday, March 19, 2013 4:30PM - 4:42PM |
J34.00007: Shear-Dependent Interactions in Rheology Modifier (RM)-Latex Suspensions Tirtha Chatterjee, Alan I. Nakatani, Antony K. VanDyk Paint viscosity, under shear is governed by its shear-induced structure which in turn controls the application properties. The micro and macroscopic structure of the RM-latex combinations under shear is central to understand paint application behavior. Using in-situ shear-small-angle neutron scattering (shear-SANS) the RM-latex structure has been studied. All studies reported here are performed on acrylic-based latex with different hydrophobically modified ethoxylated urethane (HEUR) RM varying in their hydrophobe density/chain. At a quiescent condition, latex and RM form a spherical core-shell structure, with latex particles being the core and adsorbed RMs on the surface forming the shell. The shell thickness decreases with increasing RM hydrophobe density/chain. Under shear, the solvent (D2O/H2O) is squeezed out (hydrodynamic squeezing) from the swollen RM chains and the shell structure becomes denser and \textit{anisotropic} due to differing degrees of compression along the flow and vorticity directions. An effective shear-dependent latex-RM hydrodynamic volume fraction has been calculated using SANS structural data. High shear viscosity calculated on the basis of effective hydrodynamic volume using existing models do not match with the experimental data. This suggests the existence of RM molecule mediated interactions even at high shear rate. [Preview Abstract] |
Tuesday, March 19, 2013 4:42PM - 4:54PM |
J34.00008: Multi-body effects in Charged Colloids - Polyelectrolyte systems Victor Pryamitsyn, Venkat Ganesan Multibody effects upon the electrostatic interaction between particles, polyelectrolyte molecules and monovalent ions were analyzed within Poisson-Boltzmann approximation. The numerical self-consistent field (SCF) theory for a polymer - nanoparticles systems was developed for a mixture of quenched polyelectrolytes and charged and uncharged particles and the pseudo-spectral method was used to solve polymer SCF equations in three dimensions within the Grand Canonical Ensemble for polymer and ions. A calculation of the free energies of a single particle and of two particles in polyelectrolyte solutions allowed us to calculate respectively the particle insertion free energy and particle-particle interactions as a function of the properties of solution, polymer-particle interaction and particle size. By explicitly calculating the free energy of three particles after subtraction of the contributions from two-body interaction allowed us to calculate effective contribution of 3-body particle-particle interactions in polyelectrolyte -particles systems. We have found that the polyelectrolyte mediated two body interactions are repulsive for the larger particle-particle distances and lower polymer concentrations. Interestingly, such an electrostatic repulsion exists even if particles have [Preview Abstract] |
Tuesday, March 19, 2013 4:54PM - 5:06PM |
J34.00009: A density functional approach to model highly charged spherical colloids in electrolyte mixtures Bharat Medasani, Zaven Ovanesyan, Marcelo Marucho We present a classical density functional (DFT) approach to study the effects of ion size asymmetry, ion-ion correlation and solvent excluded volume on the structural and thermodynamic properties of strongly interacting charged systems. The hard sphere correlation effects are modeled non-perturbatively with weighted density approximation, where as electrostatic correlations are modeled perturbatively within the mean spherical approximation. The present DFT approach is able to describe macro-ions in electrolytes comprising neutral hard sphere mimicking water molecules and ions with dissimilar valence and realistic sizes and densities. We applied the theory to study spherical electric double layers and obtained results in good agreement with simulations. We calculated ion profiles, integrated charge, mean electrostatic potential, ionic coordination number, zeta potential, and inverse differential capacity at different conditions. For higher surface charge on macromolecule, charge inversion is noticed and when the counter-ions are bigger than co-ions, surface charge amplification is observed. Layering and screening effects are more pronounced when water molecules are explicitly considered. This work has potential applications in bio-electrostatics and colloidal engineering. [Preview Abstract] |
Tuesday, March 19, 2013 5:06PM - 5:18PM |
J34.00010: Measuring inter-nucleosome interactions and the roles of histone tails Steven Howell, Kurt Andresen, Isabel Jimenez-Useche, Chongli Yuan, Xiangyun Qiu Nucleosome is the first level of genome organization and regulation in eukaryotes, where negatively charged DNA is wrapped around positively charged histone proteins. Being a DNA-protein complex of biological origin, nucleosome is also a model multi-phasic nanoparticle with heterogeneous charge distributions and brush-like flexible tails of the histone proteins. In solutions of nucleosomes, electrostatic forces dominate inter-nucleosome interactions at long range while specific contacts, in particular the flexible histone tails, guides short range interactions. We have thus quantified how the ions from salts (KCl, MgCl2) modulate the inter-nucleosome pair potential by modeling the total small angle x-ray scattering profiles. We additionally elucidated the individual role of the charged tails of histones H3 and H4. We found that measured effective changes at low salt concentrations are about 1/5th of theoretically predicted renormalized charges and that H4 tail deletion suppresses the attraction at high salt concentrations to a larger extent than H3 tail deletion. [Preview Abstract] |
Tuesday, March 19, 2013 5:18PM - 5:30PM |
J34.00011: Multivalent Colloids through DNA Patchy Particles Yufeng Wang, Yu Wang, Dana Breed, Vinothan Manoharan, Lang Feng, Andrew Hollingsworth, Marcus Weck, David Pine We demonstrate a general method for creating the colloidal analogs of atoms with multiple valences: colloidal particles with chemically functionalized patches that can form highly directional specific bonds. The valences of these ``colloidal atoms'' possess all the common symmetries characteristic of hybridized atomic orbitals, including sp, sp$^{2}$, sp$^{3}$, sp$^{3}$d, sp$^{3}$d$^{2}$, and sp$^{3}$d$^{3}$. The chemical functionality of the patches is programmable and specific using DNA with single-stranded sticky ends, thereby creating colloidal atoms from which different kinds of ``colloidal molecules'' can be assembled, including the colloidal analogs of carbon dioxide and tetrahedrally coordinated methane. The bonds between these new colloidal atoms are highly directional and fully reversible with temperature. [Preview Abstract] |
Session J35: Superconductivity: Vortices II
Sponsoring Units: DCMPRoom: 343
Tuesday, March 19, 2013 2:30PM - 2:42PM |
J35.00001: Doubling of the Critical Current Density of 2G-YBCO Coated Conductors through proton irradiation Ulrich Welp, Ying Jia, Wai-Kwong Kwok, Marty Rupich, Steven Fleshler, Asfghar Kayani We report on magnetization and transport measurements of the critical current density of commercial 2G YBCO coated conductors before and after proton irradiation. The samples were irradiated along the c-axis with 4 MeV protons to a fluence of 1.5x10$^{\mathrm{16}}$ p/cm$^{\mathrm{2}}$. We find that at temperatures below 50 K, proton irradiation increases J$_{\mathrm{c}}$ by a factor of 2 in low fields and increases up to 2.5 in fields of 7 T. At 77 K, proton irradiation is less effective in enhancing the critical current. Doubling of J$_{\mathrm{c}}$ in fields of several Tesla and at temperatures below 50 K will be highly beneficial for applications of coated conductors in rotating machinery, generators and magnet coils. - Work supported by the US DoE-BES funded Energy Frontier Research Center (YJ), and by Department of Energy, Office of Science, Office of Basic Energy Sciences (UW, WKK), under Contract No. DE-AC02-06CH11357. [Preview Abstract] |
Tuesday, March 19, 2013 2:42PM - 2:54PM |
J35.00002: Vortex dynamics in Co-doped and K-doped BaFe$_{2}$As$_{2}$ with point defects Toshihiro Taen, Takahiro Ohori, Fumiaki Ohtake, Yasuyuki Nakajima, Tsuyoshi Tamegai, Kunihiro Kihou, Shigeyuki Ishida, Hiroshi Eisaki, Hisashi Kitamura The discovery of iron-based superconductors urges scientists and engineers to study not only superconducting mechanism but also possible applications. In view of this situation, it is important to study vortex dynamics for understanding fundamental properties as well as for suggesting a suitable fabrication process in this system. In particular, the interaction between vortices and defects attract tremendous attention, which is because this interaction is responsible for finite critical current density $J_c$. The interaction changes with dimensionality and morphology of defects. In cuprate superconductors, vortex manifold shows vortex glass phase with point defects and Bose glass phase with columnar defects. Besides, in both cases, $J_c$ shows pronounced enhancement compared with that in a pristine sample. We have already reported the enhancement of $J_c$ by the introduction of point or columnar defects in the case of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ crystal. In this talk, we show the results in proton-irradiated BaFe$_2$As$_2$ with electron- or hole-doping. The quantitative analysis reveals the doubling of pinning potential without changing the glassy exponent in Co-doped compounds, in addition to 2.5 times enhancement of $J_c$. Similar effects are observed in K-doped crystals. [Preview Abstract] |
Tuesday, March 19, 2013 2:54PM - 3:06PM |
J35.00003: Fundamentals of free flux flow: proposed studies J.A. Alexander, O. Gafarov, A.A. Gapud, J.Z. Wu Although much is known about free flux flow (FFF) in superconductors -- in which pinning is insignificant compared to interactions between quantized vortices -- there still remain questions concerning fundamental dynamics. Building on our previous work in correlating FFF with vortex core size (\textit{PRB} \textbf{80}, 134524), we propose three new studies examining more deeply the normal state in the vortex core and interactions between vortices. A correlation between scattering inside cores and the viscosity of FFF has not been explicitly determined; this may be investigated by probing the effect of scattering centers created by proton irradiation. Using results of previous irradiation work, one could control the extent of normal state scattering while monitoring effects on FFF. Questions also exist concerning vortex motion in channels with widths approaching that of individual vortices -- as determined solely by inter-vortex interactions. Studies have suggested that flux flow through constrictions could imitate ``jamming'' in the collective motion of \textit{grains}: Under certain conditions, it is possible for grains to form a barrier, blocking flow. More than just qualitatively comparing flux flow and granular flow to find evidence of jamming, we propose a new experiment for quantitatively modeling flux jamming by realizing the flux flow equivalent of granular jamming in a ``hopper''. In the same way, we also propose a FFF equivalent of another granular-flow phenomenon, ``non-Newtonian'' fluids, where rapid shear causes jamming. [Preview Abstract] |
Tuesday, March 19, 2013 3:06PM - 3:18PM |
J35.00004: Vortex core size due to the quasiparticle interference effect in cuprate superconductors Hong-Yi Chen We investigate the vortex core properties by solving the Bogoliubov de-Gennes equations for the t-t'-U-V Hamiltonian. The double peaks structure of the local density of states at the vortex core center characterizes the vortex core state. The local density of states maps have been numerically obtained near the slightly underdoping for the energy at the vortex core state. It is found that the field induced spin-density wave would cause the vortex core shrinking as the magnetic increases. We also found that the quasiparticle interference effect would affect the vortex core shrinking that the core size is independent the strength of the applied magnetic field. [Preview Abstract] |
Tuesday, March 19, 2013 3:18PM - 3:30PM |
J35.00005: Field-induced Dirac fermions and Fermi-surface resonance-scattering in the vortex-lattice cores of strongly type-II superconductors Tsofar Maniv, Vladimir Zhuravlev A remarkable relationship between the formation of Dirac fermions in the vortex lattice of a clean 2D strongly type-II superconductor at high magnetic fields and a peculiar magneto-quantum oscillations effect is revealed. It is shown that at the magnetic fields where the low-lying BdG quasi-particle dispersion has a Dirac cone structure, dHvA oscillations amplitude is sharply modified due to Fermi-surface resonance-scatterings occurring in core regions of the vortex lattice. A Dirac cone is created at each vortex core in the reciprocal vortex lattice at magnetic fields where the effective Zeeman spin-splitting vanishes and the chemical potential is in the middle of a Landau band (M.R.Norman and A.H.MacDonald, Phys.Rev. B54 4239 (1996); Z.Tesanovic and P.Sacramento, Phys.Rev.Lett.80 1521 (1998); T.Maniv, et al., Rev.Mod.Phys.73 867 (2001)). Under these resonance conditions coherent BdG quasi-particle scatterings are singularly enhanced leading to ``erratic,'' quasi-periodic modulation of the dHvA oscillation amplitude as a function of 1/B (V.Zhuravlev and T.Maniv, Phys.Rev. B85 104528 (2012)). For a spin-triplet superconductor in the presence of commensurate arrays of pinning centers, an ``exotic'' possibility of field-induced sub-lattices of bound Majorana fermions is discussed. [Preview Abstract] |
Tuesday, March 19, 2013 3:30PM - 3:42PM |
J35.00006: Theory of de Haas van Alphen Oscillations in Superconductors with Preformed Pair Yan He, Peter Scherpelz, Kathryn Levin We address recent observations of quantum oscillatory behavior in high temperature superconductors within a preformed pair theory of the pseudogap phase. These non condensed pairs, present in the normal and superconducting phases are shown to be reflective of a slightly distorted vortex lattice phase $^1$. Importantly they contribute a separate additive (``bosonic") component to the field dependent thermodynamics in addition to that arising from fermions. In this talk we report our findings that the bosonic component appears to display the same Lifshitz-Kosevich oscillation frequencies as also found in the mixed state of conventional superconductors (associated with gapless fermionic states). We explore the different amplitude weighting factors for the bosonic and fermionic contributions and the effects of varying the pairing symmetry from $s$ to $d$-wave. For the latter and for many properties, the bosonic component is most strongly associated with the anti-node, while the fermionic contribution comes from the node. Ref.1. Pseudogap Effects in Fermi Gases in the Presence of a Strong Effective Magnetic Field, P. Scherpelz, Dan Wulin, K. Levin and A. K. Rajagopal, ArXiv 1207.4826 [Preview Abstract] |
Tuesday, March 19, 2013 3:42PM - 3:54PM |
J35.00007: Quantum oscillations in d-wave superconductors with loop current order Luyang Wang, Oskar Vafek Coexistence of $d$-wave superconductivity and Fermi pockets in underdoped high temperature cuprate superconductors has been suggested by recent quantum oscillation experiments. The origin of Fermi pockets in the superconducting state has been under debate. Here we report numerical results of quantum oscillations of the specific heat in the vortex state of a $d$-wave superconductor in the presence of loop current order, which gives rise to Fermi pockets coexisting with nodal $d$-wave superconductivity. First, we calculate the specific heat within a lattice tight-binding model, varying the loop current order and the external magnetic field. Second, we investigate the same problem in the continuum linearized limit, performing Franz-Tesanovic transformation, and find that the Bogoliubov Dirac quasiparticles also couple to a vector-like potential which corresponds to a highly nonuniform magnetic field. The results thus found are consistent with the tight-binding calculation. While the energy spectrum is qualitatively different from Landau levels, we find oscillations of the specific heat that in an intermediate temperature range approximately follow Onsager relation. [Preview Abstract] |
Tuesday, March 19, 2013 3:54PM - 4:06PM |
J35.00008: Oscillations of the Magnetoresistance and the Critical Current in MoGe Thin Films with Hole-arrays in Square Vortex-ice Geometry Michael Latimer, Golibjon Berdiyorov, Ralu Divan, Il Woong Jung, Zhili Xiao, Francois Peeters, Wai-Kwong Kwok Resistivity measurements on MoGe thin films containing hole-arrays in square vortex-ice configuration were carried out to study the formation of a frustrated vortex state. MoGe thin films of 20 nm thick were prepared by sputter-deposition and holes with spacings of 200 nm - 400 nm and diameters from 100 nm to 300 nm were introduced into them using focused-ion-beam milling. We observed unusual matching effects: depending on the hole-hole spacing and the experimental temperature, the pinning enhancement at the half matching field can be stronger than that at the first matching field, as divulged by the deeper dip in the magnetoresistance and the higher peak in the critical current. Computer simulations within the nonlinear time-dependant Ginzburg-Landau theory reveal an origin of vortex jamming in the square vortex-ice state, indicating the first experimental realization of a square vortex-ice. [Preview Abstract] |
Tuesday, March 19, 2013 4:06PM - 4:18PM |
J35.00009: Thermal Hall effect in the underdoped cuprate superconductor YBa$_2$Cu$_3$O$_{6.5}$ Phuan Ong, Max Hirschberger, Tian Liang, Toshinao Loew, Wei Li Lee, R. Ritz, Bernhard Keimer The thermal Hall conductivity $\kappa_{xy}$ (Righi-Leduc effect) is tailor-made to probe the transport properties of Bogolyubov quasiparticles (QPs) in a superconductor because neither the phonons nor vortices contribute to the off-diagonal response. We report measurements of $\kappa_{xy}$ in untwinned crystals of underdoped YBa$_2$Cu$_3$O$_{6.5}$, extending from 100 K to 15 K in fields $H$ up to 14 T. Several key features will be described. At all temperatures $T$, the QPs are hole-like. However, there is a small negative contribution that appears just below $T_c$. Below 30 K, the curve of $\kappa_{xy}/T$ vs. $H$ approaches an apparent universal step-like profile that may reflect the behavior of long-lived Dirac excitations confined to orbits around the gap nodes in an intense magnetic field. Measurements to much lower $T$ and higher $H$ ($\sim$ 32 T) are planned. [Preview Abstract] |
Tuesday, March 19, 2013 4:18PM - 4:30PM |
J35.00010: Zero field Hall effect in chiral p-wave superconductors near the Kosterlitz-Thouless transition Chun Kit Chung, Yusuke Kato A theory of vortex dynamics developed by Ambegaokar, Halperin, Nelson, and Siggia is employed to study two-dimensional chiral $p$-wave superconducting systems. Due to unequal values of drag coefficients of opposite vorticity specific to chiral $p$-wave cases, we find that a ``convective'' term, in addition to diffusivity, should enter the dynamical equations governing vortex pair unbinding process. As a consequence, we find a matrix form dielectric function and a new contribution to Hall conductance $\sigma_{xy}$ automatically follows even in zero magnetic field. We predict both the Hall conductance and power dissipation show a peak across the Kosterlitz-Thouless transition temperature. Their frequency dependence is also discussed. It is found that a set of frequency-dependent length scales, which controls the truncation of renormalization process, depends on both the convective and diffusive motion of vortices. [Preview Abstract] |
Tuesday, March 19, 2013 4:30PM - 4:42PM |
J35.00011: Domain Wall and Reverse Domain Superconductivity in Superconducting/Ferromagnet Hybrid Structures S. Moore, J. Fedor, V. Novosad, S. Ciocys, G. Karapetrov, M. Iavarone We have investigated the effect of inhomogeneous stray fields of a ferromagnet on the nucleation of the superconducting order parameter in superconductor/ferromagnet (S/F) systems magnetically coupled. Low-temperature scanning tunneling microscopy and spectroscopy measurements were performed on a Pb/[Co/Pd] system, which has a nontrivial H-T phase diagram under externally applied magnetic fields. Conductance maps and tunneling spectroscopy of these systems show clear indications of domain wall and reverse domain superconductivity. Close to the transition temperature (T$_{c}$) and in zero applied field, we visualized the emergence of superconductivity in regions above the separation between adjacent magnetic domains on length scales of the order of the coherence length. We also find an increase in T$_{c}$ for certain values of applied field above magnetic domains of the opposite polarity. [Preview Abstract] |
Tuesday, March 19, 2013 4:42PM - 4:54PM |
J35.00012: Vortex confinement by magnetic domains in superconductor-ferromagnet bilayers Marta Z. Cieplak, Z. Adamus, M. Konczykowski, L.Y. Zhu, C.L. Chien, X.M. Cheng We use a line of miniature Hall sensors to study the effect of magnetic-domain-induced vortex confinement on the flux dynamics in a superconductor/ferromagnet bilayer. A single tunable bilayer is built of a ferromagnetic Co/Pt multilayer with perpendicular magnetic anisotropy and a superconducting Nb layer, with the insulating layer in between to avoid proximity effect. The magnetic domain patterns of various geometries are reversibly predefined in the Co/Pt multilayer using the appropriate magnetization procedure. The magnetic domain geometry strongly affects vortex dynamics, leading to geometry-dependent trapping of vortices at the sample edge, nonuniform flux penetration, and strongly nonuniform critical current density. With the decreasing temperature the magnetic pinning increases but this increase is substantially weaker than that of the intrinsic pinning. The analysis of the initial flux penetration suggests that vortices may form various vortex structures, including disordered Abrikosov lattice or single and double vortex chains, in which minimal vortex-vortex distance is comparable to the magnetic penetration depth. [Preview Abstract] |
Tuesday, March 19, 2013 4:54PM - 5:06PM |
J35.00013: Magnetic decoration imaging of a superconductor-ferromagnet bilayer Maxim Marchevsky Magnetic decoration imaging technique is used to study flux distribution and local vortex order in a superconducting Nb film deposited on a ferromagnetic yttrium-iron garnet substrate. Ambient field-cooled decoration patterns reveal concentration of vortices in the Nb along the labyrinthine magnetic domains of the garnet. Re-magnetization cycles result in a formation of the complex ``vortex foam'' structure due to a coupled magnetic dynamics of the superconductor-ferromagnet (S/F) bilayer system. We analyze density variations and spatial distribution of vortices in these structures. Flux exit patterns obtained upon removal of an external magnetic field show large-scale inhomogeneity of the vortex flow. Results are compared to the recent theoretical predictions and reported studies of S/F systems by other imaging methods. [Preview Abstract] |
Tuesday, March 19, 2013 5:06PM - 5:18PM |
J35.00014: Vortex coalescence and type-1.5 superconductivity in Sr2RuO4 Egor Babaev, Julien Garaud, Daniel Agterberg Recently vortex coalescence was reported in superconducting Sr2RuO4 by several experimental groups for fields applied along the c-axis. We argue that Sr2RuO4 is a type-1.5 superconductor with long-range attractive, short-range repulsive intervortex interaction. The type-1.5 behavior stems from an interplay of the two orbital degrees of freedom describing this chiral superconductor together with the multiband nature of the superconductivity. These multiple degrees of freedom give rise to multiple coherence lengths, some of which are larger and some smaller than the magnetic field penetration length, resulting in nonmonotonic intervortex forces. The talk is based on Phys. Rev. B 86, 060513(R) (2012) [Preview Abstract] |
Tuesday, March 19, 2013 5:18PM - 5:30PM |
J35.00015: Heirarchical mesophases of vortex matter in layered and multi-component superconductors Christopher Varney, Karl Sellin, Qingze Wang, Hans Fangohr, Egor Babaev Based on several models for Type-1.5 and hybrid Type-1/Type-2 layered superconductors, we examine the zero temperature properties of vortices with Langevin dynamics and Monte Carlo simulations. We demonstrate that inter-vortex forces with multiple length scales can result in unusual mesophases of vortex structures, such as clusters of clusters, concentric rings, clusters in a ring, and stripes in a cluster. [Preview Abstract] |
Session J36: Superconductivity: Properties and Phenomena
Sponsoring Units: DCMPChair: Zhigang Wu, Colorado School of Mines
Room: 344
Tuesday, March 19, 2013 2:30PM - 2:42PM |
J36.00001: Copper Substitution in Iron Telluride: A Phase Diagram Patrick Valdivia, Thomas Forrest, Costel Rotundu, Jinsheng Wen, Edith Bourret-Courchesne, Robert Birgeneau Investigations of superconductivity in the FeCh family (Ch$=$S,Se,Te) have produced rich physics and notable materials challenges despite the ostensible simplicity of the system. We have studied the effects of copper substitution in iron-telluride. Our interests in this system are two-fold: to compare the properties of copper substitution in iron-telluride with those in the selenium-substituted compounds, and to study if there are additional controllable factors in this system such as the total excess metal content, and the distribution of iron and copper atoms over the two sites. Our initial investigations into this phase diagram involve both diffraction and transport measurements which may be used address these research goals. [Preview Abstract] |
Tuesday, March 19, 2013 2:42PM - 2:54PM |
J36.00002: Kerr effect as evidence of gyrotropic order in the cuprates Srinivas Raghu, Pavan Hosur, Steven Kivelson, Aharon Kapitulnik, Joseph Orenstein The Kerr effect can arise in a time-reversal invariant dissipative medium that is ``gyrotropic", {\it i.e.} one that breaks spatial inversion and all mirror symmetries. Examples of such systems include electron analogs of cholesteric liquid crystals, and their descendants, such as systems with chiral charge ordering. We present arguments that the striking Kerr onset, {\it which is not invertible by application of a magnetic field}, in the pseudogap phase of a large number of cuprate high temperature superconductors is evidence of chiral charge ordering. We discuss additional experimental consequences of a phase transition to a gyrotropic system. [Preview Abstract] |
Tuesday, March 19, 2013 2:54PM - 3:06PM |
J36.00003: Anomalous Hall effect in current-carrying states of matter: topology, commensuration effects, and application to Kerr measurements in the underdoped cuprates Catherine Kallin, Edward Taylor We calculate the anomalous Hall conductivity for states characterized by patterns of spontaneous currents. Using an exact Ward identity, we find that the DC Hall conductivity is topological provided the current pattern is commensurate and the Fermi surface is fully gapped. For incommensurate patterns, the DC Hall conductivity can be infinite, analogous to the infinite conductivity of a sliding charge density wave. We also discuss the optical Hall conductivity at high frequencies, in connection with Kerr rotation experiments performed on the underdoped cuprates. [Preview Abstract] |
Tuesday, March 19, 2013 3:06PM - 3:18PM |
J36.00004: ABSTRACT HAS BEEN MOVED TO B29.00015 |
Tuesday, March 19, 2013 3:18PM - 3:30PM |
J36.00005: Thermodynamic studies of Cu$_{0.10}$TiSe$_2$ via ac-calorimetry and Hall-probe magnetometry Zuzana Pribulova, Jozef Kacmarcik, Peter Samuely, Zuzana Medvecka, Viktoria Solteszova, Petra Barancekova Husanikova, Vladimir Cambel, Goran Karapetrov TiSe$_{2}$ is a compound with the charge density wave (CDW) transition at 200 K, the CDW state is gradually suppressed when intercalated by copper and for certain amount of Cu superconductivity occurs. We report the studies of the critical fields of an optimally doped sample with a superconducting transition at $T_{\mathrm{c}}$ $\sim$ 3.9 K. Upper critical field $H_{\mathrm{c2}}$ has been derived from the specific heat measurements while the lower critical field $H_{\mathrm{c1}}$ has been extracted from local magnetization measurements using miniature Hall-probes. The temperature dependence of $H_{\mathrm{c2}}$ and $H_{\mathrm{c1}}$ and its anisotropy will be presented. Moreover, local magnetometry using array of 8 Hall-probes shows that vortices after penetration into the sample move towards the centre resulting into a dome-shape induction profile suggesting relatively low pinning. [Preview Abstract] |
Tuesday, March 19, 2013 3:30PM - 3:42PM |
J36.00006: Study of Inhomogeneous Organic Superconductors Charles C. Agosta, Christopher Conroy, Daniel Ellowitz, William Von Noppen In many anisotropic superconductors, we have found evidence that they are inhomogeneous superconductors, such as those predicted by Fulde and Ferrell and Larkin and Ovchinnikov (FFLO), at the extremes of low temperature and high magnetic field. A FFLO superconductor has an order parameter with nonzero pair momentum that oscillates periodically as a function of distance, unlike traditional superconductors where the order parameter is uniform. During the last several years, our research group at Clark University has made careful and systematic measurements of quasi-2D organic superconductors that suggest an FFLO state can be stabilized in three different organic conductors if a magnetic field is applied precisely parallel to the conducting layers. We will compare our results with theoretical expressions that we have modified from the current literature, with the goal of extracting quantitative results from the phase diagram data such as the Maki parameter and scattering times. We will also describe improvements to our pulsed magnetic field - tunnel diode oscillator penetration depth apparatus. [Preview Abstract] |
Tuesday, March 19, 2013 3:42PM - 3:54PM |
J36.00007: Superconductivity in the misfit compound of (LaSe)$_{1.14}$(NbSe$_2)$: STM/S, calorimetric and magnetization studies P. Samuely, P. Szabo, J. Kacmarcik, Z. Pribulova, T. Samuely, J.G. Rodrigo, C. Marcenat, T. Klein, L. Cario (LaSe)$_1.14$(NbSe$_2)$ is a low temperature superconductor with $T_{c}$ around 1.2 K belonging to the family of the lamellar chalcogenides. Electron transfer from the LaSe to the NbSe$_2$ slab results in a natural layered system of the insulating LaSe and (super) conducting NbSe$_2$ sheets. In our previous investigations of the anisotropic transport [P. Szab\'{o} et al., Phys. Rev. Lett. 86, 5990 (2001)] indications have been found that this system behaves as a stack of Josephson-coupled superconducting NbSe$_2$ sheets separated by insulating LaSe layers. We test this hypothesis by STM/S measurements at subkelvin temperatures and in magnetic fields. Superconducting energy gap obtained by STM opens at the same temperature and field where the interlayer resistivity starts to increase before drop to zero value. Before any conclusions are made homogeneity of the superconducting parameters is to be tested. STM indicates large areas without any gap but calorimetric measurements have shown the bulk superconductivity and magnetization revealed extremely low pinning. [Preview Abstract] |
Tuesday, March 19, 2013 3:54PM - 4:06PM |
J36.00008: Microwave stimulated enhancement of the upper critical field in type-II superconducting films Antonio Lara, Ahmad Awad, Alejandro Silhanek, Victor Moshchalkov, Farkhad Aliev A few decades ago it was theoretically predicted and experimentally observed that moderate power electromagnetic fields in the GHz range could stimulate superconductivity, increasing the superconducting critical temperature and critical current. Here, on the example of Pb films without / with periodic vortex pinning centers in the form of circular Permalloy dots in the magnetic vortex state, we investigate experimentally how the microwave stimulated superconductivity phenomenon behaves in the presence of a superconducting vortex system. Namely, we present the first, to our best knowledge, experimental investigation of influence of microwave induced superconductivity on the upper critical field of type II superconducting films. An enhancement of the critical temperature of the film of up to 0.1{\%} and of the upper critical field of up to 10{\%} have been observed at a drive frequency of 6 GHz. A qualitative explanation for the observed difference in the dependence of the upper critical field on the temperature and microwave power, depending on the nearly parallel or perpendicular alignments of the field to the sample, is provided. [Preview Abstract] |
Tuesday, March 19, 2013 4:06PM - 4:18PM |
J36.00009: New Evidences for the observation of the Higgs boson in the Superconductor 2H-NbSe$_2$ Marie-aude Measson, Bertrand Clair, Yann Gallais, Maximilien Cazayous, Pierre Rodi\`ere, Laurent Cario, Alain Sacuto We provide here new evidences for the observation of the amplitude mode of the superconducting order parameter, the so-called Higgs Boson, in 2H-NbSe$_2$. We report quantitatively comparative electronic Raman measurements on the dichalcogenides 2H-NbSe$_2$, whose superconductivity (SC) coexists with a charge density wave order (CDW), and 2H-NbS$_2$, which exhibits only the SC. A SC pair breaking peak develops below T$_c$ in 2H-NbS$_2$ whose intensity is much smaller than the peak associated with the SC in 2H-NbSe$_2$. Thus, the peak observed in 2H-NbSe$_2$ below T$_c$ certainly doesn't get its intensity only from the superconducting condensate. Moreover, we measure precisely a spectral weight transfer from the amplitude mode of the CDW to the SC peak in 2H-NbSe$_2$, versus decreasing temperature. The total spectral weight for both peaks is constant within $\pm~3\%$. This result is consistent with the theory of the observation of a Higgs mode thanks to its coupling with an amplitudon developed by Littlewood and Varma. This result complements what was firstly observed by Sooryakumar et Klein under magnetic field. [Preview Abstract] |
Tuesday, March 19, 2013 4:18PM - 4:30PM |
J36.00010: Imaging the Anisotropic Nonlinear Meissner Effect in Unconventional Superconductors Steven Anlage, A.P. Zhuravel, B.G. Ghamsari, C. Kurter, J. Abrahams, S. Remillard, P. Jung, A.V. Lukashenko, Alexey Ustinov We have directly imaged the anisotropic nonlinear Meissner effect in an unconventional superconductor through the nonlinear electrodynamic response of both (bulk) gap nodes and (surface) Andreev bound states [1]. A superconducting thin film is patterned into a compact self-resonant spiral structure, excited near resonance in the radio-frequency range, and scanned with a focused laser beam perturbation. At low temperatures, direction-dependent nonlinearities in the reactive and resistive properties of the resonator create photoresponse that maps out the directions of nodes, or of bound states associated with these nodes, on the Fermi surface of the superconductor. The method is demonstrated on the nodal superconductor YBa\textunderscore 2Cu\textunderscore 3O\textunderscore 7-$\backslash $delta and the results are consistent with theoretical predictions for the bulk and surface contributions. [1] A. P. Zhuravel, \textit{et al}., \underline {arXiv:1208.1511}. ~ [Preview Abstract] |
Tuesday, March 19, 2013 4:30PM - 4:42PM |
J36.00011: X-ray edge singularity in resonant inelastic x-ray scattering (RIXS) Robert Markiewicz, John Rehr, Arun Bansil We develop a lattice model based on the theory of Mahan, Nozi{\'e}res, and de Dominicis for x-ray absorption to explore the effect of the core hole on the RIXS cross section. The dominant part of the spectrum can be described in terms of the dynamic structure function $S(q,\omega )$ dressed by matrix element effects, but there is also a weak background associated with multi-electron-hole pair excitations. The model reproduces the decomposition of the RIXS spectrum into well- and poorly-screened components. An edge singularity arises at the threshold of both components. Fairly large lattice sizes are required to describe the continuum limit. [Preview Abstract] |
Tuesday, March 19, 2013 4:42PM - 4:54PM |
J36.00012: Using photon to probe spin excitations Chunjing Jia, Cheng-Chien Chen Chen, Brian Moritz, Tom Devereaux Elementary spin excitations have attracted considerable attention in the understanding of strongly correlated materials, especially in high temperature superconductors where a full understanding of spin dynamics might reveal important information where the phase emerges in proximity of magnetic order. Photon spectroscopies, such as resonant inelastic x-ray scattering (RIXS) and optical Raman scattering, are powerful tools for the measurement of spin excitations. In this presentation, I will discuss the simulation of various spectroscopies that can reveal spin excitations, using both single- and multi-orbital models. I will show that transition metal in-direct RIXS provides information about two-magnon excitations at low energies in addition to the usual charge transfer excitations; while direct RIXS measures single spin-flip (single magnon) excitations, making it a complementary technique to inelastic neutron scattering. I also will show that Raman scattering can probe two-magnon spin excitations in correlated materials. We track the evolution of these excitations as functions of momentum and doping. These results highlight the nature of spin excitations in correlated materials and are an important step in our understanding of the corresponding experiments in real materials [Preview Abstract] |
Tuesday, March 19, 2013 4:54PM - 5:06PM |
J36.00013: Investigation of 1/f flux noise in SQUIDs and Superconducting Qubits Antonio Puglielli, Steven Sendelbach, Taylor Klaus, Robert McDermott Low-frequency 1/f flux noise is a dominant source of dephasing in the Josephson phase and flux qubits. Recent work has revealed the presence of a high density of unpaired spins at the surfaces of superconducting thin films; it is now believed that these spins are the source of the noise, although the microscopic noise mechanism is not understood. We have recently shown that the dielectric encapsulation of the SQUID loop substantially impacts the noise magnitude and noise exponent. Here we describe experiments on SQUIDs and Josephson phase qubits designed to shed light on the underlying noise mechanism, and we describe efforts to develop Josephson phase qubits with reduced levels of 1/f flux noise and improved dephasing times. [Preview Abstract] |
Tuesday, March 19, 2013 5:06PM - 5:18PM |
J36.00014: Torque magnetization study of superconducting fluctuations in single-layer cuprates: new implications for the phase diagram Guichuan Yu, R. Frink, D.-D. Xia, X. Zhao, N. Bari\v{s}i\'{c}, R.-H. He, N. Kaneko, T. Sasagawa, Y. Li, A. Shekhter, M. Greven We have studied the superconducting fluctuations above the transition temperature by angle-dependent torque magnetization in single-layer La$_{\mathrm{2-x}}$Sr$_{\mathrm{x}}$CuO$_{\mathrm{4}}$ (LSCO), Bi$_{\mathrm{2}}$(Sr,La)$_{\mathrm{2}}$CuO$_{\mathrm{6+\delta }}$ (Bi2201), and HgBa$_{\mathrm{2}}$CuO$_{\mathrm{4+\delta }}$ (Hg1201). The latter is a more ideal compound, with a maximum $T_{\mathrm{c}}$ of 97 K, more than twice the values for LSCO and Bi2201. In all three cases, the diamagnetic signal above $T_{\mathrm{c}}$ vanishes in an unusual exponential fashion, and at a rate that is universal, despite the dramatic differences in $T_{\mathrm{c}}$ [G. Yu \textit{et al}., arXiv:1210.6942v1]. These observations suggest that anomalies observed at much higher temperatures in both LSCO and Bi2201 are not associated with superconducting fluctuations. [Preview Abstract] |
Tuesday, March 19, 2013 5:18PM - 5:30PM |
J36.00015: Effect of thermal fluctuations in topological p-wave superconductors Bela Bauer, Roman M. Lutchyn, Matthew B. Hastings, Matthias Troyer We study the effect of thermal fluctuations on the topological stability of chiral p-wave superconductors. We consider two models of superconductors: spinless and spinful with a focus on topological properties and Majorana zero-energy modes. We show that proliferation of vortex-antivortex pairs above the Kosterlitz-Thouless temperature $T_{KT}$ drives the transition from a thermal Quantum Hall insulator to a thermal metal/insulator, and dramatically modifies the ground-state degeneracy splitting. Therefore, in order to utilize 2D chiral p-wave superconductors for topological quantum computing, the temperature should be much smaller than $T_{KT}$. Within the spinful chiral p-wave model, we also investigate the interplay between half-quantum vortices carrying Majorana zero-energy modes and full-quantum vortices having trivial topological charge, and discuss topological properties of half-quantum vortices in the background of proliferating full-quantum vortices. [Preview Abstract] |
Session J37: Focus Session: Fe-based Superconductors: Pressure effects
Sponsoring Units: DMP DCOMPChair: Xiao-Jia Chen, Carnegie Institution of Washington
Room: 345/346
Tuesday, March 19, 2013 2:30PM - 3:06PM |
J37.00001: Evolution of superconductivity in Ca$_{\mathrm{1-x}}$La$_{\mathrm{x}}$Fe$_2$As$_2$ under pressure Invited Speaker: Shanta Saha The evolution of superconductivity in single crystals of the aliovalent La-doped CaFe$_2$As$_2$ is studied with both quasi-hydrostatic and hydrostatic applied pressures measuring transport, magnetic, and neutron scattering properties. The application of pressure to under doped samples of Ca$_{\mathrm{1-x}}$La$_{\mathrm{x}}$Fe$_2$As$_2$ suppresses the antiferromagnetic (AFM) transition and causes an abrupt appearance of superconductivity with $T_{\mathrm{c}}$ values similar to those (about 45 K) recently been reported at ambient pressure. This superconducting phase appears under both quasi-hydrostatic and hydrostatic pressures, indicating an intrinsic property of the observed superconducting state. Unlike transition metal-doped 122 iron-superconductors where superconductivity happily coexists with AFM, the little coexistence of SC and AFM appears to mimic that found in 1111 iron-superconductors, suggesting a similar phase diagram. The unusual dichotomy between lower-$T_{\mathrm{c}}$ systems that happily coexist with AFM and tendency for the highest-$ T_{\mathrm{c}}$ systems to show phase separation provides an important clue to the pairing mechanism in iron-based superconductors. [Preview Abstract] |
Tuesday, March 19, 2013 3:06PM - 3:18PM |
J37.00002: c-axis transport in rare-earth doped CaFe$_2$As$_2$ Johnpierre Paglione, T. Drye, R. Hu, S.R. Saha The discovery of a low volume fraction phase of superconductivity in rare-earth doped Ca$_{1-x}$R$_x$Fe$_2$As$_2$ with $T_c$= 47 K has sparked controversy over the nature of the observed superconductivity. However, an important aspect to understanding the behavior in these systems lies in understanding the role of the structural collapse wherein interlayer As atoms abruptly form a bond at sufficiently low temperatures, resulting in a $\sim$10\% reduction of the $c$-axis through a first-order transition. We will present measurements of electrical transport with currents directed along the crystallographic $c$-axis, discussing the implications for the superconducting phase and the nature of the band structure change through the structural collapse transition. [Preview Abstract] |
Tuesday, March 19, 2013 3:18PM - 3:30PM |
J37.00003: Inter-plane resistivity in single crystals Ca(Fe$_{1-x}$Co$_x$)$_2$As$_2$ with doping level variation Makariy Tanatar, S. Ran, S.L. Bud'ko, P.C. Canfield, Ruslan Prozorov CaFe$_2$As$_2$ undergoes sharp first order tetragonal-to-orthorhombic phase transition on cooling below $T_{SM}$=175 K, accompanied by stripe type antiferromagnetic ordering. The transition temperature can be suppressed to zero by application of pressure, revealing collapsed tetragonal high pressure phase, and partial superconductivity. It can also be suppressed by Co substitution of Fe. This doping suppresses structural and magnetic instabilities and induces bulk superconductivity with $T_c$ up to 17~K Ca(Fe$_{1-x}$Co$_x$)$_2$As$_2$ with $x$=2.8\% [1]. Here we report systematics of the temperature-dependent inter-plane resistivity in this Co-doped series of compounds over complete doping phase diagram.\\[4pt] [1] S. Ren S.L. Bud'ko, W.E.Straszheim, J. Soh, M.G.Kim, A.Kreyssig, A.I.Goldman and P.C.Canfield, Phys. Rev. B {\bf 85}, 224528 (2012). [Preview Abstract] |
Tuesday, March 19, 2013 3:30PM - 3:42PM |
J37.00004: Thermal expansion of CaFe$_2$As$_2$: effect of annealing and cobalt doping Sergey L. Bud'ko, Sheng Ran, Paul C. Canfield Careful choice of Co concentration and annealing/quenching temperature in the Ca(Fe$_{1-x}$Co$_x$)$_2$As$_2$ series allows for tuning the ground state of the from orthorhombic-antiferromagnetic to superconducting to collapsed tetragonal [1].In this talk temperature-dependent, $c$-axis, thermal expansion measurements on several sets of Co-doped CaFe$_2$As$_2$ single crystals that were subjected to a variety of annealing conditions will be presented. These samples were chosen to cover all salient regions of the 3D $x - T_{anneal} - T$ phase diagram. The thermal expansion signatures of different types of phase transitions observed in these series will be discussed and comparison with the other measurements will be made.\\[4pt] [1] S. Ran, {\it et al.}, PRB {\bf 85}, 224528 (2012). [Preview Abstract] |
Tuesday, March 19, 2013 3:42PM - 3:54PM |
J37.00005: London penetration depth under pressure in Fe-based superconductors Kyuil Cho, Makariy A. Tanatar, Charles P. Strehlow, Ruslan Prozorov Precisely measured low - temperature London penetration depth can be used as a sensitive tool to study the superconducting gap structure. Tunnel diode resonator (TDR) technique provides the ultimate sensitivity and resolution and it has been employed to study conventional and unconventional superconductors [1]. In case of Fe-based superconductors the dome - like evolution of the superconducting properties, including the gap anisotropy, has been found as a function of doping. While easier to work with, the doping also changes the scattering, especially important in Fe-based superconductors [1]. Pressure provides potentially cleaner alternative way for systematic study of the superconducting gap evolution. However, thus far no successful measurements on London penetration depth in the pressure cell were possible due to various background contributions. We have recently developed the pressure dependent London penetration depth measurement technique by combining a TDR technique with BeCu piston cell. Technical characteristics as well as the first results will be discussed. \\ $\lbrack$1$\rbrack$ R. Prozorov and V. G. Kogan, Rep. Prog. Phys. 74, 124505 (2011). [Preview Abstract] |
Tuesday, March 19, 2013 3:54PM - 4:06PM |
J37.00006: Magnetism dependent phonon anomaly in LaFeAsO observed via inelastic x-ray scattering Steven Hahn, Gregory Tucker, Jiaqiang Yan, Ayman Said, Bogdan Leu, R.W. McCallum, Ercan Alp, Thomas Lograsso, Robert McQueeney, Bruce Harmon The phonon dispersion was measured at room temperature (above T$_{\mathrm{N}}$ ) along (0,0,L) in the tetragonal phase of LaFeAsO using inelastic x-ray scattering. Magnetostructural effects are well documented in the AFe$_{2}$As$_{2}$-based (A$=$Ca,Sr,Ba,Eu) systems. Only recently have single crystals of RFeAsO (R$=$La,Ce,Pr,Nd,Sm,Gd)-based compounds become available. The experimentally observed splitting between two A$_{\mathrm{1g}}$ phonon modes at 22 and 26 meV is only produced in spin-polarized first-principles calculations imposing various types of antiferromagnetic order. Magnetostructural effects similar to those observed in the AFe$_{2}$As$_{2}$ materials are confirmed present in LaFeAsO. This is discussed in terms of the strong antiferromagnetic correlations that are known to persist above T$_{\mathrm{N}}$ and into the tetragonal phase. [Preview Abstract] |
Tuesday, March 19, 2013 4:06PM - 4:18PM |
J37.00007: Ru $L_2$ edge X-ray resonant magnetic scattering from Ba(Fe$_{0.795}$Ru$_{0.205})_2$As$_2$ compound Min Gyu Kim, J. Soh, J.C. Lang, Y. Choi, A. Thaler, E. Bourret-Courchesne, S.L. Bud'ko, P.C. Canfield, A. Kreyssig, A.I. Goldman, R.J. Birgeneau We have investigated the magnetic polarization of the Ru 4$d$ dopant states in Ba(Fe$_{0.795}$Ru$_{0.205})_2$As$_2$ using Ru $L_2$ edge x-ray resonant magnetic scattering (XRMS). We observed a XRMS signal at Q $=$ (1/2, -1/2, 3), which is consistent with the magnetic propagation vector for the stripe AFM ordering found in the parent BaFe$_2$As$_2$ compound. We find that the temperature dependence of the XRMS signal follows closely the temperature dependence of the Fe order as determined by our previous neutron diffraction measurement. Our observations show evidence that the Ru 4$d$ states may be spin-polarized. [Preview Abstract] |
Tuesday, March 19, 2013 4:18PM - 4:30PM |
J37.00008: High Pressure Low Temperature Studies of the Iron-Based Superconductor SrFe$_{2}$As$_{2}$ Gary Chesnut, Walter Uhoya, Jeffrey Montgomery, Antonio dos Santos, Jamie Molaison Iron-based superconductors are a critical clue in understanding the mechanism behind high temperature superconductivity. It is well-known that superconductivity is highly influenced by magnetic fields. Recent neutron scattering experiments were performed on SrFe$_{2}$As$_{2}$ to examine the nuclear and magnetic structure to a temperature of 89 K and a pressure of 4.3 GPa. The structural phase transition from tetragonal to orthorhombic was observed at T$_{\mathrm{o}} =$ 196 K with an increase in orthorhombic distortion with decreasing temperature. The neutron diffraction experiments revealed subtle, but interesting results at elevated pressures. [Preview Abstract] |
Tuesday, March 19, 2013 4:30PM - 4:42PM |
J37.00009: Pressure induced structural modifications in NaFe$_{1.99}$Co$_{0.01}$As$_2$ superconductor Elissaios Stavrou, Xiao-Jia Chen, Alexander Goncharov, A. Wang, Y. Yan, X. Luo, X. Chen NaFe$_{1.99}$Co$_{0.01}$As$_2$ superconductor with the tetragonal ThCr$_2$Si$_2$-type structure ($I4/mmm$) has been studied using x-ray diffraction and Raman spectroscopy up to 25 GPa (at RT). Recently, it was found that, for this compound, $T_c$ increases with pressure to a maximum of 32 K at 2.5 GPa. With further compression $T_c$ decreases up to 6 GPa, the highest pressure superconductivity has been detected. We report that, although NaFe$_{1.99}$Co$_{0.01}$As$_2$ remains in the ambient pressure phase, the lattice parameters evolution with pressure shows distinct behavior below and above a critical pressure Pc=2.5 GPa. This is accompanied by a subtle change of Raman spectra at Pc. Below Pc, a-axis increases while both the c-axis and the c/a axial ratio decrease. In contrast above Pc, both axes show a normal decrease and c/a remains almost constant. The different behavior of c-axis, below and above Pc, can be viewed as a modification of the initial tetragonal phase (T) to a collapsed tetragonal (CT) one. This is in line with previous studies on 122 iron-based superconductors. We conclude that the high compressibility of c-axis, in the T phase, enhances superconductivity since layers are brought together. Above Pc, compression of CT phase seems to have the opposite effect. [Preview Abstract] |
Tuesday, March 19, 2013 4:42PM - 4:54PM |
J37.00010: Magneto-transport properties of single crystal LaFeAsO at ambient and high pressure Colin McElroy, James Hamlin, Benjamin White, M. Brian Maple Electrical resistivity and magneto-transport measurements were performed on single crystals of LaFeAsO, which were grown using a NaAs flux. The magneto-transport measurements were made at ambient pressure in magnetic fields up to 9 T using the van der Pauw technique, which yielded the magnetoresistance and the Hall coefficient, from which the carrier density and mobility were inferred. The dominant charge carriers were identified as electrons, and a second anomaly was observed below the spin-density wave (SDW) transition. In order to study the evolution of these two anomalies with pressure, electrical resistivity measurements were performed under applied pressures up to 36.7 GPa. [Preview Abstract] |
Tuesday, March 19, 2013 4:54PM - 5:06PM |
J37.00011: Fe moments in the pressure-induced collapsed tetragonal phase of (Ca$_{0.67}$Sr$_{0.33}$)Fe$_{2}$As$_{2}$ Jason Jeffries, Nicholas Butch, Joseph Bradley, Yuming Xiao, Paul Chow, Shanta Saha, Kevin Kirshenbaum, Johnpierre Paglione The tetragonal AEFe$_{2}$As$_{2}$ (AE=alkaline earth element) family of iron-based superconductors exhibits magnetic order at ambient pressure and low temperature. Under pressure, the magnetic order is suppressed, and an isostructural volume collapse is induced due to increased As-As bonding across the mirror plane of the structure. This collapsed tetragonal phase has been shown to support superconductivity under some conditions, and theoretical calculations suggest an unconventional origin. Theoretical calculations also reveal that enhanced As-As bonding and the magnitude of the Fe moments are correlated, suggesting that the Fe moments can be quenched in the collapsed tetragonal phase. Whether the Fe moments persist in the collapsed tetragonal phase has implications for the pairing mechanism of the observed, pressure-induced superconductivity in these compounds. We will present pressure-dependent x-ray emission spectroscopy (XES) measurements that probe the Fe moments through the volume collapse transition of (Ca$_{0.67}$Sr$_{0.33}$)Fe$_{2}$As$_{2}$, and compare these measurements with the occurrence of superconductivity. [Preview Abstract] |
Tuesday, March 19, 2013 5:06PM - 5:18PM |
J37.00012: Interplay of superconductivity with the SDW order and Eu$^{2+}$ AFM order in the Ca$_{\mathrm{1-x}}$Eu$_{\mathrm{x}}$Fe$_2$As$_2$ system at ambient and under pressures Keshav Shrestha, Kui Zhao, Ben Jawdat, Liangzi Deng, Xiyu Zhu, Yuyi Xue, Bing Lv, Paul Chu Single crystals of Eu doped Ca$_{\mathrm{1-x}}$Eu$_{\mathrm{x}}$Fe$_2$As$_2$ (0$\le $ x $\le $1) with size up to 5 x 5 mm size were grown from FeAs self-flux technique. Detailed magnetic and resistivity data a systematical evolution of a spin-density-wave (SDW) transition from $\sim$ 170K at x$=$0 to $\sim$ 190K at x $=$1. Moreover, the Eu$^{2+}$ antiferromagnetic (AFM) emerged at 3.7K at a threshold doping x $\sim$ 0.2, and systematically increased up to $\sim$ 20K with the increase of the Eu content. High pressure was applied to some of these compounds to explore the competition among SDW, collapsed phase and superconductivity. Superconductivity up to 18K was observed in samples without the structural ``collapsed tetragonal'' phase before the emergence of the superconductivity signal. The data suggest that the superconductivity in the doped Ca122 under pressure is not associated with the structural collapsed-tetragonal phase. The complex phase diagram of the SDW, Eu$^{2+}$ AFM order and superconductivity at ambient and under pressure will be presented and its implication will be discussed. [Preview Abstract] |
Tuesday, March 19, 2013 5:18PM - 5:30PM |
J37.00013: Concentration dependence of magnetic characteristics in EuFe$_2$(As$_{\mathrm{1-x}}$P$_{\mathrm{x}}$)$_2$ single crystals Takanari Kashiwagi, Takuya Ishikawa, Tomoki Gota, Youhei Jono, Akihiko Nozawa, Kasumi Tashima, Kazuo Kadowaki In order to understand the structural and magnetic characteristics of EuFe$_{2}$(As$_{\mathrm{1-x}}$P$_{\mathrm{x}}$)$_{2}$, we have performed electron spin resonance (ESR) and magnetization measurements. The experimental results of the doping dependence of both measurements for the single crystal of EuFe$_{2}$(As$_{\mathrm{1-x}}$P$_{\mathrm{x}}$)$_{2}$ samples suggest the change of the exchange interaction and/or the anisotropy of Eu-site. The angular dependence of the ESR signal in the \textit{ab}-plane clearly shows the change of the in-plane anisotropy by the doping of P. The details will be discussed in the meeting. [Preview Abstract] |
Session J38: Focus Session: Materials for Electrochemical Energy Storage II
Sponsoring Units: GERAChair: Shengbai Zhang, Rensselaer Polytechnic Institute
Room: 347
Tuesday, March 19, 2013 2:30PM - 2:42PM |
J38.00001: Structural phase transition and Li-ion diffusion in Li$_7$La$_3$Zr$_2$O$_{12}$ Khang Hoang, Noam Bernstein, Michelle Johannes Garnet-type Li$_7$La$_3$Zr$_2$O$_{12}$ (LLZO) is a promising candidate for solid electrolytes in Li-ion battery applications because of its high ionic conductivity and electrochemical and chemical stability. The material has a low-conductivity tetragonal phase and a high-conductivity cubic phase. It has been reported that the cubic phase can be stabilized at ambient conditions, usually with the incorporation of a certain amount of supervalent impurities. In this talk, we present results from density-functional theory and variable cell shape molecular dynamics simulations, and discuss the origin of structural phase transition, effects of extrinsic impurities, and diffusion of Li ions in LLZO. By identifying relevant mechanisms and critical concentrations of the impurities (Li vacancies) for achieving the high-conductivity phase, this work shows how controlled synthesis could be used to improve the material's electrolytic performance. [Preview Abstract] |
Tuesday, March 19, 2013 2:42PM - 2:54PM |
J38.00002: ABSTRACT WITHDRAWN |
Tuesday, March 19, 2013 2:54PM - 3:06PM |
J38.00003: Redox potential of liquid water: A first-principles theory Michael Lucking, Yiyang Sun, Damien West, Shengbai Zhang A first-principles molecular dynamic method is proposed to calculate the absolute redox potentials of liquid water. The key of the method is the evaluation of the difference between the vacuum level and the average electrostatic potential inside liquid water, which employs an average over both time and space. By avoiding the explicit use of the Kohn-Sham level, such as the position of the valence band maximum, as the reference energy for the excited electrons, we are able to calculate water redox potentials accurately. The results using the PBE functional are in good agreement with experiment. We attribute the success of the method to the accurate charge density given by density functional calculation under the local or semi-local approximations. This establishes the validity to apply these effective and efficient approximations to study both the energetics and dynamics of the redox processes at more complex systems such as solid/solution interfaces. [Preview Abstract] |
Tuesday, March 19, 2013 3:06PM - 3:18PM |
J38.00004: Models of novel battery architectures Paul Haney, Dmitry Ruzmetov, Alec Talin We use a 1-dimensional model of electronic and ionic transport, coupled with experimental data, to extract the interfacial electrochemical parameters for LiCoO2-LIPON-Si thin film batteries.~ TEM imaging of batteries has shown that charge/discharge cycles can lead to breakdown of the interfaces, which reduces the effective area through which further Li ion transfer can occur.~ This is modeled phenomenologically by changing the effective cross sectional area, in order to correlate this structural change with the change in charge/discharge I-V curves.~ Finally, by adopting the model to radial coordinates, the geometrical effect of nanowire architectures for batteries is investigated. [Preview Abstract] |
Tuesday, March 19, 2013 3:18PM - 3:30PM |
J38.00005: Paraquinone-Hydroquinone Couple for Flow Battery Saraf Nawar, Brian Huskinson, Michael Aziz At present, there is an ongoing search for the storage of energy from intermittent renewable sources like wind and solar. Flow batteries have gained attention due to their potential viability in inexpensive storage of large amounts of energy. Because of its high reversibility, low toxicity, and low component costs, the paraquinone/hydroquinone redox couple could be a viable candidate for use in a grid-scale storage device. In this report, we will present half-cell data for the 1,4-parabenzoquinone/1,4-hydroquinone redox couple and related couples in sulfuric acid. We will present results from a flow battery with maximum current density of up to 200 mA/cm2 using a mixture of 1,4-parabenzoquinone and 1,4-hydroquinone as the cathode material and hydrogen as the anode material. We report the effects of reactant concentration, reactant flow rate to the electrode, and temperature on the performance of the fuel cell. [Preview Abstract] |
Tuesday, March 19, 2013 3:30PM - 3:42PM |
J38.00006: Exploring the electronic structure and dynamics of lithium compounds through first-principles interpretation of X-ray absorption spectra David Prendergast, Tod Pascal, Xin Li, Jinghua Guo, Yi Luo In situ X-ray spectroscopy will reveal fundamental details of elecltrochemistry in working cells, provided that the data is interpretable. To this end, we are developing first-principles methods to simulate core-level absorption spectra of molecules, condensed phases, and interfaces with explicit inclusion of dynamics. We validate this approach by application to various lithium compounds that may be present in the solid electrolyte interphase (SEI) and to lithiation of graphite in the anode. Our calculations reveal that instantaneous broken symmetry about the x-ray excited atom may be evident in the resulting spectroscopy and highlights both dynamical and static disorder in these materials. Furthermore, we observe complex anisotropic interactions upon charge transfer between lithium and graphite that contradict a simplistic view of intercalation in terms of complete electron transfer and the rigid band approximation. [Preview Abstract] |
Tuesday, March 19, 2013 3:42PM - 3:54PM |
J38.00007: Characterization of silicate based cathodes for Li Ion Batteries Ajay Kumar, Gholam-Abbas Nazri, Maryam Nazri, Vaman Nail, Prem Vaishnava, Ratna Naik The silicate compounds Li$_{\mathrm{2}}$MSiO$_{\mathrm{4}}$, where M$=$ Mn, Fe, Co and Ni have gained interest as electrode materials for Lithium ion batteries due to their high theoretical capacity (\textgreater 330mAh/g), high thermal stability due to strong Si-O covalent bonds, environmental friendliness, and low cost. However, these materials intrinsically have low electrical conductivity. To improve conductivity of these classes of electrode materials, we synthesized Li$_{\mathrm{2}}$MnSiO$_{\mathrm{4}}$ and Li$_{\mathrm{2}}$FeSiO$_{\mathrm{4}}$ by solid state reaction in an argon atmosphere. The lithium transition metal silicates were compounded with graphene nano-sheets and the composites were used as positive electrode in a coin cell configuration. . The materials structure-composition, morphology, conductivity and electrochemical performance were characterized by XRD, XPS, SEM, TEM and electrochemical techniques.The detail structure-composition analysis and electrochemical performance of the silicate electrodes will be reported. [Preview Abstract] |
Tuesday, March 19, 2013 3:54PM - 4:06PM |
J38.00008: Solid state electrochemical studies on single crystals with hexafluoro metal centers Qifan Yuan, Yao Zhang, Sarah True, Victoria Soghomonian Electrochemical energy storage is of importance for current and future storage schemes. Our electrochemical studies on hydrothermally synthesized single crystals of metal hexafluoride, [NH$_{\mathrm{4}}$]$_{\mathrm{3}}$[V$_{\mathrm{x}}$M'$_{\mathrm{1-x}}$F$_{\mathrm{6}}$], probe the redox chemistry of the V center as a function of substitution, temperature, and contact configuration. The various compositions are probed by XRD and IR spectroscopy. The measured 2 and 4 point resistivity is around 10$^{\mathrm{7}} \quad \Omega^{\mathrm{\thinspace }}$cm. Cyclic voltammograms were obtained by both 2 and 3 point geometries, and current peaks corresponding to the electrochemical reaction recorded. To understand the observed voltammograms of the various configurations measured, the potential distribution in the crystal is calculated numerically, and equipotential lines extracted. Preliminary analysis indicates the extent of the space charge for 2 versus 3 contact measurements, and the influence of the space charge region on the electrochemical reaction when performed at micron scales. For a fixed sweep rate, the amplitude of the current peak diminishes as the temperature is increased, suggesting a dissipation of the space charge. [Preview Abstract] |
Tuesday, March 19, 2013 4:06PM - 4:18PM |
J38.00009: Enhanced Lithiation of Graphitized SiC: In Situ X-ray Scattering Study at Electrolyte / Graphene / SiC(0001) Interface Sudeshna Chattopadhyay (Bandyopadhyay), Albert Lipson, Hunter Karmel, Jonathan Emery, Vinayak Dravid, Mark Hersam, Michael Bedzyk, Paul Fenter, Timothy Fister, Michael Thackeray Silicon carbide is an inert material and not traditionally viewed as a promising electrode material. However, we observed a large enhancement to the electrochemical lithiation capacity for SiC anodes that were electrically activated by the combination of surface graphitization and substrate doping. In-situ X-ray scattering studies for lithiation at the electrolyte/EG/SiC interface show that the interfacial structure of the proposed anode system is stable in the electrolyte and graphene layers remain unaltered. While a decrease in the SiC Bragg peak intensity during lithiation indicates changes to the bulk crystallinity, the emergence of a diffuse scattering feature suggests that lithiation is associated with the development of substrate defects. Characterization via multiple depth resolved spectroscopies shows that Li penetrates the activated SiC upon lithiation. These results illustrate that the electrochemical capacity of a traditionally inert material can be increased substantially by effecting the surface and bulk conductivity [1].\\[4pt] [1] Chattopadhyay, Lipson et al., Chem. Mater. 24, 3038 (2012); Lipson, Chattopadhyay et al., J. Phys. Chem. C 116, 20949 (2012). [Preview Abstract] |
Tuesday, March 19, 2013 4:18PM - 4:30PM |
J38.00010: Dynamic dimer formation between superionic fluorines in PbF$_{2}$ Nobutaka Nakamura, Kazuo Tsumuraya Recently Tsumuraya $et\ al$.(J. Phys. Soc. Jpn. 81,055603(2012).) have elucidated the formation of the dynamic dimers in the superionic conductor $\alpha $-CuI with the first principles molecular dynamics (MD) method. They, for the first time in research, confirmed the dimer formation through the analyses the origin of the correlation peaks of the partial pair distribution functions and the partial angle distribution functions. The present study elucidates the dynamic structure of the superionc fluorines in PbF$_{2}$ crystal with the MD method through identifying the origins of the correlation peaks. The fluorines form the dynamic 32$f$-8$c$ and 4$b$-8$c$ dimers. [Preview Abstract] |
Tuesday, March 19, 2013 4:30PM - 4:42PM |
J38.00011: First-principles study of lithium ion diffusion in crystalline $\beta $-Li$_3$PS$_4$ for solid state electrolytes Myung-Soo Lim, Seung-Hoon Jhi The safety and stability are major issues to resolve in developing high-capacity lithium secondary batteries, particularly for application to electric vehicles. Solid-state lithium-ion electrolytes have been studied as a substitute of liquid electrolytes in order to enhance the stability and increase the energy density. However, low ion-mobility and poor material integrity are limiting the use of the solid electrolytes. We study the lithium-ion diffusion in crystalline $\beta $-Li$_{3}$PS$_{4}$ using first-principles methods and the nudged elastic band method. Considering diffusion paths through both interstitials and vacancy exchanges, we calculate the migration energies of lithium ions. Our results suggest that lithium ion diffusion is likely to occur through the zigzag-shaped paths along the $b$-direction that comprises of two lithium ion sites with fractional occupancy factors. We discuss the implication of our calculations for understanding the lithium ion diffusion in solid electrolytes. [Preview Abstract] |
Tuesday, March 19, 2013 4:42PM - 4:54PM |
J38.00012: Structural and Electrochemical Impacts of Oxygen Doped and Surfactant Coated Activated Carbon Electrodes in Li-ion Batteries John Collins, Gerald Gourdin, Deyang Qu, Michelle Foster Passive charge and discharge dynamics are necessary for advancing Li-ion batteries. Surfactant adsorption on activated carbon has been shown to promote advancements in the discharge capacity, time and cycle-ability of electrochemical systems---specifically by enhancing diffusion pathways for ion insertion/de-insertion and suppressing pore blockage from precipitates known to form during charge/discharge states. Enhancement of surfactant chemisorption on activated carbon is achieved through oxygen doping of the carbon surface. In addition, doping alters the degree of Faradaic processes occurring in solution, resulting in prolonged reduction at the carbon surface\textbf{.} The work presented describes how surface oxygen groups on a granulated activated carbon have been manipulated using nitric acid in a controlled, stepwise fashion. A nonionic surfactant was applied to oxidized and non-oxidized samples at various concentrations. The composition and structure of the activated carbon surface was characterized using DRIFTS, Raman Spectroscopy, SEM and Porosimetry. The charge/discharge Li insertion capacities along with correlating surface microstructure changes were analyzed for all treated electrodes at progressive oxidation stages. [Preview Abstract] |
Tuesday, March 19, 2013 4:54PM - 5:06PM |
J38.00013: Simulation of dendrite formation in lithium-ion batteries Ning Sun, Dilip Gersappe The design of the next generation of energy storage technologies requires both a fundamental understanding of the physical and chemical reactions taking place in a complex electrochemical environment and the factors that limit the performance of these systems. We have developed a Lattice-Boltzmann model to simulate 2-D dendrite formation during charging and discharging processes on the anode of lithium-ion batteries. Our results show that the formation of dendrites is strongly influenced by the morphology of the anode, and operating conditions, in particular the charging current density. Our simulation is able to recover the structures that form on Li anodes, including mossy and dendritic structures as a function of parameters such as the curvature of the interface and the applied current density. We also show that we can observe a linear relationship between the log current density (J) and the log dendrite formation onset time (ts) in the low current density region, which also agrees with experiment data quite well. We study additional effects such as the role of the separator and the Solid Electrolyte Interphase (SEI) layer on the formation of dendrites. [Preview Abstract] |
Tuesday, March 19, 2013 5:06PM - 5:18PM |
J38.00014: Surface Structure and Stability in Li$_3$PS$_4$ and Li$_3$PO$_4$ Electrolytes from First Principles Nicholas Lepley, N.A.W. Holzwarth Crystalline solid electrolyte materials continue to show considerable promise for lithium ion battery applications. Recent experiments on these materials\footnote{Chengdu Liang, ORNL, (private communication).} suggest that in some cases surface effects may play a significant role with regard to both stability and ionic conductivity. In this study, we extend our previous modeling work\footnote{N. A. W. Holzwarth, N. D. Lepley, Y. A. Du, {\em{J. Power Sources}} {\bf{196}}, 6870 (2011).} to an examination of idealized surfaces of several phases of Li$_3$PX$_4$ (X=O,S). Our preliminary results suggest that energy contributions from the surface affect the relative phase stability in Li$_3$PS$_4$, although this is not observed in the phosphate analogue. Our presentation will focus on surface energies and structures, as well as examining the calculated stability of the interface between the electrolyte and lithium metal. [Preview Abstract] |
Tuesday, March 19, 2013 5:18PM - 5:30PM |
J38.00015: Synthesis and Performance of LiFe$_{\mathrm{1-x}}$Mn$_{\mathrm{x}}$PO$_{4}$ in Lithium-ion Battery Khadije Bazzi, Maryam Nazri, Prem Vaishnava, Vaman Naik, Gholam-Abbas Nazri, Ratna Naik Olivine-type lithium transition metal phosphates (i.e. LiFePO$_{4})$ have been intensively investigated as promising electrode materials for rechargeable lithium-ion batteries. There have been attempts to improve energy density and voltage quality of phosphate based electrode. In this study, we have partially substituted Fe$^{\mathrm{II}}$/Fe$^{\mathrm{III}}$ redox center with Mn$^{\mathrm{II}}$/Mn$^{\mathrm{III}}$ in LiFePO$_{4}$ that provides over 600 mV higher voltage. We prepared various compositions of LiFe$_{\mathrm{1-x}}$Mn$_{\mathrm{x}}$PO$_{4}$ (x$=$0, 0.2, 0.4, 0.6, 0.8 and 1) between the two end members (LiFePO$_{\mathrm{4}}$ - LiMnPO$_{4})$. Due to intrinsic low electronic conductivity of lithium transition metal phosphates, we coat these materials with a uniform conductive carbon through a unique sol-gel process developed in our laboratory. In addition, we made a composite of the carbon coated phosphate with carbon nano-tubes to develop a highly conductive matrix electrode. We report the materials structure, morphology, electrical conductivity and electrochemical performances of LiFe$_{\mathrm{1-x}}$Mn$_{\mathrm{x}}$PO$_{4}$ using XRD, Raman spectroscopy, SEM, TEM, XPS, electrical conductivity and galvanostatic charge/discharge measurements. [Preview Abstract] |
Session J39: Matter at Extreme Conditions: Theory and Simulations
Sponsoring Units: GSCCM DCOMP DMPChair: Joseph Hooper, Naval Postgraduate School
Room: 348
Tuesday, March 19, 2013 2:30PM - 2:42PM |
J39.00001: Pressure-Constrained Deformation and Superior Strength: Compressed Graphite versus Diamond Yi Zhang, Changfeng Chen, Wei Zhou, Hong Sun The discoveries of compressed carbon phases and their ability to crack diamond anvil have generated great interest in the mechanical properties of carbon allotropes under high pressure. Significant progress has been made recently in structural identification of compressed graphite; however, its surprisingly high strength approaching or exceeding that of diamond remains unexplained. Here we explore this novel phenomenon and show by first-principles calculations that high-pressure confinement suppresses usual ambient or low-pressure deformation modes toward low-density carbon allotropes, and promotes alternative mechanisms for structural evolution leading to high-density compressed graphite phases that exhibit superior strength surpassing that of diamond. This finding explains the puzzling experimental observation and suggests new principles for structural deformation under high-pressure confinement. It also imposes stringent tests on widely used empirical hardness formulas that are unable to account for changes in pressure-constrained structural evolution and their influence on material strength. [Preview Abstract] |
Tuesday, March 19, 2013 2:42PM - 2:54PM |
J39.00002: Atomistic Simulation of Orientation Dependence in Shock-induced Initiation of Pentaerythritol Tetranitrate (PETN) Tzu-Ray Shan, Ryan Wixom, Ann Mattsson, Aidan Thompson Predicting the behavior of energetic materials requires a detailed description of how chemical reactions initiate during initial stages of detonation. In this talk, the dependence of the reaction initiation mechanism of pentaerythritol tetranitrate (PETN) on shock orientation and shock strength is investigated with molecular dynamics simulations using a reactive force field and the multi-scale shock technique. In the simulations, a single crystal of PETN is shocked along [110], [001], and [100] orientations with shock velocities in the range 3-10 km/s. Major reactions occur with shock velocities of 6 km/s or stronger, and reactions initiate through the dissociation of nitro (NO$_{2})$ and nitrate (NO$_{3})$ groups from the PETN molecules. The most sensitive orientation is [110], while [100] is the most insensitive. For the [001] orientation, PETN decomposition via nitro group dissociation is the dominant reaction initiation mechanism, while for [110] and [100] orientations the decomposition is via mixed nitro and nitrate group dissociation. For shock along the [001] orientation, we find that CO-NO$_{2}$ bonds initially acquire more kinetic energy, facilitating nitro dissociation. For the other two orientations, C-ONO$_{2}$ bonds acquire more kinetic energy, facilitating nitrate group dissociation. [Preview Abstract] |
Tuesday, March 19, 2013 2:54PM - 3:06PM |
J39.00003: Microstructural Evolution and Grain Growth at High Speed Frictional Interfaces J.L. Milhans, J.E. Hammerberg, R. Ravelo, T.C. Germann, B.L. Holian We have examined the effect of evolution of grain morphology on the frictional force at polycrystalline Al-Al interfaces as a function of grain size and sliding velocity in the velocity range 40-250 m/s for grain sizes of 13.5 and 20 nm. Sample sizes for NonEquilibrium Molecular Dynamics (NEMD) simulations ranged from 10 - 140 M atoms. For velocities below a size dependent critical velocity above which a fluid layer forms, we find enhanced grain coarsening leading to a highly strained, graded final steady state microstructure that exhibits a dynamic morphhology characterized by grain growth and breakup at time scales greater than 5-10 ns. We find that the frictional force is insensitive to the initial grain size distribution that evolves to this new nonequilibrium steady state. We discuss mechanisms for grain size and shape evolution and the emergence of a dynamic length scale and compare these results to single crystal simulations in the same sliding regime. [Preview Abstract] |
Tuesday, March 19, 2013 3:06PM - 3:18PM |
J39.00004: Shock response near the elastic to plastic transition in single crystal and porous silicon J. Matthew Lane, Tracy J. Vogler We use molecular dynamics simulation methods to study the onset of the plastic wave transition in single crystal silicon, and characterize the altered response due to various degrees of porosity from 5 to 50 percent. Non-elastic response near onset of plasticity follows a mechanism similar to one shown previously in germanium, in which a propagating densification transition is driven by the release of shear stress in the material. This transition mechanism can be characterized as a partial transition from the ambient diamond structure to a distorted body center tetragonal ($\beta$-tin) structure. We show that this onset region is strongly influenced by porosity and large scale defects. Sandia National Laboratories is a multi program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Tuesday, March 19, 2013 3:18PM - 3:30PM |
J39.00005: Molecular dynamics study of the shock response in hydroxyl-terminated polybutadiene melts Markus G. Froehlich, Thomas D. Sewell, Donald L. Thompson All-atom molecular dynamics (MD) simulations using the non-reactive OPLS-AA force field were performed to study the detailed structural, mechanical, and spectroscopic response of hydroxyl-terminated polybutadiene (HTPB) melts subjected to supported shock waves. A combination of Monte Carlo and MD techniques was used to generate thoroughly equilibrated initial configurations, for monodisperse systems with chain lengths ranging from 64 to 256 backbone carbons per chain. Properties characterizing the size, shape and orientation of single chains, as well as the vibrational density of states, were evaluated prior to and following shock passage for four impact velocities between 1.0 and 2.5 km/s. The structural properties and global scaling behaviors of the unshocked systems are in excellent agreement with literature data. Results for the shocked systems, obtained using a geometric binning approach that provides spatio-temporal resolution in the reference frame centered on the shock front, indicate a transition to a glass-like state with a concomitant increase by several orders of magnitude of structural relaxation times in the shocked material. [Preview Abstract] |
Tuesday, March 19, 2013 3:30PM - 3:42PM |
J39.00006: ABSTRACT WITHDRAWN |
Tuesday, March 19, 2013 3:42PM - 3:54PM |
J39.00007: The volume isotope effect in ice under high pressure Stefano de Gironcoli, Koichiro Umemoto, Renata Wentzcovitch The volume isotope effect (VIE) in ice has recently received considerable attention [1,2]. Ice Ih and XI, prototypical forms of low-pressure ice, have anomalous VIE, i.e., the volume of D$_{2}$O ($V_{D2O}$) is larger than that of H2O ($V_{H2O}$) [1]. In contrast, the VIE in ice VIII at 0 GPa was reported to be normal, i.e., $V_{D2O} < V_{H2O}$ [2]. Here we clarify the origin of this behavior in different forms of ice. Furthermore, we predict a reversal in the VIE in ice VIII under high pressure. [1] B. Pamuk et al., Phys. Rev. Lett. 108, 193003 (2012). [2] E. D. Murray and G. Galli, Phys. Rev. Lett. 108, 105502 (2012). [Preview Abstract] |
Tuesday, March 19, 2013 3:54PM - 4:06PM |
J39.00008: New transitions of MgSiO3 post-perovskite under ultrahigh pressure Koichiro Umemoto, Shunqing Wu, Min Ji, Cai-Zhuang Wang, Kai-Ming Ho, Renata Wentcovitch Understanding the behavior of MgSiO$_{3}$ post-perovskite (PPV) under extreme pressures is fundamental for modeling the interiors of super-Earth type exoplanets and the cores of solar giants. Previously, MgSiO$_{3}$ PPV was predicted to dissociate into MgO and MgSi$_{2}$O$_{5}$ and then into MgO and SiO$_{2}$ (Umemoto et al., Science 311, 983 (2006); Umemoto and Wentzcovitch, EPSL 311, 225 (2011)). Using the adaptive genetic algorithm, we predict new phase transitions in MgSiO$_{3}$. The phase diagram calculated using the quasi-harmonic approximation shows that some transitions can occur in some super-Earths type exoplanets. [Preview Abstract] |
Tuesday, March 19, 2013 4:06PM - 4:18PM |
J39.00009: Elasticity of ferropericlase at lower mantle conditions Renata Wentzcovitch, Zhongqing Wu, Joao Francisco Justo The discovery of spin-state changes (crossovers) in ferropericlase (Fp) and in silicate perovskite (Pv) under pressure has raised new questions about Earth's mantle properties. Despite extensive experimental work on the elasticity of Fp throughout the crossover, inconsistencies reported in the literature are still not explained. We introduce here a theoretical framework for thermoelasticity across spin-state changes, apply it to Fp by combining it with predictive first principles DFT$+$U calculations, and contrast results with available data on samples with various iron concentrations. We explain why the shear modulus of Fp should not soften throughout the spin crossover under hydrostatic conditions and show the importance of constraining well the elastic properties of minerals at lower mantle conditions and likely compositions without extrapolations. [Preview Abstract] |
Tuesday, March 19, 2013 4:18PM - 4:30PM |
J39.00010: Dielectric properties of water and their impact on the Earth's deep carbon cycle Ding Pan, Leonardo Spanu, Brandon Harrison, Dimitri Sverjensky, Giulia Galli Knowledge of the dielectric constant of water as a function of pressure (P) and temperature (T) plays a critical role in understanding the chemistry of aqueous systems, and in particular of fluids in the Earth's mantle. By using \emph{ab initio} molecular dynamics, we computed the dielectric constant of water at T = 1000 and 2000 K, between 1 and 12 GPa, under conditions of the Earth's upper mantle. By comparing our results with available experimental data and empirical models, we discuss how the changes in the molecular dipole moments and hydrogen-bond network upon compression affect the dielectric constant of the liquid. Based on the calculated dielectric constants, the solubility products of carbonate minerals were predicted. At P $\sim$ 10 GPa and T = 1000 K, we found that MgCO$_3$ (magnesite) is slightly soluble in water at the millimolal level, which suggests that water in the Earth's mantle has the capacity to store and transport significant quantities of oxidized carbon. [Preview Abstract] |
Tuesday, March 19, 2013 4:30PM - 4:42PM |
J39.00011: Structure change, layer sliding, and metallization in high-pressure MoS2 Erio Tosatti, Liliana Hromadova, Roman Martonak Based on ab initio calculations and metadynamics simulations, we predict that 2H-MoS$_2$, a layered insulator, will metallize under pressures in excess of 20-30 GPa. In the same pressure range, simulations and enthalpy optimization predict a structural transition. Reminiscent of this material's frictional properties, free mutual sliding of layers takes place at this transition, where the original 2H$_c$ stacking changes to a 2H$_a$ stacking typical of 2H-NbSe$_2$, a transformation which explains for the first time previously mysterious X-ray diffraction data. Phonon and electron phonon calculations suggest that metallic pristine MoS$_2$ will require ultrahigh pressures in order to develop superconductivity. [Preview Abstract] |
Tuesday, March 19, 2013 4:42PM - 4:54PM |
J39.00012: Electronic structure and topological transition of SnTe at high pressure Quan Li, Dan Zhou, Yanming Ma, Changfeng Chen Recent x-ray diffraction measurements and first-principles calculations have revealed intriguing structural evolution of tin telluride (SnTe) under high pressure. Here we report on a systematic study of the electronic band structure, density of states, Fermi surface and charge density of SnTe at high pressure using first-principles density functional theory calculations. Our results unveil an electronic topological transition in the cubic Fm-3m phase of SnTe with its Fermi surface changing from disconnected pockets to inter-connected quasicubic tubes near the L points of the Brillouin zone under high pressure. The pressure-induced quasicubic tubular Fermi surface is similar to that previously obtained via carrier doping. The induced change in electronic charge distribution stabilizes the Fm-3m structure and thus suppresses the transition to the rhombohedral structure, which explains experimental observations. Furthermore, our calculations show that pressure-induced electronic topological transition is also present in the orthorhombic Cmcm and Pnma phases of SnTe in the pressure range of 5 to 18 GPa, but this transition is absent in the high-pressure (above 18 GPa) Pm-3m phase. [Preview Abstract] |
Tuesday, March 19, 2013 4:54PM - 5:06PM |
J39.00013: Achieving unusual oxidation state of matter under high pressure Xiaoli Wang, Haiqing Lin, Yanming Ma, Maosheng Miao Pressure has many effects to matter including the reduction of the volume, the increase of the coordination number and the broadening of the band-widths. In the past, most of the high-pressure studies focused on structural and electronic state phase transitions. Using first principles calculations and a bias-free structural search method, we will demonstrate that high pressure can lead to high oxidation state of elements that can never be achieved under ambient condition, making high pressure technique a nice tool to explore many traditional topics in solid state and molecular chemistry. As an example, we will show that Hg can transfer the electrons in its outmost d shell to F atoms and form HgF$_{4}$ molecular crystals under pressure, thereby acting as a true transition metal. Group IIB elements, including Zn, Cd, and Hg are usually defined as post-transition metals because they are commonly oxidized only to the $+$2 state. Their d shells are completely filled and do not participate in the formation of chemical bonds. Although the synthesis of HgF$_{4}$ molecules in gas phase was reported before, the molecules show strong instabilities and dissociate. Therefore, the transition metal propensity of Hg remains an open question. [Preview Abstract] |
Tuesday, March 19, 2013 5:06PM - 5:18PM |
J39.00014: Dynamic structure of superionic protons in hydrogen fluoride crystal Yoshiyuki Ohde, Kazuo Tsumuraya Hydrogen fluoride crystal forms zig-zag chains of hydrogen fluoride molecules forming covalent bond between them. Goldman et al.(J. Chem. Phys.125,044501(2006).) have found the superionic state of the protons in the hydrogen fluoride crystal at 900 K and beyond the pressures at 33~GPa. The present study elucidates the dynamic structure of the protons in the superionic state of the crystal at the extreme conditions with the first principles molecular dynamics method. The strong covalent bond between the proton and the fluorine in the conductor has shown a different dynamic structure from that in the $\alpha $-CuI; The protons in the conductor are bonded with the nearest fluorine and the other protons are located at incommensurate sites of the bcc fluorine lattice. This is a different dynamic structure from the formation of the incommensurate dynamic copper dimers in the $\alpha $-CuI.(Tsumuraya $et \ al$. J. Phys. Soc. Jpn. 81,055603(2012).) [Preview Abstract] |
Tuesday, March 19, 2013 5:18PM - 5:30PM |
J39.00015: ABSTRACT WITHDRAWN |
Session J40: Quantum Simulation II
Sponsoring Units: DAMOPChair: Khan W. Mahmud, University of Maryland
Room: 349
Tuesday, March 19, 2013 2:30PM - 2:42PM |
J40.00001: Simulation of a Non-Equilibrium Localization Transition of Photons in a Superconducting Circuit-QED Dimer Darius Sadri, James Raftery, Andrew Houck, Hakan Tureci, Sebastian Schmidt, Devin Underwood, Will Shanks, Srikanth Srinivasan, Mikola Bordyuh The exponential scaling of Hilbert space dimension with number of quantum degrees of freedom, while serving as a resource for quantum computation, makes simulation of large quantum systems on classical computers prohibitive, particularly when interactions with an environment are included. Quantum simulation promises to make possible the investigation of rich quantum behavior on a controlled quantum mechanical device (effectively a specialized quantum computer), deepening our understanding of fascinating physics such as quantum phase transitions, non-equilibrium quantum dynamics, and quantum chaos. Superconducting circuit Quantum Electrodynamics (cQED) is a promising framework for the realization of such simulators. As a first step, we have constructed a quantum simulator for a conjectured dissipation-driven localization transition of light in a dimer using cQED techniques. A proper understanding of the physics and signature of this transition has been made possible by our development of a new classical simulator based on the stochastic quantum jump method, taking advantage of a fractal structure in our Hamiltonian to enable a study of the very large Hilbert spaces demanded by this problem. We present results of these simulations, and discuss possible future directions. [Preview Abstract] |
Tuesday, March 19, 2013 2:42PM - 2:54PM |
J40.00002: Quantum simulations of cooper pairing and driven nonlinear Schr\"{o}dinger equation with stationary light Priyam Das, Mingxia Huo, Changsuk Noh, B. M. Rodriguez-Lara, Dimitris G. Angelakis Strongly correlated states of photons generated in strongly coupled light-matter interfaces, such optical waveguides interacting with ensembles of cold atoms, have recently emerged as promising routes for a new kind of quantum simulators. In this work, we present two of our most recent results along this line, motivated by earlier proposals on strongly interacting stationary polaritons and a proposal to create an effective polaritonic lattice potential. In the first part, we show how to realize an optically tunable two-component Bose-Hubbard model and discuss the feasibility of generating an effective Fermi-Hubbard model of polaritons. This allows one to simulate and detect the 1D analog of the BEC-BCS crossover through correlation measurements. In the second part, we show how a similar setup allows one to study nonlinear transport properties. In the semi-classical regime, the system is formally analogous to a Bose-Einstein condensates in optical lattices or propagation of EM fields in photonic Kerr media, allowing for simulations of similar effects with distinct advantages due to the photonic nature of the proposed system. We conclude by proposing how one of the signature effects of nonlinear dynamics, bistablity, can be experimentally observed in our set up. [Preview Abstract] |
Tuesday, March 19, 2013 2:54PM - 3:06PM |
J40.00003: Experimental investigation of a nonequilibrium delocalization-localization crossover of photons in circuit quantum electrodynamics James Raftery, Darius Sadri, Mykola Bordyuh, Devin Underwood, William Shanks, Srikanth Srinivasan, Sebastian Schmidt, Hakan Tureci, Andrew Houck We report measurements of the time-dynamics of a Jaynes-Cummings dimer. The dimer is fabricated in the circuit quantum electrodynamics (cQED) architecture, with two coupled resonators each coupled to a single transmon qubit. Such a system is predicted to exhibit three distinct behavioral regimes: delocalized, in which photons can oscillate between the two cavities; localized, in which photons are locked into a single cavity; and exiguous, in which extremely low photon numbers lead to the disappearance of locking. Dissipation in the system drives crossovers between the regimes. The experimental measurements of the on and off-site correlation functions will be presented. [Preview Abstract] |
Tuesday, March 19, 2013 3:06PM - 3:18PM |
J40.00004: On the phase transition of light in cavity QED lattices Marco Schiro, Mykola Bordyuh, Baris Oztop, Hakan Tureci Systems of strongly interacting atoms and photons, that can be realized wiring up individual cavity QED systems into lattices, are perceived as a new platform for quantum simulation. While sharing important properties with other systems of interacting quantum particles here we argue that the nature of light-matter interaction gives rise to unique features with no analogs in condensed matter or atomic physics setups. By discussing the physics of a lattice model of delocalized photons coupled locally with two-level systems through the elementary light-matter interaction described by the Rabi model, we argue that the inclusion of counter rotating terms, so far neglected, is crucial to stabilize finite-density quantum phases of correlated photons out of the vacuum, with no need for an artificially engineered chemical potential. We show that the competition between photon delocalization and Rabi non-linearity drives the system across a novel $Z_2$ parity symmetry-breaking quantum criticality between two gapped phases which shares similarities with the Dicke transition of quantum optics and the Ising critical point of quantum magnetism. We discuss the phase diagram as well as the low-energy excitation spectrum and present analytic estimates for critical quantities. [Preview Abstract] |
Tuesday, March 19, 2013 3:18PM - 3:30PM |
J40.00005: Phonon mediated quantum spin simulator made from a two-dimensional Wigner crystal in Penning traps Joseph Wang, Adam Keith, J. K. Freericks Motivated by recent advances in quantum simulations in a Penning trap, we give a theoretical description for the use of two-dimensional cold ions in a rotating trap as a quantum emulator. The collective axial phonon modes and planar modes are studied in detail, including all effects of the rotating frame. We show the character of the phonon modes and spectrum, which is crucial for engineering exotic spin interactions. In the presence of laser-ion coupling with these coherent phonon excitations, we show theoretically how the spin-spin Hamiltonian can be generated. Specifically, we notice certain parameter regimes in which the level of frustration between spins can be engineered by the coupling to the planar modes. This may be relevant to the quantum simulation of spin-glass physics or other disordered problems. [Preview Abstract] |
Tuesday, March 19, 2013 3:30PM - 3:42PM |
J40.00006: Goldstone and Higgs modes of photons inside an cavity and their detections Yu Yixiang, Yu Chen, Jinwu Ye, Wuming Liu It was well known that a broken global continuous symmetry leads to two associated collective modes: a massless Goldstone mode and a massive Anderson-Higgs amplitude mode. The two modes have been detected in various condensed matter systems and recently also in cold atom systems. The Higgs mode in particle physics was finally detected in two recent LHC experiments. In this work, we show that the two modes can also be detected in optical systems inside a cavity with only a few (artificial) atoms. We demonstrate this connection by studying the $U(1)$ Dicke (Tavis-Cummings) model where $N$ qubits (atoms) coupled to a single photon mode. We perform both $1/J=2/N$ expansion and exact diagonization (ED) study on the model. We determine the Goldstone and Higgs modes and theirs corresponding spectral weights from the system's energy spectrum and also from various photon and atom correlation functions. We find nearly perfect agreements between the results achieved from the $1/J$ calculations with those from the ED studies in all these physical quantities even when $N$ gets down even to $N = 2$. The experimental detections of both modes are also discussed. [Preview Abstract] |
Tuesday, March 19, 2013 3:42PM - 3:54PM |
J40.00007: Coherent radiation from a collection of molecules interacting with surface plasmons Michael Stopa, Semion Saikin, Alan Aspuru-Guzik A collection of molecules interacting coherently with a radiation field has dramatically different absorption and emission properties than the same collection of molecules interacting incoherently with the field. In the former case, the collective states of the molecules become important and these consist of states which radiate super-classically (Dicke superradiance) as well as states which are dark. Treated as two-level systems such a collection of molecules can be thought of as a set of spins. The product state of those spins can be transformed to a basis of states of good total ``angular momentum'' J, and good J$_{\mathrm{z}}$ (z-component of total angular momentum). Here, we construct a numerical, invertible transformation between the direct product basis and the total J basis for N total molecules. For an arbitrary product state we calculate the rate of transition via radiation out of an arbitrary state in first order perturbation theory. For an ensemble of initial states we calculate the statistical distribution of the radiance (as a function of the J$_{\mathrm{z}}$ quantum number and disorder in the couplings) of the initial state. We show that the average radiance is approximately equal to the classical value but that the distributions have an asymmetric tail toward superradiance. [Preview Abstract] |
Tuesday, March 19, 2013 3:54PM - 4:06PM |
J40.00008: Exciton-Polaritons condensates with flat bands in a two-dimensional kagome lattice Na Young Kim, Naoyuki Masumoto, Yoshihisa Yamamoto, Sven Hoefling, Alfred Forchel Microcavity exciton-polariton condensates have provided immense opportunity to investigating hydrodynamic vortex properties, superfluidity, and low energy quantum state dynamics. Recently, exciton-condensates have been trapped in various artificial periodic potential geometries: one-dimensional, two-dimensional (2D) square, triangular, and hexagonal lattices. A 2D kagome lattice has been of interest for many decades, which exhibits spin frustration, giving rise to magnetic phase order in real materials. In particular, flat bands in the 2D kagome lattice are physically interesting in that localized states in the real space are formed. Here, we realize exciton-polariton condensates in a 2D kagome lattice potential and examine their photoluminescence properties. Above quantum degeneracy threshold values, we observe meta-stable condensation in high-energy bands; the third band exhibits a signature of weaker dispersive band structures, flat band. We perform single-particle band structure calculation to compare measured band structures. [Preview Abstract] |
Tuesday, March 19, 2013 4:06PM - 4:18PM |
J40.00009: Spinor condensates of ortho- and para-positronium Yi-Hsieh Wang, Charles W. Clark In 1994, Platzman and Mills [1] considered the possibility of making a Bose-Einstein condensate (BEC) of positronium atoms (Ps). There are four low-lying states of Ps: a singlet, often called parapositronium (p-Ps); and three triplet states, often referred to as orthopositronium (o-Ps). The lifetime against electron- positron annihilation for o-Ps is a thousand times longer than that of p-Ps. By converting a long-lived triplet o-Ps BEC to a p-Ps condensate with a magnetic field, strong $\gamma$-ray emission can be generated as the outcome of the annihilation of coherent p-Ps atoms. However, inelastic scattering processes which convert p-Ps atoms to o-Ps may deplete the p-Ps population and further quench the $\gamma$ emission. We investigate this possibility by treating the system as a spinor condensate, and use the coupled time dependent Gross-Pitaevskii (GP) equations to take into account possible population-exchanging scatterings and annihilation processes in the p-Ps/o-Ps BEC mixture. This GP simulation is used to predict the $\gamma$-ray yield in realistic experimental scenarios. \\[4pt] [1] P. M. Platzman and A. P. Mills, Jr.,{\em Phys. Rev. B} {\bf 49}, 454 (1994) [Preview Abstract] |
Tuesday, March 19, 2013 4:18PM - 4:30PM |
J40.00010: Quantum simulation of non-equilibrium dynamical maps with trapped ions Philipp Schindler, Marcus M\"uller, Daniel Nigg, Thomas Monz, Julio T. Barreiro, Esteban A. Martinez, Markus Hennrich, Sebastian Diehl, Peter Zoller, Rainer Blatt Dynamical maps are central for the understanding of general state transformations of physical systems. Prime examples include classical nonlinear systems undergoing transitions to chaos, or single particle quantum mechanical counterparts showing intriguing phenomena such as dynamical localization. Here, we extend the concept of dynamical maps to an open-system, many-particle context and experimentally explore the stroboscopic dynamics of a complex many-body spin model in a universal quantum simulator using up to five ions. We generate quantum mechanical long range order by an iteration of purely dissipative maps, reveal the characteristic features of a combined coherent and dissipative non-equilibrium evolution, and develop and implement various error detection and reduction techniques that will facilitate the faithful quantum simulation of larger systems. [Preview Abstract] |
Tuesday, March 19, 2013 4:30PM - 4:42PM |
J40.00011: Quantum simulations of neutrino oscillations and the Majorana equation Changsuk Noh, Blas Rodriguez-Lara, Dimitris Angelakis Two recent works on quantum simulations of relativistic equations are presented. The first is on neutrino oscillations with trapped ions as a generalization of Dirac equation simulation in 1 spatial dimension. It is shown that with two or more ion qubits it is possible to mimic the flavour oscillations of neutrinos. The second part is on quantum simulations of the Majorana equation based on the earlier work by Casanova et al. (PRX 1, 021018). We show that by decoupling the equation, it is possible to simulate with a smaller number of qubits given that one can perform complete tomography, including the spatial degrees of freedom. [Preview Abstract] |
Tuesday, March 19, 2013 4:42PM - 4:54PM |
J40.00012: Phase Diagram of a driven-dissipative Bose-Hubbard model Alexandre Le Boit\'e, Giuliano Orso, Cristiano Ciuti In recent years, quantum fluids of light in nonlinear optical systems have attracted a lot of interest [1]. In particular, a considerable activity is presently devoted to non-equilibrium many-body phenomena with light, such as superfluid propagation and generation of topological excitations. We present here recent theoretical results on strongly correlated photons in arrays of nonlinear cavities, described by a driven-dissipative Bose-Hubbard model. We have determined the mean-field phase diagram, studied the collective excitations and quantum correlations [2], finding interesting properties which are absent in the equilibrium case.\\[4pt] [1] I. Carusotto and C. Ciuti, Rev. Mod. Phys. (in press, 2012), arXiv:1205.6500.\\[0pt] [2] A. Le Boit\'e, G. Orso, C. Ciuti, in preparation. [Preview Abstract] |
Session J41: Interacting Bosons in Optical Lattices
Sponsoring Units: DAMOPChair: Tigran Sedrakyan, University of Minnesota
Room: 350
Tuesday, March 19, 2013 2:30PM - 2:42PM |
J41.00001: Characterizing boson density wave and valence bond orders in a lattice by its dual vortex degree of freedoms Yan Chen, Jinwu Ye A duality transformation in quantum field theory is usually established first through partition functions. It is always important to explore the dual relations between various correlation functions in the transformation. Here, we explore such a dual relation to study quantum phases and phase transitions in an extended boson Hubbard model at $1/3$ ($2/3$) filling on a triangular lattice. We develop systematically a simple and effective way to use the vortex degree of freedoms on dual lattices to characterize both the density wave and valence bond symmetry breaking patterns of the boson insulating states in the direct lattices. In addition to a checkerboard charge density wave (X-CDW) and a stripe CDW, we find a novel CDW-VBS phase which has both local CDW and local valence bond solid (VBS) orders. Implications on QMC simulations are addressed. The possible experimental realizations of cold atoms loaded on optical lattices are discussed. [Preview Abstract] |
Tuesday, March 19, 2013 2:42PM - 2:54PM |
J41.00002: Expansion dynamics of interacting bosons in homogeneous lattices Stephan Langer, Jens P. Ronzheimer, Michael Schreiber, Simon Braun, Sean Hodgman, Ian P. McCulloch, Fabian Heidrich-Meisner, Immanuel Bloch, Ulrich Schneider Due to independent real-time control of Hamiltonian parameters in optical lattices, the non-equilibrium transport properties of interacting bosons and fermions can be studied in experiments with ultra-cold atomic gases (see [1] for a sudden expansion experiment with fermions). In this work, we experimentally and numerically investigate the expansion of initially localized bosons in homogeneous one- and two-dimensional optical lattices. Dimensionality has a crucial influence, since one-dimensional systems expand ballistically both in the non-interacting and the strongly interacting limit, separated by a pronounced minimum in the expansion velocity at intermediate interaction strengths. For two-dimensional and sufficiently strongly coupled one-dimensional systems, even weak interactions lead to a dramatic suppression of the expansion, indicative of diffusive dynamics. In the case of one dimension, we find an excellent agreement between the experimental results and time-dependent density-matrix renormalization group simulations. [1] Schneider et al. Nature Phys. 8, 213 (2012) [Preview Abstract] |
Tuesday, March 19, 2013 2:54PM - 3:06PM |
J41.00003: Phase diagram of two-species hard-core bosons in a two-dimensional optical lattice Kalani Hettiarachchilage, Val\'ery Rousseau, Ka-Ming Tam, Mark Jarrell, Juana Moreno We study the finite temperature phase diagram as a function of doping for strongly correlated two-species hard-core bosons in a two-dimensional optical lattice by using Quantum Monte Carlo simulations. This model contains a repulsive interspecies interaction and different hopping terms between nearest neighbors of the two species. The phase diagram shows several competing phases such as an anti-ferromagnetically ordered Mott insulator, a coexistent, a phase separated, a superfluid and a normal liquid phases. Among them, coexistence of anti-ferromagnetic and superfluid phases near half filling and a phase separated region inside superfluid region away from half filling are of main interests. Mott behaviors of heavy species and Mott and superfluid behaviors of light species at low temperatures create this novel phase separation region. At high temperatures only a normal liquid phase appears. [Preview Abstract] |
Tuesday, March 19, 2013 3:06PM - 3:18PM |
J41.00004: Fractional charge separation in the hard-core Bose Hubbard Model on the Kagome Lattice Xue Feng Zhang, Sebastian Eggert We consider the hard core Bose Hubbard Model on a Kagome lattice with fixed (open) boundary conditions on two edges. We find that the fixed boundary conditions lift the degeneracy and freeze the system at 1/3 and 2/3 filling at small hopping. At larger hopping strengths, fractional charges spontaneously separate and are free to move to the edges of the system, which leads to a novel compressible phase with solid order. The compressibility is due to excitations on the edge which display a chrial symmetry breaking that is reminiscent of the quantum Hall effect. Large scale Monte Carlo simulations confirm the analytical calculations. [Preview Abstract] |
Tuesday, March 19, 2013 3:18PM - 3:30PM |
J41.00005: Thermodynamics of the Two-Dimensional Hubbard Model James LeBlanc, Emanuel Gull The application of a numerically exact continuous time impurity solver with the DCA dynamical mean field theory has allowed us to study the thermodynamics of the two-dimensional Hubbard model for finite, but large cluster sizes. Variation in cluster size, upwards of 50-sites, allows for extrapolation to the thermodynamic limit. We present results relevant to cold gas systems, such as entropy, double occupancy and nearest-neighbour spin correlations as well as discuss the implications of these calculations on pseudogap physics of the High-Tc Cuprate superconductors away from half filling. [Preview Abstract] |
Tuesday, March 19, 2013 3:30PM - 3:42PM |
J41.00006: Bosonic Mott Insulator with Pseudo-spin Meissner Currents Karyn Le Hur, Alexandru Petrescu We introduce a two-component bosonic Mott insulator that can support chiral Meissner edge currents as a result of time-reversal symmetry breaking due to the application of a uniform magnetic field. The key ingredient is the presence of two layers exhibiting both charge (total density) and pseudo-spin (relative density) degrees of freedom. This then allows for a Mott phase characterized by pseudospin edge currents of Meissner type [1]. A simple example can be built from a ladder system [2]. We determine the temperature scale for the existence of such a phase as a function of the interlayer Josephson coupling and interaction. We show that it is possible to probe this phase by introducing gauge fields parallel to the layers, and that in the low-field limit the system exhibits a Meissner effect, in which interlayer currents are suppressed, and the overall current circulation in the layers opposes the applied field. For higher field values the currents organize themselves in vortices, as a result of a commensurate-incommensurate transition.\\[4pt] [1] Alexandru Petrescu and Karyn Le Hur, in preparation \\[0pt] [2] E. Orignac and T. Giamarchi, Phys. Rev. B 64 p. 144515 (2001); F. Crepin, N. Laflorencie, G. Roux and P. Simon, Phys. Rev. B 84, 054517 (2011). [Preview Abstract] |
Tuesday, March 19, 2013 3:42PM - 3:54PM |
J41.00007: Experimental predictions based on LOAF theory in dilute Bose atomic gases Bogdan Mihaila We discuss possible new experimental signatures of correlations in dilute Bose gases with tunable interactions within the framework of LOAF theory. The leading-order auxiliary field (LOAF) theoretical framework is a non-perturbative approximation treating the contributions of the normal and anomalous densities on equal footing [Cooper et al. Phys. Rev. Lett. 105, 240402 (2012)]. LOAF is a conserving and gapless approximation, satisfies Goldstone's theorem, yields a Bose-Einstein transition that is second order, and can be applied outside the regime of weakly-interacting particles. [Preview Abstract] |
Tuesday, March 19, 2013 3:54PM - 4:06PM |
J41.00008: Quadrature interferometry for nonequilbrium ultracold atoms in optical lattices Philip Johnson, Eite Tiesinga We propose an interaction-based interferometric technique for making time-resolved measurements of quadrature operators of nonequilibrium ultracold atoms in optical lattices. The technique creates two subsystems of magnetic atoms in different spin states and lattice sites--the arms of the interferometer. A Feshbach resonance turns off atom-atom interactions in one spin subsystem, making it a well-characterized reference state, while atoms in the other subsystem undergo nonequilibrium many-body dynamics for a variable hold time. The nonequilibrium evolution can involve a variety of Hamiltonians, including systems with tunneling and spin-orbit couplings using artificial gauge fields. Interfering the subsystems via a second beam-splitting operation, time-resolved quadrature measurements are directly obtained by detecting relative spin populations. Analyzing a simple application of the interferometer, we obtain analytic predictions for quadratures for deep optical lattices with negligible tunneling. As a second, distinct application, we show that atom-atom interaction strengths can in principle be determined with super-Heisenberg scaling $n^{-3/2}$ in the mean number of atoms per lattice site $n$, making it possible to test the physics of interaction-based quantum metrology. [Preview Abstract] |
Tuesday, March 19, 2013 4:06PM - 4:18PM |
J41.00009: Utilizing nonequilibrium effects to probe the Mott-insulator-superfluid transition of a trapped gas of interacting bosons Lev Vidmar, S. Langer, I. McCulloch, U. Schollwoeck, U. Schneider, F. Heidrich-Meisner An increased effort has been lately devoted to explore and establish the possible links between equilibrium and nonequilibrium properties of interacting quantum many-body systems. Recent experiments on optical lattices have shown the possibility of measuring the expansion velocity of an initially trapped system, which after the sudden release of the trap expands in an empty lattice [1]. Recent theoretical studies of interacting fermions indicated that the measurement of expansion velocity may provide information about the initial state [2]. In our work, we show that measuring the expansion velocity of an initially trapped gas of interacting bosons allows one to distinguish between a superfluid and a Mott insulating state in the initial ground state in 1D. We perform time-dependent DMRG calculations of the Bose-Hubbard model in a harmonic trap and a box trap. We derive a state diagram of a trapped system as a function of Coulomb repulsion and density from the expansion velocity. The resulting diagram is consistent with the state diagram obtained by measuring equilibrium properties such as local density fluctuations and on-site compressibility [3]. [1] Schneider et al. Nature Physics 8, 213 (2012) [2] Langer et al., PRA 85, 043618 (2012) [3] Rigol et al, PRA 79, 053605 (2009) [Preview Abstract] |
Tuesday, March 19, 2013 4:18PM - 4:30PM |
J41.00010: Effects of Dissipation in a BEC Dimer T. Pudlik, H. Hennig, D. Witthaut, D.K. Campbell Recently\footnote{Holger Hennig, Dirk Witthaut, and David K. Campbell, Phys. Rev. A, to appear} we have shown that a ``global phase space'' (GPS) approach provides valuable understanding of the long-time coherence and Einstein-Podolsky-Rosen entanglement of a Bose-Einstein Condensate trapped in a two-well optical lattice (``BEC dimer''). In particular, the GPS approach allows one to distinguish purely quantum effects from those which are captured by semi-classical methods. The GPS approach in Ref. (1) was applied in the limit of zero dissipation. In the present contribution, we extend the approach to allow for dissipation and again compare the results with relevant experiments. We also report preliminary results on a BEC trimer, for which the semi-classical phase space is no longer completely integrable, in contrast to the the dimer. [Preview Abstract] |
Tuesday, March 19, 2013 4:30PM - 4:42PM |
J41.00011: Higgs boson in two dimensional superfluid and Mott insulator states Kun Chen, Yuan Huang, Longxiang Liu, Youjin Deng, Lode Pollet, Nikolay Prokof'ev We find that despite strong decay into Goldstone modes the Higgs boson survives as a well-defined resonance in the two-dimensional relativistic field theory realized in the cold atomic system near the quantum critical point between the superfluid (SF) and Mott insulator(MI) states. Using scaling analysis of analytically continued results from quantum Monte Carlo simulations we construct universal spectral functions for scalar response both for SF and MI phases and reveal that they share similar properties: a resonant peak followed by a broader secondary peak before saturating to a near plateau behavior at higher frequencies, i.e. the Higgs amplitude mode is present in the MI phase under the correlation length scale. Our simulations of a trapped system of ultra-cold $^{87}Rb$ atoms explain recent experimental data and how the signal is modified by tight confinement. [Preview Abstract] |
Tuesday, March 19, 2013 4:42PM - 4:54PM |
J41.00012: How the scaling behavior changes near the quantum phase transition point? Hao Lee, Shiang Fang, Daw-Wei Wang With unbiased quantum Monte Carlo simulation (worm algorithm), we investigate off-diagonal~long-ranged correlation and density-density correlation in two-dimensional Bose-Hubbard model. For a finite size system, we show how the Bose-Einstein condensate and the off-diagonal long-range order can appear before the presence of superfluidity in higher temperature, and demonstrate the difference of the definition of Tc for various model independent methods. Furthermore, we systematically explore the critical behaviors such as the decaying behavior of the single-particle~correlation~function near the classical (BKT)~and quantum phase transition point. We define the regime when the critical behavior works and observe how this regime changes when the critical temperature approaches zero near the quantum critical point (SF-Mott). Our results show the higher order behavior beyond the universal scaling regime, and provides a lot of insight to future experiments on critical behavior near various quantum phase transition. [Preview Abstract] |
Tuesday, March 19, 2013 4:54PM - 5:06PM |
J41.00013: Real space renormalization of the Mott-insulator to Bose-glass transition in the disordered Bose-Hubbard model Anthony Hegg, Frank Kruger, Philip Phillips We show the explicit breakdown of self-averaging, due to rare region Griffiths physics, in the disordered Bose-Hubbard model . The real space renormalization flow of the disorder is toward a Gaussian distribution with vanishing relative variance in the Mott insulator, whereas the Bose glass distribution becomes distinctly non-Gaussian with diverging relative variance. We explore distributions which correspond to a non-trivial fixed point in the renormalization group equations. [Preview Abstract] |
Tuesday, March 19, 2013 5:06PM - 5:18PM |
J41.00014: Anomalous hall phases in a bosonic Mott insulator Clement Wong, Rembert Duine Spin-orbit coupled systems that break time-reversal symmetry can exhibit the anomalous hall phase, which support a hall conductivity in the absence of a magnetic field. These topological phases are in a sense the building blocks of topological insulators and bears similarities to chiral topological superconductors. Recently, it has become possible to engineer spin-orbit couplings in cold atomic systems, making it possible to study these systems in the strongly interacting regime, for bosons and fermions. With these motivations, we study spin-orbit coupled bosons in an optical lattice in the Mott-insulating phase using a strong-coupling perturbation theory. We show that quite generally, strong interactions can induce an anomalous Hall phase even for a topologically trivial spin-orbit coupling. For the spin orbit coupling in experiment Lin et. al. [Nature (London) 471, 83 (2011)], we compute the quasiparticle dispersions, spectral weights, renormalized momentum space texture and the associated interaction-generated Berry curvature. Our results have implications for the Mott-insulating phases with textured magnetic order. [Preview Abstract] |
Tuesday, March 19, 2013 5:18PM - 5:30PM |
J41.00015: Superfluid-insulator transition in a disordered two-dimensional quantum rotor model with random on-site interactions Taeyang An, Min-Chul Cha We study the superfluid-insulator quantum phase transition in a disordered two-dimensional quantum rotor model with random on-site interactions in the presence of particle-hole symmetry. Via worm-algorithm Monte Carlo calculations of superfluid density and compressibility, we find the dynamical critical exponent $z\approx 1.13(2)$ and the correlation length critical exponent $1/\nu \approx 1.1(1)$. These exponents suggest that the insulating phase is a incompressible Mott glass rather than a Bose glass. [Preview Abstract] |
Session J42: Focus Session: Physics of Glasses and Viscous Liquids II
Sponsoring Units: DCPChair: Mark Ediger, University of Wisconsin
Room: Hilton Baltimore Holiday Ballroom 3
Tuesday, March 19, 2013 2:30PM - 3:06PM |
J42.00001: Theoretical and Computational Studies of Dynamical Heterogeneity and Growing Length Scales in Supercooled Liquids Invited Speaker: David Reichman In this talk I review recent progress made by our group and collaborators in elucidating quantitative aspects connected to growing length and time scales in supercooled liquids. In particular, I focus on extraction of static length scales and the relationship between soft modes and dynamical heterogeneity. Connections to jamming are discussed. If time permits, I will discuss recent work on the mean-field theory of growing dynamical length scales in supercooled liquids with respect to critical fluctuations and the putative upper critical dimension. [Preview Abstract] |
Tuesday, March 19, 2013 3:06PM - 3:42PM |
J42.00002: Equilibrium study of a liquid-glass transition Invited Speaker: Ludovic Berthier The liquid-glass transition in dense fluids is characterized by several crossover temperatures, but glasses are obtained without crossing any sharp singularity. The existence of an underlying phase transition, predicted theoretically in some limiting cases, is therefore only supported by uncontrolled extrapolations of macroscopic observables. Here we use a specific random pinning field to induce a liquid-glass transition in a simulated fluid. We discover a range of control parameters for which the transition can be crossed at thermal equilibrium, which allows us to probe for the first time the microscopic nature of an equilibrium glass. Our results, obtained for a range of modest system sizes, suggest that the glass transition is of the random first order type. [Preview Abstract] |
Tuesday, March 19, 2013 3:42PM - 4:18PM |
J42.00003: Static and dynamic length scales in glass-forming liquids Invited Speaker: Gilles Tarjus A recurring question about glass formation concerns the collective nature of the dynamics as one cools or compresses a liquid. If the phenomenon is indeed collective, it should be characterized by the development of nontrivial correlations to which one or several typical length scales might be associated. One source of growing correlations has been clearly identified in connection with the increasingly heterogeneous character of the dynamics as one approaches the glass transition. An associated length, commonly referred to as ``dynamic'', can be extracted from multi-point space-time correlation functions. In addition, several theories of the glass transition posit the existence of a growing ``static'' length accompanying a liquid's dynamical slowdown. This length is not detectable through standard measurements on pair density correlations, which have been shown to display only unremarkable behavior as the relaxation slows down. However, a number of proposals have been put forward for unveiling such a nontrivial static length. Through computer simulation of model glass-forming liquids, we address the two following central questions: (i) Are the variations with temperature or pressure of these various lengths correlated? (ii) Is the increase of the relaxation time due to the growth of any of the above lengths, or, with less compelling consequences, is it at least correlated to it? While our results rule out the existence of a general principle tying together the evolutions of dynamic and static lengths in glass-forming systems, we discuss how the answer to the above questions depends on the dynamic regime under consideration as well as on the type of material. [Preview Abstract] |
Tuesday, March 19, 2013 4:18PM - 4:30PM |
J42.00004: Correlations between Elementary Relaxation Steps in a Model Glass Former Dieter Bingemann, Nai Chien Yeat Despite decades of research the dramatic slowdown of the dynamics in glasses upon cooling remains a mystery. We identify individual, sudden, local, structural relaxation events in a deeply supercooled binary Lennard-Jones system through statistical analysis of the particle trajectories. Correlations between these events in space and time show that (a) relaxation events often occur in clusters (cooperatively rearranging regions), (b) events follow each other in space and time (facilitation), (c) regions with the longest waiting times between events are encapsulated in layers with increasingly shorter waiting times (dynamic heterogeneity), (d) the length scales of these fast and slow regions show very little temperature dependence. Focusing on individual relaxation events we find that large-amplitude vibrations of atoms surrounding each event open a gateway for structural relaxation, hinting at a potential molecular mechanism for the dramatic slowdown of glass dynamics. [Preview Abstract] |
Tuesday, March 19, 2013 4:30PM - 4:42PM |
J42.00005: Diffusion in Crowded Environments: Monte Carlo and Molecular Dynamics Studies Robin Selinger, Prithviraj Nandigrami, Andrew Konya, Jennifer Toth Anomalous diffusion is sometimes, but not always, observed in dense multicomponent mixtures, e.g. in diffusion of proteins in a lipid membrane [1]. To investigate this phenomenon, we carry out 2-d simulation studies using both on-lattice Monte Carlo and off-lattice Molecular Dynamics. ``Tracer'' particles are emitted from a source along one side of the simulation cell and absorbed by a sink along the other side, diffusing through a chamber containing ``crowder'' particles whose number remains constant. On-lattice Monte Carlo studies show that equilibrium tracer flux drops linearly with crowder density, showing non-Fickian behavior well below the percolation threshold. Molecular dynamics studies in the same geometry also show non-Fickian behavior, but tracer flux is a nonlinear function of crowder density. We compare our results with analytical calculations and experimental studies, and discuss implications for understanding diffusion-mediated processes in cell membranes.\\[4pt] [1] J. A. Dix and A. S. Verkman, Annu. Rev. Biophys. 37, 247 (2008). [Preview Abstract] |
Tuesday, March 19, 2013 4:42PM - 4:54PM |
J42.00006: Crystallization and glass formation in multi-component liquids Kai Zhang, Minglei Wang, Stefanos Papanikolaou, Jan Schroers, Corey O'Hern When a liquid is cooled faster than the critical cooling rate, crystallization is avoided, and amorphous solids are formed. What sets the critical cooling rate? We perform molecular dynamics simulations of model metallic alloys---polydisperse spheres with hard-sphere and modified Lennard-Jones interactions---to study the critical cooling rate as a function of the particle size ratio, stoichiometry, and strength of the attractive interactions. We also characterize the structural properties of glassy and crystalline states that form at rapid and slow cooling/compression rates, respectively, using local order parameters, position correlation functions, and Voronoi and other tessellations. [Preview Abstract] |
Tuesday, March 19, 2013 4:54PM - 5:30PM |
J42.00007: What controls the relaxation time? Lessons learnt from simple liquids' quasiuniversality Invited Speaker: Jeppe Dyre The relaxation time of a supercooled liquid is extremely temperature and density dependent, approaching hours upon cooling or compression. Is this quantity controlled by the entropy, is it controlled by high-frequency elastic properties as assumed in the shoving and related elastic models, or by another physical property? It is far from certain that there is a simple and generally valid answer to this question for glass-forming liquids with quite different chemistry, but as physicists we like to think that this is the case. The talk summarizes recent results [1] on the quasiuniversality of simple liquids, where a simple liquid is defined as a system with strong virial / potential-energy correlations in the equilibrium NVT fluctuations. Such systems, which include e.g. the Lennard-Jones liquid, have good isomorphs. An isomorph is a curve in the phase diagram along which structure, dynamics, and some thermodynamic properties in reduced units are invariant to a good approximation [2-5]. It was recently conjectured [1] that simple liquids have almost the same isomorphs in the sense that these systems are characterized by a quasiuniversal one-parameter family of reduced-coordinate constant-potential-energy manifolds encoding all isomorph invariants. The entropy is the logarithm of the area of this manifold and the high-frequency elastic properties are basically the surface's curvature. Since the relaxation time is also encoded in the manifold, both quantities will appear to ``control'' the relaxation time, as will any isomorph invariant.\\[4pt] References: [1] J. C. Dyre, arXiv:1208.1748 (2012).\\[0pt] [2] N. Gnan et al., J. Chem. Phys. 131, 234504 (2009).\\[0pt] [3] N. Gnan et al., Phys. Rev. Lett. 104, 125902 (2010).\\[0pt] [4] U. R. Pedersen et al., Phys. Rev. Lett. 105, 157801 (2010).\\[0pt] [5] T. Ingebrigtsen et al., Phys. Rev. X 2, 011011 (2012). [Preview Abstract] |
Session J43: Focus Session: Protein Misfolding and Aggregation I
Sponsoring Units: DCP DBIOChair: Elsa Yan, Yale University
Room: Hilton Baltimore Holiday Ballroom 2
Tuesday, March 19, 2013 2:30PM - 3:06PM |
J43.00001: Role of sequence and membrane composition in structure of transmembrane domain of Amyloid Precursor Protein Invited Speaker: John Straub Aggregation of proteins of known sequence is linked to a variety of neurodegenerative disorders. The amyloid $\beta $ (A$\beta )$ protein associated with Alzheimer's Disease (AD) is derived from cleavage of the 99 amino acid C-terminal fragment of Amyloid Precursor Protein (APP-C99) by $\gamma $-secretase. Certain familial mutations of APP-C99 have been shown to lead to altered production of A$\beta $ protein and the early onset of AD. We describe simulation studies exploring the structure of APP-C99 in micelle and membrane environments. Our studies explore how changes in sequence and membrane composition influence (1) the structure of monomeric APP-C99 and (2) APP-C99 homodimer structure and stability. Comparison of simulation results with recent NMR studies of APP-C99 monomers and dimers in micelle and bicelle environments provide insight into how critical aspects of APP-C99 structure and dimerization correlate with secretase processing, an essential component of the A$\beta $ protein aggregation pathway and AD. [Preview Abstract] |
Tuesday, March 19, 2013 3:06PM - 3:42PM |
J43.00002: Spontaneous Formation of Oligomers and Fibrils in Large-Scale Molecular Dynamics Simulations of A-beta Peptides Invited Speaker: Carol Hall Protein aggregation is associated with serious and eventually-fatal neurodegenerative diseases including Alzheimer's and Parkinson's. While atomic resolution molecular dynamics simulations have been useful in this regard, they are limited to examination of either oligomer formation by a small number of peptides or analysis of the stability of a moderate number of peptides placed in trial or known experimental structures. We describe large scale intermediate-resolution molecular dynamics simulations of the spontaneous formation of fibrils by systems containing large numbers ( 48) of peptides including A-beta (16-22), and A-beta ( 17-42) peptides. We trace out the aggregation process from an initial configuration of random coils to proto-filaments with cross-$\beta $ structures and demonstrate how kinetics dictates the structural details of the fully formed fibril. Fibrillization kinetics depends strongly on the temperature. Nucleation and templated growth via monomer addition occur at and near a transition temperature above which fibrils are unlikely to form. Oligomeric merging and structural rearrangement are observed at lower temperatures. [Preview Abstract] |
Tuesday, March 19, 2013 3:42PM - 4:18PM |
J43.00003: Exploring the Free Energy and Conformational Landscape of Peptides Upon Aggregation and Amyloid Formation Invited Speaker: Roland Winter Using various physical-chemical tools and perturbation parameters, the effects of temperature, pressure as well as lipid interfaces and confining geometries on the various stages of the aggregation and fibrillation reaction of amyloidogenic peptides have been studied. First we show data on the experimentally derived static structure factor obtained for the protein insulin which has been analyzed with a statistical mechanical model based on the DLVO potential. The data reveal that the protein self-assembles into equilibrium clusters already at low concentrations in the pre-nucleation phase. Then, mechanistic details about the nucleation process and concurrent aggregation pathways of insulin and more disease related amyloidogenic peptides, such as IAPP and PrP, and the differential stability of the aggregate structures formed are discussed. Also solvational perturbations, accomplished by the addition of various salts and cosolvents have been explored. They exert pronounced and diversified effects on the unfolding, non-native assembly and fibril formation, which ultimately manifest in morphological variations of mature aggregates and fibrils. Finally, the presence of lipid interfaces and soft-matter confinement will be discussed, which drastically change the aggregation pathway as well as the kinetics of peptide aggregation. Using various model membrane systems, the influence of different membrane characteristics on the lipid-protein interaction has been revealed. [Preview Abstract] |
Tuesday, March 19, 2013 4:18PM - 4:54PM |
J43.00004: Chaotic (``Non-Pathway'') Aggregation of $\beta $-Amyloid Congener Peptides Invited Speaker: Stephen C. Meredith We compared A$\beta_{21-30}$ and A$\beta _{16-34}$, with or without N-terminal Cys or cyclization. All A$\beta_{21-30}$ variants were monomeric and unstructured. In contrast, A$\beta_{16-34}$ and Cys-A$\beta_{16-34}$ formed fibrils -- the latter more rapidly, due to disulfide bond formation. NMR showed no long-range nOes. In serial NOESYs, after changing pH (3 to 7.4) to initiate aggregation, some chemical shifts did not change, while others changed dramatically. In addition, although signals diminished globally with aggregation, the decay rates for individual peaks varied over $\sim$ 4-fold range. We attribute selective signal loss to conformational constraints restricting local tumbling and/or static structural heterogeneity. Signal decays for A$\beta_{16-34}$ and Cys-A$\beta_{16-34}$ differed in three ways: 1) Decay rates for Cys-A$\beta_{16-34}$ \textgreater\ A$\beta_{16-34}$; 2) variances for rate constants of Cys-A$\beta_{16-34}$ \textless\ A$\beta_{16-34}$ across replicate experiments; 3) smaller variances of rate constants within single experiments for Cys-A$\beta_{16-34}$ than A$\beta_{16-34}$. These results indicate both acceleration and ordering of aggregation by the disulfide bond in Cys-A$\beta_{16-34}$ compared to which aggregation of A$\beta_{16-34}$ was chaotic and disordered. Our results highlight several essential differences between protein folding and unfolded protein aggregation.\\[4pt] In collaboration with Laura M. Luther, Joshua T.B. Williams, Andrew J. Hawk, and Joseph R. Sachleben, The University of Chicago. [Preview Abstract] |
Tuesday, March 19, 2013 4:54PM - 5:30PM |
J43.00005: Challenges for understanding protein aggregation through computer simulations Invited Speaker: Normand Mousseau The first computer simulations of protein aggregation were performed a little more than decade ago. Over the years, the community of computer biochemists, chemists and physicists has grown considerably and the simulations becoming more realistic and often closer to experiments, due both to a better understanding of the onset of aggregation and to ever more powerful computers. In view of this expansion both in terms of papers and system size, what have been the real contribution of these simulations to our understanding of amyloid diseases? In this talk, I will present a personal view of the progress that has been accomplished over the last decade. I will aslo discuss some of the challenges that must still be overcome for computer simulations to move to the next level of contributions to this fundamental problem. [Preview Abstract] |
Session J44: Biological Networks
Sponsoring Units: DBIORoom: Hilton Baltimore Holiday Ballroom 1
Tuesday, March 19, 2013 2:30PM - 2:42PM |
J44.00001: Network complexity: when interaction strengths matter more than topology Mikhail Tikhonov, William Bialek A typical cellular network has thousands of microscopic parameters that cannot all be equally relevant to the network function; yet discarding them and considering only the topology of interactions is unacceptably coarse. How much do quantitative details matter? We present a toy model where the appropriately mesoscopic level of description can be found exactly. We define a measure of network complexity and find that both the choice of topology and strength of interactions can affect the complexity dramatically, but optimizing interaction strengths typically has a stronger effect. We further show that a larger network is not automatically more complex; constructing a high-complexity network always requires a careful adjustment of the strengths of interactions. This suggests that the challenge of ``evolving a complex network'' does not reduce to making new connections and constructing a dense topology of interactions. Evolution acting on ``numbers on arrows'' (interaction strengths), even within the confines of a fixed topology, is a much more effective way of increasing complexity than adding new components and connections of some typical strength. [Preview Abstract] |
Tuesday, March 19, 2013 2:42PM - 2:54PM |
J44.00002: Ising models of strongly coupled biological networks with multivariate interactions Lina Merchan, Ilya Nemenman Biological networks consist of a large number of variables that can be coupled by complex multivariate interactions. However, several neuroscience and cell biology experiments have reported that observed statistics of network states can be approximated surprisingly well by maximum entropy models that constrain correlations only within pairs of variables. We would like to verify if this reduction in complexity results from intricacies of biological organization, or if it is a more general attribute of these networks. We generate random networks with p-spin $(p>2)$ interactions, with N spins and M interaction terms. The probability distribution of the network states is then calculated and approximated with a maximum entropy model based on constraining pairwise spin correlations. Depending on the M/N ratio and the strength of the interaction terms, we observe a transition where the pairwise approximation is very good to a region where it fails. This resembles the sat-unsat transition in constraint satisfaction problems. We argue that the pairwise model works when the number of highly probable states is small. We argue that many biological systems must operate in a strongly constrained regime, and hence we expect the pairwise approximation to be accurate for a wide class of problems. [Preview Abstract] |
Tuesday, March 19, 2013 2:54PM - 3:06PM |
J44.00003: Add HOC?: dendritic nonlinearities shape higher-than-pairwise correlations and improve coding in noisy (spiking) neural populations Joel Zylberberg, Eric Shea-Brown Recent experiments with relatively large neural populations show significant higher-order correlations (HOC): the data are poorly fit by pair-wise maximum entropy models, but well-fit by higher-order models. We seek to understand how HOC are shaped by the properties of networks and of the neurons therein, and how these HOC affect population coding. In our presentation, we will demonstrate that dendritic non-linearities similar to those observed in physiology experiments are equivalent to beyond-pairwise interactions in a spin-glass-type statistical model: they can either increase, or decrease, the magnitude of the HOC relative to the pair-wise correlations. We will then discuss a population coding model with parameterized pairwise- and higher-order interactions, revealing the conditions under which the beyond-pairwise interactions (dendritic nonlinearities) can increase the mutual information between a given set of stimuli, and the population responses. For jointly Gaussian stimuli, coding performance can be slightly improved by shaping the output HOC via dendritic nonlinearities, if the neural firing rates are low. For skewed stimulus distributions, like the distribution of luminance values in natural images, the performance gains are much larger. [Preview Abstract] |
Tuesday, March 19, 2013 3:06PM - 3:18PM |
J44.00004: Automated adaptive model inference to predict biological network dynamics Bryan Daniels, Ilya Nemenman Dynamical models of cellular regulation often consist of large and intricate networks of interactions at the molecular scale. Since individual interaction parameters are usually difficult to measure, these parameters are often estimated implicitly, using statistical fits. This can lead to overfitting and degradation in the quality of models' predictions. Here we study phenomenological models that adapt their level of detail to the amount of available data, leading to accurate predictions even when microscopic details are not well understood. The model search is made computationally efficient by testing an ordered, nested set of models and by using a model class that can be solved using linear regression in log-space. We test the method on synthetic data and find that phenomenological models inferred this way often outperform detailed, ``correct'' molecular models in making predictions about responses of the system to signals yet unseen. [Preview Abstract] |
Tuesday, March 19, 2013 3:18PM - 3:30PM |
J44.00005: Using dynamics to identify network topology Sahand Jamal Rahi, Krasimira Tsaneva-Atanasova To elucidate the topology of a signaling pathway, generally, experimentalists manipulate a cell's molecular architecture, for example, by knocking out genes. Molecular biology techniques, though, are not only invasive and labor-intensive, they have also often been eluded by the complexity of biological networks, e.g., in the case of the gonadotropin-releasing hormone (GnRH) system. Inspired by the rapidly accumulating examples of oscillatory signaling in biology, we explored whether such dynamical stimuli can be used to discriminate different topologies of adaptive pathways, which are ubiquitous in biology. Responses to static inputs are nearly indistinguishable given strong measurement noise. Sine function stimuli, widely used in physics, are difficult to implement in standard microfluidics or optogenetics set-ups and do not simplify the mathematical analysis because of the nonlinearities in these systems. With periodic on-off pulses, which can be easily produced, however, simple adaptive circuit motifs and detailed models from the literature robustly reveal distinct output characteristics, which manifest in how the period of maximal output varies with pulse width. Our calculations provide a framework for using existing methods to discover difficult to reveal mechanisms. Furthermore, our results constrain the possible design principles of the presumed frequency decoders in biological systems where pulsatile signaling has recently been discovered. [Preview Abstract] |
Tuesday, March 19, 2013 3:30PM - 3:42PM |
J44.00006: Characterization of genotype-phenotype mapping of biological networks reconciles robustness-evolvability paradox Chenghang Du, Hao Chen, Chen Zeng Typical biological system is both highly robust and highly evolvable. Yet robustness appears against changes whereas evolvability for changes. The concurrence of these two seemingly incompatible features is a central paradox for contemporary evolutionary biology. Using a Boolean model of yeast cell cycle networks, we quantitatively determine (1) the genotype-phenotype mapping. Here genotype stands for the network structure and phenotype for its dynamics; (2) the precise topology of neutral network, i.e. the interconnecting network of networks of different structures but the same dynamics; and (3) the number of new phenotypes in the vicinity of a neutral network. Our results demonstrate that both biological genotype and phenotype are atypical. We next show via sampling that all neutral networks exhibit a similar topology that is simply connected, fractal and sloppy (stiff in certain dimensions but diffusive otherwise). This percolating nature of neutral network leads to a positive correlation between robustness and evolvability and hence resolves the paradox. A likely explanation for such a correlation is that higher robustness results in a larger neutral network, measured by its designability and radius of gyration, which in turn accesses more new phenotypes. [Preview Abstract] |
Tuesday, March 19, 2013 3:42PM - 3:54PM |
J44.00007: Robustness of Network Measures to Link Errors John Platig, Michelle Girvan, Ed Ott Researchers studying biological networks use a variety of measures to identify ``important'' nodes in their networks. However, the robustness of these measures in the presence of link inaccuracies stemming from noisy data has not been well characterized. Here we present two simple models of false and missing links and their effect on different commonly used centrality measures, focusing particularly on degree centrality, betweenness centrality, and dynamical importance. We show that, compared to degree centrality, betweenness centrality and dynamical importance are much more robust in the face of noise if the false positives are randomly distributed. When the noise has more structure, the differences in the robustness levels of the various metrics can change dramatically. [Preview Abstract] |
Tuesday, March 19, 2013 3:54PM - 4:06PM |
J44.00008: Cascade-likeness is an intrinsic property of biological processes Hao Chen, Guanyu Wang, Chenghang Du, Rahul Simha, Chen Zeng A central theme in systems biology is to reveal the intricate relationship between structure, dynamics, and function of biological networks. The biological function is usually realized by the transformation of the relevant molecules through their interacting network. We name this trajectory of transformation as a biological process. In contrast to the structure-centric approach, we take a process-centric view to address such questions as what a biological process looks like and how it differs from an arbitrary process. As an example, we studied a simple Boolean model for the cell cycle process of budding yeast to characterize a large number of putative processes. This computational task was made possible by some highly efficient algorithms we developed. Our results demonstrated that the biological process is very robust and highly designable. Moreover, we uncovered two dynamical rules that dramatically enhance the robustness and designability. Finally, all processes in a system of small size were enumerated and highly designable processes are cascade-like. This implies that cascade-likeness is an intrinsic property of biological processes. [Preview Abstract] |
Tuesday, March 19, 2013 4:06PM - 4:18PM |
J44.00009: Autonomous Boolean Models of Regulatory Networks Mengyang Sun, Xianrui Cheng, Joshua Socolar Autonomous Boolean network (ABN) models have been developed to represent directly the connectivity, logic, and timing of updates in regulatory networks. [1] An ABN is a Boolean network in which the sequence of updates of nodes is determined internally by time delay parameters associated with each link. We propose a method to convert a given ODE model into an ABN that is applicable when the ODE dynamics produces clearly separated high and low values at each node. The ODE parameters are mapped into ABN logic and delay parameters using only local information about each link. Using the example of Ingolia's ODE model of the regulatory network that maintains segment boundaries in the \textit{Drosophila} embryo [2], we show that the resulting ABN model captures both the biologically relevant outcomes and the transient dynamics of the ODE model, and that the ABN framework provides direct insights into the mechanism supporting the biological function. [1] X. Cheng, M. Sun, and J. E. S. Socolar, 2012, J. R. Soc. Interface, (DOI: 10.1098/rsif.2012.0574) [2] Ingolia NT., 2004, PLoS Biol. 2, 805-815. (DOI:10.1371/journal.pbio.0020123) [Preview Abstract] |
Tuesday, March 19, 2013 4:18PM - 4:30PM |
J44.00010: Epigenetic landscapes explain partially reprogrammed cells and identify key reprogramming gene Alex Lang, Hu Li, James Collins, Pankaj Mehta A common metaphor for describing development is a rugged epigenetic landscape where cell fates are represented as attracting valleys resulting from a complex regulatory network. Here, we introduce a framework for explicitly constructing epigenetic landscapes that combines genomic data with techniques from physics, specifically Hopfield neural networks. Each cell fate is a dynamic attractor, yet cells can change fate in response to external signals. Our model suggests that partially reprogrammed cells (cells found in reprogramming experiments but not in vivo) are a natural consequence of high-dimensional landscapes and predicts that partially reprogrammed cells should be hybrids that coexpress genes from multiple cell fates. We verify this prediction by reanalyzing existing data sets. Our model reproduces known reprogramming protocols and identifies candidate transcription factors for reprogramming to novel cell fates, suggesting epigenetic landscapes are a powerful paradigm for understanding cellular identity. [Preview Abstract] |
Tuesday, March 19, 2013 4:30PM - 4:42PM |
J44.00011: Phage-bacteria infection networks: From nestedness to modularity Cesar O. Flores, Sergi Valverde, Joshua S. Weitz Bacteriophages (viruses that infect bacteria) are the most abundant biological life-forms on Earth. However, very little is known regarding the structure of phage-bacteria infections. In a recent study we re-evaluated 38 prior studies and demonstrated that phage-bacteria infection networks tend to be statistically nested in small scale communities (Flores et al 2011). Nestedness is consistent with a hierarchy of infection and resistance within phages and bacteria, respectively. However, we predicted that at large scales, phage-bacteria infection networks should be typified by a modular structure. We evaluate and confirm this hypothesis using the most extensive study of phage-bacteria infections (Moebus and Nattkemper 1981). In this study, cross-infections were evaluated between 215 marine phages and 286 marine bacteria. We develop a novel multi-scale network analysis and find that the Moebus and Nattkemper (1981) study, is highly modular (at the whole network scale), yet also exhibits nestedness and modularity at the within-module scale. We examine the role of geography in driving these modular patterns and find evidence that phage-bacteria interactions can exhibit strong similarity despite large distances between sites. [Preview Abstract] |
Tuesday, March 19, 2013 4:42PM - 4:54PM |
J44.00012: Continuum neural dynamics models for visual object identification Vijay Singh, Martin Tchernookov, Ilya Nemenman Visual object identification has remained one of the most challenging problems even after decades of research. Most of the current models of the visual cortex represent neurons as discrete elements in a largely feedforward network arrangement. They are generally very specific in the objects they can identify. We develop a continuum model of recurrent, nonlinear neural dynamics in the primary visual cortex, incorporating connectivity patterns and other ~experimentally observed features of the cortex. The model has an interesting correspondence to the Landau-DeGennes theory of a nematic liquid crystal in two dimensions. ~We use collective spatiotemporal excitations of the model cortex as a signal for segmentation of contiguous objects from the background clutter. The model is capable of suppressing clutter in images and filling in occluded elements of object contours, resulting in high-precision, high-recall identification of large objects from cluttered scenes. [Preview Abstract] |
Tuesday, March 19, 2013 4:54PM - 5:06PM |
J44.00013: The parameter landscape of a mammalian circadian clock model Craig Jolley, Hiroki Ueda In mammals, an intricate system of feedback loops enables autonomous, robust oscillations synchronized with the daily light/dark cycle. Based on recent experimental evidence, we have developed a simplified dynamical model and parameterized it by compiling experimental data on the amplitude, phase, and average baseline of clock gene oscillations. Rather than identifying a single ``optimal'' parameter set, we used Monte Carlo sampling to explore the fitting landscape. The resulting ensemble of model parameter sets is highly anisotropic, with very large variances along some (non-trivial) linear combinations of parameters and very small variances along others. This suggests that our model exhibits ``sloppy'' features that have previously been identified in various multi-parameter fitting problems. We will discuss the implications of this model fitting behavior for the reliability of both individual parameter estimates and systems-level predictions of oscillator characteristics, as well as the impact of experimental constraints. The results of this study are likely to be important both for improved understanding of the mammalian circadian oscillator and as a test case for more general questions about the features of systems biology models. [Preview Abstract] |
Tuesday, March 19, 2013 5:06PM - 5:18PM |
J44.00014: Effect of Transcranial Magnetic Stimulation on Neuronal Networks Ahmet Unsal, Ravi Hadimani, David Jiles The human brain contains around 100 billion nerve cells controlling our day to day activities. Consequently, brain disorders often result in impairments such as paralysis, loss of coordination and seizure. It has been said that 1 in 5 Americans suffer some diagnosable mental disorder. There is an urgent need to understand the disorders, prevent them and if possible, develop permanent cure for them. As a result, a significant amount of research activities is being directed towards brain research. Transcranial Magnetic Stimulation (TMS) is a promising tool for diagnosing and treating brain disorders. It is a non-invasive treatment method that produces a current flow in the brain which excites the neurons. Even though TMS has been verified to have advantageous effects on various brain related disorders, there have not been enough studies on the impact of TMS on cells. In this study, we are investigating the electrophysiological effects of TMS on one dimensional neuronal culture grown in a circular pathway. Electrical currents are produced on the neuronal networks depending on the directionality of the applied field. This aids in understanding how neuronal networks react under TMS treatment. [Preview Abstract] |
Tuesday, March 19, 2013 5:18PM - 5:30PM |
J44.00015: Biochemical response and the effects of bariatric surgeries on type 2 diabetes Roland Allen, Tyler Hughes, Jia Lerd Ng, Roberto Ortiz, Michel Abou Ghantous, Othmane Bouhali, Abdelilah Arredouani A general method is introduced for calculating the biochemical response to pharmaceuticals, surgeries, or other medical interventions. This method is then applied in a simple model of the response to Roux-en-Y gastric bypass (RYGB) surgery in obese diabetic patients. We specifically address the amazing fact that glycemia correction is usually achieved immediately after RYGB surgery, long before there is any appreciable weight loss. Many studies indicate that this result is not due merely to caloric restriction, and it is usually attributed to an increase in glucagon-like peptide 1 (GLP-1) levels observed after the surgery. However, our model indicates that this mechanism alone is not sufficient to explain either the largest declines in glucose levels or the measured declines in the homeostatic model assessment insulin resistance (HOMA-IR). The most robust additional mechanism would be production of a factor which opens an insulin-independent pathway for glucose transport into cells, perhaps related to the well-established insulin-independent pathway associated with exercise. Potential candidates include bradykinin, a 9 amino acid peptide. If such a substance were found to exist, it would offer hope for medications which mimic the immediate beneficial effect of RYGB surgery. [Preview Abstract] |
Session J45: Focus Session: Physics of Proteins II
Sponsoring Units: DBIOChair: Wouter Hoff, Oklahoma State University
Room: Hilton Baltimore Holiday Ballroom 4
Tuesday, March 19, 2013 2:30PM - 2:42PM |
J45.00001: Probing Single-Molecule Protein Conformational Folding-Unfolding Dynamics: The multiple-State and Multiple-Channel Energy Landscape H. Peter Lu, Zhijiang Wang, Yufan He The folding-unfolding dynamics of protein provides an important understanding of the protein conformational dynamics and functions. We have used single-molecule fluorescence resonance energy transfer combined with statistical data analysis to characterize enzyme and signaling protein fundamental conformational dynamics of Calmodulin (CaM) and kinase (6-Hydroxymethyl-7,8-dihydropterin pyrophosphokinase, HPPK). The concentration dependence of FRET efficiency of GdmCl indicates the unfolding conformational transition of the proteins. At 2M of denaturant solvent, the majority of the HPPK and CaM protein molecules are under fluctuating folding-unfolding conformational changes, spending about half time in their native state and half time in their unfolded state. We obtained the fluctuation rates from the autocorrelation function analyses of the protein conformational fluctuation trajectories, and we have identified multiple intermediate states involving in bunched time dynamics and the related energy landscape. We had also analyzed the protein folding-unfolding pathways using detailed balance theoretical model analysis in order to understand the complex multiple-state and multiple-channel protein dynamics. [Preview Abstract] |
Tuesday, March 19, 2013 2:42PM - 2:54PM |
J45.00002: Thermodynamics and kinetics of apoazurin folding under macromolecular crowding effect and chemical interference Fabio Zegarra, Margaret Cheung Proteins fold in a cellular milieu crowded by different kinds of macromolecules. They exert volume exclusion impacting protein folding processes in vivo. Folding processes, however, has been studied by chemical denaturation under in vitro conditions. The impact of the two factors as an attempt to advance the understanding of folding mechanism in vivo is not understood. Here, we investigate the folding mechanisms of apoazurin affected by the macromolecular crowding and chemical interference by using coarse-grained molecular simulations. Crowding agents are modeled as hard-spheres and the chemical denaturation effects are implemented into an energy function of the side chain and backbone interactions. Protein folding stability, mechanism, and kinetics rates of apoazurin under chemical interference and macromolecular crowding conditions are being investigated. [Preview Abstract] |
Tuesday, March 19, 2013 2:54PM - 3:06PM |
J45.00003: Using Electronic Properties of Adamantane Derivatives to Analyze their Ion Channel Interactions: Implications for Alzheimer's Disease Jason Bonacum The derivatives of adamantane, which is a cage-like diamondoid structure, can be used as pharmaceuticals for the treatment of various diseases and disorders such as Alzheimer's disease. These drugs interact with ion channels, and they act by electronically and physically hindering the ion transport. The electronic properties of each compound influence the location and level of ion channel hindrance, and the specific use of each compound depends on the functional groups that are attached to the adamantane base chain. Computational analysis and molecular simulations of these different derivatives and the ion channels can provide useful insight into the effect that the functional groups have on the properties of the compounds. Using this information, conclusions can be made about the pharmaceutical mechanisms, as well as how to improve them or create new beneficial compounds. Focusing on the electronic properties, such as the dipole moments of the derivatives and amino acids in the ion channels, can provide more efficient predictions of how these drugs work and how they can be enhanced. [Preview Abstract] |
Tuesday, March 19, 2013 3:06PM - 3:42PM |
J45.00004: A scoring framework for predicting protein structures Invited Speaker: Xiaoqin Zou We have developed a statistical mechanics-based iterative method to extract statistical atomic interaction potentials from known, non-redundant protein structures. Our method circumvents the long-standing reference state problem in deriving traditional knowledge-based scoring functions, by using rapid iterations through a physical, global convergence function. The rapid convergence of this physics-based method, unlike other parameter optimization methods, warrants the feasibility of deriving distance-dependent, all-atom statistical potentials to keep the scoring accuracy. The derived potentials, referred to as ITScore/Pro, have been validated using three diverse benchmarks: the high-resolution decoy set, the AMBER benchmark decoy set, and the CASP8 decoy set. Significant improvement in performance has been achieved. Finally, comparisons between the potentials of our model and potentials of a knowledge-based scoring function with a randomized reference state have revealed the reason for the better performance of our scoring function, which could provide useful insight into the development of other physical scoring functions. The potentials developed in the present study are generally applicable for structural selection in protein structure prediction. [Preview Abstract] |
Tuesday, March 19, 2013 3:42PM - 3:54PM |
J45.00005: Structure of a protein (H2AX): a comparative study with knowledge-based interactions Miriam Fritsche, Dieter Heermann, Barry Farmer, Ras Pandey The structural and conformational properties of the histone protein H2AX (with143 residues) is studied by a coarse-grained model as a function of temperature (T). Three knowledge-based phenomenological interactions (MJ [1], BT [2], and BFKV [3]) are used as input to a generalized Lennard-Jones potential for residue-residue interactions. Large-scale Monte Carlo simulations are performed to identify similarity and differences in the equilibrium structures with these potentials. Multi-scale structures of the protein are examined by a detailed analysis of their structure functions. We find that the radius of gyration ($R_{g})$ of H2AX depends non-monotonically on temperature with a maximum at a characteristic value $T_{c}$, a common feature to each interaction. The characteristic temperature and the range of non-monotonic thermal response and decay pattern are, however, sensitive to interactions. A comparison of the structural properties emerging from three potentials will be presented in this talk. \\[4pt] [1] S. Miyazawa and R.L. Jernigan, Macromolecules 18, 534 (1985).\\[0pt] [2] M.R. Betancourt and D. Thirumalai, Protein Sci. 2, 361 (1999).\\[0pt] [3] U. Bastolla et al. Proteins 44, 79 (2001). [Preview Abstract] |
Tuesday, March 19, 2013 3:54PM - 4:06PM |
J45.00006: Investigation of phonon-like excitation in hydrated protein powders by neutron scattering Xiang-qiang (Rosie) Chu, Eugene Mamontov, Hugh O'Neill, Qiu Zhang, Alexander Kolesnikov Detecting the phonon dispersion relations in proteins is essential for understanding the intra-protein dynamical behavior. Such study has been attempted by X-ray in recent years [1,2]. However, for such detections, neutrons have significant advantages in resolution and time-efficiency compare to X-rays. Traditionally the collective motions of atoms in protein molecules are hard to detect using neutrons, because of high incoherent scattering background from intrinsic hydrogen atoms in the protein molecules. The recent availability of a fully deuterated green fluorescent protein (GFP) synthesized by the Bio-deuteration Lab at ORNL opens new possibilities to probe collective excitations in proteins using inelastic neutron scattering. Using a direct time-of-flight Fermi chopper neutron spectrometer, we obtained a full map of the meV phonon-like excitations in the fully deuterated protein. The Q range of the observed excitations corresponds to the length scale close to the size of the secondary structures of proteins and reflects the collective intra-protein motions. Our results show that hydration of GFP seems to harden, not soften, the collective motions. This result is counterintuitive but in agreement with the observations by previous neutron scattering experiments [3].\\[4pt] [1] PRL 101, 135501 (2008).\\[0pt] [2] Soft Matter 7, 9848 (2011).\\[0pt] [3] J. Phys. Chem. B 113, 5001 (2009). [Preview Abstract] |
Tuesday, March 19, 2013 4:06PM - 4:18PM |
J45.00007: Biological Signaling: the Role of ``Electrostatic Epicenter'' in ``Protein Quake'' and Receptor Activation Aihua Xie, Sandip Kaledhonkar, Zhouyang Kang, Johnny Hendriks, Klaas Hellingwerf Activation of a receptor protein during biological signaling is often characterized by a two state model: a receptor state (also called ``off state'') for detection of a stimuli, and a signaling state (``on state'') for signal relay. Receptor activation is a process that a receptor protein is structurally transformed from its receptor state to its signaling state through substantial conformational changes that are recognizable by its downstream signal relay partner. What are the structural and energetic origins for receptor activation in biological signaling? We report extensive evidence that further support the role of ``electrostatic epicenter'' in driving ``protein quake'' and receptor activation. Photoactive yellow protein (PYP), a bacterial blue light photoreceptor protein for the negative phototaxis of a salt loving \textit{Halorhodospira halophia}, is employed as a model system in this study. We will discuss potential applications of this receptor activation mechanism to other receptor proteins, including B-RAF receptor protein that is associated with many cancers. [Preview Abstract] |
Tuesday, March 19, 2013 4:18PM - 4:30PM |
J45.00008: Fluctuation-allosteric regulation of protein function: Continuum elastic model and its geometrical implications Michael S. Dimitriyev, Paul M. Goldbart, T.C.B. McLeish In many proteins, function is strongly modified by the binding of some small ligand to the protein surface. We address the issue of {\it fluctuation\/} allostery, in which thermal motion of the protein medium far from the binding site is a key factor in determining the activity of the protein (i.e., the strength with which it functions). We develop a simple, coarse-grained model in which the protein is viewed as a homogeneous, isotropic, elastic continuum of specified shape, and the binding of the ligand is regarded as a small alteration of this shape. We construct a perturbative approach to the response of the thermal fluctuations to a shape-alteration as a diagnostic of the impact on protein activity that ligand binding causes. At leading order in the size of the ligand, we show how this response is determined via familiar geometrical properties of the ligand shape. Thus, we find that there are \lq\lq sweet spots\rq\rq\ for ligand binding---determined by the overall shape of the protein and location of its active site---for which the effects of ligand binding are qualitatively enhanced. To simplify the analysis whilst retaining the essential geometrical ideas, we present results for the case of a scalar field rather than the true vector displacement field of elasticity. [Preview Abstract] |
Tuesday, March 19, 2013 4:30PM - 4:42PM |
J45.00009: Predicting Allostery Wiring Diagrams within Motor Proteins Riina Tehver Motor proteins are intricate molecular machines that make use of allostery as a basis of their function. Fundamental questions in trying to understand the operational mechanism of the motors is, therefore, how allostery communicated is within the proteins, what are the pathways that transmit allosteric signals, how to model and predict them. We have proposed a normal-mode analysis based perturbation model that predicts the pathways based on the structure and chemical composition of the molecules. We use the model to investigate allosteric pathways (allostery wiring diagrams) within motor proteins myosin V and VI. [Preview Abstract] |
Tuesday, March 19, 2013 4:42PM - 4:54PM |
J45.00010: Direct evidence on the force-stabilized calcium binding of the gelsolin G6 domain Yi Cao, Chunmei Lv, Xiang Gao, Meng Qin, Wei Wang Many proteins are subjected to forces in vivo. However, how force controls the structure, ligand binding and function has only been studied recently with the invention of single molecule force spectroscopy techniques. Generally, force will destabilize the native conformation of a protein and decrease its affinity to ligands. Here we show, for the first time, that force can also increase the ligand binding affinity. We used single molecule force spectroscopy by atomic force microscopy (AFM) to study the effect of calcium binding on the unfolding of the G6 domain of gelsolin. We found that at saturated calcium concentration, the unfolding forces of G6 are $\sim$ 40 pN, which are significantly higher than those of apo G6 of $\sim$ 20 pN. At intermediate calcium concentrations, the unfolding forces show a unimodal distribution, indicating fast inter-conversion rate between apo and holo G6. More strikingly, we found that if the binding constant of G6 is independent of force, the predicted unfolding forces based on the kinetic parameters obtained from apo and holo G6 are significantly lower than the experimentally obtained ones. To reconcile such discrepancy, we proposed a new model, in which we considered that the binding affinity of calcium to G6 is also force dependent. Fitting this model to experimental data clearly indicates that G6 has much higher calcium binding affinity at higher forces. We proposed that such a special force stabilized calcium binding may be important for the function of gelsolin in vivo. [Preview Abstract] |
Tuesday, March 19, 2013 4:54PM - 5:06PM |
J45.00011: THz Microscopy of Anisotropy and Correlated Motions in Protein Crystals Katherine Niessen, Gheorghe Acbas, Edward Snell, Andrea Markelz We introduce a new technique, Crystal Anisotropy Terahertz Microscopy (CATM) which can directly measure correlated intra-molecular protein vibrations. The terahertz (THz) frequency range (5-100 cm$^{\mathrm{-1}})$ corresponds to global correlated protein motions, proposed to be essential to protein function [1, 2]. CATM accesses these motions by removal of the relaxational background of the solvent and residue side chain librational motions. We demonstrate narrowband features in the anisotropic absorbance for hen egg-white lysozyme (HEWL) single crystals as well as HEWL with triacetylglucosamine (HEWL-3NAG) inhibitor single crystals. The most prominent features for the HEWL crystals appear at 45 cm$^{\mathrm{-1}}$, 69 cm$^{\mathrm{-1}}$, and 78 cm$^{\mathrm{-1}}$ and the strength of the absorption varies with crystal orientation relative to the THz polarization. Calculations show similar anisotropic features, suggesting specific correlated mode identification is possible. 1. Hammes-Schiffer, S. and S.J. Benkovic, Relating Protein Motion to Catalysis. Annu. Rev. Biochem., 2006. 75: p. 519-41. 2. Henzler-Wildman, K.A., et al., Intrinsic motions along an enzymatic reaction trajectory. Nature, 2007. 450(7171): p. 838-U13. [Preview Abstract] |
Tuesday, March 19, 2013 5:06PM - 5:18PM |
J45.00012: Intrinsic Mean Square Displacement in Lysozyme Derya Vural, Henry R. Glyde, Liang Hong The internal dynamics of proteins is the essential interest of biophysics. The mean square displacement (MSD) of hydrogen in proteins and its associated hydration water is obtained by molecular dynamic (MD) simulation. The MSD as currently determined depends on the time of the MD simulation. A method is proposed in this paper to obtain the intrinsic MSD $\langle r^2\rangle$ of hydrogen in the proteins. The intrinsic MSD is independent of the simulation time and defined as the infinite time value of calculated MSD that appears in the Debye-Waller factor. The method consists of fitting a model to the incoherent intermediate scattering function. The model contains the intrinsic MSD and a rate constant characterizing the motions of H in the protein. The method is illustrated by obtaining the intrinsic MSD $\langle r^2\rangle$ of lysozyme in $100$ ns and $1$ $\mu$s MD simulations. [Preview Abstract] |
Session J46: Focus Session: X-ray and Neutron Instruments and Measurement Science
Sponsoring Units: GIMSChair: Albert Migliori, Los Alamos National Laboratory
Room: Hilton Baltimore Holiday Ballroom 5
Tuesday, March 19, 2013 2:30PM - 3:06PM |
J46.00001: Coherent Imaging Invited Speaker: Stefano Marchesini |
Tuesday, March 19, 2013 3:06PM - 3:18PM |
J46.00002: Coherent Diffraction Imaging of Device Nanostructures Edwin Fohtung, Jong Woo, Martin Holt, Stephan Hruszkewycz, Na Lei, Dafine Ravelosona, Ross Harder, Eric Fullerton, Ian McNulty, Oleg Shpyrko We employ x-ray coherent diffraction imaging to map the lattice strain distribution, elastic properties and device responses to external stimuli such as magnetic and electric field in a host of device nanostructures. For the case isolated Ni (001) nanowire grown vertically on an amorphous SiO2 /Si substrate we utilize the retrieved projection of lattice distortion to predict the Young's Modulus of the wire based on the elasticity theory [1]. We also image for the first time, the evolution of magnetostriction in these wires in the presence of an external magnetic field. For extended ferroelectric thin films, we utilized the recently developed Bragg Ptychography [2] to image the evolution of ferroelectricity [3].\\[4pt] [1] E. Fohtung et al., Appl. Phys. Lett. 101, 033107 (2012).\\[0pt] [2] S. O. Hruszkewycz et al., Nano Lett. 12, 5148 (2012).\\[0pt] [3] E. Fohtung et al., in preparation (2012). [Preview Abstract] |
Tuesday, March 19, 2013 3:18PM - 3:30PM |
J46.00003: Development of a 10 nm spatial resolution Hard X-ray Microscope for the Nanoprobe beamline at NSLS-II Evgeny Nazaretski, Hanfei Yan, Jungdae Kim, Kenneth Lauer, Kazimierz Gofron, Deming Shu, Yong Chu We present recent progress on the development of an x-ray microscope for the Hard X-ray Nanoprobe (HXN) beamline at NSLS-II. We discuss design approach suitable for achieving sub-10 nm spatial resolution x-ray fluorescence and diffraction measurements. Different types of focusing optics e.g. Multilayer Laue Lenses (MLL) and Zone Plates (ZP) will be implemented in the microscope yielding diverse scientific applications for the targeted spatial resolutions of 10 nm and 30 nm respectively. We discuss modular design of the microscope that enables multi-functionality and includes the possibility to regulate temperature at the sample location. The design of the microscope is greatly based on our in-depth evaluation of numerous commercially available components; detailed studies of their performance in terms of mechanical stability, resolution, and thermal characteristics. Also, our design approach greatly relies on extensive experience acquired during construction and subsequent using of a prototype scanning MLL-based microscope. [Preview Abstract] |
Tuesday, March 19, 2013 3:30PM - 3:42PM |
J46.00004: Application of ultra-small-angle X-ray scattering / X-ray photon correlation spectroscopy to relate equilibrium or non-equilibrium dynamics to microstructure Andrew Allen, Fan Zhang, Lyle Levine, Jan Ilavsky Ultra-small-angle X-ray scattering (USAXS) can probe microstructures over the nanometer-to-micrometer scale range. Through use of a small instrument entrance slit, X-ray photon correlation spectroscopy (XPCS) exploits the partial coherence of an X-ray synchrotron undulator beam to provide unprecedented sensitivity to the dynamics of microstructural change. In USAXS/XPCS studies, the dynamics of local structures in a scale range of 100 nm to 1000 nm can be related to an overall hierarchical microstructure extending from 1 nm to more than 1000 nm. Using a point-detection scintillator mode, the equilibrium dynamics at ambient temperature of small particles (which move more slowly than nanoparticles) in aqueous suspension have been quantified directly for the first time. Using a USAXS-XPCS scanning mode for non-equilibrium dynamics incipient processes within dental composites have been elucidated, prior to effects becoming detectable using any other technique. [Preview Abstract] |
Tuesday, March 19, 2013 3:42PM - 3:54PM |
J46.00005: Solution measurements yield atomic scale resolution Derek Mendez, Jongmin Sung, Daniel Ratner, Clement Levard, Marc Michel, Gordon Brown, Sebastian Doniach A conventional measure on a solution of identical non-interacting particles (e.g. a dilute solution of proteins) is the scattering averaged over all particle orientations. Such scattering results in a 1-D profile, e.g. the standard powder diffraction rings. Here, we aim to recover information that is averaged out in such a measurement. By recording many short, bright X-ray pulses one can obtain the scattering fluctuation, i.e. the 2-photon correlation function. Intensity correlations arise from double scattering events in which two photons from an incoming beam scatter off the same particle, belonging to an ensemble of randomly oriented particles. The double scattering must occur during a single exposure, and before the scatterer has undergone significant diffusion. At wide angles, such correlations have the potential to yield {\AA}-scale single-particle structural information. The problem is to extract correlated events from a background of uncorrelated single-photon scattering events. This is done by forming statistics over an ensemble of correlation measurements and comparing to correlations between uncorrelated pairs of exposures. Samples range from naturally occurring nano-minerals measured using focused synchrotron X-rays, to biomolecules measured using a free electron laser. [Preview Abstract] |
Tuesday, March 19, 2013 3:54PM - 4:06PM |
J46.00006: Interpreting SAXS spectra of non-spherical nonane-water nanodroplets using a new particle form factor Abdalla Obeidat, Fawaz Hrahsheh, Gerald Wilemski, Harshad Pathak, Barbara Wyslouzil The structure of nanodroplets plays a critical role in many natural phenomena involving atmospheric nucleation and aerosol formation. Here, we review our theoretical efforts to interpret experimental measurements of small angle x-ray scattering (SAXS) from nonane/water nanodroplets formed in supersonic nozzles. We simulated nonane/water nanodroplets using classical molecular dynamics (MD) and found that they have a nonspherical Russian-Doll (RD) structure consisting of a roughly spherical water droplet partially wetted by a large nonane lens. We have developed an exact analytical expression for the particle form factor P(q) of a lens-on-sphere droplet with sharp interfaces and uniform lens and sphere densities for use in fitting the experimental data. The model was validated by comparing it with exact results for P(q) based on the MD simulations. Excellent agreement was found. The fits of the measured SAXS spectra generated with this model are good and generally better than those based on simpler structural models, but the resulting particle size distributions do not produce mass balance for either water or nonane. Further work is needed to resolve this discrepancy. [Preview Abstract] |
Tuesday, March 19, 2013 4:06PM - 4:18PM |
J46.00007: Radiation Induced Defect Clusters in Fe and Fe-alloys Investigated by X-Ray Diffuse Scattering Measurements and Molecular Dynamics and Monte Carlo Simulations Ben Larson, Jon Tischler, Hongbin Bei, Roger Stoller, Haixuan Xu, Yanwen Zhang We have initiated fundamental investigations of 15 MeV Ni-ion induced defect clusters in single crystal Fe and Fe-Cr using diffuse scattering measurements near Bragg reflections combined with molecular dynamics (MD) and self-evolving atomistic kinetic Monte Carlo (SEAK-MC) simulations. Synchrotron x-ray diffuse scattering measurements performed near the (002) reflection of \textless 001\textgreater oriented Fe and Fe(30{\%})Cr single crystals are analyzed within the so-called asymptotic regime using scattering cross-sections based on MD simulated local lattice distortions and SEAK-MC generated interstitial and vacancy cluster configurations. Measurements for Ni-ion irradiations of Fe and Fe-Ni with doses corresponding to 0.2 and 1 displacements per atom (DPA) at ambient temperature will be presented and discussed in connection with the local Bragg scattering interpretation of defect cluster diffuse scattering in ion-irradiated Cu. Methods for calculating diffuse scattering cross sections directly from MD simulations of atomic displacements around vacancy and interstitial loops within the single defect approximation will be considered and the importance of such approaches for complex defect clusters will be addressed. [Preview Abstract] |
Tuesday, March 19, 2013 4:18PM - 4:30PM |
J46.00008: Investigation of the experimental effects on the quality of the rapid acquisition pair distribution function (RA-PDF) data Ahmad S. Masadeh Series of experiments have been carried out to investigate the quality of the recently developed rapid acquisition atomic pair distribution function (RA-PDF) method, which combines the uses of high energy X-rays and an image plate area detector. Image plate data for simple elements (C, Mg, Al, Si, Ni, Cu, Zn, Ag, and Pb) have been analyzed, using (RA-PDF) technique. The affect of undiscriminated Compton and fluorescence is investigated for a wide range of materials with atomic Z numbers ranging from 6(Carbon) and 82 (Pb). We find the RA-PDF method is capable of obtaining high quality PDFs where quantitatively reliable structure information can be extracted. [Preview Abstract] |
Tuesday, March 19, 2013 4:30PM - 4:42PM |
J46.00009: Test of the Cross Correlation Method for Efficient Single Crystal Diffuse Neutron Scattering with Elastic Discrimination Stephan Rosenkranz, John Paul Castellan, Rich Vitt, Raymond Osborn, Rick Riedel, Mariano Ruiz-Rodriguez, Loren Funk Single crystal diffuse scattering provides a powerful probe of the complex disorder associated with many emergent phenomena of great interest. It provides a determination not only of the local distortions around a point defect but also of the length scale and morphology of short-range order on the nanoscale. However, obtaining accurate models of the local structure usually demands measurements over large volumes of reciprocal space with sufficiently high momentum and energy resolution. In order to overcome limitations of current instrumentation, we propose to utilize the cross-correlation method at pulsed neutron sources. This concept that combines the high efficiency of white-beam Laue diffraction for measuring large volumes of reciprocal space with energy discrimination produced by the use of a statistical chopper is currently being implemented in a dedicated instrument, { \it Corelli }, under construction at the Spallation Neutron Source. Here, we present our detailed investigation of the effectiveness of this method for measuring weak diffuse signals, based on full experiment simulations as well as actual measurements of the diffuse scattering from powder and single crystal samples obtained utilizing the cross correlation method on a prototype instrument. [Preview Abstract] |
Tuesday, March 19, 2013 4:42PM - 4:54PM |
J46.00010: Design of Ultra Small Angle Neutron Scattering (KIST-USANS) at HANARO Cold Neutron Guide, CG4B Man-Ho Kim The ultra small angle neutron scattering instrument can measure the lower limit of scattering vector to near Q $\sim$ 2.0x10$^{-5}$ {\AA}$^{-1}$ while the upper limit can reach to an intermediate scattering vector Q $\sim$ 10$^{-2}$ {\AA}$^{-1}$ of a typical small angle neutron scattering (SANS) depending on the contrast of sample. USANS is useful when measuring objects that are micron to submicron in size while SANS is useful when measuring objects that are micron to nano in size. When both USANS and SANS were used together, we could measure sizes from micron to nano scale, which is useful when studying the hierarchical structures in the wide scale of Q and total cross-section, d$\Sigma $/d$\Omega $(Q). Recently, KIST has developed the USANS (so called KIST-USANS) at HANARO cold neutron guide hall of KAERI. We will present the instrument design, performance, future plan, and some examples of measurements that cover approximately 11 orders of magnitude in the d$\Sigma $/d$\Omega $(Q) and 4 orders in the Q. [Preview Abstract] |
Tuesday, March 19, 2013 4:54PM - 5:06PM |
J46.00011: Photon Source Capabilities of the Jefferson Lab THz to VUV FEL G.P. Williams, S.V. Benson, D. Douglas, P. Evtushenko, F.E. Hannon, C. Hernandez-Garcia, J.M. Klopf, R.A. Legg, G.R. Neil, M.D. Shinn, C.D. Tennant, S. Zhang Jefferson Lab operates a sub-picosecond photon science R\&D facility with peak and average brightness values that are many orders of magnitude higher than storage rings in the THz - VUV range. It also has multiphoton capabilities that provide unique opportunities for out of equilibrium dynamical studies at time-scales down to $\sim$ 100 fs FWHM. The facility is based on a superconducting energy recovered linac which is operated with CW RF that powers oscillator-based IR and UV Free Electron Lasers (FELs) with diffraction limited sub-picosecond pulses with $>$ 10$^{13}$ photons per pulse (1.0\% BW) at pulse repetition frequencies up to 75 MHz. Details of the facility and its present performance will be presented along with some example science applications. In addition we will discuss on-going upgrades to the facility that will allow 10 eV lasing in the fundamental. Finally we will present two potential upgrades including the design of an oscillator-based VUV-FEL that would produce 6 $\times$ 10$^{12}$ coherent (0.5\% BW) 100 eV photons per pulse at multi-MHz repetition rates in the fundamental, and a dual FEL configuration that would allow simultaneous lasing at THz and UV wavelengths. [Preview Abstract] |
Tuesday, March 19, 2013 5:06PM - 5:18PM |
J46.00012: Picosecond Time-Resolved Strain Rosette at Atomic Length Scale Maria I. Campana, G. Jackson Williams, Soo Heyong Lee, Donald Walko, Eric Landahl Ultrafast optical absorption in a crystalline solid generates coherent motions of strain, which propagate through the bulk at the speed of sound. Energy relaxation dynamics of the excited lattice system and the subsequent transport properties of the strains have been actively studied. Recently, these high-speed transient dynamics have been studied using laser based pump-probe techniques and time resolved x-ray diffraction (TRXD). However, the interpretation of these studies always assumes a uniaxial spatial profile for the strain (i.e. strain is exerted only along the direction of surface normal of the sample). This assumption comes from a symmetry argument originally given by Thomsen: if the illuminated area of the pump laser beam on the sample surface is much larger than the optical penetration depth, strain gradient along surface normal is expected to be much steeper than along lateral direction, and therefore, the strain generated is usually assumed to be one dimensional. While this assumption simplifies the analysis of the data, (and makes possible such applications as picosecond ultrasonics for the in-situ measurement of semiconductor heterostructure thickness), it overlooks any physical processes that take place along transverse direction. Here we report the experimental generation and detection of the transverse component of the impulsively generated strain in a single GaAs crystal using TRXD. Our analysis is based on a strain rosette applied to three non-collinear Bragg reflections. [Preview Abstract] |
Tuesday, March 19, 2013 5:18PM - 5:30PM |
J46.00013: A high-energy x-ray precession camera at the Advanced Photon Source A. Kreyssig, D.K. Pratt, M. Ramazanoglu, G. Tucker, D.S. Robinson, L.C. Lang, R.J. McQueeney, A.I. Goldman A key distinguishing feature of the APS is the capability for high-energy x-ray scattering, which has been exploited for numerous powder sample applications. The instrumentation for high-energy single-crystal diffraction measurements at the APS, however, remains underdeveloped. High-energy x-rays offer several advantages: (1) absorption effects are minimized and the entire bulk of the sample is probed and; (2) a large range of reciprocal space can be imaged when used together with even a modestly sized area detector. We have developed a high-energy x-ray precession camera (HEXPC) for imaging of reciprocal-space planes. This technique is highly suited to studies of Bragg and diffuse scattering with its flexibility in dynamic range, resolution and scattering vector range. These capabilities have been demonstrated by studies of single crystals and quasicrystals. \\[4pt] The work at the Ames Laboratory was supported by US DOE, Office of Basic Energy Sciences, DMSE, contract DE-AC02-07CH11358. [Preview Abstract] |
Tuesday, March 19, 2013 5:30PM - 5:42PM |
J46.00014: SESAME as a Model Project for Other Regions Herman Winick UNESCO became the umbrella organization for SESAME at its Executive Board 164th session, May 2002. The following comments about SESAME were made by this board: ``a quintessential UNESCO project combining capacity building with vital peace-building through science'' and ``a model project for other regions.'' Now that SESAME is well underway, other regions (e.g.; Africa and Central Asia) should be made aware of this progress, and they should be welcomed to join SESAME as a first step in developing similar projects in their region. Students and scientists from other regions should be encouraged to attend SESAME Users' meeting, schools, workshops, etc. where they can learn about synchrotron radiation sources, beamlines, and science. They should be invited to join SESAME scientists in designing and commissioning accelerators and beamlines, gaining relevant experience for their own projects and helping SESAME in the process. [Preview Abstract] |
Session J47: Invited Session: Mechanics, Dynamics, and Organization in Cell Growth and Division
Sponsoring Units: DBIOChair: Ned Wingreen, Princeton University
Room: Hilton Baltimore Holiday Ballroom 6
Tuesday, March 19, 2013 2:30PM - 3:06PM |
J47.00001: Protein shaping of cell shape Invited Speaker: K.C. Huang |
Tuesday, March 19, 2013 3:06PM - 3:42PM |
J47.00002: Dislocation-Mediated Elongation of Bacteria Invited Speaker: David Nelson |
Tuesday, March 19, 2013 3:42PM - 4:18PM |
J47.00003: Mechanical interplay between membrane tension and cellular dynamics Invited Speaker: Kinneret Keren Replace this text with your abstract. [Preview Abstract] |
Tuesday, March 19, 2013 4:18PM - 4:54PM |
J47.00004: Mechanical stress inference for two dimensional cell arrays Invited Speaker: Madhav Mani |
Tuesday, March 19, 2013 4:54PM - 5:30PM |
J47.00005: Cell Membranes and Out of Equilibrium Cell Phases Invited Speaker: Nir Gov |
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