Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session A1: Graphene - Spectroscopies
Sponsoring Units: DCMPChair: Amir Yacoby, Harvard University.
Room: Spirit of Pittsburgh Ballroom A
Monday, March 16, 2009 8:00AM - 8:36AM |
A1.00001: Infrared spectroscopy of gated structures based on single- and bi-layer graphene Invited Speaker: Infrared spectroscopy was employed to investigated the charge dynamics in graphene integrated in tunable gated devices (Nature Physics 4, 532 (2008)). These measurements verified that electrons in single-layer graphene behave like Dirac quasiparticles but most importantly revealed several unexpected results that are beyond the theoretical predictions for idealized graphene. Several of our findings including, a systematic enhancement of the Fermi velocity at low energy and also the ``residual'' conductivity at frequencies below 2E$_F$ are indicative of many-body interactions. Recent infrared study of bilayer graphene uncovered a pronounced asymmetry in the optical conductivity upon injection of electrons and holes (arXiv:0807.3776). We believe this result is suggestive of a marked asymmetry between the valence and conduction bands in bilayer samples. [Preview Abstract] |
Monday, March 16, 2009 8:36AM - 9:12AM |
A1.00002: Scanning tunneling microscopy and spectroscopy of graphene on graphite Invited Speaker: Graphene, a single atomic layer of crystalline carbon, exhibits fascinating electronic properties owing to low energy quasiparticles that resemble relativistic Dirac fermions. Recent experiments on graphene deposited on insulating substrates revealed that substrate induced potential fluctuations obscure the Dirac fermion nature of the carriers. Using low temperature scanning tunneling microscopy (STM) and spectroscopy (STS) we demonstrated that substrate induced potential fluctuations are substantially reduced when graphene is deposited on graphite and that in these samples the intrinsic structural and electronic properties of graphene become accessible. We observed the honeycomb structure and the V shaped density of states that vanishes at the Dirac point, characteristic of Dirac fermions. In finite magnetic field we observed the appearance of a single sequence of Landau levels, with square root dependence on level index and field, further attesting to the Dirac fermion nature of the charge carriers. The experiments give access to the fundamental parameters of the electronic spectrum in graphene including the Fermi velocity, electron-phonon coupling constant and electron-electron interactions. In addition they revealed the appearance of a small gap at the Dirac point and an anomaly at the Fermi energy. Work in collaboration with E.Y. Andrei and A. Luican. \\[4pt] [1] G. Li and E.Y. Andrei, Nature Phys. 3, 623 (2007). \\[0pt] [2] G. Li, A. Luican, and E.Y. Andrei, http://arxiv.org/abs/0803.4016 [Preview Abstract] |
Monday, March 16, 2009 9:12AM - 9:48AM |
A1.00003: Tuning the properties of Dirac fermions in graphene Invited Speaker: Graphene, a one atom thick layer of carbon, the supposedly ideal Dirac material, has been under the radar of theorists and experimentalist for many decades. Although novel physical properties were envisioned, graphene, as any other 2D material, was presumed not to exist in its free state because of long wavelength fluctuations will easily destroy purely 2D membranes. The recent success in isolating a single sheet of graphene has certainly challenged this view. In this talk I will present our experimental work in this field using a combination of spectroscopic and microscopy tools. I'll present experimental evidence of what drives the stability of a graphene membrane and show comparison between exfoliated and epitaxial graphene. I will then discuss the nature of fermions in graphene sheets and how their peculiar electronic structure can be tuned by engineering small terraces of graphene down to nm size, where the physics gets dominated by quantum confinement. The implications of our study on the properties of Dirac materials and their potential role for applications are discussed. [Preview Abstract] |
Monday, March 16, 2009 9:48AM - 10:24AM |
A1.00004: The infrared conductivity of graphene Invited Speaker: The discovery of graphene is probably one of the most important events in modern condensed matter physics. Besides being a material that is only one atom thick, it has electronic properties which are usual when compared with ordinary metals and semiconductors. These unusual properties are reflected in its infrared conductivity. We will discuss the physical processes that affect the low frequency conductivity of graphene. We show that while the standard model of graphene is capable of explaing most of the features, it also fails in some aspects, indicating that we still do not have a full understanding of the physical mechanisms that control the electronic properties of this amazing material. [Preview Abstract] |
Session A2: Transport in Oxide Interfaces
Sponsoring Units: DCMPChair: Darrell Schlom, Cornell University
Room: Spirit of Pittsburgh Ballroom BC
Monday, March 16, 2009 8:00AM - 8:36AM |
A2.00001: A Novel Approach to Opening Quantum Mechanical Limitations of Semiconductor Electronics Invited Speaker: At interfaces between insulating oxides, robust electron gases have been generated that feature unique electronic properties, such as electric-field tunable metal-insulator transitions. Inspired by the properties of such systems we propose a novel device architecture that may resolve fundamental limitations of standard, semiconductor electronics. [Preview Abstract] |
Monday, March 16, 2009 8:36AM - 9:12AM |
A2.00002: Oxide Nanoelectronics On Demand Invited Speaker: Electronic confinement at nanoscale dimensions remains a central means of science and technology. I will demonstrate nanoscale lateral confinement of a quasi-two-dimensional electron gas at the LaAlO$_{3}$/SrTiO$_{3}$ interface and show how it can be exploited to create a variety of electronic devices. Using a conducting AFM probe it is possible to create tunnel junctions and field-effect transistors (FETs) with feature sizes comparable to the diameter of a single-wall nanotube. These devices can be modified or erased without complex or irreversible lithographic procedures. This new, on-demand nanoelectronics platform has the potential for widespread technological application. [Preview Abstract] |
Monday, March 16, 2009 9:12AM - 9:48AM |
A2.00003: Role of the surface in the interfacial metal-insulator transition in LaAlO$_3$/SrTiO$_3$ Invited Speaker: The observed metal-insulator transition in thin films of LaAlO$_3$ on SrTiO$_3$ depends critically on the film thickness: a reversible transition consistently works best with films 3 unit cells thick. Using first-principles density functional calculations, the role of the surface in the interfacial metal insulator transition will be examined. Water adsorbs strongly to the surface, dissociating and causing an unusual striped reconstruction. The adsorbates allow the creation of wires and devices at the interface. The positively charged AFM removes OH adsorbates, changing the interface from insulating to metallic. The negatively charged AFM removes H adsorbates, reversing the process. [Preview Abstract] |
Monday, March 16, 2009 9:48AM - 10:24AM |
A2.00004: Potential Barrier Lowering and Electrical Transport at the LaAlO$_{3}$/SrTiO$_{3}$ Interface Invited Speaker: Interfacial phenomena form the basis for modern-day devices and continue to be an area of fundamental interest in condensed matter research. Advances in oxide thin film fabrication have enabled the synthesis of atomically precise oxide interfaces and hence have allowed for controlled investigation of interfacial phenomena in these materials. With the rich variety of functionalities exhibited by transition-metal oxides, a wide array of novel properties may be achieved at oxide heterointerfaces. An exemplary study is the discovery of metallicity at the interface of two band insulators, LaAlO$_{3}$ (LAO) and SrTiO$_{3}$ (STO), which has stimulated many subsequent experimental as well as theoretical studies. However, there is still intense debate on the origin of metallicity, specifically whether it arises from electronic reconstruction or oxygen vacancies. Using a combination of vertical transport measurements across and lateral transport measurements along the LAO/STO heterointerface, we demonstrate that significant potential barrier lowering and band bending are the cause of interfacial metallicity. Transport measurements across the heterointerface, indicate that barrier lowering and enhanced band bending extends over 2.5 nm into LAO as well as STO. We explain the origins of high-temperature carrier saturation, lower carrier concentration, and higher mobility in the sample with the thinnest LAO film on a STO substrate. Lateral transport results suggest that parasitic interface scattering centers limit the low-temperature lateral electron mobility of the metallic channel. \\[4pt] *In collaboration with Franklin Wong, Miaofang Chi, Rajesh Chopdekar, Brittany Nelson-Cheeseman and Nigel Browning. [Preview Abstract] |
Monday, March 16, 2009 10:24AM - 11:00AM |
A2.00005: Modulation Doping of Electrons and Holes at Oxide Interfaces Invited Speaker: |
Session A3: Fe-based Superconductors: Pairing Symmetry
Sponsoring Units: DCMPChair: Fu-Chun Zhang, University of Hong Kong
Room: 301/302
Monday, March 16, 2009 8:00AM - 8:36AM |
A3.00001: Andreev reflection spectroscopy of iron-based superconductors Invited Speaker: After a reign of over two decades by the cuprate superconductors, several new families of iron-based high-temperature superconductors have recently been discovered. Essential to a superconductor is the nature of the superconducting gap, its value, its structure, and its temperature dependence. Point contact Andreev reflection (PCAR) spectroscopy operating in the ballistic limit is one of few techniques that can quantitatively measure the gap of these new Fe-based superconductors and its temperature dependence. In SmFeAsO$_{1-x}$F$_{x}$ (0.15 $\le $ x $\le $ 0.30), we have determined a single gap 2$\Delta $/k$_{B}$T$_{C} \quad \approx $ 3.5-3.6 close to the BCS s-wave prediction and with a BCS-like temperature dependence.\footnote{T. Y. Chen, Z. Tesanovic, R. H. Liu, X. H. Chen, and C. L. Chien, Nature, \textbf{453}, 1224 (2008)} These results will be compared with various theoretical possibilities and those obtained by other measurements, such as ARPES and penetration depth. While the principles of the PCAR spectroscopy are well established, poor contact control and ballistic heating might lead to the appearance of spurious gaps and pseudogaps in PCAR measurements. In collaboration with T. Y. Chen, S. X. Huang and Z. Tesanovic at JHU and R. H. Liu and X. H. Chen at USTC. [Preview Abstract] |
Monday, March 16, 2009 8:36AM - 9:12AM |
A3.00002: Pairing symmetry of iron-based superconductors revealed by ARPES Invited Speaker: The recent discovery of superconductivity in iron-arsenic compounds with a transition temperature ($T_{c}$ ) as high as 56 K ended the monopoly of copper oxides in the family of high-$T_{c}$ superconductors. In this talk I will report our angle-resolved photoemission observation of the superconducting gap, including its momentum, temperature, and Fermi surface (FS) dependence in single crystals Ba$_{0.6}$K$_{0.4}$Fe$_{2}$As$_{2}$ ($T_{c}$ = 37 K). We found two nodeless and nearly isotropic superconducting gaps around their respective FS sheets: a large gap ($\Delta \quad \sim $ 12 meV) on the two small hole-like and electron-like FS sheets, and a small gap ($\sim $ 6 meV) on the large hole-like FS. The isotropic pairing interactions are strongly orbital dependent, as the ratio 2$\Delta $/$k_{B}T_{c}$ switches from weak to strong coupling on different bands. In addition, we have observed a dispersion kink that is likely related to a spin mode. These results reveal the importance of inter-band interactions in the pairing mechanism, and support the anti-phase $s$-wave pairing symmetry in the Fe-based superconductors. [Preview Abstract] |
Monday, March 16, 2009 9:12AM - 9:48AM |
A3.00003: Magnetic penetration depth of oxy-ferropnictide superconductors Invited Speaker: The determination of the symmetry of the superconducting order parameter is an important first step toward uncovering the mechanism of superconductivity in any material. In this regard, measurements of the magnetic penetration depth $\lambda$ have played an important role. Although not a true bulk probe, like specific heat, penetration depth measurements in the Meissner state probe a few thousand Angstroms below the crystal surface and so should be reasonably representative of the bulk. In this talk I will present data for the in-plane magnetic penetration depth of three different families oxy-ferropnictide superconductors, measured on single crystals using a sensitive radio frequency tunnel diode oscillator technique. Our results for samples of SmFeAsO$_{1-x}$F$_y$ ($x\simeq y \simeq 0.2$) with $T_c \simeq 45$\,K show that $\lambda(T)$ has an exponential temperature dependence suggesting that the Fermi surface is fully gapped. However, data for Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ ($T_c\simeq $22 K) show power-law behavior, $\Delta \lambda (T) \sim T^{1.7}$, at low temperature, possibly suggestive of line-nodes. Finally, data for LaFePO ($T_c\simeq 6$K), also show a power-law behavior for $\lambda(T)$, but in this case the temperature dependence is almost perfectly linear down to $T\simeq 100$mK. The results suggest that the gap symmetry may not be universal in all the oxy-ferropnictide superconductors. [Preview Abstract] |
Monday, March 16, 2009 9:48AM - 10:24AM |
A3.00004: Antiferromagnetic Correlation and the Pairing Mechanism of the Cuprates and Iron Pnictides : a View From the Functional Renormalization Group Studies Invited Speaker: We study the pairing symmetry of the iron pnictide superconductor using the functional renormalization group method. By comparing the results for the cuprates and the iron pnictides a coherent picture emerges. It suggests that antiferromagnetic correlation causes pairing for both materials. In collaboration with Fa Wang, Hui Zhai, Ying Ran, and Ashvin Vishwanath, University of California, Berkeley. [Preview Abstract] |
Monday, March 16, 2009 10:24AM - 11:00AM |
A3.00005: Superconductivity, magnetism, and pairing symmetry in Fe-based superconductors Invited Speaker: We analyze antiferromagnetism and superconductivity within the renormalization group(RG) technique in novel Fe-based superconductors using the itinerant model of small electron and hole pockets near (0; 0) and ($\pi$,$\pi$), respectively, originating from the two strongly hybridized orbitals. We find that, for this model, the bare interactions in the Cooper channel are repulsive, and superconductivity does not occur at the mean-field level. However, under RG the effective interaction in the superconducting channel changes sign and becomes attractive. Furthermore, the effective interactions in antiferromagnetic and superconducting channels logarithmically flow towards the same absolute values at low energies, i.e., both must be treated on equal footings. The magnetic instability comes first for equal sizes of the two pockets, but looses to superconductivity upon doping. The superconducting gap has no nodes, but changes sign between the two Fermi surfaces (extended s-wave symmetry). We argue that the T dependencies of the spin susceptibility and NMR relaxation rate for such state are exponential only at very low T, and can be well fitted by power-laws over a wide T range below Tc. We further show that below Tc excitonic resonance appears in the spin excitations spectrum. \\[4pt] [1] M. Korshunov, and I. Eremin, Phys. Rev. B 78, 140509(R) (2008) \\[0pt] [2] A.V. Chubukov, D. Efremov, I. Eremin, Phys. Rev. B 78, 134512 (2008). \\[0pt] [3] M.M. Korshunov and I. Eremin, Europhys. Lett. 83, 67003 (2008). [Preview Abstract] |
Session A4: Polymers and Energy Applications
Sponsoring Units: DPOLYChair: Nitash Balsara, University of California, Berkeley
Room: 306/307
Monday, March 16, 2009 8:00AM - 8:36AM |
A4.00001: Ion solvation and its effects on the miscibility of binary polymer blends Invited Speaker: We study the effects of adding salt ions on the miscibility of a binary blend of polymers having different dielectric constants. The competition between the preference of the ions to be solvated by the component of the higher dielectric constant and the entropic tendency for the ions to be distributed uniformly results in non-trivial effects on the miscibility. We first study the thermodynamics of the polymer blend-ion mixture using a simple Born model in a uniform dielectric medium of the average composition of the polymer blend. We then study the effect of local enrichment of the higher dielectric constant polymer near the ion. We find that when the dielectric constants of the polymers are both low, adding salt decreases the miscibility, while when the dielectric constants of the polymers are both high, the addition of salt enhances the miscibility. When the blend consists of a high dielectric constant polymer and a low dielectric constant polymer, miscibility is decreased if the low dielectric constant component is the majority and is increased if the high dielectric constant component is the majority. The effect becomes significant at ion concentrations corresponding to an order of one ion per polymer chain. The quantitative change in the effective $\chi$ parameter depends on the functional form of the composition dependence of the dielectric constant of the mixture. We also illustrate the difference between fixed ion concentration and fixed chemical potential of the ions. [Preview Abstract] |
Monday, March 16, 2009 8:36AM - 9:12AM |
A4.00002: The Nanostructure of Nafion for Fuel-Cell Membranes from Small-Angle Scattering and NMR Analysis Invited Speaker: We have investigated the long contentious nanometer-scale structure of the Nafion ionomer used in proton exchange membranes of H$_{2}$/O$_{2}$ fuel cells. Using a simple algorithm based on 3D numerical Fourier transformation, we have quantitatively simulated previously published small-angle scattering data of hydrated Nafion. The characteristic ''ionomer peak'' arises from long, parallel but otherwise randomly packed water channels surrounded by the partially hydrophilic sidebranches, forming inverted-micelle cylinders. The channels are stabilized by the considerable stiffness of the Nafion backbones, detected by $^{13}$C and $^{19}$F NMR. An upper limit of 300 nm to the persistence length of the water channels has been estimated from $^{2}$H NMR of $^{2}$H$_{2}$O in the channels. At 20 vol{\%} water, the water channels have diameters between 1.8 and 3.5 nm, with a 2.4-nm average. The hydration-induced changes in small-angle scattering patterns and in the surface-to-volume ratio have also been analyzed in quantitative detail. Nafion crystallites ($\sim $10 vol{\%}), which form physical crosslinks crucial for the mechanical properties of Nafion films, are elongated and parallel to the water channels, with cross sections of $\sim $(5 nm)$^{2}$. Simulations for a dozen other models of Nafion, including Gierke's cluster and the polymer-bundle model, do not match the scattering data. The water-channel model is the first without constrictions of $\sim $1.2 nm diameter; it can explain important features of Nafion, including fast diffusion of water and protons through Nafion and its persistence at low temperatures. [Preview Abstract] |
Monday, March 16, 2009 9:12AM - 9:48AM |
A4.00003: New Materials and Approaches for Solution-Processed Organic Solar Cells Invited Speaker: Organic solar cells have been proposed as low-cost and sustainable alternatives for power generation. To realize the low cost aspects of organic solar cells, conventional vacuum deposition technologies are to be replaced with solution processing. Our group has focused on the development of solution processable conductive polymers. Conductive polymers, like polyaniline, are generally doped with small-molecule acids. Though highly conductive, such materials are not processable. To overcome this intractability, polymer-acid dopants have replaced small-molecule acids. While the introduction of polymer acids render the conductive polymer solution processable, such gains in processability are accompanied by reduced conductivities. With a post-processing solvent-annealing treatment, however, we have been able to dramatically improve the electrical properties of polymer-acid doped conductive polymers; these polymers make efficient anodes in organic solar cells. To further improve the efficiencies of organic solar cells, we have introduced fractional amounts of additives within the active layer of the device. Depending on the hydrophobicity of the additives, they preferentially segregate into the electron donor phase, effectively enhancing phase separation between the electron donor and electron acceptor. This change in morphology increases charge separation; we see a two-fold increase in the short-circuit current in such devices over those without additives in the active layer. [Preview Abstract] |
Monday, March 16, 2009 9:48AM - 10:24AM |
A4.00004: Charge Transport and Storage within Radical Redox Polymers as Electroactive Materials in Energy Devices Invited Speaker: |
Monday, March 16, 2009 10:24AM - 11:00AM |
A4.00005: Polymers as active components in harnessing solar energy Invited Speaker: In the last couple of decades molecular and polymeric photovoltaic cells have attracted considerable attention as a possible low cost alternative to conventional semiconductor solar cells. While considerable advances in improving device efficiencies have been made, significant challenges in developing efficient, reliable and low cost solar cells using polymers as an active component remain. Some of these advances and challenges as well as on going efforts to mass manufacture solar cells modules will be discussed. [Preview Abstract] |
Session A5: Nanostructuring with Ions
Sponsoring Units: FIAPChair: John Melngailis, University of Maryland
Room: 401/402
Monday, March 16, 2009 8:00AM - 8:36AM |
A5.00001: High aspect ratio 3D nanopatterning using Proton Beam Writing Invited Speaker: Proton beam writing (PBW) is a new direct write lithography using MeV protons, and is unique because of its ability to fabricate 3D structures of high aspect ratio structures directly in resist material like PMMA, SU-8 and HSQ. The introduction by CIBA, Singapore of a dedicated PBW facility, capable of writing at the micro- and nano- scale has facilitated high aspect ratio nanostructuring. PBW has demontrated high aspect ratio walls in HSQ down to the 20nm level. In recent experiments details down to sub 20 nm have been achieved in PMMA. Monte-Carlo calculations have shown that structuring down to the nanometer level is feasible. All this is possible because of the virtual absence of proximity effects (unwanted resist exposure by stray secondary electrons). The design and performance of this unique nanoprobe facility will be discussed. Two potential fields of application (eg nanofluidics and nanowire integration) of PBW will be discussed. Currently nanofluidics devices have typically only \textbf{one} critical dimension below 100 nm. Here we will introduce PBW as a powerful technique to fabricate molds for replication of PDMS nanofluidic circuits down to the sub 100 nm level in \textbf{two} dimensions. Initial chips with dimension down to 150 nm have successfully been used to study DNA folding in quasi-1d nanochannels in tandem with fluorescence imaging. Since the size of these PDMS nanochannels is not limited by the PDMS or PBW further miniaturization down to the sub 100 nm level is a realistic goal and initial results will be discussed. Nanowires are a potential building block for nano-electronic devices, and one critical problem is the integration of nanowires to form contacts. Porous alumina templates and high energy ion-tracks have been used for the production of nanowire templates in a random orientation. Since PBW is the only true 3D direct write nanolithographic technique it can be used to fabricate nanowire templates in a controlled manner. [Preview Abstract] |
Monday, March 16, 2009 8:36AM - 9:12AM |
A5.00002: Left-handed metamaterials operating in the visible: negative refraction and negative radiation pressure Invited Speaker: Forty years ago, V. Veselago derived the electromagnetic properties of a hypothetical material having simultaneously-negative values of electric permittivity and magnetic permeability [1]. Such a material, denominated ``left-handed'', was predicted to exhibit a negative index of refraction, as well as a number of other counter-intuitive optical properties. For example, it was hypothesized that a perfect mirror illuminated with a plane wave would experience a negative radiation pressure (pull) when immersed in a left-handed medium, as opposed to the usual positive radiation pressure experienced when facing a dielectric medium such as air or glass. Since left-handed materials are not available in nature, considerable efforts are currently under way to implement them under the form of artificial ``metamaterials'' -- composite media with tailored bulk optical characteristics resulting from constituent structures which are smaller in both size and density than the effective wavelength in the medium. Here we show how surface-plasmon modes propagating in a stacked array of metal-insulator-metal (MIM) waveguides can be harnessed to yield a volumetric left-handed metamaterial characterized by an in-plane-isotropic negative index of refraction over a broad frequency range spanning the blue and green. By sculpting this material with a focused-ion beam we realize prisms and micro-cantilevers which we use to demonstrate, for the first time, (a) in-plane isotropic negative-refraction at optical frequencies, and (b) negative radiation pressure. We predict and experimentally verify a negative ``superpressure'', the magnitude of which exceeds the photon pressure experienced by a perfect mirror by more than a factor of two. 1) V. Veselago, \textit{ Sov. Phys. Usp. }10, p.509 (1968). [Preview Abstract] |
Monday, March 16, 2009 9:12AM - 9:48AM |
A5.00003: Direct writing of electronic circuits and micromachining by focused ion beam (FIB) implantation Invited Speaker: The maskless implantation of FIBs in semiconductors creates a local doping. In n-type conducting sheets, p-lines are written to insulate n-regions laterally from each other or vice versa. In this way, conducting areas can be biased with respect to each other. Narrow paths are easily driven into depletion, creating lateral transistor channels. The advent of multi-focussed-ion-beams allows a more parallel writing of such integrated circuits. For ion beam milling, a new long-life Bismuth (Bi) source is developed and employed [1]. Bi is the heaviest, non-radioactive element and has thus a maximal impact on the material to be sputtered locally. It is non-toxic, well available, mono-isotopic, and inexpensive, has a low melting temperature, and comes even in clusters and the single charged particles make up 95{\%} of the whole FIB-beam. This means that the chromatic errors of the electrostatic Einzel-lenses in the FIB system are not important. Since heavy ions are slower than light ones at the same energy, Bi penetrates to a minimal depth into the target, leaving minimal contaminations. The sputter rate is about 5 times higher than the one of the usual Ga. Since Bi is the only element in this source, it is not necessary to separate it from other ions by a mass filter. Bi is thus a good candidate to improve the performance of sputter-FIBs ultimately, up to replacing Ga. We developed FIB - liquid metal ion sources of nearly all metallic elements in the periodic table. In this way, practically all dopants can be introduced into semiconductors after epitaxial growth in a full ultra-high vacuum process, which enhances the flexibility of the material choice enormously. \\[4pt] [1] P. Mazarov, A. Melnikov, R. Wernhardt, and A.D. Wieck, Long-life bismuth liquid metal ion source for focussed ion beam micromachining application, Appl. Surf. Sci. 254, 7401 (2008). [Preview Abstract] |
Monday, March 16, 2009 9:48AM - 10:24AM |
A5.00004: Helium Ion Imaging and Milling at the Nanometer Dimensions Invited Speaker: The helium ion microscope (HeIM) is a new, powerful instrument for nano-metrology and nanotechnology. As an emerging imaging and measurement tool it offers several advantages over the traditional scanning electron microscope (SEM) currently in use in research and manufacturing facilities across the world. First, resolution 2 to 4 times better than that from comparable SEMs is theoretically possible, due to the very high source brightness and the short wavelength of the helium ions. Ion images with unprecedented resolution have been routinely collected on a wide range of samples with sub-nanometer features. More importantly, the interaction volume of the helium ion beam in the sample is substantially different in its size and shape from that of the electron beam in an SEM. As a consequence, the signals generated, especially secondary electrons, reveal more surface details. Imaging by the HeIM can further benefit from the superb depth of field and the fact that He ion imaging is less susceptible to sample charging. In addition, it is possible to compensate for charging by the use of an electron flood gun. Scattered He ions produced as a result of Rutherford scattering of the incident ions on the target nuclei can provide material contrast information that can be used for quantitative compositional analysis. Beyond imaging, the HeIM is a potent tool for milling and modifying surface structures at the nanometer scale, due to the relatively low mass of the helium ion, the narrow ion beam, and especially the low beam currents. It is possible to drill close to 10 nm diameter holes and mill other nanoscale structures that cannot be fabricated with any other method. It is expected that, as with the electron beam, it is feasible to expose resist and deposit various materials with He ion beam irradiation. The work is at its exploratory stage, and likely soon will yield more exciting results. This presentation will report on some of the newest research work on the NIST helium ion microscope. * Contribution of the National Institute of Standards and Technology; not subject to copyright [Preview Abstract] |
Monday, March 16, 2009 10:24AM - 11:00AM |
A5.00005: High speed focused ion and electron beam nanofabrication Invited Speaker: Both focused ion beams and electron beams can be used for direct, maskless, resistless nanofabrication as well as for lithography. So far the direct fabrication has been limited to applications such as photomask repair, circuit restructuring, failure analysis, and the creation of various highly specialized structures. Recent developments in maskless fabrication, so far aimed mainly at to resist exposure, suggest that this picture might change. For example, IMS in Vienna, Austria is developing an instrument that can be characterized as an ion beam or electron beam dot matrix printer. The total current on the sample available from this kind of instrument is at least three orders of magnitude larger than from a single beam instrument. This may lead to new applications of charged particle beam fabrication, as well as enable applications considered in the past but rejected because of very low throughput. An example of one such application is the direct writing of the identity in RFID tags using ion beam implantation. Recently we have also shown that electron beams can be used to deposit relatively pure platinum from an inorganic precursor gas, Pt(PF$_{3})_{4}$. Such metal deposits can be used as contacts to carbon nanotubes, semiconductor nano wires, organic fibers, or other structures where conventional lithography is impractical. [Preview Abstract] |
Session A6: Computational Physics in Research and Teaching: GRC Topics and Themes
Sponsoring Units: FEd DCOMPChair: Wolfgang Christian, Davidson College
Room: 406
Monday, March 16, 2009 8:00AM - 8:36AM |
A6.00001: Unstable Periodic Orbits as a Unifying Principle in the Presentation of Dynamical Systems in the Undergraduate Physics Curriculum Invited Speaker: Unstable periodic orbits are a ubiquitous feature of a wide variety of dynamical systems that exhibit behavior that may be characterized as chaotic, complex or turbulent. In physics pedagogy, they may be used as a bridge to the understanding of symbolic dynamics, topological entropy and pressure, and the dynamical zeta function formalism. As high-performance scientific computation makes the application of this methodology possible to ever-larger dynamical systems, this methodology emerges as an effective way to introduce students -- from secondary school, to undergraduate education, to graduate education -- to advanced concepts in the theory of dynamical systems. [Preview Abstract] |
Monday, March 16, 2009 8:36AM - 9:12AM |
A6.00002: Innovations in Teaching with Computers: What Works, What Doesn't, and How We Can Tell Invited Speaker: Over the past few decades, members of the physics education community, including researchers in physics education, have been actively developing an ever-expanding array of computer-based tools with which to improve the effectiveness of physics teaching. The 2008 Gordon Research Conference (GRC) on Physics Research and Education served to showcase several such examples of incorporating computation into the undergraduate physics curriculum. In light of these recent advances, it has become increasingly important to ask what it means for such innovations to ``work'' and what evidence is needed to assess whether or not such innovations ``work'' as intended. This presentation will provide an overview of several interrelated themes from the 2008 GRC that currently drive efforts in research and curriculum development. These themes include: How can computer-based modeling, interactive simulations and visualizations, and virtual experimentation enhance student learning of physics? How can these techniques be employed to improve the ability of students to design, perform, and learn from physical experiments? How are physics education researchers utilizing computer-based tools to probe student understanding, characterize student thinking about ``what it means to learn physics,'' and to develop new innovations? [Preview Abstract] |
Monday, March 16, 2009 9:12AM - 9:48AM |
A6.00003: Molecular dynamics simulation: A tool for exploration and discovery Invited Speaker: The exploratory and didactic aspects of science both benefit from the ever-growing role played by computer simulation. One particularly important simulational approach is the molecular dynamics method, used for studying the nature of matter from the molecular to much larger scales. The effectiveness of molecular dynamics can be enhanced considerably by employing visualization and interactivity during the course of the computation and afterwards, allowing the modeler not only to observe the detailed behavior of the systems simulated in different ways, but also to steer the computations in alternative directions by manipulating parameters that govern the actual behavior. This facilitates the creation of potentially rich simulational environments for examining a multitude of complex phenomena, as well as offering an opportunity for enriching the learning process. A series of relatively advanced examples involving molecular dynamics will be used to demonstrate the value of this approach, in particular, atomistic simulations of spontaneously emergent structured fluid flows (the classic Rayleigh--B\'enard and Taylor--Couette problems), supramolecular self-assembly of highly symmetric shell structures (involved in the formation of viral capsids), and that most counterintuitive of phenomena, granular segregation (e.g., axial and radial separation in a rotating cylinder). [Preview Abstract] |
Monday, March 16, 2009 9:48AM - 10:24AM |
A6.00004: Computation in the Classroom: Open Source Physics Resources Invited Speaker: There are many resources available to instructors who wish to better integrate computational physics into the undergraduate curriculum, but it is difficult find and sort through these materials. The Open Source Physics Collection on ComPADRE (www.compadre.org/osp/), the digital library for the physics community, is an attempt to provide a coherent and organized collection of computational resources for physics instruction. This talk will provide concrete examples of computational physics resources used in the classroom and will describe effective uses of these materials ranging from ready-to-run simulations to code that students modify themselves. The resources are an outgrowth of the Open Source Physics Project, generously supported by NSF DUE-0442581. [Preview Abstract] |
Monday, March 16, 2009 10:24AM - 11:00AM |
A6.00005: Astrophysical Computation in Research, the Classroom and Beyond Invited Speaker: In this talk I review progress in the use of simulations as a tool for astronomical research, for education and public outreach. The talk will include the basic elements of numerical simulations as well as advances in algorithms which have led to recent dramatic progress such as the use of Adaptive Mesh Refinement methods. The scientific focus of the talk will be star formation jets and outflows while the educational emphasis will be on the use of advanced platforms for simulation based learning in lecture and integrated homework. Learning modules for science outreach websites such as DISCOVER magazine will also be highlighted. [Preview Abstract] |
Session A7: Systems Biology of Natural and Synthetic Circuits
Sponsoring Units: DBPChair: Jan Skotheim, Standford University
Room: 407
Monday, March 16, 2009 8:00AM - 8:36AM |
A7.00001: Feedback and Modularity in Cell Cycle Control Invited Speaker: Underlying the wonderful diversity of natural forms is the ability of an organism to grow into its appropriate shape. Regulation ensures that cells grow, divide and differentiate so that the organism and its constitutive parts are properly proportioned and of suitable size. Although the size-control mechanism active in an individual cell is of fundamental importance to this process, it is difficult to isolate and study in complex multi-cellular systems and remains poorly understood. This motivates our use of the budding yeast model organism, whose Start checkpoint integrates multiple internal (e.g. cell size) and external signals into an irreversible decision to enter the cell cycle. We have endeavored to address the following two questions: What makes the Start transition irreversible? How does a cell compute its own size? I will report on the progress we have made. Our work is part of an emerging framework for understanding biological control circuits, which will allow us to discern the function of natural systems and aid us in engineering synthetic systems. [Preview Abstract] |
Monday, March 16, 2009 8:36AM - 9:12AM |
A7.00002: to be determined by you Invited Speaker: |
Monday, March 16, 2009 9:12AM - 9:48AM |
A7.00003: Signal integration, gain, and integral feedback in the \textit{Escherichia coli} chemotaxis network Invited Speaker: Bacteria are able to sense chemicals in their environment, allowing cells to swim towards nutrients (attractant chemicals) and away from repellents (toxic chemicals). The chemotaxis network of the model bacterium \textit{Escherichia coli} possesses remarkable signaling properties including high sensitivity to small changes in chemical concentration over a wide range of ambient concentrations. These signaling properties rely on the architecture of the circuit, including elements that implement signal integration, gain, and integral feedback. All of these elements rely on receptor clustering, which occurs at multiple length scales. At a small scale, the chemotaxis receptors form stable homodimers which then assemble into larger complexes in which receptors of different chemical specificities are intermixed, with trimers of dimers believed to be the smallest signaling unit. At a larger scale, $\sim $10,000 receptors form large polar and lateral receptor clusters. I will discuss recent experimental and theoretical progress in understanding how the biophysics of chemotaxis receptors leads to the remarkable signaling properties of the chemotaxis network. [Preview Abstract] |
Monday, March 16, 2009 9:48AM - 10:24AM |
A7.00004: Dissecting the nitrogen assimilation system of E. coli: from molecules to physiology Invited Speaker: Nitrogen assimilation is a major branch of cellular metabolism. For enteric bacteria such as E. coli, all of the nitrogen groups needed in biosynthesis are converted from ammonia by a relatively simple system comprised of 3 enzymes and 3 intermediate metabolites. This system is intricately regulated, at both the transcriptional and post-translational levels according to the nitrogen and carbon/energy status of the cell. While specific pieces of this regulation have been known for a long time, the strategy of regulation relating nitrogen influx to cellular demand is poorly understood. Clearly, the paradigm of end-product feedback inhibition well-established for the regulation of individual metabolic pathways is inadequate since there are too many products involving nitrogen. Through extensive experimental studies including quantitative characterization of the levels of key metabolites and enzymes for a carefully chosen spectrum of growth conditions and mutants, we obtain a dynamic picture of how the cell matches its rate of nitrogen assimilation with physiological needs through the intermediate metabolites. [Preview Abstract] |
Monday, March 16, 2009 10:24AM - 11:00AM |
A7.00005: Building a genetic transistor in yeast: How protein sequestration generates a tunable ultrasensitive or all-or-none response Invited Speaker: Protein sequestration, where an active protein (A) is bound in an inactive complex by an inhibitor, is a common molecular mechanism in natural regulatory circuits. The inhibitor serves as a molecular sink that can buffer and titrate low concentrations of A. If sufficient protein A is produced, then the sink is saturated and A will exhibit an ultrasensitive or all-or-none response. Theory demonstrates that this ultrasensitivity grows both as a function of inhibitor concentration and increased binding affinity. Although protein sequestration can theoretically generate tunable ultrasensitive responses, this regulatory principle has never been tested experimentally. We used a synthetic genetic circuit in budding yeast to show that sequestration of a basic leucine zipper transcription factor (C/EBPa) by a dominant-negative inhibitor converts a graded transcriptional response into an ultrasensitive response, with apparent Hill coefficients up to 12. We developed a simple quantitative model for this genetic network that demonstrates how the threshold and degree of ultrasensitivity depend upon the abundance of the inhibitor, exactly as observed in our experimental results. Many proteins in natural regulatory networks involve the formation of inactive protein-protein complexes, e.g. stoichiometric inhibitors of kinases and dominant-negative inhibitors of transcription factors. Our results demonstrate that protein sequestration can provide potent and tunable ultrasensitivity in genetic networks. Ultrasensitive or all-or-none responses are critical for robust bistable or oscillatory genetic networks, and our findings suggest that protein sequestration might play an unappreciated role in facilitating the evolution of bistable or oscillatory circuits in natural systems. [Preview Abstract] |
Session A8: Quantum Information meets Many-Particle Physics
Sponsoring Units: GQIChair: David DiVincenzo, IBM Watson
Room: 414/415
Monday, March 16, 2009 8:00AM - 8:36AM |
A8.00001: Preparing ground states of quantum many-body systems on a quantum computer Invited Speaker: The simulation of quantum many-body systems is a notoriously hard problem in condensed matter physics, but it could easily be handled by a quantum computer [4,1]. There is however one catch: while a quantum computer can naturally implement the dynamics of a quantum system --- i.e. solve Schr\"odinger's equation --- there was until now no general method to initialize the computer in a low-energy state of the simulated system. We present a quantum algorithm [5] that can prepare the ground state and thermal states of a quantum many-body system in a time proportional to the square-root of its Hilbert space dimension. This is the same scaling as required by the best known algorithm to prepare the ground state of a classical many-body system on a quantum computer [3,2]. This provides strong evidence that for a quantum computer, preparing the ground state of a quantum system is in the worst case no more difficult than preparing the ground state of a classical system. \begin{thebibliography}{1} \bibitem{AT03b} {\sc D.~Aharonov and A.~Ta-Shma}, {\em Adiabatic quantum state generation and statistical zero knowledge}, Proc. 35th Annual ACM Symp. on Theo. Comp., (2003), p.~20. \bibitem{B82a} {\sc F.~Barahona}, {\em On the computational complexity of ising spin glass models}, J. Phys. A. Math. Gen., 15 (1982), p.~3241. \bibitem{BBBV97a} {\sc C.~H. Bennett, E.~Bernstein, G.~Brassard, and U.~Vazirani}, {\em Strengths and weaknessess of quantum computing}, SIAM J. Comput., 26 (1997), pp.~1510--1523, quant-ph/9701001. \bibitem{Llo96b} {\sc S.~Lloyd}, {\em Universal quantum simulators}, Science, 273 (1996), pp.~1073--1078. \bibitem{PW08a} {\sc D.~Poulin and P.~Wocjan}, {\em Preparing ground states of quantum many-body systems on a quantum computer}, 2008, arXiv:0809.2705. \end{thebibliography} [Preview Abstract] |
Monday, March 16, 2009 8:36AM - 9:12AM |
A8.00002: Topological Quantum Order from Symmetry and the Role of Temperature Invited Speaker: What does a fractional quantum Hall liquid and Kitaev's proposals for topological quantum computation have in common? It turns out that they are physical systems that exhibit degenerate ground states with properties seemingly different than ordinary (Landau-type) phases of matter, such as ferromagnets. For example, those (topologically quantum ordered) states cannot be characterized by (local) order parameters such as magnetization. How does one characterize this new order? I will present a unifying framework which will allow us to engineer physical systems displaying topological quantum order. What are the physical properties of these new orders? How robust are they to temperature effects? What are they useful for? Topologically quantum ordered states of matter seem to be ideal physical systems to store and manipulate quantum information since they are believed to be robust against decoherence with an environment, and thus appropriate for building a quantum computer and quantum memories. I will discuss the role of temperature in the protection of quantum information. Have we finally found a new technological application for quantum Hall liquids? [Preview Abstract] |
Monday, March 16, 2009 9:12AM - 9:48AM |
A8.00003: Tensor-entanglement renormalization Invited Speaker: Traditional mean-field theory is a simple generic variational approach for analyzing various symmetry breaking phases. However, this simple approach only applies to symmetry breaking states with short-range entanglement. Tensor-entanglement renormalization group (TERG) is a generic approach for studying 2D quantum phases with long-range entanglement (such as topological phases) based on a new class of trial wavefucntions - the tensor product states (TPS), also known as projected entangled pair states (PEPS). Those TPS (PEPS) are built from local tensors. They can describe both states with short-range entanglement (such as the symmetry breaking states) and states with long-range entanglement (such as topological/quantum order). TERG is a real space renormalization group algorithm that can efficiently simulate expectation values for TPS wave functions in 2D and higher dimensions. As an attempt in this direction, we demonstrate our algorithm by studying several simple 2D quantum spin models, including both symmetry breaking phase transitions and topological phase transitions. However, as any variational method, the TERG approach could not find all the degenerate ground states for gapped systems and generally could not give out (approximately) correct critical exponent for critical systems. To solve these problems, we study the renormalization group flow of a Lagrangian (partition function) by representing its path integral through a tensor network. Using a tensor-entanglement-filtering renormalization group (TEFRG) method that removes local entanglement and coarse grains the lattice, we show that the renormalization flow of the tensors in the tensor network has a nice fixed-point structure. The isolated fixed-point tensors characterize various phases. The tensor fixed points can describe both the symmetry breaking phases and topological phases. The ground state degeneracy for gapped systems can be easily read out from the fixed point tensor. The scaling dimensions, the central charge and dynamic correlation functions for the critical systems that describe the continuous phase transitions between symmetry breaking and/or topological phases can also be calculated from the TEFRG approach. [Preview Abstract] |
Monday, March 16, 2009 9:48AM - 10:24AM |
A8.00004: Can multi-particle systems be too entangled to be useful for quantum computation? Invited Speaker: In the context of ``quantum information meets many-particle physics'', we pose the question of the role of entanglement in the quantum computational power of many-particle quantum systems (1). It is often argued that entanglement is at the root of the speedup for quantum compared to classical computation, and that one needs sufficient entanglement for this speedup to be manifest. In measurement-based quantum computing, the need for a highly entangled initial state is particularly obvious. In this work we show that, remarkably, quantum states can be too entangled - in the sense of having a too large geometric entanglement - to be useful for the purpose of computation. What is more, we can prove that this phenomenon occurs for the dramatic majority of all states: the fraction of pure states on n qubits not subject to the problem is smaller than e$^{-n{^2}}$. Our results show that computational universality is actually a rare property in quantum states. For the proof we make use of a link between the ``quantum probabilistic method'' and ideas on quantum many-body systems. This work stresses a new aspect of the question concerning the role entanglement plays for quantum computational speed-ups. We will also investigate a new classification of primitives from projected entangled pair states (PEPS) that can be used in order to systematically construct new models for measurement-based computation (2,3). In an outlook, I will - if time allows - mention other recent group activities related to quantum information and many-particle physics, including dynamical area laws and relaxation statements (4,5). \\[4pt] (1) ``Most quantum states are too entangled to be useful as computational resources'', Phys. Rev. Lett., in press (2009)\\[0pt] (2) ``Quantum computational webs'', arXiv:0810.2542\\[0pt] (3) ``Novel schemes for measurement-based quantum computation'', Phys. Rev. Lett. 98, 220503 (2007)\\[0pt] (4) ``Exact relaxation in a class of nonequilibrium quantum lattice systems'', Phys. Rev. Lett. 100, 030602 (2008)\\0[pt] (5) ``Area laws for the entanglement entropy'', Rev. Mod. Phys., in press (2009). [Preview Abstract] |
Monday, March 16, 2009 10:24AM - 11:00AM |
A8.00005: Quantum-limited metrology and many-body physics Invited Speaker: Questions about quantum limits on measurement precision were once viewed from the perspective of how to reduce or avoid the effects of the quantum noise that is a consequence of the uncertainty principle. With the advent of quantum information science came a paradigm shift to proving rigorous bounds on measurement precision. These bounds have been interpreted as saying, first, that the best achievable sensitivity scales as $1/N$, where $N$ is the number of particles one has available for a measurement and, second, that the only way to achieve this Heisenberg-limited sensitivity is to use quantum entanglement. I will review these results and discuss a new perspective based on using nonlinear quantum dynamics to improve sensitivity. Using quadratic couplings of $N$ particles to a parameter to be estimated, one can achieve sensitivities that scale as $1/N^2$ if one uses entanglement, but even in the absence of any entanglement at any time during the measurement protocol, one can achieve a super-Heisenberg scaling of $1/N^{3/2}$. Such sensitivity scalings might be achieved in Bose-Einstein condensates or in nanomechanical resonators. [Preview Abstract] |
Session A9: Systems Far from Equilibrium I
Sponsoring Units: GSNPChair: Royce Zia, Virginia Polytechnic Institute and State University
Room: 303
Monday, March 16, 2009 8:00AM - 8:12AM |
A9.00001: Directed motion and useful work from an isotropic non-equilibrium distribution Daniel Kosov, Maxim Gelin Since the Maxwell demon thought experiment, the extraction of useful work and directed motion from unbiased non-equilibrium distributions has been the source of fascination, intrigue, and confusion. Being a fundamental scientific problem, it is also of signficant practical interest for various biological and nanotechnological applications. We propose a new type of ``motor'' driven by the heat flow between non-equilibrium velocity and equilibrium coordinate distributions. Namely, we demonstrate that a gas of classical particles trapped in an external asymmetric potential undergoes a quasiperiodic motion, if the temperature of its initial velocity distribution Tne differs from the equilibrium temperature Teq. The magnitude of the effect is determined by the value of Tne - Teq, and the direction of the motion is determined by the sign of this expression. The ``loading'' and ``unloading'' of the gas particles change directions of their motion, thereby creating a possibility of shuttle-like motion. The system works as a Carnot engine where the heat flow between kinetic and potential parts of the non-equilibrium distribution produces the useful work. Phys.Rev. E 77, 011115 (2008) [Preview Abstract] |
Monday, March 16, 2009 8:12AM - 8:24AM |
A9.00002: Coherent transport and heat, entropy fluctuations in a thermal Brownian motor Ronald Benjamin We investigate the heat, entropy and work fluctuations in a thermal Brownian motor driven by spatially inhomogeneous temperature. We show that the total heat, entropy production and the work fluctuations satisfy the fluctuation therorem in the steady state over finite time trajectories. The transport coherence of the motor, as determined by the Peclet number is also investigated as a function of various parameters of the system. [Preview Abstract] |
Monday, March 16, 2009 8:24AM - 8:36AM |
A9.00003: Exact results for currents in nonadiabatic stochastic pumps Jordan Horowitz Biological systems abound with examples of molecular machines: assemblies of molecules that perform specific useful mechanical tasks, such as the motor proteins kinesin and myosin. Remarkably, the first steps in developing useful artificial molecular motors have been taken with the synthesis and manipulation of molecular complexes such as catenanes and rotaxanes. These developments have spurred an interest in developing theoretical frameworks which describe these mesoscopic machines that operate in the presence of thermal noise. In this talk I will analyze a generic model of molecular machines known as stochastic pumps in which useful directed motion (or current) is produced by the variation of external parameters. The main result is an exact expression for the current in the presence of nonadiabatic pumping. This expression connects to a variety of results from the field of brownian ratchets and leads to a surprising ``no-pumping'' theorem: a set of conditions that guarantee no excess or pumped current. These predictions also agree with the observations on catenanes, interlocked ring molecules, made by Leigh et. al. [Nature, 424, 174 (2003)]. [Preview Abstract] |
Monday, March 16, 2009 8:36AM - 8:48AM |
A9.00004: Rapidly forced quantum Brownian motion Mustansir Barma, Malay Bandyopadhyay We study the steady state behaviour of a confined quantum Brownian particle which is subjected to a space-dependent, rapidly oscillating time-periodic force. To leading order in the period of driving, the result of the oscillating force is to produce an effective static potential which has a quantum contribution V$_{q}$ which adds on to the classical result. This is shown by using a coherent state representation of bath oscillators which leads to a c-number generalized quantum Langevin equation. We evaluate V$_{q}$ exactly in the case of an Ohmic dissipation bath and show that it takes on different forms in different regimes, determined by the ratio of the thermal wavelength to the spatial spread of the driving force. [Preview Abstract] |
Monday, March 16, 2009 8:48AM - 9:00AM |
A9.00005: Deciding how far is far from equilibrium Antonio Cadilhe, Arthur Voter Nonequilibrium systems have both fundamental and technological interest for their unusual behavior with research efforts mainly focused on their properties. Surprisingly, little research effort has been put on diagnosing how far a system is from equilibrium. Clearly, addressing such an issue is of fundamental and technological relevance. To this end, we present results of how a particle in contact with a heat reservoir is being driven away from equilibrium by a time dependent potential well. The methodology can be straightforwardly extended to systems with more particles and under the influence of more realistic potentials. [Preview Abstract] |
Monday, March 16, 2009 9:00AM - 9:12AM |
A9.00006: Random Sequential Adsorption on patterned substrates: jammed state structure and kinetic properties N. A. M. Araujo, J. F. Marques, A. Cadilhe, V. Privman The irreversible adsorption on a patterned substrate is studied through extensive Monte Carlo simulations. As a pattern, we adopted square cells positioned at the vertices of a square lattice. Particles attempting adsorption can only stick to the substrate if they do not overlap previously adsorbed ones (excluded volume interaction) and if their geometrical centers land inside a cell. Once a particle is adsorbed, it does not detach from or diffuse on the substrate, thus representing an extended random sequential adsorption model. The distribution of particles sizes follows a truncated gaussian-size distribution with values of the size dispersion varying from zero (monodisperse) to $20\%$ (polydisperse) of the mean particle radius. We address the influence of both the pattern and size dispersion on the jammed state structure. We also present results on how the kinetics of approach to the jammed state is affected by the particular values taken by parameters like cell size and cell-cell separation and show that they can lead to either exponential or power-law functional dependences. [Preview Abstract] |
Monday, March 16, 2009 9:12AM - 9:24AM |
A9.00007: Phase Transition with Non-Thermodynamic States in Reversible Polymerization Eli Ben-Naim, Paul Krapivsky We investigate a reversible polymerization process in which individual polymers aggregate and fragment at a rate proportional to their molecular weight. We find a nonequilibrium phase transition despite the fact that the dynamics are perfectly reversible. When the strength of the fragmentation process exceeds a critical threshold, the system reaches a thermodynamic steady state where the total number of polymers is proportional to the system size. The polymer length distribution has a sharp exponential tail in this case. When the strength of the fragmentation process falls below the critical threshold, the steady state becomes non-thermodynamic as the total number of polymers grows sub-linearly with the system size. Moreover, the length distribution has an algebraic tail and the characteristic exponent varies continuously with the fragmentation rate. [Preview Abstract] |
Monday, March 16, 2009 9:24AM - 9:36AM |
A9.00008: Stochastic continuum theory of active nematics Shradha Mishra, Francesco Ginelli, Hugues Chate, Sanjay Puri, Sriram Ramaswamy We derive a stochastic continuum theory of active nematics by direct coarse-graining of a generic microscopic model and study it numerically. This allows to clarify the microscopic origin of the various terms found and to determine the non-trivial structure of the noises. We show in particular that two terms coupling density and order the non-equilibrium active current argued before to be at the origin of giant density fluctuations, and a multiplicative conserved noise are necessary to obtain a faithful description of the original model. [Preview Abstract] |
Monday, March 16, 2009 9:36AM - 9:48AM |
A9.00009: Fokker-Planck Dynamics in the Energy Domain Gcina Mavimbela, Horacio E. Castillo, Claudio Chamon We derive a Fokker-Planck Equation (FPE) in the energy domain for a system in an infinite heat bath by coarse-graining its microscopic Master Equation. The resulting FPE carries information on the dynamics through a function $\lambda(E)$, which is a sum over all possible transitions given a state of energy E. We investigate the effects of changing the assumptions about the transition rates without changing the Hamiltonian of the model. By determining the eigenvalues of the equivalent Schrodinger Equation (SE), we get the relaxation spectrum of the FPE. We find that in the thermodynamic limit the equivalent SE approaches the classical limit, and we use the WKB approximation to solve it. We illustrate the use of the method by applying it to several examples, including a system of harmonic oscillators, and a paramagnet in an external magnetic field. [Preview Abstract] |
Monday, March 16, 2009 9:48AM - 10:00AM |
A9.00010: Doing the Impossible: Very Rare Events in the Harmonic Measure David Adams, Leonard Sander, Robert Ziff We have developed a method of obtaining accurate data of rare events using biased sampling of random walkers. We have obtained the harmonic measure, analogous to the perpendicular electric field on a charged conductor, for percolation, Ising model, and Diffusion Limited Aggregation (DLA) clusters. We measured probabilities down to 10$^{-300}$ for percolation and Ising model clusters. These small probabilities allowed us to verify the theoretical predictions for the harmonic measure made by Duplantier. For DLA, which has no theory, we obtained probabilities down to 10$^{-100}$. The previous lowest probability was obtained using iterative conformal maps and was limited to small clusters and comparatively high probabilities. For all systems we have obtained the generalized dimension Dq, the singularity spectrum f(alpha), and the distribution of probabilities. [Preview Abstract] |
Monday, March 16, 2009 10:00AM - 10:12AM |
A9.00011: Anisotropic 2-dimensional Robin Hood model Sergey Buldyrev, Gabriel Cwilich, Fredy Zypman We have considered the Robin Hood model introduced by Zaitsev[1] to discuss flux creep and depinning of interfaces in a two dimensional system. Although the model has been studied extensively analytically in 1-d [2], its scaling laws have been verified numerically only in that case. Recent work suggest that its properties might be important to understand surface friction[3], where its 2-dimensional properties are important. We show that in the 2-dimensional case scaling laws can be found provided one considers carefully the anisotropy of the model, and different ways of introducing that anisotropy lead to different exponents and scaling laws, in analogy with directed percolation, with which this model is closely related[4]. We show that breaking the rotational symmetry between the \textbf{x} and \textbf{y} axes does not change the scaling properties of the model, but the introduction of a preferential direction of accretion (``robbing'' in the language of the model) leads to new scaling exponents. [1] S.I.Zaitsev, Physica \textbf{A189}, 411 (1992) [2] M. Pacuzki, S. Maslov and P.Bak, Phys Rev. \textbf{E53}, 414 (1996) [3] S. Buldyrev, J. Ferrante and F. Zypman Phys. Rev \textbf{E64}, 066110 (2006) [4] G. Odor, Rev. Mod. Phys. \textbf{76}, 663 (2004) . [Preview Abstract] |
Monday, March 16, 2009 10:12AM - 10:24AM |
A9.00012: A Cellular Automaton Model of Catastrophic Failure C. A. Serino, W. Klein We introduce a two-dimensional cellular automaton model for studying the catastrophic failure of materials under stress. Our model is similar to the Olami-Feder-Christensen earthquake model [Z.\ Olami \emph{et al.}, Phys.\ Rev.\ Lett.\ \textbf{68}, 1244 (1992)] except that after a site fails $f$-times, it no longer can receive stress from its neighbors. In the limit that the interaction range, $R$, goes to infinity, our model is equivalent to the global load sharing fiber bundle model of Pierce [F.\ T.Pierce, J.\ Text.\ Ind.\ \textbf{17}, 355 (1926)] and Daniels [H.\ E.\ Daniels, Proc.\ Roy.\ Soc.\ London A \textbf{183}, 405 (1945)]. By varying the interaction range, we observe two qualitatively different failure modes. For $R \gg 1$ catastrophic failure resembles a nucleation-like event which grows symmetrically from a single initiating site and fails every site in the lattice. In contrast, for $R \approx 1$ a percolating cluster of failed sites spans the system despite the many active sites that persist, even after catastrophic failure. We use the stress-fluctuation metric to study the ergodicity of our model and hence the validity of equilibrium descriptions of fracture. [Preview Abstract] |
Monday, March 16, 2009 10:24AM - 10:36AM |
A9.00013: Heat transport in quantum spin chains: the relevance of integrability Jinshan Wu, Mona Berciu Heat transport in quantum spin chains is investigated through the master equation in Lindblad form derived from the Schroedinger equation of a system coupled with two baths via the projector operator technique. We find that the Fourier's Law of heat transport is obeyed in some systems. Although a general proof has not been established, after a survey of various quantum spin chains, our results suggest the criteria of anomalous heat transport is not the integrability of the Hamiltonian, but whether or not it can be mapped to non- interacting fermions. [Preview Abstract] |
Monday, March 16, 2009 10:36AM - 10:48AM |
A9.00014: Hard-core Bosons in time-varying traps Aditya Raghavan, Marcos Rigol, Stephan Haas We present a study of the time evolution of hard-core bosons (HCBs) in a one-dimensional, time-varying optical trap. Previous results have shown that one-dimensional HCBs can form superfluid and Mott-insulator phases. Using an exact numerical approach, we study the dynamics of the system when the trap curvature is modulated. We find the dynamics to be markedly different in the two phases, and address its relevance in the observation of these phases in optical lattice experiments. [Preview Abstract] |
Monday, March 16, 2009 10:48AM - 11:00AM |
A9.00015: Control of Transport Behavior in spin-1/2 Heisenberg Systems Lea Santos A complete understanding of transport behavior in many-body systems is one of the utmost challenges in fundamental studies of nonequilibrium statistical mechanics. In the classical domain, it is widely believed that chaotic systems should show diffusive transport, whereas integrability should be associated with ballistic transport. In the quantum domain, the conditions that determine specific transport behaviors are still under debate. Here, we analyze transport of local magnetization in finite spin-1/2 Heisenberg systems. By adjusting parameters in the Hamiltonian, these quantum systems may show both integrable and chaotic limits. We provide examples of chaotic systems leading to diffusive and also to ballistic transport. In addition, we develop schemes to control the transport behavior in these systems, showing that quantum control methods may be used to induce a transition from diffusive to ballistic transport. [Preview Abstract] |
Session A10: Focus Session: Ferroelectrics I
Sponsoring Units: DMPChair: Matthew Dawber, Stonybrook University
Room: 304
Monday, March 16, 2009 8:00AM - 8:36AM |
A10.00001: First-principles modeling of closure domains in ferroelectric ultrathin films Invited Speaker: Capacitors based on ferroelectric perovskites are potentially attractive for nanoelectronic devices. However, in many cases their use depends on the stability of a ferroelectric state with an out-of-plane polarization. The interplay between mechanical and electrical boundary conditions, and the local chemistry at the surface or interface might give rise to exotic patterns of the polarization, especially in the thin film regime. The screening of the interfacial polarization charges by real metallic electrodes has been extensively discussed in the literature. We have carried out first-principles computations on two other screening mechanisms in ultrathin capacitors made of a few unit cells of BaTiO$_3$ with metallic SrRuO$_3$ electrodes. First, the simulation of the energetic, structural, and electronic properties of ferroelectric domains in short circuit. \footnote{P. Aguado-Puente and J. Junquera, Phys. Rev. Lett. {\bf 100}, 177601 (2008).} The domains are stabilized down to two unit cells, adopting the form of a domain of closure, common in ferromagnetic thin films. The domains are closed by the in-plane relaxation of the atoms in the first SrO layer of the electrode, that behaves more like SrO in highly polarizable SrTiO$_3$ than in metallic SrRuO$_3$. Even if small, these lateral displacements are essential to stabilize the domains, and might provide some hints to explain why some systems break into domains while others remain in a monodomain configuration. An analysis of the electrostatic potential reveals preferential points of pinning for charged defects at the ferroelectric-electrode interface, possibly playing a major role in films fatigue. The closure domain structure, predicted also by other phenomenological and effective Hamiltonian models, is more general than expected. Second, the possibility of screening by a surface state or metallization of the first few layers of the ferroelectric without a top electrode is also explored. [Preview Abstract] |
Monday, March 16, 2009 8:36AM - 8:48AM |
A10.00002: Polarization Switching at the Intrinsic Coercive Field of PbTiO$_{3}$ by Changing Oxygen Partial Pressure M. Highland, T.T. Fister, M.-I. Richard, D.D. Fong, J.A. Eastman, S.K. Streiffer, P.H. Fouss, G.B. Stephenson, Carol Thompson Previously we have found that changing the partial pressure of oxygen in the gas above ultrathin PbTiO$_{3}$ films on SrRuO$_{3}$ can induce inversion in the sign of the polarization. Here we present x-ray measurements that allow us to determine the polarization magnitude and domain distribution during switching. For films of thickness above $\sim $5 nm, switching occurs by the usual mechanism of nucleation and growth of 180\r{ } domains having the same polarization magnitude but opposing signs. However, in thinner films switching of the polarization occurs without nucleation; the polarization magnitude decreases to zero and changes sign uniformly without domain formation, indicating that the intrinsic coercive field (E$_{IC})$ is reached. Since E$_{IC}$ has never been reached in oxide ferroelectrics using applied electric field, our results suggest that the barrier to nucleation is large during chemical switching. Work supported by the Dept. of Energy under Contract DE-AC02-06CH11357. [Preview Abstract] |
Monday, March 16, 2009 8:48AM - 9:00AM |
A10.00003: Understanding the surface reconstruction during chemical switching of ultrathin PbTiO$_3$ films from density functional theory Jun He, Brian Stephenson, Serge Nakhmanson First-principles calculations are used to understand the structure and energetics of the newly discovered 4$\times$1 surface reconstruction that forms under reducing conditions during chemical switching of the ferroelectric polarization in ultrathin films of PbTiO$_3$ with SrRuO$_3$ bottom electrodes coherently strained to SrTiO$_3$ (001). Relaxed surface structures are obtained for polar films with various oxygen stoichiometries in the outermost PbO layer. To model the behavior of many-unit-cell thick films, which are observed to have polarizations near the bulk value, the lowest unit cell(s) of the PbTiO$_3$ film are forced to be polar. The observed surface reconstructions are compared with experimental synchrotron x-ray measurements. [Preview Abstract] |
Monday, March 16, 2009 9:00AM - 9:12AM |
A10.00004: First-principles calculations of structural energetics of Pb-O divacancies in PbTiO$_3$ and their role in up-down asymmetry of thin films. O. Paz, S. P. Beckman, D. Vanderbilt, K. M. Rabe Defects have been proposed as an important influence on the performance of technologically relevant ferroelectric (FE) materials, even at low concentrations. In PbTiO$_3$, $V_\textrm{Pb}$-$V_\textrm{O}$ vacancy pairs (VPs) are neutral defects, with a local electric dipole moment that can couple to the bulk polarization. To investigate the role that these defects might play in imprint behavior (i.e., a history-dependent up-down asymmetry), we carried out first-principles supercell calculations of VPs in PbTiO$_3$. While previous works considered a cubic host,% \footnote{S. P\"oykk\"o and D. J. Chadi, Appl. Phys. Lett. \textbf{76}, 499 (2000).}$^,$% \footnote{E. Cockayne and B. P. Burton, Phys. Rev. B \textbf{69}, 144116 (2004).} we study the tetragonal FE case for periodic arrays of 1$^\textrm{st}$, 2$^\textrm{nd}$ and 3$^\textrm{rd}$-neighbor VPs at densities as low as 3\%. Our lowest energy VP is a 2$^\textrm{nd}$-neighbor one oriented so as to break the bulk symmetry between the up and down polarization states. Other VP arrangements are higher in energy by at least 0.2 eV. Atomic relaxation is of particular importance in lowering the energy of the 2$^\textrm{nd}$ below that of the 1$^\textrm{st}$-neighbor VP. Berry-phase polarization calculations reveal that the total polarization is only slightly modified for VPs whose dipoles are aligned with the polarization, but is significantly suppressed by antialigned VPs. [Preview Abstract] |
Monday, March 16, 2009 9:12AM - 9:24AM |
A10.00005: Study on the electro-optic effect in Pb(Zr,Ti)O$_{3}$ (001) film using spectroscopic ellipsometry Tae Dong Kang, Xiao Bo, Vitaliy Avrutin, \"Umit \"Ozg\"ur, Hadis Morko\c{c}, Jun Woo Park, Ho Suk Lee, Hosun Lee, Xiaoyu Wang, David Smith Spectroscopic ellipsometry was applied to study electro-optic effect in lead zirconate titanate$^{ }$(PZT) thin films grown epitaxially on Nb-doped SrTiO$_{3}$(001) substrates by$^{ }$RF magnetron sputtering. Multilayer model analysis was applied$^{ }$to extract the ordinary and extraordinary refractive indices of the PZT thin film with electric$^{ }$field applied along the (001) direction. The effective linear and$^{ }$quadratic coefficients at a wavelength of 632.8 nm were estimated to be -134.6$\times $10$^{-12}$~m/V and 8.5$\times $10$^{-18}$~m$^{2}$/V$^{2}$, respectively, while the individual linear electro-optic$^{ }$coefficients $r_{33}$ and $r_{13}$ were -157.1 and 22 pm/V, respectively. We attributed existence of the linear electro-optic effect in unpoled PZT$^{ }$films to the presence of a built-in polarization$^{ }$and simultaneous poling during ellipsometric measurements. [Preview Abstract] |
Monday, March 16, 2009 9:24AM - 9:36AM |
A10.00006: A first-principles study of enhancement of polarization with electric field in tetragonal PbTiO$_3$ Anindya Roy, Massimiliano Stengel, David Vanderbilt We present first-principles calculations of the electric polarization and $c/a$ ratio of ferroelectric tetragonal PbTiO$_3$ as an external electric field is applied parallel to the polarization direction. The work is motivated in part by experimental observations\footnote{ A. Grigoviev {\it et al.}, Phys.\ Rev.\ Lett.\ \textbf{100}, 027604, (2008). } showing an anomalous enhancement (i.e., nonlinear piezoelectric response) in the $c/a$ ratio for electric fields above $\sim$200\,MV/m in PbZr$_{0.2}$Ti$_{0.8}$O$_3$. Working here with pure PbTiO$_3$ for reasons of computational convenience, we calculate the strain response to an electric field applied parallel to the spontaneous polarization. We focus mainly on the case of fixed in-plane lattice constants, which appear most suited to the experimental conditions, but we also study the case in which all lattice constants are relaxed. In addition explore the effects of artificial negative pressure, with and without the in-plane epitaxial constraint. Two ab-initio computer codes are used for this purpose, and the results are compared to each other and to the experiment. [Preview Abstract] |
Monday, March 16, 2009 9:36AM - 9:48AM |
A10.00007: Polarization reversal and backswitching kinetics in epitaxial ferroic thin films Jianheng Li, Bruce Wessels Polarization reversal and backswitching kinetics were investigated in epitaxial ferroelectric BaTiO3 thin films. Using the electro-optic effect to monitor domain dynamics, the dynamic response was measured as a function of bias pulse magnitude and temperature. The dynamics followed a Kohlrausch-Williams-Watts (KWW) stretched exponential function in time. From the measurements the activation field for polarization reversal and the activation energy for domain motion were determined. The measured activation energy of 6-12 kJ/mol is in good agreement with prior experiments on bulk material and recent theoretical calculations using molecular dynamics simulations. [Preview Abstract] |
Monday, March 16, 2009 9:48AM - 10:00AM |
A10.00008: Thickness dependent ferroelectric properties of ultrathin BaTiO$_{3}$ thin films D.A. Felker, H.W. Jang, C.B. Eom, M.S. Rzchowski The thickness dependence of the coercive field and the spontaneous polarization were studied for epitaxial trilayer heterostructures with SrRuO$_{3}$ electrodes and ultrathin (001) BaTiO$_{3}$ ferroelectric layers grown on TiO$_{2}$ terminated (001) SrTiO$_{3}$ substrates. The BaTiO$_{3}$ thickness ranged from 2.4 (6 unit cells) to 50 nm. The 3.2 nm (8 unit cells) sample provides the thinnest direct electrical measurement of ferroelectricity in BaTiO$_{3}$ in a device structure, showing the thickness dependence of ferroelectric properties down into the ultrathin regime and providing an experimental upper bound on the critical thickness. The coercive field increases dramatically in thinner samples consistent with the Kay-Dunn model of domain nucleation and propagation. Below 10 nm the spontaneous polarization decreases with decreasing thickness due to decreasing screening of the depolarization field by the electrodes. For barriers thicker than 10 nm the polarization decreases due to strain relaxation in the ferroelectric barrier. We discuss these measurements, as well as the temperature dependence of the coercive fields and hysteresis loops. [Preview Abstract] |
Monday, March 16, 2009 10:00AM - 10:12AM |
A10.00009: Temperature dependence of the dielectric properties of strained barium strontium titanate films for tunable microwave applications Lisa Alldredge, Wontae Chang, Steven Kirchoefer, Jeffrey Pond Understanding strain effects is critical to achieve desirable dielectric properties in ferroelectric films, which are of interest for tunable microwave applications. Sputter-deposited Ba$_{1-x}$Sr$_{x}$TiO$_{3}$ films on (001) MgO were studied in various strain states: in-plane or out-of-plane tetragonal lattice distortions. The optimal system calibration for microwave measurements changes greatly with temperature, requiring frequent recalibration. A temperature-dependent interpolation calibration technique was developed to increase the efficiency of measurements taken as a function of temperature. The films showed significant differences in the ferroelectric phase transition due to lattice distortions, with a strong temperature dependence of the in-plane dielectric behavior for films under tensile strain and a weak temperature dependence for films under compressive strain. We believe that films under tensile strain have polarizations aligned parallel to the applied electric field and so the in-plane dielectric properties are strongly coupled with the field, while films under compressive strain have polarizations perpendicular to the field, resulting in minimal influence on the in-plane dielectric behavior. [Preview Abstract] |
Monday, March 16, 2009 10:12AM - 10:24AM |
A10.00010: Density functional study of ferroelectric-electrode interfacial effects on the stability of ferroelectricity in thin-films Wissam A. Al-Saidi , Alexie Kolpak, Ilya Grinberg, Andrew Rappe Ferroelectric (FE) thin-films are very promising materials for various technological applications. The continuous demand of miniaturization of devices based on FE thin-films by the micro-electronic industry demands an understanding of the critical thickness of ferroelectricity in thin films. Using an ab initio density-functional approach, we study the properties of several capacitor-like structures which are based on PbTiO3 and BaTiO3 ferroelectric materials. Different electrodes are used in our study to gain a thorough understanding of the electrode-ferroelectric interfaces, and the role of the interfacial chemical bonding and charge transfer in stabilizing the FE polar phase. We finally used our ab initio results to develop a phenomenological predictive model based on a Landau-Ginzburg-Devonshire functional. [Preview Abstract] |
Monday, March 16, 2009 10:24AM - 10:36AM |
A10.00011: Boundary conditions on ferroelectricity in ultrathin SrTiO$_3$ films on silicon Alexie Kolpak, Fred Walker, James Reiner, Charles Ahn, Sohrab Ismail-Beigi The properties of SrTiO$_3$ films expitaxially grown on Si(001) are strongly influenced by the electronic structure of the interface. Using density functional theory, we demonstrate the presence of an intrinsic interface dipole, the direction of which is independent of the particular combination of Sr, Ti, O, and Si atoms at the interface, and therefore independent of growth conditions. As a result of this intrinsic dipole, a local, positive polarization is induced in the SrTiO$_3$ interfacial region, fixing the electrostatic boundary conditions at the interface and preventing the formation of a negatively polarized state with a single domain. We suggest ways in which this constraint on the ferroelectric behavior can be overcome by interfacial cation doping, allowing for the integration of ferroelectricity with traditional silicon-based devices. [Preview Abstract] |
Monday, March 16, 2009 10:36AM - 10:48AM |
A10.00012: Strain Effect in the Problem of Critical Thickness for Ferroelectric Memory A.M. Bratkovsky, A.P. Levanyuk We account for inhomogeneous strains while calculating \emph {two} characteristic thicknesses arising in the problem of critical thickness for ferroelectric memory. One of them marks the stability limit of metastable single domain state under zero external voltage with respect to small fluctuations (spinodal point of the single domain state.) The second one appears when free energies of the single and multidomain state become equal while the latter is considered within one- sinusoidal approximation [1]. At this thickness the single domain state remains metastable, but one may hope that the lifetime of this state becomes suitable for the memory applications[2]. We use the Landau approach for elastically isotropic solid with a single electrostriction constant to illustrate general behavior. It is found that the effect of the elastic strains is qualitatively different for free-standing films versus films on substrates.\\[3pt] [1] A.M. Bratkovsky and A.P. Levanyuk, arXiv: 0801.1669.\\[0pt] [2] A.M. Bratkovsky and A.P. Levanyuk, Phys. Rev. Lett. {\bf 100}, 149701 (2008). [Preview Abstract] |
Monday, March 16, 2009 10:48AM - 11:00AM |
A10.00013: Unusual polarization patterns in flat epitaxial ferroelectric nanoparticles Ivan Naumov, Alexandr Bratkovsky We investigate the effects of a lattice misfit strain on a ground state and polarization patterns in flat perovskite nanoparticles (nanoislands of BaTiO$_{3}$ and PbZr$_{0.5}$Ti$_{0.5}$O$_{3}$) with the use of an \textit{ab-initio} derived effective Hamiltonian. We show that the strain strongly controls the balance between the depolarizing field and the polarization anizotropy in determining the equilibrium polarization patterns. Compressive strain favors 180 $^0$ stripe/tweed domains while a tensile strain leads to in-plane vortex formation, with the unusual intermediate phase (s) where both ordering motifs coexist [1]. The results may allow to explain contradictions in recent experimental data for ferroelectric nanoparticles. [1] Ivan Naumov and Alexander M. Bratkovsky, Phys. Rev. Lett. 101, 107601 (2008). [Preview Abstract] |
Session A11: Superlattices, Nanostructures, Materials: Synthesis, Growth and Characterization
Sponsoring Units: DCMPChair: Joe Tischler, Naval Research Laboratory
Room: 305
Monday, March 16, 2009 8:00AM - 8:12AM |
A11.00001: Coverage-Dependent Faceting of Au Chains on Si(557) F.J. Himpsel, I. Barke, F. Zheng, S. Bockenhauer, K. Sell, V. v. Oeynhausen, K.H. Meiwes-Broer The structural and electronic phase diagram of Au on Si(557) is established using scanning tunneling microscopy (STM) and angle-resolved photoemission (ARPES). Five phases consisting of altogether seven facets are observed in the sub-monolayer regime. Four of them consist of two coexisting structures. In order of increasing Au coverage the five phases are: Si(111)7$\times$7 + Si(112), Si(557)1$\times$2-Au, Si(111)5$\times$2-Au + Si(335)-Au, Si(111)$\sqrt{3}$$\times$$\sqrt{3}$-Au + Si(335)-Au, and Si(111)$\sqrt{3}$$\times$$\sqrt{3}$-Au + Si(5 5 11)-Au. The relative surface areas of the five phases and seven facets are determined accurately by depositing a Au wedge ranging from 0 to 0.8 monolayer and performing automatic pattern recognition on large-scale STM images. Angle-resolved photoemission spectra are decomposed into contributions from the five phases. The Fermi wave vectors of various facets are identified. Using Si(557)1$\times$2-Au as reference we find a coverage of 3 Au chains per unit cell for the frequently-studied Si(111)5$\times$2-Au surface (instead of the widely-used value of 2 Au chains). The impact of this finding on structural models is discussed. [Preview Abstract] |
Monday, March 16, 2009 8:12AM - 8:24AM |
A11.00002: The Alignment of Gold Nanorods in Macroscopic Domains Jake Fontana, Peter Palffy-Muhoray, Ashish Agarwal, Nicholas Kotov The uniform alignment of nanoparticles in domains with macroscopic length scales is critical to the production of self-assembled composite metamaterials for optical applications. We describe methods of self assembly leading to films and suspensions with a high loading of orientationally ordered nanoparticles in macroscopic domains. The nanoparticles are short aspect ratio gold nanorods, with both plasmon peaks in the visible spectrum. Orientational order can be achieved via applied electric field, mechanical strain, as well as via interactions with anisotropic hosts and among nanoparticles. We have determined the orientational order in our samples from polarized absorbance measurements. [Preview Abstract] |
Monday, March 16, 2009 8:24AM - 8:36AM |
A11.00003: Role of the three Si suboxides at the surface of Si quantum dots and in Si/SiO$_2$ quantum wells on optical response Pierre Carrier The Si/SiO$_2$ interface structure has been extensively studied in the past, especially for MOSFET applications. Recent applications of Si/SiO$_2$ nanostructures in solar cells and LEDs are now investigated using Si quantum dots (QD) or Si/SiO$_2$ quantum wells (QW). The Si/SiO$_2$ interface contains three Si suboxides, each bonded to 1, 2, or 3 oxygen atoms, respectively referred to as Si$^{1+}$, Si$^{2+}$, and Si$^{3+}$. Models that contain all three suboxides are difficult to construct; results in the literature on oxygenated Si QD usually include Si$^{1+}$ and Si$^{2+}$ only. The models presented here contain the 3 suboxides and are based on a Si/SiO$_2$ surface model originally constructed by Pasquarello \textit{et al.}, Appl.\ Phys.\ Lett.\ \textbf{68}, 625 (1996). This model was used later by the author in the study of Si/SiO$_2$ QW [Phys.\ Rev.\ B \textbf{65}, 165339 (2002)] and is now extended to Si QD. It is shown that the band gap or optical response depends strongly on the Si suboxide atomic configuration at the surface of QD or at the interface of QW. Trends on the band gap variations as function of the three suboxides will be discussed. All models (QW and QD) are structurally relaxed using the program PARSEC [Phys.\ Rev.\ Lett.\ \textbf{72}, 1240 (1994)]. [Preview Abstract] |
Monday, March 16, 2009 8:36AM - 8:48AM |
A11.00004: Formation of Colloidal Nanoparticle Superlattices in a Two Solvent System Chenguang Lu, Austin Akey, Irving Herman A two solvent system consisting of a high boiling point solvent and a low boiling point solvent was found to greatly aid the self-assembly of nanoparticle superlattices. Nanoparticle mixtures were prepared under multiple suitable solvent evaporation conditions and the products were analyzed by SEM and TEM. The formation process of various binary nanoparticles superlattices was investigated to elucidate the optimal conditions for self-assembly. Superlattice formation in this two solvent system was further investigated with various spatial confinement conditions. Here, the capillary effect during the evaporation of solvents may be the driving factor in the self-assembly. Micrometer scale superlattices of CdSe nanoparticles were fabricated with this technique. [Preview Abstract] |
Monday, March 16, 2009 8:48AM - 9:00AM |
A11.00005: Modeling the Self-assembly of Nanorod Superlattices Alexey Titov, Petr Kral Colloidal semiconductor CdSe/CdS nanorods (NR) of diameters of 3-10 nm and lengths of 4-40 nm typically self-assemble into nematic and smectic phases, which are parallel to the substrate, or simple hexagonal (SH) superlattices, which are perpendicular to the substrate [1-2]. We model the formation of these structures by semi-classical means, starting from the forces between the nanorods, their coupling to the substrate and to the external electric fields. We determine the conditions under which superlattices with different number of particles, number of monolayers, aspect ratios of nanorods, etc. can be observed [3], and show that the obtained results agree well with the available experimental data. Our previous results of modeling superlattices of self-assembled monodisperse nanoparticles are also presented. \\[3pt] [1] L. S. Li and A. P. Alivisatos, Adv. Mater. 15, 408 (2003).\\[0pt] [2] D. V. Talapin et al., J. Am. Chem. Soc. 126, 12984 (2004).\\[0pt] [3] A. V. Titov and P. Kral, Nano Lett. 8, 3605 (2008). [Preview Abstract] |
Monday, March 16, 2009 9:00AM - 9:12AM |
A11.00006: Morphology of Cu$_2$S-CdS and Ag$_2$S-CdS Nanorod Heterostructures Denis Demchenko, Bryce Sadtler, Haimei Zheng, A. Paul Alivisatos, Lin-Wang Wang A partial cation exchange has been used to synthesize Cu$_2$S-CdS and Ag$_2$S-CdS nanocrystal heterostructures, with two very different morphologies. Cu$^+$ cation exchange takes place preferentially at the ends of CdS nanorods, Cu$_2$S segments grow into the nanorod from both ends. Ag$^+$ exchange is non-selective, Ag$_2$S islands nucleate and grow over the entire surface of the nanorod. This leads to very different patterns, striped Ag$_2$S-CdS superlattice with several equidistant Ag$_2$S segments in a CdS nanorod, and an asymmetric Cu$_2$S-Cds heterostructure with Cu$_2$S segments at the ends of the CdS nanorod. We use first-principles calculations to obtain formation energies of the different epitaxial interfaces between Cu(Ag)$_2$S and different facets of CdS nanorods. Comparison of chemical and elastic contributions to the interface formation energy for the Cu(Ag)$_2$S-CdS shows that the relative stability of the interfaces determines the nucleation of Cu(Ag)$_2$S and the resulting morphology. Furthermore, since two end facets of CdS nanorod are not crystallographically equivalent a controlled asymmetric nucleation of Cu$_2$S can occur. [Preview Abstract] |
Monday, March 16, 2009 9:12AM - 9:24AM |
A11.00007: Self-assembly of molecular wires Andreas Riemann Scanning Tunneling Microscopy (STM) has been used to study the self-assembly of the naturally occurring amino acid L-methionine on different surfaces. It has been found that methionine forms highly regular structures on an Ag(111) surface under UHV conditions as well as on a graphite surface under ambient conditions. Methionine arranges itself into an array of molecular wires of uniform width and separation. The spacing of these wires can be controlled by means of the deposition amount. Molecular mechanics calculations are used to suggest a model for the methionine configuration on the surfaces. The width of the wires is determined by two methionine molecules arranged with their carboxyl group facing each other. The regular separation of individual wires suggests a long range interaction between these wires. [Preview Abstract] |
Monday, March 16, 2009 9:24AM - 9:36AM |
A11.00008: An STM Study of Atomic Co Wires Nader Zaki, Denis Potapenko, Peter Johnson, Danda Acharya, Percy Zahl, Peter Sutter, Richard Osgood Due to stronger electron-electron interactions, 1-D systems are predicted and, in some cases, have been shown to exhibit unique and exotic electronic properties. One route to the formation of 1-D systems is by self-assembly using low-index vicinal crystal surfaces. In this regard, we have successfully formed 1-atom wide Co wires using Cu(775), a 7-atom wide stepped array with (111) terraces. Contrary to a recently reported DFT prediction, the Co wires are not laterally encapsulated but are positioned exactly at the step edge. We will present STM studies of this system performed at room temperature and STS measurements made at low temperature. While vicinal Cu(111) does exhibit ``frizz'' at the steps when scanning above cryogenic temperatures, the Co wires pin the edges, visually accentuating their presence under STM. Furthermore, we observe a lower local density of states for the Co wires as compared with the Cu steps, which also serves to differentiate the two metals. Cu(111) possess a surface projected bandgap which may electronically decouple the wire electrons that reside in this gap. However, we also see resonances at the Fermi level which suggests electronic coupling between the vicinal Cu surface and the Co electrons. [Preview Abstract] |
Monday, March 16, 2009 9:36AM - 9:48AM |
A11.00009: III-V nanowires grown in a simple, homebuilt system. M. D. Schroer, J. R. Petta Semiconductor nanowires are promising experimental platforms for studying quantum transport due to their built-in one-dimensional confinement of charge carriers. To enable the study of III-V semiconducting nanowires, we built a simple tube furnace based MOCVD reactor. Growth of InP and InAs nanowires using trimethylindium, di-tert-butylphosphine and triethylarsenic has been studied as a function of temperature, pressure, precursor concentration and growth substrate. At optimal growth conditions, wires of 20-100 nm in diameter and up to 10 $\mu$m in length are achievable on InAs substrates. Characterization was performed using SEM, EDS and TEM; both wurtzite and zincblende structures have been observed. We will also present transport measurements of nanowires grown using this system. \vspace{8pt} [Preview Abstract] |
Monday, March 16, 2009 9:48AM - 10:00AM |
A11.00010: Fabrication of nanowire-nanotube hybrid arrays in porous aluminum oxide membranes Zuxin Ye, Haidong Liu, Isabel Schultz, Wenhao Wu, D. G. Naugle, I. Lyuksyutov Fabrication of ordered nanowire-nanotube hybrid arrays embedded in porous anodic aluminum oxide (AAO) membranes is demonstrated. Arrays of TiO$_{2}$ nanotubes were first deposited into the pores of AAO membranes by an electroless sol-gel technique. For subsequent electrochemical deposition of Co nanowires into the TiO$_{2}$ nanotubes, a thick Au layer was first evaporated on one surface of the membrane to serve as the cathode. Co nanowires were then electrochemically deposited into the TiO$_{2}$ nanotubes through the other surface to form the hybrid nanowire-nanotube arrays. SEM and TEM measurements showed a high Co nanowire filling factor and a clean interface between the Co nanowires and the TiO$_{2}$ nanotubes. The TiO$_{2}$ nanotubes were found to be composed of nanometer sized TiO$_{2}$ crystals, while the Co nanowires were polycrystalline with Co crystal size comparable to the nanowire diameter. This technique can be extended to the fabrication of hybrid arrays of various materials. This work was supported by DOE No. DE-FG02-07ER46450, NSF No. DMR-0606529, and the Robert A. Welch Foundation A-0514. [Preview Abstract] |
Monday, March 16, 2009 10:00AM - 10:12AM |
A11.00011: First Observation of Quantum Size Effects in Metal Films on Insulator Hawoong Hong, Aaron Gray, Ruqing Xu, T.-C. Chiang Quantum size effects of metal films have been drawing lot of attention among surface science community. These quantum size effects have been reported for metal films on semiconductors and metals. Here the first observation of the quantum size effects in metal films on insulators will be reported. Pb thin films, which exhibit the most dramatic quantum-size-effects were chosen again for this effort. Sapphire (001) substrates were used after they were annealed at 1500 $^{o}$C in air and cleaned in a UHV chamber by heating. X-ray diffraction was measured during and after the deposition-annealing processes. Most of the structural aspects observed with Pb on Si(111) also appeared in Pb films on sapphire. The preferred island heights (or magic heights) appeared as 7, 10, 12, 14 layers from the preliminary analysis. This contrasts the magic heights on the Si substrates (5, 7, 9,{\ldots}). This difference is coming from the phase shifts of confined electrons through the different interfaces. This magic selection of island-heights stays quite strong even at elevated temperatures up to 200$\sim $300 $^{o}$C. Time resolved 3-D reciprocal space mapping also showed very strong ordering between islands during deposition and annealing of the films. [Preview Abstract] |
Monday, March 16, 2009 10:12AM - 10:24AM |
A11.00012: Surface Dislocation of Al Films on Ag(111) Bo Xu, Erkuang Zhu, Chao Lu, Yongjun Tian Ordered dislocation structures of metal surfaces are of particularly interests because they can provide templates for building nanostructures with novel electronic, magnetic, and catalytic properties. Here we report two dislocation structures formed for Al on Ag(111). Depending on substrate temperate, Al films demonstrate distinct surface structures. At room temperature, Al nanocrystals with the (111) orientation are formed. At 500 K, a herringbone reconstruction, similar with the well known Au(111) reconstruction surface, is formed, while at 600K, a trigonal reconstruction surface is formed. Molecular self assembly processes on these surfaces are investigated. [Preview Abstract] |
Monday, March 16, 2009 10:24AM - 10:36AM |
A11.00013: Theoretical Analysis of Equilibrium Surface Segregation in Ternary III-V and II-VI Semiconductor Nanostructures Sumeet Pandey, Tejinder Singh, Dimitrios Maroudas We present an atomic-scale analysis of equilibrium surface segregation in ternary compound (III-V and II-VI) semiconductor nanostructures. The analysis is based on a computational scheme for compositional and structural relaxation that combines Monte Carlo with conjugate-gradient methods according to properly modified/extended parameterizations of the valence-force-field (VFF) description; the VFF parameterizations employed in the analysis are validated by comparison with first-principles density functional theory calculations. We report equilibrium concentration distributions in slabs of In$_{x}$Ga$_{1-x}$As and ZnSe$_{1-x}$S$_{x}$ as a function of composition, x, slab thickness, and slab surface crystallographic orientation, as well as in In$_{x}$Ga$_{1-x}$As and ZnSe$_{1-x}$S$_{x}$ nanocrystals with well-defined surface facets as a function of x and nanocrystal size. The results are discussed in the context of synthesis of core/shell structures of ternary compound semiconductor nanocrystals for increased quantum-dot photoluminescence efficiency. [Preview Abstract] |
Monday, March 16, 2009 10:36AM - 10:48AM |
A11.00014: Growth and optical properties of highly oriented ZnSSe alloy nanowires Sui Kong Hark, Yao Liang ZnS, ZnSe and their alloys are important semiconductors for optical applications in the UV-blue spectral region. Nanowires, nanobelts and nanotubes of ZnS and ZnSe, but rarely their alloys, had been synthesized, typically as a random, inhomogeneous assembly. For future basic studies and applications, it is necessary to control the orientation and composition of the nanowires. We have grown ZnSSe alloy nanowires epitaxially on GaAs substrates by metal-organic chemical vapor deposition. Their orientation was adjusted by changing the crystallographic orientation of the substrate. Through controlled alloying, we have also achieved band gap engineering. The nanowires were characterized by SEM, HRTEM and XRD. Their optical properties were studied by Raman, cathodoluminescence and photoluminescence spectroscopy. In addition to the nanowires, the growth conditions and optical properties of ZnSSe alloy nano-tetrapods were studied. [Preview Abstract] |
Session A12: Directed Organization of Nanostructured Films
Sponsoring Units: DMP DCMPChair: Ray Phaneuf, University of Maryland
Room: 308
Monday, March 16, 2009 8:00AM - 8:36AM |
A12.00001: Controlled Synthesis of Functional Nanostructures Invited Speaker: |
Monday, March 16, 2009 8:36AM - 8:48AM |
A12.00002: Kinetic Monte Carlo Simulations of Nanostructure Evolution During Unstable Growth on Patterned GaAs(001) Chuan-Fu Lin, Krista Cosert, Ajmi Hammouda, Hung-Chih Kan, Ray Phaneuf We present results of kinetic Monte Carlo simulations, which include a diffusion barrier, lateral atom interaction energy, and Ehrlich-Schwoebel barrier to investigate unstable growth for comparison with our observations on patterned GaAs(001) surfaces at typical growth conditions [1-3]. Our results show a profound change in the mode by which an initial lithographic pattern evolves during growth, with growth mounds dominating at low temperatures and island nucleation and growth at higher temperatures. We describe the use of height-height correlation maps as a tool to facilitate the statistical characterization of the evolution of periodic patterns during growth, and correlate peaks in the maps with the change in growth mode with temperature. \\[0pt] [1] T. Tadayyon-Eslami et al., \textit{PRL} \textbf{97}, 126101 (2006) \\[0pt] [2] H.-C. Kan et al., \textit{PRB} \textbf{73}, 195410 (2006) \\[0pt] [3] H.-C. Kan et al., \textit{PRL} \textbf{92}, 146101 (2004) [Preview Abstract] |
Monday, March 16, 2009 8:48AM - 9:00AM |
A12.00003: Nanopatterning as a Probe of Unstable Growth on GaAs(001) Krista Cosert, Chuan-Fu Lin, Ajmi Hammouda, Hung-Chih Kan, Kanakaraju Subrumaniam, Chris Richardson, Ray Phaneuf We report on observations of unstable growth on nanopatterned GaAs(001) surfaces. For growth at 500$^{o}$C, 1 ML/sec and an As$_{2}$/Ga beam equivalent pressure ratio of 10:1, we find that grooves oriented at right angles to [110] produce a build up of ridges of GaAs at the upper edges, while for grooves oriented at right angles to [1\underline {1}0] no ridges form; instead cusps evolve at the bottoms of such grooves [1]. The cusp-forming grooves show a pronounced initial amplification of depth during growth which changes with length/width ratio, and become more narrow. The ridge-forming grooves instead broaden during growth. We compare these experimental observations with kinetic Monte Carlo simulations in which a small anisotropic Ehrlich-Schwoebel barrier is included. [1] T. Tadayyon-Eslami, H.-C. Kan, L. C. Calhoun and R. J. Phaneuf, \textit{Phys. Rev. Lett. }\textbf{97}, 126101 (2006) [Preview Abstract] |
Monday, March 16, 2009 9:00AM - 9:12AM |
A12.00004: Optimization of air-assisted CVD growth of vertically-aligned ZnO nanowires, guided by structural analysis using X-ray scattering Jong G. Ok, A. John Hart ZnO nanowires (ZNWs) are of significant interest for applications ranging from optical sensors to vibrational energy harvesters, due to properties including UV photoluminescence and piezoelectricity. We have studied low-pressure growth of ZNWs using a vapor transport method in air flowing within a tube furnace, giving vertically-aligned ZNW arrays on sapphire substrates seeded by Au catalysts. The growth rate and the average length of ZNWs depend on the flow rate of air and the total growth time, while multiple parameters such as catalyst thickness, pressure, and temperature also interdependently affect the ZNW characteristics. Grazing incidence small-angle X-ray scattering (GI-SAXS) measurements enable non-destructive quantification of ZNW diameter and alignment. By fitting GI-SAXS images using analytical models of the array as a population of solid cylinders having a Gaussian diameter distribution, we establish precise relationships between the structural characteristics and the growth conditions; for example, we determine rates of radial growth and size distribution broadening in comparison to axial growth. Control of the temperature gradient within the furnace also enables growth of well-aligned arrays at substrate temperatures as low as 600 $^{\circ}$C. [Preview Abstract] |
Monday, March 16, 2009 9:12AM - 9:24AM |
A12.00005: Influence of impurities on phase transition in quasi-one-dimensional nanowires on Si surface Geunseop Lee, Woosang Lee, Hyungjoon Shim, Sang-Yong Yu, Ja-Yong Koo We investigated using low-energy electron diffraction the influence of impurity doping on the structural phase transition in an array of quasi-one dimensional In nanowires on Si(111). A clean Si(111)4$\times$1-In surface, in its pristine form, undergoes a structural phase transition into a 8$\times$2 phase below 120 K. Introducing various impurities (hydrogen, oxygen, and alkali metals) on the surface at room temperature was found to affect the 4$\times$1-to-8$\times$2 structural phase transition by changing the transition temperature (T$_c$). Adsorption of the two types of the gases affected the transition in opposite ways: hydrogen adsorption lowered the T$_c$, whereas oxygen adsorption raised the T$_c$. Dosing of different alkali metals (Na, K, and Li) all decreased the T$_c$. Usually, impurities are expected to suppress the phase transition into the symmetry-broken phase (the low-temperature phase) by acting as random fluctuations in structure. In this sense, the increase in T$_c$ by the oxygen adsorption is an exceptional case enhancing the phase transition. Possible mechanisms leading to different influences of the various impurities on the structural phase transition of this In/Si(111) will be discussed. [Preview Abstract] |
Monday, March 16, 2009 9:24AM - 9:36AM |
A12.00006: Numerical simulations of VLS heteroepitaxial nanowire growth Vivek Shenoy, Klaus Schwarz, Jerry Tersoff Nanowires are particularly attractive for designing heterostructures, as effective radial strain relaxation allows heterostructures with a wider range of material combinations. The electrical, optical, and thermal properties of the nanowire are highly dependent on the accurate control of the locations and thicknesses of such heterostructures. However, in the case of nanowire growth from a metal seed particle, the composition of the seed particle will vary for growth of different materials due to alloying, which may cause problems in controlling interface abruptness. Also, recent experiments have shown that in many cases, growth instabilities do not allow for the formation of nanowires with desired morphology and material combinations. We have developed a continuum model for the growth of heteroepitaxial nanowires, and we use it to study the factors that control interface abruptness and instabilities during growth. Our model includes the following features that are critical for capturing the composition profiles in nanowires: 1) the differences of the attachment rates of the alloy components at the catalyst-wire interface, 2) the possibility of a miscibility gap in the alloy phases of the catalyst and the nanowire, 3) composition dependence of the surface energies of the nanowire and nanowire-catalyst interface and 4) anisotropy in surface energies leading to faceted morphologies. [Preview Abstract] |
Monday, March 16, 2009 9:36AM - 9:48AM |
A12.00007: Three Dimensional, Single-crystal, Oxide NANOFENCES for Epitaxial Growth of Electronic, Magnetic or Electromagnetic Nanoscale-Devices Amit Goyal, Sung-Hun Wee, Karren More, Eliot Specht A unique, three-dimensional (3D), single-crystal, MgO, NANOFENCE comprised of single crystal MgO nanowire units was synthesized via epitaxial growth on (100) SrTiO3 substrates. Individual single crystal MgO nanowire units comprising the nanofence were observed to have high aspect ratios with small diameters of 10-20 nm and long lengths from 100 nm up 1 $\mu $m. X-ray diffraction shows that the 3D MgO nanofence has an epitaxial relation with (100) SrTiO3 substrates with only a single {\{}100{\}}$<$100$>$ orientation and with full-width-half-maximum values of (200) $\omega $-scan and (110) $\phi $-scan with 4.5o and 5.5o, respectively. Such nanofences offer a single crystal, 3D nanotemplate for epitaxial growth of wide-ranging, 3D, electronic, magnetic and electromagnetic nanodevices. [Preview Abstract] |
Monday, March 16, 2009 9:48AM - 10:24AM |
A12.00008: Hierarchical Assembly of Epitaxial Quantum Dot Nanostructures on Templated Substrates Invited Speaker: Using the focused ion beam (FIB), we have modified the local topography and chemistry of Si(100) surfaces, and demonstrated control of the geometry, size, location and proximity of epitaxial Ge(Si) quantum dot (QD) nanostructures which are nucleated on these templated surfaces. We show how QDs can be located with a precision $\sim $ 10 nm using local Ga$^{+}$ FIB doses $\sim $ 10$^{14}$ cm$^{-2}$, and how QD size and morphology can be modified by local surface chemistry. We further describe how growth kinetics can control formation of more complex nanostructures with internal length scales bridging the $\sim $ 10 nm dimensions necessary for application to potential nanoelectronic device architectures and dimensions that are accessible through external lithography. In particular, we describe the self assembly of ``quantum dot molecule'' (QDM) Ge$_{x}$Si$_{1-x}$ nanostructures where a four-fold QD structures form around shallow strain relieving pits. Positional control of these QDMs using external lithographic templating allows formation of hierarchically assembled systems with length scales ranging from $\sim $ 10 nm in QD size and proximity, through the $\sim $ 100 nm dimensions of the QDM, to the micro/macro-scopic dimensions accessible with external lithography. We also describe methods for electronic and magnetic functionalization of these nanostructures by separation of ion species from alloy liquid metal sources in a mass selecting FIB column. This allows generation of ion beams comprising electronically non-invasive species for nanoscale surface templating (e.g. Si, Ge), electronic doping (e.g. As, B), or spin doping (e.g. Mn). Application of such structures to potential novel nanoelectronic device structures will be discussed. This work is done in collaboration with J. Floro, J. Graham, M. Gherasimova, J. Thorp (UVa), F. Ross (IBM), A. Portavoce (CNRS), M. Kammler (U. Duisburg) and J. Gray (U. Pittsburgh). [Preview Abstract] |
Monday, March 16, 2009 10:24AM - 10:36AM |
A12.00009: Fabrication Methods for Positioning of Quantum Dots Rebecca Kramer, Rupert Oulton, Volker Sorger, Nitipat Pholchai, Xiang Zhang Quantum dot positioning is highly useful in terms of integrating nanoemitters into nanostructures, such as nanocavities and quantum dot waveguides. Demonstration of control over the positioning of quantum dots has proven difficult, and consequently construction of single-photon emitting systems has been hindered. We report the ability to reliably position nanoscale functional objects, specifically quantum dots, within a well-defined location. Programmed assembly of DNA linked quantum dots on both gold and silver substrates is obtained by Electron Beam Lithography patterning and a series of surface chemical functionalizations. A single quantum dot was successfully positioned within 100 nm of the desired location in 36 percent of the experiments. Furthermore, the method was completely reproducible within 500 nm accuracy. This method has the potential to functionalize quantum dots in even smaller pattern sizes. [Preview Abstract] |
Monday, March 16, 2009 10:36AM - 10:48AM |
A12.00010: Stress-driven self-assembly of Si-based nanomembranes for on-chip applications Francesca Cavallo, Rudeesan Songmuang, Yongfeng Mei, Armando Rastelli, Oliver Schmidt A new field of Si technology based on transferable and engineered nanomembranes has developed with the realization of the fact that properties of bulk Si are preserved in nm-thin layers released from the substrate surface. We demonstrate the ability to pattern Si-based films with nano- scale features, and fold them into a predetermined 3D configuration by finely tuning the strain distribution in the membranes by well-established deposition processes, i.e., MBE, PVD, and thermal oxidation. Our major contributions are the fabrication of integrated microtube resistors based on Si:B/SiGe:B tubes; the use of the Ge condensation technique to tailor the strain distribution in SiGe films on insulator; the manufacturing of fully scalable and CMOS compatible all- semiconductor and hybrid tubes; the fabrication of linear and circular networks formed by interconnected wrinkled structures; the experimental demonstration of light emission from Ge and Si nanoparticles integrated in a tube wall; the observation and investigation of the waveguiding effect along the axis of SiOx/Si tubes. [Preview Abstract] |
Monday, March 16, 2009 10:48AM - 11:00AM |
A12.00011: Isotropic and anisotropic strain-induced self-assembled oxide nanostructures Marta Gibert, Patricia Abellan, Alessandro Benedetti, Felip Sandiumenge, Teresa Puig, Xavier Obradors The apparition of new functionalities based on size- and shape-dependent properties requires strategies for the formation of well-defined structures at nanometric scale. We present a bottom-up low-cost chemically-derived methodology based on the control of strain and surface energies anisotropies in CeO2/LAO system to tune the lateral aspect ratio, orientation and kinetics of interfacial oxide nanostructures. Self-organized uniform square-based nanopyramids form under isotropic strain [1]. In contrast, highly elongated nanostructures (long/short axis $\sim $20) grow induced by biaxial anisotropic strain and anisotropic surface energies. Island's distinct crystallographic orientation is the clue of their differentiated shape, and also influences their distinct evolution. The kinetically-limited coarsening of isotropic nanodots contrasts with the ultrafast kinetics of anisotropic islands. Experimental analyses are based on AFM, TEM, XRD and RHEED, and simulations based on a thermodynamic model enables us to confirm the equilibrium shape of each sort of island's shape in relation to its misfit strain and surface characteristics. [1] Gibert, M. et al., \textit{Adv.Materials} \textbf{19} (22), 3937 (2007). [Preview Abstract] |
Session A13: Focus Session: Metropolis Thesis Prize and Multiscale Modeling
Sponsoring Units: DCOMPChair: Jorge Jose, SUNY Buffalo
Room: 309
Monday, March 16, 2009 8:00AM - 8:12AM |
A13.00001: Flexibility and Direction Reversal in Flapping Locomotion Saverio Spagnolie, Michael Shelley In order to better understand the role of flexibility in the flapping of wings and fins in Nature, experimentalists at NYU have studied a heaving foil with passive pitching. We analyze this system numerically, having constructed a high-order accurate numerical scheme to solve the full Navier-Stokes equations in two-dimensions to study the dynamics. We are able to reproduce qualitatively the results of the experiments: by increasing the flapping frequency, we find regions of improved performance when compared to a rigid wing, regions of under-performance, and a bi-stable regime where the flapping wing can move horizontally in either direction. The numerical simulations have led to predictions of other modes of flapping locomotion, which have subsequently been observed in experiments. We also find that a symmetry breaking transition to forward flapping flight, as observed in experiments of a heaving foil with no pitching, may be directed with only very slight flexibility. [Preview Abstract] |
Monday, March 16, 2009 8:12AM - 8:24AM |
A13.00002: Modeling the combined effect of surface roughness and shear rate on slip flow of simple fluids Anoosheh Niavarani, Nikolai Priezjev Molecular dynamics (MD) and continuum simulations are carried out to investigate the combined effect of shear rate and surface roughness on interfacial slip in simple fluids. For weak wall-fluid interaction energy, the nonlinear shear rate dependence of the slip length in a flow past atomically flat surfaces is obtained from MD simulations. Both the magnitude of the slip length and the slope of its rate-dependence are significantly reduced in the presence of periodic surface roughness. Continuum simulations are used to reproduce the behavior of the effective slip length in a flow over periodically corrugated surface at low shear rates. The continuum analysis includes the functional form of the slip length vs. local shear rate computed from MD simulations. The discrepancy between MD and continuum results at higher shear rates is explained by examination of the local velocity profiles and pressure distribution along the wavy surface. [Preview Abstract] |
Monday, March 16, 2009 8:24AM - 8:36AM |
A13.00003: KMC simulations in 3+1 dimensions and the effects of attachment probabilities and potential gradients on island morphologies Christopher Fleck, Judith Yang, Alan McGaughey, Jun Ren Thin film growth and nano-oxidation have received significant attention lately, especially given the interesting nature of Cu$_{2}$O growth. Our long-term vision is for a comprehensive, fundamental understanding of a gas-surface reaction via coordinated multi-scale theoretical and in situ experimental efforts. The link between the theoretical and experimental efforts is our kinetic Monte Carlo (kMC) code that simulates general behavior of the irreversible nucleation and growth of epitaxial islands. This simulation was originally a versatile 2+1 dimensional kMC code (Thin Film Oxidation or TFOx) that considered a wide range of elementary steps, including deposition, adsorption, dissociation of gas molecules, surface diffusion, aggregation, desorption, and substrate-mediated indirect interactions between static adatoms. Recently, TFOx has been extended to a 3+1 dimensional kMC code composed of a C++ console program and Python GUI, such that parameterized testing, parallel execution, and 3D growth capabilities are feasible. Emphasis has been placed on the affects of the potential gradient, multilayer nucleation and sticking parameter on the 3D island morphology. [Preview Abstract] |
Monday, March 16, 2009 8:36AM - 9:12AM |
A13.00004: Nicholas Metropolis Award for Outstanding Doctoral Thesis Work in Computational Physics Talk: Understanding Nano-scale Electronic Systems via Large-scale Computation Invited Speaker: Nano-scale physical phenomena and processes, especially those in electronics, have drawn great attention in the past decade. Experiments have shown that electronic and transport properties of functionalized carbon nanotubes are sensitive to adsorption of gas molecules such as H2, NO2, and NH3. Similar measurements have also been performed to study adsorption of proteins on other semiconductor nano-wires. These experiments suggest that nano-scale systems can be useful for making future chemical and biological sensors. Aiming to understand the physical mechanisms underlying and governing property changes at nano-scale, we start off by investigating, via first-principles method, the electronic structure of Pd-CNT before and after hydrogen adsorption, and continue with coherent electronic transport using non-equilibrium Green’s function techniques combined with density functional theory. Once our results are fully analyzed they can be used to interpret and understand experimental data, with a few difficult issues to be addressed. Finally, we discuss a newly developed multi-scale computing architecture, OPAL, that coordinates simultaneous execution of multiple codes. Inspired by the capabilities of this computing framework, we present a scenario of future modeling and simulation of multi-scale, multi-physical processes. [Preview Abstract] |
Monday, March 16, 2009 9:12AM - 9:24AM |
A13.00005: Role of Adatom Relaxations in Computing Lattice-gas Energies: Multisite Interactions Rajesh Sathiyanarayanan, T. L. Einstein In simple lattice-gas models, only nearest-neighbor pair interactions are used to model adatom interactions. However, multisite interactions, such as trios and quartos, are necessary to understand certain surface properties like the orientation dependence of step stiffness and the equilibrium shape of islands. Strong multisite interactions are found to be present on a variety of metallic surfaces. Unlike pair interactions, the relaxations of adatoms in a multisite interaction are not along bond directions. Hence, these adatoms can shift significantly from their high-symmetry positions, making multisite interactions more sensitive to relaxations. Using VASP calculations, we showed that trios are very sensitive to lateral adatom relaxations on Pt(111) and Cu(100)\footnote[2]{Rajesh Sathiyanarayanan \textit{et al.}, Surf. Sci. \textbf{602} (2008) 1243.}. Our recent calculations on Cu(110) indicate that in addition to trios, quartos also undergo a big change due to adatom relaxations. Such findings severely limit the effectiveness of lattice-gas models in characterizing surface interactions. We discuss alternate approaches to this problem. [Preview Abstract] |
Monday, March 16, 2009 9:24AM - 9:36AM |
A13.00006: Rapid and Accurate Estimates of Alloy Phase Diagrams for Design and Assessment Teck Tan, Duane Johnson Based on first-principles cluster expansion (CE), we obtain rapid but accurate assessments of alloy T vs c phase diagrams from a mean-field theory that conserves sum rules over pair correlations. Such conserving mean-field theories are less complicated than the popular cluster variation method, and better reproduce the Monte Carlo (MC) phase boundaries and T$_{c}$ for the nearest-neighbor Ising model [1]. The free-energy f(T,c) is a simple analytic expression and its value at fixed T or c is obtained by solving a set of n non-linear coupled equations, where n is determined by the number of sublattices in the groundstate structure and the range of pair correlations included. While MC is ``exact,'' conserving mean-field theories are 10 to 10$^{3}$ faster, allowing for rapid phase diagram construction, dramatically saving computation time. We have generalized the method to account for multibody interactions to enable phase diagram calculations via first-principles CE, and its accuracy is showed vis-\`a-vis exact MC for several alloy systems. The method is included in our Thermodynamic ToolKit (TTK), available for general use in 2009. [1] V. I. Tokar, Comput. Mater. Sci.\textbf{ 8} (1997), p.8 [Preview Abstract] |
Monday, March 16, 2009 9:36AM - 9:48AM |
A13.00007: Chemistry effects on dislocation mobility in refractory bcc metals Nicholas Kioussis, Zhengzheng Chen, Gang Lu, Nasr Ghoniem Using a novel concurrent multiscale approach we demonstrate that the \textit{local environment} of transition-metal solutes in refractory bcc metals has a large effect on the mobility and slip paths of dislocation. The results reveal that solid solutes or nano-clusters of different geometries may lead to solid-solution hardening (SSH) or softening (SSS), in agreement with experiment, including spontaneous dislocation glide and activation of new slip planes. The underlying electronic mechanism is also studied by the multiscale approach. Solutes nano-cluster can affect Peierls potential surface (PPS) dramatically. The results indicate that it is the change of the anisotropy of the lattice resistance induced by solutes that result in the different behavior of the dislocation according to the different geometries of solutes nano-clusters. [Preview Abstract] |
Monday, March 16, 2009 9:48AM - 10:00AM |
A13.00008: Computational studies of thermal evolutions of extended interstitial defects in silicon. Hyoungki Park, John W. Wilkins Annealing induces the nucleation of extended defect clusters in silicon and their evolution, where clusters grow by capturing or interchanging interstitials, and change their crystallographic structure in order to minimize the formation energy. Extensive molecular dynamics (MD) simulations and first-principle nudged elastic band (NEB) simulations explore the thermal transitions from one structure to another of three energetically competing extended interstitial defects: two rod-like defects $\{311\}$ and $\{111\}$, and Frank dislocation loop. MD simulations capture critical sequences of atomistic processes during transitions from $\{311\}$ and $\{111\}$ defects to Frank loops as their atomic configurations and habit planes change, and massively parallelized NEB simulations within the local density approximation reveal the energetics of reaction barriers. [Preview Abstract] |
Monday, March 16, 2009 10:00AM - 10:12AM |
A13.00009: Multiscale Modeling of Catalysis and its Application to Hydrogen Production through the Water Gas Shift Reaction on Nanoparticles Altaf Karim, James T. Muckerman We describe a density functional kinetic Monte Carlo approach enabling us to study and simulate the steady-state condition of the water gas shift (WGS) reaction on Cu and Au nano-particles supported on ZnO(0001) surfaces. We have adopted a multiscale modeling paradigm in which density functional theory can be used to determine the behavior of systems at much larger length and time scales by coupling it with kinetic Monte Carlo methods. In the first step, density functional theory is used to obtain the energetics of the relevant atomistic processes of the WGS reaction on Cu and Au nanoparticles. Subsequently, the kinetic Monte Carlo method is employed, which accounts for the spatial distribution, fluctuations, and evolution of chemical species under steady-state conditions. Our simulations show that, in agreement with experiments, the hydrogen production rate strongly depends on size and structure of the nanoparticles. [Preview Abstract] |
Monday, March 16, 2009 10:12AM - 10:24AM |
A13.00010: Preparation of nanoporous systems for the study of the mechanical properties of silica aerogels by Molecular Dynamics simulations John S. Rivas Murillo, Martina E. Bachlechner, Ever J. Barbero This presentation focuses on the application of the Molecular Dynamics technique to study the mechanical properties of silica aerogels through the simulation of a tension test. It covers multiple areas, including aspects related to the preparation of a well-relaxed nanoporous system from the expansion of an amorphous bulk sample and the influence of the initial configuration of the system on the final results of the simulated tension test. The results presented here will help to develop a more complete procedure to prepare a proper sample for the study of the mechanical properties of a nanoporous system by using Molecular Dynamics. Comparison of the simulation results and previously published experimental data is provided [Preview Abstract] |
Monday, March 16, 2009 10:24AM - 10:36AM |
A13.00011: A hierarchical DPD thermostat to avoid over and/or underdamping at long wavelengths in MD simulations Kevin Green, Colin Denniston, Martin Muser In this talk, we present a new approach to use dissipative particle dynamics as a thermostat in molecular dynamics simulations. The main idea is to have DPD act on groups of atoms so that damping can be tuned as a function of length scale. This allows one to achieve a quality factor of vibrations, which is barely wavelength dependent. The number of floating point operations per time step is orders of magnitude less for the new approach than for regular DPD or any other thermostat acting on individual particles. In addition, the method avoids both underdamping of natural and/or DPD dynamics at long wavelengths L and overdamping which is unavoidable at large L for Langevin or Nose-Hoover based thermostats. Thus correlation times for observables that live on long wavelengths L, are of order L, rather than of order L$^2$ as for conventional thermostats. [Preview Abstract] |
Monday, March 16, 2009 10:36AM - 10:48AM |
A13.00012: Determination of the ground state structures of binary alloys via global space group optimziation (GSGO) with no restrictions on composition:Al-Sc. Giancarlo Trimarchi, Arthur J. Freeman, Alex Zunger Here, we extend the GSGO evolutionary algorithm scheme to survey crystal structures of binary A-B systems {\em without} constraint on the A$_{p}$B$_{q}$ composition. At each generation of the randomly started evolutionary sequences, the formation energy convex hull for the actual population is determined. The search proceeds by replacing the structures farthest away from the convex hull with new ones produced via mating and mutation with no constraints on composition. As a test of this new procedure, we searched the ground state compounds of the Al-Sc alloy whose lattice types are not easily inferred from that of the Al and Sc constituents, respectively fcc and hcp solids. Repeated, independent evolutionary sequences with six and eight atoms in the supercell were performed yielding as ground states respectively B$8_{2}$, B2, and C$15$, and D$0_{19}$, B2, and L$1_{0}$, as known from experiment and previous {\em ab-initio} studies. This yields a synthesis of the final convex hull. [Preview Abstract] |
Monday, March 16, 2009 10:48AM - 11:00AM |
A13.00013: Non-crystalline state of silicon studied by multicanonical simulation combined with first-principles calculation Yoshihide Yoshimoto By combining multicanonical ensemble molecular dynamics and first-principles calculations, non-crystalline state of silicon is studied. This attempt contrast with quenching molecular dynamics simulations whose speed is usually by far quicker than that of experimental quenchings. To make the molecular dynamics simulation tractable, a model interatomic potential is used. The parameter, however, is determined by first-principles calculation so that the discrepancy between the first-principles interatomic potential and the model one is minimized on the typical configuration set of the multicanonical ensemble. Because multicanonical ensemble represents the whole thermodynamics of the system, the obtained model will conserve the thermodynamics to a maximum extent. (thermodynamic downfolding of an interatomic potential [1]) The transition between amorphous silicon and liquid silicon, and the density maximum of liquid silicon as a function of temperature will be discussed. (Silicon has similar structure to that of water) [1] Y. Yoshimoto, J. Chem. Phys., 125, 184103 (2006) [Preview Abstract] |
Session A14: Instabilities, Turbulence and Nonlinear Flows
Sponsoring Units: DFDChair: Guenter Ahlers, University of California, Santa Barbara
Room: 315
Monday, March 16, 2009 8:00AM - 8:12AM |
A14.00001: Search for the ``ultimate state" in turbulent Rayleigh-B\'enard convection for Rayleigh numbers up to $4\times 10^{13}$ and Prandtl numbers near 0.8. Guenter Ahlers, Denis Funfschilling, Eberhard Bodenschatz Measurements of the Nusselt number $Nu$ over the Rayleigh-number range $10^{10} < Ra < 4\times 10^{13}$ for N$_2$ (Prandtl number $Pr = 0.72$) and SF$_6$ ($Pr = 0.78$ to 0.82) are reported. They were made at pressures up to 15 bars and near-ambient temperatures for a cylindrical sample of height $L=2.2$ m and diameter $D = 1.1$ m in a new High-Pressure Convection Facility (HPCF) constructed at the Max Planck Institute for Dynamics and Self-Organization in G\"ottingen, Germany. The data can be represented well by a power law with an effective exponent of 0.31. They do not show the transition to an ``ultimate regime" reported by Chavanne et al. [Preview Abstract] |
Monday, March 16, 2009 8:12AM - 8:24AM |
A14.00002: Large-scale circulation and Nusselt number in turbulent rotating Rayleigh-B\'enard convection. Jin-Qiang Zhong, Richard Stevens, Herman Clercx, Detlef Lohse, Guenter Ahlers We present measurements of the large-scale circulation (LSC) and the Nusselt number $Nu$ of turbulent Rayleigh-B\'enard convection in a cylindrical cell of aspect ratio 1 and rotated about a vertical axis at a rate $\Omega$. The side-wall temperatures at eight equally spaced azimuthal positions in the horizontal mid- plane were fit to a cosine function that gave the azimuthal LSC orientation $\theta(t)$ ($t$ is the time), the temperature amplitude $\delta(t)$, and the rms amplitude $\delta T(t)$ of the fluctuations about the fits. The LSC precessed in an azimuthal direction opposite to that of the imposed rotation. The precession rate $\omega= d\theta/dt$ showed a sharp transition at a Rossby number $Ro^* \simeq 2.5$. As $\Omega$ increased, $<\delta(t)>_t$ decreased and $<\delta T(t)>_t$ increased beginning at $Ro^*$. At $Ro^*$ $Nu$ began to increase with increasing $\Omega$. At high $Ro$ $| \omega |$ was proportional to but much smaller than $\Omega$. [Preview Abstract] |
Monday, March 16, 2009 8:24AM - 8:36AM |
A14.00003: Geometry of turbulence: a stroll through 61,506 dimensions Predrag Cvitanovic, John F. Gibson, Jonathan Halcrow We propose to use a hierarchy of exact unstable invariant solutions of the Navier-Stokes equations -- corresponding to the recurrent coherent structures observed in experiments -- to construct a description of the spatio-temporally chaotic dynamics of turbulent fluid flows as a walk through the space of such structures. This description should allow us to obtain quantitative predictions of transport properties of fluid flows such as bulk flow rate and mean wall drag. [Preview Abstract] |
Monday, March 16, 2009 8:36AM - 8:48AM |
A14.00004: Lagrangian and Eulerian Turbulence: intermittency and Universality Luca Biferale We present the result of a high resolution numerical simulations of homogeneous and isotropic turbulence at $R_{\lambda} \sim 600$. We discuss a phenomenological bridge-relation able to capture intermittent fluctuations in inertial and viscous scales for both Eulerian and Lagrangian ensembles. [Preview Abstract] |
Monday, March 16, 2009 8:48AM - 9:00AM |
A14.00005: Large-eddy simulation of swirling reacting flows Marcel Ilie Turbulent, swirling flows are encountered frequently in various chemical engineering processes. In combustion processes swirling flames are of interest due to the fact that provide enhanced mixing and reduce the pollutants formation. The challenge in understanding turbulent swirling flows stems mainly from the complexity of the flow field which is subject to vortex breakdown, recirculation and flow instability. In general the flow instabilities arise at high swirl numbers and can be used to control the performance of combustors. In the present study a large-eddy simulation (LES) approach with Smagorinsky eddy viscosity subgrid scale model is used to predict the swirling flame. The conserved scalar mixture fraction-based thermo-chemical variables are described using the steady laminar flamelet model. The present study shows that LES together with a laminar flamelet model provides a good prediction of the structure of turbulent swirling flames. Also LES captured very well the complex flame structures involving vortex breakdown which leads to swirl-induced recirculation zones, flow instability, and the occurrence of localized extinction. Also, the present study shows that the formation of an elongated recirculation (bluff-body stabilized) zone is strongly dependent on the swirl number and the ratio of momentum in the swirling annulus and central fuel jet. [Preview Abstract] |
Monday, March 16, 2009 9:00AM - 9:12AM |
A14.00006: Methods to Approach Velocity Data Reduction and Their Effects on Conformation Statistics in Viscoelastic Turbulent Channel Flows Gaurab Samanta, Antony Beris, Robert Handler, Kostas Housiadas Karhunen-Loeve (KL) analysis of DNS data of viscoelastic turbulent channel flows helps us to reveal more information on the time-dependent dynamics of viscoelastic modification of turbulence [Samanta et. al., J. Turbulence (\textit{in press}), 2008]. A selected set of KL modes can be used for a data reduction modeling of these flows. However, it is pertinent that verification be done against established DNS results. For this purpose, we did comparisons of velocity and conformations statistics and probability density functions (PDFs) of relevant quantities obtained from DNS and reconstructed fields using selected KL modes and time-dependent coefficients. While the velocity statistics show good agreement between results from DNS and KL reconstructions even with just hundreds of KL modes, tens of thousands of KL modes are required to adequately capture the trace of polymer conformation resulting from DNS. New modifications to KL method have therefore been attempted to account for the differences in conformation statistics. The applicability and impact of these new modified KL methods will be discussed in the perspective of data reduction modeling. [Preview Abstract] |
Monday, March 16, 2009 9:12AM - 9:24AM |
A14.00007: Study of Influence of Rapid Pressure in MHD Turbulence Saikishan Suryanarayanan, Aarthi Sekaran Turbulence, under the influence of magnetic field is characterized by anisotropy. Relatively limited work has been done in understanding and modeling magnetohydrodynamic (MHD) turbulence.The rapid distortion theory (RDT), which has been employed to study hydrodynamic turbulence, is a limiting case where the gradients of the mean velocity are very high compared to the gradients of the fluctuating field. When analyzed in a spectral framework, this leads to the independent evolution of each Fourier mode. RDT has been used to understand production and more importantly the ``rapid'' part of the pressure strain redistribution, as the other terms in the Reynolds stress evolution equation become negligible in the rapid distortion limit. Earlier work attempts to characterize the effect of the rapid pressure based on the geometry of the symmetric part of the mean velocity gradient tensor. This work deals with the application of RDT to MHD turbulence. The application of Elsasser variables reorganizes the MHD equations in a form similar to conventional Navier-Stokes. The current work is a numerical study of the Elsasser variable evolution equation in the rapid distortion limit and attempts to understand the role of the rapid magnetic pressure in the evolution of the Reynolds stresses for different mean distortions and magnetic fields. [Preview Abstract] |
Monday, March 16, 2009 9:24AM - 9:36AM |
A14.00008: Anisotropic Particles in Fluid Flow Monica Kishore, Nicholas T. Ouellette, Jerry Gollub Anisotropic particles are common in natural flows. In previous work [1] the dynamics of neutrally buoyant finite-sized spherical particles with Stokes numbers up to 0.08 were examined in 2D flows with Reynolds numbers of 72-220. Here, we extend this work to neutrally buoyant, high-aspect-ratio anisotropic particles of mm to cm length in a 2D cellular flow. The particle trajectories and orientations are tracked simultaneously with the underlying velocity field, which is measured using much smaller tracer particles. These methods allow us to compare the relative velocity and orientation of anisotropic particles to various features of the flow field. We find, for example, that the long axes of the particles preferentially align with the instantaneous direction of maximum compression, and that this alignment increases with particle aspect ratio. [1] N.T. Ouellette, P.J.J. O'Malley, and J.P. Gollub, Phys. Rev. Lett. 174504 (2008). [Preview Abstract] |
Monday, March 16, 2009 9:36AM - 9:48AM |
A14.00009: Statistics of preferential particle concentration in free-surface Jason Larkin, Walter Goldburg, Mahesh Bandi Particles floating on a turbulent surface of water cluster into temporally complex patterns. We experimentally study the statistics of this preferential particle concentration for various Reynolds numbers, for both transient and steady-state dynamics. The probability density function for particle concentration exhibits a power-law with an exponential cut-off. We will discuss our preliminary analysis as to how this distribution depends upon the Reynolds number and the spatial-scale $r$ at which the system is coarse-grained. [Preview Abstract] |
Monday, March 16, 2009 9:48AM - 10:00AM |
A14.00010: Vortex Street behind an Oscillating Wire on a Soap Film Aaron Meyer, Ildoo Kim, X.L. Wu A von K\"arm\"an vortex street, a periodic array of vortices behind a bluff body is normally characterized by a single frequency $f_{0}$ at which the vortices shed. In this study, von K\"arm\"an vortex streets are generated on a 2D soap film using a glass-covered metal wire in a static magnetic field. When the wire is driven with electric current to make an oscillatory motion with frequency $f_{e}$, transverse to the mean flow, vortices shed at a frequency $f'$ differs from $f_{0} $. It is seen that with oscillation, $f_{0}$ is suppressed, $f'/f_{e}$ becomes a rational number, and vortices are rearranged to form an exotic spatial structure. This ``frequency- locking'' phenomena show some features of the sine-circle map, but the relevancy to the physical system is not clear. When the amplitude of the oscillation is large enough, the system becomes chaotic. In this chaotic regime, the energy power spectrum resembles that of 2D decaying turbulence. [Preview Abstract] |
Monday, March 16, 2009 10:00AM - 10:12AM |
A14.00011: An Anomalous Behavior in Vortex Shedding in a Flowing Soap Film ILDOO KIM, X.L. Wu It is generally believed that von K\"arm\"an vortex street is characterized only by Reynolds number $Re=UD/\nu$, where $U$ is the mean flow speed, $D$ is the size of the body which generates the vortex street, and $\nu$ is the kinematic viscosity. In this study, we present experimental data in a flowing soap film showing that changing $U$ with fixed $D$ and changing $D$ with fixed $U$ are not equivalent to each other, suggesting that $Re$ alone is not sufficient to characterize vortex shedding by a bluff body. The velocity of eyes of the vortices relative to the mean flow, normalized by $U$, increases when we increase $D$, but decreases when we increase $U$. It is also found that the longitudinal spacing between the eyes is a linear function of $D$, but independent of $U$. [Preview Abstract] |
Monday, March 16, 2009 10:12AM - 10:24AM |
A14.00012: Rayleigh-Taylor Instability in Nonlinear Optics Shu Jia, Jason W. Fleischer We demonstrate, theoretically and experimentally, an all-optical Rayleigh-Taylor instability. By applying a polar (Madelung) transformation to the nonlinear Schr\"{o}dinger equation for paraxial beams, we identify fluid density with light intensity and fluid velocity with the gradient of the optical phase. Pressure is obtained by using a self-defocusing nonlinearity in a photorefractive crystal, while acceleration is created by imposing a refractive index gradient. In this way, we are able to control the effective gravity, pressure, and input density ratio. The perturbed interface at the output is then studied as functions of these parameters. Observations of the characteristic spatial period show excellent agreement with analytical calculations from perturbation theory. In this case, wave diffraction, rather than viscosity or surface tension, sets the scale for long-wave growth. Further, we show that compressibility effects are important and demonstrate that care must be taken regarding shock-wave formation. The results hold for any Schr\"{o}dinger fluid, e.g. superfluids and quantum plasma, and lay the foundation for a variety of fluid-inspired instabilities in nonlinear optics. [Preview Abstract] |
Monday, March 16, 2009 10:24AM - 10:36AM |
A14.00013: Dispersive shock waves with negative pressure Wenjie Wan, Dmitri Dylov, Christopher Barsi , Jason Fleischer Dispersive shock waves (DSWs) arise from nonlinear wave breaking and mode dispersion and are a fundamental type of fluid behavior. In normal fluid systems, the pressure is positive and repulsive, so that the underlying particles resist compression. Examples include water, plasma, and optical beams with self-defocusing nonlinearity. However, there are systems in which the interactions are attractive, resulting in an effectively negative pressure. Here, we demonstrate that dispersive shock waves can arise in these negative-pressure systems by considering the equivalent optical problem with self-focusing nonlinearity. Using partially-coherent light, to prevent the competition of modulation instability, we experimentally observe DSWs formed in a self-compressive beam in a photorefractive crystal. We characterize the nonlinear speed and profile of the DSWs and show that statistical de-phasing by the incoherent beam causes an effective Landau damping of the waves. Observations are supported both by analytic theory and numerical simulation. [Preview Abstract] |
Monday, March 16, 2009 10:36AM - 10:48AM |
A14.00014: Convective instability in pipe flow through a sudden expansion James Seddon Flow through a sudden expansion in a pipe has been the subject of a lot of recent scientific interest. The geometry occurs in many industrial processes, from heat exchangers to combustion chambers, and is closely related to the physiological problem of flow through a stenosis. The inlet flow from the upstream pipe is Poiseuille, which forms a central jet surrounded by a recirculating eddy in the expanded downstream pipe. Recently we showed that this kind of flow passes through a symmetry breaking bifurcation before the onset of both intermittent and fully periodic time-dependent effects. We have now investigated the intermittency in more detail and find that the flow becomes convectively unstable. A wave packet emerges from the laminar state and grows to a maximum size of several diameters before decaying. [Preview Abstract] |
Session A15: Soft Matter, Fluid Structure and Properties
Sponsoring Units: DFDChair: Wendy Zhang, University of Chicago
Room: 316
Monday, March 16, 2009 8:00AM - 8:12AM |
A15.00001: Impact of a viscous drop Wendy W. Zhang, Robert D. Schroll, Christophe Josserand, Stephane Zaleski Recent experiments [1] reveal that reducing the ambient air pressure entirely suppresses the splash generated by the impact of an oil drop at several m/s onto a dry smooth wall. Motivated by these observations, we simulate two types of drop impact: impact onto a smooth, dry solid wall and head-on collision of two identical liquid drops. In both cases we make the additional simplification that impact simply arrests the downward fall and redirects the liquid radially outwards in a thin, expanding sheet. It does not break the drop surface. Since experiments suggest that splash is created by airflow deforming the thin sheet, we focus on the time-evolution of the thin liquid sheet but restrict ourselves to the simpler situation of negligible airflow effects. In this regime, we find that the ejected sheet is always characterized by two different lengthscales. Surface tension controls the rim size. The thickness over the rest of the sheet is controlled by a different mechanism. Impact onto a solid surface creates a pancake whose thickness is controlled by viscous dissipation. Head-on collision creates a sheet that thins continuously with distance from the collision center. Its thickness is controlled by the kinematics of impact.\\[0pt][1] Stevens, Keim, Zhang \& Nagel, FC03 APS DFD meeting (2007) [Preview Abstract] |
Monday, March 16, 2009 8:12AM - 8:24AM |
A15.00002: Focused impact through layers of aqueous cornstarch solution Bin Liu, Jun Zhang, Michael Shelley A layer of aqueous cornstarch solution, when punched with a solid sphere, will create a thickened mass on the sphere that transmits the impact towards the bottom. As a consequence, the mass can leave an imprint on the bottom, if composed of a soft molding clay. The impact transmitted through the fluid layer is more localized for slower speeds of the sphere, giving rise to an imprint with sharper curvature. Our work shows that a layer of shear-thickening fluid may help to focus the impact rather than dissipate it when punched slowly enough. [Preview Abstract] |
Monday, March 16, 2009 8:24AM - 8:36AM |
A15.00003: Drop pinch-off of concentrated surfactant solutions in the lamellar phase Itai Cohen, Patrick Spicer, Marco Caggioni, John Savage Droplet pinch-off in air is a common phenomenon that occurs all around us. At the point of pinch-off, the drop radius shrinks to zero in a finite amount of time. The pressure exerted by the interface is inversely proportional to the minimum radius and becomes singular at Pinch-off. In Newtonian fluids, this finite time singularity gives rise to universal features in the pinch-off process that can be described by similarity solutions for the fluid air interface. In this talk I will address the question of how this process is altered when observed in concentrated surfactant solutions that are in the lamellar phase. Remarkably we find that pinch-off in these systems is a mix between universal and non-universal behavior. [Preview Abstract] |
Monday, March 16, 2009 8:36AM - 8:48AM |
A15.00004: Watching the Paint Dry: Dynamics of Drying in Porous Media Lei Xu, Simon Davies, Andrew Schofield, David Weitz What is the dynamics of drying in porous media? It has been difficult to visualize due to the non-transparency of the media. We study this phenomenon in an optical index matched colloidal system with confocal microscopy. We observe abrupt air invasions which result from the strong flow from menisci in large pores to menisci in small pores. The size and structure of the air invasions are in accord with 3D invasion percolation. By varying the particle size and contact angle we unambiguously demonstrate that capillary pressure dominates the drying process. [Preview Abstract] |
Monday, March 16, 2009 8:48AM - 9:00AM |
A15.00005: Monitoring Three-dimensional Fluid Configurations in Porous Media Amber Krummel, David Weitz The spatial and time resolution of confocal microscopy affords the ability to collect three-dimensional images during the course of two-phase flow experiments. We fully instrument the microscope with precise flow and pressure measurements, such that we can begin to understand the origins and consequences of the three-dimensional fluid configurations that evolve in the sample. The porous media used in this work is composed of slightly sintered, borosilicate glass beads that are 150 microns in diameter. [Preview Abstract] |
Monday, March 16, 2009 9:00AM - 9:12AM |
A15.00006: Shear banding fluids in microchannels: high shear rheology, slippage and Poiseuille flow instability Philippe Nghe, Guillaume Degre, Patrick Tabeling, Armand Ajdari We characterize by Particle Image Velocimetry the Poiseuille flow a semi-dilute solution of wormlike micelles (a CTAB and sodium nitrate aqueous solution) in pressure resistant microchannels. At low shear rates, we observe a parabolic profile. Increasing the pressure driving the flow, the fluid separates into two phases above a critical shear rate at the wall. This is the so called shear-banding regime. Deducing the non-linear rheology from the velocity profiles by a local calculation, we are able to measure the stress versus shear rate curve at least one order of magnitude above the dynamical range attainable in Couette geometries, independently from the slippage, revealing a strongly shear-thinning structure. In addition, by extrapolation of the velocity profiles to the wall position, we measure an absence of slippage at the wall. Looking into more details to the increase in velocity fluctuations in the downstream direction, we characterize a supercritical instability in this shear-banded Poiseuille flow, localized at the interface between the two phases with a wavelength comparable to the confining dimension. [Preview Abstract] |
Monday, March 16, 2009 9:12AM - 9:24AM |
A15.00007: Nonlinear Dynamics in Viscoelastic Jets Trushant Majmudar, Matthieu Varagnat, Gareth McKinley Instabilities in free surface continuous jets of non-Newtonian fluids, although relevant for many industrial processes, remain poorly understood in terms of fundamental fluid dynamics. Inviscid, and viscous Newtonian jets have been studied in considerable detail, both theoretically and experimentally. Instability in viscous jets leads to regular periodic coiling of the jet, which exhibits a non-trivial frequency dependence with the height of the fall. Here we present a systematic study of the effect of viscoelasticity on the dynamics of continuous jets of worm-like micellar surfactant solutions of varying viscosities and elasticities. We observe complex nonlinear spatio-temporal dynamics of the jet, and uncover a transition from periodic to quasi-periodic to a multi-frequency, broad-spectrum dynamics. Beyond this regime, the jet dynamics smoothly crosses over to exhibit the ``leaping shampoo'' or the Kaye effect. We examine different dynamical regimes in terms of scaling variables, which depend on the geometry (dimensionless height), kinematics (dimensionless flow rate), and the fluid properties (elasto-gravity number) and present a regime map of the dynamics of the jet in terms of these dimensionless variables. [Preview Abstract] |
Monday, March 16, 2009 9:24AM - 9:36AM |
A15.00008: Relating shear banding and orientational order in wormlike micellar solutions Matthew Helgeson, Matthew Reichert, Eric Kaler, Norman Wagner Shear banding has been observed in a variety of complex fluids, including polymer solutions, colloidal suspensions and, most prominently, wormlike micelles (WLMs). However, accurate modeling of shear banding fluids remains a challenge, due to the inability to identify the mechanism(s) leading to banding. Using a novel approach that combines measurements of phase behavior, rheology, and spatially-resolved microstructure on model WLMs, we present the first complete study of local rheology and microstructure through the shear banding transition for model WLMs in the vicinity of an equilibrium isotropic-nematic transition (I-N). The rheology of such fluids is well-described by the Giesekus constitutive equation with incorporated stress diffusion, which allows simultaneous description of rheology, flow kinematics, and spatially-resolved microstructure under shear. The results show that shear banding coincides with a first-order, shear-induced transition to a paranematic state at critical values of micellar orientation and alignment, which can be related directly to a non-monotonic constitutive relation. Furthermore, the model allows for the construction of non-equilibrium state diagrams that elucidate a number of experimental observations in shear banding fluids. [Preview Abstract] |
Monday, March 16, 2009 9:36AM - 9:48AM |
A15.00009: Structure and Phase Behavior of Ion--Dipole Mixtures Wonki Roh, Erik Luijten It is well established that Coulombic interactions induce a liquid--liquid transition in ionic solutions. By contrast, the occurrence of phase separation driven by anisotropic dipolar interactions is still a matter of debate, with our recent simulation results excluding this phase separation for a large region of the temperature--density plane. These observations naturally lead to the question whether phase separation takes place in mixtures that contain ions as well as dipolar particles. Employing large- scale grand-canonical Monte Carlo simulations, we investigate four prototypical ion--dipole mixtures: \emph{ion-dominated} systems in which the dipole moment is either strong or weak, and \emph{dipole-dominated} systems with strong or weak dipolar strength. We focus on the low-temperature regime and search for phase separation by varying the chemical potentials of the ions as well as the dipolar particles. Depending on temperature and on the magnitude of the dipole moment, remarkable liquid structures are found that may have implications not only for the behavior of ion--dipole mixtures, but also for self-assembly in suspensions containing charged and dipoloar colloids. [Preview Abstract] |
Monday, March 16, 2009 9:48AM - 10:00AM |
A15.00010: Surface Layering Near Room Temperature in a Nonmetallic Liquid Sudeshna Chattopadhyay, Benjamin Stripe, Patrick Shively, Geunnadi Evmenenko, Pulak Dutta, Steven Ehrlich, Haiding Mo Oscillatory density profiles (layers) have been observed at the free surfaces of many liquid metals at and above room temperature [1]. A surface-layered state has been previously reported only in one dielectric liquid, tetrakis(2-ethylhexoxy)silane (TEHOS), and only at lower temperatures [2]. We have used x-ray reflectivity to study a molecular liquid, pentaphenyl trimethyl trisiloxane. Below T$\sim $ 267K (well above the freezing point for this liquid), density oscillations appear at the surface. This liquid has a higher $T_{c}$ ($\sim $1200K) than TEHOS ($\sim $950K), so that layers appear at $T/T_{c} \quad \approx $ 0.2 in both cases. Our results indicate that surface order is a universal phenomenon in both metallic and dielectric liquids, and that the underlying physics is likely to be the same since layers always appear at T$<\sim $0$.$2$T_{c}$ as theoretically predicted [3] \\[3pt] REFERENCES: \\[0pt] [1]. e.g. O. M. Magnussen \textit{et al}., Phys. Rev. Lett. \textbf{74}, 4444 (1995) \\[0pt] [2]. H. Mo et al. \textit{Phys. Rev. Lett.} \textbf{96}, 096107 (2006); \textit{Phys. Rev. B} 76, 024206 (2007) \\[0pt] [3]. e.g. E. Chac\'{o}n et al., Phys. Rev. Lett. \textbf{87}, 166101 (2001) [Preview Abstract] |
Monday, March 16, 2009 10:00AM - 10:12AM |
A15.00011: Structural effect of sugars on water Simcha Srebnik, Ravit Matza, Iliya Kusner, Yoav D. Livney The modulation of the structure of liquid water by solutes has tremendous consequences in numerous fields, particularly on the stability of proteins. However, the reasons for the differences in effects of similar solutes are still unclear. Recently, Livney and coworkers [1] found a strong relationship between the hydration layer of sugars and its effect on the phase transition of a model polymer, which may be explained by the strong interaction between water and sugars leading either to cooperative structuring of the water and thus to large hydration numbers, or disrupting water structure near the sugar, resulting in lower hydration. Using atomistic Monte Carlo simulation, we studied the compatibility of various sugars with an ideal tetrahedral water structure, as embodied in hexagonal ice. Our simulations suggest the following order of compatibility with ideal water structure: galactose $>$ glucose $>$ mannose. In agreement, experimental measurements of isentropic compressibility show the same order of hydration numbers and kosmotropic effect. A simple physical model of the binary system is used to shed further insight on the structuring effect of sugars on water. 1. Shpigelman, A.; Portnaya, I.; Ramon, O.; Livney, Y. D. J Polym Sci Part B: Polym Phys 2008, 46, 2307-2318. [Preview Abstract] |
Monday, March 16, 2009 10:12AM - 10:24AM |
A15.00012: Reconstructing the dynamical solvent structure around a model `hydrated electron' using inelastic x-ray scattering R. Coridan, G.H. Lai, N. Schmidt, P. Abbamonte, G.C.L. Wong, R. Godewat, S. Garde, M. Krisch, A.Q.R. Baron The structure and dynamics of water on femtosecond timescales is relevant to many topics in physical chemistry such as electron solvation. We computationally reconstruct the {\AA}-scale spatial and fs-scale temporal evolution of density fluctuations in water using high-resolution inelastic x-ray scattering (IXS). The imaginary part of density propagator $\chi $(q,$\omega )$ is directly extracted from the IXS data, and the real part recovered using Kramers-Kronig relations. The resultant complex-valued $\chi $(q,$\omega )$ is the Fourier transform of the real-space density-density response function $\chi $(r,t) which measures the dynamical density fluctuations of water due to a point-like instantaneous pulse. We use this density propagator and linear-response theory to reconstruct a model of the hydrated electron. The water density fluctuations as the electron `diffuses' through bulk water can be observed. Moreover, preliminary data on the solvent response to changes in the electronic wave function will be presented. [Preview Abstract] |
Monday, March 16, 2009 10:24AM - 10:36AM |
A15.00013: Direct measurement of negative square gradient coefficients for density fluctuations in all-atom simulations of common liquids Colin Denniston, Lingti Kong, Dan Vriesinga We perform all-atom simulations of common liquids such as water (TIP3P) and organic liquids such as short-chain olefins. We show that square gradient coefficients for the mass density can be measured directly in a linear response measurement to sinusoidal forces at several different wavelengths. Surprisingly, in all fluids measured, the square gradient coefficient is negative implying that density gradients lower the free energy of the system. However, stability is maintained at any wavelength greater than the separation between molecules due to the global mass conservation constraint. We suggest that this provides a mechanism for the molecular scale cut-off of pressure singularities that arise in situations such as droplet pinch-off. [Preview Abstract] |
Monday, March 16, 2009 10:36AM - 10:48AM |
A15.00014: Chiral Structures of Thermoresponsive Soft Spheres in Hollow Cylinders Matthew A. Lohr, Ahmed Alsayed, Zexin Zhang, Arjun G. Yodh We experimentally observe the formation of closely packed crystalline structures in hollow cylinders. ~The structures have varying degrees of chiral order. The systems are created from aqueous suspensions of thermoresponsive N-isopropylacrylamide (NIPA) microgel particles packed in micron-diameter glass capillaries. We categorize these structures according to classifications used by Erickson for tubular packings of hard spheres [1]. By varying the temperature-tunable diameter of these particles, the system's volume fraction is changed, permitting observations of the resilience of these structures and their melting transitions. Melting of these thermal crystalline structures is observed. [1] R. O. Erickson, Science 181 (1973) 705-716. [Preview Abstract] |
Monday, March 16, 2009 10:48AM - 11:00AM |
A15.00015: Nonlinear Transverse Wave Excitations in Fluid Flows Dillon Scofield, Pablo Huq The interplay of inertia and dissipation in flows with nonlinear transverse wave excitations is described by including a vortex field into the stress-energy balance equation. The theory uses an acoustic spacetime which allows limiting the speed of propagation of fluid transverse waves to a maximum speed. In the low speed limit, the theory reduces to the Navier-Stokes equations. By examining other limiting cases we show that the Navier-Stokes theory neglects terms involved with the transport of vorticity and the dissipation of energy due to the vortex field. Comparison of the theory to experiment, relative to the Navier-Stokes theory, shows that the presence of the vortex field accounts for the observed relative increase in energy- dissipation, extended lifetime of vortex structures, and excitation structure of the transverse wave field. [Preview Abstract] |
Session A16: Quantum Fermi Gases and Feshbach Resonances
Sponsoring Units: DAMOPChair: Martin Zwierlein, Massachusetts Institute of Technology
Room: 317
Monday, March 16, 2009 8:00AM - 8:12AM |
A16.00001: Theory of rf Spectroscopy in ultracold Fermi Gases William Schneider, Vijay Shenoy, Mohit Randeria We calculate the rf spectroscopy line shape for three states of the ultracold Fermi gas: (a) the equal spin population superfluid state in the BCS-BEC crossover, (b) the normal Fermi liquid state in a highly imbalanced gas, and (c) the normal Fermi liquid state for a repulsive, balanced gas. We address the question of how rf spectroscopy can make a sharp distinction between a normal Fermi liquid and a paired superfluid at $T = 0$. We also describe the role of final state interactions and of finite temperature effects. [Preview Abstract] |
Monday, March 16, 2009 8:12AM - 8:24AM |
A16.00002: BCS-BEC crossover in an optical lattice Parag Ghosh, Roberto Diener, Mohit Randeria We model fermions with an attractive interaction in an optical lattice with a single-band Hubbard model away from half-filling with on-site attraction $U$ and nearest neighbor hopping $t$. We use a large $N$ theory with Sp($2N$) symmetry to study the fluctuations beyond mean field theory. At $T=0$, we calculate across the crossover various observables, including chemical potential, gap, ground state energy, speed of sound and compressibility. The superfluid density $\rho_s$ is found to have non-trivial $U/t$ dependence in this lattice system. We show that the transition temperature $T_c$ scales with the energy gap in the weak coupling limit but crosses over to a $t^2/U$ scaling in the BEC limit, where phase fluctuations controlled by $\rho_s$ determine $T_c$. [Preview Abstract] |
Monday, March 16, 2009 8:24AM - 8:36AM |
A16.00003: Pseudogap phenomena in the BCS-BEC crossover regime of atomic Fermi gases Shunji Tsuchiya, Ryota Watanabe, Yoji Ohashi We study pseudogap behavior of atomic Fermi gases in the BCS-BEC crossover. Including paring fluctuations, we calculate the fermionic density of states above the superfluid transition temperature Tc, based on the strong coupling theory developed by Nozieres and Schmitt-Rink. We show that the gap structure appears in the density of states above Tc in the crossover region, and it evolves as the attractive interaction strength increases. We also clarify the temperature dependence of the pseudogap, which disappears as the temperature rises, and determine the pseudogap region in the phase diagram. We discuss the origin of the pseudogap by examining the behaviors of quantities such as spectral function and self-energy. [Preview Abstract] |
Monday, March 16, 2009 8:36AM - 8:48AM |
A16.00004: Universal properties of ultracold Fermi gases Shizhong Zhang, Anthony Leggett We present some general considerations on the properties of a two-component ultra-cold Fermi gas along the BEC-BCS crossover. It is shown that the interaction energy and the free energy can be written in terms of a single dimensionless function $h({\xi,\tau})$, where $\xi=-(k_Fa_s)^{-1}$ and $\tau=T/T_F$. The function $h(\xi,\tau)$ incorporates all the many-body physics and naturally occurs in other physical quantities as well. In particular, we show that the average rf-spectroscopy shift $\overline{\d\o}(\xi,\tau)$ and the molecular fraction $f_c(\xi,\tau)$ in the closed channel can be expressed in terms of $h(\xi,\tau)$ and thus have identical temperature dependence. The conclusions should have testable consequences in future experiments. [Preview Abstract] |
Monday, March 16, 2009 8:48AM - 9:00AM |
A16.00005: Probing the Spectral Function Using Momentum Resolved Radio Frequency Spectroscopy in Trapped Fermi Gases Qijin Chen, Kathy Levin A measurement of the centrally important spectral function has not been possible in ultracold Fermi gases until recent momentum resolved radio frequency (RF) spectroscopy experiments in $^{40}$K. These new experiments can be exploited to test many-body theories underlying general quantum simulations performed on Fermi gases. Here we establish the underlying physics of these RF measurements. We show that, by providing a clear dispersion signature of pairing, they remove ambiguity plaguing the interpretation of previous RF studies. Our calculated spectral intensities are in semi-quantitative agreement with the data. [Preview Abstract] |
Monday, March 16, 2009 9:00AM - 9:12AM |
A16.00006: Temperature and final state effects in radio frequency spectroscopy experiments on atomic Fermi gases Yan He, Chih-chun Chien, Qijin Chen, Kathy Levin We present a simple and systematic characterization of the radio frequency (RF) spectra of homogeneous, paired atomic Fermi gases at general temperatures, $T$, in the presence of final state interactions. The spectra, consisting of possible bound states and positive as well as negative detuning ($\nu$) continua, satisfy exactly the zeroth- and first-moment sum rules at all $T$. We show how to best extract the pairing gap and how to detect the $\nu <0$ continuum arising from thermally excited quasiparticles, not yet seen experimentally. We explain semi-quantitatively recent RF experiments on ``bound-bound'' transitions, predicting effects of varying temperature. [Preview Abstract] |
Monday, March 16, 2009 9:12AM - 9:24AM |
A16.00007: The Higgs resonance in fermionic condensates Roman Barankov The Higgs mode appears in the spectrum of fermionic condensates described by the BCS model as a result of the energy dispersion of interaction. Specifically, the mode enters the spectral gap of quasi-particle excitations when the pairing of fermions is enhanced at the Fermi energy. Conversely, it becomes a resonance in the quasi-particle continuum with a finite lifetime, when the pairing is suppressed on the energy scale small compared to the equilibrium gap. The exponential decay of the mode converts into algebraic decay for a smooth suppression. We confirm our analytical results by numerical analysis of the pairing dynamics. [Preview Abstract] |
Monday, March 16, 2009 9:24AM - 9:36AM |
A16.00008: Dynamic Cluster Monte Carlo Study of the Single-Particle Spectra of Strongly-Interacting Fermion Gases Shiquan Su, Daniel E. Sheehy, Juana Moreno, Mark Jarrell We study Feshbach-resonantly interacting fermions near unitarity within the context of the attractive Hubbard model. Our principal focus is the single-particle spectral function for such strongly-interacting fermions, recently probed in radio-frequency spectroscopy and photoemission experiments in cold-atom systems. To obtain quantitatively-accurate results on unitary gases, we apply the Dynamical Cluster Approach (DCA) and the Maximum Entropy method to study this system both in the pair-formation temperature region and in the low-temperature condensed state. Different Quantum Monte Carlo approaches emphasizing different observables are used as the quantum solver in the DCA approach, and the data from different approaches are compared to each other. [Preview Abstract] |
Monday, March 16, 2009 9:36AM - 9:48AM |
A16.00009: Feshbach-Einstein condensates Valy Rousseau, Peter Denteneer We investigate the phase diagram of a two-species Bose-Hubbard model describing atoms and molecules on a lattice, interacting via a Feshbach resonance. We identify a region where the system exhibits an exotic super-Mott phase and regions with phases characterized by atomic and/or molecular condensates. Our approach is based on a recently developed exact quantum Monte Carlo algorithm, the Stochastic Green Function (SGF) algorithm with tunable directionality. We confirm some of the results predicted by mean-field studies, but we also find disagreement with these studies. In particular, we find a phase with an atomic but no molecular condensate, which is missing in all mean-field phase diagrams. [Preview Abstract] |
Monday, March 16, 2009 9:48AM - 10:00AM |
A16.00010: Strongly-correlated fermionic matter in the dilute limit Bogdan Mihaila, Andres Cardenas We study ``the ground-state properties of the many-body system composed of spin-1/2 fermions interacting via a zero-range, infinite scattering length contact interaction.'' The above is referred to sometimes as the \emph{George Bertsch problem}, and is of particular interest in astrophysics in connection with the equation of state for neutron matter and has been revisited recently with the advent of experimental studies of the BCS to BEC crossover in ultracold fermionic atom gases. We will show that new insights into the solution to this problem are obtained in the context of a coupled-cluster (exp S) expansion approach to calculating the equation of state for dilute fermionic systems and that present state-of-the-art Monte Carlo calculations have not yet provided the definitive answer. [Preview Abstract] |
Monday, March 16, 2009 10:00AM - 10:12AM |
A16.00011: Superfluid equation of state of dilute composite bosons or how to include $3$ and $4$-body problems in the {\it many}\,body problem Xavier Leyronas, Roland Combescot We show how the $3$ and $4$-body problems emerge in the BEC limit of the BEC-BCS crossover, where we treat explicitely dimers as made of two fermions. We give the argument leading, at zero temperature, to the calculation of the equation of state. We find that, when expanding the chemical potential in powers of the density $n$ up to the Lee-Huang-Yang order, proportional to $n^{3/2}$, the result is identical to the one of elementary bosons in terms of the dimer-dimer scattering length $a_M$, the composite nature of the dimers appearing only in the next order term proportional to $n^2$. [Preview Abstract] |
Monday, March 16, 2009 10:12AM - 10:24AM |
A16.00012: A study of momentum entanglement and negativity in Bardeen-Cooper-Schrieffer states at finite temperature Chun KIt Chung, Chi Kwong Law We study the momentum entanglement between the spin-up and spin- down particles of the homogeneous Bardeen-Cooper-Schrieffer (BCS) state at finite temperature. To achieve this, we construct from the BCS state the partial transposition $\rho_2^{\mathrm{T} _A}$ of the two particle density matrix in momentum space. The structure of $\rho_2^{\mathrm{T}_A}$ and its corresponding negativity ${\cal N}_2$ are examined. We show that $\rho_2^ {\mathrm{T}_A}$ consists of infinitely many decoupled $2 \times 2 $ submatrices, and momentum entanglement coexists with the pairing order parameter $\Delta$. It is found that pairs with momenta slightly above a surface related to the Fermi energy contribute this entanglement most significantly. We propose an entanglement witness operator as a measurable quantity to detect momentum entanglement in BCS states. [Preview Abstract] |
Monday, March 16, 2009 10:24AM - 10:36AM |
A16.00013: Matterwave Probe for Detecting Fermi Superfluidity Satyan Bhongale, Han Pu We propose a matter wave probe for detecting BCS type superfluidity within a trapped two-component Fermi gas. While, previous theoretical/experimental attempts have addressed Fermi superfluidity via a global measurement, for example by demonstrating a vortex lattice, the current proposal allows for a local measurement of the pairing gap. For this, we study the phase diagram of a mixture of Bose-Einstein condensate and an interacting two-component Fermi gas. We identify regions of the parameter space where the Bose-Fermi mixture is unstable resulting in phase separation. We show that, under proper conditions, by employing a tunable scattering resonance, the phase separation phenomenon can be exploited as a robust probe of ``local'' fermion superfluidity. [Preview Abstract] |
Monday, March 16, 2009 10:36AM - 10:48AM |
A16.00014: Detecting Onset of BCS-Superfluidity Using a BEC Probe B. Ramachandhran, S.G. Bhongale, H. Pu Recent experiments\footnote{Zwierlein et al. Nature 435, 1047, (2005), and references therein} have used Feshbach resonance to tune the interactions in a two-component ultracold Fermi gas to obtain (Bardeen-Cooper-Schrieffer) BCS-type pairing and hence superfluidity. For this degenerate gas, we propose using Bose Einstein Condensate (BEC) as a matter wave probe of the BCS superfluid state. Towards this end, we explore the phase diagram of a 3-dimensional mixture of BEC and a two-component superfluid fermi gas at finite temperature. In particular, we identify the regime in which the homogenous mixture becomes unstable against phase separation. We show that, under proper conditions, this spatial phase separation phenomenon occuring in the presence of the BEC can be used to probe the ``local'' value of the superfluid Gap parameter and possibly help detect the onset temperature of the BCS superfluidity. [Preview Abstract] |
Monday, March 16, 2009 10:48AM - 11:00AM |
A16.00015: ABSTRACT WITHDRAWN |
Session A17: Focus Session: Photons and Quantum Dots
Sponsoring Units: GQIChair: Thaddeus Ladd, Stanford University
Room: 318
Monday, March 16, 2009 8:00AM - 8:36AM |
A17.00001: Excited-State Spectroscopy and Control of Single Spins in Diamond Invited Speaker: Nitrogen Vacancy (NV) defect centers in diamond are a promising system for spin-based applications in quantum information and communication at room temperature. Using a combination of optical microscopy and spin resonance, the spin of individual NV centers can be initialized, manipulated and read out. These techniques have been used to study the long room temperature spin coherence times of NV centers as well as their interactions with nearby electrical and nuclear spins. There remain significant challenges, however, both in understanding the physics of these defects as well as the development of technologies based on their quantum properties. In particular, knowledge of the detailed structure of the orbital excited-state, which continues to be an active research area, is critical to ultra-fast quantum control schemes. Here we present recent experiments using single-spin resonant spectroscopy of the excited-state of an NV center at room temperature.\footnote{G. D. Fuchs, V. V. Dobrovitski, R. Hanson, A. Batra, C. D. Weis, T. Schenkel, and D. D. Awschalom, \emph{Phys. Rev. Lett} \textbf{101}, 117601 (2008).} We observe these spin levels over a broad range of magnetic fields allowing for a direct measurement of the zero-field splitting, g-factor and transverse anisotropy splitting. The latter of these is nearly zero in the ground-state spin levels, but plays an important role in the excited-state. In addition, we find strong hyperfine coupling between the nitrogen nuclear spin and the NV electronic spin in the excited-state. These findings will be discussed in the context of quantum control of single and coupled spins in diamond. [Preview Abstract] |
Monday, March 16, 2009 8:36AM - 8:48AM |
A17.00002: Time-resolved measurements of single electron spins using continuous wave lasers Patrick Irvin, Yanjun Ma, Jeremy Levy, Jesse Berezovsky, David D. Awschalom Applications such as spin-based quantum computing require that the dynamics of single spins are monitored. Single spins produce a small signal and measurement is further complicated by the background from the large number of neighboring spins. We have developed a time-resolved Kerr rotation technique that uses continuous wave lasers. This technique is able to resonantly address a particular spin. Furthermore, it provides an efficient means of data collection that allows for more signal averaging. Finally, we decrease the background and increase the light-matter interaction by utilizing a solid-immersion lens. We will describe our recent efforts to measure single spins in GaAs/AlGaAs fluctuation-type quantum dots. [Preview Abstract] |
Monday, March 16, 2009 8:48AM - 9:00AM |
A17.00003: A photonic cluster state machine gun Terry Rudolph, Netanel Lindner, Sophia Economou A method is developed to convert certain single photon sources into devices capable of emitting large strings of photonic cluster state in a controlled and pulsed ``on demand'' manner. Such sources greatly alleviate the resources required to achieve linear optical quantum computation. Standard spin errors, such as dephasing, are shown to affect only 1 or 2 of the emitted photons at a time. This allows for the use of standard fault tolerance techniques. Using realistic parameters for current quantum dot sources, we conclude high entangled-photon emission rates are achievable, with Pauli-error rates less than 0.2{\%}. For quantum dot sources the method has the added advantage of circumventing the problematic issue of obtaining identical photons from independent, non-identical quantum dots. By using recently controlled-phase gates between two spins in neighboring quantum dots, a two-dimensional cluster can be generated. [Preview Abstract] |
Monday, March 16, 2009 9:00AM - 9:12AM |
A17.00004: Complete quantum control of a single quantum dot spin using ultrafast optical pulses David Press, Thaddeus Ladd, Bingyang Zhang, Yoshihisa Yamamoto We demonstrate a complete set of ultrafast all-optical single-qubit operations on a single electron spin in a quantum dot [\textit{Nature} \textbf{456}, 218 (2008)].~ First, the spin is initialized by optical pumping into a pure spin-state with 92{\%} fidelity.~ Next, a single-qubit gate is implemented by rotating the spin about any arbitrary axis using a sequence of two ultrafast optical pulses separated by a time delay.~ Finally, the spin is measured by detecting single-photon photoluminescence.~ As a manifestation of controlling the spin with optical pulses, we demonstrate six complete Rabi oscillations between the two spin states, and a complete set of Ramsey interference fringes.~ The fidelity of our $\pi $/2- and $\pi $-rotations exceed 90{\%}.~ The single-qubit gate is completed in 38 ps, potentially allowing for approximately 10$^{5}$ operations within the qubit's expected microsecond coherence time, and quantum information processing with clock speeds exceeding 10 GHz. [Preview Abstract] |
Monday, March 16, 2009 9:12AM - 9:24AM |
A17.00005: All-optical coherent control and spin-echo of electron spins bound to neutral donors in GaAs Susan Clark, Kai-Mei Fu, Qiang Zhang, Thaddeus Ladd, Colin Stanley, H.C. Holland, Yoshihisa Yamamoto Electron spins bound to neutral donors in GaAs are promising systems for quantum information processes. These electron spins form three-level Lambda-type systems that can be manipulated quickly by ultrafast light pulses and have potentially long storage times, making them natural candidates for quantum information manipulation and storage. Unlike quantum dots, they are extremely homogenous, making multi-qubit interactions and entanglement more accessible. Here, we report on our efforts to coherently control these electron spins using fast pulses and an all-optical spin-echo technique. Using three, off-resonant, small-angle (pi/3), ultrafast (2 ps) pulses, we have demonstrated that the spins exhibit an echo signal indicating T2 coherence times much longer than the previously measured 1 ns T2* bulk dephasing time. The visibility of the echo signal for different pulse delays gives us insights into the T2 decoherence time and decoherence processes in this system. Currently, we are measuring coherences as long as 4 microseconds. [Preview Abstract] |
Monday, March 16, 2009 9:24AM - 9:36AM |
A17.00006: Time-resolved luminescence of hierarchically self-assembled GaAs/AlGaAs quantum dots Botao Zhang, Armando Rastelli, Oliver Schmidt, Jeremy Levy, David Snoke, Albert Heberle Hierarchically self-assembled GaAs/AlGaAs quantum dots are promising building blocks for quantum information processing and novel lasers because they combine the tight confinement of InGaAs dots with the size homogeneity and a shorter emission wavelength of the GaAs/AlGaAs system at which many photodetectors are especially sensitive. So far, the emission dynamics of these structures has been unexplored. With a streak camera connected to a confocal microscope, we have measured the luminescence dynamics after direct optical picosecond excitation into the quantum dot states at a sample temperature of 10 K. Ensembles of high-density quantum dots (30 dots/$\mu $m$^{2})$ with well-separated transitions give information on state filling as well as intra- and interband relaxation. Single quantum dots on low-density samples (0.5 dots/$\mu $m$^{2})$ with microelectronvolt emission line widths reveal furthermore the time scale of biexciton formation and decay, as well as coherent effects. [Preview Abstract] |
Monday, March 16, 2009 9:36AM - 9:48AM |
A17.00007: Highly-reduced Fine-structure splitting in InAs/InP quantum dots offering efficient on-demand 1.55 $\mu$m entangled photon emitter Lixin He, M. Gong, C-F Li, G-C Guo, A. Zunger There has been intense recent interest in finding efficient entangled photon sources, including the demonstration of generation of ``event-ready'' entangled photon pairs via a biexciton cascade process using an (In,Ga)As/GaAs quantum dot(QD). However, a genuine finite energy difference between photons with different polarizations, known as the fine structure splitting (FSS), can destroy the entanglement of the photon pairs. To achieve entanglement from (In,Ga)As/GaAs QD, it was, indeed, necessary to Cherry-pick a sample with extremely small FSS from a large number of samples, or to apply strong in-plane magnetic field. Furthermore, the emission wavelength of (In,Ga)As/GaAs QD (880 - 950 nm) is mismatched with the 1.55 $\mu$m needed for communications using the optical fibers. Using theoretical modeling of the fundamental causes of FSS in QDs, we predict that the intrinsic FSS of InAs/InP QDs is an order of magnitude smaller than that of InAs/GaAs dots, and better yet, their excitonic gap matches the 1.55 $\mu$m fiber optic wavelength,therefore offer efficient on-demand entangled photon emitters for long distance quantum communication. [Preview Abstract] |
Monday, March 16, 2009 9:48AM - 10:00AM |
A17.00008: Estimation of extrinsic detection efficiency using intrinsic detection sensitivity of the commercial single photon detector Kiyotaka Hammura, Xiulai Xu, Frederic Brossard, David Williams The detection efficiency ($DE$) of the commercial single-photon-receiver based on InGaAs gate-mode avalanche photodiode is estimated using the detection sensitivity ($DS$). Instalment of a digital-blanking-system (DBS) to reduce dark current makes the difference between $DS$, which is an efficiency of the detector during its open-gate/active state, and the total/overall detection efficiency ($DE$). By numerical simulations, it is found that the average number of light-pulses, blanked by DBS, following a registered pulse is 0.333. $DS$ is estimated at 0.216, which can be used for estimating $DE$ for an arbitrary photon arriving rate and a gating frequency of the receiver. [Preview Abstract] |
Monday, March 16, 2009 10:00AM - 10:12AM |
A17.00009: Quantum key distribution with an unknown and untrusted source Yi Zhao, Bing Qi, Hoi-Kwong Lo The security of a standard bi-directional ``plug \& play'' quantum key distribution (QKD) system has been an open question for a long time. This is mainly because its source is equivalently controlled by an eavesdropper, which means the source is unknown and untrusted. Qualitative discussion on this subject has been made previously. In this paper, we present the first quantitative security analysis on a general class of QKD protocols whose sources are unknown and untrusted. The securities of standard BB84 protocol, weak+vacuum decoy state protocol, and one-decoy decoy state protocol, with unknown and untrusted sources are rigorously proved. We derive rigorous lower bounds to the secure key generation rates of the above three protocols. Our numerical simulation results show that QKD with an untrusted source gives a key generation rate that is close to that with a trusted source. Our work is published in [1]. \\[4pt] [1] Y. Zhao, B. Qi, and H.-K. Lo, Phys. Rev. A, 77:052327 (2008). [Preview Abstract] |
Monday, March 16, 2009 10:12AM - 10:24AM |
A17.00010: Spectral study of type-0/type-I spontaneous parametric down-conversion in a PPKTP waveguide Jun Chen, Aaron Pearlman, Alexander Ling, Alan Migdall, Jingyun Fan Compared with their bulk-crystal counterparts, SPDC in second-order ($\chi ^{(2)})$ nonlinear optical waveguides has been used to generate correlated photons that are naturally emitted into a single spatial mode in a collinear geometry, easing the effort in efficient photon collection and leading the potential to make chip-scale devices for quantum-information-processing applications. Here towards building chip-scale devices for quantum-information-processing applications, we performed the first spectral characterization of correlated two-photon, and single-photon emission for both type-0 and type-I spontaneous parametric down-conversion (SPDC) in a periodically-poled KTiOPO$_{4}$ (PPKTP) waveguide. [Preview Abstract] |
Monday, March 16, 2009 10:24AM - 10:36AM |
A17.00011: Deterministic generation of entangled photon pairs from a semiconductor quantum dot Andreas Muller, Wei Fang, John Lawall, Glenn Solomon Optical tuning based on the AC Stark effect is used to cancel the fine-structure splitting in a single self-assembled InAs quantum dot. Under this condition, polarization anisotropy vanishes, and photon pairs emitted from the biexcitonic radiative cascade become polarization-entangled. Entanglement is verified by well-known criteria applied to the two-photon density matrix that was reconstructed experimentally via quantum state tomography. Our approach uses a planar optical microcavity for efficient background laser discrimination, and yields triggered polarization-entangled photons deterministically. [Preview Abstract] |
Monday, March 16, 2009 10:36AM - 10:48AM |
A17.00012: Indistinguishable photons from independent semiconductor single-photon devices Thaddeus Ladd, Kaoru Sanaka, Alexander Pawlis, Klaus Lischka, Yoshihisa Yamamoto We demonstrate quantum interference between single photons generated by the radiative decay processes of excitons that are bound to isolated fluorine donor impurities in ZnSe/ZnMgSe quantum-well nanostructures. Single photon generation is confirmed by auto-correlation experiments, and indistinguishability of single photons from independent devices is confirmed via a Hong-Ou-Mandel dip. These results indicate that donor impurities in appropriately engineered semiconductor structures can portray atom-like homogeneity and coherence properties, potentially enabling scalable technologies for future large-scale optical quantum computers and quantum communication networks. [Preview Abstract] |
Session A18: Bulk Block Copolymers I
Sponsoring Units: DPOLYChair: Thomas Epps, University of Delaware
Room: 319
Monday, March 16, 2009 8:00AM - 8:12AM |
A18.00001: Order-order transition among lamellae, \textit{Fddd}, and gyroid in diblock copolymer melts Mikihito Takenaka, Myung Im Kim, Tsutomu Wakada, Satoshi Akasaka, Shotaro Nishitsuji, Kenji Saijo, Hirokazu Hasegawa, Kazuki Ito We firstly found a Disorder-Gyroid-\textit{Fddd}-Lamellae transition behavior found poly(styrene-$b$-isoprene) (S-I) diblock copolymer melts in previous study. In this study, we will present the dynamics of order-order transition (OOT) among lamellae, \textit{Fddd}, and gyroid. we investigated the dynamics of OOT by using time-resolved small angle X-ray scattering with Synchrotron radiation X-ray source. We found that \textit{Fddd} structure was formed as a metastable structure during the OOT from lamellae to gyroid induced by temperature jump. [Preview Abstract] |
Monday, March 16, 2009 8:12AM - 8:24AM |
A18.00002: Determination of\textit{ Fddd} phase boundary in polystyrene-\textit{block}-polyisoprene diblock copolymer Myung Im Kim, Satoshi Akasaka, Tsutomu Wakada, Mikihito Takenaka, Hirokazu Hasegawa We previously reported the discovery of a novel bicontinuous microdomain structure with \textit{Fddd} symmetry in polystyrene-\textit{block}-polyisoprene (SI) diblock copolymer. In this study, we investigated the phase behavior of eight SI diblock copolymer samples having slightly different compositions (0.627$\le f_{PI} \quad \le $0.653) by SAXS and TEM to determine the phase boundary of the \textit{Fddd} structure in the phase diagram of SI. The SI having the lowest $f_{PI}$ (= 0.627) showed only disorder-lamella (L) transition but no order-order transition. The SI having the largest $f_{PI}$ (= 0.653) showed disorder-gyroid (G)-L transition with decreasing temperature, but did not show \textit{Fddd} phase. The other six SI samples having $f_{PI}$ values between these two exhibited disorder-G-\textit{Fddd}-L transition with decreasing temperature. Consequently, we could determine the compositional region where \textit{Fddd} phase is thermally stable, which is in good agreement with that predicted by SCFT. [Preview Abstract] |
Monday, March 16, 2009 8:24AM - 8:36AM |
A18.00003: Stabilization of Bicontinuous Phases in Diblock Copolymer Systems Fernando Escobedo, Francisco Martinez-Veracoechea We used a coarse-grained description of the copolymer chains (i.e., dissipative particle dynamics fluid), together with continuum-space Monte Carlo and Molecular Dynamics methods, to study systems of diblock copolymers melts that have been ``filled'' with selective additives (i.e., homopolymer, and nanoparticles). Approximate phase boundaries were found via free-energy calculations. We focus on the stabilization of bi-continuous phases and the strikingly different phase behavior observed when the nature of the selective filler is changed. Our results elucidate the origins of the packing frustration that limits the viability of the gyroid, double-diamond, and plumber's nightmare phases and provide insights for overcoming it. Attention is also focused on directly determining the areas of phase diagram where macro- phase separation occurs. We compare the particle-based simulation results with the results obtained by means of self- consistent filed theory calculations. [Preview Abstract] |
Monday, March 16, 2009 8:36AM - 8:48AM |
A18.00004: Structure-Properties Relationship in Proton Conductive Sulfonated Polystyrene-Polymethyl Methacrylate Block Copolymers. Laurent Rubatat, Chaoxu Li, Herve Dietsch, Antti Nykainen, Janne Ruokolainen, Raffaele Mezzenga We report on the dependence of proton conductivity on the morphologies of sulfonated polystyrene-poly(methyl methacrylate) (sPS-PMMA) diblock copolymers. Three diblock copolymers of varying molecular weight and block volume fraction were studied, for each one several sulfonation degrees of the PS block were considered. The investigation of the morphologies of the self-assembled sPS-PMMA diblocks was carried out by means of small angle neutron scattering and transmission electron microscopy. Depending on molecular weight and sulfonation degrees, isotropic phase (ISO), lamellar phase (LAM), cylindrical hexagonal phase (HEX) and hexagonally perforated lamellae (HPL) were observed. Proton conductivity, normalized by the volume fraction of the conductive domains (formed by PS, sPS and water), was shown to rise monotonically with the following sequence of morphologies: ISO to HEX to HPL to LAM. [Preview Abstract] |
Monday, March 16, 2009 8:48AM - 9:00AM |
A18.00005: Morphology of Sulfonated Styrenic Pentablock Copolymer Solutions and Membranes Arun Kota, Karen Winey We report a systematic investigation of the morphology of sulfonated styrenic pentablock copolymer solutions and membranes obtained from Kraton Polymers LLC. The polymer studied was poly((t-butyl-styrene)-b-(ethylene-r-butylene)-b-(styrene-r-styrene sulfonate)-b-(ethylene-r-butylene)-b-(t-butyl-styrene)). Small angle x-ray scattering (SAXS) revealed that the solutions exhibited micellar morphologies. The solution SAXS data was modeled using the Kinning-Thomas model to obtain radius of the micelle core, the radius of closest approach between two micelles and the volume fraction of micelles. The membranes exhibited anisotropic morphologies with different d-spacings in-plane and through-plane. A good linear correlation was observed between the radius of closest approach between two micelles in the solutions and the d-spacings in the membranes. Efforts are underway to characterize the type of morphology in the membranes using electron microscopy and correlate them to the transport properties. [Preview Abstract] |
Monday, March 16, 2009 9:00AM - 9:12AM |
A18.00006: Thermodynamic Behavior of Poly(styrene-b-styrene sulfonate) Block Copolymers With Varying Counterions Kevin Cavicchi, Kevin Pollack A series of poly(styrene-b-styrene sulfonate) (PS-b-PSS) block copolymers have been prepared by RAFT polymerization. The counterions in the PSS block have been varied by neutralizing the sulfonate groups with alkyl amines or quaternary ammonium ions. The choice of counterion has a strong effect on the lipophilicity of the PSS block. This presentation will focus on the resulting morphology and bulk thermodynamic behavior of these polymers as a function of the PSS counterion. The use of these materials for preparing ion-exchange membranes will be discussed. [Preview Abstract] |
Monday, March 16, 2009 9:12AM - 9:24AM |
A18.00007: Salt Doping in PEO Containing Block Copolymers: Counterion and Concentration Effects Wen-Shiue Young, Thomas Epps Salt-doped poly(ethylene oxide)-based block copolymers are promising candidates for lithium battery polymer electrolytes, which require high ionic conductivities and adequate mechanical integrity. We studied the phase behavior of poly(styrene-b-ethylene oxide) block copolymers doped with various lithium salts over a range of [EO]:[Li] ratios. Small-angle X-ray scattering, transmission electron microscopy, and differential scanning calorimetry experiments were used to characterize the phase behavior of our samples. Specimens were prepared in an argon atmosphere and rigorously dried to reduce the effects of moisture uptake on phase behavior. We found that we can tune the copolymer microstructure by varying the lithium counterion as well as the salt doping ratio. Using strong segregation theory, we estimated an effective interaction parameter for the salt-doped copolymers, which varies linearly with salt concentration, where the slope is influenced by the nature of the counterion. [Preview Abstract] |
Monday, March 16, 2009 9:24AM - 9:36AM |
A18.00008: Ion transport through block copolymer electrolytes Scott Mullin, Ashoutosh Panday, Nitash Balsara Poly(styrene)-\textit{block}-poly(ethylene oxide) (SEO) is a candidate material for electrolytes for rechargeable lithium metal batteries. The PS phase suppresses lithium dendrite growth on the anode during recharge, and the PEO phase solvates lithium bis(trifluoromethane)sulfonimide (LiTFSI) salt to form conducting pathways. Complete electrochemical characterization of PEO/LiTFSI mixtures requires measurement of conductivity, salt diffusion coefficient, and lithium ion transference number. The present study covers SEO copolymers that exhibit lamellar and cylindrical morphologies in the absence of salt. The addition of salt affects morphology but the relationships between morphology and electrochemical characteristics have not yet been clarified. Some aspects of these relationships will be presented. [Preview Abstract] |
Monday, March 16, 2009 9:36AM - 9:48AM |
A18.00009: Morphology of Novel Semicrystalline Ethylene-$\alpha$-Olefin Block Copolymers Sheng Li, Richard Register, Brian Landes In semicrystalline block copolymers, the solid-state structure can be set either by block incompatibility or by crystallization of one or more blocks. Depending on the block interaction strength, a wide array of solid-state morphologies may be observed, ranging from spherulitic to confined crystallization within preexisting microphase-separated domains. Dow Chemical has recently developed a novel chain shuttling polymerization process to produce olefin block copolymers with alternating amorphous and semicrystalline chain segments, where each block exhibits the most-probable distribution. We examined the melt and solid-state morphologies of these novel olefin block copolymers, having a high octene content in the amorphous block, using two- dimensional synchrotron small-angle and wide-angle x-ray scattering on specimens oriented by channel die compression. Multiblock and diblock copolymers with near-symmetric compositions showed well-ordered lamellar structures at room temperature with long periods exceeding 100 nm, with little dependence on thermal history, indicating the presence of a mesophase-separated melt which templates crystallization. [Preview Abstract] |
Monday, March 16, 2009 9:48AM - 10:00AM |
A18.00010: Self-assembly of crystalline bioinspired block copolymers A.M. Rosales, H.K. Murnen, R.N. Zuckermann, R.A. Segalman Polypeptoids are sequence-specific biologically inspired polymers based on N-substituted glycines for which monodisperse, polymeric molecular weights can be achieved. Sequence control allows for a degree of tunability in both the self-assembly and thermal properties not available in classical polymer systems. We demonstrate that a series of homopolypeptoids are thermally stable to 300C and are crystalline with melting transitions ranging from 150C to 250C. The introduction of defects at precise locations in the polymer sequence (as a side chain substitution) allows crystallization and hence the melting temperature to be suppressed. Symmetric block copolymers with two crystalline polypeptoid blocks exhibit co-crystallization of the two blocks but distinct melting behaviors, indicating a disordered melt. If samples are carefully prepared to allow for microphase separation, block copolymer lamellae with long range order are formed with an order-disorder transition temperature well below the melting transition temperature of the polymer. [Preview Abstract] |
Monday, March 16, 2009 10:00AM - 10:12AM |
A18.00011: Crystallization, Crystal Orientation and Morphology of Poly(ethylene oxide) under 1D Defect-Free Nanoscale Confinement Ming-Siao Hsiao, Joseph X. Zheng, Ryan M. Van Horn, Roderic P. Quirk, Edwin L. Thomas, Bernard Lotz, Stephen Z. D. Cheng One-dimensional (1-D) defect-free nanoscale confinement is created by growing single crystals of PS-b-PEO block copolymers in dilute solution. Those defect-free, 1-D confined lamellae having different PEO layer thicknesses in PS-b-PEO lamellar single crystals (or crystal mats) were used to study the polymer recrystallization and crystal orientation evolution as a function of recrystallization temperature (T$_{rx})$ because the T$_{g}^{PS}$ is larger than T$_{m}^{PEO}$ in the PS-b-PEO single crystal. The results are summarized as follows. First, by the combination of electron diffraction and known PEO crystallography, the crystallization of PEO only takes place at T$_{rx}<$-5$^{o}$C. Meanwhile a unique tilted PEO orientation is formed at T$_{rx }>$-5$^{o}$C after self-seeding. The origin of the formation of tilted chains in the PEO crystal will be addressed. Second, from the analysis of 2D WAXD patterns of crystal mats, it is shown that the change in PEO c-axis orientation from homogeneous at low T$_{rx}$ to homeotropic at higher T$_{rx}$ transitions sharply, within 1$^{o}$C. The mechanism inducing this dramatic change in crystal orientation will be investigated in detail. [Preview Abstract] |
Monday, March 16, 2009 10:12AM - 10:24AM |
A18.00012: Gradient Architecture as Means of Phase Diagram Manipulation in Copolymers: Accessing Both LCOT and UCOT in High Molecular Weight Styrene/n-Butyl Acrylate Systems Michelle Mok, Wesley Burghardt, Christopher Ellison, John Torkelson Traditionally, phase transitions of block copolymers could only be tuned through molecular weight and relative block length. Here, we introduce comonomer sequence design through gradient compositions as a means of further manipulating phase diagram boundaries. In such gradient copolymers, the reduced repulsion between chain segments allows access to phase transitions even at high molecular weights (MW). Rheological and x-ray scattering studies were performed to study the impact of comonomer sequence on phase behavior in styrene/n-butyl acrylate (S/nBA) systems. In S/nBA block copolymers, only upper critical ordering behavior was observed. In contrast, by using a gradient architecture of higher MW we observed both upper and lower ordering transitions similar to those seen in very weakly segregating S/n-butyl methacrylate block copolymers, where such dual ordering transitions were first detected by Russell et al. This is the first study to access a miscibility gap in gradient copolymers. Access to such behavior is very rare in blends and block copolymers, limited to low MW and/or very weakly segregating systems. [Preview Abstract] |
Monday, March 16, 2009 10:24AM - 10:36AM |
A18.00013: Effects of polydispersity on the order-disorder transition of diblock copolymer melts Tom Beardsley, Mark Matsen The effect of polydispersity on an AB diblock copolymer melt is investigated using lattice based Monte Carlo simulations with parallel tempering (PT) techniques. We consider melts where the B blocks are monodisperse and the A blocks are polydisperse with a Schultz-Zimm distribution. Expanding our previous work on polydisperse melts of symmetric composition, we now construct a polydisperse phase diagram, investigating the size of the domains and locations of the order-disorder (ODT) and order-order (OOT) transitions. The PT method has yielded a number of benefits over single-processor temperature scans, including: simulating a number of temperatures simultaneously, annealing out defects in the configurations more readily and capturing the distinctive spike in the heat capacity that occurs at the ODT, allowing the location of the transition to be determined more accurately than in previous studies. The results are compared to those of experiment and to the predictions of self-consistent field theory (SCFT). [Preview Abstract] |
Monday, March 16, 2009 10:36AM - 10:48AM |
A18.00014: Dynamics of Disordered PI-PtBS Diblock Copolymer Hiroshi Watanabe Viscoelastic ($G^*$) and dielectric ($\varepsilon''$) data were examined for a LCST-type diblock copolymer composed of polyisoprene (PI; M = 53K) and poly(\textit{p}-\textit{tert}- butyl styrene) (PtBS; M = 42K) blocks disordered at $T \quad \le 120 \textrm{C}^{\circ}$. Only PI had the type-A dipole parallel along the chain backbone. Thus, the $\varepsilon''$ data reflected the global motion of the PI block, while the $G^*$ data detected the motion of the copolymer chain as a whole. Comparison of these data indicated that the PI block relaxed much faster than the PtBS block at low $T$ and the dynamic heterogeneity due to PtBS was effectively quenched to give a frictional nonuniformity for the PI block relaxation. The $\varepsilon''$ data were thermo-rheologically complex at low $T$, partly due to this nonuniformity. However, the block connectivity could have also led to the complexity. For testing this effect, the $\varepsilon''$ data were reduced at the iso- frictional state defined with respect to bulk PI. In this state, the $\varepsilon''$ data of the copolymer at low and high $T$, respectively, were close to the data for the star-branched and linear bulk PI. Thus, the PI block appeared to be effectively tethered in space at low $T$ thereby behaving similarly to the star arm while the PI block tended to move cooperatively with the PtBS block at high $T$ to behave similarly to the linear PI, which led to the complexity of the $\varepsilon''$ data. The PtBS block also exhibited the complexity (noted from the $G^*$ data), which was well correlated with the complexity of the PI block. [Preview Abstract] |
Monday, March 16, 2009 10:48AM - 11:00AM |
A18.00015: Morphology of Renewable Polylactide / Soybean Oil Blends Compatibilized by Block Copolymers Megan Robertson, Kwanho Chang, Marc Hillmyer Renewable composites derived from polylactide and soybean oil (soy) were prepared by melt blending. The blend morphology was tuned with the addition of poly(isoprene-b-lactide) block copolymers. Due to the extreme differences in the viscosities of soy and polylactide, a critical block copolymer block ratio was found to induce a phase inversion in which the morphology changed from soy droplets in a polylactide matrix to polylactide droplets in a soy matrix, even though soy was the minority component. This transition was not only due to the thermodynamic interactions between the block copolymer and the two immiscible phases, but also was a result of shear forces acting on the mixture during melt blending. The droplet size of the soy droplets in the polylactide matrix was also highly dependent on the block copolymer composition. In binary polylactide/soy blends there was a limiting concentration of soy which could be incorporated into the polylactide matrix (5 percent of the total blend weight) due to the mismatch in viscosities resulting in the loss of soy during mixing. The addition of block copolymer with an appropriate block ratio allowed full incorporation of soy up to 20 percent of the total blend weight. [Preview Abstract] |
Session A19: The Physics of Polymer Nanocomposites: Polymer Nanoparticle Interactions
Sponsoring Units: DPOLYChair: Amalie Frischknecht, Sandia National Laboratories
Room: 320
Monday, March 16, 2009 8:00AM - 8:12AM |
A19.00001: Functional polymer colloids stabilized by type-purified single-wall carbon nanotubes. Erik K. Hobbie, Jeffrey A. Fagan, Jan Obrzut Emulsion polymerization of a methacrylate monomer in aqueous biological suspensions of type-purified single-wall carbon nanotubes (SWNTs) is used to synthesize polymer colloids coated by nearly pure metallic or semiconducting SWNTs. The polymer-nanocomposite spherical particles are 1-100 micrometers in diameter, are marginally stable in ethanol, and retain the color and unique optical characteristics of the purified SWNT coating in the absence of any surfactant. By assembling these functional polymer colloids on microelectronic circuits, we characterize the electronic properties of the SWNT-polymer nanocomposite microspheres and relate this to the band structure of the purified SWNT coating, demonstrating their potential use as microscopic optical and electronic components that can be easily manipulated using standard methods of colloidal self-assembly. [Preview Abstract] |
Monday, March 16, 2009 8:12AM - 8:24AM |
A19.00002: Theoretical analysis of dispersing of aggregated nanorods in shear flow in presence of AC electric or magnetic fields Victor Pryamitsyn, Venkat Ganesan Efficient dispersion of nanotubes in polymeric matrices is a critical problem confronting the development of modern polymer nanocomposites. The nanotube-nanotube interactions usually promote aggregation, which also depends on factors such as the chemical makeup of the polymer matrix and the size of nanotubes. High intensity mechanical mixing such shear pulverization are commonly used for dispesion of nanotubes. The main disadvantage of such processes is the degradation of polymer matrix, which may downgrade the final properties of PNC's. In this work, we theoretically explore a novel strategy to reduce the shear stresses required for dispersion of rodlike fillers. Explicitly, we found that simultaneous applications shear flow and AC electric field oriented at an angle to each other may cause rotational instabilities of the rods suspension and lead to the dispersion of the rods. We demonstrate this idea through Brownian dynamics simulations of aggregating nanorods and a complementary theoretical analysis using a 2D Smoluchowski equation. Our results suggest that an optimal dispersion may be achieved at an shear-E field orientation of $\beta=-45^{\circ} $ with an optimal amplitude of AC electric field which is proportional to the rotation Peclet number of nanorods suspension. [Preview Abstract] |
Monday, March 16, 2009 8:24AM - 8:36AM |
A19.00003: Effective Interactions, Structure and Phase Behavior of Polymer Nanocomposites with Nonspherical Fillers Lisa M. Hall, Kenneth S. Schweizer The Polymer Reference Interaction Site Model is applied to study polymer-mediated inter-nanoparticle interactions, fluid structure, and miscibility of nonspherical filler particles in a melt of adsorbing freely-jointed chains. The behavior of hard rod, disk, and cube-like nanoparticles are compared. The depletion contact aggregation, dispersed, and polymer bridging mediated nanoparticle network states of organization are sensitive to filler shape. A detailed study of thin rod fillers, including the rod-rod potential of mean force and second virial coefficient, B$_{2}$, as a function of polymer-rod and rod-rod attraction strengths, has also been performed. A primary goal is to identify design rules for dispersing nanotubes in polymer melts. Shortening the spatial range of rod-rod attraction compared to polymer-rod attraction increases miscibility. The transition from positive to negative B$_{2}$ at low polymer-rod interfacial attraction (entropic depletion) occurs more readily (at higher attraction strength) as rod-rod attraction is increased. However, the transition to negative B$_{2}$ at high polymer-rod attraction strength, driven by polymer-induced enthalpic bridging of rods, is relatively invariant to inter-rod attraction strength. Increasing rod length reduces the stabilizing consequences of polymer adsorption and the attendant steric repulsion. [Preview Abstract] |
Monday, March 16, 2009 8:36AM - 9:12AM |
A19.00004: Polymer-tethered nanoparticle ``shape amphiphiles": A new class of macromolecular building block for self-assembly Invited Speaker: Fabricating complex ordered structures from nanoparticles requires controlling nanoparticle interactions for self-assembly over multiple length scales. Here we exploit both building block shape and interaction anisotropy for self-assembly, and explore the use of polymer ``tails'' attached to nanoparticle ``head groups'' to create a new kind of amphiphile that self assembles into structures like those seen in surfactant and block copolymer systems, but with important differences arising from nanoparticle shape, and tethered nanoparticle geometry and topology. We investigate the impact of nanoparticle size polydispersity and show that it can both help and hinder formation of certain complex phases. Using simulation, we investigate tethered spheres, rods, cubes, triangles, and other shapes, and provide design rules for the predicted self-assembly of a range of chiral and achiral structures, including helical scrolls, gyroid, square arrays, and ionic crystal-like structures. [Preview Abstract] |
Monday, March 16, 2009 9:12AM - 9:24AM |
A19.00005: Control of the Spatial Distribution of Nanoparticles in Fluorescent Polymer Nanocomposites Chelsea Chen, Peter Green In a brush-coated nanoparticle (NP) / polymer nanocomposite system, the spatial distribution of the NPs is largely determined by the entropic and enthalpic interactions between the brush and polymer host chains. We examined the miscibility between polystyrene (PS) homopolymer chains and a fluorescent polymer poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene]~(MEH-PPV) and found that in thin films, low molecular weight (MW) PS and MEH-PPV are miscible, whereas high MW mixtures exhibit phase separation. Consequences of this behavior were examined with regard to the effect on the morphology of nanocomposite thin films composed of MEH-PPV and thiol terminated polystyrene grafted Au nanoparticles of varying sizes. We were able to achieve complete dispersion, as well as interfacial segregation, of the Au-PS nanoparticles within MEH-PPV hosts. Through control of the morphology, we were able to ``tune'' the optical properties of the MEH-PV/Au-PS nanocomoposites. [Preview Abstract] |
Monday, March 16, 2009 9:24AM - 9:36AM |
A19.00006: Importance of Chain Connectivity in the Formation of Non-covalent Interactions between Polymers and Single-Walled Carbon Nanotubes Dias Linton, Brad C. Miller, Huimin Li, Charles Feigerle, Bobby G. Sumpter, Mark D. Dadmun Our work is focused on understanding and utilizing non-covalent electron donor-acceptor (EDA) interactions between polymers and SWNT to optimize interfacial adhesion and homogeneity of nanocomposites without modifying the SWNT native surface. Nanocomposites with polymer bound electron donating 2-(dimethylamino)ethyl methacrylate or electron accepting acrylonitrile and cyanostyrene moieties leads to improved SWNT dispersion if the interacting functional group is a minor component of a copolymer matrix. Correlation of experimental (Raman mapping, Raman D* band peak shifts, and optical microscopy) and computational results indicates that chain connectivity is critical in controlling the accessibility of the functional groups to form EDA interactions. Thus, controlling the amount of e$^{-}$ donating or withdrawing moieties throughout the polymer chain will direct the extent of EDA interaction, which enables tuning the SWNT dispersion. [Preview Abstract] |
Monday, March 16, 2009 9:36AM - 9:48AM |
A19.00007: The Structure of Amphiphilic Polymers Interacting with Carbon Nanotubes Yachin Cohen, Meirav Granite, Wim Pyckhout-Hintzen, Aurel Radulescu Dispersion of single-walled carbon nanotubes, necessary for their beneficial utilization, is often based on amphiphilic copolymers. We have successfully utilized the following systems: an alternating copolymer of styrene and sodium maleate, exhibiting alternating hydrophobic and hydrophilic groups, amphiphilic block copolymers such as Pluronic F108 and a synthetic short polypeptide (FFDD)$_{6}$, containing alternating hydrophobic blocks of two phenylalanine (FF) and hydrophilic block of two aspartic acid (DD). Cryo-transmission electron microscopy images reveal isolated, very small bundles of carbon nanotubes, with diameters range from 1 to 5 nm and approximately 500 nm length. Small-angle neutron scattering experiments were conducted at different D$_{2}$O/H$_{2}$O content of the dispersing medium. The scattering patterns suggest a complex entity with an heterogeneous structure. For the alternating copolymer, loose adsorption of polymer coils is indicated, contrary to published ideas on ``polymer wrapping'' of nanotubes. For the Pluronic block copolymers, the data suggest that even below the critical micellization temperature there is a dense coating on the nanotube surface and the hydrophilic blocks are highly extended. The polypeptide also forms a dense coating with an apparently ``spongy'' structure. [Preview Abstract] |
Monday, March 16, 2009 9:48AM - 10:00AM |
A19.00008: Chain Expansion in Polymer-Nanoparticle Melts A. L. Frischknecht, E. S. McGarrity, M. E. Mackay We apply a self-consistent version of the polymer reference interaction site model (PRISM) theory to a model of spherical nanoparticles in a polymer melt. The average radius of gyration of the chains in the blend clearly increases (over that from chains in a neat melt at the same density) with increasing nanoparticle volume fraction. The amount of chain expansion also depends on the magnitudes of the attractive interactions in the system. The bulk modulus of the blend decreases with increasing nanoparticle volume fraction. We compare our theoretical results to experimental scattering data from polystyrene blended with various nanoparticles and to previous simulation results. (This work was performed, in part, at the Center for Integrated Nanotechnologies, a U.S. Department of Energy, Office of Basic Energy Sciences user facility. Sandia National Laboratories is a multi-program laboratory operated by Sandia Corporation, a Lockheed-Martin Company, for the U. S. Department of Energy under Contract No. DE-AC04-94AL85000.) [Preview Abstract] |
Monday, March 16, 2009 10:00AM - 10:12AM |
A19.00009: Effects of nanoparticles on chain dynamics and glass transition in athermal polymer nanocomposites Hyun Joon Oh, Peter Green Chain relaxation dynamics and the glass transition of mixtures of polystyrene (PS) homopolymer with PS-grafted gold nanoparticles were examined using broadband dielectric spectroscopy, differential scanning calorimetry and capacitive scanning dilatometry. Through changes in the nanoparticle core size, D, grafting density, $\sigma $, degree of polymerization of grafted chains, N, and the nanoparticle concentration, $\phi $, both the chain relaxation time, $\tau $, and the T$_{g}$ could be induced to undergo significant changes, increases or decreases, in magnitude. These results will be discussed in light of dynamics in other polymer/nanoparticle systems. In addition, the role of particle size and the role of the melt/brush interfacial interactions on the dynamics will be discussed. [Preview Abstract] |
Monday, March 16, 2009 10:12AM - 10:24AM |
A19.00010: Dynamics in Nanoparticle Liquids Peter Mirau, Michael Jespersen, Richard Vaia, Robert Rodriguez, Emmanuel Giannelis Nanoscale Ionic Materials (NIMS) are organic-inorganic hybrids in which a nanometer-sized core is functionalized with a covalently attached corona and an ionically tethered canopy. NIMS can be engineered to be liquids at ambient temperature in the absence of solvent and are of interest for a variety of applications. We have used NMR relaxation and pulse-field gradient NMR to measure the dynamics of NIMS made from a 20 nm silica core modified with propyl sulfonic acid groups and amino-terminated ethylene oxide/propylene oxide block copolymers. Carbon NMR studies show that the block copolymer canopy is quite mobile both in the bulk and the nanoparticle liquid. The carbon spin-lattice relaxation times as a function of temperature are fit to a model with rapid librational motions and slower reorientation of the copolymer. Neither the correlation times for reorientation of the block copolymer nor the self-diffusion coefficient are influenced by the presence of the silica nanoparticle core. These data suggest that the liquid-like behavior in NIMS is due to rapid exchange of the block copolymer canopy between the ionically modified nanoparticles. [Preview Abstract] |
Monday, March 16, 2009 10:24AM - 10:36AM |
A19.00011: Hierarchically Structured Block Copolymer $\backslash $ Silicate Nanocomposites Ross Behling, Eric Cochran In this contribution we functionalized MMT with a bromine terminated alkylamine and subsequently polymerized polystyrene-$b$-poly(tert-butyl acrylate) via graft-from atom transfer radical polymerization. Hierarchical assembly of these composites was facilitated through targeted graft density, polymer block size, and MMT intrinsic properties. The high graft density on the MMT surface results in confinement effects which force growing polymer chains into highly extended conformations. Block copolymer (BCP) brushes were chosen because they inherently offer an easily tunable method for producing self-assembled structures on the order of tens of nanometer. Montmorillonite (MMT) in its raw state is composed of negatively charged tactoid stacks 100-300 nm in diameter and 1-2 nm thick resulting in BCP$\backslash $silicate nanocomposites particles arranged under shear into structures spanning several hundreds of nanometers. The equilibrium structures were influenced by the MMT platelet curvature and the BCP interaction parameter, $\chi $. BCP nanocomposites were observed via transmission electron microscopy to display novel morphologies with multiple systems exhibiting interpenetrating networks reminiscent of ``worm micelles''. [Preview Abstract] |
Monday, March 16, 2009 10:36AM - 10:48AM |
A19.00012: Effect of Copolymer-Nanoclay Interactions on Intercalation Kinetics Loan Vo, Haris Retsos, Emmanuel Giannelis We use X-ray diffraction to measure the melt intercalation kinetics of a series of surface-modified clay nanoparticles (nanoclay) with styrene-butadiene-rubber (SBR). Since SBR is a copolymer, both the styrene and the butadiene components interact with the nanoparticles contributing to the nanoclay miscibility and the intercalation kinetics. We are able to directly measure the butadiene-nanoclay interaction strength by using dielectric relaxation spectroscopy to probe the butadiene-nanoclay interfacial relaxation mode, and by varying the nanoclay surfactant and copolymer composition, we can indirectly measure the styrene-nanoclay interaction strength. We will present the spectroscopy results and discuss the relation to the intercalation kinetics. [Preview Abstract] |
Monday, March 16, 2009 10:48AM - 11:00AM |
A19.00013: Polymer Nanocomposites Containing Carbon Nanotubes and Exfoliated Nanoplatelets Hung-Jue Sue, Dazhi Sun We report a simple and efficient method to disperse carbon nanotubes (CNTs) into an epoxy matrix through exfoliated nanoplatelets. Pre-oxidized CNTs were first dispersed in the presence of exfoliated nanoplatelets in water, followed by re-dispersion in epoxy matrix. Both individual CNTs and nanoplatelets are exfoliated and well dispersed in epoxy, which is confirmed by high-resolution transmission electron microscopy. The possible mechanisms responsible for the CNT dispersion in polymers are proposed. The epoxy nanocomposites containing CNTs and nanoplatelets show exceptional mechanical properties: significant improvements in both modulus and strength without reduction in ductility have been found from tensile testing. The implication of the present findings for the engineering applications of the CNT-based polymer nanocomposites is also discussed. [Preview Abstract] |
Session A20: Spatially Confined Polymer Materials
Sponsoring Units: DPOLYChair: Joanne Budzien, Sandia National Laboratories
Room: 321
Monday, March 16, 2009 8:00AM - 8:36AM |
A20.00001: Field-Based Simulations of Confined Block Copolymers Invited Speaker: This presentation will discuss field-theoretic simulation methods that can be used to analyze the self-assembly behavior of thin block copolymer films, including films that are laterally confined on a flat substrate and curved films on a spherical manifold. Our studies of lateral confinement have revealed strategies for epitaxially templating microdomain patterns with long-range in-plane order and minimal defects (``graphoepitaxy"), and methods for diversifying the set of stable 2D lattice structures. On the sphere, we have found defective ground state block copolymer morphologies that are analogous to spherical crystalline packings in other contexts, e.g. the Thompson problem and viruses. The methods and findings have applications in block copolymer lithography and in dispersion technology of polymer-stabilized nanoparticles and colloids. [Preview Abstract] |
Monday, March 16, 2009 8:36AM - 8:48AM |
A20.00002: Diblock copolymers in thin films Marianne Heckmann, Barbara Drossel We investigate the phase behaviour of diblock copolymers which are confined to thin films. We employ both self-consistent field (SCFT) theory and Strong Stretching Theory (SST) in order to compute the free energies of different possible morphologies. We show that only the simplest structures which are combinations of the bulk morphologies are stable when cylinder- or sphere-forming copolymers are confined between hard walls interacting with the monomers. We present and compare phase diagrams computed with SCFT and SST for different sets of parameters. [Preview Abstract] |
Monday, March 16, 2009 8:48AM - 9:00AM |
A20.00003: Block copolymers confined in nano-pores Andrei Zvelindovsky, Marco Pinna Using cell dynamics simulation we investigate morphologies block copolymers confined in the nano-pores of various geometries such as spherical cavities, cylinder pores and confinement formed by concentric spheres and cylinders. We examine the influence of molecular composition, parameters of confinement such as curvature and characteristic size, as well as preferential interaction of copolymer blocks with the surfaces. Several block copolymer morphologies are investigated: lamellae, cylinders, spheres and bicontinuous. Deviation from the bulk structure develops under influence of confinement. [Preview Abstract] |
Monday, March 16, 2009 9:00AM - 9:12AM |
A20.00004: Order and Disorder in a Lamella Forming Fluid Near an Attractive Substrate Andrew Croll, An-Chang Shi, Kari Dalnoki-Veress By using wedged thin films, we have measured the effect of interfaces on the ordering of a diblock copolymer in real space. Symmetric diblocks can form lamellae and the strong preference of the substrate for one of the blocks can induce this ordering well above the order-disorder transition (ODT) temperature. However, the induced order is decays away from the substrate $\sim \exp (-x/\xi)$. The lengthscale, $\xi$, diverges as the temperature is decreased at ODT, and two distinct regions are observed in the scaling of $\xi$ with the temperature attributable to the two interfaces of the film (air and substrate). The dynamics of ordering and disordering near the substrate is found to be exponential in time, though the mechanisms differ. In disordering the distance from the substrate is unimportant, and all layers relax identically. However, the timescale of the formation of lamellae is found to vary significantly with the distance from the substrate. [Preview Abstract] |
Monday, March 16, 2009 9:12AM - 9:24AM |
A20.00005: Crystallization behavior of polyethylene under nanoscopic cylindrical confinement Woojung Cho, Euntaek Woo, June Huh, Younggyu Jeong, Kyusoon Shin Due to the increasing usage of nanoscopic materials, fundamental understanding of the crystallization is more and more demanded. In this presentation, we will discuss the crystallization behavior of linear polyethylene confined in cylindrical nanopores. We observed the crystallization mechanism transition from homogeneous nucleation to heterogeneous nucleation upon the tightening of the confinement. We also found that the crystalline structure, examined by x-ray diffraction and calorimetry, shows deviations from those of bulk and the crystal formation is severely influenced by the imposed confinement. [Preview Abstract] |
Monday, March 16, 2009 9:24AM - 9:36AM |
A20.00006: Managing bond tension in spreading macromolecules. Sergey Sheyko, Insun Park, Alper Nese, Krzysztof Matyjaszewski, David Shirvaniants, Michael Rubinstein Mechanical activation of chemical bonds plays a vital role in biology, chemistry, and engineering. Unlike other activation stimuli, such as light and temperature, mechanical activation is site and direction specific. However, in a typical experiment, macroscopic stress is distributed over myriads of different molecules. This results in significant and ill-defined variation of both the magnitude and direction of forces at individual chemical bonds. Here, we show how to achieve a great degree of control over bond tension in flowing polymer films. The distinctive feature of this finding is that the mechanical tension is controlled on three different length scales. First, chemical bonds are activated within a narrowly defined area of a macroscopic film. Second, only certain molecules are activated within a mixture of molecules. Third, the tension can be focused to a specific bond within a flowing macromolecule. It is demonstrated that the focused tension breaks covalent bonds with a molecular-scale precision. [Preview Abstract] |
Monday, March 16, 2009 9:36AM - 9:48AM |
A20.00007: Spreading of Polymer Films at the Molecular Scale: Conformation, Orientation, and Fractionation. Michael Barrett, Alper Nese, Krzysztof Matyjaszewski, Sergei Sheiko Previously, we have reported that comb-like polymer macromolecules undergo a plug-flow with an insignificant contribution of molecular diffusion (\textit{Phys. Rev. Lett}. \underline {93}, 206103, \textbf{2004}). It was also suggested that the composition of the flowing polymer melt was the same both inside the fluid reservoir (drop) and in the precursor film. This work called into question the macroscopic picture of polymer spreading. Through molecular imaging by AFM, we observe that macromolecules spread at different velocities depending on their size. We show that flow causes the molecules to align perpendicular to the flow direction We have also identified specific molecular conformations, such as hairpins, that become more abundant in spreading films. Lastly, we demonstrate that chain entanglements hinder permeation of long macromolecules from the drop to precursor film. These findings shed light on the molecular mechanism of spreading of polymer melts on natural, i.e. heterogeneous, substrates. [Preview Abstract] |
Monday, March 16, 2009 9:48AM - 10:00AM |
A20.00008: Polarization Anisotropy of DNA in Nanochannels Fredrik Persson, Fredrik Westerlund, Jonas Tegenfeldt, Anders Kristensen Studies of DNA in nanoscale confinement, where the dynamics and statics of the DNA extension or position is investigated as a function of e.g. DNA contour length, degree and shape of the confinement as well as buffer conditions, has yielded new insight in the physical properties of DNA. Our work extends this field by not only studying the location of fluorescent dyes along a confined DNA molecule but also monitor the orientation of the dyes by measuring the polarization of the fluorescence emission. By choosing a dye which fluorescence polarization is related to the DNA backbone and measuring the emission that is polarized parallel and perpendicular to the extension axis of the stretched DNA, information on the local spatial distribution of the DNA backbone can be obtained. The result is analogous to linear dichroism (LD) on a single-molecule level, and obtained in a highly parallel fashion. [Preview Abstract] |
Monday, March 16, 2009 10:00AM - 10:12AM |
A20.00009: ABSTRACT WITHDRAWN |
Monday, March 16, 2009 10:12AM - 10:24AM |
A20.00010: Newtonian drop deformation in a viscoelastic matrix under shear Shahriar Afkhami, Pengtao Yue, Yuriko Renardy When a Newtonian drop is sheared in a viscoelastic matrix, the direct numerical simulation produces a viscoelastic ``wake'' at the interface at the front and back of the drop. In the case of a drop reaching a stationary state, the stresses in the wake and the viscous shear balance out the interfacial tension force. When a viscoelastic drop is sheared in a Newtonian matrix, numerical simulations exhibit an overshoot in the transient evolution of drop deformation. Experimental observations also show that an overshoot can occur when a Newtonian drop is sheared in a viscoelastic matrix. However, these overshoots do not appear for 3D drop computations. In this work, we investigate why drop simulations in 3D do not display overshoots. [Preview Abstract] |
Session A21: Semiconductors: 2D Electrons and Transport
Sponsoring Units: FIAPChair: Michael Zudov, University of Minnesota
Room: 323
Monday, March 16, 2009 8:00AM - 8:12AM |
A21.00001: Temperature dependence of microwave photoresistance in 2D electron systems Michael Zudov, Anthony Hatke, Loren Pfeiffer, Ken West We report on studies of the temperature dependence of microwave- induced resistance oscillations in a high-mobility two- dimensional electron system. We find that the oscillations decay exponentially with increasing temperature, as $\exp(- \alpha T^2)$, where $\alpha$ scales with the inverse magnetic field. This observation suggests that the temperature dependence originates {\em primarily} from the modification of the single particle lifetime, likely through electron-electron interaction effects. The relevance of our findings to existing theories will be discussed. [Preview Abstract] |
Monday, March 16, 2009 8:12AM - 8:24AM |
A21.00002: Temperature dependence of Hall-field induced resistance oscillations in 2D Electron Systems Anthony Hatke, Hung-Sheng Chiang, Michael Zudov, Loren Pfeiffer, Ken West A few years ago a new class of resistance oscillations was discovered in two-dimensional electron systems subject to weak magnetic fields and strong dc electric fields [1]. These oscillations, termed Hall field induced resistance oscillations (HIRO), are believed to originate from commensurability between the cyclotron diameter and real-space separation between Hall field tilted Landau levels. Here, we study temperature dependence of these oscillations in a very-high mobility two-dimensional electron system. Our results suggest that, in contrast to Shubnikov-de Haas effect, HIRO are sensitive to electron-electron interactions modifying the single particle lifetime. [1] C. L. Yang, J. Zhang, R. R. Du, J. A. Simmons, and J. L. Reno, Phys. Rev. Lett. {\bf 89}, 076801 (2002) [Preview Abstract] |
Monday, March 16, 2009 8:24AM - 8:36AM |
A21.00003: Non-linear transport in microwave-irradiated 2D electron systems at the cyclotron resonance subharmonics Hung-Sheng Chiang, Anthony Hatke, Michael Zudov, Loren Pfeiffer, Ken West We study microwave photoresistivity oscillations in a high mobility two-dimensional electron system subject to strong dc electric fields. We find [1] that near the second subharmonic of the cyclotron resonance the frequency of the resistivity oscillations with dc electric field is twice the frequency of the oscillations at the cyclotron resonance, its harmonics, or in the absence of microwave radiation. This observation is discussed in terms of the microwave-induced sidebands in the density of states and the interplay between different scattering processes in the separated Landau level regime. [1] A.~T.~Hatke, H.-S. Chiang, M.~A.~Zudov, L.~N.~Pfeiffer, and K.~W.~West, Phys. Rev. Lett. accepted for publication. [Preview Abstract] |
Monday, March 16, 2009 8:36AM - 8:48AM |
A21.00004: Observation of Fractional Microwave-Induced Resistance Oscillations using Co-Planar Waveguide on High-Mobility 2DES Kristjan Stone, Rui-Rui Du, Loren Pfeiffer, Ken West The microwave-induced resistance oscillations (MIRO) are commonly observed in high-mobility GaAs 2D electron systems (2DES) irradiated by microwaves. Usually this is accomplished using an antenna or waveguide, where the electromagnetic components (E$_{\omega }$ and H$_{\omega })$ coincide with the 2DES plane. We explore MIRO in a co-planar waveguide (CPW) geometry, in which E$_{\omega }$ is the dominant excitation component in the 2DES plane. Our samples are Hall bars of high-mobility, $\mu $= (6 - 12) $\times $10$^{6}$ cm$^{2}$/Vs, GaAs/Al$_{x}$Ga$_{1-x}$As quantum wells with electron densities ranging from 3 to 5 $\times $10$^{11}$cm$^{-2}$. Microwaves from a tunable source (2 - 40 GHz) were fed in, via a semi-rigid coax cable, to an impedance-matched CPW across the length of the Hall bar, and brought out via a similar semi-rigid coax to a power sensor. Using this CPW geometry, we are able to simultaneously measure the photoconductivity and the microwave transmission across the sample. In a temperature range of 2.0 K - 5.0 K, we observed fractional MIRO associated with $\varepsilon $ =1/2, 1/3, 1/4, and 1/5, where $\varepsilon =\omega $/$\omega _{c}$, and $\omega _{c}$ is the cyclotron frequency. Experimental data as well as a brief discussion will be presented. The work at Rice was funded by NSF DMR-0706634. [Preview Abstract] |
Monday, March 16, 2009 8:48AM - 9:00AM |
A21.00005: Photoconductivity of a 2D electron gas at large filling factors Ivan Dmitriev, Maxim Khodas, A.D. Mirlin, D.G. Polyakov, Maxim Vavilov We study non-equilibrium dc conductivity of a 2D electron gas, placed in a classically strong perpendicular magnetic field in the presence of in-plane microwave field and generic Gaussian disorder potential. Focusing the consideration on the bilinear response in the microwave field, we identify four different mechanisms essential for the linear dc resistance. We employ two specific models of the disorder relevant for ultra-high mobility samples and show that the relative strength of the above mechanisms strongly depends on the spatial range of the disorder potential. In particular, when large angle scattering dominates the transport and temperature is sufficiently high, the contribution of the ``displacement'' mechanism can overcome the ``inelastic'' contribution, which is dominant at low temperature. For smooth disorder, characterized by small angle scattering, the ``displacement'' contribution is strongly suppressed. Other contributions are responsible for the microwave-induced corrections to the non-diagonal part of the conductivity tensor and only weakly depend on the nature of the disorder. We discuss the ways to distinguish experimentally the contributions of the above mechanisms according to their different polarization and temperature dependence. [Preview Abstract] |
Monday, March 16, 2009 9:00AM - 9:12AM |
A21.00006: Warming in systems with discrete spectrum: spectral diffusion of two dimensional electrons in magnetic field Sergey Vitkalov, Natalia Romero Kalmanovitz, Alexey Bykov Warming in complex physical systems, in particular global warming, attracts significant contemporary interest. It is essential, therefore, to understand basic physical mechanisms leading to overheating. It is well known that application of an electric field to conductors heats electric charge carriers. Often an elevated electron temperature describes the result of the heating. This paper demonstrates that an electric field applied to a conductor with discrete electron spectrum produces a non-equilibrium electron distribution, which cannot be described by temperature. Such electron distribution changes dramatically the conductivity of highly mobile two dimensional electrons in a magnetic field, forcing them into a state with a zero differential resistance. Most importantly the results demonstrate that, in general, the effective overheating in the systems with discrete spectrum is significantly stronger than the one in systems with continuous and homogeneous distribution of the energy levels at the same input power. [Preview Abstract] |
Monday, March 16, 2009 9:12AM - 9:24AM |
A21.00007: Effect of parallel magnetic field on the zero-differential resistance state Natalia Romero, Sean McHugh, Myriam P. Sarachik, Sergey A. Vitkalov, A. A. Bykov The non-linear zero-differential resistance state (ZDRS) that occurs for highly mobile two-dimensional electron systems in response to a dc bias in the presence of a strong magnetic field applied perpendicular to the electron plane is suppressed and disappears gradually as the magnetic field is tilted away from the perpendicular at fixed filling factor $\nu$. Good agreement is found with a model that considers the effect of the Zeeman splitting of Landau levels enhanced by the in-plane component of the magnetic field. [Preview Abstract] |
Monday, March 16, 2009 9:24AM - 9:36AM |
A21.00008: Magnetoresistance of two-dimensional electrons in Si/SiGe quantum wells in in-plane magnet field at 20 mK T.M. Lu, L. Sun, D.C. Tsui, S. Lyon, W. Pan, M. Muhlberger, F. Schaffler, J. Liu, Y.H. Xie We have measured the magnetoresistance of two-dimensional electrons in two modulation-doped Si/SiGe quantum wells in an in-plane magnetic field at 20mK. It was found that the ratio of the saturation resistance in high in-plane magnetic field to the zero-magnetic-field resistance is dependent on the electron density. At high electron density, the ratio is approximately 1.8. As the electron density decreases and is close to the metal-insulator transition, the ratio is strongly enhanced and appears diverging at a sample dependent characteristic density. The field at which the magnetoresistance saturates as a function of density is linear at high density. It deviates from this linear dependence and appears to extrapolate to zero when the electron density is below $\sim $0.7x10$^{11}$/cm$^{2}$. [Preview Abstract] |
Monday, March 16, 2009 9:36AM - 9:48AM |
A21.00009: Electron and Hole Transport in 40 MilliKelvin Germanium $<100>$ Kyle Sundqvist Ultrapure germanium at milliKelvin temperatures presents a charge transport regime which is rarely encountered. In this case, thermal phonons play a negligible role and the scattering of electrons and holes is dominated by spontaneous phonon emission. As these carriers are always hot, typical assumptions of thermal equilibrium are no longer valid. Furthermore, for fields of only a few $V/cm$, the emission of optical and intervalley phonons is highly inelastic such that carrier distributions may differ substantially from the form of a displaced Maxwellian. We present simulation results of transport processes of carriers in germanium $<100>$ at a temperature of $40 ~mK$. These studies were performed in order to provide a deeper understanding of processes occurring in detectors of the Cryogenic Dark Matter Search (CDMS), which seeks to detect weakly-interacting massive particles (WIMPs) in the halo of our galaxy. As CDMS measures both the ionized charge and the energy in non-thermalized phonons created by particle interactions, we will describe the applicability of these transport simulation results to a wide variety of measured phenomena. [Preview Abstract] |
Monday, March 16, 2009 9:48AM - 10:00AM |
A21.00010: Magnetotransport in Zener Tunneling Regime in a High-Mobility Two-Dimensional Hole System Yanhua Dai, Zhuoquan Yuan, Changli Yang, R.R. Du, M. J. Manfra, L. N. Pfeiffer, K. W. West Magnetotransport in two-dimensional electron systems (2DES) under a DC-current bias has recently revealed a number of interesting phenomena, including current-induced Zener oscillations [1] and current-induced spin-polarization in Rashba 2DES. We have measured the DC-current induced magnetotransport in high-mobility 2D holes in a C-doped (100) GaAs/Al$_{0.4}$Ga$_{0.6}$As quantum well (QW). The QW has a width of 15 nm and a carrier density p $\sim $ 2 x 10$^{11}$/cm$^{2 }$and a mobility $\mu $= 7 x 10$^{5}$ cm$^{2}$/Vs at T = 300 mK. We observe sharp features in the differential resistance, which we interpret as the Zener tunneling peak and valley associated with commensuration transition of Landau orbits. In a gated Hall bar we are able to tune the carrier density to p $>$ 2.6 x 10$^{11}$/cm$^{2}$, and observe strong positive magnetoresistance, which can be attributed to the inter-subband scattering with light holes. We will discuss the roles that electron - electron scattering plays in the Zener oscillations observed in electron and hole systems. The work at Rice was supported by NSF DMR-0706634. [1] C. L. Yang et al, Phys. Rev. Lett. 89, 076801 (2002). [Preview Abstract] |
Monday, March 16, 2009 10:00AM - 10:12AM |
A21.00011: Strongly Temperature-dependent Compressibility of Dilute 2D Holes near the Metal-Insulator Transition Xuan Gao, Naoto Masuhara, Greg Boebinger, Loren Pfeiffer We used the capacitance measurement to study the compressibility of dilute 2D holes in a 10nm wide GaAs quantum well for $T$=0.01-0.7K. The sample exhibits the $B$=0 metal-insulator transition (MIT) at a critical density $p_{c} \quad \sim $ 1.0 $\times $ 10$^{10 }$/cm$^{2}$. Deep in the metallic state, the sample capacitance decreases slowly as hole density $p$ increases, due to the (negative) exchange contribution to the compressibility of an interacting 2D system. As $p$ is reduced below $p_{c}$ at low-$T$, the capacitance of sample diminishes rapidly as a result of the incompressible nature of the insulator state, similar to previous studies (Dultz and Jiang, PRL 84, 4689 (2000); Allison et al., PRL 96, 216407 (2006)). On the other hand, we found that temperature has a strong effect near the MIT, in contrast to literature. In our system, the compressibility of insulator state increases with $T$ and remains positive, while the behavior of metallic phase is more complex. Notably, for metallic phase with $p$ slightly above $p_{c}$, the sign of compressibility can change from positive to negative as $T$ increases. This strongly $T$-dependent compressibility is possibly related to the competition between two phases with distinctive compressibility in our system, which is more strongly interacting than samples studied previously. [Preview Abstract] |
Monday, March 16, 2009 10:12AM - 10:24AM |
A21.00012: Branched flow and caustics in random media with magnetic fields Jakob Metzger, Ragnar Fleischmann, Theo Geisel Classical particles as well as quantum mechanical waves exhibit complex behaviour when propagating through random media. One of the dominant features of the dynamics in correlated, weak disorder potentials is the branching of the flow. This can be observed in several physical systems, most notably in the electron flow in two-dimensional electron gases {[}1], and has also been used to describe the formation of freak waves {[}2]. We present advances in the theoretical understanding and numerical simulation of classical branched flows in magnetic fields. In particular, we study branching statistics and branch density profiles. Our results have direct consequences for experiments which measure transport properties in electronic systems {[}3].\\ \\ {[}1] e.g. M. A. Topinka \emph{et al}., Nature \textbf{410}, 183 (2001), M. P. Jura \emph{et al.}, Nature Physics \textbf{3}, 841 (2007)\\ {[}2] E. J. Heller, L. Kaplan and A. Dahlen, J. Geophys. Res., \textbf{113}, C09023 (2008)\\ {[}3] J. J. Metzger, R. Fleischmann and T. Geisel, \emph{in preparation} [Preview Abstract] |
Monday, March 16, 2009 10:24AM - 10:36AM |
A21.00013: Possible competing ground states in high mobility electron-hole bilayers K. Das Gupta, A.F. Croxall, C.A. Nicoll, M. Thangaraj, H.E. Beere, I. Farrer, D.A. Ritchie, M. Pepper Recently it has become possible to fabricate independently contacted high mobility electron-hole bilayers (EHBL) with densities $<5\times10^{10}{\rm cm}^{-2}$ and a separation 10-20 nm in a GaAs/AlGaAs system. In these EHBLs the interlayer interaction can be stronger than the intralayer interactions. Excitonic superfluidity in such EHBLs was first predicted almost forty years ago. Since then theoretical works have indicated the possibility of a very rich phase diagram, containing a superfluid, charge density waves, Wigner crystals and a BCS-BEC crossover. However this system has been extremely difficult to fabricate in practice. Very recent experiments have revealed novel features in the interlayer scattering (Coulomb drag) below $\sim 1{\rm K}$. The Coulomb drag shows strong non-monotonic deviations from a $\sim T^2$ behaviour expected for Fermi-liquids at low temperatures. Simultaneously an insulating behaviour in the single layer resistances also appears in both layers inspite of electron mobilities $> 10^6{\rm cm}^{2}{\rm V}^{- 1}{\rm s}^{-1}$ and hole mobilities $> 10^5{\rm cm}^{2}{\rm V}^{-1}{\rm s}^{-1}$. The experimental results may indicate a competition between an excitonic ground state and charge-density- waves. [J. Keogh {\it et al} APL, 87,202104 (2005). A.F.Croxall {\it et al} arXiv:0807.0117 (to appear in JAP), A.F. Croxall {\it et al} arXiv:0807.0134v3 (to appear in PRL)] [Preview Abstract] |
Monday, March 16, 2009 10:36AM - 10:48AM |
A21.00014: Anomalous plateau formation and improved quantization in charge pumping under the application of a perpendicular magnetic field Samuel Wright, Mark Blumenthal, Godfrey Gumbs, Adam Thorn, Michael Pepper, T.J.B.M. Janssen, Stuart Holmes, Dave Anderson, Geb Jones, Christine Nicoll, Dave Ritchie We present experimental results of high frequency quantized charge pumping through a quantum dot formed by the electric field arising from applied voltages in a GaAs/AlGaAs system in the presence of a perpendicular magnetic field $B$. Improved quantization and robustness in gate voltage are seen as $B$ is increased. Under application of even higher $B$ fields, the formation of anomalous plateaus in the pumped current are seen. [Preview Abstract] |
Monday, March 16, 2009 10:48AM - 11:00AM |
A21.00015: Thermal transport size effects in self-assembled Germanium quantum dots in single-crystal silicon Jean-Numa Gillet Superlattices with low thermal conductivity have been used to design 1-D thermoelectric materials. With them, it is challenging to obtain a thermoelectric figure of merit \textit{ZT} $>$ 1. Self-assembly is used to fabricate Ge quantum-dot (QD) arrays. High \textit{ZT} is expected in these self-assembled Ge QDs arrays in Si since they are single crystals. We prove that high-density 3-D Ge QD arrays in diamond-cubic Si exhibit low thermal conductivity. This property can be used to design 3-D thermoelectric devices. To study the thermal behavior of these 3-D `phononic crystal' nanocomposites, we create an atomistic model of a supercell consisting of Si unit cells. Inside each supercell, we substitute Si atoms with Ge atoms to form a QD. The thermal conductivity has been shown to reduce below 0.2 W/m/K. Such a result is realized by ensuring minimum group velocities. Further reduction is expected from multiple scattering. We are concerned with the size dependence of the thermal conductivity upon the Ge volumic composition $f$. From preliminary results with constant $f$, we obtain an exponential-like thermal-conductivity decrease when the supercell size is increased. [Preview Abstract] |
Session A22: Focus Session: Spins in Group III-V and II-VI Semiconductors
Sponsoring Units: GMAG DMP FIAPChair: Paul Crowell, University of Minnesota
Room: 324
Monday, March 16, 2009 8:00AM - 8:36AM |
A22.00001: Electrically Injected Spin Polarized Lasers Invited Speaker: The ability to electrically modulate orthogonal polarization states in spin-polarized lasers opens up avenues for a wide range of applications such as photochemical spectroscopy, optical switches, and communications with enhanced security [1]. This has motivated us to investigate the properties of quantum well (QW) [2] and quantum dot (QD) [3] spin-polarized vertical cavity surface emitting lasers (spin-VCSELs). The laser heterostructures are grown by molecular beam epitaxy (MBE). The active region consists of In0.2Ga0.8As/GaAs QWs [2] or InAs QDs [3]. VCSELs are fabricated using standard micro-fabrication techniques. The FM Schottky tunnel contact is realized with Fe or MnAs re-grown by MBE. The QW spin-VCSELs exhibit a maximum threshold current reduction of 11 {\%} and output degree of circular polarization of 23 {\%} at 50 K. The corresponding values observed in QD spin VCSELs at 200 K are 8 {\%} and 14 {\%}, respectively. Inhibition of the D'yakonov-Perel spin scattering process results in higher operating temperatures for spin-lasers with QD active region. In addition, we have demonstrated electrical modulation of the output polarization with a peak modulation index of 0.6. The spin polarization of carriers in the active region of a spin laser gives rise to large gain anisotropy at biases near threshold. As a result, the output polarization can be much larger than the spin polarization of the injected carriers. This is contrary to the linear relation between carrier spin orientations in the active region and the polarization of photons emitted upon their radiative recombination in spin light emitting diodes. The exact magnitude of the output polarization in spin lasers and the parameters upon which it depends have been analytically determined and are in excellent agreement with those obtained from measurements. These results will be described and discussed. \\[4pt] References: \\[0pt] [1] M. Holub et al., J. Phys. D. 40, R179 (2007). \\[0pt] [2] M. Holub et al., Phys. Rev. Lett. 98, 146603 (2007). \\[0pt] [3] D. Basu et al., Appl. Phys. Lett. 92, 091119 (2008). \\[0pt] [4] M. I. D'yakonov et al., Sov. Phys Solid State. 13, 3023 (1971) [Preview Abstract] |
Monday, March 16, 2009 8:36AM - 8:48AM |
A22.00002: Electrical Spin injection from Fe into ZnSe. Aubrey Hanbicki, G. Kioseoglou, M.A. Holub, O.M.J. van 't Erve, B.T. Jonker The wide bandgap semiconductor ZnSe is an opto-electronic material with a comparable spin lifetime and small lattice mismatch to GaAs. Novel spintronic devices that incorporate ZnSe/GaAs heterostructures will require the facile transport of spin information across several heterointerfaces including spin injection into the ZnSe. We have electrically injected spin-polarized electrons from a ferromagnetic Fe contact into a ZnSe epilayer grown on a GaAs heterostructure. The injected carriers proceed through 300 nm of ZnSe and recombine in the GaAs emitting light characteristic of the bulk GaAs exciton. We measure spin polarizations in excess of 40{\%} in the GaAs based on analysis of the circular polarization of the electroluminescence. We report results as a function of applied magnetic field, device current and temperature. The spin injection process and transport through the ZnSe layer sustains significant spin populations in this heterostructure. This work was supported by core programs at NRL. [Preview Abstract] |
Monday, March 16, 2009 8:48AM - 9:00AM |
A22.00003: Tuning spin-current across a Semiconductor/Ferromagnet junction by resonance tunneling Pengke Li, Hanan Dery We present a theory of spin-dependent transport in a hybrid semiconductor/ferromagnet system which includes an asymmetric double barrier region at the interface (e.g., GaAs/AlGaAs/GaAs/Fe). The system has two electron confinement regions with one being a thin quantum well between a heterostructure barrier and a Schottky barrier. The second confinement region is a two dimensional electron gas (2DEG) at the heterointerface with the bulk semiconductor generated by intentional doping. The I-V curve has two current peaks when electrons tunnel into the ferromagnet. These peaks are due to resonance tunneling of electrons whose energy matches the energy of the quasi-bound state in the quantum well. The first peak is governed by tunneling of delocalized electrons from the bulk semiconductor and the second by escape from the 2DEG. These resonances are met at different bias levels and correspond to opposite spin polarization of the current. [Preview Abstract] |
Monday, March 16, 2009 9:00AM - 9:12AM |
A22.00004: Triggering phase-coherent spin packets by pulsed electrical spin injection across an Fe/GaAs Schottky barrier B. Beschoten, L.R. Schreiber, C. Schwark, G. Guentherodt, C. Adelmann, C.J. Palmstrom, X. Lou, P.A. Crowell The precise control of spins in semiconductor spintronic devices requires an electrical means for generating spin packets with a well-defined initial phase. So far, ultrafast laser pulses have successfully been used to trigger the ensemble phase of optically generated spin packets. However, electrical methods for ensemble phase triggering remain challenging. Here, we use fast current pulses to inject phase triggered electron spin packets across an Fe/GaAs Schottky barrier into n-GaAs. We demonstrate phase coherence by the observation of multiple Larmor precession cycles for current pulse widths down to 500 ps at 20 K. We show that the current pulses are broadened by the charging and discharging time of the Schottky barrier. At high frequencies, the observable spin coherence is limited only by the finite band width of the current pulses, which is on the order of 2 GHz. [Preview Abstract] |
Monday, March 16, 2009 9:12AM - 9:48AM |
A22.00005: The importance of Fe interface states for ferromagnet-semiconductor based spintronic devices Invited Speaker: I present our recent theoretical studies of the bias-controlled spin injection, detection sensitivity and tunneling anisotropic magnetoresistance in ferromagnetic-semiconductor tunnel junctions. Using first-principles electron transport methods we have shown that Fe 3{\it d} minority-spin surface (interface) states are responsible for at least two important effects for spin electronics. First, they can produce a \emph{sizable} Tunneling Anisotropic Magnetoresistance in magnetic tunnel junctions with a \emph{single} Fe electrode. The effect is driven by a Rashba shift of the resonant surface band when the magnetization changes direction. This can introduce a new class of spintronic devices, namely, Tunneling Magnetoresistance junctions with a single ferromagnetic electrode that can function at room temperatures. Second, in Fe/GaAs(001) magnetic tunnel junctions they produce a \emph{strong} dependence of the tunneling current spin-polarization on applied electrical bias. A dramatic \emph{sign reversal} within a voltage range of just a few tenths of an eV is found. This explains the observed sign reversal of spin-polarization in recent experiments of electrical spin injection in Fe/GaAs(001) and related reversal of tunneling magnetoresistcance through vertical Fe/GaAs/Fe trilayers. We also present a theoretical description of electrical spin-detection at a ferromagnet/semiconductor interface. We show that the sensitivity of the spin detector has \emph{strong} bias dependence which, in the general case, is \emph{dramatically different} from that of the tunneling current spin-polarization. We show that in realistic ferromagnet/semiconductor junctions this bias dependence can originate from two distinct physical mechanisms: 1) the bias dependence of tunneling current spin-polarization, which is of \emph{microscopic} origin and depends on the specific properties of the interface, and 2) the \emph{macroscopic} electron spin transport properties in the semiconductor. Our numerical results show that the magnitude of the voltage signal can be tuned over a wide range from the second effect {\it alone} and thus identifies a universal method for enhancing electrical spin-detection sensitivity in ferromagnet/semiconductor tunnel contacts. [Preview Abstract] |
Monday, March 16, 2009 9:48AM - 10:00AM |
A22.00006: Three Terminal Spin Extraction Resistance in Fe/GaAs Heterostructures E.S. Garlid$^1$, T. Kondo$^1$, Q. Hu$^{1,2}$, C.J. Palmstr\O m$^{1,2}$, P.A. Crowell$^1$ Spin transport measurements have been difficult to interpret in two terminal Fe/GaAs/Fe devices where current flows in both the injector and detector electrodes. This is due to the strong non-monotonic dependence of the spin accumulation on the Fe/GaAs interface bias, which affects the spin injection and detection efficiencies. To address this, we measured the four terminal non-local spin valve resistance and the three terminal spin extraction resistance in epitaxial Fe/GaAs heterostructures with a systematically varied Schottky barrier doping profile. Lateral devices were fabricated from epitaxial Fe/n$^+$/n-GaAs (100) heterostructures in which the thickness of the n$^+$ layer (n$^+=5\times 10^{18}$) was varied from 5 to 50 nm while n $=5\times 10^{16}$ in the 2.5 $\mu$m channel. The three terminal resistance measured using a single contact as the injector and detector is $\approx 100\times$ larger than the non-local spin valve resistance, an effect which cannot be attributed to spin relaxation in the channel. In the case of a three terminal measurement, we obtain both a large spin accumulation as well as an enhanced detection sensitivity under forward bias conditions. This can be analyzed by considering the measured non-local spin polarization as a function of bias, as well as the electric fields at the Fe/GaAs interface in the presence of a charge current. Supported by ONR and the NSF MRSEC, and NNIN programs. [Preview Abstract] |
Monday, March 16, 2009 10:00AM - 10:12AM |
A22.00007: Contributions to oblique Hanle linewidths in Fe/GaAs non-local spin valve transport Chaffra Awo-Affouda, O. M. J. van 't Erve, G. Kioseoglou, A. T. Hankbicki, M. Holub, C. H. Li, B. T. Jonker The transport Hanle effect linewidth is commonly used to determine spin lifetimes in spin- polarized transport structures. We show that the magnetic domain structure of the ferromagnetic contacts used to inject and detect the spin current introduces asymmetries to the Hanle lineshape. In addition, the nuclear spin polarization can produce anomalous narrowing and broadening of the Hanle linewidth depending upon the orientation of the transport spin and the applied field. These contributions can significantly impact the apparent spin lifetime extracted from the Hanle curve, but are not included in the analysis typically applied. [Preview Abstract] |
Monday, March 16, 2009 10:12AM - 10:24AM |
A22.00008: High Optical Polarization from Electrical Spin Injection into an InGaAs QW C.H. Li, G. Kioseoglou, M. Holub, O.M.J. van 't Erve, B.T. Jonker, T. Ali, I. Khan, M. Yasar, A. Petrou We have fabricated spin light emitting diodes (LEDs) with Fe as the spin injector and 100{\AA} In0.1Ga0.9As/GaAs QWs as the detector. The emission efficiency from the InGaAs QW is extremely high, with a narrow linewidth of 4meV at 5K. The free exciton exhibits 25{\%} optical polarization due to the injection of spin polarized carriers from the reverse-biased Fe Schttky contact. At low biases, a feature 10meV below the free exciton appears which exhibits a much larger polarization with a peculiar magnetic field dependence. Similar to that of the free exciton, the circular polarization of this lower energy feature first increases with magnetic field, and reaches a maximum of 67{\%} at 2.5T, indicating injection from Fe. However, this behavior is superposed on a large diamagnetic background of 21{\%}/T which dominates above 2.5T. The intensity and polarization of this feature is strongly bias dependent, and the feature disappears above 15K, suggesting that it originates from a weakly bound complex. The origin of this feature and its dependence on the magnetic field will be discussed at the meeting. Supported by ONR, NRL core funds, and NSF. [Preview Abstract] |
Monday, March 16, 2009 10:24AM - 10:36AM |
A22.00009: Spin polarization control through resonant states in an Fe/GaAs Schottky barrier Atsufumi Hirohata, Shuta Honda, Hiroyoshi Itoh, Jun-ichiro Inoue, Hidekazu Kurebayashi, Theodossis Trypiniotis, C. H. W. Barnes, J. A. C. Bland We show that \textit{the IRSs }\textit{(Interface Resonant States)}\textit{ within the Schottky barrier }play an important role for the negative spin polarization of the current and its bias dependence, and compare with our experimental results [1]. We have calculated the spin polarization $P$ of the tunnel conductivity using a full-orbital tight-binding model, and have shown that the IRSs within the Schottky barrier in the GaAs layer influence significantly the spin-dependent tunneling across the interface. It has been clearly shown that the band matching of the IRSs plays a crucial role on the spin polarization. The theoretical results account well for earlier experimental results including the tunneling of photo-excited electrons. The present results suggest that the spin polarization can be controlled by the Schottky barrier heights, and that a spin-switch device with bias control may also be promising. Quantitative performance of the device, however, needs more quantitative calculations including effects of atomic disorder for example. [1] H. Kurebayashi \textit{et al.}, \textit{Appl. Phys. Lett.} \textbf{91}, 102114 (2007). [Preview Abstract] |
Monday, March 16, 2009 10:36AM - 10:48AM |
A22.00010: Time dependent analysis of spin transport in lateral semiconductor/ferromagnet structures with non-collinear magnetization Yang Song, Hanan Dery We model the transport in lateral semiconductor channels beneath ferromagnetic contacts with non-collinear magnetization directions. We quantify the effects of the mixing conductance and of the spin polarization across the interface, of the electrical field in the channel and of the resistance ratio between the channel and the interface. We focus on a non-local spin valve geometry in which two contacts are biased and collinear and a third terminal is non-collinear and ``semi'' floating (connected in series with a capacitor). This structure can be used for memory devices with multi-valued stored bits by rotating the magnetization in one of the terminals and detecting the transient current signal that flows through the non-collinear terminal. The shape and magnitude of this current signal is strongly influenced by the relation between the non- collinear magnetization direction and the (2D) spin accumulation in the channel that is being set by the biased (collinear) contacts. [Preview Abstract] |
Monday, March 16, 2009 10:48AM - 11:00AM |
A22.00011: Magnetic force detection of non-equilibrium electron spin-polarization in n-GaAs Vidya Bhallamudi, Gang Xiang, Mark Brenner, Youngwoo Jung, Yuri Obukhov, Denis Pelekhov, Steve Ringel, P. Chris Hammel Magnetic Force Microscope (MFM) offers an alternative to optical and electrical techniques for detecting and imaging spin-polarized electron populations in semiconductor spintronic devices. Unlike other methods, MFM has the advantage of being material non-specific as it directly detects spins in the semiconductor through their magnetic dipole coupling to micromagnetic tip. However, it is challenging to achieve the high sensitivity required for sensing small non-equilibrium spin populations, orders of magnitude smaller than those in ferromagnetic materials. Here we present our progress developing a high sensitivity cryogenic MFM for imaging optically injected electronic spins in GaAs. Spins are created in an epitaxially grown n-GaAs membrane by circularly polarized laser shone from an optical fiber. A cantilever scans over the membrane and detects the magnetic force due to the optically injected spins. Micro-magnetic tip generating large field gradient is used for enhancing the signal. We will show simulation results for the expected forces, taking spin relaxation, diffusion and local tip field into account. The status of spin imaging experiment will also be presented. [Preview Abstract] |
Session A23: Plasmons and Optical Absorption
Sponsoring Units: DCMPChair: David Singh, Oak Ridge National Laboratory
Room: 325
Monday, March 16, 2009 8:00AM - 8:12AM |
A23.00001: Ternary cobalt spinel oxides for solar driven hydrogen production: Theory and experiment Aron Walsh, Kwang-Soon Ahn, Sudhakar Shet, Muhammad N. Huda, Todd Deutsch, Heli Wang, John A. Turner, Yanfa Yan, Mowafak M. Al-Jassim, Su-Huai Wei Discovery of a chemically stable, light absorbing and conductive metal oxide with band edges aligned to water redox potentials has been a goal of physical scientists for the past forty years. Despite an immense amount of effort, no solution has been uncovered. We will present the results of our combined theoretical and experimental exploration of a series of unconventional ternary cobalt spinel oxides, which offer chemical functionality through substitution on the tetrahedral spinel A site. First-principles predictions of the substitution of group 13 cations (Al, Ga, In) in Co$_3$O$_4$ to form a series of homologous CoX$_2$O$_4$ spinel compounds are combined with experimental synthesis and photoelectrochemical characterization. Ultimately, while tunable band gaps in the visible range can be obtained, the material performance is limited by poor carrier transport properties associated with small polarons. Future design pathways for metal oxide exploration will be briefly discussed. [Preview Abstract] |
Monday, March 16, 2009 8:12AM - 8:24AM |
A23.00002: Multiple electron generation in a sea of electronic states Wayne Witzel, Andrew Shabaev, Alexander Efros, Carl Hellberg, Jacobs Verne In traditional bulk semiconductor photovoltaics (PVs), each photon may excite a single electron-hole, wasting excess energy beyond the band-gap as heat. In nanocrystals, multiple excitons can be generated from a single photon, enhancing the PV current. Multiple electron generation (MEG) may result from Coulombic interactions of the confined electrons. Previous investigations have been based on incomplete or over-simplified electronic-state representations. We present results of quantum simulations that include hundreds of thousands of configuration states and show how the complex dynamics, even in a closed electronic system, yields a saturated MEG effect on a femtosecond timescale. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Monday, March 16, 2009 8:24AM - 8:36AM |
A23.00003: Quantum confined stark effect in non-identical InAs/GaAs coupled quantum dots: Dependence on vertical electrical field Muhammad Usman, Gerhard Klimeck InAs/GaAs coupled quantum dot (QDs) have gained much attention for optical and quantum computing applications. Due to strain, originating from assembly of lattice- mismatched semiconductors, quantum dot tend to grow in the vertical direction. These stacked QDs are strongly coupled through strain field, which is atomistically inhomogeneous and penetrates deep into GaAs buffer layer surrounding the dots. Piezoelectric field must be taken into account to properly model the experimentally observed symmetry breaking and a global shift in the energy spectra of the system. Vertical electrical field applied in the growth direction results in the red shift of emission spectra due to the quantum confined stark effect. Previous studies are based on kp method that ignore the crystal symmetry, optical anisotropy and piezoelectricity effects. In this work, we apply a twenty band sp$^{3}$d$^{5}$s* atomistic tight binding model to study the experimentally observed red shift of emission spectra resulting from an applied electrical field. We quantitatively compare the results for coupled QDs with the results for single QD. [Preview Abstract] |
Monday, March 16, 2009 8:36AM - 8:48AM |
A23.00004: Experimental Kataura plot from individual Single-Wall Carbon nanotubes on silicon substrate. Ya-Ping Hsieh, Mario Hofmann, Chi-Te Liang, Mildred S. Dresselhaus, Jing Kong The dependence of Raman scattering of individual carbon nanotubes on excitation energy was investigated. For this carbon nanotubes were grown on Silicon substrate and their Raman spectra were analyzed for a multitude of different laser excitation wavelength. Resonance windows for several tubes within one family were measured to obtain the energy of maximum intensity of the RBM feature. By carefully calibrating these RBM peak positions, this experimental data can generate an experimental Kataura plot, which was compared to the theoretical prediction. Finally, a relation between RBM frequency and diameter was obtained based on the experimental Kataura plot. These results will improve the chirality assignment of carbon nanotubes grown on silicon substrate. [Preview Abstract] |
Monday, March 16, 2009 8:48AM - 9:00AM |
A23.00005: Effect of Multi-Resonance Subband Structure on the Kerr Nonlinearity of Quantum-Cascade Lasers Jing Bai This work focuses on the investigation of the optical Kerr lensing effect in quantum-cascade lasers with multiple resonance levels. The Kerr refractive index $n_{2}$ is obtained through the third-order susceptibility at the fundamental frequency \textit{$\chi $}$^{(3)}$(\textit{ $\omega $}). Resonant two-photon processes are found to have almost equal contributions to \textit{$\chi $}$^{(3)}$(\textit{$\omega $}) as the single-photon processes, which result in the predicted enhancement of the positive $n_{2}$, and thus may enhance mode-locking of quantum-cascade lasers. Moreover, an isospectral optimization strategy for further improving $n_{2}$ through the band-structure design is also demonstrated, in order to boost the multimode performance of quantum-cascade lasers. Simulation results show that the optimized stepwise multiple-quantum-well structure has a twofold enhancement on $n_{2}$ over the original flat quantum-well structure. This leads to a refractive-index change $\Delta n$ of about 0.01, which is at the upper bound of those reported for typical Kerr medium. This stronger Kerr refractive index may be important for quantum-cascade lasers ultimately to demonstrate self-mode-locking. [Preview Abstract] |
Monday, March 16, 2009 9:00AM - 9:12AM |
A23.00006: Anisotropic Electronic Screening due to Fermi Surface Nesting in Graphite James Reed, Young Il Joe, Diego Casa, Thomas Gog, Y. Q. Cai, Peter Abbamonte We used inelastic X-ray scattering to measure the imaginary part of the density-density Green's function, Im[$\chi(\vec k;\omega)$], of a single crystal graphite sample along six direction in the Basal plane from [100] to [110]. To place Im[$\chi(\vec k;\omega)$] on an absolute scale we calculate a scaling coefficient using the optical sum rule. The real part of $\chi(\vec k;\omega)$ is calculated via the Kramers-Kronig transformation. We use an inversion algorithm to map the data into real space at various time intervals with attosecond time resolution. The images of the density response we produced show hexagonal anisotropy, which arises from scattering between the K and K' points of the Brillouin zone. Analysis of the data at $\omega=0$ provides us with the anisotripic induced electron density around a static impurity as function of distance. Integration of the local density around $\vec r=0$ gives the effective charge of the impurity from which we deduced the background dielectric constant, $\epsilon_{\infty}$, to be approximately 2.23. [Preview Abstract] |
Monday, March 16, 2009 9:12AM - 9:24AM |
A23.00007: Plasmons in the presence of Tamm-Shockley states with Rashba splitting at noble metal surfaces Abdel-Khalek Farid, Eugene Mishchenko Au(111) or similar noble metal surfaces feature Tamm-Shockley surface states that are known to possess considerable spin-orbit splitting of the Rashba type of order $\Delta=0.1$ eV. When interacting with an electromagnetic field such states are expected to have resonances when the frequency of the field is near the energy of the spin-orbit splitting $\Delta$. They originate from the intersubband transitions between spin-split subbands and can be observed in the frequency dependence of the surface impedance. Plasmons in thin metal films are gapless and can be strongly affected by these spin resonances, acquiring significant modification of the spectrum when it intersects the $\omega=\Delta$ line. Finally, an interesting demonstration of the intersubband resonances can be achieved when metal films are coated with ionic dielectrics that have a frequency of longitudinal/transverse optical phonons above/below $\Delta$. The dielectric function between the two optical phonon frequencies is negative which forbids propagation of conventional plasmon-polaritons. However, the presence of spin-orbit-split surface states allows plasmon-polaritons to exist in this otherwise forbidden range of frequencies. [Preview Abstract] |
Monday, March 16, 2009 9:24AM - 9:36AM |
A23.00008: Phases Shift in Sub-wavelength Plasmonic Hole on Thin Metal Film. Jun Xu, Hyungjin Ma, Nicholas X. Fang While recent study of extraordinary transmission of light through sub-wavelength plasmonic nanostructures shows promise of novel nanophotonic elements in sensing and display, the origin of such phenomena is still under hot debate. In this paper, we measured the phase delay of the squeezed light emerging from individual plasmonic holes. Near-field Scanning Optical Microscope (NSOM) has been used to measure the interference of transmitted and scattered light of an isolated sub-wavelength hole on thin metal film. Our results indicate that even with a 30nm perforated film, the observed phase shift can be as large as 300 degrees, well beyond the prediction from earlier theoretical models. Counter intuitively, the measured phase shift is sensitive to the wavelength, the film thickness but insensitive to hole diameter. Also, full scale simulation by COMSOL has been done to show the more detail features inside the metal film. Our study may provide new insight to compact and efficient optoelectronic devices. [Preview Abstract] |
Monday, March 16, 2009 9:36AM - 9:48AM |
A23.00009: Coupling between Surface Plasmon Resonance and electric current in Au stripes Miguel Angel Garcia, Aida Serrano, Jose de la Venta Surface Plasmon Resonance (SPR) is the most outstanding feature of noble metal films. SPR consists on a collective oscillation of the conduction electrons when excited optically in the appropriate geometrical and energy conditions. The electrical current passing trough the metal film involves also the movement of conduction electrons. Thus, coupling effects are expected between SPR and electrical resistivity. A modification of the SPR when a electrical current passes through the film, could allow the modulation of an optical signal by a electrical one. Similarly, when the film is illuminated at the SPR conditions, the oscillation of the conduction electrons and local heating can induce an enhancement of the electric resistivity that can be used to translate an optical signal into a electric one. Those effects could be useful in the development of new fast optoelectronic transducers. We present here results on Au stripes illuminated to induce the SPR while electric currents flow with different orientation with respect to the light polarization [Preview Abstract] |
Monday, March 16, 2009 9:48AM - 10:00AM |
A23.00010: Surface plasmon resonance enhanced binding of metal nanoparticles K.L. Chan, M.J. Zheng, K.W. Yu The interparticle force between metallic nanoparticles illuminated by laser light has been studied theoretically. When the distances between the particles are sufficiently small, the excitation of surface plasmon modes within these particles can lead to strongly enhanced laser fields. As a result, there are strongly enhanced light-induced binding forces between these nanoparticles. For physically realizable laser power, these forces can exceed the van der Waals forces by several orders of magnitude. In our theoretical calculations, we considered the interparticle force and potential between two approaching metal nanoparticles. The metal particles are routinely modelled as Drude metallic spheres, and the interparticle force has been captured conveniently by an approximate multiple image formula between two spheres. When the incident light frequency is near the surface plasmon resonant frequency, we find that the force varies nonmonotonically with the distance and a stable local minimum in the potential energy can be found, signifying a binding between the particles. On the experimental and technological side, these studies are also crucial to optical spectroscopy in the nanoscales. Work supported by the General Research Fund of the Hong Kong SAR Government. [1] J. P. Huang, K. W. Yu, G. Q. Gu Electrorotation of a pair of spherical particles, Phys. Rev. E, Vol. 65, art. no. 021401 (2002). [Preview Abstract] |
Monday, March 16, 2009 10:00AM - 10:12AM |
A23.00011: Giant Electric Field Generation in Nano-Metallic Cylinder Chains Due to Plasmon Propagation Angela Camacho, Juan Carlos Arias We present a study of the superficial plasmons propagation in a chain of nano-metalic cylinders by analyzing: size effect and coupling between the particles. Particularly, we focus on the main features of electric fields in the inter-cylinder regions due to their relationship with SERS(Surface-enhanced Raman Scattering). Giant electric fields have been observed in spherical nano-particles showing an enormous increasing of the cross section, which offers very interesting applications in molecular physics. We calculate the external radiation effect on chains of cylinders lateral and vertically coupled and examine the Plasmon formation in them. Specially, we study the Plasmon propagation depending on the particle size, the separation between them and the type of coupling. We find enhanced electric fields in the inter-particles regions showing the charge accumulation and border effects in cylinders, which are strongly dependent on the two above proposed parameters, and we also extend our results to possible Surface Enhanced Raman Scattering geometric effect to make a comparison with the spherical nano-particles. [Preview Abstract] |
Monday, March 16, 2009 10:12AM - 10:24AM |
A23.00012: Optical properties of medium size noble and transition metal nanoparticles Juan C. Idrobo, Sokrates T. Pantelides Using first-principles methods within time dependent density functional theory and the local density approximation (TDLDA) the absorption spectra of medium size ($\sim $20-80 atoms) silver, gold and copper nanoparticles have been calculated. The nanoparticles are fcc fragments with different aspect ratios. We find that in the case of Ag nanoparticles is well reproduced by classical electrodynamics theory based in Mie's formalism, using the dielectric function of bulk Ag and taking into account the nanoparticle shape. For the case of Cu and Au, there is a similarity in the overall features of the quantum mechanical and classical spectra, but no detailed agreement. We will discuss the role that the d-electrons among all the different elements and the surface states play in controlling the optical properties of the nanoparticles. This work was supported by GOALI NSF grant (DMR-0513048), DOE, the Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, and Alcoa Inc. [Preview Abstract] |
Monday, March 16, 2009 10:24AM - 10:36AM |
A23.00013: Electronic Coupling and Optimal Gap Size within a Metal Nanoparticle Dimer Ke Zhao, Claudia Troparevsky, Di Xiao, Adolfo Eguiluz, Zhenyu Zhang We study the electronic coupling between two metal nanoparticles using density functional theory methods. We show that a continuous change in the particle separation leads to an abrupt transition from strong to weak electronic coupling, which defines an optimal separation for the dimer. While in the weak-coupling regime the dimer behaves like isolated clusters, its crossing into the strong-coupling regime is signified by two distinct phenomena, namely, the onset of a net magnetic moment, and a maximum in the static polarizability. We also show that as the system switches over from strong to weak coupling regime, the response to an applied electric field is nonlinear even for very small fields. The strong dependence of the coupling on the atomic structure of the nanoparticles and their orientation is also discussed. Our study is expected to have an impact on a variety of systems composed of aggregates of nanoparticles. [Preview Abstract] |
Monday, March 16, 2009 10:36AM - 10:48AM |
A23.00014: Extraordinary Optical Transmission through Circular Nanotrenches in Ag Films Feng Wang, Min Xiao, Qihuo Wei This work reports studies on the extraordinary transmission of normally incident light through sub-wavelength circular nanotrenches in Ag films. The concentric periodic nanotrenches are perforated through 100nm thick Ag films by using focused-ion beam (FIB). Far-field transmission measurements show that under the illumination of linearly polarized white light, the transmitted light is not linearly polarized and exhibits broad-band enhanced transmission with the center wavelength varying with the periodicity of the trenches. These spectroscopic experimental results can be reproduced qualitatively through finite-difference time domain (FDTD) simulations. Especially, simulations show that the transmitted light is radially polarized at low frequencies, while azimuthally polarized at high frequencies. These interesting polarization statuses can be explained as a result of competition between transmission of s and p polarized light through periodic gratings of nanotrenches. [Preview Abstract] |
Session A24: Focus Session: Nanotube Synthesis
Sponsoring Units: DMPChair: Avetik Harutyunyan, Honda Research
Room: 326
Monday, March 16, 2009 8:00AM - 8:36AM |
A24.00001: Thermodynamic instabilities in nano-catalysts and their effects on the diameter of grown nanotubes Invited Speaker: Fe and Fe:Mo nanoclusters are becoming the standard catalysts for growing single-walled carbon nanotubes (SWCNTs) via chemical vapor decomposition (CVD). Contrary to the Gibbs-Thomson formalism, experimental results show that reducing the size of the catalyst beyond a certain limit requires increasing the (minimum) growth temperature. This apparent paradox is addressed in terms of solubility of C in Fe nanoclusters. By using first principles calculations, an innovative thermodynamic model is constructed to determine the behavior of the phases competing for stability. As a function of particle size, there are three scenarios: steady state-, limited-, or no-growth of SWCNTs, corresponding to unaffected, reduced, and zero solubility of C in the clusters. The results are extended to Fe-Mo binary catalysts. The 15+ year long-standing question about the effects of Mo concentration on the growth capability is finally answered. Phys. Rev. Lett. {\bf 100}, 195502 (2008), Phys. Rev. B, {\bf 77}, 115450 (2008), Phys. Rev. B {\bf 75}, 205426 (2007). [Preview Abstract] |
Monday, March 16, 2009 8:36AM - 8:48AM |
A24.00002: CVD grown SWCNTs on Si substrate from DPN patterned catalyst precursor. Irma Kuljanishvili, Rachel Koltun, Scott Mayle, Venkat Chandrasekhar, Dmitriy Dikin , Sergey Rozhok Much interest has been generated around patterning and synthesis of high quality single wall carbon nanotubes (SWCNTs) into desired architectures. Here we report our work, undertaken to elucidate a simple method for delivering catalyst nanoparticles on defined locations on Si substrate via direct writing approach. We applied the Dip Pen Nanolithography (DPN) approach to pattern catalyst nanoparticles in selective locations on the substrate and developed a successful recipe for the subsequent CVD growth to produce high quality SWCNTs into scalable array geometries. Key parameters for successful implementation of this technology into devices or circuit architectures will be discussed. We will present our results on patterning, synthesis and characterization of SWCNTs as-grown on the substrate. Raman spectroscopy analysis, electrical and thermal properties of individual SWCNTs prepared into complex nanodevices will be presented in progress. [Preview Abstract] |
Monday, March 16, 2009 8:48AM - 9:00AM |
A24.00003: Effective Growth of Boron Nitride Nanotubes by Thermal-CVD Chee Huei Lee, Ming Xie, Derek Meyers, Jiesheng Wang, Yoke Khin Yap The synthesis of boron nitride nanotubes (BNNTs) are challenging as compared to the growth of carbon nanotubes (CNTs). Most of reported techniques required unique setup and temperatures $>$1300 $^{\circ}$C. Here we show that clean and long multiwalled BNNTs can be grown by simple catalytic thermal CVD. This was obtained by a growth vapor trapping approach inspired by the whisker nucleation theory. Based on our new findings, we have achieved patterned growth of BNNTs at desired locations. High resolution TEM shows that these BNNTs are highly crystallized. Besides, the tangential vibrational mode predicted by theory was detected in our BNNTs. This vibration mode could be the fingerprint for BNNTs with high crystallinity. [Preview Abstract] |
Monday, March 16, 2009 9:00AM - 9:12AM |
A24.00004: Co-optimizing carbon nanotube synthesis: control of diameter, structural quality, and growth kinetics along with simultaneous cost minimization Eric R. Meshot, Desire\'e L. Plata, Christopher M. Reddy, Philip M. Gschwend, A. John Hart We employ a decoupled CVD method that not only facilitates control of mean diameter and structural quality of vertically aligned CNTs, but also co-optimization of kinetics for efficient growth to ``forest'' heights of several millimeters. The growth substrate temperature (Ts) governs agglomeration of the catalyst film which primarily determines CNT diameter, while structural quality monotonically increases with Ts. Independent heating (Tp) of the reactant mixture generates a strikingly diverse population of active hydrocarbons. These analyses, in concert with real-time laser measurements of forest growth rate and height suggest that select products of gas treatment promote growth, while excessive gas-phase pyrolysis of hydrocarbons adversely affects the CNT structure. Further, we directly inject select compounds in the absence of thermal treatment, thus minimizing energetic costs. [Preview Abstract] |
Monday, March 16, 2009 9:12AM - 9:24AM |
A24.00005: Diameter and Geometry Control of Vertically Aligned SWNTs through Catalyst Manipulation Rong Xiang, Erik Einarsson, Jun Okawa, Yoichi Murakami, Shigeo Maruyama We present our recent progress on manipulating our liquid-based catalyst loading process, which possesses greater potential than conventional deposition in terms of cost and scalability, to control the diameter and morphology of single-walled carbon nanotubes (SWNTs). We demonstrate that the diameter of aligned SWNTs synthesized by alcohol catalytic CVD can be tailored over a wide range by modifying the catalyst recipe. SWNT arrays with an average diameter as small as 1.2 nm were obtained by this method. Additionally, owing to the alignment of the array, the continuous change of the SWNT diameter during a single CVD process can be clearly observed and quantitatively characterized. We have also developed a versatile wet chemistry method to localize the growth of SWNTs to desired regions via surface modification. By functionalizing the silicon surface using a classic self-assembled monolayer, the catalyst can be selectively dip-coated onto hydrophilic areas of the substrate. This technique was successful in producing both random and aligned SWNTs with various patterns. The precise control of the diameter and morphology of SWNTs, achieved by simple and scalable liquid-based surface chemistry, could greatly facilitate the application of SWNTs as the building blocks of future nano-devices. [Preview Abstract] |
Monday, March 16, 2009 9:24AM - 9:36AM |
A24.00006: Ethanol-promoted growth of dense vertically aligned small-diameter carbon nanotubes Yongyi Zhang, John Gregoire, John Hart We report the use of a small concentration of ethanol in addition to ethylene as the carbon source for growth of vertically aligned CNT ``forests.'' In our system, adding ethanol promotes the catalyst lifetime from approximately 20 minutes to nearly 60 minutes, and accordingly increases the forest height from 2.5 mm to over 5 mm, with CNT diameters of approximately 5 nm (2-3 walls). Spread composition films deposited by gradient sputtering, combined with non-destructive mapping of CNT diameter and alignment by small-angle X-ray scattering, enable high-throughput discovery of necessary and sufficient conditions for growth of small-diameter CNTs. Compared with the widely known water-assisted ``super growth'' process, we find that ethanol enables much finer control of the dewpoint and thus offers more consistent and tunable results. Also, ethanol is a weaker oxidant than water and therefore creates fewer structural defects due to unwanted etching of the CNT walls. [Preview Abstract] |
Monday, March 16, 2009 9:36AM - 9:48AM |
A24.00007: Manufacturing thin films of densely packed horizontally aligned carbon nanotubes Sameh Tawfick, A. John Hart Dense packing of carbon nanotubes (CNTs) over long-range dimensions is necessary to replicate their outstanding properties in functional thin films. We present a continuous method for transforming pillars of vertically aligned (VA) CNTs into densely packed, horizontally aligned (HA) CNT ribbons and sheets, which can be directly used on wafer-scale dimensions and/or patterned by photolithography and plasma etching to achieve feature dimensions down to the micron scale. In this process, a small roller is used to ``topple'' millimeter-tall VA-CNT microstructures and to simultaneously compress them, thus increasing the packing fraction of CNTs from 2{\%} to 60{\%}. We formulate design guidelines for selection of pattern geometry, roller diameter and material, and the kinetics of the rolling motion. This enables precise control of the HA-CNT film topography and thickness, and the packing density and orientation of the CNTs. Nanoindentation of the HA-CNT films reveals that the initial tortuosity of the VA-CNT forest determines the ultimate achievable densification. Electrical conductivity of ribbons is characterized using dc-four-point testing of lithographically-patterned CNT ribbons with Au contacts. The HA-CNT structures are easily transferred to other substrates, enabling integration with CMOS and MEMS fabrication, and with alternative substrates such as flexible plastics. [Preview Abstract] |
Monday, March 16, 2009 9:48AM - 10:00AM |
A24.00008: Incremental Growth of Single-Wall Carbon Nanotube Arrays Explored by Pulsed CVD Jeremy Jackson, Alex Puretzky, Igor Merkulov, Christopher Rouleau, Karren More, Norbert Thonnard, Gyula Eres, David Geohegan Gas pulses of variable duration and peak flux were used to explore the incremental growth and evolution of alignment of vertically-aligned carbon nanotubes arrays (VANTAs) by typical chemical vapor deposition within a tube furnace. Time-resolved reflectivity from Fe/Al catalyst-coated Si substrates was used to follow the growth of the arrays after the arrival of successive acetylene gas pulses injected into fast argon-hydrogen flows at 6 Torr total pressure. The evolution of alignment of the arrays measured with the in situ optical reflectivity data was correlated with SEM images for growth resulting from single- and multiple-pulse growth. The incremental length per pulse was varied from 20 nm to several microns in less than a second, corresponding to growth rates ranging up to 7 microns/second. Effects of repeated renucleation of growth along the nanotube wall structure were measured by HRTEM and Raman spectroscopy. [Preview Abstract] |
Monday, March 16, 2009 10:00AM - 10:12AM |
A24.00009: A model of dry-drawing of multiwall carbon nanotube forest into self-assembled sheets and yarns Alexander Kuznetsov, Alexandre Fonseca, Ray Baughman, Anvar Zakhidov A dry-state technique to produce highly-oriented, free-standing multiwalled carbon nanotube (MWNT) sheets and yarns which are mechanically strong, transparent, and highly oriented has been developed recently [1,2]. A model which allows to describe the main features of the process of dry-drawing self-assembly of vertically oriented multiwall carbon nanotube (MWCNT) forest into horizontal MWCNT sheets or yarns is developed in this presentation. The model is based on two main concepts: 1. self-strengthening of nanotube bundle interconnects during the bending-pulling process; 2. rearrangement of bundles by accordion-type stretching motion accompanied by detachment of bundles (at the top and bottom of the forest). This detachment occurs due to unzipping and self-strengthening of interconnects beyond a critical force, which permits to pull the next bundle from the forest, keeping the process of dry-drawing continuous. Developed model determines the parameters of CNT forest for which the dry-drawing is possible. It also allows to estimate such properties of the produced sheets and yarns as length, density, strength and electrical conductivity. [1] M. Zhang et al., Science \textbf{2005}, 309, 1215. [2] M. Zhang et al., Science \textbf{2004}, 306, 1358. [Preview Abstract] |
Monday, March 16, 2009 10:12AM - 10:24AM |
A24.00010: Termination mechanism of carbon nanotube forest growth Mostafa Bedewy, Eric R. Meshot, Yongyi Zhang, Haicheng Guo, Eric Verploegen, Wei Lu, A. John Hart Understanding the termination event in the growth of carbon nanotubes (CNTs) by chemical vapor deposition (CVD) is a roadblock in the pursuit of ultra-long CNTs, which would be useful for many of applications. Our previous in situ measurements show that vertically-aligned CNT ``forest'' growth terminates abruptly, which is not predicted by widely suggested models of diffusion-limited growth. In this work, we complement forest height measurements with mass and density measurements, and with spatial mapping of CNT diameter, alignment, and spacing along the forest sidewall by small-angle and ultra-small-angle synchrotron X-ray scattering (SAXS, USAXS). Accordingly, we reveal that the areal density of growing CNTs begins to decay long before the forest height terminates, indicating that gradual deactivation of catalyst particles is collectively responsible for the limitation to CNT forest height. Considering that mechanical and surface interactions among CNTs create the self-supporting forest structure, a gradual decay of CNT density can lead to an abrupt loss of CNT alignment at the forest base when the CNT-CNT spacing increases sufficiently. This proposed mechanism is supported by finite element models of CNT-CNT buckling and contact. [Preview Abstract] |
Monday, March 16, 2009 10:24AM - 10:36AM |
A24.00011: Clarifying the Rules for the Highly Efficient Growth of Carbon Nanotubes Don Futaba, Jundai Gotou, Satoshi Yasuda, Takeo Yamada, Motoo Yumura, Kenji Hata In water-assisted chemical vapor deposition (CVD), the addition of a growth enhancer, e.g. water, to the ambient of normal hydrocarbon dramatically improves growth efficiency resulting in vertically aligned forests [1]. Here, we present a generalized picture of water-assisted CVD (Super-growth) by demonstrating that highly efficient growth of carbon nanotubes (CNTs) is possible by, essentially, a countless number of growth enhancers exemplified here by alcohols ethers, esters, ketones, aldehydes, and even carbon dioxide. From an extensive investigation, we found that the key for highly efficient growth is to use two essential ingredients: 1) a carbon source not containing oxygen, and 2) a growth enhancer containing oxygen. We believe that this new understanding of CNT synthesis further cultivates and expands the world of CVD where innumerable new and completely unexplored growth ambients can emerge that would lead to further scientific discovery [1] K. Hata \textit{et al}, Science, \textbf{306}, 1241 (2004). [Preview Abstract] |
Monday, March 16, 2009 10:36AM - 10:48AM |
A24.00012: ABSTRACT WITHDRAWN |
Session A25: Focus Session: Graphene I: Electronic Properties
Sponsoring Units: DMPChair: Jules Carbotte, McMaster University
Room: 327
Monday, March 16, 2009 8:00AM - 8:12AM |
A25.00001: Proximity induced supercurrent in multilayer graphene Akinobu Kanda, Hidenori Goto, Sho Tanaka, Yukitoshi Nagai, Youiti Ootuka, Shunsuke Odaka, Hisao Miyazaki, Kazuhito Tsukagoshi We report experimental study on gate-dependent superconducting proximity effect in multilayer graphene. In our sample, multilayer graphene (MLG), obtained by the micromechanical cleavage of Kish graphite, is placed on a SiO$_{2}$/p$^{+}$-Si substrate, and two superconducting (Ti/Al) electrodes are connected to the top of the MLG. Dependence of the critical supercurrent on MLG length and temperature will be discussed. [Preview Abstract] |
Monday, March 16, 2009 8:12AM - 8:24AM |
A25.00002: Doping Dependent Magnetic Structure of Graphene Nanostructures Somnath Bhowmick, Umesh Waghmare, R. Shankar, Vijay Shenoy Graphene nanostructures bounded by zigzag edges are predicted to have interesting magnetic structure. We investigate how doping of the nanostructures by holes affects their magnetism. By a detailed mean-field analysis of the Hubbard model, and supported by first principles calculations, we show that doping dramatically changes the magnetic structure. In the case of a zigzag terminated nanoribbon, there is a range of doping that depends on the width of the nanoribbon, where magnetizations of both the zigzag edges are parallel (``ferro'' structure) as opposed to the undoped case where the magnetization on the two zigzag edges are anti-parallel (``anti-ferro''). We explain these results by means of a continuum field theory. We also study doping dependence of magnetic structure of other zigzag terminated nanostructures such as nanodots and find this to be a generic phenomenon in these systems. [Preview Abstract] |
Monday, March 16, 2009 8:24AM - 9:00AM |
A25.00003: A subnanometer view of the chemistry and electronic structure of graphene and graphite Invited Speaker: We have used UHV, low and variable temperature Scanning Tunneling Microscopy (STM) to investigate the structure and electronic properties of single sheets of graphite (graphene) on both insulating and metallic surfaces. STM as well as Scanning Tunneling Spectroscopy reveal variations in electron tunneling images for these one atom thick samples that differ markedly from those of ordinary, multi-layer graphite. These kinds of investigations have been extended to the study of surface chemical reactions where the Scanning Tunneling Microscope/Atomic Force Microscope is used as a ``camera'' to observe defect growth in single and multiple graphene sheets due to treatment with oxygen. ``Healing'' of defects in ordinary graphite has also been observed in samples exposed to acetylene at elevated temperatures using STM. The chemistry on these model surfaces is expected to reveal many features that determine the nanoscale properties of graphene. [Preview Abstract] |
Monday, March 16, 2009 9:00AM - 9:12AM |
A25.00004: ABSTRACT WITHDRAWN |
Monday, March 16, 2009 9:12AM - 9:24AM |
A25.00005: Interactions of Aromatic Compounds with Graphene Hugo Romero, Humberto Gutierrez, Peter Eklund We have used back-gated graphene field effect transistors (FETs) on Si/SiO$_{2}$ substrates to study the interactions of graphene with six-membered ring aromatic compounds C$_{6}$H$_{2n}$ ($n$ = 3-6). Electronic detection occurs through the shift of the of the source-drain resistance maximum (``Dirac peak'') with gate voltage. The size of the \textit{positive} shift of the Dirac peak is found to be linear in to the $\pi $ electron count of the molecule, suggesting the coupling between these $\pi $ electrons and those in the graphene may be responsible for the observed effects. [Preview Abstract] |
Monday, March 16, 2009 9:24AM - 9:36AM |
A25.00006: Slow imbalance relaxation and thermoelectric transport in graphene Igor Aleiner, Matthew Foster We compute the electronic component $(\kappa)$ of the thermal conductivity and the thermoelectric power $(\alpha)$ of monolayer graphene, within the hydrodynamic regime, taking into account the slow rate of carrier population imbalance relaxation. Interband electron-hole generation and recombination processes are inefficient due to the non-decaying nature of the relativistic energy spectrum. As a result, a population imbalance of the conduction and valence bands is generically induced upon the application of a thermal gradient. The thermoelectric response of a graphene monolayer depends upon the ratio of the sample length to an intrinsic length scale $l_Q$, set by the imbalance relaxation rate. At the same time, the metallic contacts required for the thermopower determination (under open circuit boundary conditions) can crucially influence its measurement, since carrier exchange with the contacts also relaxes the imbalance. These effects are especially pronounced for clean graphene, where the thermoelectric transport is limited exclusively by intercarrier collisions. For specimens shorter than $l_Q$ joined to sufficiently resistive contacts, we show that the population imbalance extends throughout the sample; $\kappa$ and $\alpha$ asymptote toward their zero imbalance relaxation limits. In the opposite limit of a graphene slab longer than $l_Q$, we show that at non-zero doping $\kappa$ and $\alpha$ approach intrinsic values characteristic of the infinite imbalance relaxation limit. [Preview Abstract] |
Monday, March 16, 2009 9:36AM - 9:48AM |
A25.00007: Electron localization in graphene quantum dots Vadym Apalkov, Prabath Hewageegana We study theoretically a localized state of an electron in a graphene quantum dot with a sharp boundary. Due to Klein's tunneling, the relativistic electron in graphene cannot be localized by a confinement potential. In this case electron states in a graphene quantum dot become resonances with finite trapping time. We consider these resonances as the states with complex energy. To find the energies of these states we solve the time-independent Schrodinger equation with outgoing boundary conditions. The imaginary part of the energy determines the width of the resonances and the electron trapping time. We show that if the parameters of the confinement potential satisfy a special condition, then an electron can be strongly localized by such quantum dot, i.e., the trapping time becomes infinitely large. We show how a deviation from this condition affects the electron trapping time. We also analyze the energy spectra of an electron in a graphene quantum ring with a sharp boundary. We show that in this case the condition of strong trapping can be tuned by varying parameters of confinement potential, e.g. internal radius of the ring. [Preview Abstract] |
Monday, March 16, 2009 9:48AM - 10:00AM |
A25.00008: Dirac Quasiparticle Interference in Graphene in External Magnetic Field Rudro Biswas, Laila Mattos, Hari Manoharan, Alexander Balatsky Recent experiments [1] on graphene have been able to probe impurity scattering effects locally using scanning tunneling microscopy. The spatial Fourier transforms of the local density of states (FT-STS) display prominent features corresponding to both intervalley and intravalley scattering of massless Dirac quasiparticles. We provide low energy calculations using the effective mass model to describe these experimental observations. In addition, we are able to explain the striking change in the shape of the chiral scattering peaks when graphene is placed in a perpendicular magnetic field. [1] L.S. Mattos, et al., unpublished. [Preview Abstract] |
Monday, March 16, 2009 10:00AM - 10:12AM |
A25.00009: Effects of the electron-phonon interaction on spectroscopies of graphene E.J. Nicol, J.P. Carbotte, S.G. Sharapov We examine the effect of the electron-phonon interaction in graphene on the electronic density of states, the quasiparticle spectrum and the optical conductivity. A simplified model of coupling to an Einstein mode at 200 meV is employed and self-consistent calculations are performed. The results will be discussed in relation to tunneling, ARPES and optical conductivity experiments on graphene. [Preview Abstract] |
Monday, March 16, 2009 10:12AM - 10:24AM |
A25.00010: Thermally assisted self-trimming of graphene nanoribbon edges Teng Yang, David Tom\'anek, Savas Berber Edge morphology is known to play a key role in the conductance of graphene ribbons. We use a combination of {\em ab initio} density functional total energy and molecular dynamics calculations to investigate thermally induced reconstruction occurring at graphene edges. The calculated total energy surfaces suggest that among all nanoribbon sites, atoms at edge defect sites require least energy to be displaced. At elevated temperatures, these atoms will primarily participate in diffusion and related processes at the edge that will gradually reduce the edge roughness and thus lower the edge energy. We explore various scenarios leading to such self-trimming of edges, including concerted migration processes and unravelling of chains at the edge. Close inspection of our results suggests that the preferential mechanisms and activation barriers for trimming of rough armchair and zigzag edges may be different. In selected scenarios, Joule heating of nanoribbons may not only straighten rough edges, but also modify the preferred edge morphology. [Preview Abstract] |
Monday, March 16, 2009 10:24AM - 10:36AM |
A25.00011: Fermi surface of graphene on Ru(0001) Thomas Brugger, Hugo Dil, J\"{u}rg Osterwalder, Thomas Greber, Bin Wang, Marie-Laure Bocquet, Sebastian G\"{u}nther, Joost Wintterlin The structure of a single layer graphene on Ru(0001) is compared with that of a single layer hexagonal boron nitride nanomesh on Ru(0001). Both are corrugated sp$^2$ hybridized networks and display a $\pi$-band gap at the $\overline{\rm{K}}$ point of their $1\times1$ Brillouin zone. In contrast to $h$-BN/Ru(0001), g/Ru(0001) has a distinct Fermi surface which indicates that 0.1 electrons per $1\times1$ unit cell are transferred from the Ru substrate to the graphene. Photoemission from adsorbed xenon on g/Ru(0001) identifies two distinct Xe 5p$_{1/2}$ lines, separated by 240 meV, which reveals a corrugated electrostatic potential energy surface like on $h$-BN/Rh(111) [1]. These two Xe species are related to the topography of the template and have different desorption energies.\\[4pt] [1] H. Dil, J. Lobo-Checa, R. Laskowski, P. Blaha, S. Berner, J. Osterwalder, and T.Greber, Science \textbf{319}, 1824 (2008). [Preview Abstract] |
Monday, March 16, 2009 10:36AM - 10:48AM |
A25.00012: Complex refractive index of graphene measured by picometrology Xuefeng Wang, David Nolte The complex refractive index $\tilde {n}_g $ of graphene remains unresolved because the traditional technique, ellipsometry, fails when applied to graphene with its sub-nanometer thickness, dielectric anisotropy, and small transverse sample size. Here we apply interferometric picometrology to measure $\tilde {n}_g $ at 488 nm, 532 nm and 633 nm. A strong dispersion of $\tilde {n}_g $ was found in the visible region. $\tilde {n}_g $ varies from 2.4-1.0i at 532 nm to 3.0-1.4i at 633 nm at room temperature. The dispersion is five times stronger than bulk graphite (2.67-1.34i to 2.73-1.42i from 532 nm to 633 nm). In experiments, Graphene is deposited on a substrate with complex reflection coefficient $\tilde {r}$ tuned near an antinode condition. As a dielectric film, graphene modifies $\tilde {r}$ of the substrate into $\tilde {r}'$. Picometrology measures both the amplitude and the phase change of $\tilde {r}$, and$_{ }$therefore acquires the full information needed to calculate $\tilde {n}_g $. This is accomplished by scanning a normal-incidence focused Gaussian beam (1.5 $\mu $m width) over the graphene and monitoring the asymmetric diffraction of the reflected beam. Picometrology measures the complex change of $\tilde {r}$ with a quadrant detector that simultaneously monitors both intensity and axis shift of the reflected beam and calculates $\tilde {n}_g $. The strong dispersion of graphene is reported here for the first time, and it is likely caused by the strongly modified quantum level structure of the single atomic layer. [Preview Abstract] |
Monday, March 16, 2009 10:48AM - 11:00AM |
A25.00013: Zero-bias conductance anomaly in point-contact junctions on graphite Wan Kyu Park, Cesar Chialvo, Rich Jones, Sam Johnson, Nadya Mason, Laura Greene The electronic properties of graphene, a two-dimensional carbon allotrope, continue to attract great interest because of the interesting underlying physics and application potential of this novel electronic material. An ideal single-layer graphene is known to show a linear behavior in the electronic density of states (DOS) around the Fermi level. The ability to engineer the DOS of single- and multi-layer graphene is considered as a fundamental requirement for the realization of electronic devices. To investigate the electronic DOS in graphene/graphite, we adopt a spectroscopic technique based on nanoscale point-contact junctions, where differential conductance spectra are taken at around the liquid helium temperature. A common feature observed in all junctions on both Kish graphite and HOPG is an anomalous conductance dip at zero bias. The conductance curves show a logarithmic bias dependence in their slopes, exhibiting a systematic evolution as a function of magnetic field and contact pressure. We discuss possible origins of these behaviors including the possibility of modification in the electronic DOS of graphite. [Preview Abstract] |
Session A26: Focus Session: Computational Nanoscience I: Inorganic Nanostructures and Interfaces
Sponsoring Units: DMP DCOMPChair: Peihong Zhang, SUNY Buffalo
Room: 328
Monday, March 16, 2009 8:00AM - 8:36AM |
A26.00001: Interface electrostatics in ferroelectric capacitors from first principles Invited Speaker: Capacitors based on ferroelectric perovskites are potentially attractive for applications in nanoelectronics, such as non-volatile random-access memories and high-permittivity gate dielectrics. Thin-film geometries are sought after for optimal efficiency and information storage density. However, in such a regime, strong size effects arise that generally deteriorate the overall performance of the device. Understanding the properties of the oxide/electrode interface is crucial to overcoming these deleterious effects. In this talk I will present our recently-developed methodologies for working at fixed electric displacement field in first-principles density-functional calculations. I will show that application of fixed-$D$ methods to ferroelectric capacitors provides enhanced flexibility for the study of interface-specific issues. I will demonstrate this technique by presenting results for a range of systems based on PbTiO$_3$ or BaTiO$_3$ as ferroelectric, and Pt or SrRuO$_3$ as electrode. Based on a microscopic analysis of interface bonding and electrostatics, I will discuss possible routes to the realization of devices that are free from size effects. [Preview Abstract] |
Monday, March 16, 2009 8:36AM - 8:48AM |
A26.00002: Multiexciton absorption in CdSe nanocrystals Alberto Franceschetti, Yong Zhang Efficient multiple-exciton generation (MEG) has been recently reported in semiconductor nanocrystals. In this process, a single absorbed photon generates two or more electron-hole pairs. The MEG efficiency has so far been evaluated assuming that the change (bleaching) of the absorption spectrum due to MEG is linearly proportional to the number of excitons ($N_X$) that are present in the nanocrystal. We have examined this assumption using atomistic pseudopotential calculations for colloidal CdSe nanocrystals ranging in size from 3 to 4.6 nm. We found that the bleaching of the first absorption peak, $\Delta \alpha_{1S}$, depends non-linearly on $N_X$, due to carrier-carrier interactions. When a single exciton is present in the nanocrystal, the 1S exciton peak is already 65-75\% bleached. This non-linearity mandates an upper bound of 1.5 to the value of the normalized bleaching that can be attributed to MEG, significantly smaller than the limit of 2.0 predicted by the linear scaling assumption. Thus, measured values of the normalized bleaching in excess of 1.5 in CdSe nanocrystals cannot be due entirely to MEG, but must originate in part from other mechanisms. [Preview Abstract] |
Monday, March 16, 2009 8:48AM - 9:00AM |
A26.00003: Electronic Structure of Thiolate Covered Gold Nanoparticles Yan Li, Giulia Galli, Francois Gygi We present ab initio calculations of the structural, electronic, and bonding properties of thiolate-covered gold nanoparticles that have been crystallized in recent experiments (Au$_{102}$(MBA)$_{44}$)[1]. We simulated exactly the same system as investigated experimentally (1596 atoms) and the results of our structural optimization confirm the stability of the experimentally determined structure. We find that the crystallized solid is a semiconductor with a sizable energy gap ($\sim$0.5 eV, within DFT), and electronic states at the valence band maximum and conduction band minimum are extended over the gold nanoparticle core and the interface. This energy gap appears to be insensitive to the type of adsorbate. We find a tendency of the adsorbate to exert ``pull-out'' forces on the surface gold atoms, and our analysis of chemical bonding supports the hypothesis that gold s electrons are donated to the MBA radicals so as to form a highly stable 58-electron, filled electronic shell structure. Finally, comparisons between adsorption energies in the case of gold nanoparticles and flat surfaces show that a finite curvature of the interface enhances the stability of Au-S bonds.\\[0pt] [1]Yan Li, Giulia Galli and Francois Gygi, ACS Nano, 2, 1896(2008) [Preview Abstract] |
Monday, March 16, 2009 9:00AM - 9:12AM |
A26.00004: Covalently Bonded Aromatic Molecules on Gold using a GW Approach Isaac Tamblyn, Su Ying Quek, Stanimir A. Bonev, Jeffrey B. Neaton Frontier molecular orbital energies dictate the nature of optical absorption, chemical reactivity, and charge injection at metal-organic interfaces. Recent work [1] on the conductance of benzenediamine-Au single-molecule junctions has shown that standard methods based on density functional theory fail to correctly position molecular orbital energies relative to the Au Fermi level, resulting in a pathological overestimate of the conductance for this class of systems. In this work, we use many-electron perturbation theory within the GW approximation to compute quasiparticle energies of aromatic molecules covalently bonded to a gold surface, taking particular care to assess dynamical screening beyond standard plasmon-pole approximations. We discuss results for benzene on Au(111) bonded via amine (-NH2) and thiol (-SH) link groups. These data are compared with more approximate model self-energy corrections applied to these systems [1], and also recent experiments. [1] Quek et al, Nano Lett. 7, 3477 (2007). [Preview Abstract] |
Monday, March 16, 2009 9:12AM - 9:48AM |
A26.00005: First-principles studies of interfacial charge separation in nano-materials photovoltaic heterojunction Invited Speaker: Charge separation is a crucial process that must be understood in order to make substantial improvements in nano-materials based PV cells. In our work, first principles quantum mechanical calculations are employed to shed light on this process for some important nano-material heterojunctions. I will first present our work on the interfacial charge separation in Fullerene/P3HT and CNT/P3HT heterojunctions. Our findings indicate that in the fullerene system a two-step process is operative, involving an adiabatic electron transfer and an exciton dissociation via quasi-degenerate states localized on the fullerene. For the nanotubes, on the other hand, while such a two-step process is not necessary for efficient charge separation, the presence of metallic nanotubes lead to undesirable charge traps. Secondly, I will discuss how we are addressing the difficulty in employing standard DFT approaches for investigating inorganic-organic PV interfaces, which are composed of two distinct materials with very different electronic environments. I will discuss a QMC scheme for obtaining many-body corrections to the Kohn-Sham level alignments and its application to a CdSe/Oligothiophene hybrid PV interface, with the aim of tailoring its behavior by controlling the conjugation length. [Preview Abstract] |
Monday, March 16, 2009 9:48AM - 10:00AM |
A26.00006: Nanoscale phase stability reversals in titanium oxide polymorphs Paul Kent, Daniel Hummer, James Kubicki, Jeffrey Post, Peter Heaney A surprising discovery of nanoscience is the reversal in relative stability of materials with the same composition but different structures as particles evolve from the nanoscale to the macroscopic. These reversals can be problematic, as they frequently induce the precipitation of metastable contaminant phases during the synthesis of compounds of interest. To investigate the fundamental origins of this phenomenon, we have (1) performed a series of large scale density functional calculations to characterize the energetics of the stability reversal between rutile and anatase nanoparticles, and (2) experimentally monitored the hydrothermal crystallization of titania nanoparticles using in-situ x-ray diffraction. Although the phenomenon of stability reversal is commonly explained as a domination of the crystalline surface energy as a fraction of the total energy, we find computationally that the refined average surface structures cannot account for the stabilization of nano-anatase relative to nano-rutile, even for 3nm sized particles. Instead we find that defects associated with the edges and corners of nanocrystals contribute significantly to the energy and must be included in any description of the stability reversal. [Preview Abstract] |
Monday, March 16, 2009 10:00AM - 10:12AM |
A26.00007: Domain boundary formation in helical multishell gold nanowire Takeo Hoshi, Takeo Fujiwara Helical multishell gold nanowire (Y. Kondo and K. Takayanagi, Science 289, 606 (2000)) is studied by molecular dynamics simulation with electronic structure (``ELSES'' http://www.elses.jp/), so as to explore formation mechanism of helical domain boundary. We have proposed a model for the formation of helical multishell gold nanowires with molecular dynamics simulation with electronic structure (Y. Iguchi, T. Hoshi, T. Fujiwara PRL 99, 125507 (2007)). In this paper, we show simulation results with lager samples, of which the rod length is more than 10 nm and the number of rod atoms is more than one thousand. Unlike the results of shorter rods in the previous paper, a well-defined domain boundary between helical and (non-)helical regions appears, when an atom moves from a inner shell into rod surface. The inserted atoms on the rod surface causes a surface reconstruction on rod surface and introduces a helical region with a domain boundary. Such an inserted atom is a possible candidate of mechanism for forming a helical rod from an ideal (non-helical) one. [Preview Abstract] |
Monday, March 16, 2009 10:12AM - 10:24AM |
A26.00008: Plasmon resonance of gold nanoparticles: the effect of surfactants and solvents Jeremy Neal, Peter Palffy-Muhoray Metallic nanoparticles dispersed in host materials have many potential applications due to their unique optical properties. These properties are determined not only by the size, shape and composition of the particles, but also by their environment. Metallic nanoparticles are typically coated with surfactants to prevent aggregation; these surfactants can also significantly affect their optical response. The role of surfactant coatings has been studied previously, but the results are incomplete. We have obtained theoretical expressions to describe and have carried out numerical simulations to determine the effects of solvents and surfactants on the optical response. [Preview Abstract] |
Monday, March 16, 2009 10:24AM - 10:36AM |
A26.00009: Investigation of atomic oxygen embedment into copper surface by DFT calculation Minyoung Lee, Alan McGaughey, Susan Sinnott, Simon Phillpot, Judith Yang In the oxidation of a Cu(100) surface, the Cu$_2$O islands grow both into the substrate and parallel to the surface. To investigate the oxide growth into the copper surface, we analyzed oxygen embedment using DFT calculations. Using the nudged elastic band method, we calculated energy barriers for oxygen embedment for different oxygen coverages and different surface morphologies. As the oxygen coverage increases from 0.25 monolayers (ML) to 1.0 ML, the energy barrier decreases and we find an energetically favorable site between the top and second copper layers at an oxygen coverage of 1.0 ML. The different surface morphologies [$c$(2$\times$2), missing-row reconstruction and $c$(2$\times$2) with 0.25 ML disordered copper vacancy] have comparable energetics and no energetically favorable site for oxygen embedment is predicted. To find the energetically favorable transition states on the missing-row reconstructed Cu(100) surface, we will investigate oxygen embedding paths with and without point defects on the top copper layer. [Preview Abstract] |
Monday, March 16, 2009 10:36AM - 10:48AM |
A26.00010: \textit{Ab initio} vibrational dynamics of Ag$_{27}$Cu$_{7}$ nanoalloy. Marisol Alcantara Ortigoza, Rolf Heid, Klaus P. Bohnen, Talat S. Rahman We have carried out calculations of the vibrational dynamics of the 34-atom nanoalloy, Ag$_{27}$Cu$_{7}$, using density functional perturbation theory, which furnishes a powerful and reliable method to asses the linear response of the charge density to ionic perturbations. We find that the D$_{5h}$ core-shell structure of Ag$_{27}$Cu$_{7}$ [1,2] is dynamically stable, since all modes have non-zero frequencies affirming that the structure does not surrender itself to structural transitions as a result of the small perturbations in the charge density led by vibrations. The phonons of Ag$_{27}$Cu$_{7}$ range from 2.6 to 28.5 meV and are relatively evenly distributed. There are, however, three $\sim $3.0 meV gaps between 2.8-5.6, 15.0-18.7, and 23.6-26.8 meV. In modes whose frequency is below 7.0 meV, Ag atoms participate the most while Cu atoms show a very small displacement. The opposite is true for four modes whose frequency is above 24 meV. We present the displacement patterns of the main modes and find the mode with highest energy to be a radial \textit{breathing} mode of Cu atoms with respect to the center of the cluster. [1] G. Rossi \textit{et al}., PRL. \textbf{93}, 105503 (2004), [2] M. Alc\'{a}ntara Ortigoza and T. S. Rahman, PRB \textbf{77}, 195404 (2008). Work supported in part by U.S. DOE under Grant DE-FG02-07ER46354. [Preview Abstract] |
Monday, March 16, 2009 10:48AM - 11:00AM |
A26.00011: Clusters, Platelets, and Nanowires of Mo-S, and Their Assemblies P. Murugan, Vijay Kumar, Y. Kawazoe, N. Ota Nano-structures of MO-S are useful for removal of S in petroleum industry, as solid state lubricants in space technology, and in Pt-free fuel cell research. We study by first principles density functional calculations Mo-S nanoclusters, nanowires, their assemblies, and triangular nano-platetlets all of which have been produced in laboratory. Mo-S clusters have Mo polyhedral structures and sulfur atoms cap this metal polyhedron. These structures have high stability due to strong Mo-Mo and Mo-S bonding. Some of the polyhedral clusters have non-zero magnetic moments due to the partially occupied 4d states in Mo atoms. Mo$_{6}$S$_{8}$ octahedral cluster has ultra-high stability and it could be condensed to form Mo-S nanowire or nanorod. However, for high S contents, we show that triangular platelets become more stable. The Mo-S nanowires are good electronic conductors and are interesting for miniature devices. Assembly of nanowires stabilizes in a hexagonal structure with vdW interactions. The trigonal void between the nanowires can be occupied by Li atoms to develop materials for Li-ion battery applications. References: P. Murugan, V. Kumar, Y. Kawazoe, and N. Ota, Appl. Phys. Lett. (2008); Nano letters. (2007); J. Phys. Chem. A (2007). [Preview Abstract] |
Session A27: Focus Session: Advances in Scanned Probe Microscopy I: Low Temperatures
Sponsoring Units: GIMSChair: Cyrus Hirjibehedin, London Centre for Nanotechnology
Room: 329
Monday, March 16, 2009 8:00AM - 8:12AM |
A27.00001: Development of an Ultra Low Temperature Scanning Tunneling Microscope Young Jae Song, Alexander Otte, Young Kuk, Joseph Stroscio In this talk we give an update on the next generation of ultra low temperature, high magnetic field (15T) scanning tunneling microscope (STM). With this system, we plan to extend the capability of STM to include higher energy resolution ($\sim $1$\mu $eV) for scanning tunneling spectroscopy (STS) with operation at 20 mK. To realize this energy resolution in STS, we constructed an ultra high vacuum dilution refrigerator (DR) for STM applications. It operates with two independent modes of He3-He4 mixture gas condensation: a traditional 1K pot condenser, or a Joule-Thomson condenser for possible lower noise operation. This eliminates potential vibration problems during operation of the DR. To match the very low limit of thermal noise in this system, our new system includes extensive vibration isolation and RF shielding. Our STM sample holder has five isolated electrical contacts. This allows four-probe macroscopic electrical measurements to be performed simultaneously with microscopic STM measurements. The current progress and performance of this new system will be discussed. [Preview Abstract] |
Monday, March 16, 2009 8:12AM - 8:24AM |
A27.00002: Design and construction of a millikelvin scanning tunneling microscopy system Mark Gubrud, Barry Barker, Michael Dreyer, Dan Sullivan We are developing a scanning tunneling microscopy and spectroscopy system for work at millikelvin temperatures, intended for studies of superconductor, semiconductor, and other materials and systems of interest to quantum computing research. Our approach incorporates recent advances in this field as well as original insights and innovations to help achieve low noise and low effective operating temperatures. [Preview Abstract] |
Monday, March 16, 2009 8:24AM - 8:36AM |
A27.00003: Direct evidence of the surface state contribution to the Kondo resonance Qing Li, Shiro Yamazaki, Toyoaki Eguchi, Howon Kim, Se-Jong Kahng, Jinfeng Jia, Qikun Xue, Yukio Hasegawa We performed low temperature scanning tunneling microscopy/spectroscopy on the isolated single 5, 10, 15, 20-tetrakis-(4-bromophenyl)-porphyrin-Co (TBrPP-Co) molecules adsorbed on the Si(111)- $\sqrt 3 \times \sqrt 3 $ Ag substrate. On this substrate, all the TBrPP-Co molecules show a square shape, indicating a planar conformation with a spin-active Co atom caged at its center. As the substrate supports a two-dimensional surface state and does not have bulk state near the Fermi level, the observed Fano-shaped peak near the Fermi level taken above the single molecule is a direct evidence of the contribution of the surface state electrons to the Kondo resonance. The long decay length ($\sim $ 1.4 nm) of the resonance also support for the surface state contribution. [1] Q. Li, S. Yamazaki, T. Eguchi, Y. Hasegawa, H. Kim, S.-J. Kahng, J. F. Jia, and Q. K. Xue, Nanotechnology 19, 465707 (2008). [Preview Abstract] |
Monday, March 16, 2009 8:36AM - 8:48AM |
A27.00004: Kondo Effect in a Co-Porphyrin on Au(111) probed by Scanning Tunneling Spectroscopy Se-Jong Kahng, Howon Kim, Won Jun Jang, Jung Heum Jeon, Won-joon Son, Seungwu Han Kondo effect is a core topic in condensed matter physics, exhibiting a localized state raised by the interaction between a single magnetic impurity and Fermi electrons in metals. We have studied Kondo effect in a Co-porphyrin on Au(111) using low-temperature scanning tunneling spectroscopy. A localized state is observed at Fermi level from the spectra measured above the Co atom. The spectra were fitted by Fano line shape, revealing the Kondo temperature of the system $\sim $ 400K. By taking spectra at points along some symmetry directions, decaying behavior of the Kondo effect could be analyzed. With the help of simulated d-electron orbital, the observed decaying behavior is accounted for. Our study implies that lattice reconstruction in a system can induce d-electron orbital distortion, resulting in magnetic asymmetries. [Preview Abstract] |
Monday, March 16, 2009 8:48AM - 9:00AM |
A27.00005: Imaging the Quantum Berry Phase C.R. Moon, L.S. Mattos, B.K. Foster, H.C. Manoharan Geometric phase operations are attractive to quantum information technology because they are time-independent and relatively insensitive to topological perturbations. However, in most coherent devices where these operations could be performed, electron wave functions are inaccessible to local probes. Here, we demonstrate Berry phase rotations on two-dimensional electron wave functions by using atomic manipulation to adiabatically alter their confinement potential. By consecutively changing the boundary of a quantum corral, we traverse a closed circuit in deformation space that engenders a net $\pi$ phase shift in two electron eigenstates. With scanning tunneling microscopy, we trace both the energetic and spatial evolution of these states and directly track their accrual of geometric phase, revealing information that would be obscured in other two-dimensional electron devices. This enables the determination of the two-point transconductance through the device, thus making contact to other nanostructures such as semiconductor quantum dots, where this promising technique for phase control can be implemented using only voltages controlling appropriately patterned gates. [Preview Abstract] |
Monday, March 16, 2009 9:00AM - 9:12AM |
A27.00006: Evolution of Single-Molecule Vibrational Modes from Tunneling to Quantum Point Contact W. Mar, W. Ko, C. R. Moon, B. K. Foster, L. S. Mattos, H. C. Manoharan A detailed understanding of how molecular junctions form and evolve is vital for emerging fields such as molecular electronics. We present high-precision scanning tunneling microscopy studies tracing the evolution of molecular junctions from the tunneling regime to quantum point contact. We employ a model system of CO molecules on Cu(111) and are able to extend inelastic spectroscopy into the point contact regime, thus following the energy shifts of specific vibrational modes as the molecular contact is formed. We observe surprising non- monotonic shifts, confirmed by simultaneous noise measurements traceable to molecular motion. In point contact, we also observe a novel ``nucleonic gating'' effect in which the carbon nucleus controls a measurable dc molecular conductance shift. This shows that the electrical properties of molecular wires can be profoundly altered by their isotopic makeup. We extend these measurements to geometries where the three-dimensional approach vector of the tip relative to the target molecule is finely controlled, a technique not possible in break junction measurements. [Preview Abstract] |
Monday, March 16, 2009 9:12AM - 9:48AM |
A27.00007: Low Temperature Scanning Tunneling Microscopy of High Temperature Superconductors: What We Gain By Taking a Closer Look Invited Speaker: Scanning tunneling microscopy (STM) and spectroscopy have been applied to a wide variety of experimental systems. In this talk I will focus on one which was discovered at nearly the same time as STM -- high temperature superconductors. After two decades of intense research these materials still hold many mysteries, mainly due to the rich variety of states of matter that may coexist, cooperate, or compete with superconductivity. I will present the unique perspective that STM is capable of bringing to our study of these materials through atomic-scale temperature dependent mapping of the density of states. After describing widely observed spatial ``checkerboard'' patterns which we have found to have a distinct doping dependence suggestive of charge density wave order, I will demonstrate how local variations of this order can help us understand nanoscale inhomogeneity in these materials. Taken together, these results not only show the power of STM to untangle complex nanoscale phenomena but also suggest a new path towards understanding high temperature superconductivity. [Preview Abstract] |
Monday, March 16, 2009 9:48AM - 10:00AM |
A27.00008: Vortex excitation in nano-sized Pb island structures using low temperature scanning tunneling microscopy Takahiro Nishio, Shizeng Lin, Kousuke Miyachi, Toshu An, Toyoaki Eguchi, Yukio Hasegawa Vortex behaviors in nano-size superconductors have attracted a lot of attention since there are various novel phenomena due to the size and shape effects. Using scanning tunneling microscopy/spectroscopy (STM/S) at low temperature ($<$2 K) we have visualized vortex phases on atomically-flat nano-sized Pb islands formed on the Si(111)-7x7 substrate and measured the critical magnetic fields for vortex penetration and expulsion [1]. In this study we demonstrate the excitation of a vortex with additional pulsed tunneling current from an STM probe tip. We found that probability of the excitation depends on the amount of the tunneling current, the pulse duration and a tip position in the island. These dependences suggest that the formation of normal state region below the tip due to the excess tunneling current induces the vortex penetration. Experimental details and theoretical results will be explained in the presentation. [1] T. Nishio \textit{et al}., PRL \textbf{101}, 167001(2008). [Preview Abstract] |
Monday, March 16, 2009 10:00AM - 10:12AM |
A27.00009: Spatial and temperature-evolved tunneling spectroscopic studies of La$_{0.7}$Ca$_{0.3}$MnO$_{3}$(LCMO) films and LCMO/organic-semiconductor heterostructures with spin-polarized scanning tunneling microscopy (SP-STM) C.R. Hughes, A.D. Beyer, N.-C. Yeh We report studies of spatially resolved tunneling spectra (TS) of La$_{0.7}$Ca$_{0.3}$MnO$_{3}$ (LCMO) (T$_{c}$ = 260K) epitaxial films and related heterostructures of tris(8-hydroxyquinoline) aluminum (Alq3)/(LCMO) using a UHV, variable temperature STM. At 77K with a Pt/Ir tip we observe sharp spatial transitions between two cluster types with disparate normalized conductance. The majority type region exhibits high conductance peaks at high bias (+/- 2V) and a low energy gap, consistent with band structure calculations. The minority type region reveals moderate conductance over the entire bias range, from -3V to +3V. In contrast, spin-polarized tunneling spectra taken with Cr-coated STM tips show a spatially varying low bias gap in all regions. Further experiments using SP-STM on LCMO under varying temperatures and applied magnetic fields and on Alq3/LCMO structures to study the spin transport length in Alq3 will be reported. [Preview Abstract] |
Monday, March 16, 2009 10:12AM - 10:24AM |
A27.00010: Degeneracy lifting of zero energy (Majorana) modes in a chiral p-wave superconductor due to the tunneling between vortices. Meng Cheng, Roman Lutchyn, Victor Galitski, Sankar Das Sarma We study lifting of the degeneracy of the zero energy (Majorana) modes in a chiral $p_x+ip_y$ superconductor caused by tunneling between states localized in two different vortex cores. Using Bogoliubov-de Gennes equations, we analytically calculate the energy splitting of the Majorana modes as a function of the distance between two vortices. Our result may have applications in testing Majorana state by tunneling spectroscopy and the realization of topological quantum computation in chiral p-wave superconductors. [Preview Abstract] |
Monday, March 16, 2009 10:24AM - 10:36AM |
A27.00011: The role of magnetic anisotropy in the Kondo effect Alexander Otte, Markus Ternes, Kirsten von Bergmann, Sebastian Loth, Harald Brune, Christopher Lutz, Cyrus Hirjibehedin, Andreas Heinrich The Kondo effect is a fascinating many-body phenomenon, the origin of which is often unclear. Using a Scanning Tunneling Microscope operating at 0.5~K, we study inelastic spin excitations on individual atoms bound atop a thin insulating Cu$_{2}$N layer. We find that, unlike previously studied Fe and Mn atoms, the spins of Co and Ti atoms are Kondo screened in this environment. By applying strong magnetic fields in various directions we are able to precisely analyze the magneto- crystalline anisotropy experienced by the spins, and consequently their orientations relative to the surface. We show that the anisotropy plays a major role in determining whether or not a spin becomes Kondo screened, and how the Kondo effect is influenced by a magnetic field. [Preview Abstract] |
Monday, March 16, 2009 10:36AM - 10:48AM |
A27.00012: Tunneling through a single magnetic atom: spin-dependent elastic and inelastic processes C.F. Hirjibehedin, A. Mody, X. Shi, A. Fisher, A.F. Otte, M. Ternes, S. Loth, C.P. Lutz, A.J. Heinrich Recent low-temperature scanning tunneling microscopy and spectroscopy studies have used inelastic electron tunneling to probe the spin excitations of magnetic atoms, molecules, and bulk surfaces. Here we describe the mechanisms that drive these spin excitations using a combination of resonant two-step and three-step virtual processes, with the latter including a simple exchange coupling between the tunneling electron and the electrons that comprise the atomic spin. Our description predicts the existence of a sum rule that includes a previously unnoticed type of spin-dependent elastic scattering, and evidence of both are seen in the observed spectra. We discuss the key factors that determine the relative strength of the inelastic tunneling, providing insight on when such processes can be observed and potentially how they might be enhanced. [Preview Abstract] |
Monday, March 16, 2009 10:48AM - 11:00AM |
A27.00013: Implementation of a cryogenic scanning microwave impedance microscope Keji Lai, Worasom Kundhikanjana, Michael Kelly, Zhi-xun Shen We have implemented a near-field scanning microwave impedance microscope in a variable temperature (2-300K) cryostat equipped with 9T magnet. Reflected microwave signals at 1GHz from a shielded cantilever probe were detected using room-temperature electronics. During the tip-sample approach, a small oscillating voltage was applied to the z-piezo and the modulated microwave signals were monitored to locate the sample surface. The approaching curve toward bulk dielectric materials can be quantitatively simulated by finite-element analysis. We have obtained the first low-T and high-B microwave images on a patterned silicon wafer with ion-implanted stripes. The results show clear impedance contrast in both the capacitive and loss channels. In particular, high-loss regions were seen between the heavily doped areas and the insulating substrate, allowing us to visualize the local conductivity variation. With this novel instrument, we expect to study electronic inhomogeneity in complex materials and explore local properties during phase transitions. [Preview Abstract] |
Session A28: Bionanotechnology
Sponsoring Units: FIAPChair: Xinsheng (Sean) Ling, Brown University
Room: 330
Monday, March 16, 2009 8:00AM - 8:12AM |
A28.00001: Mesoscopic light reflection spectroscopy of weakly disordered dielectric media: Nanoscopic to mesoscopic light transport properties of a single biological cell and ultra-early detection of cancer Prabhakar Pradhan, Hariharan Subramanian, Dhwanil Damania, Hemant Roy, Vadim Backman We have developed a mesoscopic partial wave spectroscopy (PWS) method to measure the nanoscopic light transport properties of weakly disordered dielectric mesoscopic systems such as biological cells. Using mesoscopic theory, we have statistically quantified the light reflection coefficient and its correlation due to nanoscale refractive index fluctuations within a biological cell, and the results are consistent with the prediction of mesoscopic light transport theory. Finally, using these parameters, we have characterized the nanoscale optical disorder strength within the biological cell. Results of precancerous cell studies and cancer detection by the technique will be discussed. [Preview Abstract] |
Monday, March 16, 2009 8:12AM - 8:24AM |
A28.00002: Stick-Slip Motion of DNA in a Solid Nanopore Binquan Luan, Glenn Martyna Nanopore technology is a potential solution for the low-cost and high-throughput DNA sequencing. Till now, in a typical experiment DNA driven by an electric field translocates through a nanopore too fast to be detected at a single-base resolution. The recently proposed DNA transistor (Appl. Phys. Lett. 91, 153103 (2007)) holds the promise to trap DNA inside a nanopore and translocate single-stranded DNA (ssDNA) at a single-base resolution. Using extensive all-atom molecular dynamics simulations, we modeled the process of ssDNA's translocation through the DNA transistor when ssDNA is pulled by an optical tweezer. We found a stick-slip type of motions of DNA when both the stiffness of an optical tweezer and the pulling velocity are below critical values. This irregular motion of DNA is quantitatively characterized using the Tomlinson model. In a typical slip event, ssDNA advances one nucleotide spacing, while in a stick state the base of DNA can be conveniently measured. The duration of a stick state depends on the strength of a trapping field in the DNA transistor, the stiffness of an optical tweezer and the pulling velocity. Therefore, the controlled stick-slip motion of DNA is ideal for DNA sequencing methods using a solid nanopore. [Preview Abstract] |
Monday, March 16, 2009 8:24AM - 8:36AM |
A28.00003: Ionic Dependence of the Conformation and Dynamics of DNA Confined in Slit-like Nanofluidic Channels Yongqiang Ren, Walter Reisner, Derek Stein Due to the growth in nanobiofluidic technology for DNA manipulation and analysis there is growing interest in understanding the physics of DNA in nanoconfined environments. Using fluorescence video microscopy we study the ionic dependence of static and dynamic properties of DNA molecules confined in slit-like nanofluidic channels with varying channel height. We observe an abrupt transition from the de Gennes regime to the Odijk regime for three different ionic strengths for both the radius of gyration and the relaxation time. The cross-over channel height between the regimes increases with decreasing ionic strength. By direct measurement of the distribution function for the radius of gyration we can clearly show that the DNA molecules perform 2-D self-avoiding random walks for both the de Gennes and Odijk regimes. We also investigate the possible effects of electrostatic DNA-channel wall interactions. [Preview Abstract] |
Monday, March 16, 2009 8:36AM - 8:48AM |
A28.00004: Development of a nanopore-based electrical device for controlling the translocation of DNA with single base resolution and mass-production methods for sub-5 nm nanopores Hongbo Peng, Stephen Rossnagel, Stanislav Polonsky, Gustavo Stolovitzky During the last 10 years, nanopores have been proposed or demonstrated as sensors for rapid analysis of biomolecules (DNA, RNA, protein, etc.) or interactions between these biomolecules. Application of nanopores to low-cost DNA sequencing is particularly attractive as there is great need to reduce the cost for sequencing a whole human genome to \$1000. A key issue in the field of nanopore DNA sequencing is to control the DNA translocation. Here we will report the development of what we call a DNA transistor: a nanopore-based electrical device for controlling the translocation of DNA with single base resolution. The key part of this device is a free standing membrane, made of metal/dielectric/metal/dielectric/metal, with the thickness of each layer about 2 or 3 nm. A nanometer size (about 3 nm) pore is made through the membrane. Voltage biases are applied on the metal layers to modulate electrical field inside the nanopore. Our ongoing experiments test if the modulated electrical field can trap or translocate DNA in a controllable way. We will also report the development of methods to make uniform sub-5nm nanopores across standard 200 mm Si wafers using an industrial processing facility. [Preview Abstract] |
Monday, March 16, 2009 8:48AM - 9:00AM |
A28.00005: Single-Molecule Denaturation Mapping of Genomic DNA in Nanofluidic Channels Walter Reisner, Niels Larsen, Anders Kristensen, Jonas O. Tegenfeldt, Henrik Flyvbjerg We have developed a new DNA barcoding technique based on the partial denaturation of extended fluorescently labeled DNA molecules. We partially melt DNA extended in nanofluidic channels via a combination of local heating and added chemical denaturants. The melted molecules, imaged via a standard fluorescence videomicroscopy setup, exhibit a nonuniform fluorescence profile corresponding to a series of local dips and peaks in the intensity trace along the stretched molecule. We show that this barcode is consistent with the presence of locally melted regions and can be explained by calculations of sequence-dependent melting probability. We believe this melting mapping technology is the first optically based single molecule technique sensitive to genome wide sequence variation that does not require an additional enzymatic labeling or restriction scheme. [Preview Abstract] |
Monday, March 16, 2009 9:00AM - 9:12AM |
A28.00006: Translocation Studies of Single Strand-DNA Oligomer Complexes with ds-DNA Markers Using Solid-State Nanopores Venkat Balagurusamy, Paul Weinger, Sungcheol Kim, Xinsheng Sean Ling We have designed short oligomers of single strand DNA of about 130 bases long each with 12-bases long sticky ends that are complimentary to those on one end of other oligomers to form ds-DNA regions by Watson-Crick base-pairing in these regions. Such a design facilitates the formation of a chain of single strands of DNA with ds-DNA regions interspersed. In order to slow down the translocation speed of these complexes through solid-state nanopores that could enable one to identify the ds-DNA region markers in the blockage current signal during translocation, we have attached these ss-DNA complexes with a polystyrene bead on one end. We present the results of our preliminary studies that show that the signature of these ds-DNA region markers could be identified. [Preview Abstract] |
Monday, March 16, 2009 9:12AM - 9:24AM |
A28.00007: Magnetophoresis of Fe$_{3}$O$_{4}$ Nanorods David Tan, Jitkang Lim, Caitlin Lanni, Frederick Lanni, Robert Tilton, Sara Majetich The magnetophoretic motion of a nanorod is quite different from that of a nanosphere. In large particles, motion is predicted from the balance of magnetic and viscous drag forces, but for nanoparticles random thermal forces lead to Brownian motion as well. Due to magnetic and diffusive anisotropy, a nanorod has advantages over a nanosphere for the single particle guidance and tracking, which would be important for studies within living cells. We have investigated the magnetophoretic behavior of nanorods and nanospheres both theoretically and experimentally. Peclet number analysis shows that 300 nm x 20 nm nanorods are more likely to be in the convective than diffusive regime than nanospheres of equal volume, for the same field and field gradient. Experimental studies of nanorod motion were made using Fe$_{3}$O$_{4}$ nanorods coated with poly (diallyldimethylammonium chloride) and fluorescein-5-isothiocyanate (FITC) tagged bovine serum albumin (BSA) and dispersed in saline solution. The motion of the nanorods was observed with and without magnetic field gradients using fluorescence microscopy. Fluorescence micrograph showed the nanorods undergo magnetophoretic motion toward the higher field gradient region with a velocity of about 28$\mu $m/sec. The controlled motion of magnetic nanorods within HeLa cancer cells has been demonstrated. [Preview Abstract] |
Monday, March 16, 2009 9:24AM - 9:36AM |
A28.00008: Chip-based Magnetic Resonance System for Medical Diagnosis Hakho Lee, Tae-Jong Yoon, Ralph Weissleder We have developed a chip-based, diagnostic magnetic resonance (DMR) system that can perform rapid, quantitative and multi-channeled detection of biological targets. The measurement is based on the effect of molecularly targeted magnetic nanoparticles on NMR (nuclear magnetic resonance) signals. With magnetic nanoparticles bound to their intended detection targets, the overall spin-spin relaxation time of bulk samples will be significantly shortened, as the particles efficiently dephase spins of surrounding water protons. Because the signal detection relies on NMR, the interference from media becomes negligible, making it possible to perform measurements in native biological samples (e.g., blood, sputum and urine). As proof of concept, we have developed a first DMR prototype by integrating microcoils, microfluidic channels and a permanent magnet. The microcoils, used as an NMR probe, are arranged in an array format for multiplexed, parallel detection. The microfluidic channels provide on-chip mixing between magnetic nanoparticles and biological samples and confine the mixture to microcoils for high filling factor. Here, we demonstrate clinical utility of the DMR system by measuring proteins at exquisite sensitivities ($\sim $1 pM), identifying the disease condition of human sera, and profiling cancer cells according to their cell-surface markers. [Preview Abstract] |
Monday, March 16, 2009 9:36AM - 9:48AM |
A28.00009: Hyperpolarized Long-\emph T$\bf_1$ Silicon Nanoparticles for Magnetic Resonance Imaging Maja Cassidy, Jacob Aptekar, Alexander Johnson, Robert Barton, Menyoung Lee, Alexander Ogier, Chinh Vo, Chandrasekhar Ramanathan, David Cory, Alison Hill, Ross Mair, Matthew Rosen, Ronald Walsworth, Charles Marcus Nanoparticles are currently being widely investigated as targetable contrast agents for magnetic resonance imaging (MRI). Silicon is a promising material system for use as a magnetic resonance imaging agent due to its long bulk (T$\bf_1$) times and receptivity to hyperpolarization. We present studies of the nuclear relaxation (T$\bf_1$) times of silicon nanoparticles as a function of particle size, dopant concentration and fabrication method. The T$\bf_1$ times of these particles are found to be remarkably long (depending on size and dopant concentration), allowing for them to be transported and administered on practical time scales. In addition, we discuss the particles' receptivity to hyperpolarization, via low temperature microwave induced dynamic nuclear polarization. [Preview Abstract] |
Monday, March 16, 2009 9:48AM - 10:00AM |
A28.00010: Magnetic detection of biotin-streptavidin binding using InAs quantum well $\mu $-Hall sensor Khaled Aledealat, K. Chen, G. Mihajlovic, P. Xiong, G. Strouse, P.B. Chase, S. von Moln\'ar, M. Field, G.J. Sullivan Magnetic sensors are a key component in any high-sensitivity, rapid-response, and portable platform for magnetic biosensing. InAs quantum well micro-Hall sensors have shown high potential for such a role due to their low noise level and capability to detect single micron- sized or smaller superparamagnetic beads suitable for biosensing$^{1}$. Here we present successful selective biotinylation of InAs micro-Hall sensors and directed self-assembly of 350 nm streptavidin-coated superparamagnetic beads via the biotin-streptavidin interaction. Two Hall crosses with three and two beads produced detection signals with S/N ratio of 21.3 dB and 18.4 dB respectively. In addition, our progress for \textit{in situ} detection of micron-sized magnetic beads using microfluidic channel will be presented. $^{1}$G. Mihajlovic et al., APL 87, 112502 (2005) This work was supported by NIH NIGMS GM079592. [Preview Abstract] |
Monday, March 16, 2009 10:00AM - 10:12AM |
A28.00011: Single Nanometric Memory Unit Based On a Protein-Nanoparticle Hybrid Izhar Medalsy, Arnon Heyman, Oded Shoseyov, Danny Porath Proteins as an isolating template and nanoparticle (NP) as an electric storage component can form a single addressable unit cell isolated from the conductive surface and adjacent NPs. This setup gives rise to a wide range of nanoelectronic applications. Here we demonstrate, by Conductive AFM, a single nanometric memory unit using individual protein-NP hybrids. SP1 is a boiling-stable ring-shaped protein, 11 nm in diameter. Mutants of SP1 were synthesized allowing its selective attachment to gold surface and the formation of 2D arrays using methods such as phospholipids trough and Langmuir Blodgett. The SP1 inner pore was connected to Si NP forming a chargeable entity embedded in an isolating unit over a conductive surface. Each NP holds three charging states: natural, positive and negative. The charging life times are 10 min in ambient and days in vacuum. Using this setup, and the relative long charging time, we were able to apply a read and write operations on individual 5nm Si NP embedded in a stable protein. [Preview Abstract] |
Monday, March 16, 2009 10:12AM - 10:24AM |
A28.00012: Characterization of the Uptake of Quantum Dots by Algae Priyanka Bhattacharya, Sijie Lin, Xiaoqian Sun, David Brune, Pu-Chun Ke The exposure of living systems to nanoparticles is inevitable due to a dramatic increase in their release into the environment, the most likely pathways being through inhalation, ingestion and skin uptake. The extremely small size of the nanoparticles may facilitate their tissue and cellular uptake by plants and animals, resulting in either positive (drug delivery, antioxidation) or negative (toxicity, cellular dysfunction) effects. Here we report the effects of quantum dots uptake by algae, the single-celled plant species and major food sources for aquatic organisms. In our studies, the presence of quantum dots in algal cells was detected using fluorescence microscopy and electron microscopy. Using spectrophotometry we found a supralinear increase of the uptake with the concentration of quantum dots, with a saturation of the uptake occurring beyond a concentration of 15 mg/mL. Using a bicarbonate indicator we further evaluated the effects of quantum dots uptake on algal photosynthesis and respiration. Such study facilitates our understanding of the environmental impact of nanomaterials. [Preview Abstract] |
Monday, March 16, 2009 10:24AM - 10:36AM |
A28.00013: Thermochemical nanolithography of multi-functional templates for selective assembly of bioactive proteins Debin Wang, Vamsi Kodali, William Underwood, Jonas Jarvholm, Takashi Okada, Simon Jones, Mariacristina Rumi, Zhenting Dai, William King, Seth Marder, Jennifer Curtis, Elisa Riedo Atomic force microscopy based techniques have been successful in generating protein nano-arrays on various substrates. However, several challenges still exist in terms of resolution, writing speed, cost, substrate choice, protein bioactivity, multi-component patterning, and surface passivation. Recently, we have developed the use of thermochemical nanolithography combined with post covalent functionalization and molecular recognition on a polymer surface of a single chip to produce multiplexed nanopatterns at speeds of mm/s. These patterns can then be functionalized under native conditions to create tailored nano-assemblies of two different species of proteins coexisting on the same surface. The proteins attach selectively and strongly to the nanopatterns via covalent and/or specific interactions, while retaining their ability to interact specifically with other proteins in buffered solution. At present, this method has produced nanopatterns of bio-active proteins with features as small as 40 nm on polymer films. This technique opens up new possibilities in nanoscale manipulation of biological macromolecules as well as many molecular biophysics studies such as inter-protein interactions. [Preview Abstract] |
Monday, March 16, 2009 10:36AM - 10:48AM |
A28.00014: Directional Growth of Polymeric Nanowires Prem Thapa, Bret Flanders This work establishes an innovative electrochemical approach to the template free growth of conducting polypyrrole and polythiophene wires. These polymeric wires exhibit a knobby structure, but persistent growth in a given direction up to 30 $\mu $m in length. A long-range component of the applied voltage signal defines the growth-path. Moreover, the presence of this component enables the growth of amorphous nanowires with wire-like geometries. Such wires are employed in a non-invasive methodology for attaining strong mechanical attachments to live cells. This capability is of potential use in the electro-mechanical probing of cell physiological processes. [Preview Abstract] |
Monday, March 16, 2009 10:48AM - 11:00AM |
A28.00015: A novel nanoarchitecture with optical, solar, medical and biochemical utility M.J. Naughton, K. Kempa, Z.F. Ren We discuss a nanoscale platform offering widespread utility in nanophotonics, photovoltaics, visual prosthetics, and biological and chemical sensing. As a subwavelength wave-guide architecture, these nanostructures can be used in array form for high efficiency solar cells, as well as in a wide range of nanoscale manipulations of light without deleterious plasmonic effects. They are also being developed as a high electrode-density (10$^{8}$/cm$^{2})$ retinal implant. Finally, a modification of the basic structure enables the fabrication of a highly sensitive ``nanocavity'' biochemical sensor. We will report on aspects of each application. We also thank the following collaborators: N. Argenti, D. Cai, T.C. Chiles, P. Dhakal, Y. Gao, T. Kirkpatrick, Y.C. Lan, G. McMahon, J.I. Oh, B. Rizal, J. Rybczynski. [Preview Abstract] |
Session A29: Spin Glasses and Disordered Magnetic Materials
Sponsoring Units: GMAGChair: Ben Ueland, National Institute of Standards and Technology, Center for Neutron Research
Room: 333
Monday, March 16, 2009 8:00AM - 8:12AM |
A29.00001: Probing the relation between structural glasses and 3-spin spin glasses using one-dimensional models Derek Larson, Helmut G. Katzgraber, A.P. Young Motivated by a proposed connection between 3-spin spin glasses and structural glasses, we have performed Monte Carlo simulations on a one-dimensional long-range Ising glass with power-law interactions involving 3-spins. Varying the exponent of the power-law interactions is analogous to changing the space dimension of a corresponding short-range 3-spin model. We present results of a finite-size scaling analysis of the two-point correlation length, and compare our results with the prediction of Moore and Yeo that the three-spin model is in the same universality class as an Ising spin glass in a magnetic field. [Preview Abstract] |
Monday, March 16, 2009 8:12AM - 8:24AM |
A29.00002: Study of the de Almeida-Thouless line using power-law diluted one-dimensional Ising spin glasses Helmut G. Katzgraber, Derek A. Larson, A.P. Young We test the existence of a spin-glass state in an externally-applied (random) magnetic field via Monte Carlo simulations of a power-law diluted one-dimensional Ising spin glass. The model has the advantage over conventional short-range models in that by tuning the exponent of the power-law interactions we are able to scan the full range of possible behaviors from the infinite-range to the non-mean-field regime. Furthermore, due to the average fixed connectivity very large linear system sizes can be studied. An analysis of the two-point correlation length shows that the system in the non-mean-field universality class does not order in a field. This suggests that there is no de Almeida-Thouless line for short-range Ising spin glasses below the upper critical dimension. [Preview Abstract] |
Monday, March 16, 2009 8:24AM - 8:36AM |
A29.00003: Reentrant and Forward Phase Diagrams of the Anisotropic Three-Dimensional Ising Spin Glass Can G\"uven, A. Nihat Berker, Michael Hinczewski, Hidetoshi Nishimori The spatially uniaxially anisotropic d=3 Ising spin glass is solved exactly on a hierarchical lattice.[1] Five different ordered phases, namely ferromagnetic, columnar, layered, antiferromagnetic, and spin-glass phases, are found in the global phase diagram. The spin-glass phase is more extensive when randomness is introduced within the planes than when it is introduced in lines along one direction. Phase diagram cross-sections, with no Nishimori symmetry, with Nishimori symmetry lines, or entirely imbedded into Nishimori symmetry, are studied. The boundary between the ferromagnetic and spin-glass phases can be either reentrant or forward, that is either receding from or penetrating into the spin-glass phase, as temperature is lowered. However, this boundary is always reentrant when the multicritical point terminating it is on the Nishimori symmetry line. [1] C. G\"uven, A.N. Berker, M. Hinczewski, and H. Nishimori, Phys. Rev. E 77, 061110 (2008). [Preview Abstract] |
Monday, March 16, 2009 8:36AM - 8:48AM |
A29.00004: The Blume-Emery-Griffiths Spin Glass and Inverted Tricritical Points V. Ongun \"Oz\c{c}elik, A. Nihat Berker The Blume-Emery-Griffiths spin glass is studied by renormalization-group theory in d=3.[1] The boundary between the ferromagnetic and paramagnetic phases has first-order and two types of second-order segments. This topology includes an inverted tricritical point, first-order transitions replacing second-order transitions as temperature is lowered. The phase diagrams show disconnected spin-glass regions, spin-glass and paramagnetic reentrances, and complete reentrance, where the spin-glass phase replaces the ferromagnet as temperature is lowered for all chemical potentials. [1] V.O. \"Oz\c{c}elik and A.N. Berker, Phys. Rev. E 78, 031104 (2008). [Preview Abstract] |
Monday, March 16, 2009 8:48AM - 9:00AM |
A29.00005: Quenched-Vacancy Induced Spin-Glass Order G\"ul G\"ulpinar, A. Nihat Berker The ferromagnetic phase of an Ising model in d=3, with any amount of quenched antiferromagnetic bond randomness, is shown to undergo a transition to a spin-glass phase under sufficient quenched bond dilution.[1] This general result, demonstrated here with the numerically exact renormalization-group solution of a d=3 hierarchical lattice, is expected to hold true generally, for the cubic lattice and for quenched site dilution. Conversely, in the ferromagnetic-spinglass-antiferromagnetic phase diagram, the spin-glass phase expands under quenched dilution at the expense of the ferromagnetic and antiferromagnetic phases. In the ferro-spinglass phase transition induced by quenched dilution reentrance is seen, as previously found for the ferro-spinglass transition induced by increasing the antiferromagnetic bond concentration. [1] G. G\"ulpinar and A.N. Berker, arXiv:0811.0025v1 [cond- mat.dis-nn] (2008). [Preview Abstract] |
Monday, March 16, 2009 9:00AM - 9:12AM |
A29.00006: Nonequilibrium spin glass dynamics with the Janus computer David Yllanes, F. Belletti, A. Cruz, L.A. Fernandez, A. Gordillo-Guerrero, M. Guidetti, A. Maiorano, F. Mantovani, E. Marinari, V. Martin-Mayor, J. Monforte, A. Munoz Sudupe, D. Navarro, G. Parisi, S. Perez-Gaviro, J.J. Ruiz-Lorenzo, S.F. Schifano, D. Sciretti, A. Tarancon, R. Tripiccione The out of equilibrium evolution of the Edwards-Anderson spin glass is followed for a tenth of a second, effectively halving the (logarithmic) temporal gap between previous simulations and experiments. In fact, we have been able to make safe predictions about the behavior at experimental times, using mild extrapolations. This work has been made possible by Janus, a special purpose computer designed by our collaboration. We have thoroughly studied the spin glass correlation functions and the growth of the coherence length for $L\!=\!80$ lattices in $3D$,using $L\!=\!24,40$ lattices to check for finite size effects. We present clear evidence for a replicon correlator. Our main conclusion is that these spin glasses follow non-coarsening dynamics, at least up to the experimentally relevant time scales. [Preview Abstract] |
Monday, March 16, 2009 9:12AM - 9:24AM |
A29.00007: Chaotic Spin Correlations in Frustrated Ising Hierarchical Lattices Ne\c{s}e Aral, A. Nihat Berker Spin-spin correlations are calculated in frustrated hierarchical Ising models that exhibit chaotic renormalization-group behavior. [1] The spin-spin correlations, as a function of distance, behave chaotically. The far correlations, but not the near correlations, are sensitive to small changes in temperature or frustration, with temperature changes having a larger effect. On the other hand, the calculated free energy, internal energy, and entropy are smooth functions of temperature. The recursion-matrix calculation of thermodynamic densities in a chaotic band is demonstrated. The spectrum of Lyapunov exponents is calculated as a function of frustration. [1] N. Aral and A.N. Berker, arXiv:0810.4586v1 [cond-mat.dis-nn] (2008). [Preview Abstract] |
Monday, March 16, 2009 9:24AM - 9:36AM |
A29.00008: Tunable domain pinning in a Random-Field Ising Ferromagnet D. M. Silevitch, G. Aeppli, T.F. Rosenbaum The diluted magnetic salt $\mathrm{Li(Ho,Y)F}_4$ was shown recently [Nature {\bf 448} 567-570 (2007)] to be the first ferromagnetic realization of the random-field Ising model, where the strength of the random fields can be tuned by an external magnetic field. These random-field effects can be used to continuously and reversibly vary the pinning potential of the magnetic domains, allowing us to tune the hysteretic behavior. Magnetization measurements reveal enhanced pinning in the random-field regime as well as a temperature-dependent crossover into a regime dominated by quantum fluctuations. [Preview Abstract] |
Monday, March 16, 2009 9:36AM - 9:48AM |
A29.00009: A strongly disordered spin glass and minimum spanning trees Thomas Jackson, Nicholas Read We investigate the ground state structure of a strongly
disordered spin glass model proposed by Newman and Stein (NS). In
the strong disorder limit, frustration is negligible and the
problem of identifying ground states is equivalent to the minimum
spanning tree (MST) problem in combinatorial optimization: given
an edge-weighted graph, the MST is the subset of edges that
connects all vertices, has no cycles, and minimizes the total
edge weight. Here the weights are quenched random variables, and
we use a relation between Kruskal's greedy algorithm for finding
the MST and percolation. We solve this random MST on the Bethe
lattice with appropriate boundary conditions, which defines a
mean-field theory valid above $d_c=6$ (NS proposed $d_c=8$).
Above $d_c$, NS showed that the spin glass model has infinitely
many ground states, but only a single pair below $d_c$. For
$d |
Monday, March 16, 2009 9:48AM - 10:00AM |
A29.00010: ABSTRACT WITHDRAWN |
Monday, March 16, 2009 10:00AM - 10:12AM |
A29.00011: Overlap as a Measure of Spin-Glass Memory and a Probe of Free Energy Landscape Wen Luo, Michael Mihalco, Thomas E. Stone, Susan R. McKay The degree of history dependence and the structure of the free energy landscape of the spin glass are both indicators of the complexity of this ordered phase. Using the Ising antiferromagnet on a triangular lattice, diluted with quenched random ferromagnetic bonds, we probe these indicators through repeated cycling between two temperatures. We consider cases in which both temperatures are within the spin-glass phase, and systematically vary the temperature difference between initial and final states. These results are compared with the same cycling pattern with one temperature inside and the other outside of the spin-glass phase. The average overlap between low-temperature states provides a quantitative measure of the system's memory, and is non-zero when the system remains within the spin-glass phase during cycling. A plot of the overlaps of the low temperature states and their differences in internal energy shows no simple relationship between overlap and internal energy. States with almost identical internal energies often have very little overlap. [Preview Abstract] |
Monday, March 16, 2009 10:12AM - 10:24AM |
A29.00012: Quantum Effects for Interaction of Electron with coupled magnetic local spin chains Fatih Dogan, Lucian Covaci, Wonkee Kim, Frank Marsiglio In this talk, we will look at time dependent interaction of an electron with ferromagnetic chain. We will show that ferromagnetic interactions between magnetic spins cause the electron interacting with them to change its energy and depending on the strength of interactions form a bound state. These effects are visible through the resulting state of the electron. Experimental suggestions will be given to observe this quantum behavior. [Preview Abstract] |
Monday, March 16, 2009 10:24AM - 10:36AM |
A29.00013: Avalanche Spatial Structure: Viewing Crackling Noise through Windows Yan-Jiun Chen, Stefanos Papanikolaou, James P. Sethna, Gianfranco Durin, Stefano Zapperi In imaging experiments of Barkhausen noise in thin films, magnetic avalanches at the boundaries present challenges to analysis. Large avalanches are removed from the distribution, and the portion inside the viewing window may sometimes be treated as smaller avalanches. We analyze the scaling behavior of different categories of avalanches in artificially-windowed simulations of Barkhausen noise to examine the effect of window size on scaling relations. In passing, we discuss the average spatial shapes of avalanches, multivariable scaling functions, and the use of nonlinear-least-squares methods for exploring and reporting universal scaling functions. [Preview Abstract] |
Monday, March 16, 2009 10:36AM - 10:48AM |
A29.00014: Avalanche Average Shapes: Mean-field temporal average avalanche shape Stefanos Papanikolaou, Christopher R. Myers, Francesca Colaiori, Karen E. Daniels, Gianfranco Durin, Stefano Zapperi, James P. Sethna The average temporal shape of avalanches has been a fruitful application of universality and critical scaling, with experimental and theoretical investigations particularly in the field of magnetic Barkhausen noise. The mean-field shapes of these avalanches have been thought to come in two forms: inverted parabolas for the infinite-range model and one lobe of a sinusoid for the single-degree of freedom ABBM model. We show that the infinite-range model can be mapped onto the earlier ABBM model, and that the average shape for both mean field theories is an inverted parabola, seemingly resolving the ambiguity. However, we also propose a new mean-field model including the effects of local saddle-node bifurcations on the dynamics, and analyze both its predictions for dynamical exponents and temporal average shapes. We compare with experimental results on sheared granular materials. [Preview Abstract] |
Monday, March 16, 2009 10:48AM - 11:00AM |
A29.00015: Permutation Symmetric Critical Phases in Disordered Non-Abelian Anyonic Chains Lukasz Fidkowski, Gil Refael, Han-Hsuan Lin, Paraj Titum Topological phases supporting non-abelian anyonic excitations have been proposed as candidates for topological quantum computation. We study disordered non-abelian anyonic chains based on the quantum groups $SU(2)_k$, a hierarchy that includes the $\nu=5/2$ FQH state and the proposed $\nu=12/5$ Fibonacci state, among others. We find that for odd $k$ these anyonic chains realize infinite randomness critical {\it phases} in the same universality class as the $S_k$ permutation symmetric multi-critical points of Damle and Huse (arXiv:cond-mat/0207244). Indeed, we show that the pertinent subspace of these anyonic chains actually maps to the ${Z}_k \subset S_k$ symmetric sector of the Damle-Huse model, and this ${Z}_k$ symmetry stabilizes the phase. [Preview Abstract] |
Session A30: Focus Session: Vanadates, Iridates and Other Oxides
Sponsoring Units: DMP GMAGChair: Stephen Nagler, Oak Ridge National Laboratory
Room: 334
Monday, March 16, 2009 8:00AM - 8:12AM |
A30.00001: Electronic band structure of metallic phase V$_{2}$O$_{3}$ O. Krupin, J. Denlinger, B.J. Kim, Ravi S. Singh, J.W. Allen V$_{2}$O$_{3}$ has an archetypal strongly correlated paramagnetic metallic (PM) phase which becomes insulating with alloying or decreasing temperature. Recent progress has been made experimentally to measure the true bulk V 3d density of states of PM phase V$_{2}$O$_{3}$ using high-energy angle-integrated photoemission, and theoretically to quantitatively describe the observed prominent quasiparticle peak near E$_{F}$ using LDA+DMFT. Theoretical predictions of the k-resolved electronic band structure of V$_{2}$O$_{3}$ have been made, but experimental measurement has proven to be very challenging and elusive. We present intermediate-energy soft x-ray angle-resolved photoemission measurements of the PM-phase V$_{2}$O$_{3}$ (0001) cleaved surface that reveal for the first time distinct k-resolved band dispersions within the coherent quasiparticle peak and a corresponding three-fold symmetric Fermi surface topology. The agreement of these measurements to theoretical calculations will be discussed. [Preview Abstract] |
Monday, March 16, 2009 8:12AM - 8:24AM |
A30.00002: Concurrent structural and magnetic phase transition in nanopowder V$_{2}$O$_{3}$ J. P. Carlo, Y. J. Uemura, V. Blagojevic, M. L. Steigerwald, L. E. Brus, S. J. L. Billinge, W. Zhou, G. M. Luke, A. A. Aczel, G. J. MacDougall, P. W. Stephens V$_{2}$O$_{3}$, which has been the subject of investigations for well over 30 years, is a classic example of a Mott-Hubbard transition system. This first-order metal-insulator transition, near 160K, is accompanied by a rhombohedral-monoclinic structural as well as a paramagnetic-antiferromagnetic magnetic transition. We report on structural synchrotron x-ray characterization of V$_{2}$O$_{3}$ nanopowder (dia $\approx $ 10-50 nm) at NSLS, and magnetic characterization via muon spin relaxation at TRIUMF. We find that, just as in bulk V$_{2}$O$_{3}$, the structural and magnetic transitions are concurrent, and that in contrast to the abrupt and hysteretic transition witnessed in the bulk, in the nanopowder sample the transition occurs with phase separation over a broad temperature range. [Preview Abstract] |
Monday, March 16, 2009 8:24AM - 8:36AM |
A30.00003: Metal-insulator transition in AV$_{10}$O$_{15}$ (A=Ba,Sr) T. Katsufuji, T. Suzuki, T. Kajita In AV$_{10}$O$_{15}$ (A=Ba, Sr), the V ions take a mixed- valence state, V$^{2.8+}$ ($3d^{2.2}$), and form a modified triangular lattice, in which V triangles are periodically missing from a normal triangular lattice. It is known that BaV$_ {10}$O$_{15}$ undergoes a structural phase transition at around 120 K. We have succeeded in growing large single crystals of AV$_{10}$O$_{15}$ (A=Ba, Sr) by a floating-zone method. We found a large jump of electrical resistivity by $\sim 10^{3}$ times at the structural transition temperature (123 K) of BaV$_ {10}$O$_{15}$, which can be regarded as a metal-insulator transition presumably dominated by a charge/orbital ordering of V. We also found an antiferromagnetic ordering at 43 K in the same compound. On the other hand, SrV$_{10}$O$_{15}$ did not show any structural anomaly down to the lowest temperature, and a spin-glass behavior was observed. These results indicate a strong correlation between the structural anomaly (charge/orbital ordering) and the magnetism in this series of compounds. We also measured the optical reflectivity of BaV$_ {10}$O$_{15}$ and found the opening of a charge gap in the optical conductivity spectrum at low temperatures. [Preview Abstract] |
Monday, March 16, 2009 8:36AM - 8:48AM |
A30.00004: Charge dynamics in thermally and doping induced insulator-metal transitions of (Ti$_{1-x}$V$_x$)$_2$O$_3$ Masaki Uchida, Jun Fujioka, Yoshinori Onose, Yoshinori Tokura Charge dynamics of (Ti$_{1-x}$V$_x$)$_2$O$_3$ with $x=0-0.06$ has been investigated by measurements of charge transport and optical conductivity spectra in a wide temperature range of $2-600$ K with the focus on the thermally and doping induced insulator-metal transitions (IMTs). The optical conductivity peaks for the interband transitions in the 3$d$ $t_{2g}$ manifold are observed in the both insulating and metallic states, while their large variation (by $\sim 0.4$ eV) with change of temperature and doping level scales with that of the Ti-Ti dimer bond length, indicating the weakened singlet bond in the course of IMTs. The thermally and V-doping induced IMTs are driven with the increase in carrier density by band-crossing and hole-doping, respectively, in contrast to the canonical IMT of correlated oxides accompanied by the whole collapse of the Mott gap. [Preview Abstract] |
Monday, March 16, 2009 8:48AM - 9:00AM |
A30.00005: Intersite correlations and the metal insulator in cluster dynamical mean field theory: cluster size, interaction strength, and the location of the transition line C. Lin, A. Millis To gain insight into the physics of the metal insulator transition and the effectiveness of cluster dynamical mean field theory we have used one, two and four site dynamical mean field theory (both CDMFT and DCA) to solve a model of electrons coupled to a classical phonon field. A partial density of states is defined encoding a generalized nesting property of the band structure; variations in this density of states account for differences between dynamical cluster approximation and cellular-DMFT implementation of cluster DMFT, and for differences in behavior between single band (cuprate-like) and multiband (manganite-like) models. The cluster size dependence of the metal to polaronic insulator phase boundary is determined along with electron spectral functions and cluster correlation functions. Over most of the interaction strength regime the single-site and multi-site approximations are found to yield similar results. Important cluster size effects occur only in the metal insulator transition region, where short-ranged orrelations are found to significantly reduce the critical interaction strength required to drive a metal polaron insulator transition. In the cluster approximations the physics of the metal-insulator transition is shown to be Slater-like (driven by band filling). The minimal cluster size required to capture the metal-polaron insulator transition is shown to depend sensitively on the carrier concentration. Implications for the theoretical treatment of doped manganites are discussed. [Preview Abstract] |
Monday, March 16, 2009 9:00AM - 9:12AM |
A30.00006: Study of phase separated manganites by DC transport and infrared spectroscopy A. Zimmers, T. Wolf, J. Lesueur, R.P.S.M. Lobo, A. Kushwaha, R.C. Budhani We present confined geometry measurements of manganites La$_{0.325}$Pr$_{0.3}$Ca$_{0.375}$MnO$_{3}$ (LPCMO) and La$_{2/3}$Sr$_{1/3}$MnO$_3$ (LSMO). As reported previously, due to electronic phase separation, LPCMO microwires show a step-like metal insulator transition as temperature is lowered and as magnetic field is sweped. We will show how this feature evolves as a function of the width and shape of the microwires. On the contrary, LSMO microwires are found to have a smooth transition in all wire sizes down to a width of 140$nm$. We have created extra disorder in the LSMO sample by irradiating it using 150keV oxygen ions with a dose of 2 x 10$^{14}$ ions/cm$^2$. As expected, the microwires T$_{MI}$ transitions are lowered by 120K but the transitions remain stepless. The fact that the electronic homogeneity in LSMO sample is robust, even when irradiated, enabled us start an optical study of artificially phase separated highly correlated materials. We will show preliminary results comparing reflectivity spectra of micropatterned samples to effective medium approximation models. [Preview Abstract] |
Monday, March 16, 2009 9:12AM - 9:24AM |
A30.00007: Spectral Function of Manganite Systems Juan Salafranca, Elbio Dagotto Using a double exchange model with Jahn Teller distortions, we calculate the one particle spectral function of manganite systems. The relative contribution of the different interactions is established by means of Monte Carlo and self consistent mean field calculations. In particular, we examine the effects of electron-electron and electron-phonon couplings. We discuss the relevance of our results in relation to recent Photoemission experiments on layered manganites [1], where similarities of manganites spectra with that of cuprates was observed.\\[3pt] [1] N. Mannella {\it et al.} Nature {\bf 438} 474 (2005) [Preview Abstract] |
Monday, March 16, 2009 9:24AM - 9:36AM |
A30.00008: Anomalous nuclear relaxation in the ferromagnetic phase of the bilayered manganite La$_{1.2}$Sr$_{1.8}$Mn$_{2}$O$_{7}$. Michael Hoch, Philip Kuhns, William Moulton, Arneil Reyes, Jun Lu, John Mitchell In contrast to ferromagnetic (FM) 3D manganites, $^{55}$Mn NMR spectra obtained for the FM phase of the colossal magnetoresistance bilayer manganite La$_{1.2 }$Sr$_{1.8 }$Mn$_{2}$O$_{7}$ show a broad distribution of hyperfine fields implying a large distribution of local environments at Mn sites. The hyperfine distribution may be linked to orbital ordering effects. $^{55}$Mn spin -- lattice relaxation rates have a surprisingly weak dependence both on temperature and applied magnetic field. Significant departures of the relaxation rate from Korringa temperature dependence below 20 K provide evidence for non-Fermi liquid behavior in this quasi-2D metal. At temperatures approaching $T_{C }$ from below, further anomalous behavior is found consistent with spin polaron formation in the range where colossal magnetoresistance starts to appear. [Preview Abstract] |
Monday, March 16, 2009 9:36AM - 9:48AM |
A30.00009: Anomalous metallic state: Decisive effect of dilute impurity doping on ferromagnetic BaIrO$_3$ single crystals O.B. Korneta, S. Chikara, T.F. Qi, S. Parkin, G. Cao, W.H. Song BaIrO$_3$ is a quasi one-dimensional system, where a {CDW} and a ferromagnetic state with T$_{C}$=183K coexist. The ground state of the system is critically linked to the lattice and orbital degrees of freedom due to extended $5d$-orbitals. The central findings of this study are: (1) An occurrence of a 2D-metallic state with a linear temperature dependence of resistivity at low temperatures in slightly oxygen-deficient samples; (2) Unusual temperature dependence of resistivity above the Curie temperature in the rare-earth doped BaIrO3; (3) High sensitivity of the resistivity to applied pressure ($<12$ Kbar), which results in changes in resistivity by a few orders of magnitude in these doped samples. The results of the resistivity, heat capacity, magnetization, thermoelectric power and structural measurements as a function of temperature, magnetic field, and pressure will be presented and discussed. [Preview Abstract] |
Monday, March 16, 2009 9:48AM - 10:00AM |
A30.00010: Non-Fermi-liquid behavior in nearly ferromagnetic SrIrO$_{3}$ single crystals T.F. Qi, S. Chikara, O.B. Korneta, S. Parkin, L.E. De Long, G. Cao, P. Schlottmann We report magnetic, electric transport, and calorimetric properties of
single-crystal SrIrO$_{3}$ as a function of temperature $T$ and applied
magnetic field $H$. We find that SrIrO$_{3 }$is a non-Fermi-liquid metal near a
ferromagnetic instability, as characterized by the following properties: (1)
small saturation moment and no evidence for long-range order down to 1.7 K,
(2) strongly enhanced magnetic susceptibility that diverges as $T^{\gamma }$
at low temperatures with 1/2$<\gamma <$1, depending on the applied
field, (3) heat capacity $C(T$,$H)\sim \quad -T $ln $T $that is readily enhanced in low
applied fields, and (4) $T^{3/2}$ dependence of electrical resistivity over
the range 1.7 K$ |
Monday, March 16, 2009 10:00AM - 10:12AM |
A30.00011: Magnetic Soft Mode Behavior of the Field-Dependent Specific Heat of SrIrO$_{3}$ Lance De Long, Daheng He, Vinayak Bhat, Gang Cao Previous work\footnote{Cao et al., Phys. Rev. B \textbf{76}, 100402(R),(2007)} indicates SrIrO$_{3}$ is a strongly exchange-enhanced paramagnet (Wilson ratio = 75) exhibiting non-Fermi liquid (NFL) behavior at low magnetic fields, and a cross-over to weak ferromagnetism (0.025 $\mu _{B}$/Ir at $\mu _{o}$H = 7.0 T and T = 1.7 K) at applied fields $\mu _{o}$H $\approx $ 3 T and temperatures T $<$ 4 K. Measurements of the specific heat performed in constant field for 1.8 $<$ T $<$ 4K have been used to extract the field dependence C$_{P}$(H,T$_{o})$(constant T$_{o})$, which exhibits a Schottky-like peak as a function field in the range 1.0 $< \quad \mu _{o}$H $<$ 1.5 T at increasing temperatures 1.8 $<$ T$_{o} \quad <$ 3.9 K, respectively. Fits of C$_{P}$(H,T$_{o})$ imply a nonmagnetic ground state is separated from magnetic excited states by an energy splitting $\Delta $(H,T)/k$_{B}$ = T* that decreases from 7.5 to 2 K as $\mu _{o}$H increases from 0 to 8 T. The Schottky peak field increases as $\mu _{o}$H* = 0.94 T + (0.03 T/K$^{3})$T$^{3.1}$. We discuss how a semi-classical two-level model reproduces the NFL-weak ferromagnet cross-over with applied field. [Preview Abstract] |
Monday, March 16, 2009 10:12AM - 10:24AM |
A30.00012: Temperature-dependent electronic structure evolution of spin-orbit coupling induced Mott insulator Sr$_{2}$IrO$_{4}$ S. J. Moon, W. S. Choi, T. W. Noh, H. Jin, Y. S. Lee, G. Cao Recently, the effect of spin-orbit coupling in 5$d$ transition metal oxides attracted lots of attention. It was found that the cooperative interaction of spin-orbit coupling and electron correlation could realize unique Mott insulating ground state of Sr$_{2}$IrO$_{4}$. We investigated temperature-dependent optical conductivity spectra of 5$d$ Mott insulator Sr$_{2}$IrO$_{4}$. We observed drastic changes of the optical conductivity spectra. As temperature increases, the Mott gap was significantly reduced and spectral weight redistribution between the Ir $t_{2g}$ bands occurred. The electronic structure changes accompanied the change of optical phonon modes. These experimental observations could be understood in terms of the effects of spin-orbit and electron-lattice coupling. [Preview Abstract] |
Monday, March 16, 2009 10:24AM - 10:36AM |
A30.00013: LS separation of J=1/2 Mott insulator observed by magnetic X-ray diffraction Shigeki Fujiyama, B.J. Kim, H. Ohsumi, T. Komesu, D. Hirai, K. Ohashi, S. Sakai, T. Arima, H. Takagi Spin-orbit coupling is a key concept to realize spin Hall effect in insulating materials. A perovskite iridate Sr$_2$IrO$_4$ is known to show insulating transport properties in spite of large spatial extent of 5d electrons. The origin of this unconventional behavior is argued to be a strong LS coupling which results in $J_{\mathrm{eff}}=1/2$ Mott insulator. We performed off-resonant magnetic X-ray scattering in the antiferromagnetically ordered state of this material and found that considerable contribution from orbital moment for the ordered moment. The estimated ratio, $\langle \psi | L_z | \psi | \rangle / \langle \psi | S_z | \rangle$ is 5. This is consistent with $J_{\mathrm{eff}}=1/2$ which expects the ratio as 4. This justifies unconventional $J_{\mathrm{eff}}=1/2$ Mott insulator for this transition metal oxides. [Preview Abstract] |
Monday, March 16, 2009 10:36AM - 10:48AM |
A30.00014: Magnetization reversal in Sr$_3$Ir$_2$O$_7$ : DM interactions vs. magnetic single-ion anisotropy Hosub Jin, Jaejun Yu Recently the unusual insulating ground state of Sr$_2$IrO$_4$ was shown to be a consequence of a novel quantum ground state of $j_{\mathrm{eff}}$=1/2. Another compound in its series Sr$_3$Ir$_2$O$_7$ with double layers of the IrO$_2$ planes exhibits anomalous magnetic responses such as magnetization reversal during field-cooling processes. We performed density-functional theory calculations to investigate the electronic and magnetic properties of Sr$_3$Ir$_2$O$_7$. Similarly to the case of Sr$_2$IrO$_4$, both spin-orbit (SO) and on-site Coulomb interactions are found to be responsible for the insulating ground state. Based on the analysis of our first-principle calculations for the various spin and lattice configurations, we found that the non-zero angular momentum state originating from large SO interactions with lattice distortions plays a crucial role in determination of both single-ion anisotropy and Dzyaloshinski-Moriya interactions, which explains anomalous magnetic responses in Sr$_3$Ir$_2$O$_7$. [Preview Abstract] |
Monday, March 16, 2009 10:48AM - 11:00AM |
A30.00015: Post-perovskite transition and magnetic and charge transport properties of the correlated 4$d$ post-perovskite CaRhO$_3$ K. Yamaura, Y. Shirako, H. Kojitani, M. Arai, D.P. Young, M. Akaogi, M. Nakashima, T. Katsumata, Y. Inaguma, E. Takayama-Muromachi A high-quality polycrystalline sample of the correlated 4$d$ post-perovskite CaRhO$_3$ (Rh$^{4+}$: 4$d^5$ $S = 1/2$) was attained under a moderate pressure of 6 GPa for the first time. It is obvious that the perovskite CaRhO$_3$ transforms into a layered phase, in which RhO$_6$ octahedra are connected by shearing the edge along a-axis and the corner along c-axis. The Rh-O layer stacks up alternatively with the Ca layer along b-axis. The characteristic structure suggests an electronic anisotropy toward 2D, which may be essential for unusual magnetism. The sample was subjected for measurements of charge transport and magnetic properties. The data clearly indicate it goes into an antiferromagnetically ordered state below $\sim90$ K in an unusual way, being a strikingly contrast to what was observed for the perovskite phase. [Preview Abstract] |
Session A31: Focus Session: Molecular Nanomagnets
Sponsoring Units: GMAG DMPChair: Michael Pechan, Miami University
Room: 335
Monday, March 16, 2009 8:00AM - 8:36AM |
A31.00001: Molecular Spintronics using Molecular Nanomagnets Invited Speaker: A revolution in electronics is in view, with the contemporary evolution of two novel disciplines, spintronics and molecular electronics. A fundamental link between these two fields can be established using molecular magnetic materials and, in particular, single-molecule magnets [1], which combine the classic macroscale properties of a magnet with the quantum properties of a nanoscale entity. The resulting field, molecular spintronics aims at manipulating spins and charges in electronic devices containing one or more molecules. In this context, we want to fabricate, characterize and study molecular devices (molecular spin-transistor, molecular spin-valve and spin filter, molecular double-dot devices, carbon nanotube nano-SQUIDs, etc.) in order to read and manipulate the spin states of the molecule and to perform basic quantum operations. The talk will discuss this--still largely unexplored--field and present our the first important results [2,3].\\[4pt] [1] L. Bogani \& W. Wernsdorfer, Nature Mat. 7, 179 (2008).\\[0pt] [2] J.-P. Cleuziou, W. Wernsdorfer, V. Bouchiat, T. Ondar\c{c}uhu, M. Monthioux, Nature Nanotech. 1, 53-59 (2006).\\[0pt] [3] N. Roch, S. Florens, V. Bouchiat, W. Wernsdorfer, F. Balestro, Nature 453, 633 (2008). [Preview Abstract] |
Monday, March 16, 2009 8:36AM - 8:48AM |
A31.00002: Entrapment of magnetic micro-crystals for on-chip ESR studies Nickolas Groll, Sylvain Bertaina, Mekhala Pati, Naresh S. Dalal, Irinel Chiorescu On-chip Electronic Spin Resonance (ESR) of magnetic molecules requires the ability to precisely position nanosized samples in antinodes for a maximum magnetic coupling. A method is developed to entrap micro-crystals containing spins in a well defined location on the substrate surface. Through the use of photolithography, this method has achieved positioning of single to tens of crystals with micron scale resolution. The method has allowed Q-band EPR measurements of a 175 micron diameter single crystal of BDPA at 34 GHz. Polycrystalline diluted Cr$^{5+}$ spin $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} $ systems [1] have been entrapped in 500 micron squares for which the lower limit of the EPR measurement sensitivity was approached. This method gives way to on-chip ESR measurements at dilution refrigerator temperatures by allowing the samples to be positioned inside an on-chip superconducting cavity. [1] N. Sarita et al, Phys. Rev. Lett. 99, 137601 (2007). [Preview Abstract] |
Monday, March 16, 2009 8:48AM - 9:00AM |
A31.00003: Experimental determination of the dipolar field in Mn$_{12}$-acetate Sean McHugh, R. Jaafar, M.P. Sarachik, Y. Myasoedov, H. Shtrikman, E. Zeldov, R. Bagai, G. Christou Crystals of the molecular magnet Mn$_{12}$-acetate are known to contain a small fraction of defect (minor species) molecules with a small anisotropy barrier against spin reversal. The lower barrier leads to faster magnetic relaxation and lower coercive field. We exploit the low coercive fields of the minor species, and the location of the minor species tunneling resonances, to make a direct determination of the dipole field in Mn$_{12}$-ac. We find that the dipolar field of a fully magnetized crystal is $51.5 \pm 8.5$ mT, consistent with theoretical expectations. [Preview Abstract] |
Monday, March 16, 2009 9:00AM - 9:12AM |
A31.00004: Magnetization barrier reduction in Mn$_{12}$ single-molecule magnets Gage Redler, Changhyun Koo, Saiti Datta, Christos Lampropoulos, Theocharis C. Stamatatos, George Christou, Stephen Hill High-frequency electron paramagnetic resonance (HFEPR) and AC susceptibility data will be presented for a new high-symmetry Mn$_{12}${\-}Ac complex, [Mn$_{12}$O$_{12}$(OAc)$_{16}$(MeOH)$_{4}$]$_{~}\cdot $~MeOH, in which the acetic acid solvent is replaced by a single methanol. The results are compared with those of several other Mn$_{12}$ single-molecule magnets (SMMs), including Mn$_{12}${\-}Ac$_{~}\cdot _{~}$2CH$_{3}$COOH. AC susceptibility studies indicate that Mn$_{12}${\-}Ac$_{~}\cdot _{~}$MeOH has a relatively large effective barrier, $U_{eff}$~$\sim $~74 K, in comparison to Mn$_{12}${\-}Ac$_{~}\cdot _{~}$2CH$_{3}$COOH. Meanwhile, EPR studies suggest more-or-less identical zero-field-splitting parameters for the two complexes. Based on these findings, we discuss the factors that can lead to reductions in $U_{eff}$ in various Mn$_{12}$ SMMs. [Preview Abstract] |
Monday, March 16, 2009 9:12AM - 9:24AM |
A31.00005: The role of quantum tunneling in magnetic avalanches in Mn$_{12}$-acetate Xiang Ma, Bo Wen, S. McHugh, M.P. Sarachik, Y. Myasoedov, H. Shtrikman, E. Zeldov, R. Bagai, G. Christou Steps occur in the hysteresis loop of the molecular magnet, Mn$_{12}$-ac due to quantum tunneling at ``resonant" magnetic fields where the energies of levels on opposite sides of the anisotropy barrier corresponding to different spin projections cross. The effect of quantum tunneling is also evident when magnetic relaxation occurs abruptly as a magnetic avalanche where spin reversal occurs along a narrow front that travels at subsonic speed. In particular, studies have shown that the ignition temperature displays minima and the velocity of the avalanche front shows maxima at the resonant fields. We report measurements of the avalanche speeds triggered in an external magnetic field applied at an angle with respect the c-axis of the crystal, where the transverse component provides a symmetry-breaking field that increases the tunneling rate and magnetic relaxation. [Preview Abstract] |
Monday, March 16, 2009 9:24AM - 9:36AM |
A31.00006: Tuning magnetization avalanches in Mn$_{12}$-acetate Bo Wen, S. McHugh, Xiang Ma, M. P. Sarachik, Y. Myasoedov, H. Shtrikman, E. Zeldov, R. Bagai, G. Christou We report the results of a systematic study of magnetic avalanches (abrupt magnetization reversals) in the molecular magnet Mn$_{12}$-acetate using a micron-sized Hall sensor array. Measurements were taken for: (a) fixed magnetic field (constant barrier against spin reversal); and (b) fixed energy release obtained by adjusting the barrier and $\Delta M$. A detailed comparison with the theory of magnetic deflagration of Garanin and Chudnovsky [1] will be presented and discussed. [1] D. A. Garanin and E. M. Chudnovsky, Phys. Rev. B 76, 054410 (2007) [Preview Abstract] |
Monday, March 16, 2009 9:36AM - 9:48AM |
A31.00007: Non-adiabatic spin transition in the presence of phonon bottleneck effect Lei Chen, Irinel Chiorescu We present a study on deviations of the magnetization cycle of a two-level spin system from a reversible function into an opened hysteresis cycle due phonon bottleneck effect combined with Landau-Zener transitions. In the case of large zero-field level repulsion the magnetization curves can be described by a simple phonon-bottleneck model, in agreement with recent experiments on molecular magnets (V15 and Ru2 [1]). In the case of small tunneling gaps, as for large spin systems (Mn12 or Fe8), the spin will tunnel with a probability given by the Landau-Zener mechanism. The phonon-bottleneck model is here generalized into a model able to blend the non-adiabatic dynamics of spins with the presence of a non-equilibrium phonon bath [2]. Bloch equations are written in the eigenbasis of the effective spin Hamiltonian, assumed to be a two-level system at low temperatures, with a relaxation term driven by the phonon-bottleneck mechanism. \\[0pt] [1] L. Chen et al, Applied Phys. Lett. 89, 252502 (2006) \\[0pt] [2] L. Chen, I. Chiorescu, cond-mat/0810.2502 [Preview Abstract] |
Monday, March 16, 2009 9:48AM - 10:00AM |
A31.00008: Geometric-Phase Effect in the Thermally Assisted Resonant Tunneling of Mn$_{12}$-tBuAc J.R. Friedman, E. H. da Silva Neto, C. Lampropoulos, G. Christou, N. Avraham, Y. Myaesoedov, H. Shtrikman, E. Zeldov Mn$_{12}$-tBuAc, like the better-known single-molecule magnet Mn$_{12}$-Ac, relaxes between up and down spin states by thermally assisted resonant tunneling when a longitudinal magnetic field (H$_{L})$ brings energy levels into resonance. In Mn$_{12}$-Ac, tunneling is induced by a second-order transverse anisotropy produced by local solvent disorder. Such disorder makes the observation of any possible geometric-phase interference effect impractical. Mn$_{12}$-tBuAc, in contrast, has negligible solvent disorder and an intrinsic fourth-order transverse anisotropy. We present experimental data on the transverse-field (H$_{T})$ dependence of the magnetic relaxation rate for Mn$_{12}$-tBuAc. When on resonance (H$_{L}$=0), the rate increases as a function of H$_{T }$ in a series of steps and plateaus due to abrupt changes in the dominant tunneling pair of levels. Surprisingly, a similar effect occurs when off resonance (i.e. large H$_{L})$. Detailed numerical simulations show that the experimental results, both on and off of resonance, can be well described if the fourth-order anisotropy is included in the spin Hamiltonian. The results can be understood as arising from a geometric-phase effect that occurs when H$_{T}$ is applied along the hard axis. Support: NSF grant {\#}DMR-0449516. [Preview Abstract] |
Monday, March 16, 2009 10:00AM - 10:12AM |
A31.00009: GGA+U study of exchange interactions in a Mn5 single-molecule magnet Emalee Popoff, Salvador Barraza-Lopez, Kyungwha Park, Hui-Lien Tsai Electronic structure of a single-molecule magnet (SMM) Mn5 is investigated using GGA+U formalism. There are two types of Mn ions in the SMM Mn5: Mn3+ (S=2) and Mn2+ (S=5/2). In a prototype single-molecule magnet Mn12, superexchange interactions between Mn ions through oxygen anions are known to be antiferromagnetic. Our calculation on Mn5, however, showed that the Mn ions are all ferromagnetically coupled to each through various ligands. This results in the ground state spin of S=11, which is in good agreement with experiment. We discuss the nature of the ferromagnetic coupling between the Mn ions by analyzing calculated projected density of states. We also present calculated exchange coupling constants considering various broken symmetry states. [Preview Abstract] |
Monday, March 16, 2009 10:12AM - 10:24AM |
A31.00010: ABSTRACT HAS BEEN MOVED TO SESSION S1 |
Monday, March 16, 2009 10:24AM - 10:36AM |
A31.00011: Magnetism and magnetic anisotropies of small organic molecules Jaime Ferrer, Diego Carrascal, Lucas Fernandez Seivane The ability to enhance and tailor the magnetism of small atomic cluster and molecules will determine whether nanospintronics can be used as a storage technology. We present here our ab initio studies on the magnetism of small organic molecules containing transition metal atoms. We focus specially on 5d atoms like gold, platinum and iridium. These have a large spin-orbit interaction, which generates large magnetic anisotropies in small atomic clusters[1]. [1] L. Fernandez Seivane and J. Ferrer, Phys. Rev. Lett. 99, 183401 (2007). [Preview Abstract] |
Monday, March 16, 2009 10:36AM - 10:48AM |
A31.00012: Anisotropic exchange in tetranuclear Co$^{II }$complexes Saiti Datta, Junjie Liu, Jon Lawrence, Christopher C. Beedle, David N. Hendrickson, Stephen Hill High-frequency electron paramagnetic resonance (HFEPR) studies of the tetranuclear Co$^{II}$ complex [Co(hmp)(dmb)Cl]$_{4}$ (\textbf{1) }reveal the presence of significant zero-field-splitting (ZFS) within the ground state spin multiplet. Meanwhile, low-temperature hysteresis measurements of \textbf{1} provide evidence for slow magnetization relaxation, suggesting that it could be a single-molecule magnet (SMM). However, HFEPR studies of a Zn analog of \textbf{1}, doped with a small quantity of Co$^{II}$, show the ground state of the Co$^{II}$ ions to be an effective spin \textit{S$\prime $}~=~1/2 Kramers doublet with a highly anisotropic $g$-tensor.$^{ }$To understand the origin of the ZFS within the ground state spin multiplet of \textbf{1}, as well as the slow magnetization relaxation, we consider the effect of anisotropic and antisymmetric exchange interactions between the ions within the tetranuclear complex. Our model provides an explanation for the ZFS in the ground state observed via HFEPR, and can also account for qualitative features observed through magnetic measurements. [Preview Abstract] |
Monday, March 16, 2009 10:48AM - 11:00AM |
A31.00013: Magnetization studies of a new single molecule magnet [Net4]3[Mn3Zn2(salox)3O(N3)6Br2]. John Henderson, Enrique del Barco, Changhyun Koo, Stephen Hill, Patrick Feng, David Hendrickson, Motohiro Nakano We present magnetization studies of a novel S=6 single molecule magnet (SMM) [Net4]3[Mn3Zn2(salox)3O(N3)6Br2] (Mn3). The results reveal extremely clean changes in the magnetization associated to the high crystalline quality of the compound. The sample allows a detailed study of the role of molecular symmetry on the nature of the magnetic quantum tunneling relaxation of these molecules. Experiments at temperatures down to 30 mK carried out in a 3D vector superconducting magnet will be presented. The possible role of phonons as originators of the tunneling relaxation of the magnetization in this SMM will be used to explain interesting experimental observations. [Preview Abstract] |
Session A32: Magnetization and Spin Dynamics
Sponsoring Units: GMAGChair: Ion Garate, University of Texas at Austin
Room: 336
Monday, March 16, 2009 8:00AM - 8:12AM |
A32.00001: Nanomagnetic Spin Fluids Skomski Ralph, A. Enders, R. D. Kirby, D. J. Sellmyer The dynamics of conventional magnets is governed by the static micromagnetic response to an external magnetic field, with corrections due to thermal excitations. For example, permanent magnets undergo aging (magnetic viscosity), and magnetic recording media lose some of the stored bits due to thermal excitation. Essential deviations from this Arrhenius (or N\'{e} el- Brown) behavior occur on a length scale below about 2 nm. The relaxation no longer obeys Kramer's escape-rate theory and must be replaced by path-integral considerations with nontrivial activation-entropy contributions. This presentation investigates several theoretical and experimental aspects of unusual magnetization dynamics in small-scale wires, thin films and dots. The first explicit example is the formation of liquid-like droplets, observed in ultrathin films with perpendicular magnetic anisotropy and characterized by 180$^{\circ}$ domains of well-defined chirality. (The one-dimensional equivalent of this phenomenon is a hard-core gas with particle-number conservation.) The second example is of theoretical nature and links the phenomenon of slow magnetization dynamics to the concept of fractional kinetics. A general feature of the considered nanomagnets is their resemblance to fluids (liquids or gases), as opposed to the glassy dynamics of conventional magnets. [Preview Abstract] |
Monday, March 16, 2009 8:12AM - 8:24AM |
A32.00002: Interface Magnetization Switching and Demagnetization in \textit{Fe/Al} films and \textit{Fe/Pt} nanoparticles Wei Lai, Tetiana Nosach, Yu Gong, Yuhang Ren, Chaehyun Kim, Savas Delikanli, Hao Zeng We report on the reversal and demagnetization processes of the Fe interface layer magnetization in thin films and nanoparticles of \textit{Fe/Al} and \textit{Fe/Pt} by time-resolved magnetization-induced second-harmonic generation. The results are compared with those of the bulk magnetization as obtained from magneto-optic Kerr effect. We realize that switching and demagnetization characteristics are distinctly different between bulk and interface layers because of the interface-derived anisotropy and the dipole interactions. In particular, the surface and interface magnetism will dominate the behaviors of nanoscale structures. [Preview Abstract] |
Monday, March 16, 2009 8:24AM - 8:36AM |
A32.00003: Magnetization hysteresis studies in Sm$_{1-x}$Gd$_x$Al$_2$ alloys U.V. Vaidya, S. Venkatesh, V.C. Rakhecha, S. Ramakrishnan, A.K. Grover SmAl$_2$ (T$_c$ $\sim$ 125 K, $\mu_{sat}$= 0.23 $\mu_B$/f.u.) is known to exhibit magnetic compensation when doped with Gd ($<$ 3 at.\%). In such stoichiometries though the magnetization gets closer to zero, there exists a large spin polarization. This makes such materials attractive candidates for applications. We have performed detailed magnetization hysteresis and other studies in the series Sm$_{1-x}$Gd$_x$Al$_2$. In x=0.02 alloy, the loops are shifted (notion of exchange bias) along negative H-axis for temperatures just above T$_{comp}$ , and along positive H-axis for temperatures T $<$ T$_{comp}$. We argue that the change in the sign of exchange bias is due to the magnetic contribution of conduction electron polarization as well as that of local magnetic moments reversing the signs. At T$_{comp}$ the width of the hysteresis loop collapses. In the given series, one can set up the system in either spin-surplus or orbital-surplus state and control the exchange bias field. The compositions with 0.03 $\leq$ x $<$ 0.06 do not exhibit zero cross over of magnetization and remain spin surplus. Our various studies and analysis shall be presented. [Preview Abstract] |
Monday, March 16, 2009 8:36AM - 8:48AM |
A32.00004: New spin modes in itinerant ferromagnets John Feldmann, Kevin Bedell We are theoretically investigating new collective spin phenomena that could exist in itinerant ferromagnetic materials such as MnSi. We postulate a ground state that is in agreement with experimental findings for these materials and derive a set of new spin collective modes. These modes would affect properties of these materials such as specific heat and spin transport and would be experimentally observable. [Preview Abstract] |
Monday, March 16, 2009 8:48AM - 9:00AM |
A32.00005: Scaling collapse of the irreversible magnetization of ferromagnetic thin films R. Das, A.F. Hebard The irreversible magnetization, \textit{$\Delta $M}, defined as the difference of field-cooled magnetization $M_{FC}$ and zero-field-cooled magnetization $M_{ZFC}$, has been measured for a variety of ferromagnetic thin films as a function of magnetic field $H$ at different temperatures $T$. Isotherms of \textit{$\Delta $M} show maxima\textit{ $\Delta $M}$_{max}$ at characteristic temperature-dependent fields $H_{m}(T)$. At very low and high magnetic fields the values of $M_{FC}$ and $M_{ZFC}$ converge and \textit{$\Delta $M} is observed to approach zero in these limits. If \textit{$\Delta $M/$\Delta $M}$_{max}$ is plotted as a function of $H/H_{m}$ for a given ferromagnetic system, the graphs for different temperatures collapse onto the same curve. This scaling collapse is clearly seen for three different ferromagnetic thin-film systems: polycrystalline gadolinium, phase separated manganites, and single domain Ni nanomagnetic grains embedded in an insulating host. Similar scaling behavior has also been observed in spin-glass material [1]. These results represent a heretofore unrecognized scaling behavior that appears to apply to a broad range of ferromagnetic systems. [1] V. S. Zotev, G. G. Kenning, and R. Orbach, Phys. Rev. B \textbf{66}, 014412 (2006) [Preview Abstract] |
Monday, March 16, 2009 9:00AM - 9:12AM |
A32.00006: Atomistic spin-dynamics simulations from first principles theory Olle Eriksson In this talk I will present recent developments in atomistic spin-dynamics simulations using first principles theory. Details of the implementation will be give and simulations of spin-glass materials (Cu-Mn) and diluted magnetic semiconductors (Mn doped GaAs) will be presented. [Preview Abstract] |
Monday, March 16, 2009 9:12AM - 9:24AM |
A32.00007: Dynamics of Einstein - de Haas Effect: Application to Magnetic Cantilever. Reem Jaafar, E.M. Chudnovsky, D.A. Garanin Local time dependent theory of Einstein - de Haas effect is developed. We show that internal elastic twists that accompany dynamics of spins enter equations of elasticity in the universal form that does not require precise knowledge of spin-lattice interactions. As long as the space-time dependence of the magnetization is known, local elastic deformations can be computed rigorously without any unknown parameters. The theory is applied to the description of the motion of a magnetic cantilever caused by the oscillation of the domain wall. Theoretical results are compared with a recent experiment on Einstein - de Haas effect in a microcantilever. [Preview Abstract] |
Monday, March 16, 2009 9:24AM - 9:36AM |
A32.00008: Colored thermal noise in spin valves Jiang Xiao, Gerrit Bauer, Sadamichi Maekawa, Arne Brataas We report a theoretical study of the thermal electrical noise in spin valves. There are two independent noise sources in spin valves: 1) thermal agitation of charge carriers causing Johnson-Nyquist noise, 2) thermal agitation of the magnetization that contributes to the electric noise by spin and charge pumping. The noise power spectrum from the latter consists of two absorption lines at zero frequency and at the ferromagnetic resonance on top of a white noise background. The relative intensities depend on the magnetization configuration. [Preview Abstract] |
Monday, March 16, 2009 9:36AM - 9:48AM |
A32.00009: Numerical evidence for unstable magnons at high fields in the square lattice Heisenberg antiferromagnet Olav F. Sylju{\aa}sen We have found numerical evidence for decaying magnons in the square lattice spin-1/2 Heisenberg antiferromagnet when it is exposed to a strong external magnetic field. The results are obtained using Quantum Monte Carlo simulations combined with a Bayesian inference technique to obtain dynamics, and are consistent with earlier predictions from spin wave theory. [Preview Abstract] |
Monday, March 16, 2009 9:48AM - 10:00AM |
A32.00010: Low-Frequency Magnetization Noise in Spin-Valve Structures Arif Ozbay, Aisha Gokce, Edmund Nowak, Thomas Flanagan, Ryan Stearrett, Cathy Nordman We report on 1/f resistance noise due to thermally driven fluctuations of the domain structure in GMR and MTJ sensors. Resistance noise from both the free layer (FL) and reference layer (RL) is evident. A near linear scaling of the normalized noise power with the sensor's sensitivity is observed. For a given sensitivity, the RL exhibits higher noise than the FL. This appears correlated to the larger imaginary (dissipative) component in the resistance susceptibility of the RL. In addition, we find that the imaginary component is larger for layers that exhibit pronounced magnetic hysteresis, suggestive of connection between the noise and hysteresis. A model based on equilibrium magnetization fluctuations is in good quantitative agreement with the measured noise power over most of the sensor's magnetoresistive response. A magnetic 1/f noise parameter is defined which can be used to compare magnetoresistive sensors having differing sizes, sensitivities, and under different biasing conditions. [Preview Abstract] |
Monday, March 16, 2009 10:00AM - 10:12AM |
A32.00011: Phase Diagram of Equilibrium Domain-Wall Solutions in Finite-Size ECC Media Sonali Mukherjee, Luc Berger Reversal in ECC media where hard and soft anisotropy magnetic material are exchange coupled has been studied because it has high thermal stability with low reversal field. Using Euler-Lagrange condition, we have studied the field evolution of domain-wall solutions in ECC for various anisotropy ratios of hard and soft phase and soft-phase length scales. We find that there exist 3 critical fields. At the field H1s, the domain-wall solution Es (surface domain-wall) and E1(soft-phase domain-wall) start existing. The nucleation field Hn,is the field where the energy of Es and the unreversed uniform solution E0 coincide. Above Hn, Es ceases to exist. The domain-wall propagation field Hdw is the field where the energy of soft domain wall E1 and hard domain wall E2 coincide. Above Hdw, E1 and E2 cease to exist. The reversal field is the field at which no domain-wall solutions exist anymore and is the maximum of Hn and Hdw fields. The field Hn is found to reduce with increasing soft-phase length ls, and Hdw is found to be independent of ls for ls greater than eh where eh is the domain- wall width of the hard phase. For hard/soft anisotropy ratio kh/ks less than 5, the nucleation field is always dominant. When kh/ks is greater than 5, there exits a soft-phase length lsc, at which the fields Hn and Hdw become equal. When ls is greater than lsc, Hdw dominates the reversal and, when ls is smaller than lsc, Hn is the reversal field. [Preview Abstract] |
Monday, March 16, 2009 10:12AM - 10:24AM |
A32.00012: Nature of magnetic ordering in Ni(OH)$_{2}$ nanoplates James Rall, Mohindar Seehra Nickel hydroxides are important for their potential applications in rechargeable batteries and as precursors for NiO and Ni catalysts. $\beta $--Ni(OH)$_{2}$ has the CdI$_{2}$ layered structure with Ni atoms forming a hexagonal unit cell. Here, we report on the magnetic ordering in 17 nm $\times $ 4 nm nanosheets of $\beta $--Ni(OH)$_{2}$ . Measurements of the magnetization M as a function of temperature (2K to 300K) and magnetic field H up to $\pm $65kOe are reported. M vs. T data in H =100 Oe for the ZFC case shows a peak in M at T$_{N}$ = 24 K characteristic of antiferromagnetic (AF) ordering; however for T $>$ T$_{N}$, the Curie-Weiss ($\chi $ = C/(T - $\theta ))$ fit yields $\theta $ = 26K characteristic of ferromagnetism. Following Takada (J. Phys. Soc. Jpn. 21, 2745, 1966), we measured M vs. H loops from T = 2K to 25K and observed a metamagnetic transition at H$_{c}$ = 56 kOe at 2K, with H$_{c}$ decreasing with increasing T. These results suggests strong ferromagnetic coupling among Ni within (001) sheets and a weaker antiferromagnetic coupling in the neighboring (001) sheets, and [001] as the easy axis. This model is used to determine the exchange constants consistent with the observed Curie-Weiss variation. [Preview Abstract] |
Monday, March 16, 2009 10:24AM - 10:36AM |
A32.00013: First-principles calculations of laser-induced spin manipulation in small magnetic clusters with CO Chun Li, Georgios Lefkidis, Wolfgang H{\"u}bner We present a fully {\it ab initio} controlled ultrafast magnetooptical switch and transfer mechanism in small magnetic clusters exploiting spin-orbit-coupling enabled $\Lambda$-processes [1-3]. Two-magnetic-center clusters with CO attached to one of the magnetic atoms are studied to achieve a mapping of the laser-induced spin manipulation to the IR spectrum of CO. The predicted spin-state-dependent CO frequencies can facilitate experimental monitoring of the processes. The lower electronic states of the clusters exhibit a very high degree of spin localization either at the Co or the Ni site. Spin flip on one magnetic atom and transfer from one magnetic center to the other are realized in structurally optimized magnetic clusters with fidelities that reach 99.8\%. \newline\newline[1] R. G\'{o}mez-Abal, O. Ney, K. Satitkovitchai and W. H\"{u}bner, Phys. Rev. Lett. 92, 227402 (2004)\newline[2] G. Lefkidis and W. H\"{u}bner, Phys. Rev. B 76, 014418 (2007)\newline[3] T. Hartenstein, C. Li, G. Lefkidis and W. H\"{u}bner, J. Phys. D: Appl. Phys. 41, 164006 (2008). [Preview Abstract] |
Monday, March 16, 2009 10:36AM - 10:48AM |
A32.00014: Magnetic properties of a doped quasi-triangular lattice material, Cu$_{2(1-x)}$Zn$_{2x}$(OH)$_{3}$NO$_{3}$/(C$_{7}$H$_{15}$COO) Jian Wu, Anup K. Gangopadhyay, S.A. Solin Cu$_{2}$(OH)$_{3}$NO$_{3}$,is a geometrically frustrated layered compound in which spin S=1/2 Cu$^{2+}$ ions are arranged on a slightly distorted triangular lattice. The magnetic properties of the pure compound and of the compound intercalated with alkanecarboxylate have been extensively studied.[1] However, the effects of intralayer doping remain unexplored. The substitution of non-magnetic ions such as Zn$^{2+}$ for Cu$^{2+}$ will ultimately drive the ordering temperature toward zero [2] which may provide a candidate system possessing an exotic spin-liquid ground state. We have prepared powder samples of the Cu$_{2(1-x)}$Zn$_{2x}$(OH)$_{3}$NO$_{3}$ family and systematically investigated them by magnetic susceptibility measurements. The ordering temperature decreases from 11K to 5.6K while the C-W temperature increases from -5.1K to +2.8K as the Zn concentration increases from 0 to 65\%. To enhance the 2-dimensional characteristic and reduce the interlayer interaction, we introduce an alkanecarboxylate C$_{7}$H$_{15}$COO into the interlayer space. The experimental results we have obtained indicate that this new class of materials have much higher frustration levels $|\Theta_{cw}$ /T$_{c}|\sim$ 20 and order at a lower temperature than the doped parent compounds.\newline[1] M. A. Girtu et al, Phys Rev B 61,4117(2000).\newline[2] M. Mekata et al, J. Phys. Soc. Japan 56, 4544(1987). [Preview Abstract] |
Monday, March 16, 2009 10:48AM - 11:00AM |
A32.00015: Coupling between optically-induced coherent spin and lattice dynamics in epitaxial Fe films Vladimir Stoica, Don Walko, Eric Landahl, Yuelin Li, Roy Clarke Spin dynamics excitation using femtosecond optical pulses in ferromagnetic thin films is a powerful technique to study spin dependent interactions in solids. One topic of interest is the temporal separation of relaxation processes related to fundamental interaction mechanisms, which include spin-orbit, spin-lattice and exchange coupling. Establishing experimentally the relaxation timescales for these couplings is an important step that assists the development of new spintronic applications. We employ time-resolved magneto-optical and X-ray diffraction probes to separate spin and lattice dynamics in epitaxial Fe samples grown by molecular beam epitaxy on Ge and MgO substrates. We study the connection between the magnetic and lattice relaxation transients excited by optical pulses. We find that coherent spin precession dynamics correlates well with thermo-elastic strain relaxation from picosecond to nanosecond time scales. [Preview Abstract] |
Session A33: Superconductivity: ARPES
Sponsoring Units: DCMPChair: Barrett Wells, University of Connecticut
Room: 403
Monday, March 16, 2009 8:00AM - 8:12AM |
A33.00001: Temperature dependent ARPES study of the pseudogap of Pb-Bi2201 Makoto Hashimoto, Ruihua He, Kiyohisa Tanaka, Jean-pierre Testaud, Worawat Meevasana, Rob Moore, Donghui Lu, Yoshiyuki Yoshida, Hiroshi Eisaki, Zahid Hussain, Zhi-Xun Shen The pseudogap phenomena in the high-Tc cuprates have been extensively studied because of possible intimate connection with the unknown mechanism of superconductivity. We have studied the ARPES spectra of optimally-doped Pb-Bi2201 (Tc = 34 K) at SSRL BL5-4, from the superconducting state (10 K) to the normal state above the pseudogap temperature (160 K). We have revealed how the band structure changes with the pseudogap opening, and found that the superconducting gap alone cannot explain the ARPES spectra in the antinodal region. Based on the results, in the presentation, we would like to discuss possible origins of the pseudogap. This work is supported by the DOE Office of Basic Energy Science, Division of Materials Science and Engineering. ARPES experiments were performed at the Stanford Synchrotron Radiation Laboratory (SSRL), which is operated by the Department of Energy Office of Basic Energy Science. [Preview Abstract] |
Monday, March 16, 2009 8:12AM - 8:24AM |
A33.00002: Pseudo-gap in electron doped cuprates: Spin fluctuation origin and close relation with superconducting gap Seung Ryong Park, D.J. Song, C.S. Leem, Chul Kim, S.K. Choi, Y.K. Kim, C. Kim, K.J. Choi, J.H. Kim, K.M. Song, Jung Hoon Han, Y. Yoshida, H. Eisaki A natural candidate for the cause of PG in electron doped cuprates could be spin fluctuation. However, there has not been any careful calculation based on the spin fluctuation model, at least to our best knowledge. Recently, entire dynamic spin susceptibility of electron doped cuprate was obtained by using inelastic neutron scattering. Therefore, one could use electron-spin coupling which is proportional to the dynamic spin susceptibility and calculate the spectral function based on the electron-spin fluctuation coupling model. In this presentation, we first show calculated ARPES spectral function based on electron-spin fluctuation coupling model with the magnetic susceptibility as the input. We could identify the origin of PG as electron-spin fluctuation from this simulation, and we also could extract a rough value for the electron-spin fluctuation coupling strength g in electron doped cuprates by comparison between experimental data and simulation results. [Preview Abstract] |
Monday, March 16, 2009 8:24AM - 8:36AM |
A33.00003: Temperature dependent ARPES study of the superconducting gap in overdoped Bi2212 H.-B. Yang, J.D. Rameau, G.D. Gu, P.D. Johnson High-resolution angle-resolved photoemission (ARPES) is used to probe the temperature dependence of the superconducting gap around the Fermi surface in overdoped Bi2212. Lucy-Richardson deconvolution is applied to reduce the error from experimental resolution. Normalizing by the Fermi function then allows the observation of the true gap in the spectral function. Numerical simulation is also used to compare the experimental results with theoretical models. We have investigated the temperature dependence of the gap around the nodal region and the anti-nodal region, with temperature going from the superconducting state into the normal state. [Preview Abstract] |
Monday, March 16, 2009 8:36AM - 8:48AM |
A33.00004: ABSTRACT WITHDRAWN |
Monday, March 16, 2009 8:48AM - 9:00AM |
A33.00005: ARPES study of YBa$_2$Cu$_3$O$_{7-\delta}$ from the over to the underdoped regime by in situ K evaporation G. Levy, David Fournier, M.A. Hossain, J.D.F. Mottershead, J.L. McCheyney, A. Bostwick, E. Rotenberg, W.N. Hardy, R. Liang, G.A. Sawatzky, I.S. Elfimov, D.A. Bonn, A. Damascelli Unravelling the nature of the electronic excitations in the underdoped regime of the Cuprates is a key element for understanding the fundamental mechanism behind HTSC. The YBCO phase diagram has been studied with photoelectron spectroscopy (ARPES) using a new in situ electron doping approach based on controlled potassium deposition [1] onto as-cleaved samples. All of the compounds studied ($\delta$=0,0.5,0.66) exhibit heavily over-doped nature with well defined LDA-like Fermi surfaces and evolve toward disconnected Fermi arcs in the underdoped regime. These results are consistent with previous ARPES measurement on the HTSCs and in contrast with the quantum oscillations observations [2]. \\[4pt] [1] M. A. Hossain et al., \textit{Nat. Phys.}, 4:527, 2008\\[0pt] [2] N. Doiron et al., \textit{Nature}, 447:565, 2007 [Preview Abstract] |
Monday, March 16, 2009 9:00AM - 9:12AM |
A33.00006: Quasiparticles in Bi-2212 I.M. Vishik, W.-S. Lee, K. Tanaka, B. Moritz, E.A. Nowadnick, T. Sasagawa, T. Fujii, T.P. Devereaux, Z.-X. Shen From ARPES measurements, much has been learned about the single-particle excitations of the high-Tc cuprate superconductors, and collective properties can be inferred from these experiments too. ~The gap in the spectrum below Tc is related to the superconducting gap, and the superfluid density, the other hallmark of superconductivity, has been demonstrated to correlate closely with the weight of the antinodal quasiparticle peak. ~The momentum, temperature, and doping dependence of quasiparticle lifetime yields information about scattering processes, which are related to ground state properties. ~In Bi-2212 quasiparticles are present on the entire Fermi surface over a wide doping range. ~We present ARPES studies of the quasiparticles in Bi-2212 as a function of doping, momentum, and temperature, and discuss connections to other experiments. [Preview Abstract] |
Monday, March 16, 2009 9:12AM - 9:24AM |
A33.00007: ARPES study on Tl-based Cuprates Wei-Sheng Lee, Kiyohisa Tanaka, Inna Vishik, Donghui Lu, Rob Moore, Hiroshi Eisaki, Akira Iyo, Thomas Devereaux, Zhu-Xun Shen Here we report the angle-resolved photoemission measurements on nearly optimally multi-layer Tl-based superconducting copper oxides, including Tl$_2$Ba$_2$CaCu$_2$O$_8$ (Tl-2212), TlBa$_2 $Ca$_2$Cu$_3$O$_9$ (Tl-1223), and a comparison to the data of single layer Tl$_2$Ba$_2$CuO$_6$ (Tl-2201). Consistent with other optimally-doped cuprates, a hole-like Fermi surface and sharp quasi-particle peak in the superconducting state is observed. The renormalization effect due to the coupling of bosonic modes is also observed, which exhibits intriguing materials dependence. Implications of the observed material dependent renormalization effect will also be discussed. [Preview Abstract] |
Monday, March 16, 2009 9:24AM - 9:36AM |
A33.00008: Anomalous behavior of the nodal scattering rate of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ near the Fermi energy Theodore Reber, Nick Plumb, John Douglas, Zhe Sun, Qiang Wang, Yoshihiro Aiura, Hiroshi Eisaki, Hideki Iwasawa, Michael Hermele, Daniel Dessau The scattering rate as determined by the width of a band is a direct measure of the imaginary part of a particle's self-energy. Though the dispersion of a band can also used to extract the particle's self energy, the scattering rate is superior, because the ambiguity due to determining the underlying bare band is not included. The excellent momentum and energy resolution of low photon energy ARPES allows us to study the scattering rate of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+ \delta }$ near the Fermi energy. Our studies show an anomalous feature that warrants continued study. [Preview Abstract] |
Monday, March 16, 2009 9:36AM - 9:48AM |
A33.00009: Novel feature in the nodal electron self-energy and strong temperature dependence of the Fermi velocity in the high temperature superconductor Bi2212 N.C. Plumb, T.J. Reber, J.D. Koralek, Z. Sun, J.F. Douglas, Y. Aiura, K. Oka, H. Eisaki, D.S. Dessau Using low-photon energy angle-resolved photoemission (ARPES), we study the low-energy dispersion along the superconducting node in Bi2212 as a function of temperature. Less than 10 meV below the Fermi energy, the high-resolution data reveals a novel ``kink''-like feature in the real part of the electron self-energy. The kink is strongest below the superconducting critical temperature and appears to vanish as the temperature is raised. A corollary of this finding is that the Fermi velocity, as measured over this small energy range, varies rapidly with temperature -- increasing by approximately 35{\%} from 50 to 200 K. This is in contrast to the slope of the dispersion at only slightly deeper energy, which changes little by comparison and whose behavior is ostensibly dominated by the well-known 70-meV kink. We discuss some possible physical origins of the new low-energy feature, including the possibility that it may arise from bosonic mode couplings and/or nonanalytic corrections to Fermi liquid theory in 2D. [Preview Abstract] |
Monday, March 16, 2009 9:48AM - 10:00AM |
A33.00010: ARPES investigation of two leg ladder compounds Sr$_{14-x}$Ca$_x$Cu$_{24}$O$_{41}$ Gey-Hong Gweon, Takao Sasagawa, Takami Tohyama, Matthew Brunner, James Hinton, Jacob Stanley The so-called ``two leg ladder compounds'' Sr$_{14-x}$Ca$_x$Cu$_{24}$O$_{41}$ are interesting since it is well accepted that they form a resonating valence bond state. The crystal structure of two leg ladder compounds consists of one-dimensional motifs (ladders and chains), and it is within the ladders that the resonating valence bond state arises. As $x$ is varied, these compounds go through a metal-insulator transition, and become a superconductor, albeit under pressure. So far, a high resolution ARPES study shedding light on the near-Fermi-level electronic structure of these interesting compounds has been missing. Here, we report our ARPES results, providing the first view of the near-Fermi-level quasi-one-dimensional electronic structure arising from the ladders. We discuss line shape features that are indicative of strong electron correlations. [Preview Abstract] |
Monday, March 16, 2009 10:00AM - 10:12AM |
A33.00011: ARPES matrix element and the waterfall effect in the cuprates. Susmita Basak, Tanmoy Das, Jouko Nieminen, Matti Lindroos, Hsin Lin, Robert Markiewicz, Arun Bansil The high-energy kink (HEK) or the 'waterfall' effect as seen in angle-resolved photoemission spectra (ARPES) in the cuprates has the potential of revealing important information about the dressing of quasiparticles by electronic excitations [1,2,3]. However, recently it has been suggested that matrix element effects radically modify the experimental spectra in $\rm{Bi_2Sr_2CaCu_2O_8}$ (Bi2212), and it has been questioned whether the HEK exists [4]. Here we discuss how the interplay between the matrix element and self-energy effects shapes the ARPES spectra. Both the ARPES matrix element and the self-energy are found to be necessary for understanding the experimental spectra. Work supported in part by the USDOE. [1] R. S. Markiewicz {\it et al.}, Phys. Rev. B {\bf{76}}, 174514 (2007). [2] A. Macridin {\it et al.},Phys. Rev. Lett. {\bf 99}, 237001 (2007). [3] Tanmoy Das {\it et al.}, cond-mat:0807.4257. [4] D.S. Inosov {\it et al.}, Phys. Rev. Lett. {\bf 99}, 237002 (2007). [Preview Abstract] |
Monday, March 16, 2009 10:12AM - 10:24AM |
A33.00012: ABSTRACT WITHDRAWN |
Monday, March 16, 2009 10:24AM - 10:36AM |
A33.00013: LASER ARPES studies on Sr$_{2}$RuO$_{4}$ Chul Kim, Seung-Ryong Park, C. S. Leem, D. J. Song, Y. K. Kim, S. K. Choi, W. S. Jung, Y. Y. Ko, C. Kim LASER ARPES was performed on Sr$_{2}$RuO$_{4}$. With advance of LASER ARPES, we could perform bulk sensitive ARPES with 1meV energy resolution. With these advantages, intensive studies on Sr$_{2}$RuO$_{4}$ enlightened us more knowledge on layered structures. [Preview Abstract] |
Monday, March 16, 2009 10:36AM - 10:48AM |
A33.00014: Fermiological interpretation for collective spin and charge orderings in underdoped La-based cuprates by ARPES Ruihua He, Kiyohisa Tanaka, Sung-Kwan Mo, Hong Yao, Makoto Hashimoto, Erez Berg, Takao Sasagawa, Tadashi Adachi, Masaki Fujita, Teppei Yoshida, Norman Mannella, Worawat Meevasana, Yoji Koike, Kazuyoshi Yamada, Atsushi Fujimori, Steve Kivelson, Zahid Hussain, Zhi-Xun Shen By using ARPES interpretation for the origin of the collective spin and charge orderings in underdoped La-based cuprate superconductors is carefully examined. A Fermi surface nesting wave vector is identified which closely matches the collective ordering wave vector observed by neutron scattering for doping around 1/8. The pseudogap state is found to be essential for the development of such nesting instability of the electronic system toward density wave formation. A systematic doping dependence study will be presented that allows further insights into this issue. [Preview Abstract] |
Monday, March 16, 2009 10:48AM - 11:00AM |
A33.00015: Mott gap collapse in the cuprates -- apparent or real? R. S. Markiewicz, Tanmoy Das, A. Bansil We have recently introduced a model self-energy for the cuprates, which includes an antiferromagnetic (AFM) transition dressed by spin and charge fluctuations.[1] This model correctly reproduces many `strong coupling' features in the angle-resolved photoemission (ARPES) and optical spectra, including the waterfall effect and the doping dependence of the optical Mott gap. Here we discuss a dichotomy between Slater and Mott physics in the cuprates, with a Slater-like AFM gap collapse with doping in the coherent bands while the Mott-like gap persists in the incoherent bands. By analyzing the spectral weights, we show that there is an anomalous spectral weight transfer which is rather large to be consistent with strong coupling physics, but which is reasonably described by our intermediate coupling model. Work supported in part by the USDOE. [1] Tanmoy Das, R. S. Markiewicz, and A. Bansil, arXiv:0807.4257. [Preview Abstract] |
Session A34: Superconductivity: Spin Properties
Sponsoring Units: DCMPChair: John Tranquada, Brookhaven National Laboratory
Room: 404
Monday, March 16, 2009 8:00AM - 8:12AM |
A34.00001: Evidence for Local Moment Magnetism in Superconducting Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ John Tranquada, G.Y. Xu, G.D. Gu, M. Huecker, B. Fauque, T.G. Perring, C. Stock, L.-P. Regnault We have used inelastic neutron scattering to measure the dynamic spin susceptibility in optimally-doped Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ (T$_{c}$ = 91 K). Four crystals with a total mass of 19 g were measured on the MAPS spectrometer at ISIS for temperatures of 10 K and 100 K. We have been able to identify the magnetic excitations in the energy range 20-90 meV. The magnetic nature of the scattering has been confirmed with spin-polarization analysis on IN22 at the ILL. While we see temperature-dependent changes for energies around 40 meV that are consistent with earlier studies, we find that the Q-integrated signal shows a much weaker variation with temperature. The absolute magnetic cross section is quite comparable to that of spin fluctuations in stripe ordered La$_{1.875}$Ba$_{0.125}$CuO$_{4}$. As the magnetism in the latter system has been shown to have a dominant contribution from local moments [1], we argue that the same must be true for Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$. [1] M. Huecker et al., Phys. Rev. B (accepted); cond-mat/0503417v3. [Preview Abstract] |
Monday, March 16, 2009 8:12AM - 8:24AM |
A34.00002: Phase Separation and Magnetism in High Temperature Superconductors Samuel Emery, Barret Wells, Joseph Budnick, William Hines, Linda Udby, Niels Hessel Anderson, Kim Lefmann, Christof Niedermayer, Fangcheng Chou Previous work by our group has determined that the low temperature phase diagram of super-oxygenated, La2CuO4 consists of only a few line phases that are either superconducting (SC) or magnetic. Samples with doping levels between the stable phases will segregate into separate domains; this raises the question as to the nature of the interaction between SC and magnetic domains. The application of a magnetic field has been shown to enhance the magnetic scattering associated with a spin density wave order parameter. In our phase separated samples, the magnitude of this enhancement varies strongly with no apparent dependence on hole concentration or phase fractions. Disorder in cuprate superconductors also seems to favor magnetic order over superconducting order, and may be responsible for the variations we find. We present muon and neutron results of the enhancement of magnetic order by magnetic field and oxygen disordering in superoxygenated La2-xCuO4+y. [Preview Abstract] |
Monday, March 16, 2009 8:24AM - 8:36AM |
A34.00003: Stripe order in $\bf La_{2-x}Ba_{x}CuO_4$ at ambient and high pressure. M. Huecker, J. S. Wen, Z. J. Xu, G. D. Gu, J. M. Tranquada, M. v. Zimmermann The pronounced stability of the charge and spin stripe order in $\rm La_{2-x}Ba_{x}CuO_4$ at $\rm x=1/8$ doping still is a poorly understood peculiarity. A combination of electronic and structural interactions is likely, however it has been difficult to clearly separate the involved mechanisms. One approach is to explore how stripe order fades away for dopings $\rm x \neq 1/8$. We have performed high energy (100 keV) x-ray diffraction and static magnetization experiments on single crystals between x=0.095 and 0.155. To our surprise, at ambient pressure stripes exist in a much broader range of doping around $\rm x=1/8$ than expected. In the underdoped region charge stripe order always coincides with a structural transition associated with a rotation of the octahedral tilt axis. However, for $\rm x=1/8$ and high pressure we have been able to show that stripe order also occurs in the absence of this structural phase, which motivates us to discuss stripes in terms of an electronic liquid crystal phase. [Preview Abstract] |
Monday, March 16, 2009 8:36AM - 8:48AM |
A34.00004: The local magnetic field response of $\mathrm{La_{2-x}Sr_{x}CuO_{4}}$ on the overdoped side of the phase diagram as revealed by $\mathrm{\mu SR}$ G.J. MacDougall, A.A. Aczel, S.-J. Kim, J.A. Rodriguez, C.R. Wiebe, G.M. Luke, T. Ito, P.L. Russo, A.T. Savici, Y.J. Uemura, H. Kim, S. Wakimoto, R.J. Birgeneau There is now a large body of evidence from local probes which point to intrinsic heterogeneity in the cuprates. One example is the recent reports of an inhomogenous magnetic field response in the underdoped materials, as revealed by muon spin rotation experiments. In an effort to explore this field response, we have measured the $\mathrm{/mu SR}$ spectra of $\mathrm{La_{2-x}Sr_{x}CuO_{4}}$ in a number of applied magnetic fields and with $x$ ranging from the underdoped to heavily overdoped materials. I will summarize these efforts, and present the resultant data in the context of existing literature. Possible interpretations and directions for future research will be discussed. [Preview Abstract] |
Monday, March 16, 2009 8:48AM - 9:00AM |
A34.00005: The emergence of coherent magnetic excitations in the pseudogap state of underdoped superconducting La$_{2-x}$Sr$_{x}$CuO$_{4}$ S.M. Hayden, O.J. Lipscombe, B. Vignolle, T.G. Perring, C.D. Frost We use inelastic neutron scattering to measure the magnetic
excitations in the underdoped superconductor
La$_{2-x}$Sr$_{x}$CuO$_{4}$ (x=0.085, $T_c$=22~K) over wider
energy ($5 |
Monday, March 16, 2009 9:00AM - 9:12AM |
A34.00006: Spin Excitations in La$_{1.75}$Sr$_{0.25}$Zn$_{0.01}$Cu$_{0.99}$O$_{4}$ Stephen Wilson, B. K. Freelon, Z. Yamani, W. Buyers, C. Rotundu, S. Wakimoto, Robert Birgeneau Recently, reports of a dramatic enhancement in the dynamic, incommensurate, spin density wave response in overdoped La$_{2-x}$Sr$_{x}$Cu$_{1-y}$Zn$_{y}$O$_{4}$ [PRB \textbf{72}, 064521 (2005)] have provided considerable insight toward the microscopic understanding of Zn-induced magnetic states and their interaction with the superconducting phase. As a continuation of these studies, in this talk we will present our recent inelastic neutron scattering studies of spin excitations within the Zn-doped cuprate, La$_{1.75}$Sr$_{0.25}$Zn$_{0.01}$Cu$_{0.99}$O$_{4 }$(LSCZO x=0.25, y=0.01). The dispersion of the spin density wave excitations throughout the Brillouin zone will be presented along with the local spin susceptibility from $\Delta $E=0-45meV. Particular focus will be given to the modifications in the magnetic behavior induced via Zn-substitution in this overdoped LSCZO system. [Preview Abstract] |
Monday, March 16, 2009 9:12AM - 9:24AM |
A34.00007: Magnetic signatures of the pseudogap phase and doping dependence of the antiferromagnetic resonance in HgBa$_{2}$CuO$_{4+\delta }$ Yuan Li, Victor Baledent, Neven Barisic, Philippe Bourges, Yongchan Cho, Benoit Fauque, Klaudia Hradil, Richard Mole, Yvan Sidis, Guichuan Yu, Xudong Zhao, Martin Greven We present our latest experimental results for the pseudogap phase of the model high-Tc superconductor HgBa$_{2}$CuO$_{4+\delta }$. Refinement of our polarized neutron diffraction experiments [Y. Li \textit{et al.}, Nature \textbf{455}, 372 (2008)] using sizable single-crystals of this structurally simple compound consistently show a novel magnetic order below the pseudogap temperature. Furthermore, DC magnetic susceptibility measurements on the highest-quality crystals exhibit a response with highly anisotropic temperature- and doping-dependence, in which the pseudogap anomaly is only visible when the magnetic field is applied along the $c$-axis. These findings strongly suggest that the new magnetic order competes with the superconductivity. Building on our initial work for optimally-doped HgBa$_{2}$CuO$_{4+\delta }$ [G. Yu \textit{et al., }arXiv: 0810:5759], we also briefly discuss preliminary results for the doping dependence of the antiferromagnetic resonance. [Preview Abstract] |
Monday, March 16, 2009 9:24AM - 9:36AM |
A34.00008: Spin correlations and magnetic excitation spectrum of electron-doped Nd$_{2-x}$Ce$_{x}$CuO$_{4\pm \delta }$ near the magnetic quantum critical point Eugene Motoyama, Guichuan Yu, Yuan Li, Daniel Petitgrand, Klaudia Hradil, Richard Mole, Patrick Mang, Inna Vishik, Owen Vajk, Martin Greven One of the most intriguing issues in the field of high-T$_{c}$ superconductivity is the electron-hole asymmetry: the hole- or electron-doping of the parent Mott insulators leads to superconductors with differing properties. For the comparatively less-studied electron-doped materials, the antiferromagnetic phase extends much further with doping and appears to overlap with the superconducting phase. Our previous inelastic neutron scattering measurements show that in the compound Nd$_{2-x}$Ce$_{x}$CuO$_{4\pm \delta }$, genuine long-range antiferromagnetism does not extend as far as previously thought, and may not coexist with bulk superconductivity; the system features a magnetic quantum critical point at x$\sim $0.13, very close to the composition above which superconductivity is first observed [Motoyama \textit{et al.}, Nature \textbf{445}, 186 (2007)]. Here we present new measurements for the instantaneous spin correlations and the magnetic excitation spectrum that aim to refine our understanding of the physics in this interesting doping regime. [Preview Abstract] |
Monday, March 16, 2009 9:36AM - 9:48AM |
A34.00009: Studies of broken time reversal symmetry states in high temperature superconductors using high resolution Sagnac interferometry Elizabeth Schemm, Hovnatan Karapetyan, Jing Xia, Martin M. Fejer, Aharon Kapitulnik Using a cryogenic fiber Sagnac interferometer, we measure polar Kerr effect to high precision in several high-$T_c$ superconductors, concentrating on $\mathrm{YBa_2Cu_3O}_{6+x}$. Previous work on $\mathrm{YBa_2Cu_3O}_{6+x}$ showed non-zero Kerr rotations on the order of $\sim$ 1 $\mu$rad, appearing near the pseudogap temperature $T^*$ and marking what appears to be a true phase transition. We continue this study on single crystals and oriented films to further understand the dependence of the observed Kerr signal on crystal direction, as well as to further probe the anomalous response of this effect to magnetic field training. [Preview Abstract] |
Monday, March 16, 2009 9:48AM - 10:00AM |
A34.00010: Giant impact of magnetic impurity on stability of stripe order in high-Tc cuprate Masaki Fujita, Masanori Enoki, Satoshi Iikubo, Kazuyoshi Yamada To study the stability of stripe orders in cuprate oxides thought impurity-effect, we performed neutron-scattering experiments on La$_{1.88-y}$Sr$_{0.12+y}$Cu$_{1- y}$M$_{y}$O$_4$ ($y$=0 for M=Cu, Zn and $y$=0.01 for M=Ga, Fe). Well-defined incommensurate peaks from charge-density-wave order was induced in the low- temperature-orthorhombic phase (LTO) by impurity substitution, and the scattering intensity is much stronger in the Fe-doped system than in the Zn-doped one. Integrated intensity of magnetic peak from spin-density-wave order was also enhanced by Fe-doping, while it does not change so much by Zn -and Ga- substitutions. These results indicate that the static spin and charge stripe orders can be realized in the LTO phase, and the stability is effectively induced by doping the magnetic impurity. [Preview Abstract] |
Monday, March 16, 2009 10:00AM - 10:12AM |
A34.00011: Anomalous normal state magneto-resistance in the noncentrosymmetric superconductor Li$_{2}$Pt$_{3}$B B. J. Taylor, C. McElroy, T. A. Sayles, A. C. Mota, M. Brian Maple The isotypic superconducting compounds Li$_{2}$Pd$_{3}$B and Li$_{2}$Pt$_{3}$B crystallize in a structure lacking physical inversion symmetry. In contrast to the inversion symmetry breaking superconductors CePt$_{3}$Si, CeRhSi$_{3}$, and UIr, to date, no evidence of magnetic order has been reported in either Li$_{2}$Pd$_{3}$B or Li$_{2}$Pt$_{3}$B. Through detailed magnetoresistive and magnetization measurements of Li$_{2}$Pt$_{3}$B, we have observed behavior suggestive of a link between normal state electronic transport and magnetic behavior, and properties of the superconducting ground state. Corresponding changes in magnetoresistive and magnetic behavior are found wherein both properties exhibit two distinct features at high and intermediate temperatures. Remarkably, both features evolve as a function of magnetic field and temperature towards the $T$ = 0 value of the superconducting upper critical field H$_{c2}$(0)~$\approx $~1.5 T. Polycrystalline samples of Li$_{2}$Pt$_{3}$B with the highest reported values to date of the superconducting critical temperature, T$_{c}$ = 3K , and residual resistivity ratio, RRR $\approx $ 2, were used in this study. [Preview Abstract] |
Monday, March 16, 2009 10:12AM - 10:24AM |
A34.00012: Glassy effects in magnetotransport in La$_{1.97}$Sr$_{0.03}$CuO$_4$ thin films X. Shi, J. Jaroszy\'nski, D. Popovi\'c, C. Panagopoulos, G. Logvenov, A. Bollinger, I. Bozovic We have studied the in-plane magnetoresistance (MR) in atomically smooth, MBE grown La$_{1.97}$Sr$_{0.03}$CuO$_4$ thin films. The MR was measured at temperatures between 0.6~K and 8~K and in $B$ up to 9~T, both parallel and perpendicular to the $c$-axis. The MR exhibits strong dependence on magnetic field history, such as hysteresis and memory at low $T$, similar to the results on $c$-axis transport in single crystal samples [1]. Here, however, the difference between field-cooled and zero-field cooled MR vanishes above $T \sim$ 5~K, independent of the magnitude and orientation of $B$. Low $T$ resistance noise measurements also will be discussed. The results suggest that the glassiness observed in the films may also originate from the slow charge dynamics at low temperatures. [1] I. Rai\v{c}evi\'c, \emph{et. al.}, PRL \textbf{101}, 177004 (2008). [Preview Abstract] |
Monday, March 16, 2009 10:24AM - 10:36AM |
A34.00013: Novel positive magnetoresistance in lightly doped La$_{2}$CuO$_4$ I. Rai\v{c}evi\'c, D. Popovi\'c, C. Panagopoulos, T. Sasagawa We have measured magnetoresistance (MR) in single crystals of La$_{1.97}$Sr$_{0.03}$CuO$_4$ and La$_{2}$Cu$_{0.97}$Li$_{0.03}$O$_4$ at temperatures 0.050$\leq T$(K)$\leq 70$ and fields $0\leq B$(T)$\leq 18$ parallel and perpendicular to the c-axis. Our study reveals an unusual, low-field positive MR in both in-plane and c-axis transport in both materials and for both \emph{B} orientations. However, while the positive c-axis MR may persist up to \emph{T} as high as 40 K, the large positive in-plane MR occurs only at very low $T\ll T_{sg}$ ($T_{sg}$ -- spin glass transition temperature). In that regime, where the noise spectroscopy disclosed slow and correlated charge dynamics [1], both the in-plane and c-axis positive MR exhibit signatures of glassiness, such as memory and hysteresis. We discuss the possible mechanism responsible for this novel low-field positive MR that appears to be intimately related to the emergence of the charge glass dynamics. [1] I. Rai\v{c}evi\'c \textit{et al.}, Phys. Rev. Lett. \textbf{101}, 177004 (2008). [Preview Abstract] |
Monday, March 16, 2009 10:36AM - 10:48AM |
A34.00014: Magnetic hysteresis studies of Tl-2223 substituted by Fe and Zn Aly Abou-Aly, Ramadan Awad, Ibrahim Ibrahim, Ahmed Faraj Effect of Fe and Zn substitutions on the magnetic hysteresis of Tl-2223 are investigated in high magnetic fields up to 9 Tesla and at different temperatures (T= 6, 20, 40 and 80 K). The results of magnetic hysteresis loops show that the area of these loops decreases as Fe-content increases, whereas it increases for Zn-substitutions till x = 0.2 and then decreases for x $>$ 0.2. The magnetization difference $\Delta $M is found to decay exponentially with temperature at low magnetic fields, according to $\Delta $M $\alpha $ exp (-T/T$_{0})$. The characteristic temperature T$_{0}$ is found to be varied from 6 K to 40 K and it is related to the applied magnetic field B according to T$_{0} \quad \alpha $ B$^{-1/n}$. The critical current density is calculated for the prepared samples from magnetic hysteresis measurements and compared with that determined from ac magnetic susceptibility. The results are discussed according to the flux motion and flux pinning. [Preview Abstract] |
Monday, March 16, 2009 10:48AM - 11:00AM |
A34.00015: Neutron Scattering Study of Magnetic Field Effect on the Stripe Order in LBCO Zhijun Xu, Jinsheng Wen, Guangyong Xu, Markus H\"ucker, John Tranquada, Genda Gu We have been investigating the relationship of stripe order to high-temperature superconductivity in cuprates. In particular, our neutron scattering results indicate that spin-stripe order is present in La$_{2-x}$Ba$_{x}$CuO$_{4}$ (LBCO) over a substantial range of doping about x = 1/8, where the bulk superconductivity is anomalously suppressed. Focusing on the x = 1/8 composition, we have recently studied the impact on stripe order of a magnetic field applied along the c-axis [1]. Applying a field up to 7 T, we observed a small enhancement of the intensity of the incommensurate antiferromagnetic superlattice peaks and a slight increase in the ordering temperature. In measurements of the spin dynamics, the field had no significant impact on the small spin gap ($\sim $ 0.5 meV) found in the ordered phase [2]. [1] Jinsheng Wen \textit{et al}., arXiv:0810.4085. [2] J.M. Tranquada \textit{et al}., arXiv:0809.0711. Work supported by Office of Science, U.S. DOE, under Contract No. DEAC02-98CH10886 [Preview Abstract] |
Session A35: Focus Session: Iron Pnictides and Other Novel Superconductors I: Synthesis and New Materials
Sponsoring Units: DMPChair: David Larbelestier, Florida State University
Room: 405
Monday, March 16, 2009 8:00AM - 8:12AM |
A35.00001: Superconductivity in layered pnictides BaIr$_{2}$P$_{2}$ and BaRh$_{2}$P$_{2}$ Daigorou Hirai, Tomohiro Takayama, Ryuji Higashinaka, Hiroko Aruga-Katori, Hidenori Takagi The exploration of new superconductors, triggered by the discovery of LaFeAs(O,F), has concentrated mostly on Fe-based pnictides. A variety of non-Fe pnictides, isostructural to Fe pnictide superconductors, have been known for a long time but not yet fully explored in terms of possible superconductivity. Further exploration of non-Fe pnitide superconductors is important for understanding the key factors in realizing the high $T_{c}$ in the Fe pnictides. We report new Ir and Rh pnictide superconductors, isostructural to BaFe$_{2}$As$_{2}$, BaRh$_{2}$P$_{2}$ and BaIr$_{2}$P$_{2}$, with $T_{c}$ = 1.0 and 2.1 K respectively. This discovery demonstrates the presence of superconductivity over a surprisingly broad range of transition metal compounds with ThCr$_{2}$Si$_{2}$-type structure from Fe to Ir. [Preview Abstract] |
Monday, March 16, 2009 8:12AM - 8:24AM |
A35.00002: Superconductivity and enhanced susceptibility in SrFe$_2$As$_2$ single crystals S. R. Saha, N. P. Butch, K. Kirshenbaum, J. Paglione Single crystals of SrFe$_2$As$_2$ grown using a self-flux solution method were characterized via x-ray, transport, magnetization and specific heat studies, revealing a superconducting transition at 21 K which appears far below the magnetostructural transition at 198 K as evidenced by transitions in resistivity and susceptibility. We present experiments which probe the nature of this phase and its relation to the enhancement of magnetic susceptibility in superconducting samples. [Preview Abstract] |
Monday, March 16, 2009 8:24AM - 8:36AM |
A35.00003: Growth and properties of single crystalline F-doped and Co-doped NdFeAsO. M. E. Tillman, S. Kim, R. Prozorov, M. A. Tanatar, S. L. Bud'ko, P. C. Canfield We report the results of single crystal growth of F-doped and Co-doped, NdFeAsO.~ A high pressure furnace using a 19mm truncation edge length cubic anvil capable of reaching 3.3 GPa and 2000 C was used to grow crystals of doped NdFeAsO from on- and off-stoichiometry melts.~ Single crystals with linear dimensions of $>$1mm can be grown using this technique. Anisotropic transport, thermodynamic and spectroscopic properties [1-3] have been measured and the results of these measurements as well as crystal size and doping level will be correlated to growth profile and initial stoichiometry. [1] T. Kondo et al., Phys. Rev. Lett. \textbf{101}, 147003 (2008), [2] C. Liu, et al., arXiv:0806.2147, [3] C. Martin et al., arXiv: 0807.0776. [Preview Abstract] |
Monday, March 16, 2009 8:36AM - 8:48AM |
A35.00004: Transport, magnetic and thermal properties of $M$FePO $M$ = La, Pr, and Nd single crystals Ryan Baumbach, James Hamlin, Lei Shu, Diego Zocco, Nicole Crisosto, M. Brian Maple The recent discovery of $T_{c}$ values near 26 K in the compound LaFeAsO$_{1-x}$F$_{x}$ induced a torrent of publications on what are now recognized as a new class of Fe-based high temperature superconductors. To date, the phosphorus based versions of these compounds have recieved little attention due to their comparitively low $T_{c}$ values. In this work we report the low temperature electrical resistivity, magnetic susceptibility, and specific heat data of single crystalline PrFePO and NdFePO. We also report the effect of annealing on the properties of LaFePO, PrFePO, and NdFePO. A systematic comparison of the occurence of superconductivity in the series $M$FePO and $M$FeAsO (where $M$ is a lanthenide) points to a possible difference in the origin of the superconductivity in these two series of compounds. [Preview Abstract] |
Monday, March 16, 2009 8:48AM - 9:00AM |
A35.00005: New ternary phosphide superconductors with the ThCr2Si2 parent structure Nicholas Berry, Cigdem Capan, Gabriel Seyfarth, Zachary Fisk Compounds with the ThCr2Si2 structure have been studied extensively for over 20 years for their interesting magnetic, superconducting, and heavy fermion properties. We have grown single crystals of new ternary phosphides in this structure in the form of AX2P2, with A being an Alkaline Earth Metal and X a transition metal. We have characterized the properties of these new materials with X-ray, heat capacity, resistivity, and susceptibility measurements and have discovered new superconductors. [Preview Abstract] |
Monday, March 16, 2009 9:00AM - 9:12AM |
A35.00006: Magnetic, Thermodynamic, and Transport Properties of Layered Arsenides BaRu$_{2}$As$_{2}$ and SrRu$_{2}$As$_{2}$ R. Nath, Y. Singh, D.C. Johnston As part of our effort to search for novel superconductors related to the FeAs high-$T$ superconductors, we have synthesized polycrystalline samples of BaRu$_{2}$As$_{2}$ and SrRu$_{2}$As$_{2}$. The magnetic, transport and thermodynamic properties of the samples were investigated by means of magnetic susceptibility $\chi(T)$, electrical resistivity $\rho(T)$, and heat capacity $C_{p}(T)$ measurements. The temperature dependence of $\rho$ indicates metallic character for both compounds with a residual resistivity ratio $\rho$(310 K)/$\rho$(2 K) of 17 and 7 for the Ba and Sr compounds, respectively. The $C_{p}(T)$ results indicate a low density of states at the Fermi level with the low-$T$ Sommerfeld coefficient $\gamma \simeq 4.9$ and $4.13$ mJ/mole K$^{2}$ for the Ba and Sr compounds, respectively. The Debye temperature $\Theta_{D}$ was estimated to be $270$ K and $260$ K for the Ba and Sr compounds, respectively. The $\chi (T)$ was found to be diamagnetic with a small absolute value for both the compounds. No evidence for superconductivity, a spin density wave, or a structural transition was observed from the $C_{p}(T)$ and $\rho(T)$ measurements down to $2$ K. However, the $\chi(T)$ data for SrRu$_{2}$As$_{2}$ exhibit a cusp at $\sim 190$ K, possibly an indication of a structural and/or magnetic transition.\\ \noindent * Supported by the USDOE under contact No. DE-AC02-07CH11358. [Preview Abstract] |
Monday, March 16, 2009 9:12AM - 9:24AM |
A35.00007: Robust Ferromagnetism in Ultrathin Films of CeFeAsO Lifeng Yin, Zheng Gai, Thomas Zac Ward, Paul C. Snijders, Jian Shen The mechanism of magnetic order in iron-oxypnictides has attracted lots of research efforts, but the conclusions are so distinct: local moment ground state with antiferromagnetic fluctuation vs. itinerant ground state with ferromagnetic fluctuation, although a spin-density-wave (SDW) ground state is generally observed in polycrystalline parent compounds. Because single-crystal growth of iron-oxypnictides seems very tough, the fabrication of epitaxial thin films by pulsed laser deposition (PLD) technique should be an important solution for basic research. Here we report a successfully fabrication of c-axis oriented high quality CeFeAsO(001) heteroepitaxial ultrathin film, which shows an atomic flat surface with (4$\times $1) reconstruction at initial stage. We find a striking in-plane ferromagnetic ground state with a small Fe moment in CeFeAsO ultrathin film, followed by the development of spin glass like order with thickness. Therefore, the robust ferromagnetism in single crystalline parent compound thin film implies that the superconductivity in 1111 system should be mediated by ferromagnetic spin fluctuation. [Preview Abstract] |
Monday, March 16, 2009 9:24AM - 9:36AM |
A35.00008: Reaction Kinetics of the Formation of SmFeAsO$_{1-x}$F$_x$ High \textit{T}$_{\rm c}$ Superconductor Probed by High Energy X-ray Diffraction S. Das, Y. Singh, R. Nath, G.E. Rustan, A.I. Goldman, J.Q. Yan, A. Kreyssig, D.C. Johnston, R.W. McCallum, M.J. Kramer The recent discoveries of superconductivity with $T_{\rm c}$ up to 55 K in a new class of compounds $R$FeAsO$_{1-x}$F$_x$ ($R$ = rare earth), have attracted much attention. It is difficult to synthesize single phase high quality samples that are needed to study the intrinsic properties of these materials. The samples reported are made at high temperatures $T$ $\sim$~1100~$^{\circ}$C and usually contain impurity phases along with the $R$FeAsO$_{1- x}$F$_x$ phase. Information on the kinetics of formation of the superconducting phase would be helpful. Here we report the results of high-energy x-ray diffraction measurements versus temperature, carried out at the Advanced Photon Source. We determined the kinetics of the reaction between the appropriate mixtures of SmAs, ${\rm Fe_2O_3}$, Fe, and ${\rm SmF_3}$ to give nominal compositions of SmFeAsO and SmFeAsO$_{1-x}$F$_x$, respectively. Our results indicate that the SmFeAsO$_{1-x}$F$_x$ phase starts forming at $T~\sim$~750~$^{\circ}$C and its formation ceases by $\sim$~1000~$^{\circ}$C\@. \noindent $^*$Supported by the USDOE under Contract Nos.\ DE- AC02-07CH11358 and DE-AC02-06CH11357. [Preview Abstract] |
Monday, March 16, 2009 9:36AM - 9:48AM |
A35.00009: Growth and Properties of SmFeAsO1-xFx thin films using pulsed laser deposition Sanghan Lee, Chad Folkman, Seung Hyub Baek, David Felker, Mark Rzchowski, Chang Beom Eom, Jianyi Jiang, Eric Hellstrom The discovery of iron pnictide superconductors has been gaining interest due to their highest transition temperatures (Tc) among transition metal compound except cuprate systems. In iron pnictide system, Tc has been significantly enhanced by replacing elements. The growth of epitaxial thin film of this interesting compound is desirable for fundamental understanding of superconductivity and potential device applications. So far, there is no report with successful growth of epitaxial thin films with the same Tc of the bulk materials. Although several reports show Tc of thin film, the resistivity dose not reach zero down to at 4K. We have employed pulsed laser deposition for the growth of iron pnictides, SmFeAsO1-xFx, thin films on various single crystal substrates followed by ex--situ post annealing. In this talk, we will discuss the structural and superconducting properties of SmFeAsO1-xFx thin film including Tc, Jc and Hc2. [Preview Abstract] |
Monday, March 16, 2009 9:48AM - 10:00AM |
A35.00010: Superconducting Fe-Based Compounds (A$_{1-x}$Sr$_{x })$Fe$_{2}$As$_{2}$ with A = K and Cs with Transition Temperatures up to 37 K Kalyan Sasmal, Bing Lv, Bernd Lorenz, Arnold M. Guloy, Feng Chen, Yu-Yi Xue, Ching-Wu Chu New high-Tc Fe-based superconducting compounds, AFe$_{2}$As$_{2}$ with A = K, Cs, K/Sr, and Cs/Sr, were synthesized. The Tc of KFe$_{2}$As$_{2}$ and CsFe$_{2}$As$_{2}$ is 3.8 and 2.6 K, respectively, which rises with partial substitution of Sr for K and Cs and peaks at 37 K for 50{\%}--60{\%} Sr substitution, and the compounds enter a spin-density-wave state with increasing electron number (Sr content). The compounds represent p-type analogs of the n-doped rare-earth oxypnictide superconductors. Their electronic and structural behavior demonstrate the crucial role of the (Fe$_{2}$As$_{2})$ layers in the superconductivity of the Fe-based layered systems, and the special feature of having elemental A layers provides new avenues to superconductivity at higher Tc. [Preview Abstract] |
Monday, March 16, 2009 10:00AM - 10:12AM |
A35.00011: The superconductivity in alkaline containing iron arsenide system Changqing Jin, X.C. Wang, Q.Q. Liu, Y.X. Lv, W.B. Gao, L.X. Yang, R.C. Yu, F.Y. Li We report superconducting properties of LiFeAs that crystallize into 111 type structure showing superconducting transition at Tc 18K (http://xxx.lanl.gov/abs/0806.4688 (arXiv:0806.4688)). We will present the update experimental results on characterizations of superconductivity. The possible role of magnetic order to this system will be discussed. [Preview Abstract] |
Monday, March 16, 2009 10:12AM - 10:24AM |
A35.00012: Superconductivity in Sr-122 Iron Arsenide System by Yttrium Doping Shen V. Chong, Kazuo Kadowaki Superconductivity in SrFe$_{2}$As$_{2}$ by interlayer electron doping is achieved for the first time with the substitution of 30 to 50 atomic percent of the strontium with yttrium (Sr$_{1-x}$Y$_{x}$Fe$_{2}$As$_{2})$. In the optimum doped Y = 0.4 sample, Hall effect measurements indicate the injection of extra electrons into the system with negative Hall coefficient below room temperature down to $T_{c}$, and an electron charge carrier density reaching up to 10$^{21}$ cm$^{-3}$ above 200 K. Temperature dependent magnetization measurements indicate the suppression of the magnetic orderings observed in the parent compound at 60 and 210 K upon yttrium doping; while resistivity versus temperature ($R-T)$ measurements show the anomaly above 200 K still persists in the superconducting samples. A maximum onset $T_{c}$ at 26.4 K based on $R-T$ measurement was observed in Sr$_{0.6}$Y$_{0.4}$Fe$_{2}$As$_{2}$. No superconductivity was observed below 0.3 and above 0.5 Y-doping. [Preview Abstract] |
Monday, March 16, 2009 10:24AM - 10:36AM |
A35.00013: High-pressure growth of SmFeAsO$_{1-x}$ single crystals without fluorine doping Hyun-Sook Lee, J.H. Park, J.Y. Lee, Hu-Jong Lee, N.H. Sung, J.Y. Kim, B.K. Cho, T.Y. Koo, C.U. Jung We synthesized fluorine-free SmFeAsO$_{1-x}$ single crystals for a nominal value of $x$=0.15 at a pressure of 3.3 GPa and at a temperature of 1350-1450 $^{\circ}$C by using a self-flux method. Plate-shaped single crystals were obtained with the lateral size of a few-150 $\mu $m. Single crystals showed the superconducting transition at about 53.5 K with a narrow resistive transition width of 0.5 K. The synchrotron-irradiated X-ray diffractometry (XRD) peaks and the high-resolution scanning transmission electron microscopy (HR-STEM) images indicate the good crystallinity of our single crystals, which have a well-defined layered tetragonal structure. The chemical composition of the crystals was examined with the electron-probe X-ray microanalysis. A sharp transition, a low residual resistivity, and a large residual resistivity ratio indicate high quality of our single crystals. Fluorine-free single crystals of Fe-based superconductors are expected to provide an easier route to find the optimum conditions for the crystal formation in this family of materials. [Preview Abstract] |
Monday, March 16, 2009 10:36AM - 10:48AM |
A35.00014: Growth of superconducting FeSe films Michio Naito, Shinya Agatsuma, Shinya Ueda The recently discovered Fe arsenide and chalcogenide superconductors have provided the superconducting community with a great surprise that Fe-based compounds are not ferromagnetic but superconducting with high Tc. The superconducting Fe arsenides and chalcogenides are also interested from the viewpoint of superconducting electronics. One can see good lattice compatibility between the superconducting Fe family and the existing III-V and II-VI semiconducting family (GaAs, ZnSe). All-epitaxial super-semiconductor multilayer structures may be ideal for superconducting electronics and spintronics. Toward this goal, we have attempted to grow epitaxial thin films of the superconducting Fe family. Of this family, tetragonal $\alpha $-FeSe seems to be the easiest to grow thin films. We employed two approaches for FeSe film growth: post-annealing and MBE growth. In the post-annealing, precursor films of Fe are annealed at 500 - 600 $^{o}$C with Se vapor in an evacuated quartz tube. Annealing with elemental Se produced semiconducting FeSe$_{2}$ whereas annealing with FeSe polycrystalline pellets produced superconducting FeSe with $T_{c}$(onset) $\sim $ 10 K. In the MBE growth, we attempted the growth similar to GaAs growth, namely with the vapor rich in Se, expecting self-limiting adsorption of Se. MBE films so far obtained with the growth temperature of 330 $^{o}$C are nonsuperconducting hexagonal $\beta $-FeSe. [Preview Abstract] |
Monday, March 16, 2009 10:48AM - 11:00AM |
A35.00015: \textit{In situ} X-ray Synchrotron Diffraction Study of the Synthesis of LaFeAsO and LaFeAsO$_{1-x}$F$_{x}$ R.W. McCallum, J.-Q. Yan, G.E. Rustan, E.D. Mun, S. Das, R.C. Nath, Youwen Xu, S.L. Bud'ko, K.W. Dennis, D.C. Johnston, P.C. Canfield, M.J. Kramer, A. Kreyssig, T.A. Lograsso, A.I. Goldman The reaction path for the synthesis of LaFeAsO and LaFeAsO$_{1-x}$F$_{x}$ by nominally solid state reaction was studied by \textit{in situ} x-ray synchrotron diffraction technique and Differential Thermal Analysis (DTA) in the temperature interval 100\r{ }C $\le $ T $\le $ 1150\r{ }C. Starting materials were LaAs, Fe$_{2}$O$_{3}$, Fe and for the F containing materials LaF3. The results show that the synthesis is characterized by three temperature intervals: (1) below 400\r{ }C, Fe$_{2}$O$_{3}$ gradually transforms to Fe$_{3}$O$_{4}$. (2) In the temperature interval 400\r{ }C $<$ T $<$ 800\r{ }C, multiple intermediate reactions take place resulting in the formation of La$_{2}$O$_{3}$ and Fe --As compounds. (3) above 800\r{ }C, reaction leads to the formation of LaFeAsO. Possible reaction paths and the difference between F-free and F-doped samples will be discussed in the talk. [Preview Abstract] |
Session A36: Carbon Nanotubes: Electrical Transport and Noise
Sponsoring Units: DMPChair: Hui Tang, Yale University,
Room: 408
Monday, March 16, 2009 8:00AM - 8:12AM |
A36.00001: The Device Physics of Experimentally Validated Analytical Theory of Transport in Ballistic Carbon Nanotube Transistors Deji Akinwande, Jiale Liang, H.-S. Philip Wong We have developed a fully analytical ballistic theory of carbon nanotube field effect transistors (CNFETs) enabled by the development of an analytical surface potential capturing the temperature dependence and gate and quantum capacitance electrostatics. The analytical ballistic theory is compared to the experimental results of a ballistic transistor with good agreement. The validated analytical theory enables qualitative and quantitative intuitive insight into transport in CNFETs, provides techniques for extracting device parameters such as the bandgap and the surface potential from experimental current-voltage characteristics, and elucidates on the relatively new device physics of drain optical phonon scattering and its role in reducing the linear conductance and intrinsic gain of the transistor. These results apply to all ballistic CNFETs with a channel length that is less than the acoustic phonon mean free path. [Preview Abstract] |
Monday, March 16, 2009 8:12AM - 8:24AM |
A36.00002: Unusual behaviors of heat-treated nanotube devices with platinum contact metal Alexander A. Kane, Tatyana Sheps, Philip G. Collins Modest heat treatments typically lead to performance improvements in devices made from single-walled carbon nanotubes (SWCNTs) [1]. We report highly unusual behavior, however, in the response SWCNTs contacted with Pt electrodes. Instead of monotonically improving, the contact resistance can either increase or decrease by one order of magnitude depending on the processing temperature. Furthermore, we observe anomalous changes in the device transconductance, such that SWCNTs previously identified as metallic acquire gate-dependent characteristics. The results appear intrinsic to the Pt-SWCNT interface and are not due to contamination or environmental effects, because measurements are performed \textit{in situ} during heating in ultra-high vacuum. Complimentary electrochemical and spectroscopic testing reveals the influences of Pt-SWCNT interface chemistry. These results have particular importance for high power applications requiring refractory metal contact electrodes. [1] A. Kane et al. \textit{App. Phys. Lett}. \textbf{92} 038506 (2008). [Preview Abstract] |
Monday, March 16, 2009 8:24AM - 8:36AM |
A36.00003: Length dependent transport measurements in long channel semiconducting carbon nanotubes Tarek Ghanem, Ellen Williams, Michael Fuhrer In order to understand the intrinsic electronic properties of CNTs, it is important to eliminate the contact effects from the measurements. We accomplish this by using a conductive-tip atomic force microscope cantilever as a movable electrode to obtain length dependent transport measurements. We report on the resistance versus length$ R(L)$ for several long channel ($L$ up to 130 $\mu $m) semiconducting CNTs at room temperature. In the on state,$ R(L)$ of semiconducting CNTs is linear. In the depleted state, $R(L)$ is linear for long channel lengths ($>$ 10 $\mu $m), but non-linear for short channel lengths due to the long depletion lengths in one-dimensional semiconductors. Transport remains diffusive under all depletion conditions, due to both low disorder and high temperature. [Preview Abstract] |
Monday, March 16, 2009 8:36AM - 8:48AM |
A36.00004: Simulation of Electronic Transport in Carbon Nanotube Field Effect Transistors Dylan McGuire, Ferdows Zahid, Lei Liu, Richard Martel, Hong Guo In recent years, Carbon nanotube (CN) field effect transistors (CNFETs) with a sub-threshold slope of 40mV/dec have been demonstrated, which is less than the thermal limit of 60mV/dec\footnote{Appenzeller, J., \emph{et al.}, \emph{IEEE Trans. Elec. Dev.}, \textbf{52}, 2568, (2005)}. By exploiting inter-band tunneling, the transmission ratio comes to depend on the density of states rather than the thermal distribution of carriers in the contacts. Using tight-binding approximations to the Hamiltonian in the Keldysh non-equilibrium Green's function (NEGF) formalism, we study the transport properties of CNFETs under a tunneling mode bias. Phonon coupling effects are included through the self-consistent Born approximation (SCBA). The mode-space approach to decoupling the Hamiltonian\footnote{Venugopal, R., \emph{et al.}, \emph{J. Appl. Phys., }\textbf{92}, 3730, (2002)} is extended to include chiral nanotubes, such that a more realistic class of CNs may be treated with computational efficiency. Further, a comparison is made between the $\pi$-orbital and $\pi+\sigma$-orbital tight-binding models. Here, we find that transport is minimally affected. The geometry and electrostatic contact doping are examined to optimize device performance. [Preview Abstract] |
Monday, March 16, 2009 8:48AM - 9:00AM |
A36.00005: Effect of gate electrode contact on transport properties of carbon nanotubes Nobuhiko Kobayashi, Taisuke Ozaki, Kenji Hirose Recently, considerable effort has been devoted to developing carbon nanotube devices. One of the important issues in the developments of carbon nanotube devices is the control of contact effects of the electrodes. To detect electric signals through nanotubes, electrodes must be connected to the nanotubes. The contact with the electrodes sensitively influences their electronic structures and transport properties. Therefore, it is important to discuss the transport properties on the basis of the detailed electronic state calculations that include the effect of the contact with the electrodes. We have investigated quantum transport in carbon nanotubes bridged between metallic electrodes. The electronic states are calculated using a numerical atomic orbital basis set in the framework of the density functional theory, and the conductance is calculated using the Green's function method. We have analyzed transport properties of the finite size of carbon nanotubes bridged between metallic electrodes on a gate electrode, and discuss the contact effect of the electrodes on the transport properties. [Preview Abstract] |
Monday, March 16, 2009 9:00AM - 9:12AM |
A36.00006: Theoretical Study of Multiple-Trap Correlations in Random Telegraph Signals of a Carbon Nanotube Field-Effect Transistor Smitha Vasudevan, Jack Chan, Brian Burke, Kenneth Evans, Kamil Walczak, Mingguo Liu, Joe Campbell, Keith Williams, Avik Ghosh We develop a theoretical model to explain the observation of high amplitude, multiple-trap random telegraph signatures (RTS) in the electronic transport of a one-dimensional field effect transistor (FET) with a carbon nanotube channel. A unique RTS pattern is observed, with an initial strong blockade of the current that continues over a well-defined bias window, and subsequent reversal of the blockade through a separate RTS series. We ascribe our observations to correlated electrostatic effects between multiple charge traps along the channel, whereby one trap 'passivates' the other purely electrostatically and without any direct chemical bond. We present a robust quantum transport model that provides quantitative validation of this hypothesis. We assert that this effect, which has not been reported in bulk silicon devices, arises from the logarithmic electrostatic potential profile of the 1-D channel that allows the trap levels to slip past each other under the action of a remote gate, ultimately reversing their energy hierarchy and annihilating each other. Our results suggest that multiple-trap behavior in low-dimensional field-effect devices may be adaptable for several new transistor and sensor technologies. [Preview Abstract] |
Monday, March 16, 2009 9:12AM - 9:24AM |
A36.00007: Reversible current blockade of carbon nanotube through well resolved multitrap-interactions Tsz Wah Chan, Smitha Vasudevan, Brian Burke, Kenny Evans, Kamil Walczak, Mingguo Liu, Joe Campbell, Avik Ghosh, Keith Williams We report the observation of a new kind of multiple-trap random telegraph signal (RTS) in the electronic transport of a carbon nanotube field effect transistor at room temperature. RTS from one charge trap precedes a transition to strong current blockade continues over a well-defined bias window, and subsequently reverses to unblock through a separate RTS series arising from a second, adjacent trap thru pure electrostatic interaction. Our results suggest that multiple-trap behavior in low-dimensional field-effect devices may be adaptable for several new transistor and sensor technologies. [Preview Abstract] |
Monday, March 16, 2009 9:24AM - 9:36AM |
A36.00008: Scanning Gate Microscopy of Single-Walled Carbon Nanotubes Jiamin Xue, B.J. LeRoy The one dimensional nature of single-walled carbon nanotubes (SWCNTs) causes their low energy properties to be described by Luttinger liquid theory. Using low-temperature scanning probe microscopy and electrical transport measurements, we have investigated the electronic properties of SWCNTs. Individual SWCNTs were contacted with Pd electrodes and located using atomic force microscopy. The AFM operating at 300 mK was used to probe the SWCNTs using scanning gate microscopy. Differential conductance as a function of source-drain bias voltage and gate voltage showed Coulomb diamond patterns. Using a voltage on the AFM tip, we are able to probe the spatial dependence of the conductance. The ability to perform simultaneous electrical transport and scanning probe microscopy measurements allows us to test theoretical predictions about Luttinger liquids including spin-charge separation. [Preview Abstract] |
Monday, March 16, 2009 9:36AM - 9:48AM |
A36.00009: Electron Transport in Fluorinated Singe Wall Nanotubes Thushari Jayasekera, Junwen Li, John W. Mintmire, Vincent Meunier Taking advantage of helical symmetry, we present results for the electronic structure and electron transport properties of fluorinated chiral single-wall carbon nanotubes, within an all-electron, local density functional approach. In this talk, we discuss the effect of pairs of fluorine substituents on metallic nanotubes. Our results show that this fluorination results in resonance features in the vicinity of the Fermi level. The resonance behavior comes out as a result of the interaction of the C-C atoms those are close to the F-attached C atoms. We also discuss the change of atomic structure caused by fluorination. To our knowledge this is the first electron transport calculation that uses the helical symmetry within a first-principles approach. We find that the use of helical symmetry has important advantages in electron transport calculations of systems with local defect sites. We will also briefly discuss about the application of this technique on Si nanowire systems with defects. [Preview Abstract] |
Monday, March 16, 2009 9:48AM - 10:00AM |
A36.00010: Electron-phonon scattering effects on transport properties of carbon nanotubes - From diffusive to ballistic regimes - Hiroyuki Ishii, Nobuhiko Kobayashi, Kenji Hirose Recently, nanotechnology has fabricated various nanoscale electronic devices. In these systems, the mean free path is comparable to the system size. Therefore, it is important to understand how the transport property changes from the ballistic to the diffusive regimes by various scattering effects. In this work, we study the transport properties of carbon nanotubes using the time-dependent wave-packet approach [1]. Combining with the molecular dynamics simulations, we can treat the electron transport from diffusive to ballistic regimes from atomistic point of views [2]. We investigated the transport properties of metallic carbon nanotubes and the channel-length dependence of resistance from diffusive to ballistic regimes. The obtained mean free path and relaxation time are consistent with experimental observations. Furthermore, we investigate the mobility of semiconducting nanotubes. In the presentation, we will discuss the detail analysis of the origin of resistance. [1] H.Ishii \textit{et al.}, Phys.Rev.B 76(2007) 205432 [2] H.Ishii \textit{et al.}, to be published in Applied Physics Express. [Preview Abstract] |
Monday, March 16, 2009 10:00AM - 10:12AM |
A36.00011: Scanning gate microscopy of electronic inhomogeneities in single-walled carbon nanotube (SWCNT) devices Steven R. Hunt, Philip G. Collins The electronic properties of SWCNT devices are primarily determined by the contact metal and the SWCNT bandstructure. However, inhomogeneities such as substrate imperfections, sidewall defects, and mobile contaminants also contribute. In extreme cases, metallic SWCNTs have transistor-like behaviors due to these inhomogeneities. We investigate methods of identifying and distinguishing these different scattering mechanisms using scanning gate microscopy. For example, we can readily identify a sidewall defect in the presence of substrate charge traps, because the two types of disorder respond differently to gate electric fields. We present methods of optimizing the imaging conditions to make such distinctions. This research has been partly supported by the NSF (DMR-0801271 and ECCS-0802077). [Preview Abstract] |
Monday, March 16, 2009 10:12AM - 10:24AM |
A36.00012: Tuning the electronic states of carbon nanotube based devices under magnetic field Sebastien Nanot, Bertrand Raquet, Walter Escoffier, Jean-Marc Broto, Remi Avriller, Stephan Roche Carbon nanotubes have already demonstrated their wide potential in nanoelectronics and optoelectronics. In our study, we demonstrate that an applied magnetic field, along with a control of the electrostatic doping, drastically modifies the electronic band structure of a carbon nanotube based transistor. Several examples will be addressed in this presentation. In a parallel configuration (B parallel to the tube axis), a quantum flux threading the tube induces a giant Aharonov-Bohm conductance modulation mediated by Schottky barriers which profile is magnetic field dependent. In the perpendicular configuration, the applied magnetic field breaks the revolution symmetry along the circumference and non conventional Landau states are expected in the high field regime. By playing with a carbon nanotube based electronic Fabry-Perot resonator, we bring evidence that the electronic transmission of the device can be modified by a transverse magnetic field. The field dependence of the resonant states of the cavity reveals the onset of the first landau state at zero energy. These experiments also enlighten the outstanding efficiency of magneto-conductance experiments to probe the electronic properties of carbon based nano-materials. [Preview Abstract] |
Monday, March 16, 2009 10:24AM - 10:36AM |
A36.00013: Transport and Magnetism in Template Synthesized Hydrogenated Multiwalled Carbon Nanotubes Adam Friedman, Hyunkyung Chun, Donald Heiman, Yung Joon Jung, Latika Menon In this work, we synthesize highly disordered carbon nanotubes by CVD in porous alumina templates. We show that, due to the disorder in the nanotubes, they can easily be made to uptake hydrogen by annealing. We show that this induces ferromagnetism in the nanotubes, and we perform a magnetic study. We also measure the transport properties of the nanotubes. First, we find a rate dependent hysteretic magnetoresistance. We explain the rate dependence through strong magneto-viscosity effects, and we attribute the hysteresis to anisotropic magnetoresistance. We also discover a magnetic field-driven temperature dependent transition from positive to negative magnetoresistance in the ferromagnetic nanotubes that is not observed in similarly disordered un-hydrogenated carbon nanotubes. We attempt to explain this behavior by considering it an order-disorder transition described by the Bright model due to several scattering pathways, that are present in the ferromagnetic nanotubes that are not present in the non-ferromagnetic tubes. [Preview Abstract] |
Monday, March 16, 2009 10:36AM - 10:48AM |
A36.00014: Magnetotransport of hybrid nanoparticle-nanowire systems Dongkyun Ko, Cole Robinette, Xianwei Zhao, Fengyuan Yang, Ezekiel Johnston-Halperin Semiconductor nanowires decorated with metal nanoparticles have a number of interesting electronic and photonic properties. For example, top-gated field effect transistors based on these hybrid systems have shown charge storage when operated in a floating-gate architecture. In addition, recent measurements have demonstrated that spin relaxation and phase coherence lengths can be extracted from magnetoconductance patterns in the gating response of bare nanowires. Together, these results suggest the possibility of \textit{in situ} tuning of the spin relaxation length in hybrid systems via modulation of the floating-gate potential. Initial efforts along these lines will be presented, including gating response and low temperature magnetotransport in 50 nm diameter InP nanowires decorated with Au nanoparticles from 20 -- 250 nm in diameter. The potential utility of these systems as testbeds for the exploration of spin scattering and transport will be discussed. [Preview Abstract] |
Monday, March 16, 2009 10:48AM - 11:00AM |
A36.00015: Exploration of conductance peak splitting in carbon nanotube field effect transistors at critical field strengths Jeffrey D. Stephens, Jerome C. Licini, A.T. Charlie Johnson , Doug R. Strachan , Danvers E. Johnston , Sam Khamis Carbon nanotube field effect transistors were produced by chemical vapor deposition growth of nanotubes on oxidized silicon substrate. Samples were back gated on doped silicon and contacted with gold/chrome contacts. Conductance measurements were performed at low temperature and high magnetic field using a dilution refrigerator and a superconducting magnet. Data was taken at 0.5 Tesla increments from 0-11Tesla. The differential conductance (dI/dV) shows an interesting asymmetry with bias voltage as well as a near zero bias conductance peak. The near zero bias conductance peak demonstrates splitting at two critical magnetic field strengths on the 0.5T scale. These two critical regimes are further explored on a finer magnetic field scale. [Preview Abstract] |
Session A37: Focus Session: Fundamental Developments in Density Functional Theory I
Sponsoring Units: DCPChair: Adam Wasserman, Purdue University
Room: 409
Monday, March 16, 2009 8:00AM - 8:36AM |
A37.00001: Insights and Progress in Density Functional Theory. Invited Speaker: Density functional theory of electronic structure is widely and successfully applied in simulations throughout engineering and sciences. However, there are spectacular failures for many predicted properties, which can be traced to the delocalization error and static correlation error of commonly used approximations. These errors include underestimation of the barriers of chemical reactions, the band gaps of materials, the energies of dissociating molecular ions and charge transfer excitation energies. Typical DFT calculations also fail to describe degenerate or near degenerate systems, as arise in the breaking of chemical bonds, and strongly correlated materials. These can all be characterized and understood through the perspective of fractional charges and fractional spins introduced recently. Understanding the errors of functionals in the simplest way possible --- as violations of exact conditions for fractional charges and fractional spins -- opens the path forward for reduction of the errors and for applications of density functional theory in new frontiers. [P.~Mori-Sanchez, A.~J. Cohen, and W.~T. Yang, \textit{Phys. Rev. Lett. }100:146401(2008); \textit{Phys. Rev. B},77:115123(2008)\textit{; J. Chem. Phys.} 129:121104(2008); \textit{Science}, 321:792(2008)] [Preview Abstract] |
Monday, March 16, 2009 8:36AM - 9:12AM |
A37.00002: Semiclassical origins of density functional theory Invited Speaker: Until the seminal work of Hohenberg, Kohn, and Sham of the mid 60's, most density functional theory (DFT) was derived from semiclassical approximations. This non-empirical approach shows an intrinsic difference between solids (for which DFT was originally developed) and molecules, and explains many of its more mysterious manifestations. For example, the success of DFT for molecules has nothing to do with the uniform gas. Results include [1] a derivation of the empirical parameter in the B88 exchange functional, [2] PBEsol, a new GGA that restores the exchange gradient expansion and improves lattice constants in solids, [3] a novel approach to ``orbital-free'' DFT that, in preliminary tests, is 40 times more accurate than its DFT counterpart. The talk is aimed at a general theoretical audience. Detailed technical knowledge of DFT is neither needed, nor desirable. \\[4pt] [1] J.P. Perdew, L.A. Constantin, E. Sagvolden, and KB, Phys. Rev. Lett. 97, 223002 (2006). \\[0pt] [2] J.P. Perdew, A. Ruzsinszky, G.I. Csonka, O.A. Vydrov, G.E. Scuseria, L.A. Constantin, X. Zhou, and KB, Phys. Rev. Lett. 100, 136406 (2008). \\[0pt] [3] Peter Elliott, Donghyung Lee, Attila Cangi, KB, Phys. Rev. Lett. 100, 256406 (2008). [Preview Abstract] |
Monday, March 16, 2009 9:12AM - 9:24AM |
A37.00003: Semiclassics in Density Functional Theory Donghyung Lee, Attila Cangi, Peter Elliott, Kieron Burke Recently, we published an article [1] about the semiclassical origin of density functional theory. We showed that the density and the kinetic energy density of one dimensional finite systems with hard walls can be expressed in terms of the external potential using the semiclassical Green's function method. Here, we show a uniformization scheme for the semiclassical density and the kinetic energy density for turning-point problems.\newline \newline [1] P. Elliott, D. Lee, A. Cangi, and K. Burke, Phys. Rev. Lett. 100, 256406 (2008). [Preview Abstract] |
Monday, March 16, 2009 9:24AM - 9:36AM |
A37.00004: Correlated Thomas-Fermi: Semiclassical ground-state energies of many-electron systems Brian Landry, Adam Wasserman, Eric Heller A new semiclassical method is proposed to obtain accurate ground-state energies for many-electron systems. The method borrows its semiclassical character from Thomas Fermi theory (TF), but improves upon it by including correlation effects, at least approximately. We illustrate our method (CTF) on simple models of 1D-interacting electrons, showing that it yields dramatic improvements over TF, particularly in the strongly-correlated regime. [Preview Abstract] |
Monday, March 16, 2009 9:36AM - 9:48AM |
A37.00005: Dimensional crossover of the exchange-correlation energy at the semilocal level Lucian Constantin Commonly used semilocal density functional approximations for the exchange-correlation energy fail badly when the true two dimensional limit is approached. We show, using a quasi-two-dimensional uniform electron gas in the infinite barrier model, that the semilocal level can correctly recover the exchange-correlation energy of the two-dimensional uniform electron gas. We derive new exact constraints at the semilocal level for the dimensional crossover of the exchange-correlation energy and we propose a method to incorporate them in any exchange-correlation density functional approximation. [Preview Abstract] |
Monday, March 16, 2009 9:48AM - 10:00AM |
A37.00006: Towards a semiclassical theory of electronic structure Attila Cangi, Donghyung Lee, Peter Elliott, Kieron Burke Modern density functional theory (DFT) -formulated in the context of the Kohn-Sham scheme- evolved from ``old'' Thomas-Fermi theory to an accurate theory for predicting various properties of molecules and solids. We discuss the interrelation of semiclassical physics[1] with the fundamental gradient approximation, which is the basis of all functional construction. We speculate that applying semiclassical methods in the context of a DFT-like theory is a path towards more accurate and efficient approximations to electronic properties of condensed systems.[2]\newline \newline [1] M. V. Berry and K. E. Mount, Reports of Progress in Physics 35, 315 (1972). \newline [2] P. Elliott, D. Lee, A. Cangi, and K. Burke, Phys. Rev. Lett. 100, 256406 (2008). [Preview Abstract] |
Monday, March 16, 2009 10:00AM - 10:12AM |
A37.00007: Spin-Current-Density Functional Theory with $SU(2)$ Potentials: the Local Approximation Saeed H. Abedinpour, Giovanni Vignale In order to study spin-orbit coupled systems, spin-transfer torque devices, or even systems with pseudospin coupling like graphene, non-Abelian vector potentials and their conjugate variables, the spin-current densities, should be incorporated in the density functional theory. The general formalism for doing this has been known for some time [K. Bencheikh, J. Phys. A: Math. Gen. {\bf 36} 11929 (2003)], but explicit functionals of the spin currents are not known. Here we identify the most general form of an exchange-correlation (xc) functional of the spin-currents, which satisfies the $SU(2)$ gauge invariance and construct the local density approximation (LDA) for it. As an illustration we present the LDA functional for a two-dimensional (2D) electronic system, using as reference system a homogeneous 2D electron gas subjected to isotropic $SU(2)$ vector potentials (\emph{i.e.}, linear spin-orbit coupling). The numerical results for the xc energy of this reference system will be presented. [Preview Abstract] |
Monday, March 16, 2009 10:12AM - 10:24AM |
A37.00008: Coupling of electrons to the electromagnetic field in a localized basis Roland Allen A simple formula is obtained for coupling electrons in a complex system to the electromagnetic field. It includes the effect of intra-atomic excitations and nuclear motion, and can be applied in, e.g., first-principles-based simulations of the coupled dynamics of electrons and nuclei in materials and molecules responding to ultrashort laser pulses. Some additional aspects of nonadiabatic dynamical simulations are also discussed, including the potential of reduced Ehrenfest simulations for treating problems where standard Ehrenfest simulations will fail. [Preview Abstract] |
Monday, March 16, 2009 10:24AM - 10:36AM |
A37.00009: Density-Functional Partition Theory for Order-N Electonic-Structure and Quantum-Dynamic Computations Morrel Cohen, Roberto Car Large Complex Systems pose a challenge to first-principles electronic-structure and quantum-dynamics computations. Most widely used codes scale as N$^{3}$, where N measures the size of the system. In this talk we describe a new order-N scheme based on a new conceptual structure, density-functional partition theory. In partition theory, the system is broken up into mono- or multinuclear parts and its electron density exactly decomposed into contributions from each part. A common partition potential and partition forces acting on each part carry the influence of the rest of the system to that part. A novel functional for the partition potential and a new iteration scheme achieve linear scaling. The scheme will integrate smoothly into the current widely-used Car-Parrinello electronic- and atomic-structure and quantum-dynamics codes. [Preview Abstract] |
Monday, March 16, 2009 10:36AM - 10:48AM |
A37.00010: Performance of Long-Range Corrected Functionals Mary Rohrdanz, John Herbert Popular generalized gradient approximations (GGA) to the exchange-correlation functional are accurate and useful in many different physical systems. However they have several well-documented shortcomings, including the incorrect asymptotic behavior. One manifestation of this is that linear response time-dependent density functional calculations of vertical excitation energies with GGA functionals sharply underestimate charge-transfer excitations in large systems. Consequently, such functionals are not reliable for calculations in such situations, for example, biomolecules in solution. To circumvent this problem, a number of long-range-corrected functionals (based on GGAs) have recently been developed, which posses the correct asymptotic form by construction. We analyze the performance of some of these functionals through a battery of tests, and demostrate that there exists a functional form and parameter set that provide reasonable results for both ground-state properties and vertical excitation energies. We find these functionals suitable for general use in large condensed-phase systems. [Preview Abstract] |
Monday, March 16, 2009 10:48AM - 11:00AM |
A37.00011: Density Functional with Full Exact Exchange, Balanced Nonlocality of Correlation, and Constraint Satisfaction John P. Perdew, Viktor N. Staroverov, Jianmin Tao, Gustavo E. Scuseria We construct a nonlocal density functional with full exact exchange, while preserving the constraint-satisfaction approach and justified error cancellations of simpler semilocal functionals. This is achieved by interpolating between different approximations suitable for two extreme regions of the electron density. In a ``normal region'', the exact exchange-correlation hole around an electron is semilocal because its range is reduced by correlation and because it integrates over a narrow range to -1. ``Abnormal'' regions, where nonlocality is unveiled, include those in which exchange can dominate correlation (one-electron, nonuniform high density, and rapidly-varying limits), and those open systems of fluctuating electron number over which the exact exchange-correlation hole integrates to a value greater than -1. Regions between these extremes are described by a local hybrid mixing exact and semilocal exchange locally. [Preview Abstract] |
Session A38: Focus Session: The Chemical Physics of Biological and Biologically-inspired Solar Energy Harvesting I
Sponsoring Units: DCPChair: Greg Scholes, University of Toronto
Room: 410
Monday, March 16, 2009 8:00AM - 8:36AM |
A38.00001: Investigation of Excitonic Coherence in LHCII by 2D Electronic Spectroscopy Invited Speaker: Photosynthesis has evolved with the ability to transfer energy through a matrix of light-harvesting pigment-protein complexes with almost no loss. The accomplishment of this near unity quantum efficiency is a feat that man has yet to understand or replicate. One proposed mechanism integral to this process requires long-lived coherent superpositions of the excitons, delocalized electronic excitations, in these systems. Two-dimensional Fourier transform electronic spectroscopy, already proven to be an ideal technique for investigating these coherences, has been employed to study Light Harvesting Complex II (LHCII), the most abundant light harvesting complex in higher plants. As in other photosynthetic systems previously studied, we observe long-lived coherence lasting beyond many of the excitons' lifetimes. Furthermore, unique coherence signatures allow the energies of the individual excitons to be located in an otherwise highly congested spectrum. This technique, by which 2D FT electronic spectroscopy can pinpoint excitonic spectral positions, and the resulting implications for LHCII will be discussed. [Preview Abstract] |
Monday, March 16, 2009 8:36AM - 9:12AM |
A38.00002: Luminescent Solar Concentrators Employing Phycobilisomes Invited Speaker: At current manufacturing growth rates, it is expected to take at least 20 years to produce enough Si-based solar cells to make a significant impact on the world energy supply. Solar concentrators could alleviate manufacturing constraints by focusing light on small solar cells. Luminescent solar concentrators (LSCs) are especially promising because they do not need to track the sun to obtain high optical concentration factors. Light incident on an LSC is absorbed by dyes, re- emitted into a guided mode in the slab, and finally collected by a PV cell mounted at the edge of the slab. The maximum optical concentration of an LSC is theoretically limited by the wavelength shift between absorption and emission in the dye. In this presentation, we describe LSCs that mimic a four energy level laser design, maximizing the wavelength shift and minimizing re-absorption losses. We employ phycobilisomes - photosynthetic antenna complexes that concentrate excited states in red algae and cyanobacteria. The phycobilisomes are cast in a solid-state matrix that preserves their internal Förster energy-transfer pathways and large wavelength shift between absorption and emission. Casting is a simple fabrication technique that also eliminates any need for expensive high-index glass or plastic. By comparing the performance of intact and decoupled complexes, we establish that energy transfer within intact phycobilisomes reduces LSC self-absorption losses by approximately (48$\pm$5)\%. These results suggest that phycobilisomes are the model for a new generation of cast LSCs with improved efficiency at high optical concentrations. [Preview Abstract] |
Monday, March 16, 2009 9:12AM - 9:48AM |
A38.00003: Coherent Multidimensional Spectroscopy of Photosynthetic Complexes: Manipulating Quantum Pathways by Optical Pulse Sequences Invited Speaker: The response of chromophore aggregates to sequences of femtosecond laser pulses is simulated using the nonlinear exciton equations. The nonlinear response can be interpreted in terms of the scattering of elementary excitations, quasiparticles, rather than as transitions among eigenstates. Applications are made to the Fenna-Matthews-Olson (FMO) and the PSI light harvesting complexes. Some fundamental symmetries of multidimensional optical signals are used to design techniques that can selectively resolve coherent quantum dynamics and incoherent energy dissipation. Simulations show damped oscillations of cross peaks corresponding to evolution of coherences, without interference from incoherent population relaxation. Energy transfer pathways are seen through the redistribution of crosspeak amplitudes. Resolution is enhanced by employing specific pulse polarization configurations to generate chirality-induced signals. New pulse sequences are designed to generate signals that are induced by correlations among elementary excitations. Specific phase-matching directions can target the correlated dynamics of double excitations. Cross peaks in 2D correlation plots are interpreted in terms of quasiparticle scattering and shown to reveal the double-exciton wavefunction, projected into products of single-excitons. Uncorrelated double-exciton states do not show up in the spectra due to quantum interference among pathways. The proposed techniques amplify cooperative dynamical features and reveal information on the robustness of quantum states to fluctuating environments. In collaboration with Darius Abramavicius and Dmitri Voronine, University of California, Irvine, CA 92697. [Preview Abstract] |
Monday, March 16, 2009 9:48AM - 10:00AM |
A38.00004: Exploring Nanophotovoltaic Molecules using STM Chenggang Tao, Jibin Sun, Xiaowei Zhang, R. Yamachika, D. Wegner, Y. Bahri, G. Samsonidze, S. Louie, T. Tilly, R. Segalman, M. Crommie Composite molecular solar cells are a promising and exciting alternative to traditional silicon or gallium arsenide solar cells, but the power conversion efficiency remains low. In order to further increase this efficiency, a deeper understanding of the microscopic mechanisms at work in organic solar cells is needed. Using scanning tunneling microscopy and spectroscopy we have investigated nanophotovoltaic molecules that combine both donor and acceptor elements. Submolecular spectral resolution reveals the energy level alignment within these composite molecular structures. This information should be useful for understanding the energy conversion pathways within molecular solar cells, and for developing higher efficiency solar cell materials. [Preview Abstract] |
Monday, March 16, 2009 10:00AM - 10:12AM |
A38.00005: New highly polar semiconductor ferroelectrics for solar energy conversion devices Andrew M. Rappe, Ilya Grinberg, Joseph W. Bennett Solar energy is a promising long-term solution for future energy requirements; however, current solar energy conversion devices are plagued by low efficiency. The use of ferroelectric ABO$_3$ perovskite oxides is one approach for boosting conversion efficiency. Ferroelectric oxides possess spontaneous polarization and have been shown to produce a bulk photovoltaic effect, in which charged carriers, specifically electrons and holes, separate to prevent recombination. Once separated, the high-energy electrons are available for electrical work or for the catalytic splitting of water into hydrogen and oxygen. Currently, most solid oxide ferroelectrics have a band gap of at least 3~eV, absorbing primarily in the ultra-violet (UV) region. Since UV light comprises only 8$\%$ of the solar spectrum, new materials with a decreased band gap and large polarization would be highly desirable. We use first-principles density functional theory (DFT) calculations to investigate the ground state structures of PbTiO$_{3}$ solid solutions containing Ni, Pd and Pt. We predict that these proposed materials will display a decreased band gap when compared to PbTiO$_{3}$, while maintaining or enhancing polarization. They are promising candidates for use as semi-conducting ferroelectric substrates for solar conversion devices. [Preview Abstract] |
Monday, March 16, 2009 10:12AM - 10:24AM |
A38.00006: Femtosecond electronic relaxation in cylindrical molecular aggregates Andrew Moran, Jordan Womick, Stephen Miller Natural light harvesting systems have evolved with correlated pigment fluctuations whose fine-tuning promotes efficient energy transfer and photosynthesis. We investigate similar correlations in a double-walled cylindrical molecular aggregate with a diameter of 10 nm. A variety of nonlinear laser spectroscopies are utilized in this work. Excitons localized on different regions of the cylindrical structure are found to undergo correlated energy level fluctuations by analysis of photon echo line shapes. Particular electronic relaxation channels are resolved with a specialized coherent Raman spectroscopy. The importance of correlated pigment fluctuations for excitonic energy transfer is discussed. [Preview Abstract] |
Monday, March 16, 2009 10:24AM - 10:36AM |
A38.00007: F\"orster coupling in realistic nanoparticle circuits Patrick Rebentrost, Mike Stopa, Alan Aspuru-Guzik Semiconductor nanoparticles could potentially be used to build artificial light-harvesting systems and electrically controlled excitonic circuits. We derive a new method for computing the F\"orster coupling between nanoparticles in an arbitrary electrostatic environment. We calculate the self-consistent electronic structure of an exciton, including the electron-hole attraction, in a nanoparticle within the two band effective mass approximation. Self-interaction of the electron and the hole are removed and the eigenstate is approximated as a product state of the electron and the hole wavefunctions. The environment is incorporated via boundary conditions on Poisson's equation and arbitrary dielectric background. The transfer rate of the exciton via F\"orster coupling to a neighboring nanoparticle is computed, without making a dipole approximation, from the results of the self-consistent calculation. Departure from the usual $1/R^3$ dependence are calculated, as well as specific cases where gates or additional nanoparticles are present. [Preview Abstract] |
Monday, March 16, 2009 10:36AM - 10:48AM |
A38.00008: Non-Markovian Environmental Contributions to the Efficiency of Energy Transfer Cesar Rodriguez-Rosario, Patrick Rebentrost, Alan Aspuru-Guzik Non-Markovian environmental effects have been experimentally observed in the Fenna-Matthews-Olson photosynthetic complex, but their role is not understood. We study the dynamical contribution of the environment to the efficiency of energy transfer by considering a non-Markovian environment and its interplay with the system Hamiltonian. We focus on the role of memory effects of different orders in time, and their competition that affect the energy transfer by defining the efficiency of the non-Markovian process. This efficiency measure has applications to the study of the quantum transport efficiency and engineering of light-harvesting devices. [Preview Abstract] |
Monday, March 16, 2009 10:48AM - 11:00AM |
A38.00009: Spatially correlated fluctuations and coherence dynamics in photosynthesis Z. G. Yu, M. A. Berding, Haobin Wang Recent multicolor photon-echo experiments revealed a long-lasting quantum coherence between excitations on donor and acceptor in photosynthetic systems. Identifying the origin of the quantum coherence is essential to fully understand photosynthesis. Here we present a generic model in which a strong intermolecular steric restoring force in densely packed pigment-protein complexes results in a spatial correlation in conformational (static) variations of chromophores, which in turn induces an effective coupling between high-frequency (dynamic) fluctuations in donor and acceptor. The spatially correlated static and dynamic fluctuations provide a favorable environment to maintain quantum coherence, which can consistently explain the photon-echo measurements [1]. [1] Z. G. Yu, M. A. Berding, and Haobin Wang, Phys. Rev. E {\bf 78}, 050902 (Rapid Communications) (2008). [Preview Abstract] |
Session A39: Cellular Biomechanics I
Sponsoring Units: DBPChair: Jennifer Schwarz, Syracuse University
Room: 411
Monday, March 16, 2009 8:00AM - 8:12AM |
A39.00001: Loss of an actin crosslinker uncouples cell spreading from cell stiffening on gels with a gradient of stiffness Qi Wen, Fitzroy J. Byfield, Kerstin Nordstrom, Paulo E. Arratia, R.Tyler Miller, Paul A. Janmey We use microfluidics techniques to produce gels with a gradient of stiffness to show the essential function of the actin crosslinker filamin A in cell responses to mechanical stimuli. M2 melanoma cells null for filamin A do not alter their adherent area in response to increased substrate stiffness when they link to the substrate only through collagen receptors, but change adherent area normally when bound through fibronectin receptors. In contrast, filamin A-replete A7 cells change adherent area on both substrates and respond more strongly to collagen 1-coated gels than to fibronectin-coated gels. A7 cells alter their stiffness, as measured by atomic force microscopy, to match the elastic modulus of the substrate immediately adjacent to them on the gradient. M2 cells, in contrast, maintain a constant stiffness on all substrates that is as low as that of A7 cells on the softest gels achievable (1000 Pa). By contrasting the responses of these cell types to different adhesive substrates, cell spreading can be dissociated from stiffening. [Preview Abstract] |
Monday, March 16, 2009 8:12AM - 8:24AM |
A39.00002: Substrate Induced Osteoblast-Like Differentiation of Stromal Stem Cells Jacqueline Belizar, Reena Glaser, Matthew Hung, Marcia Simon, Vladimir Jurukovski, Miriam Rafailovich, Alice Shih We have demonstrated that Adipose-derived stem cells (ASCs) can be induced to biomineralize on a polybutadiene (PB) coated Si substrate. The cells began to generate calcium phosphate deposits after a five-day incubation period in the absence of dexamethasone. Control cells plated on tissue culture PS culture dish (TCP) did not biomineralize. In addition, the biomineralizing culture retained proliferative cells In order to determine whether the induction was transient, we transferred the cells exposed to polybutadiene after 14 and 28-day incubation periods to TCP dishes. These cells continued to biominerlize. Genetic testing is underway which will determine whether differentiation is maintained after transfer. [Preview Abstract] |
Monday, March 16, 2009 8:24AM - 8:36AM |
A39.00003: Mechanical anisotropy of viscoelasticity in biological cells Ming-Tzo Wei, H.D. Ou-Yang The mechanism that biological cells use to remodel their cytoskeletal structure in response to external stress is unclear. Experimental observations suggest that the cells remodel their skeleton in a manner that is directionally responsive to the external stress. In order to understand these directional responses, we developed a method to measure the rheological response of the cell in orthogonal directions simultaneously. To achieve controlled stimulation and detection, we used a dual-beam optical tweezer, which used a pump and probe scheme to measure the storage and loss modulus of the cellular cytoskeleton. The pump was used to manipulate extracelluar micro-particles which were attached to the actin cytoskeleton through trans-membrane integrin alpha receptors. By measuring two independent regions of the cell, we were able to generate a localized mechanical stress on the outer surface of the cell while observing the directionally specific inside response. [Preview Abstract] |
Monday, March 16, 2009 8:36AM - 8:48AM |
A39.00004: Traction forces associated with shape changes in migrating amoeboid cells Baldomero Alonso-Latorre, Juan C. del Alamo, Effie Bastounis, Rudolph Meili, Richard Firtel, Juan C. Lasheras Amoeboid motility results from the repetition of a repertoire of shape changes (motility cycle). We studied the dominant changes and their relation to the activity and localization of cytoskeletal proteins by applying Principal Component Analysis (PCA) to measurements of cell shape, traction forces and F-actin concentration in migrating\textit{ Dictyostelium} cells. Using wild-type cells (\textit{wt}) as reference, we investigated myosin II activity by studying myosin II-null (\textit{mhc-}) and essential light chain-null cells (\textit{mlc-}). Only three PCA modes are enough to represent 67{\%} of the variance of cell area: dilation/elongation, a half-moon shape and a bulging of the front/back. These modes are similar for \textit{wt, mlc-} and \textit{mhc-} but they are implemented more slowly in \textit{mhc-}. The second mode, which represents sideways protrusion/retraction and is associated to lateral asymmetry in the traction forces, is significantly less important in \textit{mhc-}. These results suggest that migration speed decreases in the absence of myosin II due to a reduced control on the spatial organization of the cell stresses. [Preview Abstract] |
Monday, March 16, 2009 8:48AM - 9:00AM |
A39.00005: Imaging spatiotemporal redistribution of cellular traction stresses during fibroblast migration on a physiologically relevant ECM mimic Zhi Pan, Yajie Liu, Kaustabh Ghosh, Dhruv Nandamudi, Danny Stemp, Toshio Nakamura, Richard Clark, Miriam Rafailovich To better understand the dynamics of cell migration, we measured the spatiotemporal redistribution of cellular traction stresses during fibroblast migration at a submicron level and correlated it with nuclear translocation on a physiologically relevant ECM mimic. We found that nuclear translocation occurred in pulses whose magnitude was larger on the low ligand density surfaces (LLDS) than on the high ligand density surfaces (HLDS). Large nuclear translocations only occurred on LLDS when the rear traction forces completely relocated to a posterior nuclear location, while such relocation took much longer time on HLDS, probably due to the greater magnitude of traction forces. Our results suggest that the reinforcement of the traction forces around the nucleus is a critical step during fibroblast migration, serving as a speed regulator, which must be considered in any dynamic molecular reconstruction model of tissue cell migration. A traction gradient foreshortening model was proposed to explain how the relocation of rear traction forces leads to pulsed fibroblast migration. [Preview Abstract] |
Monday, March 16, 2009 9:00AM - 9:12AM |
A39.00006: Achieving in-vitro axonal polarization~by using micro-patterns Sophie Roth, Jacques Brocard, Sylvie Gory-Faure, Catherine Villard Our project is based on the elaboration of in vitro neuron networks as simplified models to explore the relation between neuronal architecture and biological function. Beyond a control of soma and neurite position, our first goal was to achieve in-vitro axonal differentiation of embryonic E18 hippocampal mice neurons by the mean of geometrical growth constraints, i.e. by the use of adhesive micro-patterns on silanized glass substrates. Such a process thus excludes chemical guidance or specific adhesion mechanisms. This study explores two different types of geometrical constraints. The first one, based on the centrosome role and localization, is applied to the soma, and force a choosen neurite to differentiate into an axon with 39{\%} of efficacy (N= 160 cells, 3 different cultures). The second one derives from the suggested relationship between neurite mechanical tension and axonal differentiation, and is based on the design of wavy neurite's shape. Its efficacy reach 0.51{\%} (N= 300 cells, 3 different cultures). The combinaison of these two constraints into a final pattern yields an efficacy of 82{\%} (N= 83 cells, 2 different cultures). These results not only provide an important tool for creating neural model networks but also point out an important role of intrinsic neurite tension during axon differentiation. [Preview Abstract] |
Monday, March 16, 2009 9:12AM - 9:24AM |
A39.00007: Matrix rigidity optimizes the polarization of stem cells Assaf Zemel, Florian Rehfeldt, Andre Brown, Dennis Discher, Samuel Safran We present a theoretical model and experiments to explain the non-monotonic dependence of stress-fiber polarization in stem cells on matrix rigidity. The theory generalizes the treatment of elastic inclusions to ``living'' inclusions (cells) whose active polarizability, unlike non-living matter, depends on the feedback of cellular forces that develop in response to matrix stresses. We demonstrate experimentally that the stress fibers in adult mesenchymal stem cells, generally orient parallel to the long axis of the cells, with an anisotropy that depends non-monotonically on substrate stiffness. Consistent with these experiments, our theory predicts that the magnitude of the cellular force increases monotonically with the matrix rigidity while the polarization anisotropy shows a maximum that depends on the cell shape and the elastic modulus of the medium. These findings offer a mechanical correlate for the observation that stem cell differentiation optimizes in a range of matrix rigidities that depends on the tissue type. [Preview Abstract] |
Monday, March 16, 2009 9:24AM - 9:36AM |
A39.00008: Structure at the Leading Edge D. A. Quint, J. M. Schwarz, M. C. Marchetti The leading edge of a crawling cell is propelled forward by a polymerizing network of branched actin filaments. This emergent structural array seems to be rigid enough to support and push against the cell membrane within the appropriate time scales under which cell motility can be realized. We seek to understand how such a network can optimize its structure to generate the rigidity required, particularly focusing on the role of branching in the network. For isolated elastic beams, which model semiflexible polymers, the critical buckling load is enhanced when branched supports are included. Therefore, we conjecture that an optimal branching angle is found by looking at the competition between branching providing collective structural support, which results in polymerization with a component perpendicular to the direction of motion, and polymerization along the direction of motion. To partially test this conjecture, we simulate a directed, branched network in the absence of forces. Preliminary results indicate a lower bound on the optimal branching angle of approximately 40 degrees (to be compared with the observed 70 degree branching angle). Studies of a directed, branched network with forces will also be addressed. [Preview Abstract] |
Monday, March 16, 2009 9:36AM - 9:48AM |
A39.00009: Chemotactic strategy of Vibrio alginolyticus studied with an optical trap Suddhashil Chattopadhyay, Tuba Altindal, Xie Li, Xia-Lun Wu The canonical ``run'' and ``tumble'' mode of chemotaxis, employed by multiple flagellated cells such as \emph{Escherichia coli, }has been studied in great detail over the years. In this work we will demonstrate the usage of an optical trap for studying the chemotaxis of cells belonging to the single flagellated strain of \emph{Vibrio alginolyticus. }This method allows precise and direct observation of chemotactic response, while the cell is exposed to various chemical signals (positive/negative gradient or no chemicals). We have studied the response of the flagellar motor with a precise control on the input signal (chemical gradient), such that a cell can be forced to move up or down a chemical gradient, a control which is not attainable for free swimming cells. The optical trap does not restrict the rotational motion of the bacterium and allows the state of the motor (clockwise or counter clockwise rotation) to be monitored continuously. Our group has recently observed an active flagellar movement (called the ``flagellar flick'') that is used by cells of \emph{V. algninolyticus} for randomizing swimming direction during chemotaxis. We consider this mode in addition to ``back and forth'' swimming employed by these cells. A modified chemotactic strategy is proposed and tested using the optical trap. [Preview Abstract] |
Monday, March 16, 2009 9:48AM - 10:00AM |
A39.00010: Chemotaxis in Marine Bacterium Vibrio alginolyticus Li Xie, Suddhashil Chattopadhyay, Tuba Altindal, Xiao-Lun Wu We investigated swimming behavior of marine bacterium \emph{Vibrio alginolyticus} in an uniform chemical environment. The typical bacterial trajectory consists of consecutive run (forward swimming) and reverse (backward swimming) intervals with occasional sudden changes of swimming directions, which we call \textquotedblleft{}flagellar flicks\textquotedblright{}. This mode of chemotaxis is different from the canonical run-and-tumble strategy adopted by \emph{Escherichia coli} and may be selected for in \emph{V. alginolyticus} due to the ocean environment where nutrients are scarce and are subject to rapid turbulent dispersion. We measured the statistical distributions of run $T_{run}$ and revers $T_{rev}$ time intervals, $P(T_{run})$ and $P(T_{rev})$, and found that while the back-swimming time appears to have a well-defined time scale of $0.5\, s$, the forward swimming time is more broadly distributed, suggestive of a Poisson process. Measurements of the time interval $T_{flick}$ between two consecutive directional changes show that $P(T_{flick})$ is also peaked at a finite time, $T_{flick}\sim1\, s$, and the mean directional change is $\Delta\theta\sim70\,^{0}$. Interestingly, this $\Delta\theta$ observed is nearly optimal for efficient randomization of swimming directions. Altogether, our experiments suggest that \emph{V. alginolyticus }employs both run-and-reverse and flicking activities for chemotaxis, and this behavior presumably optimizes their foraging efficiency in a turbulent environment. [Preview Abstract] |
Monday, March 16, 2009 10:00AM - 10:12AM |
A39.00011: The flagellar ``flick'': direction randomization in single falgellated cells of V. alginolyticus Tuba Altindal, Suddhashil Chattopadhyay, Li Xie, Xiao-Lun Wu Single flagellated bacteria such as \emph{Vibrio alginolyticus}, which possess a single flagellum are believed to be unable to use an active mechanism for direction randomization, and thus follow a {}``back'' and {}``forth'' swimming pattern. Inspired by the observation that \emph{V. alginolyticus }cells were able to rapidly accumulate around a point source of a chemoattractant, we have identified a previously unknown phenomenon in which an active movement of the flagellum is used to randomize the swimming direction. Fluorescently labelled cells clearly demostrated that bending of the flagellum is responsible for imparting direction changes to the cell body. Clues obtained from high speed video, bright-field microscopy and fluorescent imaging suggests a series of steps involved in the flagellar {}``flick''. An investigation of the energetics of the proposed mechanism leads to the conclusion that the directional change may be connected to the flagellar motor, which normally propels the cell body. [Preview Abstract] |
Monday, March 16, 2009 10:12AM - 10:24AM |
A39.00012: Probing the Dynamics of Cellular Traction Forces with Magnetic Micropost Arrays Corinne Kramer, Christopher Chen, Daniel Reich Mechanical forces on living cells are associated with changes in cellular function. For example, vascular smooth muscle cells are known to undergo a mechanical feedback response to increased stress, which can result in atherosclerosis. We have recently developed a magnetic micropost array, a novel device for measuring cellular traction forces that simultaneously enables the application of localized external forces to cells. The device consists of an array of micrometer scale elastomeric posts that act as force sensors for cells cultured on their tips. An external force is applied to the adherent surface of a cell via a magnetic torque on a cobalt nanowire embedded in a single post. Initial results showed an active and non-local cellular response to applied forces in mouse fibroblast cells.\footnote{N. Sniadecki, et. al, \emph{Proc Natl Acad Sci}, {\bf 104}, 14553 (2007).} We will present results on the spatially resolved dynamics of traction forces exerted by smooth muscle cells over time in response to constant and time-varying stimulation. The observation of biochemical and mechanical regulation of the subcellular redistribution of force may allow insights into cellular mechanotransduction and control of cell function. [Preview Abstract] |
Monday, March 16, 2009 10:24AM - 10:36AM |
A39.00013: Differential cellular response to linear and strain-stiffening hydrogel substrates Jessamine P. Winer, Shaina A. Oake, Bethany C. Baumann, Paul A. Janmey Many cell types act as tensiometers, modulating their spread area, motility, and protein expression in response to the substrate stiffness. Studies of stiffness sensing typically employ linear elastic materials whose stiffness is independent of the applied strain. Biological gels, however, often stiffen in response to increasing strain. Fibroblasts and mesenchymal stem cells adherent to linearly elastic gels typically display a small, round phenotype on soft substrates and increase spread area as the elastic modulus of their substrate increases. On the strain-stiffening biopolymer gel fibrin, the same cell types are maximally spread even when the gel's low strain elastic modulus would predict a round morphology. Traction microscopy reveals that cells apply active displacements of several microns up to five cell lengths away, and atomic force microscopy shows that these displacements locally stiffen the gel by deforming it beyond its linear range. The magnitude of cell-applied strains is inversely related to the gel's low strain elastic modulus and results in long distance cell-cell communication and alignment. [Preview Abstract] |
Monday, March 16, 2009 10:36AM - 10:48AM |
A39.00014: Elasticity of the eye's crystalline lens: A Brillouin light scattering study. S. Bailey, J. Gump, R. Sooryakumar, C. Jayaprakash, M.S. Venkiteshwar, M. Bullimore, M. Twa Focusing the eye on a near object results in an increase in its optical power brought about by contraction of the ciliary muscles and an increase in the lens surface curvature. Distant vision occurs when the muscular force flattens the lens. Central to the ability of the lens to alter shape are its mechanical properties. Thus, given that hardening of the lens would impede deformation and reduce its ability to undergo the changes required for accommodation, a noninvasive approach to measure the elastic properties of the lens is valuable. We present results of Brillouin scattering from bovine and human lenses (from the organ donor program at The Ohio State University) that measure their high frequency acoustic response. These measurements are conducted with a few milli-watts of laser power and, in the case of bovine lenses, from entire intact eye globes, allow the stiffness of the lens to be mapped across its cross-section. The results will be compared to values of the shear- and bulk-moduli determined from other techniques and the implications of differences in these moduli discussed. [Preview Abstract] |
Monday, March 16, 2009 10:48AM - 11:00AM |
A39.00015: Physical Description of Mitotic Spindle Orientation During Cell Division Andrea Jim\'enez-Dalmaroni, Manuel Th\'ery, Victor Racine, Michel Bornens, Frank J\"ulicher During cell division, the duplicated chromosomes are physically separated by the action of the mitotic spindle. The spindle is a dynamic structure of the cytoskeleton, which consists of two microtubule asters. Its orientation defines the axis along which the cell divides. Recent experiments show that the spindle orientation depends on the spatial distribution of cell adhesion sites. Here we show that the experimentally observed spindle orientation can be understood as the result of the action of cortical force generators acting on the spindle. We assume that the local activity of force generators is controlled by the spatial distribution of cell adhesion sites determined by the particular geometry of the adhesive substrate. We develop a simple physical description of the spindle mechanics, which allows us to calculate the torque acting on the spindle, as well as the energy profile and the angular distribution of spindle orientation. Our model accounts for the preferred spindle orientation, as well as the full shape of the angular distributions of spindle orientation observed in a large variety of pattern geometries. M. Th\'{e}ry, A. Jim\'{e}nez-Dalmaroni, et al., Nature 447, 493 (2007). [Preview Abstract] |
Session A40: Proteins: Structure and Functions I
Sponsoring Units: DBPChair: Ching-Hwa Kiang, Rice University
Room: 412
Monday, March 16, 2009 8:00AM - 8:12AM |
A40.00001: Free energy landscapes of short peptide chains using adaptively biased molecular dynamics Vadzim Karpusenka, Volodymyr Babin, Christopher Roland, Celeste Sagui We present the results of a computational study of the free energy landscapes of short polypeptide chains, as a function of several reaction coordinates meant to distinguish between several known types of helices. The free energy landscapes were calculated using the recently developed adaptively biased molecular dynamics method followed up with equilibrium ``umbrella correction'' runs. Specific polypeptides investigated include small chains of pure and mixed alanine, glutamate, leucine, lysine and methionine (all amino acids with strong helix-forming propensities), as well as glycine, proline(having a low helix forming propensities), tyrosine, serine and arginine. Our results are consistent with the existing experimental and other theoretical evidence. [Preview Abstract] |
Monday, March 16, 2009 8:12AM - 8:24AM |
A40.00002: Structural and functional allostery wiring diagrams in GroEL/GroES Riina Tehver, Jie Chen, D. Thirumalai Repeated cycling between distinct allosteric states is required for the functions of numerous biological nanomachines. Determining the specific residues that are responsible for transmitting allosteric signals is needed to understand their operation. Using structural perturbation analysis and evolutionary correlations of mutations of residues, we determine networks of key residues in molecular chaperonin GroEL and its cochaperonin GroES. GroEL is a molecular machine that rescues aggregation-prone misfolded proteins. Its functional cycle consists of a series of large-scale allosteric transitions between the T, R, R' and R'' states. The corresponding structural rearrangements facilitate substrate protein capture, refolding, and release. The networks of residues we find provide a microscopic foundation for the cooperativity of the allosteric transitions and a linkage between substrate protein binding and ATPase activity of GroEL. [Preview Abstract] |
Monday, March 16, 2009 8:24AM - 8:36AM |
A40.00003: Decoupling of Protein Dynamics from the Solvent Viscosity Sheila Khodadadi, Armistead Kathleen, Alexei Sokolov Studies show that solvent viscosity has a strong influence on protein dynamics and activity, but the detailed mechanism of the solvent-protein interactions is not fully understood. Using dielectric spectroscopy we were able to identify a protein related relaxation process of myoglobin in water-glycerol and water-sucrose solutions. We demonstrate that the rate of biochemical reaction (taken from literature$^{1})$ follows the protein related relaxation observed in dielectric spectra. Also our results reveal decoupling of protein dynamics from solvent viscosity. This finding explains the known in literature decoupling of protein activity from solvent viscosity and demonstrates direct connection between protein dynamics and its functionality. Possible microscopic mechanisms of this decoupling are discussed at the end. 1. Kleinert, T.; \textit{et al}. \textit{Biochemistry}\textbf{ 1998}, $37$, 717. [Preview Abstract] |
Monday, March 16, 2009 8:36AM - 8:48AM |
A40.00004: Phosphorylation of histone H3 Thr 118 converts nucleosomes into a higher-mass complex Justin North, Michael Poirier, Michelle Ferdinand, Jennifer Ottesen The nucleosome is the fundamental unit of DNA compaction in eukaryotes by which 147 base pairs of DNA wrap 1.7 times around a protein complex called the histone octamer. Numerous chemical modifications are found in vivo that alter octamer surface charge and shape. One such modification is phosphorylation of histone H3 residue Thr 118 located in the dyad region of the nucleosome. We find that phosphorylated H3 T118 (H3 pT118) octamer, when reconstituted with DNA of about 200bp, suppresses nucleosome formation and promotes formation of a higher-mass DNA-protein complex. Coordinately, dephosphorylation of H3 pT118 octamer by phosphatase results in reconstitution of normal nucleosomes. DNAse I foot printing reveals that while DNA contacting the octamer surface in nucleosomes is less accessible than free DNA, the entire DNA strand is equally accessible in the higher-mass complex and is digested at one-third the rate of free DNA. [Preview Abstract] |
Monday, March 16, 2009 8:48AM - 9:00AM |
A40.00005: The Role of gp120 Flexibility in Binding A.J. Rader Current treatment of the human immunodeficiency virus (HIV) focuses on delivering several drugs to to a few specific viral protein targets. A complementary antiviral therapy involves targeting the process of viral entry. Viral entry is a dynamic process which involves a series of conformational changes by the HIV envelope glycoproteins (gp120 and gp41). The extraordinary conformational flexibility, glycosylation and strain variability of these proteins complicate the development of an effective vaccine. We present results from the graph theoretical analysis of flexibility and rigidity using the Floppy Inclusion and Rigid Substructure Topography (FIRST) software for all known HIV-1 gp120 structures. Comparisons between structures using this mechanical stability and intrinsic flexibility is used to identify a consensus rigid region that might serve as drug targets in a pre-complex conformation. Furthermore, analysis of structures with various binding partners illustrates the differential partitioning of mechanical flexibility and strain. We relate these differences in mechanical stability to thermodynamic differences in binding and stabilizing mutations. [Preview Abstract] |
Monday, March 16, 2009 9:00AM - 9:12AM |
A40.00006: AP helical stability in salt solutions Eliana Asciutto, Kan Xiong, Sanford Asher, Jeffry Madura Protein dynamics depends on the environment and the inclusion of salts in the simulation of folding/unfolding becomes extremely necessary when comparing energy barriers or reaction rates with experimental results. The aim of this study is to investigate the effects of three sodium salts: $NaClO_4$,$NaCl$ and $Na_2 SO_4$ on the helical stability of AP, a mainly alanine peptide. The dependence of the peptide helical stability on the environment has been studied using Replica Exchange Molecular Dynamics (REMD) simulations, Circular Dichroism (CD) and Ultraviolet Raman Resonance Spectroscopy (UVRS) experiments. It was found that $NaClO_4$ solution strongly stabilizes the helical states and that the order in which sodium salts stabilize the peptide helical states follows a reverse Hofmeister Series ($ClO_4^- < Cl^- < SO_4^{2-})$. Another interesting result found is that $ClO_4^- $ ions are attracted to the backbone; $Cl^-$ ions are repelled while $SO_4^{2-} $ ions are attracted to the positive side chains. A thorough investigation of the ion effects on the first and second solvation water along with the Kirkwood-Buff theory for solutions allowed us to explain the physical mechanisms involved in the observed ion specific effects. [Preview Abstract] |
Monday, March 16, 2009 9:12AM - 9:24AM |
A40.00007: Computational modeling of protein folding assistance by the eukaryotic chaperonin CCT Manori Jayasinghe, George Stan Chaperonins are biological nanomachines that promote protein folding using energy derived from ATP hydrolysis. Structurally, chaperonins are large oligomeric complexes that form double-ring construct, enclosing a central cavity that serves as folding chamber. Our focus is on the substrate binding mechanisms of the Eukaryotic chaperonin CCT and Archaeal chaperonin Thermosome. We contrast our results with the annealing action of the bacterial chaperonin GroEL of \textit{E. coli.}, currently the best studied for chaperonin machinery. CCT was suggested to be more selective towards the substrate recognition where as GroEL is more promiscuous due to the hydrophobic interactions. We study the interaction of CCT with Tubulin, one of its stringent substrates. Using molecular docking and molecular dynamics simulations, we probe binding of a $\beta$tubulin peptide (205-274) to the CCT$\gamma$ apical domain. We identify a versatile binding mechanism, involving mostly hydrophobic interactions with the helical region and electrostatic interactions with the helical protrusion region. This specific substrate-protein recognition mechanism is likely to be optimized for specific substrate protein-CCT subunit pairs. [Preview Abstract] |
Monday, March 16, 2009 9:24AM - 9:36AM |
A40.00008: Substrate protein recognition mechanism of archaeal and eukaryotic chaperonins. Pooja Shrestha, Manori Jayasinghe, George Stan Chaperonins are double ring-shaped biological nanomachines that assist protein folding. Spectacular conformational changes take place within each chaperonin ring using energy derived from ATP hydrolysis. These changes result in transitions from the open to the closed ring. Substrate proteins bind to the open ring and are encapsulated within the closed ring cavity. We focus on the substrate protein recognition mechanism of archaeal and eukaryotic chaperonins. We predict substrate protein binding sites using structural and bioinformatic analyses of functional states during the chaperonin cycle. Based on large changes in solvent accessible surface area and contact maps we glean the functional role of chaperonin amino acids. During the transition between open to closed chaperonin ring, the largest change in accessible surface area of amino acids is found in helical protrusion and two helices located at the cavity opening. Our calculations suggest that the helical protrusion and two helices constitute the substrate protein binding site. [Preview Abstract] |
Monday, March 16, 2009 9:36AM - 9:48AM |
A40.00009: Terahertz Dielectric Response for Cytochrome C Yunfen He, Jing-Yin Chen, Wenjun Zheng, Andrea Markelz Previously we demonstrated a large contrast in the terahertz dielectric response between oxidized and reduced cytochrome c and associated this difference with a change in the collective structural motions associated with protein flexibility (Chen et al. Phys. Rev. E Rapid 72, 040901 (2005).) We present calculations of the terahertz dielectric response of cytochrome c as a function of oxidation state and hydration. Molecular dynamics simulations are performed to equilibriate water content. Quasiharmonic analysis and dipole-dipole correlation analysis is performed after equilibriating the system. We compare the calculated results with the measurements to determine the relative contribution of correlated motions and diffusive motions to the measured dielectric response and how these two different contributions give rise to the observed large oxidation dependence and hydration dependence. [Preview Abstract] |
Monday, March 16, 2009 9:48AM - 10:00AM |
A40.00010: Micro-Spectroscopy of Proteins and Cells at Variable Pressure in a Micro-Capillary Sang Hoon Park, Silki Arora, Alfons Schulte Combining Raman microscopy with a micro-capillary compartment enables spectroscopic studies of small amounts of biological material at variable pressure. We present experiments employing a variety of optical probes over the pressure range from atmospheric pressure to 4 kBar in a micro-capillary which uses less than 100 nanoliters of sample. We investigate pressure effects on the Raman spectrum of poly(L-glutamic acid) and proteins in solution. A shift of the amide I band in poly(L-glutamic acid) to lower frequency with pressure may suggest significant change in secondary structure towards a-helical conformation. The micro-capillary also allows to enclose living cells and to optically interrogate them through a microscope. This is demonstrated by Raman spectroscopy of individual red blood cells. [Preview Abstract] |
Monday, March 16, 2009 10:00AM - 10:12AM |
A40.00011: Cross-correlated TIRF/AFM reveals asymmetry in self-assembled Myosin filaments - a Dyck paths model of asymmetry and implications for Motility Andre Brown, Alina Hategan, Daniel Safer, Yale Goldman, Dennis Discher Myosin-II's rod-like tail drives filament assembly with a head arrangement that should generate equal and opposite contractile forces on actin -- if one assumes that the filament is a symmetric bipole. Self-assembled myosin filaments are shown here to be asymmetric in physiological buffer based on cross-correlated images from both atomic force microscopy (AFM) and total internal reflection fluorescence (TIRF). Quantitative cross-correlation of these orthogonal methods produces structural information unavailable to either method alone in showing that fluorescence intensity along the filament length is proportional to height. This implies that myosin heads form a shell around the filament axis, consistent with F-actin binding. A motor density of $\sim$50-100 heads/micron is further estimated but with an average of 32\% more motors on one half of any given filament compared to the other, regardless of length. A purely entropic pyramidal lattice model is developed and mapped onto a Dyck path problem that qualitatively captures this lack of length dependence and the distribution of filament asymmetries. Such strongly asymmetric bipoles are likely to produce an imbalanced contractile force in cells and in actin-myosin gels, and thereby contribute to motility as well as cytoskeletal tension. [Preview Abstract] |
Monday, March 16, 2009 10:12AM - 10:24AM |
A40.00012: A new view on the role of intrinsic protein disorder Jintao Liu, James Faeder, Carlos Camacho Recent studies have found that many proteins do not have stable structure by themselves, i.e., are intrinsically disordered, which challenges the conventional view that structure determines protein function and interaction. We have analyzed the Human Protein Reference Database with the VSL2 protein disorder predictor, and find that the amount of disorder in a protein is the result of evolutionary pressure: catalytic proteins interact with substrates rapidly and highly specifically and thus exhibit low levels of disorder; transcription regulators often slide along DNA, which favors flexible or disordered structures; binding proteins have affinities that depend weakly on folding stability, and thus have a broad disorder distribution. Finally, our findings suggest that sequence/structural features such as phosphotyrosine are better indicators of multiple protein-protein interactions than disorder. [Preview Abstract] |
Monday, March 16, 2009 10:24AM - 10:36AM |
A40.00013: Structure-Based Prediction of Unstable Regions in Proteins: Applications to Protein Misfolding Diseases Will Guest, Neil Cashman, Steven Plotkin Protein misfolding is a necessary step in the pathogenesis of many diseases, including Creutzfeldt-Jakob disease (CJD) and familial amyotrophic lateral sclerosis (fALS). Identifying unstable structural elements in their causative proteins elucidates the early events of misfolding and presents targets for inhibition of the disease process. An algorithm was developed to calculate the Gibbs free energy of unfolding for all sequence-contiguous regions of a protein using three methods to parameterize energy changes: a modified G\={o} model, changes in solvent-accessible surface area, and solution of the Poisson-Boltzmann equation. The entropic effects of disulfide bonds and post-translational modifications are treated analytically. It incorporates a novel method for finding local dielectric constants inside a protein to accurately handle charge effects. We have predicted the unstable parts of prion protein and superoxide dismutase 1, the proteins involved in CJD and fALS respectively, and have used these regions as epitopes to prepare antibodies that are specific to the misfolded conformation and show promise as therapeutic agents. [Preview Abstract] |
Monday, March 16, 2009 10:36AM - 10:48AM |
A40.00014: Catching the cold: Can computational modeling explain the physical mechanisms behind cold denaturation Cristiano Dias Proteins assume a unique three-dimensional structure under physiological conditions. This structure becomes gradually unstable as temperature is raised or lowered. At about 60$^{\circ}$C the ordered structure of proteins becomes unstable. This phenomenon is called denaturation and is also observed at low temperatures, around -20$^{\circ}$C. While denaturation at high temperature is well understood, the mechanism behind denaturation at low temperature, i.e. cold denaturation, is still controversial. This mechanism depends strongly on the properties of the solvent in which the protein is immersed, i.e. water. In this talk, I will discuss a simplified model for water that we have recently proposed in the literature (to be published) and a microscopic mechanism for cold denaturation (Phys. Rev. Lett. 100, 118101 (2008) ). [Preview Abstract] |
Monday, March 16, 2009 10:48AM - 11:00AM |
A40.00015: Exact analytical solution in the Extended Zwanzig Model (EZM) for linear protein denaturation Luis Olivares-Quiroz The elucidation of the physical mechanisms underlying protein's folding and unfolding in a variety of physico-chemical conditions is one of the most challenging problems faced by molecular biology and biophysics. The \textit{Extended Zwanzig Model} (EZM) is a formalism that relates protein denaturation profiles with the energy spectrum \{$E_k$\} accessible to the system. In this work, an exact analytical solution for the EZM in the case of a protein following a linear denaturation profile as function of a reactive coordinate $\mathcal{C}$ is presented. The relevance for this solution is two fold. Given the complex functional form followed by the energy spectrum \{$E_k$\} in terms of the reactive coordinates, there is a lack of analytical solutions for the MEZ even in the most simplest cases. On the other hand, it is known that most proteins exhibit a sigmoidal denaturation pattern in terms of physical variables like temperature, pressure or concentration of chemical compounds. It is shown here that the sigmoidal denaturation profile can be approximated by a sum of linear terms and therefore, an approximate solution for the general denaturation profile can be generated from a superposition of exact linear cases. [Preview Abstract] |
Session A41: Heavy Fermions
Sponsoring Units: DMPChair: Rena Zieve, University of California, Davis
Room: 413
Monday, March 16, 2009 8:00AM - 8:12AM |
A41.00001: Realistic simulation of Kondo lattice model: application to Cerium compounds Munehisa Matsumoto, Myung Joon Han, Junya Otsuki, Sergey Savrasov In order to do a good description of heavy fermion materials at very low temperatures, we simulate the Kondo lattice model with the continuous-time quantum Monte Carlo method recently developed by one of the authors [1] combined with the conduction-electron density of states given by first-principle calculations and performing self consistency using dynamical mean field theory. We discuss our results for Cerium compounds down to the temperature range of O(1) [K] using realistic values of crystal-field and spin-orbit level splitting. [1] J. Otsuki, H. Kusunose, P. Werner, Y. Kuramoto, J. Phys. Soc. Jpn. 76, 114707 (2007). [Preview Abstract] |
Monday, March 16, 2009 8:12AM - 8:24AM |
A41.00002: Layered Kondo lattice model for quantum critical superconductor $\beta$-YbAlB$_4$ Andriy Nevidomskyy, Piers Coleman We perform a theoretical analysis of the magnetic and electronic properties of the quantum critical heavy fermion superconductor $\beta$-YbAlB$_4$. Using a combination of the realistic material modelling and single-ion crystal field analysis, we propose a layered Kondo lattice model for this system, in which two dimensional boron layers are Kondo-coupled via interlayer Yb moments in a $J_{z} = \pm 5/2$ state. This model fits the measured single ion magnetic susceptibility and predicts a substantial change in the electronic anisotropy as the system is pressure-tuned through the quantum critical point. An interesting connection is made between this model and the Kondo effect in Coulomb-blockaded quantum dots. We also calculate the Fermi surface and the angular dependence of the extremal orbits relevant to the de Haas--van Alphen measurements. [Preview Abstract] |
Monday, March 16, 2009 8:24AM - 8:36AM |
A41.00003: Interplay of Composite Pairs and Magnetism in Heavy Fermion Superconductors Rebecca Flint, Piers Coleman Superconductivity in Pu{\emph M}Ga$_5$, {\emph M} = \{Co,Rh\} and NpPd$_5$Al$_2$ can be treated within a two channel Kondo lattice model, where the electron-spin scattering develops an Andreev component, creating a composite bound state of a spin-flip and a pair of electrons. We extend this model to Ce{\emph M}In$_5$, where magnetism and superconductivity exist in close proximity by including antiferromagnetic interactions. Different crystal symmetries lead to composite pairing with either a g-wave gap or d-wave gap, while antiferromagnetism leads to RVB superconductivity with a d-wave gap. Within a symplectic large $N$ limit, we examine the effects of antiferromagnetic interactions on the composite pairing. If both gaps are d-wave, they couple linearly, mutually enhancing the superconducting transition temperature. [Preview Abstract] |
Monday, March 16, 2009 8:36AM - 8:48AM |
A41.00004: Very low temperature specific heat and magnetoresistance of PrOs$_{4}$Sb$_{12}$. Bohdan Andraka Heavy fermion character of PrOs$_{4}$Sb$_{12}$ has been concluded based on its superconducting properties, such as large values of the discontinuity in C and upper critical field slope at T$_{c}$ (1.85 K). On the other hand, normal state properties do not provide any strong evidence of the heavy fermion behavior. In particular, m* enhancement measured by the de Haas van Alphen technique in overcritical fields and below 700 mK is typical of transition metals, suggesting that the heavy fermion state is either suppressed by magnetic fields or collapses at low temperatures. We reexamine these possible scenarios using new magnetoresistance and specific heat, down to 50 mK, data. [Preview Abstract] |
Monday, March 16, 2009 8:48AM - 9:00AM |
A41.00005: Effect of ferromagnetism on unconventional superconductivity in the Pr$_{1-x}$Nd$_x$Os$_4$Sb$_{12}$ system P.-C. Ho, M. B. Maple, T. Yanagisawa, W. M. Yuhasz, N. P. Butch, A. A. Dooraghi, C. C. Robinson The filled skutterudite compound PrOs$_4$Sb$_{12}$ is a 1.85\,K heavy fermion superconductor (SC), which displays unconventional SC property, such as existence of multiple SC phases and point nodes in the SC energy gap and the appearance of internal magnetic field in its SC state. NdOs$_4$Sb$_{12}$ is a mean-field type magnet with a low Curie temperature $\sim 1$\,K. The Nd substitution in the Pr$_{1-x}$Nd$_x$Os$_4$Sb$_{12}$ has been carried out in order to investigate the ferromagnetic (FM) effect on the unconventional SC of PrOs$_4$Sb$_{12}$ and the quantum critical behavior in this system. SC state in this system disappears near $x \sim$ 0.55 and FM extends into the SC region. The x dependence of the 0-K extrapolated upper critical field $H_{c2}(x,T=0)$ has curvature breaking at $x \sim 0.3$. The $H_{c2}(x,T=0)$ data can be analyzed by multiple pair breaking effect due to magnetic field, appearance of impurity, and the exchange field generated by magnetic ions. However, when $H_{c2}(x,T=0)$ of Pr$_{1-x}$Nd$_x$Os$_4$Sb$_{12}$ is compared to that of a BCS SC La$_{3-x}$Gd$_x$In, the curvature in the regime where $x/x_{cr} < 0.5$ is significantly different. Detail analysis and comparison will be discussed. Research at CSU-Fresno supported by RC CCSA $\#$7669 and Fresno State start-up fund; at UCSD by US DOE and NSF; at Hokkaido U by MEXT, Japan. [Preview Abstract] |
Monday, March 16, 2009 9:00AM - 9:12AM |
A41.00006: The role of $f$-electrons at the Fermi surface of the heavy fermion superconductor $\beta$-YbAlB4 Eoin O'Farrell, D.A. Tompsett, S.E. Sebastian, N. Harrison, C. Capan, L. Balicas, K. Kuga, T. Matsuo, M. Tomonaga, S. Nakatsuji, G. Cs\'anyi, Z. Fisk, M.L. Sutherland We present a detailed quantum oscillation study of the fermi surface of the recently discovered Yb-based heavy fermion superconductor $\beta$-YbAlB4. We compare the data, obtained at fields from 10 to 45 Tesla, to band structure calculations performed using the local density approximation. Analysis of the data suggests that the $f$-electrons are delocalized and contribute to the fermi volume at all fields. We comment on the significance of these findings for the observed quantum critical and superconducting properties of this material. [Preview Abstract] |
Monday, March 16, 2009 9:12AM - 9:24AM |
A41.00007: Superconducting Properties of the Non-Fermi-Liquid System $\beta $-YbAlB$_{4}$. Kentaro Kuga, Yoshitomo Karaki, Yosuke Matsumoto, Yo Machida, Naoki Horie, Satoru Nakatsuji {\$}$\backslash $beta{\$}-YbAlB{\$}{\_}4{\$} is the first Yb-based heavy fermion superconductor with the transition temperature of {\$}T{\_}{\{}$\backslash $rm c{\}} = 80{\$} mK [1]. Our study using high-purity single crystals indicates that the clean limit superconductivity directly emerges from the pronounced non-Fermi-liquid state [2]. The critical fields are anisotropic and probably have a paramagnetic effect under field along the {\$}c{\$}-axis because of the divergent behavior of the Ising like {\$}c{\$}-axis susceptibility. The strong pair-breaking effect, most likely due to nonmagnetic impurities, suggests that {\$}$\backslash $beta{\$}-YbAlB{\$}{\_}4{\$} is an unconventional superconductor. We discuss the detailed properties of superconductivity and quantum criticality. [1] S. Nakatsuji, K.Kuga, Y. Machida, T. Tayama, T. Sakakibara, Y. Karaki, H. Ishimoto, S. Yonezawa, Y. Maeno, E. Pearson, G. G., Lonzarich, L.Balicas, H. Lee and Z. Fisk, \textit{Nature Physics}, \textbf{4}, 603 (2008). [2] K. Kuga, Y. Karaki, Y. Matsumoto, Y. Machida, and S. Nakatsuji,\textit{ Physical Review Letters},\textbf{ 101}, 137004 (2008). [Preview Abstract] |
Monday, March 16, 2009 9:24AM - 9:36AM |
A41.00008: Electronic Structure of PuCoGa$_{5}$ and PuSb$_{2}$ Using Angle-Resolved Photoemission J.J. Joyce, T. Durakiewicz, K.S. Graham, D.P. Moore, J.M. Wills, Jian-Xin Zhu, E.D. Bauer, J. Mitchell The Electronic structure of PuCoGa$_{5}$ and PuSb2 is investigated using angle-resolved photoemission (ARPES). Details of the sharp quasiparticle peak at the Fermi energy are presented giving insight into the details of the mechanisms which give rise to strongly correlated characteristics in these materials. Additionally, the details of the dual nature of the 5f electrons are explored via characterization of more localized 5f states well removed from the Fermi energy. The ARPES data is compared with electronic structure models which move beyond the density functional theory approach to address the strong electron correlations present in Pu compounds as well as the dual nature of the 5f electrons. [Preview Abstract] |
Monday, March 16, 2009 9:36AM - 9:48AM |
A41.00009: $^{7}$Li NMR Study of Yb$_{4}$LiGe$_{4}$: A Possible Kondo Insulator M. J. Graf, V. Lanio, P. Carretta, Yu. Grin, S. Peter We report on the temperature-dependent resistivity, magnetic susceptibility, and nuclear spin-lattice relaxation rate of polycrystalline Yb$_{4}$LiGe$_{4}$. The parent compound, Yb$_{5}$Ge$_{4}$, is known to be mixed valent. The increasing susceptibility and resistivity with decreasing temperature are consistent with a Kondo insulator. Measurements of the temperature dependent $^{7}$Li spin-lattice relaxation rate $1/T_{1}$ show an increasing rate for decreasing temperature ($T \quad >$ 50 K), followed by a broad maximum near 30 K. These results are discussed in light of the heavy-fermion like nature of the material. [Preview Abstract] |
Monday, March 16, 2009 9:48AM - 10:00AM |
A41.00010: Revealing the Ce gamma-alpha Isostructural Phase Transition Yi Wang, Louise Hector, Shunli Shang, Long-Qing Chen, Zi-Kui Liu Since its discovery eighty years ago, the gamma-alpha iso- structural phase transition in cerium has been the subject of numerous theoretical studies. Existing theories, however, yield inaccurate results. Nowhere is this more evident than with the 50-200\% disagreement between existing theoretical predictions of the critical point and experiment. We resolve this issue by explicitly incorporating finite temperature mixing of the Ce nonmagnetic and magnetic states into a novel partition function wherein all input quantities are computed with density functional theory. Unique to our approach is the calculation of vibrational properties from phonon theory. The critical behavior of the transition is shown to be controlled by the configurational mixing entropy between the magnetic and nonmagnetic states. Our theoretical framework leads to predicted values of the critical point and equation-of-state that are in remarkably close agreement with experiment and thereby places the Ce gamma-alpha phase transition on a firm theoretical foundation. [Preview Abstract] |
Monday, March 16, 2009 10:00AM - 10:12AM |
A41.00011: ABSTRACT WITHDRAWN |
Monday, March 16, 2009 10:12AM - 10:24AM |
A41.00012: Pressure Effect on The Kondo Behavior of Layered YbB$_{2}$ Bora Kalkan, Sefik Suzer, Tayyar Gungor, Engin Ozdas The structural properties and the effect of pressure on the electrical properties of polycrystalline YbB$_{2}$ were investigated via x-ray diffraction and x-ray photoemission spectroscopy (XPS) under ambient pressure and resistivity measurements up to 6.2 GPa. Rietveld refinement confirmed the hexagonal layered structure of YbB$_{2}$ in P6/mmm space group with \textit{a=b=3.2522(2) {\AA} }and \textit{c=3.7297(4) {\AA} }lattice parameters. XPS and low temperature measurements proved the magnetically ordered ground state of YbB$_{2}$ in which Yb ion has already magnetic trivalent state under ambient pressure. $\rho $(T) measurements down to 3.5 K at various pressures exhibit a typical Kondo lattice behavior at low temperatures with a resistivity minimum around 23 K and a coherence maximum around 11 K, which define the depth of the Kondo effect. Moreover, resistivity results yield the gradual weakening of the contribution of Kondo effect and the pressure dependence of Debye temperature as (d$\theta _{D}$/dP)=(1.46$\pm $0.30) K/GPa which is calculated using Bloch-Gr\"{u}neisen approximation. These results suggest that pressure increasing causes the suppressing of Kondo effect in YbB$_{2}$. [Preview Abstract] |
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