Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session T1: Entanglement Spectroscopy
Sponsoring Units: DCMPChair: Ashvin Vishwanath, University of California, Berkeley
Room: Ballroom A1
Wednesday, March 23, 2011 2:30PM - 3:06PM |
T1.00001: Momentum space entanglement in quantum spin chains Invited Speaker: I will discuss work performed in collaboration with R. Thomale and A. Bernevig ({\sl Phys. Rev. Lett.} {\bf 105}, 116805 (2010)) on entanglement spectra in spin chains. Typically, bipartite entanglement entropy and spectra have been studied in the case of spatial partitions, {\it i.e.\/} A denotes the left half of a spin chain, B the right half, $\rho^{\vphantom{\dagger}}_{\rm A}={\rm Tr}^{\vphantom{\dagger}}_{\rm B}|\Psi_0\rangle\,\langle\Psi_0|$ is the reduced density matrix, and ${\rm spec}(\rho_{\rm A})$ is the entanglement spectrum (ES). We find for the $S={1\over 2}$ Heisenberg model that a remarkable structure in the ES is revealed if the partition is performed in momentum space, {\it i.e.\/} A = left-movers and B = right-movers. Further classifying the entanglement eigenstates by total crystal momentum, we observe a universal low-lying portion of the ES with specific multiplicities separated from a higher-lying nonuniversal set of levels by an {\it entanglement gap\/}, similar to what was observed by Li and Haldane ({\sl Phys. Rev. Lett.} {\bf 101}, 010504 (2008)) for the fractional quantum Hall effect. Indeed, the momentum space ES for the Heisenberg chain is understood in terms of the proximity of the Haldane-Shastry model, which corresponds to a fixed point with no nonuniversal corrections, and whose ground state wavefunction is related to that for the $\nu={1\over 2}$ Laughlin state. We further explore the behavior of the ES as one tunes through the spin-Peierls transition in a model with next-nearest- neighbor exchange. We also discuss entanglement gap scaling and applications to other systems. [Preview Abstract] |
Wednesday, March 23, 2011 3:06PM - 3:42PM |
T1.00002: Identifying Topological Order from the Entanglement Spectrum Invited Speaker: The Schmidt decomposition reveals bipartite entanglement of a quantum state. Calculation of the entanglement entropy reduces it to a single number, which can be studied as a function of the size and shape of the entangled regions. However, this reduction discards additional information contained in the full spectrum of the entanglement, which can be presented as a set of (dimensionless) ``pseudo-energy'' levels spectrum, labeled by quantum numbers such as momentum parallel to the $(d-1)$-dimensional boundary along which the bipartite decomposition of a $d$-dimensional system is made. The nature of the entanglement is revealed by this spectrum, much as the elementary excitations and collective modes characterizes condensed-matter states. (The von Neumann entropy is equivalent to the the thermodynamic entropy of the system of pseudo-energy levels at a particular fictitious ``temperature'' $k_BT = 1$.) The previously-unrecognized importance of the spectrum (as opposed to just its entropy) became immediately apparent when the entanglement spectrum of a 2D fractional Quantum Hall state along a 1D cut was first plotted [1]. The gapless spectrum of the conformal field theory related to the topological order of the FQHE can be recognized, and is the only spectrum in model states like the Laughlin or Moore-Read wavefunctions related to cft. For realistic states, corrections due to collective-mode fluctuations give rise to high-pseudo-energy modes that are separated from the gapless (topological) modes by a finite gap. Previously, it had been believed that the extensive O(L) (``area law'') part of the entanglement entropy of this spectrum was non-universal, and topological order could only be recognized from the O(1) subleading behavior as the length L of the cut was scaled. However, while the ``pseudo-energy'' distribution appears to be non-universal, the distribution of the spectrum {\it as a function of (true) momentum} does not have this drawback, showing that the topological contribution to the O(L) part has a universal character not visible in the numerical value of the entropy itself. In general, (including also systems such as topological or Chern insulators [2]), the signature of topological order is the occurrence of gapless mode in the entanglement spectrum, providing a fingerprint from which this order can be identified. \\[4pt] [1] Hui Li and F. D. M. Haldane, Phys. Rev. Lett. {\bf 101} 246806 (2008).\\[0pt] [2] F. D. M. Haldane, BAPS.2009.MAR.T13.13. [Preview Abstract] |
Wednesday, March 23, 2011 3:42PM - 4:18PM |
T1.00003: Multiplets in the Entanglement Spectrum Invited Speaker: Often, spin chains do not have any long range order, because of quantum mechanical fluctuations. Surprisingly, there can be phase transitions between two such phases, which suggests the existence of a hidden order. In this talk, I demonstrate that the entanglement spectrum can be used to distinguish between these subtly different phases. The central idea is to reduce a one-dimensional chain to a zero-dimensional imaginary system, called the entanglement Hamiltonian. One can then understand the phases of the original spin chain simply by looking at the spectrum of the entanglement Hamiltonian, just as one deduces the properties of atoms from their spectra. The next question is what the physical meaning of the entanglement Hamiltonian is. Properties of the entanglement Hamiltonian are in fact often reflected in physical properties of the ends of a finite chain, such as the appearance of gapless degrees of freedom or surface charge; I will give some examples in higher dimensional systems such as topological insulators as well as one dimension. [Preview Abstract] |
Wednesday, March 23, 2011 4:18PM - 4:54PM |
T1.00004: Interacting Topological Insulators Invited Speaker: Topological insulators and superconductors are new phases of matter whose physics is described by non-interacting fermions. They can be understood in terms of the topological ``twisting" of the fermion's phase over the Brillouin zone, and using topology one can come up with a full classification of when such phases can occur. Strangely, this classification fails for some one dimensional systems once higher-order interactions are allowed. In this talk I will use the entanglement spectrum to understand the modified interacting classification, in one dimension. I will also discuss general one dimensional gapped models, and how matrix product states allow us to find their phases. [Preview Abstract] |
Session T2: Defects and Strain in Graphene
Sponsoring Units: DCMPChair: Brian LeRoy, University of Arizona
Room: Ballroom A2
Wednesday, March 23, 2011 2:30PM - 3:06PM |
T2.00001: Electron Interactions in Graphene Invited Speaker: Electrons confined in two dimensions (2D) can exhibit strongly correlated states. Recent experimental discovery of integer and fractional quantum Hall effect in graphene amplified interest in correlated 2D electronic systems, owning to presence of the unusual topological phase associated with zero effective mass of charge carriers. In this talk, we will discuss the role of the many-body effects due to the electron-electron interaction in graphene manifested in electron transport phenomena. In particular, we will discuss the nature unusual spontaneous symmetry breaking Landau levels graphene under the extreme quantum condition, the appearance of unique low density insulating states and fractional quantum Hall states. Employing extremely high quality samples obtained by suspending graphene and graphene on atomically flat defect free insulating substrate such as hexa-bron nitride, we now investigate various broken symmetry states under high magnetic field. The nature of these broken symmetry state can be explained generally considering underlying SU(4) symmetry in the single particle level of the Landau levels. [Preview Abstract] |
Wednesday, March 23, 2011 3:06PM - 3:42PM |
T2.00002: Spatially Resolving Spin-split Edge States of Chiral Graphene Nanoribbons Invited Speaker: A central question in the field of graphene-related research is how graphene behaves when it is patterned at the nanometer scale with different edge geometries. The most fundamental shape in this regard is the graphene nanoribbon (GNR), a narrow strip of graphene that is characterized by its width and chirality. GNRs have been predicted to exhibit a wide range of behavior that includes tunable energy gaps and unique 1D edge states with unusual magnetic structure. I will discuss a scanning tunneling microscopy and spectroscopy (STS) study of GNRs that allows us to examine how GNR electronic structure depends on the chirality of atomically well-defined GNR edges. Our STS measurements reveal the presence of 1D GNR edge states that closely match theoretical expectations for GNRs of similar width and chirality. We additionally observe width-dependent energy splitting in GNR edge states, providing compelling evidence of their magnetic nature. [Preview Abstract] |
Wednesday, March 23, 2011 3:42PM - 4:18PM |
T2.00003: Graphene under a tip Invited Speaker: The strictly two dimensional structure of graphene results in 2D charge carriers that are readily accessible by surface probes such as scanning tunneling microscopy (STM) and spectroscopy (STS), and in electronic properties that can be controlled through doping, strain and external potentials. At the same time the 2D structure causes graphene to be extremely sensitive to environmental disturbances. I will describe STM, STS and magneto-transport experiments showing that when graphene is decoupled from substrate-induced potential fluctuations the intrinsic properties of the carriers become apparent. This is clearly seen in suspended graphene devices where, in the absence of substrate induced potential fluctuations, electron-electron interactions lead to a fractional quantum Hall effect and to an insulating phase at the Dirac point [1]. We find that even for non-suspended graphene it is possible to find non-invasive substrates on which one can directly observe the sequence of quantized Landau levels [2] and to track their evolution with field and doping down to the Dirac point where interaction effects kick in [3]. When the ``substrate'' is another graphene layer with relative orientation other than that of the standard Bernal stacking, it can profoundly affect the electronic density of states transforming it from the linear massless Dirac spectrum to one containing prominent Van Hove singularities which are controlled by the degree of twist between the layers [4].\\[4pt] [1] X. Du, I. Skachko, F. Duerr, A. Luican, EYAndrei, Nature \textbf{462}, 192 (2009)\\[0pt] [2] G. Li, A. Luican and E. Y. Andrei, Phys. Rev. Lett. 102, 176804 (2009).\\[0pt] [3] A. Luican, G. Li, and E. Y. Andrei, Phys Rev. B (R) (2011)\\[0pt] [4] G. Li, A. Luican and E. Y. Andrei, Nature Physics 6, 109 (2010) [Preview Abstract] |
Wednesday, March 23, 2011 4:18PM - 4:54PM |
T2.00004: High Resolution Tunneling Spectroscopy of Graphene in Strong and Weak Disorder Potentials Invited Speaker: Using scanning tunneling spectroscopy (STS), the local density of states can be mapped in real space to give insight into the role the local disorder potential plays in determining the 2-dimensional electron gas (2DEG) properties. In this talk I describe studies using scanning tunneling spectroscopy to examine various graphene systems with varying degrees of disorder. The amount of disorder depends on how the graphene was made. In the growth of graphene on the Si-face termination of SiC in UHV, local defects are found which contribute to strong inter- and intra-valley scattering [1]. Medium disorder is found in exfoliated graphene on SiO$_{2}$. Using a back-gated exfoliated graphene device on SiO$_{2}$ we observe a Landau level spectrum and charging resonances [2] that are completely different from previous STS measurements on weak disorder graphene systems. Applying a gating potential allows us to obtain ``STS gate maps'' which show the graphene 2DEG breaking up into a network of interacting quantum dots formed at the potential hills and valleys of the SiO$_{2}$-induced disorder potential. Graphene grown on the C-face termination of SiC is shown to have weak disorder with Landau level line widths approaching thermal limits at liquid He temperatures [3]. Using a new STM system operating at 10 mK, we are able to resolve a graphene ``quartet'' of the N=1 Landau level [4]. The quartet structure shows the complete lifting of the valley and spin degeneracies, which we determine as a function of magnetic field. \\[4pt] [1] \textit{Scattering and Interference in Epitaxial Graphene}, G.~M.~Rutter, J.~N.~Crain, T.~Li, P.~N.~First, and J.~A.~Stroscio, \textit{Science} \textbf{317, }5835, 219 (2007). \\[0pt] [2] \textit{Evolution of Microscopic Localization in Graphene in a Magnetic Field: From Scattering Resonances to Quantum Dots}, S. Jung, G. M. Rutter, N. N. Klimov, D. B. Newell, I. Calizo, A. R. Hight-Walker, N. B. Zhitenev, and J. A. Stroscio, (Nature Physics in press DOI:10.1038). \\[0pt] [3] \textit{Observing the Quantization of Zero Mass Carriers in Graphene}, D. L. Miller, K. D. Kubista, G. M. Rutter, M. Ruan,W. A. de Heer, P. N. First, and J. A. Stroscio, Science \textbf{324}, 924 (2009). \\[0pt] [4] \textit{High Resolution Tunneling Spectroscopy of a Graphene Quartet}, Y. Jae Song, A. F. Otte, Y. Kuk, Y. Hu, D. B. Torrance, P. N. First, W. A. de Heer, H. Min, S. Adam, M. D. Stiles, A. H. MacDonald, and J. A. Stroscio, Nature \textbf{467}, 185 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 4:54PM - 5:30PM |
T2.00005: Influence of edge structure, substrate structure and grain boundaries on the electronic properties of graphene quantum dots and transferred graphene Invited Speaker: We have used UHV STM to study the quantum size effect gap and the effects of edge electronic structure on graphene quantum dots (GQDs) and nanoribbons [1]. GQDs on H-Si(100) exhibit the expected size-dependent gap with the exception of those with predominantly zigzag edges, which are metallic. STM spectroscopy elucidates the predicted zigzag metallic edge state, which has a characteristic decay length of 1nm. Monolayer graphene deposited in UHV on cleaved GaAs(110) and InAs(110) substrates exhibits an electronic semitransparency effect in which the substrate electronic structure can be observed `through' the graphene [2]. This effect is observed when the equilibrium graphene-substrate spacing is reduced by about 0.06nm. We have also studied the grain boundaries in graphene monolayers that have been grown on copper and then transferred to silicon dioxide substrates. On the annealed copper foils, we find many crystallographic facets, grain boundaries, and annealing twins, all of which affect the carbon species nucleation. Graphene does not grow as readily on the foil annealing twins and non-primary crystal facets, leading to varying nucleation and graphene grain boundaries in the transferred film. STM images show graphene misorientation angles of approximately 7\r{ }, 23\r{ }, and 30\r{ } at the grain boundaries. Standing wave patterns with a decay length on the order of 1 nm were observed adjacent to the grain boundaries and depend on the structure of the boundary. Spectroscopy across the boundaries showed enhanced conduction in empty states on the grain boundaries. \\[4pt] [1] K.A. Ritter and J.W. Lyding, Nat. Mat. \textbf{8}, 235 (2009). \\[0pt] [2] K.T. He, J.C, Koepke, S. Barraza-Lopez and J.W. Lyding, Nano Lett. \textbf{10}, 3446 (2010). [Preview Abstract] |
Session T3: DCMP/DMP Prize Session: Buckley, McGroddy, Davisson-Germer
Sponsoring Units: DCMP DMPChair: Ward Plummer, Louisiana State University
Room: Ballroom A3
Wednesday, March 23, 2011 2:30PM - 3:06PM |
T3.00001: Oliver E. Buckley Condensed Matter Prize Talk I Invited Speaker: This abstract not available. [Preview Abstract] |
Wednesday, March 23, 2011 3:06PM - 3:42PM |
T3.00002: Oliver E. Buckley Condensed Matter Prize Talk: High-resolution Photoemission Studies of the High Tc Superconductors Invited Speaker: In the last decade, high resolution angle-resolved photoelectron spectroscopy has evolved into one of the most powerful probes of the electronic structure of condensed matter systems. This development reflects new technological advances coupled to the enormous research effort devoted to the study of strongly correlated systems, particularly the high T$_{c}$ cuprate superconductors. Two decades after their initial discovery the latter still present some of the biggest challenges for materials science. In this talk we review some of the developments in new instrumentation and analysis techniques in photoemission and include discussion of both self-energy effects and Fermi surface studies. In the latter case, the discussion will focus on the pseudogap phase of the underdoped cuprates with particular reference to an observed particle-hole asymmetry and the possibility of hole pockets. [Preview Abstract] |
Wednesday, March 23, 2011 3:42PM - 4:18PM |
T3.00003: Oliver E. Buckley Condensed Matter Prize Talk: Energy Gaps and Their Implications on the Phase Diagram of Cuprate Superconductors Invited Speaker: In this talk, I will survey the progress made in understanding the cuprate superconductors using angle-resolved photoemission spectroscopy. I will focus on the discovery and understanding of the anomalous energy gaps, and their implication on the pairing symmetry as well as the phase diagram of cuprate superconductors. This includes the detection of the d-wave superconducting gap structure that contributes to the current consensus of d-wave pairing symmetry, the unexpected discovery of the anisotropic normal state gap in single particle spectra (also known as pseudogap) that has become a defining feature of the cuprate phase diagram, the new progress in demonstrating that the pseudogap state is a distinct phase that breaks the particle-hole (thus translational) symmetry, and the momentum dependent information on the competition between pseudogap and superconducting gap. [Preview Abstract] |
Wednesday, March 23, 2011 4:18PM - 4:54PM |
T3.00004: James C. McGroddy Prize for New Materials Talk: Geometrically Frustrated Materials Invited Speaker: Geometrical frustration occurs when interacting degrees of freedom do not ``fit'' into the lattice that they occupy and, as a result, are under-constrained at low temperature. While the early ideas behind geometrical frustration originate in Wannier's triangular antiferromagnetic Ising model and Anderson's resonating valence bond model, they are broadened here to define an entire class of magnetic materials whose structures are based on triangular or tetrahedral units. When the degree of misfit is high, conventional long range order is suppressed and thermodynamic spectral weight is pushed to energies much lower than the mean field value. Out of this low energy spectral weight, new states of matter are found to emerge experimentally, such as spin liquid on the kagome lattice and spin ice on the pyrochlore lattice. The concept of geometrical frustration can be broadened beyond magnetism to describe a frustrated soft mode that can lead to persistent negative thermal expansion and giant dielectric constants. A brief review will be given of recent work on excitations in frustrating lattices, including the prediction of, and evidence for, magnetic monopoles in spin ice, and the relevance of frustrated hopping for topological insulators. [Preview Abstract] |
Wednesday, March 23, 2011 4:54PM - 5:30PM |
T3.00005: Davisson-Germer Prize in Atomic or Surface Physics Talk: Soft X-Ray Studies of Surfaces, Interfaces and Thin Films: From Spectroscopy to Ultrafast Nanoscale Movies Invited Speaker: My talk will review the development of soft x-ray spectroscopy and microscopy and its impact on our understanding of chemical bonding, magnetism and dynamics at surfaces and interfaces. I will first outline important soft x-ray spectroscopy and microscopy techniques that have been developed over the last 30 years and their key strengths such as elemental and chemical specificity, sensitivity to small atomic concentrations, separation of charge and spin properties, spatial resolution down to the nanometer scale, and temporal resolution down to the intrinsic femtosecond timescale of atomic and electronic motions. I will then present scientific breakthroughs based on soft x-ray studies in three selected areas: the nature of molecular bonding and reactivity on metal surfaces, the molecular origin of liquid crystal alignment on surfaces, and the microscopic origin of interface-mediated spin alignments in modern magnetic devices. My talk will also cover the use of soft x-rays for revealing the temporal evolution of electronic structure, addressing the key problem of ``function,'' down to the intrinsic femtosecond time scale of charge and spin configuration changes. As examples I will present the formation and breaking of chemical bonds in surface complexes and the motion of the magnetization in magnetic devices. [Preview Abstract] |
Session T4: Keithly Award Session
Sponsoring Units: GIMSChair: Timothy Graber, University of Chicago
Room: Ballroom A4
Wednesday, March 23, 2011 2:30PM - 3:06PM |
T4.00001: Joseph F. Keithley Award For Advances in Measurement Science Talk: Beyond the Fringe: measuring ultrafast optical pulses using spectral interferometry Invited Speaker: The ability to completely characterize ultrashort electromagnetic pulses has revolutionized the field of ultrafast optics, enabling both new technology and new science. The aim of pulse characterization is to infer the electric field of the pulse from measurements of quantities that rely on standard (relatively slow) photodetectors. Since the field is a fundamental entity in Maxwell's theory, it contains the most information one may obtain about a system probed by an optical pulse, and is, in this sense, much more valuable set of data than a measurement simply of the pulse energy or even the spectral or temporal intensity, Pulse measurement methods may be categorized as spectrographic and tomographic, by which the time-frequency phase space of the pulse is mapped, or interferometric, by which the phase is determined directly. Interferometry provides a sensitivity and robust approach affording a rapid, direct reconstruction algorithm that gives a provably unique solution to the complete space-time field. An important class of self-referencing interferometric are those based on spectral shearing, whereby two frequency shifter replicas of the test pulse are generated encoding the spectral phase derivative in the spectral interferogram resulting from their superposition. The nonlinear implementation of this approach is called spectral phase interferometry for direct electric-field reconstruction (SPIDER). SPIDER has shown itself to be an adaptable and robust method, gaining widespread application in all areas of ultrafast optics, from nonlinear microscopy to attoscience. Some essential concepts and history of the field will be presented, along with recent developments, illustrating applications in ultrafast source diagnosis and certification, dynamical spectroscopy, coherent control, imaging and materials processing. [Preview Abstract] |
Wednesday, March 23, 2011 3:06PM - 3:42PM |
T4.00002: High-Energy Sub-Cycle Waveform Synthesis and Characterization Invited Speaker: The control of atomic scale electronic motion by ultrafast optical electric field waveforms strong enough to mitigate the atomic Coulomb potential has broken tremendous new ground with the advent of phase controlled high-energy few-cycle pulse sources. Currently, such sources are based on Ti:sapphire amplifiers and hollow-core fiber post-compression or optical parametric chirped pulse amplification, together with optical gating techniques. Significant control of the waveform on sub-cycle time scales, however, requires a fully phase-controlled multiple-octave-spanning spectrum. Here, we present a first fully phase-controlled multi-octave-spanning source that supports gigawatt-peak-power isolated single-cycle waveforms based on pulse synthesis of two carrier-envelope phase (CEP) stable OPCPA systems. It is especially a challenge to fully characterize such ultrawide band waveforms. We apply two-dimensional spectral shearing interferometry (2DSI), which can measure the group delay between all spectral components of the synthesized pulse. [Preview Abstract] |
Wednesday, March 23, 2011 3:42PM - 4:18PM |
T4.00003: Generation, characterization and spectroscopic use of ultrashort pulses fully tunable from the deep UV to the MIR Invited Speaker: The impressive work of Ian Walmsley has brought us invaluable new possibilities for the full characterization of ultrashort pulses. Spectroscopy of physical, chemical and biological relevance does, however, need pulses far from the 800 nm Ti:sapphire wavelength used for testing SPIDER and its advanced versions. Fortunately, optical parametric amplification (OPA) allows for easy generation of fully tunable pulses. I will review our efforts, highlighting noncollinear OPA, i.e. NOPA, for visible pulses shorter than 10 fs, mixing into the UV down to below 200 nm at 20 fs duration and novel hybrid schemes to efficiently reach the middle IR. I will show that these schemes can be used equally well from kHz to MHz repetition rates. The tunable ultrafast pulses in turn also demand improvements in characterization. The UV range led us to use difference frequency generation instead of the sum frequency mixing employed in the original SPIDER. The lack of proper beam splitters and auto-referencing led us to the use of two auxiliary pulses and the avoidance of any additional chirp added to the test pulse. We termed this zero-additional-phase SPIDER, i.e. ZAP-SPIDER. Lately, with increased use of UV pulses, we came to the conclusion, that the ubiquitous two-photon-absorption can well serve as nonlinearity, at least in UV autocorrelation measurement. How do we use this for full characterization? Hopefully, Ian will tell us! Since the proof is known to be in the eating, I will demonstrate the success of our technical efforts with examples taken from ultrafast molecular dynamics. Highly pronounced vibronic wavepackets in the product of ultrafast excited state proton transfer and the very primary processes leading to homolytic and heterolytic bond cleavage will serve as easy to comprehend illustrations. [Preview Abstract] |
Wednesday, March 23, 2011 4:18PM - 4:54PM |
T4.00004: Probing Electron Correlation with Sequential Laser--Induced Tunnel ionization Invited Speaker: Since 1964 we have known that multiphoton ionization could be approximated by tunnel ionization for long wavelength light. Aside from re-collision, since then multiple ionization has been treated as successive, independent single ionization events. Our results show that this long-held belief is false. Tunnelling is highly directional and highly sensitive to the ionization potential (Ip) of the accessible ionic states (which itself can depend on the direction of ionization). Using rare gas atoms as examples, we show that laser induced tunnelling is suppressed or enhanced depending on how the field is applied. We image the hole left by the first tunnelling electron by measuring in the spatial correlation of the second electron. Laser induced tunnelling gives us experimental access to one of the most difficult to measure properties of matter -- electron-electron correlations [1]. \\[4pt] [1] A. Fleischer, H.J. W\"{o}rner, L. Arissian, L.R. Liu, M. Meckel, A. Rippert, R. D\"{o}rner, D.M. Villeneuve, A. Staudte and P.B. Corkum, unpublished results. [Preview Abstract] |
Wednesday, March 23, 2011 4:54PM - 5:30PM |
T4.00005: Pulse Propagation through Dispersive Optical Materials Invited Speaker: It is now possible to characterize the complete time-frequency behavior of optical pulses with unprecedented precision [1, 2]. The frequency content of optical pulses determines how they propagate through dispersive optical materials. In this talk, we review recent work on methods for dramatically modifying the velocity with which light pulses propagate through material systems. This modification can be so severe that one speaks of slow light, fast light, and backwards light depending on how the magnitude and sign of the group velocity compares to the vacuum speed of light c. We review the physical processes that can be used to achieve such a strong modification of the velocity of light, and we discuss the conceptual understanding of exotic propagation effects such as backwards propagation. We also review the implications of modified pulse velocities within the context of modern optical technology.\\[4pt] [1] Kane, D.J. and R. Trebino, Opt. Lett., 18 823 (1993)\\[0pt] [2] C. Iaconis and I. A. Walmsley, Opt. Lett., 23 792 (1998). [Preview Abstract] |
Session T5: 20 Years of Quantum Information in Physical Review Letters
Sponsoring Units: GQIChair: John Preskill, California Institute of Technology
Room: Ballroom C1
Wednesday, March 23, 2011 2:30PM - 3:06PM |
T5.00001: Theory of entanglement and entanglement-assisted communication Invited Speaker: Protocols such as quantum teleportation and measurement-based quantum computation highlight the importance of entanglement as a resource to be quantified and husbanded. Unlike classical shared randomness, entanglement has a profound effect on the capacity of quantum channels: a channel's entanglement-assisted capacity can be much greater than its unassisted capacity, and in any case is given by much a simpler formula, paralleling Shannon's original formula for the capacity of a classical channel. We review the differences between entanglement and weaker forms of correlation, and the theory of entanglement distillation and entanglement-assisted communication, including the role of strong forms of entanglement such as entanglement-embezzling states. [Preview Abstract] |
Wednesday, March 23, 2011 3:06PM - 3:42PM |
T5.00002: Twenty Years of Quantum Error Correction Invited Speaker: Starting around 1991 It became clear that the emerging quantum computer would need error correction to be sensible. Almost immediately, Berthiaume, Deutsch, and Jozsa announced the first key ideas (e.g., allowed codewords should live in a well-defined subset of the Hilbert space), and in less than ten years, the problem was ``solved.'' This solution had many components, involving insights from quantum teleportation, the concept of noisy entanglement and its improvement, creative borrowings from classical binary and quaternary codes, pure group theory, and the reliable working of noisy automata. Actually, this ``solved'' problem continues to produce new difficulties and insights up to the present day, and it is increasingly central for the question of what we do next in the progress towards a functioning quantum computer. [Preview Abstract] |
Wednesday, March 23, 2011 3:42PM - 4:18PM |
T5.00003: Less Reality, More Security Invited Speaker: Bell's inequality makes a seemingly insane scenario possible --- devices of unknown or dubious provenance, even those that are manufactured by our enemies, can be safely used for secret communication. And this is for real! All that is needed to implement such a bizarre form of cryptography is a loophole-free violation of Bell's inequalities. It is on the edge of being technologically feasible. I will provide a brief overview of quantum and post-quantum cryptography and describe how studies of entanglement and the foundations of quantum theory influenced the way we may soon protect information. [Preview Abstract] |
Wednesday, March 23, 2011 4:18PM - 4:54PM |
T5.00004: Twenty Seven Years of Quantum Cryptography! Invited Speaker: One of the fundamental goals of cryptographic research is to minimize the assumptions underlying the protocols that enable secure communications between pairs or groups of users. In 1984, building on earlier research by Stephen Wiesner, Charles Bennett and Gilles Brassard showed how quantum physics could be harnessed to provide information-theoretic security for protocols such as the distribution of cryptographic keys, which enables two parties to secure their conventional communications. Bennett and Brassard and colleagues performed a proof-of-principle quantum key distribution (QKD) experiment with single-photon quantum state transmission over a 32-cm air path in 1991. This seminal experiment led other researchers to explore QKD in optical fibers and over line-of-sight outdoor atmospheric paths (``free-space''), resulting in dramatic increases in range, bit rate and security. These advances have been enabled by improvements in sources and single-photon detectors. Also in 1991 Artur Ekert showed how the security of QKD could be related to quantum entanglement. This insight led to a deeper understanding and proof of QKD security with practical sources and detectors in the presence of transmission loss and channel noise. Today, QKD has been implemented over ranges much greater than 100km in both fiber and free-space, multi-node network testbeds have been demonstrated, and satellite-based QKD is under study in several countries. ``Quantum hacking'' researchers have shown the importance of extending security considerations to the classical devices that produce and detect the photon quantum states. New quantum cryptographic protocols such as secure identification have been proposed, and others such as quantum secret splitting have been demonstrated. It is now possible to envision quantum cryptography providing a more secure alternative to present-day cryptographic methods for many secure communications functions. My talk will survey these remarkable developments. [Preview Abstract] |
Wednesday, March 23, 2011 4:54PM - 5:30PM |
T5.00005: A Brief Prehistory of Qubits Invited Speaker: In the early 1990's, alongside the early work on quantum cryptography, there existed a considerable body of research on the classical information capacity of quantum channels. The strongest and most general result known was the theorem of Holevo giving an entropic bound for the accessbile information in a mixture of quantum signals. This motivated the problem of whether the Holevo bound could be closely approached by suitable choice of code and decoding observable. If so, then the quantum (von Neumann) entropy had a straightforward informational interpretation. When this question was found to be very difficult to answer, quantum data compression and the idea of a ``qubit'' was introduced as an alternate framework for thinking about information in quantum systems and interpreting the quantum entropy. However, the mathematical ideas from the new framework proved essential to solving the original problem of showing that the Holevo bound was asymptotically achievable. This was an early example of the interplay between classical and quantum concepts of information -- an interplay that has been, to say the least, extremely fruitful. [Preview Abstract] |
Session T6: Theory in Industry
Sponsoring Units: FIAPChair: Alex Demkov, University of Texas at Austin
Room: Ballroom C2
Wednesday, March 23, 2011 2:30PM - 3:06PM |
T6.00001: From Atoms to Autos: Adventures of a Theoretical Physicist in Industry Invited Speaker: After earning a PhD in solid state theoretical physics at Cornell and following a post-doc, the author arrived at GM R{\&}D for what has turned out to be his first and only real job. An anecdotal chronicle of his adventures in the auto industry will be presented. [Preview Abstract] |
Wednesday, March 23, 2011 3:06PM - 3:42PM |
T6.00002: Challenging theoretical physics problems in the energy industry Invited Speaker: Critical reliance on technology is ubiquitous in the energy industry, where considerable resources are dedicated to fundamental research aimed at solving our most challenging problems. For example, technological challenges are found in all aspects of the oil and gas industry ranging from exploration, development, and production of oil fields, to transportation and refining of the raw materials, and all the way to the production of specialty products such as polymers and lubricants. From a scientific perspective, these activities cover a broad range of physical science disciplines. As examples, during the exploration and development of oil and gas fields, sound and electromagnetic waves are used to image the earth's interior, and drilling involves an array of sophisticated tools and detectors at the bore hole, both activities being possible thanks to geophysicists, applied mathematicians, and rock physics specialists. Similarly, the transformation of crude oil to refined products requires a fundamental understanding of physical chemistry, phase transition, and transport processes, while the design of products involves polymer physics, and special disciplines such as tribology. The goal of this talk is to present examples of problems posed by the energy industry in view of encouraging physicists to contribute to finding solution to these problems, either through their academic research, or by pursuing a challenging career as industrial physicists. Many of those problems can benefit from the unique approach provided by a rigorous physics training. [Preview Abstract] |
Wednesday, March 23, 2011 3:42PM - 4:18PM |
T6.00003: Theoretical Physics + Experiments in Superfluid He$^4$ = Commercial Oilfield Acoustic Service Invited Speaker: I will describe a specific project which involved the understanding of the basic physics of acoustics in porous and permeable fluid saturated media. The end product is a commercially available measurement of the fluid-flow resistance of porous rock in a real oil-field borehole using an acoustic technique. One key ingredient of the understanding was obtained by laboratory measurements of the acoustic properties of a porous sample saturated with superfluid He$^4$. Another key ingredient is the theoretical understanding of the properties of the frequency dependent fluid-flow resistance, and its extension to complex values of the frequency. [Preview Abstract] |
Wednesday, March 23, 2011 4:18PM - 4:54PM |
T6.00004: Industry: Theory Matters Invited Speaker: This talk will try to illustrate on a few examples a significant role that theory and modeling plays in an industrial environment of a diversified company. It is especially clear in case of novel materials and processes that may bring big benefits down the road to those with sufficient time horizon. Relevant global trends will be analyzed in this regards. [Preview Abstract] |
Wednesday, March 23, 2011 4:54PM - 5:30PM |
T6.00005: Technologically relevant, in theory Invited Speaker: This talk will give a personal perspective on the role of theoretical physicists in industry, and in particular at IBM. This role has changed over the decades, and continues to evolve. [Preview Abstract] |
Session T7: Structural and Mechanical Properties of Jammed Amorphous Materials
Sponsoring Units: GSNPChair: Bulbul Chakraborty, Brandeis University
Room: Ballroom C3
Wednesday, March 23, 2011 2:30PM - 3:06PM |
T7.00001: Length scale of dynamic heterogeneity and its relation to time scales in a glass-forming liquid Invited Speaker: The role of the length scale of dynamic heterogeneity in the enormous increase in the relaxation times of glass-forming liquids upon supercooling has received much attention recently. Using molecular dynamics simulations and finite-size scaling for a realistic glass-forming liquid, we establish that the growth of dynamic heterogeneity with decreasing temperature is governed by a growing dynamic length scale. We also perform a computational study of a four-point structure factor, defined from spatial correlations of mobility, for the same liquid and show that estimates of the dynamic correlation length and susceptibility obtained from this study are consistent with the results of the finite-size scaling analysis. However, the observed dependence of the simultaneously growing time scale of the long-time $\alpha$-relaxation on system size does not exhibit the same scaling behavior as the dynamic heterogeneity: this time scale is instead determined, for all studied system sizes and temperatures, by the configurational entropy, in accordance with the Adam-Gibbs relation. We also investigate the dependence of the time scale of the short-time $\beta$-relaxation on temperature and system size. A finite-size scaling analysis of this dependence reveals the existence of a length scale that grows as the temperature is reduced. Surprisingly, the temperature dependence of this length scale is found to be identical to that of the length scale that governs the growth of dynamic heterogeneity at the $\alpha$-relaxation time scale. This result suggests a close connection between short-time dynamics and dynamic heterogeneity at time scales of the order of the $\alpha$-relaxation time. [Preview Abstract] |
Wednesday, March 23, 2011 3:06PM - 3:42PM |
T7.00002: Point-to-Set as a measure of Correlations during Unjamming in Granular Systems Invited Speaker: There is evidence indicating that the unjamming of frictionless, soft grain packings occurs at a critrical point, however, no correlation function associated with a diverging, static length scale has been identified. To better understand the nature of this transition, we consider the soft grain packing problem as a constraint satisfaction problem [1]. Jammed configurations are mechanically stable packings with non-zero pressure. Contact forces on each grain therefore satisfy the equations of mechanical equilibrium, which are a set of local constraints, as well as a global constraint from the pressure. In general when jammed, there are more contact forces than constraints, so that an ensemble of force networks exists [2] which satisfy the constraints. These force networks make up a high-dimensional solution space that shrinks to a point at the unjamming transition, suggesting that the unjamming transition can be considered an entropy vanishing transition. We explore a new type of ``point-to-set'' correlation function which has been used to identify non-obvious length scales in other constraint satisfaction problems [3], and show that it exhibits a diverging length scale. We compare and contrast this length scale with the well established ``isostatic length'' of Wyart et. al [4].\\[4pt] [1] F. Krzakala and J. Kurchan, PRE 76, 021122 (2007)\\[0pt] [2] J.H. Snoeijer et. al, PRE 70, 061306 (2004)\\[0pt] [3] A. Montanari and G. Semerjain, Journal of Stat. Phys, 125, 1 (2006)\\[0pt] [4] M. Wyart et. al, PRE 72, 051306 (2005) [Preview Abstract] |
Wednesday, March 23, 2011 3:42PM - 4:18PM |
T7.00003: Protocol Dependence in Jammed Particulate Media: Statistics of the Density Landscape Invited Speaker: The density at which hard-sphere fluids jam into amorphous solids depends strongly on the compression protocol. Extremely fast quenching protocols bring each initial point in configuration space to the closest basin-maximum on the density landscape. In contrast, slower quench protocols allow the system to relax and explore configuration space. The protocol-dependence of the density, other structural quantities, and mechanical properties depends strongly on statistical features of the landscape. In this talk, I describe calculations of the the basin volumes associated with jammed hard sphere packings, and the critical quench rate $\Gamma^*$ above which the probabilities for obtaining jammed packings are determined by their basin volumes. Basin volumes are exponentially distributed; thus, for $\Gamma > \Gamma^*$, so are jammed packing probabilities. We discuss the implications of this result on the statistical mechanics of jammed systems. [Preview Abstract] |
Wednesday, March 23, 2011 4:18PM - 4:54PM |
T7.00004: Vibrational modes identify soft spots in a sheared model glass Invited Speaker: Both solids and fluids can flow under applied stress. In crystalline solids, flow occurs via particle rearrangements controlled by a population of dislocations, while in fluids, particle rearrangements occur everywhere throughout the material. In disordered solids, flow generally occurs via localized rearrangements, but no one has been able to identify a population of flow defects, analogous to dislocations, that are structurally different from the rest of the system and more susceptible to flow. It has therefore remained unclear whether a solid-like or fluid-like description is more appropriate for describing flow in such systems. By analyzing the low-energy vibrational modes in a model glass, we have identified a population of structural ``soft spots'' and have shown that particle rearrangements are initiated at these spots. Thus, these spots serve as good candidates for flow defects. We analyze statistical and structural features of the spots and find that the density of spots decreases with increasing packing fraction and that the population of spots changes slowly compared to the time between particle rearrangements. These results support a solid-like description of flow controlled by a population of localized flow defects in glassy materials. [Preview Abstract] |
Wednesday, March 23, 2011 4:54PM - 5:30PM |
T7.00005: Jamming Mechanisms and Density Dependence of Dynamic Heterogeneities in a Kinetically-Constrained Model Invited Speaker: Experiments on granular [1] and colloidal [2] systems show steady growth in dynamic heterogeneities as the relaxation time increases with increasing density. In glass-forming liquids, however, the scale of heterogeneities remains modest even as the relaxation time increases by more than ten orders of magnitude with decreasing temperature [3]. This difference may be attributed to the far greater dynamic range measurable in glass-forming liquids [2]. We introduce a simple lattice model [4] which suggests that this difference signals a fundamental distinction between jamming due to an increase in particle density as opposed to jamming by lowering the temperature, or the strength of external driving forces. The recently proposed spiral model [5] has a kinetic constraint that breaks its ergodicity at a critical density smaller than 1. We add to it relaxation mechanisms that mimic the effect of temperature and non-equilibrium driving. This enables us to explore its jamming phase-diagram and study unjamming by temperature or driving above the critical density, which we relate to the random close packing density in particulate systems. We separate the effects of density, temperature and driving and show that jamming resulting from increasing density gives rise to dynamic heterogeneity that grows unboundedly. Whereas decreasing temperature or driving eventually leads to a saturation of the dynamic correlation length even though the relaxation time diverges. \\[4pt] [1] A.R. Abate and D.J. Durian, Phys. Rev. E 76, 021306 (2007).\\[0pt] [2] G. Brambilla et al., Phys. Rev. Lett. 102, 085703 (2009).\\[0pt] [3] C. Dalle-Ferrier et al., Phys. Rev. E 76, 041510 (2007).\\[0pt] [4] Y. Shokef, A.J. Liu, Europhys. Lett. 90, 26005 (2010).\\[0pt] [5] C. Toninelli, G. Biroli, D.S. Fisher, Phys. Rev. Lett. 98, 129602 (2007). [Preview Abstract] |
Session T8: Shaping Regional Identities through Research Funding Policies
Sponsoring Units: FIPChair: Giulia Pancheri, INFN Frascati National Laboratories
Room: Ballroom C4
Wednesday, March 23, 2011 2:30PM - 3:06PM |
T8.00001: Europe and research: a multi-speed scenario Invited Speaker: A review of the European situation concerning physics research facilities, collaborations, networks and funding policies will be sketched, pointing out the diversity among different countries of the enlarged EU. Some special focus will be put on particle physics. The continuing integration of Europe, the higher students' mobility trained in the ``Bologna Process'' frame, the expansion of the European research infrastructures in physics, the increasing funding from the European Commission, the various research shaping and roadmap attempts, the challenge by Asia in addition to that from the USA, are all issues to be examined. [Preview Abstract] |
Wednesday, March 23, 2011 3:06PM - 3:42PM |
T8.00002: Science, Technology and Innovation in Brazil: Advances and Challenges Invited Speaker: We review the construction of the infra-structure for science and technology in Brazil, its success cases, its major advances, and its challenges, both present and future. We emphasize the budget increases, the new legal framework, new mechanisms of support for S{\&}T, and the plans for the future. Preliminary results of the discussions that took place in the 4$^{th}$ National Conference on Science, Technology and Innovation of May 2010 will also be presented. [Preview Abstract] |
Wednesday, March 23, 2011 3:42PM - 4:18PM |
T8.00003: Mega Physics Projects: National and International Initiatives: the Indian Experience Invited Speaker: Indian Physics community is now getting involved in a number of Mega physics projects, both home and abroad, national and International. I will present some information on these efforts. [Preview Abstract] |
Wednesday, March 23, 2011 4:18PM - 4:54PM |
T8.00004: Science and Technology in Africa: The African Union new initiatives and financial support perspectives Invited Speaker: Physics, which is widely considered as the most fundamental of the sciences, underpins the progress in all other branches of science and has a wide range of applications in economic development, including in health, energy research, food security, communication technology and climate change. The African Union (AU) Commission articulates the continental vision of its Member States and its programs are designed to directly contribute to its social and economic development and integration efforts. In the area of science and technology the Department has developed \textit{Africa's Science and Technology Consolidated Plan of Action} as a strategic policy document through the AU system of conference of ministers responsible for science to guide the continent on common priority programs. The programs in this plan of action that have been transformed into bankable projects under the Book of ``lighthouse projects Phase 1,'' adequately respond to Africa's challenges and development needs using science. They can be summarized into three main themes: a pan-African university (PAU) initiative (to combine higher education and scientific research as a network of differentiated PAU in each of the five African regions), African research grants (to strengthen the research capacity of the African institutions and upgrading infrastructures, consolidating their accumulated asset of scientific knowledge), popularization of science and technology and promotion of public participation (to build public understanding and raising awareness on science and technology as a driving agent for social and economic progress for Africa and its integration process) and a science and technology institutional capacity building program). This talk will provide a review these programs and their status and impact in Africa. [Preview Abstract] |
Wednesday, March 23, 2011 4:54PM - 5:30PM |
T8.00005: Science for Energy Invited Speaker: This abstract not available. [Preview Abstract] |
Session T9: Flow Instabilities, Turbulence and CFD
Sponsoring Units: DFDChair: Prabir Daripa, Texas A&M University
Room: D220
Wednesday, March 23, 2011 2:30PM - 2:42PM |
T9.00001: ABSTRACT WITHDRAWN |
Wednesday, March 23, 2011 2:42PM - 2:54PM |
T9.00002: Modal decomposition of free and forced circular jets at low and high Reynolds numbers Muralidhar Krishnamurthy, Trushar Gohil, Arun Saha Free and forced jets are important in applications such as combustion, propulsion, mixing, and aero-acoustics. Jet control for noise reduction and mixing efficiency can be achieved by manipulating the flow structures. The most energetic structures of a flow field can be objectively recovered by proper orthogonal decomposition. POD extracts a basis for modal decomposition as eigenfunctions from an ensemble of signals. In the present work, the snapshot POD method is applied to data recorded from direct numerical simulation as well as large eddy simulation in three dimensions. Free jets are reported at a Reynolds number of 1000 and 10000 and 4300 for forced jets. Results show that all of the kinetic energy of laminar flow is stored in large-scale structures while for the turbulent jet, a broader distribution of kinetic energy is obtained. At Re = 1000, 40 snapshots of the flow field are adequate to resolve the major flow structures. For Re=10000, at least 100 snapshots are required for a good spectral representation. Blooming jets arising from dual mode forcing show the formation of odd-even pairs. The first pair contains the details of branching. In addition, the higher order modes capture the inherent jet instability mechanisms. [Preview Abstract] |
Wednesday, March 23, 2011 2:54PM - 3:06PM |
T9.00003: Generalized Saffman-Taylor formula for multi-layer Hele-Shaw flows Prabir Daripa Stability theory plays a major role from fundamental science to applied sciences. It is useful in the design of many processes and engineering instruments as well as in explaining many phenomena. In this paper we review some of the author's and his collaborator's recent works on the extension of Saffman-Taylor instability which occurs at an interface between two immiscible fluids in porous media and Hele-Shaw cells when displacing fluid is less viscous than the displaced one. The growth rate of interfacial disturbances is given by a formula called Saffman-Taylor formula which plays a very important role in many areas including flows in porous media and oil recovery among many others. In this talk, we will present our results on the generalization of this formula to multi-layer flows involving many interfaces. As an application of the generalized Saffman-Taylor formula, we will derive necessary conditions for suppressing instability of two-layer flows by introducing arbitrary number of constant viscosity fluid layers in between. The important role that these conditions play in stabilization of hydrodynamic instabilities in Hele-Shaw flows will be discussed. [Preview Abstract] |
Wednesday, March 23, 2011 3:06PM - 3:18PM |
T9.00004: The stability of a droplet suspended in a straight micro-channel Haider Hekiri, Takumi Hawa CFD simulations of the dynamics of a two-dimensional, incompressible, and two coupled spherical-cap water droplets suspended in a straight micro-channel, whose channel height is D, have been conducted to investigate the stability of the droplet. FLUENT with a 2-D pressure based solver is utilized in this simulation. The suspended droplet states are measured by the location of the central of mass of the droplet. We find that there is a critical volume, Vc(D), where asymmetric droplet states appear in addition to the basic symmetric states when V $>$ Vc(D). Using the CFD it is demonstrated that when V $<$ Vc(D) the symmetric droplet states have a stable mode. However, when V $>$ Vc(D) the symmetric states become unstable and asymmetric states have a stable mode. The bifurcation of asymmetric states at Vc(D) has a pitchfork nature. The simulations clarify the relationship between the linear stability results and the experimental results of the droplet behavior. [Preview Abstract] |
Wednesday, March 23, 2011 3:18PM - 3:30PM |
T9.00005: Liquid-air interface instability due to an in-plane electric field Mikhail Pevnyi, Jake Fontana, Peter Palffy-Muhoray We report observations of an instability at the free surface of a liquid due to an in-plane electric field. The horizontal air-liquid interface in a partially filled sample cell between vertical electrodes exhibited first oscillations, then increasingly turbulent fluctuations as the strength of the horizontal electric field was increased. This behavior was observed in toluene and choloroform; the applied AC field was sinusoidal with f= 60Hz. The dynamics of the interface was probed via dynamic light scattering. We present our experimental observations, as well as a simple model and numerical simulations of the interface dynamics under the influence of the applied electric field. [Preview Abstract] |
Wednesday, March 23, 2011 3:30PM - 3:42PM |
T9.00006: Stretch-induced wrinkles in reinforced membranes Atsushi Takei, Fabian Brau, Beno\^It Roman, Jos\'e Bico We study through model experiments the buckling of a rigid stripe (or fiber) embedded in a soft membrane under compression. The compression is induced through Poisson effect when the membrane is stretched perpendicularly to the stripe. The wavelength of the wrinkles is found to depend on the material properties and the stretching strain. A balance between the bending and stretching energies of both the membrane and the stripes dictates this wavelength: \textit{$\lambda $} $\sim $ (\textit{Bd }/$E_{S}H_{S}$\textit{$\delta $ })$^{1/3}$, where $B$ is the bending stiffness, $d$ the width of the rigid band, \textit{$\delta $} the strain, and $E_{S}$ and$ H_{S}$ the Young modulus and the thickness of the membrane, respectively. The characteristic extension of the wrinkled zone is set by the wavelength. This result also applies to fibers imbedded in a thin membrane. However, in-plane buckling is observed when the thickness of the membrane is large compared with the radius of the fiber. In this last regime, we find \textit{$\lambda $} $\sim R (E_{F}$ /$E_{S}$ )$^{1/4}$, where $E_{F }$and $R$ are the Young modulus and the radius of the fiber, respectively. [Preview Abstract] |
Wednesday, March 23, 2011 3:42PM - 3:54PM |
T9.00007: Computational Parametric Study of R-M Instability Growth for an Inclined Interface Jacob McFarland, Devesh Ranjan, Jeff Greenough An inclined interface perturbation is studied for an RM instability to model upcoming experiments in the Texas A{\&}M inclined shock tube facility. Simulations were created using the ARES code developed at Lawrence Livermore National Lab. A parametric study was performed for inclination angles from 30 to 60 degrees, incident Mach numbers of 1.5 to 2.5, and high Atwood number gas pairs air-SF6 and helium/SF6. Qualitative results are examined to show the relative effects of these parameters. Interface growth rates are calculated and compared to the existing linear growth regime models. A new model is proposed based on the interface geometry and compared to the simulation results. [Preview Abstract] |
Wednesday, March 23, 2011 3:54PM - 4:06PM |
T9.00008: Drop Splashing on a Smooth Surface at Low Velocities Cacey Stevens, Sidney Nagel When a low viscosity liquid drop impacts on a smooth, dry surface, a thin fluid sheet is emitted which subsequently breaks up into a distribution of secondary droplets. Ambient gas pressure is crucial in creating this splash: splashing is completely suppressed below a threshold pressure [1]. There are several regimes that occur as the velocity and liquid viscosity are varied [2]. Here, we discuss splashing in the low velocity, low viscosity regime. We explore how the threshold pressure scales with drop size, as well as liquid viscosity. We also characterize the dependence of threshold pressure with molecular weight of the surrounding gas. \\[4pt] [1] L. Xu, S. Nagel, and W. Zhang. Phys. Rev. Lett. 94, 184505 (2005).\\[0pt] [2] L. Xu. Phys. Rev. E 75, 056316 (2007). [Preview Abstract] |
Wednesday, March 23, 2011 4:06PM - 4:18PM |
T9.00009: Elastic effects on the shear flow instabilities in viscoelastic fluids Ahmed Kaffel A linear stability analysis was applied and the stability equation is derived and solved numerically using the spectral Chebyshev collocation method. The objective is to study the elastic effects on the instability of inviscid parallel shear flows. We focus on the upper convected Maxwell model in the limit of infinite Weissenberg and Reynolds numbers. Specifically, we study the effects of elasticity on the instability of a few classes of simple parallel flows, specifically plane Poiseuille and Couette flows, the hyperbolic-tangent shear layer and the Bickley jet. The algorithm is computationally efficient and accurate in reproducing the discrete eigenvalues. We consider flows bounded by walls as well as flows bounded by free surfaces. In the inviscid, nonelastic case all the flows we study are unstable for free surfaces. In the case of wall bounded flow, there are instabilities in the shear layer and Bickley jet flows. In all cases, the effect of elasticity is to reduce and ultimately suppress the inviscid instability. The numerical solutions are compared with the analysis of the long wave limit and excellent agreement is shown. We found flows which are long wave stable, but nevertheless unstable to wave numbers in a certain finite range. While elasticity is ultimately stabilizing, this effect is not monotone; there are instances where a small amount of elasticity actually destabilizes the flow. [Preview Abstract] |
Wednesday, March 23, 2011 4:18PM - 4:30PM |
T9.00010: Thermal convection in multiphase systems Luca Biferale, Prasad Perlekar, Mauro Sbragaglia, Andrea Scagliarini, Federico Toschi We present preliminary results of a numerical study of two dimensional and three dimensional multiphase thermal convection close to the phase transition and in presence of phase coexistence. The numerical algorithm is based on a suitable implementation of multiphase Lattice Boltzmann scheme with non-ideal pressure tensor. We discuss the effects of droplets and bubbles formation on the global heat flux from bottom to top boundaries. [Preview Abstract] |
Wednesday, March 23, 2011 4:30PM - 4:42PM |
T9.00011: Vortex Sheet Model for a Turbulent Mixing Layer Ujjayan Paul, Roddam Narasimha, Meheboob Alam The primary aim of this work is to study instability induced roll up of a slightly perturbed vortex sheet in an Euler fluid. A point vortex model tends to evolve into a chaotic cloud of point vortices instead of smooth double branched spirals. The present model uses linear splines to interpolate the vortex sheet. Computer simulation of this vortex sheet is numerically prohibitive. However, the evolution of the vortex sheet can be performed conveniently using a closed form equation of motion which derived from the basic equations of vortex dynamics. The vortex sheet rolls up into a smooth double branched spiral. A vortex core is formed by regular windings of the vortex sheet and irrotational fluid in between the layers. Various statistical quantities like the growth rate and mean velocity profiles are computed along with the evolution of the vortex sheet. The problem of spontaneous appearance of singularity in an evolving vortex sheet is treated in detail. The critical time for the present vortex sheet model is calculated analytically and compared to the numerical value. [Preview Abstract] |
Wednesday, March 23, 2011 4:42PM - 4:54PM |
T9.00012: Nonlinear Deformation in Weak Turbulence Nicholas Ouellette, Douglas Kelley, Yang Liao Turbulent and chaotic flows are well known to mix efficiently: by repeatedly stretching and folding material volumes, material lines stretch exponentially quickly and gradients of an advected scalar field can become very large. By adapting a technique originally introduced to study plasticity in glassy solids, we explicitly separate stretching (a linear transformation) from folding (a nonlinear transformation) in a quasi-two-dimensional experimental flow and study them independently. We compare results from two forcing schemes: one that is dominated by rotation, and another that is dominated by shear. [Preview Abstract] |
Wednesday, March 23, 2011 4:54PM - 5:06PM |
T9.00013: Principal Direction of Scalar Transport in Wall Turbulence Chiranth Srinivasan, Dimitrios Papavassiliou Lagrangian scalar tracking in conjunction with direct numerical simulation is utilized in an infinitely long channel to study the principal direction of scalar transfer for both forwards and backwards single particle dispersion. Four regions are of interest: the viscous sub-layer, the transition region (between the viscous sub-layer and the logarithmic region), the logarithmic region and the center of channel. Fluctuating velocities of scalar markers released in the flow field are correlated forwards and backwards in time to find the components of the correlation coefficient tensor. Eigenvalues and eigenvectors are obtained for both the forwards and backwards dispersion and for fluids with Prandtl number between 0.1 and 1000. The largest eigenvalues are higher in the case of backwards dispersion compared to the case of forwards dispersion. The eigenvector inclinations relative to the yz plane are different for forwards and backwards dispersion (at times comparable to the Lagrangian timescale). [Preview Abstract] |
Wednesday, March 23, 2011 5:06PM - 5:18PM |
T9.00014: Molecular origins of continuum fluid mechanics: Atomic migrations of single-phase fluid and slip boundary conditions Alan Graham, Shihai Feng, Tony Redondo We report the results of molecular dynamics simulations of pressure-driven flows of liquid argon in circular and planar conduits. We find that in inhomogeneous shear flows the molecules migrate to the center of the conduits and establish large radial density gradients under conditions that were previously assumed to be incompressible. These are the first predictions of shear-induced migration in pure fluids subjected to inhomogeneous shear flows. These density gradients increase monotonically with P\'{e}clet number. They result in a blunted velocity profile that deviates from the parabolic profile predicted by the Navier-Stokes equations for an incompressible fluid. Comparisons with simulations where the flow exhibits zero or linear shear indicate that this phenomenon is the result of the nonlinear shear flows and the finite size of the molecules. [Preview Abstract] |
Wednesday, March 23, 2011 5:18PM - 5:30PM |
T9.00015: Search for Euler Singularity using Vortex Filaments Sahand Hormoz, Michael Brenner A promising mechanism for generating a finite-time singularity in the incompressible Euler equations is stretching of vortex filaments. An exhaustive search of all possible initial conditions involving filaments, however, is not practically feasible. In this talk, I will show that two interacting vortex filaments can not generate a singularity for any initial conditions, by analyzing the asymptotic self-similar limit of their collapse. Essentially, our approach entails a separation of the dynamics of the filament shape, from the shrinking of its core. We solve for the dynamics using a self-similar ansatz and show that the core does not shrink fast enough for a self-consistent collapse. The similarity solution allows for many different collapse geometries, consistent with the tireless effort in the past of investigating new initial conditions. Potential for a singularity at higher number of filaments is also discussed. [Preview Abstract] |
Session T10: Focus Session: Growth, Structure, Dynamics, and Function of Nanostructured Surfaces and Interfaces -- Oxides
Sponsoring Units: DMPChair: Dan Dougherty, North Carolina State University
Room: D221
Wednesday, March 23, 2011 2:30PM - 3:06PM |
T10.00001: Cross-sectional scanning tunneling microscopy and spectroscopy of fractured oxide surfaces and heterostructure interfaces Invited Speaker: Recently, interfaces between novel oxide materials have become a playground for manipulation of new functionalities. At interfaces, the broken symmetry and the spatially confined environment have been shown to modify the local interactions and generate wholly new electronic phases (e.g. magnetism, metallicity, superconductivity etc.) distinct from the composite bulk materials. However, to date our understanding of these interface driven phases is still limited. While there exists powerful spatially resolved tools for visualizing the chemical and magnetic structure of an interface, a direct observation of electronic behavior across the interface presents a major experimental challenge. After the success of creating flat fractured surfaces on Nb-doped SrTiO$_{3}$ (Nb:STO) accessible to scanning tunneling microscopy (STM) [1-3], we have further harnessed the high-sensitivity to electronic local density of states (LDOS) of the scanning tunneling spectroscopy (STS) in cross-sectional geometry to visualize complex oxide interface electronic properties. By extending XSTM/S to the interface between colossal magnetoresistant manganite La$_{2/3}$Ca$_{1/3}$MnO$_{3}$ (LCMO) and semiconducting Nb:STO, we were able to map the LDOS across the boundary to unambiguously visualize the interface by the location of the valence band and elucidate the fundamental issue of band alignment at a complex oxide heterointerface [4]. \\[4pt] [1] TeYu Chien \textit{et al.}, \textit{Appl. Phys. Lett.} \textbf{95}, 163107 (2009). \\[0pt] [2] Nathan P. Guisinger \textit{et al.}, \textit{ACS nano} \textbf{3}, 4132 (2009). \\[0pt] [3] TeYu Chien \textit{et al.}, \textit{J. Vac. Sci. Technol. B} \textbf{28}, C5A11 (2010). \\[0pt] [4] TeYu Chien \textit{et al.}, \textit{Phys. Rev. B} \textbf{82}, 041101(R) (2010). [Preview Abstract] |
Wednesday, March 23, 2011 3:06PM - 3:18PM |
T10.00002: Electrical-stress-induced transport and surface potential characterizations of metal/TiO$_{2}$/metal planar junctions Haeri Kim, Dong-Wook Kim Electric-field-induced resistive switching (RS) phenomena in metal oxides have attracted considerable research interest due to their potential use in nonvolatile memory device applications. Intensive investigations have revealed that coupled electron ion dynamics play a key role the RS mechanism. Metal/single crystal junction can be an ideal model system to study how the ionic drift and diffusion can affect the resistance. We investigated transport and local electrical properties of Pt/TiO$_{2}$ single crystal/Ti planar junctions with micron- sized gaps between the electrodes. Scanning Kelvin probe microscopy (SKPM) showed that negative (positive) electrical stress to the Pt electrodes significantly reduced (hardly affected) the Pt/TiO$_{2}$ contact resistance. The SKPM results also revealed that the electrical stress caused alteration of the local work function of TiO$_{2}$. The comparative investigations of the transport and SKPM results suggested that the electrical stress induced redistribution of ions, resulting in the change of the junction resistance. [Preview Abstract] |
Wednesday, March 23, 2011 3:18PM - 3:30PM |
T10.00003: ABSTRACT WITHDRAWN |
Wednesday, March 23, 2011 3:30PM - 3:42PM |
T10.00004: Theoretical study of Ge/BaTiO$_{3 }$ Interfaces Kurt Fredrickson, Alexander Demkov It has been shown (McKee \textit{et al.,} Phys. Rev. Lett. \textbf{81,} 3014 (1998), and R. McKee, \textit{et al.}, \textit{Science }\textbf{293}$, $468 (2001)) that perovskite oxides SrTiO$_{3}$ and BaTiO$_{3}$ (BTO) can be grown epitaxially on Si and Ge, respectively. It would be interesting to achieve the reverse, i.e. to grow for example, Ge on BTO. It is not clear, however, whether one can achieve wetting of BTO by Ge. Theoretically, the energy of the Ge (001) surface is estimated to be anywhere between 591 and 1700 erg/cm$^{2}$ and the surface energy of BTO is in the range of 1083-1496 erg/cm$^{2}$ depending on termination and environment. The missing piece of information is the energy of the Ge/BTO interface. We examine five possible Ge/BTO interface structures and calculate their energies using density functional theory to determine which one has the lowest energy, and whether wetting can be achieved. [Preview Abstract] |
Wednesday, March 23, 2011 3:42PM - 3:54PM |
T10.00005: Strain relaxation in single crystal SrTiO$_{3}$ grown on Si (001) Miri Choi, Agham Posadas, Rytis Dargis, Dina Triyoso, David Theodore, Chih-Kang Shih, Alexander A. Demkov A layer of SrTiO$_{3}$ grown directly on Si may be used as a pseudo-substrate in perovskite deposition. As grown, SrTiO$_{3}$ is compressively strained, however, by subsequent annealing in oxygen at elevated temperature, a strain relieving SiO$_{x}$ buffer layer can be grown between the substrate and the perovskite layer. We perform a systematic study of strain relaxation in SrTiO$_{3}$ films grown on Si by molecular beam epitaxy as a function of the process conditions (annealing time, temperature, and oxygen partial pressure). Using a combination of X-ray diffraction, reflection high energy electron diffraction, and transmission electron microscopy we explore the oxidation and strain relaxation of SrTiO$_{3}$. We compare the kinetics of the buried oxide growth to that predicted by the conventional Deal-Grove model. An understanding of strain relaxation of SrTiO$_{3}$ on silicon can potentially be used to control the SrTiO$_{3}$ lattice constant for lattice matching with functional oxide overlayers. [Preview Abstract] |
Wednesday, March 23, 2011 3:54PM - 4:06PM |
T10.00006: X-ray 3D atomic imaging of Pt nanocrystals supported on SrTiO$_{3}$(001) Zhenxing Feng, Alexander Kazimirov, Michael Bedzyk Ultrathin metal or metal-oxide layers deposited onto oxide surfaces have wide applications in catalysis, chemical sensing and electronics. For sub-monolayer Pt deposited on the 2x1 SrTiO$_{3}$(001) surface, atomic-force microscopy shows the formation of nanoparticles and X-ray standing wave (XSW) atomic imaging shows that these nanoparticles are composed of Pt face-centered-cubic nanocrystals with cube-on-cube epitaxy coherent to the substrate unit cell. The phase sensitivity of the XSW allows for a direct measurement of the interface offset between the two unit cells along the c-axis. Different Pt coverages lead to differences in the observed XSW image of the interfacial structure, which is explained by the Pt-Pt interaction becoming stronger than the Pt-substrate interaction as the coverage is increased from 0.2 to 0.6 ML. Proposed atomic-scale interface models are based on a published double-layer TiO$_{2}$ terminated structure for the 2x1 SrTiO$_{3}$(001) surface and density functional theory. [Preview Abstract] |
Wednesday, March 23, 2011 4:06PM - 4:42PM |
T10.00007: Dynamics of early stage nano-oxidation by \textit{ in situ} UHV-TEM Invited Speaker: Environmental stability is one of the most important properties for materials exposed to air. As dimensions of engineered systems approach nanoscale, fundamental understanding of reactions with oxygen at this length scale is critical for environmental stability as well as for processing oxide nanostructures, where surface reactions are commonly utilized. The nanoscale stages of oxidation from the nucleation of the metal oxide to the formation of the thermodynamically stable oxide represent a scientifically challenging and technologically important terra incognito. The kinetics of early stage oxidation of Cu, Cu-Au and Cu-Ni alloys were visualized using in situ ultra-high vacuum transmission electron microscopy (UHV-TEM), where the initial oxidation stages can be observed in real-time under well-controlled surface conditions. We examined the dynamic responses of thin films to variations in thermodynamic variables such as temperature, oxygen pressure, strain, and crystallographic orientation. The kinetics of the nucleation and growth of three-dimensional oxide islands demonstrate that oxygen surface diffusion is the primary mechanism for oxide growth during initial oxidation in dry oxygen, and thus bears a striking resemblance to heteroepitaxy. Compared with the behavior of Cu films, the oxidation of Cu-Au alloys revealed more complexity. For example, the oxidation of (100)-oriented Cu-Au alloys with low Au content at $\sim $ 600C results in the formation of Cu2O oxide islands with a dendritic morphology and a non-uniform lateral distribution of Au around the islands. For Cu-Ni oxidation, the addition of Ni causes the formation Cu2O and/or NiO where the oxide type(s) and the relative orientation with the film depend on the Ni concentration, oxygen partial pressure and temperature. Evolution of the shape and size of the oxide islands can be quantitatively analyzed and provide fundamental insights into the complex kinetics and energetics of oxidation. Models based on surface orientation, strain development, and diffusion will be discussed to explain the formation of some of the novel oxide nano-structures. [Preview Abstract] |
Wednesday, March 23, 2011 4:42PM - 4:54PM |
T10.00008: Controlled surface reorganization of complex oxides by laser MBE Mikhail Kareev, B. Gray, Jian Liu, E.J. Moon, J. Chakhalian We report on the fabrication of ultra-thin layers of complex oxide perovskites, which display a variety of high-order surface reconstructions. In order to obtain the observed complex surface reconstructions (e.g. 6x2, 4x2, 4x4, etc.), nearly stoichiometric complex oxide material are found to be re-arranged into specific combinations of long-ordered periodic structures. We examine details of homo- and heterogeneous growth of SrTiO3 (STO) and LaNiO3 respectively on TiO2 terminated and mixed TiO2/SrO termination STO substrates by the combination of high-pressure RHEED and AFM to investigate mechanisms behind the high order surface reconstruction. J.C. was supported by DOD-ARO under the Contract No. 0402-17291 and NSF Contract No. DMR-0747808. [Preview Abstract] |
Wednesday, March 23, 2011 4:54PM - 5:06PM |
T10.00009: Quantum confinement effects in nanocrystals of SnO$_{2}$ in MgO matrix M.B. Sahana, C. Sudakar, A. Dixit, J.S. Thakur, R. Naik, V.M. Naik We have studied the nanocrystal formations of SnO$_{2 }$in$_{ }x$SnO$_{2}$-(1-$x)$MgO composite thin films ($x$ =0 to 1 and thickness 0.5 to 1 $\mu $m) prepared by metal-organic decomposition method. We find a direct relationship between the size of SnO$_{2}$ nanocrystals and the annealing temperature. Similarly, higher concentration of Mg in $x$SnO$_{2}$-(1-$x)$MgO leads to smaller size nanocrystals of SnO$_{2}$. Under the controlled choice of composition and annealing conditions, the bandgap of SnO$_{2}$ can be continuously increased from 3.89 eV to 4.5 eV thus providing a generic approach for tuning the bandgap in nanocomposite systems over a wide range of energy. We discuss this behavior in terms of the quantum confinement effect arising from particle size being comparable to the order of Bohr radius of the material. [Preview Abstract] |
Wednesday, March 23, 2011 5:06PM - 5:18PM |
T10.00010: Detection of Dielectric Trap States in Hafnium Oxide By Single Electron Tunneling Force Spectroscopy Dustin Winslow, Jon Johnson, Clayton Williams Atomic scale detection and imaging of electronic trap states in dielectric films has recently been demonstrated.\footnote{J.P. Johnson et al, Nanotechnology \textbf{20} (2009) 055701} Standard methods typically provide characterization over a much larger scale. Single Electron Tunneling Force Spectroscopy has been employed to measure the energy levels of trap states in HfO$_{2}$ with sub-nanometer spatial resolution. Analysis of individual spectra obtained at different locations shows variation in the density of defect states. When multiple spectra taken from 40 different locations are averaged, a broad peak 0.3 eV below the conduction band is observed, which agrees with data obtained over large areas by standard measurements.\footnote{G. Ribes et al, IEEE Trans. Dev. Mat. Reliability \textbf{6}, 132 (2006).} Additional peaks, not seen by the standard methods, are also observed. The method will be described and the data discussed. [Preview Abstract] |
Wednesday, March 23, 2011 5:18PM - 5:30PM |
T10.00011: Charge Injection and Relaxation in HfO$_{2}$ Films Measured by Single Electron Tunneling Force Spectroscopy Clayton Williams, Dustin Winslow, Jon Johnson Detection and imaging of individual trap states in dielectric materials with atomic scale spatial resolution has been recently demonstrated.\footnote{J.P. Johnson et al, Nanotechnology \textbf{20 }(2009) 055701.} Spectroscopic measurements on HfO$_{2}$ films by Single Electron Tunneling Force Spectroscopy have now shown evidence of both reversible and irreversible tunneling to and from these electronic trap states. The irreversibility is small near the middle of the band gap, becoming larger at $\sim $ 0.7 eV below the conduction band and 1.3 eV above the valence band. The irreversibilty of tunneling is likely due to charge relaxation. The evidence of charge relaxation in the film and a description of this new nanometer scale spectroscopic capability will be presented. The possible mechanisms by which the relaxation takes place will also be described. [Preview Abstract] |
Session T11: Spin and Transport in Low Dimensional Semiconductors
Sponsoring Units: FIAPChair: Nikolai Klimov, National Institute of Standards and Technology
Room: D222
Wednesday, March 23, 2011 2:30PM - 2:42PM |
T11.00001: Noble Metals and Transition Metals Adsorbed on Graphene: The Pursuit of Graphene Spintronics Matheus Paes Lima, Adalberto Fazzio Via first-principles simulations we study the structural, magnetic and electron transport properties of 2D graphene in the presence of single atoms. We consider Cu, Ag and Au, as well as Mn, Fe, Co and Ni adsorbed on pristine and defective graphene containing vacancies and divacancies. To obtain the transport properties we perform ab-initio calculations based on Density Functional Theory (DFT) coupled to Non-Equilibrium Greens` Function (NEGF) formalism using the Landauer-B\"uttiker formula within the Meir-Wingreen approach. Our results show that graphene+noble metals systems have a gate controllable spin polarized current, allowing the fabrication of switchable spin filters with a moderated efficiency. In the particular case of Gold adsorbed on pristine graphene, a positive gate leads to a polarized current with excess of up electrons, while a negative gate the converse. Despite the high-spin configuration of Mn, the d levels are very far from the Fermi level. The Ni atom prefers a non-magnetic configuration. Therefore, graphene+transition metals systems present a polarized current only for Fe and Co atoms, allowing the fabrication of spin filters with very high efficiency. [Preview Abstract] |
Wednesday, March 23, 2011 2:42PM - 2:54PM |
T11.00002: ABSTRACT WITHDRAWN |
Wednesday, March 23, 2011 2:54PM - 3:06PM |
T11.00003: Hole-induced Dynamic Nuclear Polarization in Quantum Dots Wen Yang, L.J. Sham We present a microscopic theory showing that through the non- collinear hole-nuclear dipolar hyperfine interaction, an optically excited heavy hole in a quantum dot can induce a steady-state nuclear polarization. The preferential direction of the nuclear spin flip is set by the energy mismatch of relevant transitions instead of thermal relaxation. The induced nuclear polarization shows a sign dependence on the product of the nuclear Zeeman splitting and the frequency detuning of the pumping laser, leading to bidirectional hysteretic locking of the optical absorption peak onto resonance or bidirectional hysteretic shift of the peak away from zero detuning. This sheds light on a puzzling observation of bidirectional hysteretic locking of the neutral exciton absorption peak in Faraday geometry [C. Latta et al., Nature Phys. 5, 758 (2009)]. By solving the Fokker-Planck equation for the nuclear polarization distribution, we found a ~10-fold suppression of the steady- state nuclear fluctuation, in reasonable agreement with the single pump experiment in Voigt geometry [X. Xu et al., Nature 459, 1105 (2009)]. [Preview Abstract] |
Wednesday, March 23, 2011 3:06PM - 3:18PM |
T11.00004: Beating of Friedel Oscillations in Spin-Orbit Coupled System Samvel Badalyan, Alex Matos-Abiague, Giovanni Vignale, Jaroslav Fabian The interplay of different spin-orbit interaction mechanisms induces highly anisotropic modifications of the static dielectric function of a two-dimensional electron system. One of the main changes of the static polarization function is the induced shift of its singularity position, which is in opposite directions for orthogonal momentum orientations. More interestingly, we have found that in certain situations the polarization function exhibits a \textit{doubly-singula}r behavior. This new property generates a novel phenomenon--\textit{the beating of Friedel oscillation}s, which can be controlled by an external electric field. This effect is a general feature of systems with Bychkov-Rashba and Dresselhaus spin-orbit fields and should be directly observable through tunneling microscopy imaging of the density distribution around an impurity. [Preview Abstract] |
Wednesday, March 23, 2011 3:18PM - 3:30PM |
T11.00005: Cyclotron resonance in graphene and Kohn's theorem Keshav Shrivastava In 1961 Kohn has shown [1] that the cyclotron frequency is independent of the interaction. In the case of graphene there is some effort to suggest that the electron dispersion is linear in k, instead of (h/2$\pi $ )$^{2}$k$^{2}$/2m so that the Kohn theorem may not apply [2]. We find that the Kohn theorem does not use the dispersion relation and applies to graphene the same way as in some other material. We find that if e is replaced by e*=(1/2)ge, the Kohn theorem applies with the cyclotron frequency (h/2$\pi )\omega _{c}$= (1/2)geB/mc. Hence there is no interaction and all of the interaction is contained in g = (2j+1)/(2l+1) which is used only in the unperturbed Hamiltonian. The degeneracy of the levels is found to be related to the flux quantization. We have explained [3] the plateaus observed in the Hall effect resistivity of graphene without the use of interaction. Hence the Kohn's them applies to graphene. \\[4pt] [1] W. Kohn, Phys. Rev. 123, 1242-1244 (1961);\\[0pt] [2] E. A. Henriksen, et al., Phys. Rev. Lett. 104, 067404(2010). \\[0pt] [3] K. N. Shrivastava, AIP Conf. Proc. 1150, 59-67(2009); 1017,422-428(2008); Proc. SPIE 7155, 71552F(2008). [Preview Abstract] |
Wednesday, March 23, 2011 3:30PM - 3:42PM |
T11.00006: Fluctuation-dissipation theorem for chiral systems in nonequilibrium steady states Dima Feldman, Chenjie Wang We consider a three-terminal system with a chiral edge channel connecting the source and drain terminals. Charge can tunnel between the chiral edge and a third thermal. The third terminal is maintained at a different temperature and voltage than the source and drain. We prove a general relation for the current noises detected in the drain and third terminal. It has the same structure as an equilibrium fluctuation-dissipation relation with the nonlinear response $\partial I/\partial V$ in place of the linear conductance. The result applies to a general chiral system and may be useful for detecting ``upstream'' modes on the quantum Hall edges. [Preview Abstract] |
Wednesday, March 23, 2011 3:42PM - 3:54PM |
T11.00007: Spontaneous Quantum Hall States in Chirally-Stacked Few-Layer Graphene Systems Fan Zhang, Jeil Jung, Gregory Fiete, Qian Niu, Allan MacDonald Chirally stacked N-layer graphene systems with N$\geq$2 exhibit a variety of distinct broken symmetry states in which charge density contributions from different spins and valleys are spontaneously transferred between layers. We explain how these states are distinguished by their charge, spin, and valley Hall conductivities, by their orbital magnetizations, and by their edge state properties. We argue that valley Hall states have [N/2] edge channels per spin-valley. [Preview Abstract] |
Wednesday, March 23, 2011 3:54PM - 4:06PM |
T11.00008: Re-entrant Negative Coulomb Drag in a 1D Quantum Circuit Dominique Laroche, Guillaume Gervais, Mike P. Lilly, John L. Reno We report Coulomb drag measurements between tunable vertically- coupled quantum wires. The wires are fabricated in a GaAs/AlGaAs double quantum well heterostructure with a 15 nm barrier separating the quantum wells and are non-ballistic. The Coulomb drag signal is mapped out versus the number of subbands occupied in each wire, and regions of both positive and negative drag are observed. Negative Coulomb drag signals are measured in two regimes: one at low electronic density when the drag wire is close to or beyond depletion, and one at higher electronic density when the drag wire has more than a single 1D subband occupied. A discussion of the negative drag signal in terms of electron-hole asymmetry and localization is presented. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04- 94AL85000. [Preview Abstract] |
Wednesday, March 23, 2011 4:06PM - 4:18PM |
T11.00009: Dynamic control over nanoparticle plasmon resonance through variation of refractive index Hari P. Paudel, Michael N. Leuenberger In a semiconductor material it is possible to vary the index of refraction by exciting electron-hole pairs through a laser pulse. Generally the change in refractive index changes linearly with the carrier density and also increases with the lattice temperature. We present our results on the variation in index of refraction in the TiO2 shell of an Ag/TiO2 core-shell nanoparticle by exciting electron-hole pairs in TiO2 through a laser pulse. We performed bandstructure calculations using VASP to determine the variation of the optical dielectric tensor as a function of photon frequency. This change in refractive index not only affects the refraction of photons with frequency below bandgap, but also affects strongly the resonance peaks of the surface plasmons due to the Ag core. This effect can be used to dynamically control the plasmon resonance of a hybrid metal-semiconductor nanoparticle, for example for use in cancer therapy or nanoplasmonic circuits. [Preview Abstract] |
Wednesday, March 23, 2011 4:18PM - 4:30PM |
T11.00010: Transport in Coherent Quantum Hall Bilayers Allan MacDonald, Dmytro Pesin We develop an approach to describe transport in bilayer quantum Hall systems in which coherence is established spontaneously between layers. We use Landauer-Buttiker theory with phenomenological parameters which can be fit to experimental data to describe quasiparticle transport in bilayers with strong coherence. We use the above approach to calculate two-probe conductances for various experimental configurations. We also apply the formalism to describe high-current transport in a bilayer with a time-dependent condensate. To describe the transition from strong to weak coherence, we use a pheonomenological single ``relaxation length'' ansatz for contact-to-contact transmission coefficients. As an application, we consider longitudinal drag, and find a good agreement with experiment in the regime of well- developed Quantum Hall Effect. [Preview Abstract] |
Session T12: Atomic Structures and Mechanical Properties in Semiconductors
Sponsoring Units: FIAPRoom: D223/224
Wednesday, March 23, 2011 2:30PM - 2:42PM |
T12.00001: A dual approach to quantum fluctuations in frustrated lattice spin models Anirban Gangopadhyay, Victor Galitski We develop a dual approach to describe quantum dynamics in lattice spin models, which allows us to describe nonperturbative \emph{quantum} trajectories in the spin path integral. The spin path integral takes the form of a combination of traces of ``dynamic density matrices'', which belong to either special linear group $SL(2,C)$ for a quantum ferromagnet or $SU(2)$ for an antiferromagnet. We analyze the latter model of a highly frustrated quantum antiferromagnet and find a class of non-perturbative trajectories that contribute to thermodynamics of the model at low temperatures. [Preview Abstract] |
Wednesday, March 23, 2011 2:42PM - 2:54PM |
T12.00002: Potassium-induced semiconducting to metallic transition on the $\beta$-SiC(100) c(4x2) surface Barry Haycock We present new data on the potassium-induced semiconducting to metallic transition of the silicon-terminated $\beta$-SiC(100) c(4x2) surface, resulting from density functional theory simulations. In our calculations we have analysed many different K-SiC(100) structures, corresponding to K coverages ranging from 0.08 to 1.25 monolayers (ML), paying special attention to the 2/3 ML and 1 ML cases where the transition has been reported. We find that the K-SiC(100) surface is metallic in all the cases. In spite of that, the K layer shows a semiconducting density of states (DOS) up to K coverages of $\sim$1ML, beyond which the potassium layer undergoes a transition to metallic behaviour, explaining the experimental observation. We propose a new atomic model for the surface reconstruction of the 1ML case which has far lower total energy than the previously suggested model based on linear K chains after simulated annealing studies. [Preview Abstract] |
Wednesday, March 23, 2011 2:54PM - 3:06PM |
T12.00003: The role of structural ordering in the semiconducting behavior of Cr$_3$Al Zoe Boekelheide, D.A. Stewart, F.J. Wong, Y. Suzuki, F. Hellman Cr$_{1-x}$Al$_x$ displays unexpected semiconducting behavior for x $\sim$ 0.25; an ordered Cr$_3$Al structure has been proposed to explain it.[1, 2] In this work, density functional theory calculations and nonequilibrium thin film growth were used to study the role of ordering on the transport properties. The atoms in Cr$_3$Al occupy the sites of a bcc lattice, like Cr. Calculations comparing possible structures show that the proposed chemically ordered, rhombohedrally distorted Cr$_3$Al structure, with ordering along the $<$111$>$ direction, is the lowest energy of those considered. In addition, the band structure shows a pseudogap, consistent with experimentally observed transport properties. Thin films of Cr$_{1-x}$Al$_x$ were grown with various growth and annealing temperatures to vary the properties. Samples with the most rhombohedral ordering are semiconducting. Decreased rhombohedral ordering leads to lower resistivity. Samples with a tetragonal distortion due to the C11$_b$ (Cr$_2$Al) structure have metallic resistivity. References: [1] D. J. Chakrabarti and P. A. Beck, J. Phys. Chem. Solids 32, 1609 (1971) [2] F. J. A. den Broeder et al, Phys. Status Solidi A, 67, 233 (1981) [Preview Abstract] |
Wednesday, March 23, 2011 3:06PM - 3:18PM |
T12.00004: Raman Investigation of Ru$_{2}$Si$_{3}$ Single Crystals and Thin Films A. Glen Birdwell, Daniel Lenssen, Conin B. Vining, Frank Crowne, Robert Glosser Raman spectra were obtained from Ru$_{2}$Si$_{3}$ single crystals and thin films. The spectra were taken with the incident light polarization parallel and perpendicular to each axis of the single crystal. When inspecting all the spectra of multiple geometric configurations, we were able to observe a total of thirteen phonon modes. To the best of our knowledge, there are very few published results with which to compare. However, the encouraging point of this work is that when spectra were obtained on thin film material composed of multiple crystal orientations, we have a very close correspondence to the modes found from the experiments on the single crystal. [Preview Abstract] |
Wednesday, March 23, 2011 3:18PM - 3:30PM |
T12.00005: Long-range-order and Short-range-order Structures in Sn-doped Ge Thin Films Yun-Liang Soo, S.L. Chang, J.F. Lee, H.H. Cheng The Sn-doped Ge semiconductor thin films have shown direct bandgap and many other interesting physical properties with great potential for technological applications. To understand the underlying mechanism for the unique properties of these materials, information on the locations of Sn atoms in the matrix and the effects of Sn doping on the crystal structure of Ge host is an important prerequisite. Samples of Sn-doped Ge thin films of thickness around 300{\AA} and Sn concentration 4 at.{\%} to 28 at.{\%} have been prepared by molecular beam epitaxy (MBE) method. Long-range-order and short-range-order structures in these films have been probed by using x-ray diffraction (XRD) and extended x-ray absorption fine structure (EXAFS) techniques, respectively. Our x-ray results demonstrate that Sn impurity atoms are located on the substitutional sites in the Ge films with Sn concentration up to 20 at.{\%}. Variation of lattice constant as a result of Sn doping in the Ge host will also be presented. [Preview Abstract] |
Wednesday, March 23, 2011 3:30PM - 3:42PM |
T12.00006: Wurtzite-derived polytypes of kesterite and stannite quaternary chalcogenide semiconductors Shiyou Chen, Ye Luo, Xin-Gao Gong, Aron Walsh, Su-Huai Wei The I$_2-$II$-$IV$-$VI$_4$ quaternary chalcogenide semiconductors (\textit{e.g.}, Cu$_2$ZnSnS$_4$, Cu$_2$ZnGeSe$_4$) have been studied for more than 40 years, but the nature of their crystal structures has proved contentious. Literature reports exist for the stannite and kesterite structures, which are zincblende-derived structures, and wurtzite-stannite, which is a wurtzite-derived structure. In this talk we report a new wurtzite-derived structure, wurtzite-kesterite (space group $Pc$), which is the ground state for some I$_2-$II$-$IV$-$VI$_4$ compounds, but is easily confused with the wurtzite-stannite structure. We show that there is a clear relationship between the properties of the wurtzite-kesterite and zincblende-derived kesterite structures,, as well as between wurtzite-stannite and stannite. The energy stability of different structures are studied according to the strain and Coulomb energy contributions, showing a dependence on the size and ionicity of the component atoms. Electronic structure of the wurtzite-derived structures will also be discussed. [Preview Abstract] |
Wednesday, March 23, 2011 3:42PM - 3:54PM |
T12.00007: Spontaneous symmetry breaking in two coupled nanomechanical electron shuttles Chulki Kim, Jonghoo Park, Robert Blick We present spontaneous symmetry breaking in two coupled nanomechanical electron shuttles. The electron shuttles are realized as silicon nanopillars and placed between two capacitor plates in a homogeneous electric field. Instead of being mechanically coupled through a spring they exchange electrons, i.e., they shuttle electrons from the source to the drain capacitor plate. The nonzero dc current through this system by external ac excitation is caused via dynamical symmetry breaking. The oscillation frequencies of the shuttling system are mode locked to the applied voltage frequency. [Preview Abstract] |
Wednesday, March 23, 2011 3:54PM - 4:06PM |
T12.00008: ABSTRACT WITHDRAWN |
Wednesday, March 23, 2011 4:06PM - 4:18PM |
T12.00009: Vibrational Modes of thin Silicon Membranes Reimar Waitz, Olivier Schecker, Elke Scheer Membranes with thicknesses in the range of hundred nanometers and macroscopic lateral size are interesting systems to study the mechanical properties of solids on various length scales. In our experiment a piezo is used to couple in vibrations, which can be observed with a phase-shift interferometer using stroboscopic light. With this technique we image transverse modes of frequencies up to 12 MHz. In general, the observed wave pattern of the membrane deflection will be a superposition of the mode corresponding to the excitation frequency and several higher harmonics. Using a Fourier transformation in time, it is possible to separate these contributions. This way eigenmodes up to the 8th harmonic of the excitation frequency can be imaged. The influence of strain on the dispersion relation is investigated by applying a pressure difference between both sides of the membrane. The results are compared to finite-elements simulations. [Preview Abstract] |
Wednesday, March 23, 2011 4:18PM - 4:30PM |
T12.00010: Phonon-imaging measurements of CaWO$_{4}$ Elastic Constants Timothy Head, Madeleine Msall Recent use of CaWO$_{4}$ in phonon-mediated detectors for a dark matter interactions by the Cryogenic Rare Event Search using Superconducting Thermometers (CRESST) collaboration have increased interest in precise measurements of CaWO$_{4}$ elastic constants. Phonon-imaging simulations based on continuum elasticity theory show that position and shape of phonon caustics depend sensitively on the elastic constants. Spatial and Temporal phonon flux distributions arising after point heat-pulse excitation have been measured for [001] and [010] oriented CaWO$_{4}$ crystals. We report elastic constants derived from time of flight measurements along symmetry directions, and calculated by matching experimental phonon images to simulations based on continuum elasticity theory. [Preview Abstract] |
Wednesday, March 23, 2011 4:30PM - 4:42PM |
T12.00011: Kinetic Monte Carlo simulation of GaAs homoepitaxy and droplet epitaxy Kris Reyes, Denis Nothern, Joanna Millunchick, Peter Smereka We present a new model for atomistic simulation of III-V semiconductors that is based on the solid-on-solid model and allows for multiple species, atom exchanges, and local considerations for atom bonding energies. The model is validated by comparison with experimental observations of GaAs homoeptiaxial growth. In particular the simulated surface concentration and growth modes agree with experiments over a wide range of growth conditions. An important feature of this model is that Ga and As atoms are treated explicitly, resulting in the ability to realistically model Ga droplet formation under low As overpressure and their recrystallization upon exposure to As. [Preview Abstract] |
Wednesday, March 23, 2011 4:42PM - 4:54PM |
T12.00012: First principles determination of vibrational and elastic properties of quaternary compounds Cu$_2$ZnSnS$_4$ and Cu$_2$ZnSnSe$_4$ Tanju Gurel, Cem Sevik, Tahir Cagin Recently, the quaternary compounds, Cu$_2$ZnSnS$_4$ and Cu$_2$ZnSnSe$_4$, have been attracted pretty much attention because of their potential use in the field of energy harvesting applications. Several theoretical calculations have been reported about their first principles electronic, optic and transport properties. However, no lattice dynamic calculations have been published yet despite the discussions about their possible ground state crystal structures and measured low thermal conductivity values. In this systematical study, we examined the vibrational and elastic properties of the two different crystal phases, kesterite (KS) with space group $I\overline{4}$ and stannite (ST) with space group $I\overline{4}2m$, of this quaternary compounds by using density functional perturbation theory. In addition, we predicted the relaxation time dependent lattice thermal conductivity within the solution of phonon Boltzmann transport equation by making use of the acquired vibrational frequency data. [Preview Abstract] |
Wednesday, March 23, 2011 4:54PM - 5:06PM |
T12.00013: Measuring graphene's thermal coefficient of expansion with bimetallic cantilevers Hiram Conley, Nickolay Lavrik, Dhiraj Prasai, Kirill I. Bolotin We developed a method for probing the thermal coefficient of expansion of 2D materials by measuring the deflection of a bimetallic cantilever. We fabricate suspended bimetallic cantilevers composed of single-layer or multilayer graphene and either gold or silicon nitride. Because of the mismatch of the thermal expansion coefficients, these cantilevers bend when heated. We employ laser interferometry to measure the bending and to extract the thermal expansion coefficient from -170 C to 250 C. We find that this technique provides a reliable measurement of the thermal expansion coefficient for graphene. Through comparison of the coefficient of expansion obtained from our bimetallic cantilevers and that of suspended graphene we demonstrate how graphene's dimensionality is perturbed by contact with other materials. [Preview Abstract] |
Wednesday, March 23, 2011 5:06PM - 5:18PM |
T12.00014: Temperature Dependence of Cyclotron Decoherance Time in a High Mobility Two-Dimensional Electron Gas J.A. Curtis, J.D. Moore, T.T. Tokumoto, J.G. Cherian, X. Wang, J.L. Reno, A. Belyanin, J. Kono, S.A. McGill, D.J. Hilton Using time-domain THz magneto-spectroscopy, we studied the dynamics of a 2DEG ($\mu =3.4\times 10^{6}\mathrm{cm}^{2}\,\mathrm{V}^{-1}\,\mathrm{s}^{-1}$) as a function of temperature ($0.4$K-$100$K).The decoherence lifetimes increase monotonically as temperature decreases below 1 K, which we have fit to a power law ($\tau \sim T^{0.29}$). We will discuss the mechanisms that contribute to the lifetimes. The transmitted pulse amplitude increases from 0.4K-1.2K, saturates from 1.5K-25K, and decreases from 50K-100K. J. A. Curtis is supported by a US Dept. Education GAANN Fellowship (P200A090143). [Preview Abstract] |
Session T13: Focus Session: Transport and Diffusion in Non-equilibrium Systems
Sponsoring Units: GSNPChair: Daniel Lacks, Case Western Reserve University
Room: D225/226
Wednesday, March 23, 2011 2:30PM - 3:06PM |
T13.00001: Thermodiffusion (Ludwig-Soret Effect) in Geological Systems Invited Speaker: Since its discovery more than a century and a half ago, the Ludwig-Soret effect (development of concentration gradients in response to a temperature gradient) has been documented in a large variety of inorganic and organic solutions, and exploited as an industrial tool for chemical and isotopic refinement. Theoretical treatments are numerous based on kinetic and thermodynamic principles, but none adequately explains the phenomenon operating in complex naturally occurring fluids. In the geological sciences, the Ludwig-Soret effect has received extraordinary attention as an agent for small (and large) scale chemical differentiation and a probe of fundamental fluid properties of the Earth's hydrosphere, and silicate (crust-mantle) and alloy (core) interior. In particular, silicate liquids show significant isotope fractionation by thermodiffusion at temperatures greatly exceeding those where equilibrium fractionation effects are vanishingly small, and this can persist to lower temperatures even with concomitant crystallization. We review these recent findings and present new experimental work on silicate and Fe(FeS) melts, considering the underlying causes towards reconciling observed mass-dependent isotope and mass-independent chemical effects. [Preview Abstract] |
Wednesday, March 23, 2011 3:06PM - 3:18PM |
T13.00002: Galilean thermodynamics of a multicomponent fluid and its induced electromagnetic fields Sylvain Brechet, Jean-Philippe Ansermet The phenomenological theory of irreversible processes in fluid systems has been successfully applied to new research fields, for example spintronics and spincaloritronics (arXiv:1011.2323). We include the electromagnetic interaction into the thermodynamic description of a multicomponent fluid. Our analysis is performed in the Galilean limit of electromagnetism. The tensorial part of the Onsager relations accounts in particular for mutliferroic effects. [Preview Abstract] |
Wednesday, March 23, 2011 3:18PM - 3:30PM |
T13.00003: Thresholds, memory, and self-similarity on river deltas Meredith Reitz, Douglas Jerolmack The bulk dynamics of river deltas and alluvial fans result from several physical processes acting on a wide range of scales. We study a series of experimental alluvial fans to sort the relevant processes and determine the way in which their interaction drives fan behavior. We find a timescale of channel movement that depends on mass conservation, as sediment fills a wedge of space determined by a separation between conditions of grain entrainment and distrainment, in a manner analogous to the separation between static and dynamic angles of repose in dry granular systems. Channel path selection behavior shows a marked tendency for flow to reoccupy abandoned paths, in a way that can be abstracted with a random walk model in a system with absorbing states, and resulting in a predictable self-similar shoreline growth pattern. Because we isolate the processes that drive the evolution of our experimental fans, we are able to translate our findings to the study of natural fans and deltas in which the same processes operate. [Preview Abstract] |
Wednesday, March 23, 2011 3:30PM - 4:06PM |
T13.00004: Tailoring Polymer Nanocomposite Properties by Nanoparticle Assembly Invited Speaker: Novel materials based on polymer-grafted nanoparticles (NP) are the focus of this talk. Since inorganic NPs and organic polymers typically ``dislike'' each other, these ``hairy'' particles behave like block-copolymers or amphiphiles. They can, therefore, self-assemble into a range of superstructures when placed in an organic matrix. Understanding the factors controlling this this assembly state and how it affects the properties of the resulting material are our central interests in this area. As part of this global effort, here we address three questions: (i) Can we direct NP assembly using external fields, e.g., shear, with the ultimate goal of designing membranes with directional transport properties? (ii) Can we assemble grafted NPs at interfaces with the aim of compatibilizing immiscible polymer blends? (iii) Can NP assemblies result in simultaneous improvements in the Young's modulus, the yield stress and strain-to-break of an amorphous polymer in the solid-state? As with all of our work we combine theory and experiments to understand these concepts that underpin our nascent understanding in this area. [Preview Abstract] |
Wednesday, March 23, 2011 4:06PM - 4:18PM |
T13.00005: Thermodynamic Analysis of Nanoporous Membrane Separation Processes David Rogers, Susan Rempe We give an analysis of desalination energy requirements in order to quantify the potential for future improvements in desalination membrane technology. Our thermodynamic analysis makes it possible to draw conclusions from the vast array of equilibrium molecular dynamics simulations present in the literature as well as create a standardized comparison for measuring and reporting experimental reverse osmosis material efficiency. Commonly employed methods for estimating minimum desalination energy costs have been revised to include operations at positive input stream recovery ratios using a thermodynamic cycle analogous to the Carnot cycle. Several gaps in the statistical mechanical theory of irreversible processes have also been identified which may in the future lead to improved communication between materials engineering models and statistical mechanical simulation. Simulation results for silica surfaces and nanochannels are also presented. [Preview Abstract] |
Wednesday, March 23, 2011 4:18PM - 4:30PM |
T13.00006: Geometric Phase Effect in Heat Transport Jie Ren, Petter Hanggi, Baowen Li Nonlinear molecular heat-pumping devices, which operate via explicitly modulating at least two parameters, are crucial for energy control in low dimensional nano-scale systems. We have applied slow two-parameter modulations on such a molecular junctions and consequently uncovered an intrinsic heat flux contribution, additional to the known, usual dynamical heat flux (from hot to cold). This additional heat flux derives from a nontrivial geometric origin that relates to a non-vanishing, so termed finite Berry phase. It provides a free lunch for the pumped heat and even can direct heat flux against the temperature bias. In addition we are able to show that this so pumped energy exhibits a novel robust fractional quantization phenomenon. Interestingly, this additional geometric heat pump mechanism is also shown to cause a breakdown of the heat-flux fluctuation theorem, which holds true for the non-driving, stationary heat flux transfer. The validity of this theorem is guaranteed whenever (i) the geometric phase contribution vanishes and (ii) the cyclic protocol preserves the detailed balance symmetry. [Preview Abstract] |
Wednesday, March 23, 2011 4:30PM - 4:42PM |
T13.00007: Contact processes in crowded environments J.M. Schwarz, Bismayan Chakrabarti A nonequilibrium absorbing state phase transition with conserved particle number has been realized in a periodically sheared particle suspension. A diffusing (active) particle in the suspension collides with a stationary (inactive) particle and activates it. As the strain amplitude is increased, the fraction of active particles per shear cycle becomes nonzero only above some critical strain amplitude. To further study this system at higher densities, we construct a lattice model with active and inactive particles occupying some fraction of the lattice sites with each site being occupied by at most one particle. The active particles hop to empty neighboring sites and activate $k$ neighboring inactive particles at some rate $\lambda_k$. Also, active particles become inactive at some rate $\gamma$. We investigate this model for $\lambda_{k=1}=0$ and $\lambda_{k>1}>0$ to study the effects of multi-particle collisions which are likely to occur at higher densities, i.e. crowded environments. [Preview Abstract] |
Wednesday, March 23, 2011 4:42PM - 4:54PM |
T13.00008: Mixing of Diffusing Particles Eli Ben-Naim We study how the order of $N$ independent random walks in one dimension evolves with time. Our focus is statistical properties of the inversion number $m$, defined as the number of pairs that are out of sort with respect to the initial configuration. In the steady-state, the distribution of the inversion number is Gaussian with the average $\langle m\rangle \simeq N^2/4$ and the standard deviation $\sigma\simeq N^{3/2}/6$. The survival probability, $S_m(t)$, which measures the likelihood that the inversion number remains below $m$ until time $t$, decays algebraically in the long-time limit, $S_m\sim t^{-\beta_m}$. Interestingly, there is a spectrum of $N(N-1)/2$ distinct exponents $\beta_m(N)$. We also find that the kinetics of first passage in a circular cone provides a good approximation for these exponents. When $N$ is large, the first-passage exponents are a universal function of a single scaling variable, $\beta_m(N)\to \beta(z)$ with \hbox{$z=(m-\langle m\rangle)/\sigma$}. In the cone approximation, the scaling function is a root of a transcendental equation involving the parabolic cylinder equation, $D_{2\beta}(-z)=0$, and surprisingly, numerical simulations show this prediction to be exact. [Preview Abstract] |
Wednesday, March 23, 2011 4:54PM - 5:06PM |
T13.00009: Crossover behavior in models of depinning Yan-Jiun Chen, Lasse Laurson, Stefanos Papanikolaou, Stefano Zapperi, James P. Sethna We explore the behavior of models describing driven interfaces in random media. These models are useful in describing a wide range of real-world systems: disordered magnets, fluids in porous medium, pinning of flux lines in superconductors, and fluid imbibition in paper. Variations of these models have been numerically studied and classified into distinct universality classes at the depinning transition, however the exact structure of the phase space is still not known. We are investigating the crossover behavior in between various linear and nonlinear models with short-range and long-range interactions, and will report on their respective scaling functions of height-height correlation and size distributions of avalanches. [Preview Abstract] |
Wednesday, March 23, 2011 5:06PM - 5:18PM |
T13.00010: An Information-Theoretic Order Parameter for Non-Equilibrium Systems Martin Tchernookov, Ilya Nemenman In non-equilibrium statistical physics, symmetry and free energy are difficult to define, preventing application of classical machinery for analysis of phase transitions. Can one define a ``universal'' order parameter that would be measurable from experimental data, would allow identification of an onset of a phase transition, and would be meaningful independently of the underlying systems dynamics? We suggest that predictive information, which is the mutual information between the sequence of the observed past states of a system and its future states, introduced by us in 2001,\footnote{W Bialek, I Nemenman, N Tishby. Neural Computation (2001),13,2409} may serve as such order parameter. We study this suggestion in the context of a model non-stationary Langevin process. We show analytically that the predictive information attains its maximum value at the phase transition, diverging logarithmically with the length of the observed past. We demonstrate that the speed of divergence is related to traditional critical exponents. Finally, we show how the onset of a phase transition can be found empirically from data, independently of its parameterization. [Preview Abstract] |
Wednesday, March 23, 2011 5:18PM - 5:30PM |
T13.00011: Classical nucleation theory explains the critical cooling rate of cryoprotectant solutions Matt Warkentin, Robert Thorne We have measured critical cooling rates for a range of concentrations of different solutes in aqueous solutions. Our results show that the glass formability of aqueous solutions is exponential in the concentration for all solutes tested, with a different characteristic concentration for each solute. This characteristic correlates with the Stokes radius of the solute. A simple modification of critical droplet theory relates the characteristic concentration to the critical nucleation radius in pure water, and explains the relationship between the Stokes radius and the exponential characteristic. This simple, general theory of glass formability in aqueous solutions is important at a fundamental level, and will also have broad consequences for the field of cryobiology. [Preview Abstract] |
Session T14: Focus Session: Extreme Mechanics: Elasticity and Deformation II
Sponsoring Units: GSNPChair: Benny Davidovitch, University of Massachusetts, Amherst
Room: D227
Wednesday, March 23, 2011 2:30PM - 3:06PM |
T14.00001: Elasto-capillarity: The role of stretching Invited Speaker: Elasto-capillarity, the interaction between elasticity and surface tension or surface energies, has been much studied in recent years. However, to date the focus has been almost exclusively on situations where the bending stiffness of an object resists deformation by the surface tension of a liquid. In this talk I will consider some situations in which it is instead the stretching stiffness of an elastic object that resists its deformation by surface tension. I will focus on explaining recent experiments that demonstrate the wrinkling of floating elastic membranes [J. Huang \emph{et al.}, \emph{Science} \textbf{317}, 650 (2007)] but will also discuss related fundamental problems in wrinkling as well as other situations that involve an interaction between the stretching of an elastic object and capillarity. [Preview Abstract] |
Wednesday, March 23, 2011 3:06PM - 3:18PM |
T14.00002: Building blocks for the shapes of confined elastic sheets Robert Schroll, Eleni Katifori, Benny Davidovitch Several configurations, such as d-cones, minimal ridges, and developable patches, occur regularly in the configuration of elastic sheets. We dub such features ``building blocks.'' Here, we study elastic sheets confined in a manner that prohibits the sheet from taking on a single-buckle shape. We find not only building blocks where stress focuses, reminiscent of d-cones, but also ``diffuse-stress'' regions. The former is characterized by a geometrical constraint (inextensibility) while the latter is governed by a mechanical constraint: the dominance of a single component of the stress tensor. We characterize how boundary conditions and applied tension select which building blocks appear and discuss implications for the curtain problem. [Preview Abstract] |
Wednesday, March 23, 2011 3:18PM - 3:30PM |
T14.00003: Droplet Formation and Scaling in Dense Suspensions Marc Miskin, Heinrich Jaeger A drop detaching from a nozzle is a prototypical example of scaling behavior. For a pure fluid, this scaling is contingent on the fact that the material parameters remain invariant throughout the detachment. However, for a dense suspension, this assumption is invalid. We use high-speed photography to examine the formation of suspension droplets. We find that the minimum neck radius, $R_{m}$, near breakup can be described by a power law $(t_{b}- t)^{2/3}$, with a material independent exponent. By considering how particles deform the surface and appealing to topological constraints, we develop a modified version of the Laplace-Young equation relating the surface pressure to the macroscopic Gaussian curvature. This model, combined with a scaling argument, allows us to collapse all of our data for $R_{m}$ near breakup. These results open a new territory for modeling suspensions by asserting that a major stress resides at the boundary, and that it can be calculated using strictly macroscopic parameters. [Preview Abstract] |
Wednesday, March 23, 2011 3:30PM - 3:42PM |
T14.00004: Surface patterns in thermally responsive elastomeric gels Shawn Chester, Lallit Anand Many stimulus responsive elastomeric gels operate in non-isothermal, chemically saturated environments in a variety of applications. We have recently developed a three dimensional continuum level theory to describe the coupled fluid permeation and large deformation response of thermally responsive elastomeric materials. In this work, we apply our theory and numerical simulation capability to the specific case of surface wrinkles induced via swelling of a thermally responsive gel bonded on top of a compliant impermeable elastic substrate. We show that we can numerically model the swelling behavior and subsequent surface pattern formation. Also, we examine the effect of substrate thickness by varying the ratio of gel to substrate thickness. Further, we show that it is possible to modulate the amplitude of the surface wrinkles by taking advantage of the thermally responsive nature of this class of materials. [Preview Abstract] |
Wednesday, March 23, 2011 3:42PM - 3:54PM |
T14.00005: Morphologies of Equibiaxially Wrinkled Surfaces Derek Breid, Shengqiang Cai, Zhigang Suo, John Hutchinson, Alfred Crosby The morphological characteristics of a wrinkled film are largely determined by the state of stress at the onset of the instability. For surfaces compressed equibiaxially, it is well established that ridge-based structures, including herringbone or labyrinth patterns, provide the lowest energy state for stresses far exceeding critical buckling. For near-critical stresses, the equilibrium morphology is less understood. Using surface-oxidized poly(dimethylsiloxane) as a model wrinkling material, we control the applied stress by swelling the oxide film with a compatible vapor-phase solvent. The extent of swelling is controlled by the vapor pressure of the solvent and the thickness of the oxide layer, and the generated overstress in turn dictates the observed morphology. Analytical and numerical models are used to determine the deformation morphologies that provide the lowest energy state with increasing overstress. Comparison of experimental observations and theoretical predictions provides insight into the importance of substrate curvature in determining final equilibrium morphologies. [Preview Abstract] |
Wednesday, March 23, 2011 3:54PM - 4:06PM |
T14.00006: Revisiting the curvature cancellation in forced thin sheets Jin Wang, Thomas Witten We revisit the numerically observed spontaneous vanishing of mean curvature [1] on a developable cone or ``d-cone'' [2] made by pushing a thin elastic sheet into a circular container. The deflection of the d-cone is the distance by which the sheet is pushed into the container. We investigate the ratio of the two principal curvatures versus sheet thickness $h$ over a wider dynamic range than was used previously, holding the deflection and radius fixed. Instead of tending towards 1 as suggested by previous work, we find that the ratio scales as $h^{1/3}$. Scaling arguments and geometric variants support this $h^{1/3}$ finding. Thus the mean curvature does not vanish for very thin sheets as previously claimed. \\[4pt] [1] T. Liang and T. A. Witten, {\sl Phys. Rev. E} {\bf 73}, 046604 (2006). \newline [2] E. Cerda, S. Chaieb, F. Melo, and L. Mahadevan, {\sl Nature} {\bf 401}, 46 (1999). [Preview Abstract] |
Wednesday, March 23, 2011 4:06PM - 4:18PM |
T14.00007: Mechanics of Geometrically-Tuned pH-Responsive Polymers Lifeng Wang, Lin Han, Khek-Khiang Chia, Robert Cohen, Michael Rubner, Mary Boyce, Christine Ortiz Stimuli-responsive polymer materials have been extensively explored over the past two decades because of their promising applications. We consider the mechanics of mechanomutable polyelectrolyte multilayers (PEMs), which undergo reversible pH-responsive transition from a condensed, ionically crosslinked state (small pH) to a hydrated, ionized state (large pH). Instrumented indentation and micro-structurally-based finite element analysis are conducted on the PEM thin films and PEM tube forests to determine the effective elastic properties and further the mechanomutability as a result of the coupling between inherent responsive material properties and geometry. We demonstrate that geometry can be used to introduce and tailor different deformation mechanisms as a means to tune mechanomutabilibility of stiffness and dissipation in addition to the constitutive material properties. The rate-dependent stimulus-responsive mechanomutability can be finely controlled within a wide range from $\sim $ 2 -- 100 times by tailoring the tube geometrical factors at different indentation rates. These studies provide fundamental understanding and mechanics of indentation of PEM thin films and tube forests and show the tremendous potential for dynamically tuning surface and bulk properties of novel complex structured materials. [Preview Abstract] |
Wednesday, March 23, 2011 4:18PM - 4:30PM |
T14.00008: Stretch-induced compressive stresses and wrinkling in hyperelastic thin sheets Rui Huang, Vishal Nayyar, K. Ravichandar Wrinkles are commonly observed in stretched thin sheets. This paper presents a study on stretch-induced wrinkling of hyperelastic thin sheets using the finite element method. The model problem is set up for uniaxial stretching of a rectangular sheet with two clamped ends and two free edges. A two-dimensional stress analysis is performed first under the plane-stress condition to determine stretch-induced stress distributions in the elastic sheets, assuming no wrinkles. As a prerequisite for wrinkling, the development of compressive stresses in the transverse direction is found to depend on the length-to-width aspect ratio of the sheet and the applied stretch. A phase diagram is constructed with a set of different distribution patterns of the compressive stress spanning a wide range of aspect ratios and up to moderately large tensile strain ($\sim $150{\%}). Next, an eigenvalue analysis is performed to find the potential buckling modes of the elastic sheet under the prescribed boundary conditions. Finally, a nonlinear post-buckling analysis is performed to show evolution of the stretch-induced wrinkles. In addition to the aspect ratio and the applied stretch, it is found that the critical condition for wrinkling and the post-buckling behavior both depend sensitively on the sheet thickness. [Preview Abstract] |
Wednesday, March 23, 2011 4:30PM - 4:42PM |
T14.00009: Elastic Instabilities of Nematic Liquid Crystals in Spherical Geometries Vinzenz Koning, Teresa Lopez-Leon, K.B.S. Devaiah, Alberto Fernandez-Nieves, Vincenzo Vitelli We investigate elastic instabilities of nematic liquid crystals confined in spherical shells characterized by extreme thickness inhomogeneity. For shells with a uniform thickness there exists an equilibrium defects structure containing two pairs of boojums at the north and south poles. By minimizing the elastic free energy (subject to tangential boundary conditions on both bounding surfaces), we determine the locations of the defects as a function of thickness inhomogeneity. We find that the defects make an abrupt confinement transition to the thinnest hemisphere from the initial antipodal arrangement, when the thickness inhomogeneity exceeds a critical value. Our results agree well with recent experimental studies on nematic double emulsions and suggest design criteria to engineer micron scaled particles with directional binding capabilities. [Preview Abstract] |
Wednesday, March 23, 2011 4:42PM - 4:54PM |
T14.00010: Near critical phenomena in amorphous smart materials Eduard Oberaigner, Michael Fischlschweiger The importance of smart materials e.g. shape memory alloys (SMAs) for technological applications has been growing during the last 20 years. Especially modeling SMAs behavior has become of high interest in materials science for the prediction of macroscopic effects like pseudoelasticity. The key for their behavior is a displacive solid - solid phase transformation, called martensitic phase transformation. However, such a critical phenomenon requires investigations for deep relations between physical quantities nearby the region of phase transformation. The present study is focusing on a statistical mechanics approach for the description of relations between heat capacity, pseudoelasticity, volume fraction compressibility, alternatively fraction expansion coefficient, and a compressibility tensor, leading to the compliance tensor in the case of elasticity. Also a heat expansion tensor along the line of magnetic phase transitions and transformations has been formulated for shape memory alloys. The work discusses martensitic variants (which occur due to a subgroup relation between the austenitic and martensitic phase) and their asymmetry, which influences the above mentioned quantities as well and gives ideas and suggestions for model improvements. [Preview Abstract] |
Wednesday, March 23, 2011 4:54PM - 5:06PM |
T14.00011: The role of membrane viscosity in the relaxation dynamics of fluid membranes Marino Arroyo, Luca Heltai, Antonio DeSimone Fluid membranes made out of lipid bilayers are the fundamental separation structure in eukaryotic cells. Many physiological processes rely on dramatic shape and topological changes (e.g. fusion, fission) of fluid membrane systems. Fluidity is key to the versatility and constant reorganization of lipid bilayers. Here, we study the role of the membrane intrinsic viscosity, arising from the friction of the lipid molecules as they rearrange to accommodate shape changes, in the dynamics of morphological changes of fluid vesicles driven by curvature elasticity. In particular, we analyze the competition between the membrane viscosity and the viscosity of the bulk fluid surrounding the vesicle as the dominant dissipative mechanism. We consider the relaxation dynamics of fluid vesicles put in an out-of-equilibrium state, but conclusions can be drawn regarding the kinetics or power consumption in regulated shape changes in the cell. On the basis of numerical calculations, we find that the dynamics arising from the membrane viscosity are qualitatively different from the dynamics arising from the bulk viscosity. When these two dissipation mechanisms are put in competition, we find that for small vesicles the membrane dissipation dominates, with a relaxation time that scales as the size of the vesicle to the power 2. For large vesicles, the bulk dissipation dominates, and the exponent in the relaxation time vs. size relation is 3. [Preview Abstract] |
Wednesday, March 23, 2011 5:06PM - 5:18PM |
T14.00012: The star shaped pattern on broken thin sheets Nicolas Vandenberghe, Romain Vermorel, Emmanuel Villermaux We study transverse impacts of rigid objects on a thin elastic sheet made of acrylic. After impact, a transverse wave propagates on the sheet and orthoradial stresses lead to the formation of radial cracks. The result of this fragmentation process is the star shaped pattern frequently observed on broken windows. We investigate the variation of the pattern and in particular the number of radial cracks with impact speed and material properties. The formation of rayed craters by meteorite impacts will be briefly discussed. [Preview Abstract] |
Wednesday, March 23, 2011 5:18PM - 5:30PM |
T14.00013: How to make sticky tapes stickier Laurent Ponson, Shuman Xia, Guruswami Ravichandran, Kaushik Bhattacharya Thin film adhesives have become increasingly important in various applications such as packaging and coating, and we benefit daily of their adhesion properties by using various kinds of tapes. Despite the apparent simplicity of these systems, a certain number of questions remain open. In particular, important efforts have been deployed recently to understand the effect of the complex tridimensional and highly heterogeneous structure at the interface of some adhesives, such as the one encountered in nature like the geckos toes. Although inspired by these natural adhesives, we studied a much simpler system, and however largely unexplored: a thin film with spatially varying adhesion energy and elastic properties. We will show how these heterogeneities introduced at the microscale can generate quite unexpected macroscopic behaviors, and that one can this way design stronger adhesives with new properties. Beyond their practical interests, these systems involve long range elastic interactions and heterogeneities resulting in a rich and complex physics that will be illustrated through experimental examples and their theoretical interpretation. [Preview Abstract] |
Session T15: Focus Session: Spins in Semiconductors - III-V Magnetic Semiconductors
Sponsoring Units: DMP GMAG FIAPChair: Ken Burch, University of Toronto
Room: D171
Wednesday, March 23, 2011 2:30PM - 3:06PM |
T15.00001: Valence-band structure of the ferromagnetic semiconductor GaMnAs investigated by resonant tunneling spectroscopy Invited Speaker: The origin of ferromagnetism in the prototype ferromagnetic semiconductor GaMnAs is still controversial due to the insufficient understanding of its band structure and Fermi level position. Here, we investigate the valence-band (VB) structure of GaMnAs by analyzing the resonant tunneling levels of the GaMnAs quantum well (QW) in double-barrier heterostructures. The resonant levels including the heavy-hole first state (HH1) are clearly observed in the metallic GaMnAs QW with the Curie temperature (T{\_}C) of 60 K, which indicates that no holes reside in the VB of GaMnAs in the equilibrium condition. Clear enhancement of tunnel magnetoresistance induced by resonant tunneling is demonstrated. We find that the resonant levels formed in the GaMnAs QW are well explained by using the transfer matrix method with the 6x6 \textit{kp} Hamiltonian and small $p-d$ exchange Hamiltonian. The VB structure of GaMnAs is well reproduced by that of GaAs with a small exchange splitting energy of 3-5 meV and with the Fermi level lying at $\sim $30 meV higher than HH1 in the bandgap. Furthermore, we show our more recent results of resonant tunneling spectroscopy on various surface GaMnAs films (Mn concentration: 6-15{\%}, T{\_}C: 71-154 K) grown on an AlAs layer, where the resonant levels are formed by confinement of the VB holes by the surface Schottky barrier and the AlAs barrier. We systematically investigate the thickness dependence of the resonant levels in GaMnAs by precisely etching the surface of GaMnAs. We find that the p-d exchange interaction is negligibly small (3-5 meV) and that the Fermi level exists in the bandgap. This work was performed in collaboration with I. Muneta, P. N. Hai, K. Takata, and M. Tanaka, and partly supported by Grant-in-Aids for Scientific Research, the Special Coordination Programs for Promoting Science and Technology, and FIRST Program by JSPS.\\[4pt] [1] S. Ohya et al., Phys. Rev. Lett. 104, 167204 (2010).\\[0pt] [2] S. Ohya et al., arXiv:1009.2235. [Preview Abstract] |
Wednesday, March 23, 2011 3:06PM - 3:18PM |
T15.00002: On magnetism and the insulator-to-metal transition in $p$-doped GaAs Brian Chapler, R.C. Myers, S. Mack, A. Frenzel, B.C. Pursley, K.S. Burch, E.J. Singley, A.M. Dattelbaum, N. Samarth, D.D. Awschalom, D.N. Basov Although Ga$_{1-x}$Mn$_{x}$As is often described as the prototypical ferromagnetic semiconductor, many aspects of the electronic structure and nature of mediating carriers remain open. A central question in this regard is whether the insulator-to-metal transition (IMT) in $p$-doped GaAs is significantly modified when dopants are magnetic. We address this through an infrared spectroscopic study of GaAs doped with either non-magnetic Be or magnetic Mn acceptors. Through our comparison, we are able to isolate effects of magnetic dopants in GaAs from those associated with disorder and proximity to the IMT. Here we show Mn-doped samples exhibit an unusual electronic transport regime, combining elements of both metallic and insulating behavior, at doping concentrations far beyond the onset of the IMT. Be-doped films however, reveal genuine metallicity just above the IMT boundary. These results underscore the pivotal role of magnetism in transport and optical phenomena of Ga$_{1-x}$Mn$_{x}$As. [Preview Abstract] |
Wednesday, March 23, 2011 3:18PM - 3:30PM |
T15.00003: Magnetic properties of narrow gap In$_{1-x}$Mn$_{x}$Sb semiconductor films with x$>$0.10 Caitlin Feeser, John Peters, Bruce Wessels Narrow gap In$_{1-x}$Mn$_{x}$Sb magnetic semiconductors with x $<$ 0.05 have been recently shown to have interesting magnetotransport properties at room temperature.\footnote{J. A. Peters et al, PRB \textbf{82} 2010.} Calculations based on the field dependence of the magnetoresistance indicate that the carriers are highly spin polarized. To increase both the saturation magnetization and potentially the Curie temperature Tc of the alloys, we have investigated MOVPE epitaxial layers with 0.10 $<$ x $<$ 0.22. Films were ferromagnetic at room temperature, showing clear hysteresis in field dependent measurements from 5 to 300 K. Alloys with magnetization values as high as 83 emu/cm$^{3}$ for x=0.22 were measured at 5 K. Temperature dependent magnetization indicated that the Curie temperature of the films was above 400 K. These measurements indicated the presence of two magnetic species both with Curie temperatures above 300 K. The high Tc is attributed to carrier mediated ferromagnetism involving Mn and its complexes that form shallow or resonant electronic states with the valence band through correlated substitution. [Preview Abstract] |
Wednesday, March 23, 2011 3:30PM - 3:42PM |
T15.00004: Cyclotron Resonance in InMnAs and InMnSb Ferromagnetic Films Giti Khodaparast, Y.H. Matsuda, R. Shen, S. Takeyama, X. Liu , J. Furdyna, B.W. Wessels Ferromagnetic semiconductors are important materials for development of spintronic devices. While effort in this area was made primarily on GaMnAs, other ferromagnetic III-Mn-V alloys have also been developed, including the narrow gap ferromagnetic alloys such as InMnAs and InMnSb. Investigation of the electronic structure of III-Mn-V alloys by techniques such as the cyclotron resonance (CR) can shed important light on the origin of ferromagnetism and the p-d exchange interaction in III-Mn-V systems. In this work we report on CR experiments carried out on the ferromagnetic InMnAs and InMnSb films, on which clear resonance signals have been successfully observed in high magnetic fields generated by a single turn coil technique. The CR in ferromagnetic InMnSb was observed for the first time and we compare our observations with the Landau levels calculations on the basis of an 8-band $k\dot.p$ model. [Preview Abstract] |
Wednesday, March 23, 2011 3:42PM - 4:18PM |
T15.00005: Visualizing Critical Correlations Near the Metal-Insulator Transition in Ga$_{1-x}$Mn$_x$As Invited Speaker: Semiconductors have long been an ideal class of materials for studying the metal-insulator transition. Samples of the dilute magnetic semiconductor Ga$_{1-x}$Mn$_x$As with Mn doping levels near to the metal-insulator transition have been studied using low temperature cross-sectional scanning tunneling microscopy (STM). This allows us to visualize the electronic states near the Fermi level which display unique critical properties. Strong modifications to the density of states around the Fermi energy due to electron-electron interactions are observed. In this energy range, the electronic states show a diverging correlation length approaching E$_F$, where the correction to the density of states due to interactions is strongest. The distance dependence of the correlations at E$_F$ is consistent with a power law decay, expected for multifractal states near criticality in the metal-insulator transition, while away from E$_F$ the correlations fall off exponentially. These results highlight the importance of electron-electron interactions and represent some of the first experimental observations of states near the Mott-Anderson metal-insulator transition, where both disorder and interactions are important for the localization of electronic states. [Preview Abstract] |
Wednesday, March 23, 2011 4:18PM - 4:30PM |
T15.00006: Time resolved spectroscopy of MOVPE grown narrow gap III-Mn-V ferromagnetic semiconductors T. Merritt, M. Bhowmick, G.A. Khodaparast, C. Feeser, B.W. Wessels, S. McGill The emergence of III-Mn-V magnetic semiconductors, has led to a number of exciting results relevant to the spin and charge based applications. Important advances have now been made in the MOVPE growth of the narrow gap ferromagnetic structures with the Tc above room temperature. As the switching rates in electronic and optoelectronic devices are pushed to higher frequencies, understanding the dynamical behavior of non- equilibrium carriers/spins can provide valuable information about different scattering mechanisms, carrier phonon coupling, and band structures. In this work, we report several time- resolved and magneto-optical measurements on $In_{1-x}Mn_{x}As$ and $In_{1-x}Mn_{x}Sb$ ferromagnetic films with the Mn content of 4$\%$. Our measurements on the basis of several time resolved differential transmission techniques in NIR and MIR demonstrate unique and complex dynamics in these material systems where photo-induced absorption and bleaching can co- exist. [Preview Abstract] |
Wednesday, March 23, 2011 4:30PM - 4:42PM |
T15.00007: Structural and Magnetic Characteristics of p-GaAs/MnAs Nanocluster Hybrids David Rench, Peter Schiffer, Nitin Samarth A possible route towards semiconductor spintronic devices involves the controlled synthesis of hybrid materials that combine ferromagnetic (FM) nanoclusters within a doped semiconductor host lattice. We use molecular beam epitaxy of (Ga,Mn,Be)As followed by in situ annealing to synthesize a systematic set of samples wherein FM nanoclusters are embedded in a p-GaAs matrix. High resolution transmission electron microscopy (HRTEM) and magnetometry demonstrate our ability to reproducibly synthesize two distinct classes of materials: (a) type I samples consisting of uniformly distributed, small clusters ($\sim $6 nm); (b) type II samples consisting of a bimodal distribution of small ($\sim $6 nm) and large ($\sim $25 nm) clusters. HRTEM studies show that while the large clusters are clearly MnAs with NiAs structure, the smaller clusters are possibly zinc blende in structure but with a more complex composition. We analyze the magnetic behavior of these two classes of samples and show measurements of their transport properties. Supported by the ONR-MURI program. [Preview Abstract] |
Wednesday, March 23, 2011 4:42PM - 4:54PM |
T15.00008: Magnetoresistance due to inelastic spin-flip cotunneling within Coulomb blockade regime in III-V semiconductor / MnAs nanoparticle heterostructures Ryota Akiyama, Shinobu Ohya, Pham Nam Hai, Masaaki Tanaka Inelastic spin-flip cotunneling is a key to understand the spin-dependent single-electron transport in the ferromagnetic nanoparticles systems. We fabricated a heterostructure consisting of Al/ AlAs/ ferromagnetic zinc-blende (ZB) MnAs nanoparticles embedded in GaAs / GaAs:Be on a GaAs(001) substrate, where electrons are expected to go through only one nanoparticle during tunneling. By analyzing the $I-V$ data at various temperatures $T$, we found that inelastic cotunneling is dominant when $T<$60 K. The ratio of the inelastic cotunneling energy $E$ to the thermal energy \textit{kT}, estimated by the $I-V$ data, was remarkably increased with decreasing $T$. We observed clear magnetoresistance (MR) up to $\sim $3{\%} (at 1T), and MR was also increased with decreasing $T$. The shape of the \textit{MR-T }curve was quite similar to that of the \textit{E/kT - T} curve, which strongly suggests that MR is induced by the spin-flip process due to inelastic cotunneling. From the \textit{E/kT - T} curve, the energy needed for the spin-flip process is estimated to be $\sim $0.04 meV, which corresponds to $\sim $3.3{\%} of the inelastic cotunneling energy. This work was partly supported by the Grant-in-Aids for Scientific Research, Special Coordination Programs by JST, FIRST Program, and JSPS Fellowship. [Preview Abstract] |
Wednesday, March 23, 2011 4:54PM - 5:06PM |
T15.00009: Magnetism in Cr doped Si nanowires Michael Shaughnessy, C.Y. Fong, Lin Yang We carry out first principles calculations of magnetic and electronic structures of single and multiple Cr atom dopants in Si nanowires. Both unsupported isolated wires and supported wires on Si 110 surfaces are studied. The relative stability and underlying physical picture of the ferromagnetic and antiferromagnetic configurations of the local moments on the Cr atoms are studied. Results are also presented for fully noncollinear calculations. [Preview Abstract] |
Wednesday, March 23, 2011 5:06PM - 5:18PM |
T15.00010: GaMnAs-based Core-Shell Nanowires Grown by Molecular Beam Epitaxy R. Pimpinella, X. Liu, K. Tivakornsasithorn, J.K. Furdyna, M. Dobrowolska, T. Wojtowicz We have successfully fabricated GaMnAs/GaAs core-shell nanowires (NWs) by molecular beam epitaxy (MBE), by first growing Au-assisted GaAs NWs, and subsequently depositing the GaMnAs shells on the Gaas NW side facets under low temperature conditions. Scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM) show that GaMnAs grows epitaxially on the GaAs NWs, retaining good crystalline quality. SQUID magnetometry shows that the shells obtained so far are ferromagnetic below 20 K. Studies by high resolution transmission electron microscopy (HRTEM) and energy-dispersive X-ray spectroscopy are planned for the future, in order to allow us to relate the observed magnetic properties of these one-dimensional magnetic wires to their chemical and structure profiles, in the hope of designing strategies for increasing the Curie temperature of the GaMnAs shells. [Preview Abstract] |
Wednesday, March 23, 2011 5:18PM - 5:30PM |
T15.00011: Magnetic Nanostructures in the Mn-Si system Petra Reinke, Kiril Simov, Catherine Jenkins Magnetic doping of group IV semiconductors is coveted for spintronics building blocks. Theoretical assessment of magnetism in Mn-Si is promising, but many of these structures have not been realized yet. Our STM study combines the study of Mn-nanostructure growth on Si(100)(2x1) with the investigation of the magnetic signature with X-ray magnetic circular dichroism and magnetometry. Mn self-assembles into monoatomic chains on the Si(100) surface. The mechanism of chain-formation and its competition with cluster growth will be presented. The nanostructures are capped with a 10 ML Si-or Ge- layer to form delta-doped layers, and protect the Mn-nanostructure. The Mn-chains are preserved, and the growth process for the cap was studied by STM and is now well understood. The magnetic signature is presented for nanowires and nanocluster structures below about 50 K, and a dense array of Mn-chains shows the highest saturation magnetization with 2-3 $\mu _{B}{\rm g}$er Mn. The hysteresis loops indicate a superparamagnetic behavior. We will discuss the relative spin-orbital contributions and the directional dependence of the magnetic signature in relation to the Mn-nanostructure geometry. [Preview Abstract] |
Session T16: Focus Session: Magnetic Nanostructures, Materials & Effects
Sponsoring Units: DMP GMAGChair: Jiwei Lu, University of Virginia
Room: D173
Wednesday, March 23, 2011 2:30PM - 2:42PM |
T16.00001: Magnetocaloric effect and refrigerant capacity in Eu$_{8}$Ga$_{16}$Ge$_{30}$ clathrates and Eu$_{8}$Ga$_{16}$Ge$_{30}$-EuO composites H. Srikanth, A. Chaturvedi, M.H. Phan, S. Stefanoski, G.S. Nolas, V. Franco Eu$_{8}$Ga$_{16}$Ge$_{30}$ clathrates are widely known for their excellent thermoelectric properties. Recently, we have discovered the giant magnetocaloric effect (MCE) in Eu$_{8}$Ga$_{16}$Ge$_{30}$ type-VIII clathrates. The tunable MCE and refrigerant capacity (RC) have also been achieved in Eu$_{8}$Ga$_{16}$Ge$_{30 }$type-I clathrates by partial substitution of Eu with non-magnetic Sr. As an interesting host matrix the type-I clathrates are combined with EuO at different portions (80{\%}/20{\%}, 70{\%}/30{\%}, 65{\%}/35{\%}, 60{\%}/40{\%}, 40{\%}/60{\%}) for making novel composites with enhanced RC over a tunable temperature range (10-100K). We have achieved a very large RC of 794 J/kg at 5T over a 70K in the clathrate type I -- EuO (40{\%}/60{\%}) composite, which is the largest value ever achieved among the existing materials for magnetic refrigeration around 70K. This composite is very attractive for magnetic refrigeration for nitrogen liquefaction. A new potential of using the type VIII clathrate -- EuO composite (50{\%}-50{\%}) to produce refrigeration in two different temperature ranges has been proposed. [Preview Abstract] |
Wednesday, March 23, 2011 2:42PM - 2:54PM |
T16.00002: Exploration of thermal conductivity, Seebeck coefficient, and Lorenz number deviations in Ni-Fe alloy films B.L. Zink, A.D. Avery, R. Sultan, D. Bassett, G. Cotteril As electronic and spintronic systems continue to shrink, exploration of the fundamental physics affecting thermal transport in prospective materials becomes increasingly essential. For example, the potential use of spin-torque driven domain wall motion in ferromagnetic nanowires as a memory element requires application of large current densities to these tiny structures. The resulting heating could have both helpful and harmful effects, and is in general not yet well-understood. This is partly due to a gap in the fundamental knowledge of thermal properties of nanoscale systems that is due to the challenging nature of the necessary measurements. We have recently developed a micromachined thermal isolation platform that allows measurement of thermal conductivity, electrical conductivity, and thermopower (or Seebeck effect) in thin film systems. In this talk we present our recent data on thermal conductivity, resistivity, and Seebeck coefficient, for Ni-Fe alloy films with thicknesses varying from 25-100 nm. We compare our results to the predictions of the Wiedemann-Franz law and discuss variations represented by deviations from the Sommerfeld value of the Lorenz number, and conclude with our plans to extend the technique to yet smaller structures. [Preview Abstract] |
Wednesday, March 23, 2011 2:54PM - 3:06PM |
T16.00003: Thermodynamics and Magnetocaloric properties of Fe/Cr Superlattices T. Mukherjee, S. Michalski, R. Skomski, D.J. Sellmyer, Ch. Binek We explore MC properties of tailored Fe/Cr superlattices involving simple 3d metals. Our multilayers are fabricated by pulsed laser deposition with emphasis on maximizing magnetic entropy changes near room temperature. We use nanostructuring\footnote{Phys. Rev. B~\textbf{79}, 144406 (2009).} to tailor magnetic interaction and exploit geometrical confinement in order to fit the FM to paramagnetic transition temperature of the FM constituent films. In concert this leads to an optimized global metamagnetic transition maximizing the isothermal entropy change. Thermodynamic and MC properties of such Fe/Cr superlattices are studied with the help of SQUID magnetometry. Entropy changes are deduced via the Maxwell relation in single phase regions and via the Clausis-Clapeyron relations at first order metamagnetic transitions, X-ray diffraction and X-ray reflectivity are used to correlate structural data with the magnetic properties. [Preview Abstract] |
Wednesday, March 23, 2011 3:06PM - 3:18PM |
T16.00004: Magnetocaloric effect in heterostructures of Ni$_{x}$Cu$_{(1-x)}$ alloys C.A. Bauer, P.B. Jayathilaka, R.V. Rupani, D.D. Belyea, Casey W. Miller We used 99.9{\%} compound targets to co-sputter Ni$_{x}$Cu$_{(1-x)}$ thin film multilayers composed of sub-layers with differing alloying compositions on silicon and oxidized silicon substrates. Each system had a Ta underlayer and capping layer. XRD was used to determine structural properties, showing a (111) preferred orientation for all Ni$_{x}$Cu$_{(1-x)}$ layers. In-Plane XRD was used to check polycrystallinity. Energy-dispersive X-ray scattering is used to determine the relative concentrations of Nickel and Copper, with XRD data corroborating the EDX results from Vegard's law. The magnetic properties of the systems are characterized and the magnetic entropies are calculated near the relevant critical temperature. [Preview Abstract] |
Wednesday, March 23, 2011 3:18PM - 3:30PM |
T16.00005: Thermal conductivity behaviour of magnetic fluids R.A. Medina-Esquivel, J. Mendez-Gamboa, J. Tapia, J.J. Alvarado-Gil We study the thermal conductivity of five kinds of Magnetic Fluids (MFs) by varying the magnetic material volume fraction and the direction and intensity of an homogeneous magnetic field: The studied MFs are: Magnetorheological fluids (MRF), carbon coated and uncoated Fe nanofluids (CcFeNF, FeNF), ferrofluis (FF); and two kind of composed fluids; ferrofluids loaded with carbon nanotubes (FFCNTs) and ferrofluids loaded with carbon nanofibers (FFCNFs). MRF and FFCNFs fluids increase its thermal transport along the field direction; the thermal enhancement in MRF was dramatically overtaken by the FFCNFs, but in contrast; the rest of the fluids did not present thermal conductivity enhancement under the field. Theoretical models show that thermal resistance at the nanoscale level presents a very important role in the thermal transport among linked particles, this is the reason why FFCNTs, FF, FeNF, and CcFeNF did not present an increase in its thermal conductivity under the action of the magnetic field, although its chain-like structuring. We believe that these experimental finding may have significant application in the area of thermally tailored materials. [Preview Abstract] |
Wednesday, March 23, 2011 3:30PM - 3:42PM |
T16.00006: Large Seebeck coefficient in frustrated doped Mott insulators Louis-Fran\c{c}ois Arsenault, B. Sriram Shastry, Patrick S\'{e}mon, Andr\'{e}-Marie Tremblay Since calculations based on the standard Kubo formula have proven extremely difficult for electric and thermal transport, Shastry and co-workers [1] suggested two novel approximate ways to obtain the thermopower (S) in interacting systems. One method is based on the high-frequency limit. The other, based on ideas of Kelvin, is purely thermodynamical. With these we study the Hubbard model on a 3d FCC lattice, a frustrated lattice. The high dimensionality of the problem justifies the use of dynamical mean field theory (DMFT). CTQMC in the hybridization expansion and the fast IPT are the impurity solver. The Seebeck coefficient is obtained as a function of doping and temperature for different U. Within DMFT, vertex corrections vanish for transports coefficients, hence the bubble suffices. This enables us to further assess how both approximate methods compare with each other and with the DC Kubo approach. At low T, results can be interpreted in terms of effective Fermi temperatures and carrier number.\\[4pt] [1] B.S. Shastry, Rep. Prog. Phys. 72, 016501 (2009) [Preview Abstract] |
Wednesday, March 23, 2011 3:42PM - 3:54PM |
T16.00007: Spin dynamics simulations for a nanoscale Heisenberg antiferromagnetic film Zhuofei Hou, David Landau, G. Malcolm Stocks Thermoinduced magnetization(TiM) is a novel response predicted to occur in nanoscale antiferromagnetic (AF) materials. Extensive Monte Carlo simulations\footnote{G. Brown, A. Janotti, M. Eisenbach, and G. M. Stocks, Phys.Rev.B {\bf 72}, 140405(2005)} have shown that TiM is an intrinsic property of the AF classical Heisenberg model. To obtain a fundamental understanding of TiM, spin dynamics (SD) simulations are performed to study the spin wave behavior, which seems to be the cause of TiM. A classical Heisenberg model with an AF nearest-neighbor exchange interaction and uniaxial single-site anisotropy is studied. Simple-cubic lattices with two free-surfaces and periodic boundaries parallel to the surfaces are used. We applied fast SD algorithms with 4th-order Suzuki-Trotter decompositions of the exponential operator. Discrete spin wave modes due to spin wave confinement\footnote{{\em Spin Wave Confinement}, edited by S. O Demokritov (Pan Stanford Publishing, Singapore, 2008)} are found in transverse S(q,\,$\omega$) in the perpendicular direction to free surfaces. [Preview Abstract] |
Wednesday, March 23, 2011 3:54PM - 4:06PM |
T16.00008: Thermomagnonic spin transfer in textured magnets Alexey A. Kovalev, Yaroslav Tserkovnyak We study interplay between the spin-energy transport and magnetization dynamics in ferromagnetic insulators with magnetic textures. With the help of the Onsager reciprocity principle we construct a phenomenological theory capable of describing various thermomagnonic effects. Motion of domain walls by thermal gradients and generation of heat flows by magnetization dynamics are suggested. By estimating the kinetic coefficients (such as $\beta$ like viscous coupling) for realistic materials (e.g. Yttrium iron garnet), we analyze the feasibility of mentioned effects for energy related applications. [Preview Abstract] |
Wednesday, March 23, 2011 4:06PM - 4:18PM |
T16.00009: Rotating magnon wavepackets in ferromagnets and thermal Hall effect Ryo Matsumoto, Shuichi Murakami We theoretically construct the semiclassical equation of motion of the magnon wavepacket in an insulating ferromagnets, in analogy with the electron systems. We find that the magnon wave packet has nonzero angular momentum, which consists of two parts: the self-rotational motion and the revolving motion (edge current). We show that these are expressed in terms of the Berry curvature in \textbf{k}-space, i.e., these arise from the magnon band structure. Furthermore, we find that the thermal Hall effect of the magnon is totally due to the magnon edge current, and present an intuitive picture of the thermal Hall effect. We also construct the linear response theory for the thermal Hall effect, and compare the results with the previous works with an example of Lu$_2$V$_2$O$_7$. [Preview Abstract] |
Wednesday, March 23, 2011 4:18PM - 4:30PM |
T16.00010: Magnetocaloric Properties of Thin Film Heterostructures H. Kirby, C. Bauer, B.J. Kirby, J. Lau, C.W. Miller In an effort to understand the impact of nanostructuring on the magnetocaloric (MC) effect, we have studied gadolinium in MgO/W(50 {\AA})/[Gd(400 {\AA})/W(50 {\AA})]$_{8}$ heterostructures [Miller et al., J. Appl. Phys. 107, 09A903 (2010)]. The entropy change peaks at a temperature of 284 K with a value of 3.4 J/kg K for a 0--30 kOe field change. Polarized neutron reflectometry was used to determine the depth profile of the magnetic moment per Gd atom, m$_{Gd}$ in a Gd/W multilayer. Our results suggest that creating materials with Gd-ferromagnet interfaces may increase the m$_{Gd}$, leading to enhanced MC properties. Therefore SiOx/Fe(50 {\AA})/Gd(300 {\AA})/Fe(50 {\AA}) heterostructures have been investigated. [Preview Abstract] |
Wednesday, March 23, 2011 4:30PM - 4:42PM |
T16.00011: Antiferromagnetism in BaF$_{2}$/Fe$_{x}$Ni$_{1-x}$F$_{2}$ bilayers Felio Perez, Trent Johnson, David Lederman A series of crystalline BaF$_{2}$/epitaxial (110) Fe$_{x}$Ni$_{1-x}$F$_{2}$ samples were deposited on MgF$_{2}$ (110) via molecular beam epitaxy. The Fe concentration x was determined from x-ray diffraction measurements of the [110] lattice parameter. The actual thickness of each layer and the roughness of each interface were determined by fitting x-ray reflectivity data. The antiferromagnetic ordering of samples with x = 0.04, 0.27, 0.40, 0.46, 0.50, and 0.85 were studied and compared with BaF$_{2}$/NiF$_{2}$ (x=0) and BaF$_{2}$/FeF$_{2}$ (x=1) bilayers, via standard magnetometry measurements. A significant enhancement of the N\'{e}el temperature in alloys and evidence of spontaneous magnetization along c-axis were found. [Preview Abstract] |
Wednesday, March 23, 2011 4:42PM - 4:54PM |
T16.00012: MnCu$_{4}$In: a new high temperature ferromagnet Alessia Provino, Durga Paudyal, Sudesh K. Dhar, Maria Luisa Fornasini, Pietro Manfrinetti, Vitalij K. Pecharsky, Karl A. Gschneidner Jr. The new intermetallic compound MnCu$_{4}$In has been synthesized and investigated. The crystal structure, studied by single crystal and powder X-ray diffractions, shows that the compound crystallizes into its own hexagonal prototype (\textit{hP}12-$P$6$_{3}$\textit{mc}) derived from the MgZn$_{2}$-type. The measured magnetic and physical properties indicate that, in contrast to the antiferromagnetic MnCu$_{4}$Sn (MgCu$_{4}$Sn-type), MnCu$_{4}$In is a high temperature ferromagnet with $T_{C}$ = 540$^{\circ}$C. In order to understand the physics involved, the first principles calculations have been performed and compared with the MnCu$_{2}$Al-type MnCu$_{2}$In(Sn) phases and the rare earth representatives GdCu$_{4}$In and GdCu$_{2}$In. Work partially supported by the US DOE, Division of Materials Science and Engineering (Office of Basic Energy Sciences). [Preview Abstract] |
Wednesday, March 23, 2011 4:54PM - 5:06PM |
T16.00013: The layer-by-layer growth of ferromagnetic $\tau $ phase MnAl thin films by Bias Target Ion Beam Yishen Cui, Wenjing Yin, Jiwei Lu, Stuart Wolf It is well known that the metastable $\tau $ phase of MnAl has a L10 structure (chemical ordering along [001] directions) and is the only ferromagnetic phase of this binary intermetallic. In our study, alternating Al/Mn quasi-monolayer deposition was developed using a novel Bias Target Ion Beam deposition technique, that enabled precise control of the microstructural growth. We have obtained epitaxial $\tau $ phase MnAl thin films ($\sim $10 nm thick) on single crystal MgO substrates with improved saturation magnetization and anisotropy in comparison with co-sputtered ultra thin films. We will discuss the microstructure and magnetic behaviors of MnAl films in detail. [Preview Abstract] |
Wednesday, March 23, 2011 5:06PM - 5:18PM |
T16.00014: Field induced first order phase transition in the antiferromagnet Yb$_{3}$Pt$_{4}$ L.S. Wu, Y. Janssen, M.S. Kim, C. Marques, K.S. Park, M. Bennett, M.C. Aronson, S.X. Chi, J.W. Lynn Yb$_{3}$Pt$_{4}$ is an antiferromagnet that orders at T$_{N}$=2.4K. Magnetic fields B suppress T$_{N}$, and the B-T phase line T$_{N}$(B) terminates almost vertically at T=0, B$_{C}$=2.0 T. Specific heat measurements find a mean-field transition at T$_{N}$(B), and the magnetocaloric effect shows that the antiferromagnetic transition is continuous at all fields, with no associated latent heat. However, neutron diffraction measurements performed for B$\sim $B$_{C}$ find that a distinct step in the magnetization $\Delta $M occurs near the transition, with a magnitude that increases for T$<$1 K. The field dependent magnetization M(B) similarly has a metamagnetic-like step at T$_{N}$(B) below 1 K, accompanied by a sharp peak in the susceptibility whose magnitude increases but does not diverge as T$\to $0. We argue that a nonzero magnetization step $\Delta $M is required to give $\Delta $S=0 for T=0, since the vertical phase line at T=0 implies dT$_{N}$/dB=-$\Delta $M/$\Delta $S$\to -\infty $. We argue that T$_{N}$ (B) terminates at B$_{C}$ in a T=0 first order transition. [Preview Abstract] |
Session T17: Focus Session: Magnetic Oxide Thin Films - Manganite Thin Films
Sponsoring Units: GMAG DMPChair: John Freeland, Argonne National Laboratory
Room: D174
Wednesday, March 23, 2011 2:30PM - 3:06PM |
T17.00001: Tunable Percolative Transport in Manganite Thin Films using Strain and Exchange Fields Invited Speaker: Strongly correlated electronic systems are often sensitively dependent on spin-charge-orbital-lattice interactions. We will discuss recent work on manganites that have led to some fascinating new discoveries on the role of these interactions in driving electronic phase separation in strongly correlated systems. We demonstrate that substrate induced anisotropic strain effects and surface exchange coupled magnetic nanodots can be used to preferentially seed electronic domains. These effects have led to a new understanding of how order parameter tuning can lead to highly controllable electronic and magnetic properties. We find that even strain frustrated ultrathin manganite films---where no metal-insulator transition is present---can be selectively tuned with the application of magnetic nanodots at the film surface. Both the magnetoresistance and the metal-insulator transition temperature can be tuned through dot density and dimension. The strain frustrated film's metal-insulator transition and magnetoresistivity can be driven to bulk levels. We expect these results to be applicable in many other systems in which order parameters are tightly correlated. [Preview Abstract] |
Wednesday, March 23, 2011 3:06PM - 3:18PM |
T17.00002: STM study on the electronic phase separation of manganites Min Gao, Zheng Gai, Paul C. Snijders, Hangwen Guo, Thomas Z. Ward, H.-J. Gao, Jian Shen Phase separation is a key problem in understanding the exotic properties of complex oxide materials. Combing pulsed laser deposition with in situ scanning tunneling microscopy, we can investigate the electronic phase separation ofLa$_{5/8-x}$Pr$_{x}$Ca$_{3/8}$M nO$_{3}$. Current imaging tunneling spectroscopy reveals both local domain contrast and global conductivity evolving when the temperature crosses over the metal-insulator transition temperature. The domain size can be several hundred nanometers. This result confirms other experimental results and shows that the surface electronic properties of complex oxide materials can represent their bulk properties. [Preview Abstract] |
Wednesday, March 23, 2011 3:18PM - 3:30PM |
T17.00003: Nonlinear optical imaging of antiferromagnetic domains in orbital-ordered Pr$_{0.5}$Sr$_{0.5}$MnO$_{3}$ thin films Kenjiro Miyano, Naoki Ogawa, Yasushi Ogimoto Perovskite manganites develop various ordering patterns of charge, orbital, and spin, whose geometrical correlation brings out a new order, e.g., electronic polarization. As an example in pseudo-cubic systems, the CE-type charge/orbital order (CO/OO) was predicted to be multiferroic. With the use of nonlinear optics, we show that Pr$_{0.5}$Sr$_{0.5}$MnO$_{3}$ thin films exhibit symmetry breaking in the OO phase without CO. Ultrathin films prepared on LSAT(110) substrates reproduce bulk-like properties, showing successive phase transitions from paramagnetic metal, to ferromagnetic metal, and finally to A-type antiferromagnetic (AF) insulator upon cooling. Below $T_{OO}$=$T_{N}$, we detect SHG from these films and visualize the formation of AF domains with the size of several $\mu$m, which is much larger than that of phase-separated manganites. With careful examination of the magnetic point group, we can ascribe the broken symmetry to the AF spin order under the monoclinic lattice distortion concomitant with the OO, which also manifests the direction of the AF vector. [Preview Abstract] |
Wednesday, March 23, 2011 3:30PM - 3:42PM |
T17.00004: Novel Resistive Switching Behavior in Phase Separated Manganites Hangwen Guo, T. Zac Ward, Dali Sun, Paul C. Snijders, Zheng Gai, Jian Shen Electronic phase separation plays a key role in many novel phenomena in complex materials. Manganites are a prime example of this class of materials and have recently come under increase scrutiny for possible application in resistive random-access memory (RRAM) technology. Here, we will discuss our recent work on spatially confined La5/8-xPrxCa3/8MnO3. We have discovered that it is possible to drive single electronic domain formation/annihilation through electric field pulsing. By measuring the I-V curve, we find such resistive switching is different from normal RRAM mechanisms in manganites and is closely related to the nature of electronic phase separation. These findings open these systems to a new understanding of the nature of electronic phase separation and begin the development of manganites for future applications in RRAM devices. [Preview Abstract] |
Wednesday, March 23, 2011 3:42PM - 4:18PM |
T17.00005: Mesoscopic orientation-ordered percolating network in a strained manganite thin film Invited Speaker: Many unusual behaviors in complex oxides are deeply associated with the spontaneous emergence of microscopic phase separation. Recent studies on these strongly correlated materials have shown that multiple states can coexist near certain phase boundaries. In this work, a cryogenic microwave impedance microscope [1] is implemented to investigate the microscopic origin of the colossal magnetoresistance effect in manganite thin films. In a strained Nd$_{0.5}$Sr$_{0.5}$MnO$_{3}$ thin film grown on (110) SrTiO$_{3}$ surface, the filamentary ferromagnetic metallic domains emergent from the antiferromagnetic charge/orbital-ordered insulating background as increasing magnetic fields align preferentially along certain crystal axes of the substrate [2]. Such an orientation ordering is missing in a relaxed sample with partial loss of the epitaxial coherency. The mesoscopic glassy orders with a period of 100nm indicate that the substrate-induced anisotropic strain rather than the Coulomb interaction plays the dominant role in the phase separation. The microwave images also revealed drastically different domain structures between the zero-field-cool and field-cool processes, consistent with the macroscopic transport measurements in both bulk and thin film materials. \\[4pt] [1] W. Kundhikanjana \textit{et al.}, arXiv 1010.1509. \\[0pt] [2] K. Lai \textit{et al.}, \textit{Science} \textbf{329}, 190 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 4:18PM - 4:30PM |
T17.00006: Room Temperature Resistive Switching in Manganite Thin Films Luis Pe\~na, Luis Garzon, Zori\c{c}a Konstantinovic, Lluis Balcells, Carmen Ocal, Benjamin Martinez Resistive switching (RS), i.e. the switching between two distinct resistive states electrically controlled, is currently a subject of major interest because of its very promising properties for the implementation of data storage devices. In this work we report on the reversible transitions from low resistive (LR) to high resistivity (HR) states in high quality manganite thin films (LSMO) prepared by RF sputtering on top of (001) oriented STO substrates. The transitions between the LR and HR states are induced by the application of a bias voltage by means of the conducting tip of a scanning force microscope [1]. The experimental setup is arranged in order to avoid parasitic interfacial phenomena (e.g., metal diffusion) or electrode interconnections (e.g., filamentary formation). These RS experiments have been performed on few microns wide patterned LSMO structures. The magnetotransport properties and thermal stability of these LR and HR states are investigated in order to gain a deeper insight into the nature of the structural/electronic modifications generated by the application of high electric field by means of the AFM tip on the manganite film. \\[4pt] [1] C. Moreno et al. \textit{Nanoletters 10, 3828 (2010)} [Preview Abstract] |
Wednesday, March 23, 2011 4:30PM - 4:42PM |
T17.00007: Magnetic and Transport Properties of Heterostructured Films of Prussian Blue Analogues and Manganites P.A. Quintero, H. Jeen, E.S. Knowles, A. Biswas, M.W. Meisel, M.J. Andrus, D.R. Talham The magnetic and transport properties of heterostructured films consisting of Prussian blue analogues, $A_j$M$^{\prime}_k$[M(CN)$_6$]$_{\ell}\cdot n$H$_2$O (M$^{\prime}$M-PBA), where $A$ is an alkali ion and M$^{\prime}$,M are transition metals, and manganites have been studied. Specifically, NiCr-PBA and CoFe-PBA films\footnote{D.M.~Pajerowski \emph{et al.}, J.~Am.~Chem.~Soc. {\bf 132} (2010) 4058.} of $\sim$100~nm thickness have been deposited on perovskite (La$_{1-y}$Pr$_{y}$)$_{0.67}$Ca$_{0.33}$MnO$_3$ (LPCMO) manganese films\footnote{T.~Dhakal, J.~Tosado, A.~Biswas, Phys.~Rev.~B {\bf 75} (2007) 092404.} of $\sim$30~nm thickness. The effect of the ferromagnetic NiCr-PBA, $T_c \sim 70$~K, and the photo-controllable ferrimagnetic CoFe-PBA, $T_c \sim 20$~K, on the I-V properties of the LPCMO will be reported, where special attention will be given to the changes of the transition temperatures of the ferromagnetic metallic (FMM) and the charge-ordered insulating (COI) phases in the LPCMO substrate. \\ ** Supported by NSF DMR-0701400 (MWM), DMR-0804452 (AB), DMR-1005581 (DRT), DMR-0654118 (NHMFL), and by scholarship from the Organization of American States (PAQ). [Preview Abstract] |
Wednesday, March 23, 2011 4:42PM - 4:54PM |
T17.00008: Interfacial phase separation in La$_{2/3}$(Sr/Ca)$_{1/3}$MnO$_{3}$ thin films with different complex oxide capping layers Sergio Valencia, Zori\c{c}a Konstantinovic, Schmitz Detlef, Lluis Balcells, Benjamin Martinez Interfacial effects in sputtered manganite thin films with different capping layers (MgO, LAO, STO, NGO and Au) have been investigated. The interfaces have been chemically and magnetically characterized by means of local probes such as X-ray absorption spectroscopy (XAS) and X-ray magnetic circular (XMCD) and linear dichroism (XLD). Total electron yield detection at the Mn L-edge guarantees that the spectroscopic information originates from those regions closer to the film/capping interface. A complex phase separated scenario at the interface arises from the spectroscopic data. XAS shows departure of the Mn valence from bulk like values in case of STO and Au capping (Mn2+ presence) and in case of MgO and NGO (Mn4+ increase). XMCD shows concomitant depressed interface magnetization suggesting coexistence of ferromagnetic and non-magnetic phases. Finally XLD proves the presence of an antiferromagnetic (AFM) and orbital ordered (OO) phase. [Preview Abstract] |
Wednesday, March 23, 2011 4:54PM - 5:06PM |
T17.00009: Photoinduced effects in ferromagnetic state of manganites Vera Smolyaninova, Grace Yong, Rajeswari Kolagani, Amlan Biswas, Kilhwan Wang Rear-earth manganese oxides have a rich phase diagram. Transitions between different magnetic, electronic and structural states can be induced by application of external fields. Light-induced destruction of the charge ordering in some compositions of manganites, which results in significant increase of conductivity, is a well known example of such transition. Significant change in conductivity of these materials makes them attractive for photonic and opto-electronic device applications. We report a study of photoinduced properties of manganites in ferromagnetic metallic state. Since light penetration depth in these materials is small, thin films of manganites were used in this study. Photoinduced resistivity changes in these materials will be reported. Photoinduced effects in compositions with different temperatures of metal-insulator transition will be discussed. This work is supported by the NSF grant DMR-0348939. [Preview Abstract] |
Wednesday, March 23, 2011 5:06PM - 5:18PM |
T17.00010: Bulk-like electronic structure at the surface of epitaxial La$_{1-x}$Sr$_{x}$MnO$_{3}$ films Eric Monkman, Carolina Adamo, Daniel Shai, Dawei Shen, John Harter, Ilya Elfimov, Darrell Schlom, Kyle Shen We present direct measurements of the electronic structure of La$_{1-x}$Sr$_{x}$MnO$_{3}$ (LSMO) using a combined molecular beam epitaxy and angle-resolved photoelectron spectroscopy system. Our results allow for the first comparison between theory and experimental results over the entire Fermi surface in the ferromagnetic-metallic phase. We observe both of the predicted Fermi surface sheets, and find that the evolution of the Fermi surface shape with doping is consistent with a rigid-band shifting of the chemical potential. Measurements in the antiferromagnetic phase at x $>$ 0.5 allow us to determine the changes in the low energy electronic structure linked to the magnetic phase transition. The ability of this surface sensitive technique to probe the bulk electronic structure of LSMO limits the possible depth of a surface dead layer. This conclusion is supported by density functional theory calculations for LSMO slabs, which indicate that the polarity of the (001) surface is efficiently screened within $\sim$1 unit cell. [Preview Abstract] |
Wednesday, March 23, 2011 5:18PM - 5:30PM |
T17.00011: Hall effect on strain-released La$_{0.67}$Ca$_{0.33}$MnO$_{3}$ thin films Liuqi Yu, Xiaohang Zhang, S. von Moln\'ar, P. Xiong, Lingfei Wang, W.B. Wu It has been demonstrated that releasing the in-plane anisotropic strain in thin films of La$_{0.67}$Ca$_{0.33}$MnO$_{3}$ grown on orthorhombic NdGaO$_{3}$ (001) substrates can induce a charge ordering state below the Curie temperature.$^{1}$ Three LCMO films on NGO, (PLD at 735$^{o}$C and 45 Pa O$_{2}$ pressure, 45 nm thick) were annealed at 780$^{o}$C in flowing O$_{2}$ for 1, 10 and 20 hours to increase degrees of strain relaxation. Hall measurements were performed. In all three samples, the Hall resistivity takes on two distinct slopes in the paramagnetic phase: a negative slope at low fields, which varies with temperature, and a temperature-independent positive slope at high fields. Notably, the switching field for the Hall slope decreases linearly with temperature and extrapolates to the paramagnetic Curie temperatures of the samples. The observation is similar to the behavior of the nonlinear Hall effect in EuB$_{6}$ and suggests that the switches occur at a \textit{constant critical magnetization}.$^{2}$ In strain-released samples, peaks in the Hall resistivity emerge near $T_{C}$ and become more pronounced with decreasing temperature. The origins and implications of these observations will be discussed. Work supported in part by NSF DMR-0908625. $^{1}$Z. Huang et al., JAP 105, 113919 (2009) $^{2}$X. Zhang et al., PRL \textbf{103}, 106602 (2009) [Preview Abstract] |
Session T18: Focus Session: Low D/Frustrated Magnetism - Molecular Magnets II
Sponsoring Units: GMAG DMPChair: M. Sarachik, City College of New York
Room: D172
Wednesday, March 23, 2011 2:30PM - 3:06PM |
T18.00001: Recent developments in Molecular Magnetism Invited Speaker: Two hot topics in this area are highlighted namely the design of new multifunctional magnetic materials, and the study of mononuclear single-molecule magnets. In the first topic I will present the attempts to design materials exhibiting coexistence of superconductivity and magnetism using a chemical approach [1]. In the second topic I will show that single-molecule magnets based on lanthanides provides the opportunity to tune quantum tunneling effects and to use these systems as qubits in quantum computing [2]. Finally, I will highlight the relevance of these magnetic systems in molecular spintronics.\\[4pt] [1] E. Coronado, C. Mart\'i-Gastaldo, E. Navarro-Moratalla, A. Ribera, S. J. Blundell, P. J. Baker Nature Chem. 2010, 2, 1031.\\[0pt] [2] F. Luis, M. J. Mart\'inez-P\'erez, O. Montero, E. Coronado, S. Cardona-Serra, C. Mart\'i-Gastaldo, J.M. Clemente-Juan, J. Ses\'e, D. Drung, T. Schurig, Phys. Rev. B 2010, 82, 060403 R. [Preview Abstract] |
Wednesday, March 23, 2011 3:06PM - 3:18PM |
T18.00002: Random Exchange Antiferromagnetic Heisenberg Chains Christopher Landee, Fan Xiao, Brian Wells, Brian Koopman, Mark Turnbull, Matthias Thede, Andrey Zheludev Copper \textit{bis}pyridine dichloride (CPC) and copper \textit{bis}pyridine dibromide (CPB) are isostructural Heisenberg S = $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} $ linear chains in which the copper atoms are bridged by two halides with the pyridine molecules in the axial sites. The exchange strengths are 27 K (CPC) and 45 K (CPB). We have prepared mixed halide versions Cu(py)$_{2}$[Cl$_{1-x}$Br$_{x}$]$_{2}$ for values of x from 0 to 1. We will report on the effect of the exchange randomness on the susceptibilities and N\'{e}el temperatures of this family of compounds, as determined by dc-magnetometry, muon spin relaxation, and low-temperature calorimetry. [Preview Abstract] |
Wednesday, March 23, 2011 3:18PM - 3:30PM |
T18.00003: Magnetic dilution in the cadmium-doped spin ladder compound $Cd_xCu_{1-x}(quinoxaline)Br_2$ Brian Keith, Chris Landee, Mark Turnbull Both $Cu(quinoxaline)(Br_2)$ and $Cu(quinoxaline)(Cl_2)$ are examples of molecule-based magnets where the $CuX_4$ dimers are linked into ladders by quinoxaline molecules, where X is either Cl or Br. The rung exchange occurs through the bridging halides while the rail exchange occurs through the quinoxaline rings. Introducing random rung interactions into the system [$Cu (quinoxaline)(Br_2)_{1-x}(Cl_{2})_x$] has caused the spin gap to close, in contrast with the gapped pure spin ladder parents. Crystal growth of non-magnetic-doped molecular magnets, $Cd_xCu_ {1-x}(2,3-dimethylpyrazine)Br_2$, have been performed for several values of the nominal conentration, x, and have been confirmed. The magnetizations and susceptibilities of the magnetically diluted ladder assemblage are presented along with a comparison of the effects of dilution from the pure case (x=0). [Preview Abstract] |
Wednesday, March 23, 2011 3:30PM - 3:42PM |
T18.00004: Electron and Spin Transport in Mn$_{12}$ Single Molecule Magnets Bridging Gold Electrodes: How to Determine the Easy Axis Orientation Experimentally Fatemeh Rostamzadeh Renani, George Kirczenow We present a tight-binding theory of Mn$_{12}$ single molecule magnets thiol-bonded to gold electrodes. The model includes both the spatial and spin aspects of the electronic states. Spin-orbit coupling is included explicitly in the Hamiltonian and magnetic anisotropy values in agreement with experiment are obtained. We demonstrate that Mn$_{12}$ single molecule magnets strongly coupled to gold electrodes should exhibit strong spin filtering under appropriate conditions. In experiments, the orientation of the molecule's easy axis relative to leads is not controllable and it has not been feasible to measure it. Our calculations reveal the possibility of determining the easy axis orientation experimentally by means of current measurements: In the molecular junction with the easy axis parallel to leads the current is predicted to be at least two orders of magnitude larger than if the easy axis is perpendicular to the leads, for molecules thiol-bonded to the leads with similar gold-sulfur distances in the two cases. [Preview Abstract] |
Wednesday, March 23, 2011 3:42PM - 3:54PM |
T18.00005: Hall magnetometry measurements of the susceptibility of variants of Mn$_{12}$-ac Pradeep Subedi, A.D. Kent, B. Wen, M.P. Sarachik, Y. Yeshurun, A.J. Millis, S. Mukherjee, G. Christou The temperature dependence of the inverse magnetic susceptibility of both Mn$_{12}$-ac and Mn$_{12}$-ac-MeOH is found to give finite temperature intercepts, indicating a ferromagnetic phase at low temperature [1, 2]. A magnetic field applied transverse to the Ising axis suppresses the Curie temperature, T$_{CW}$, to a T = 0 quantum critical point. However, the decrease of T$_{CW}$ with transverse field, H$_{\bot }$, in Mn$_{12}$-ac does not agree with mean field calculations performed with a spin Hamiltonian that includes dipolar interactions between molecules and H$_{\bot }$ effects. We attribute the pronounced suppression of T$_{CW}$ of Mn$_{12}$-ac to the effect of random fields arising from a distribution of molecular easy axis tilts due to ligand disorder [1]. Mn$_{12}$-ac-MeOH is of interest in this regard because it appears to have no ligand disorder. We discuss these experiments as well as ongoing studies of Mn$_{12}$-toluate, a faster relaxing variant of Mn$_{12}$, which has a lower effective anisotropy barrier that permits the study of the susceptibility in larger H$_{\bot }$ at very low temperature. \\[0pt] [1] Wen et.al PRB \textbf{82}, 014406 (2010) \\[0pt] [2] Li et.al PRB \textbf{82}, 174405 (2010) [Preview Abstract] |
Wednesday, March 23, 2011 3:54PM - 4:06PM |
T18.00006: Local magnetic susceptibility study of long-range order in Mn$_{12}$-ac Bo Wen, P. Subedi, Y. Yeshurun, M.P. Sarachik, A.D. Kent, A.J. Millis, S. Mukherjee, G. Christou The magnetic susceptibility of single crystals of Mn$_{12}$-ac obeys a Curie-Weiss law, indicating a transition to a ferromagnetic phase below 1 K [1]. Measurements by Hall magnetometry have yielded different temperature intercepts ranging from 0.4 K to 0.9 K. Moreover, these values differ from those obtained for the same crystals in a SQUID-based MPMS. We have proposed two possible origins: (1) the effect of crystal's aspect ratio, reported in [2]; (2) the possibility that the local ordering temperature differs from the global average value. Here we report an ongoing study of the longitudinal magnetic susceptibility at different locations of a single crystal, performed on a Hall sensor array in both zero and finite transverse magnetic field. Preliminary results yield temperature intercepts that are lower near the end of the crystal than in the middle. \\[4pt] [1] Bo WEN et al., PRB 82, 014406 (2010). \\[0pt] [2] Shiqi LI et al., PRB 82, 174405 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 4:06PM - 4:18PM |
T18.00007: Magneto-structural study of M(TCNE)(NCMe)]X molecule-based magnets Konstantin Pokhodnya, Chris Olson, Chris Heth, John Schlueter, Gregory Halder In M$^{II}$(TCNE)(NCMe)$_{2}$]X (M=Fe, Mn, Ni; TCNE = tetracyanoethylene; X = monovalent anion) the magnetic properties can be tuned by systematic altering of transition metal from Mn to Ni, as well as adjusting the inter-plane distances via changing the anion volume (e.g., PF$_{6}$, AsF$_{6}$ and SbF$_{6})$. The magnetic properties of the molecule-based magnets are highly responsive to structural perturbations. For the series of M[TCNE]X magnets the synchrotron-based powder diffraction experiments in combination with magnetic susceptibility, all under hydrostatic pressure, were performed revealing the correlations between metal-ligand bonding and magnetic exchange and allowing the structure-magnetic property correlations to be established. The pathways toward important conductivity and band spin polarization improvements substantial for spin-polarized current injection in microelectronic applications are discussed. [Preview Abstract] |
Wednesday, March 23, 2011 4:18PM - 4:30PM |
T18.00008: Experimental determination of the Weiss temperature of several variants of Mn12-ac Shiqi Li, Lin Bo, Bo Wen, P. Subedi, Y. Yeshurun, A.D. Kent, M.P. Sarachik, A.J. Millis, C. Lampropoulos, S. Mukherjee, G. Christou We report measurements of the susceptibility in the temperature range from 3.5 K to 6.0 K of a series of Mn12-ac and Mn12-ac-MeOH samples in the shape of rectangular prisms. The susceptibility obeys a Curie-Weiss Law, where the temperature intercept varies systematically with sample aspect ratio. Using published demagnetization factors, we obtain the Curie-Weiss intercept for an infinitely long sample corresponding to intrinsic ordering temperatures Tc $\sim $ 0.85 K and $\sim $ 0.74 K for Mn12-ac and Mn12-ac-MeOH, respectively [1]. The difference in Tc for the two materials suggests an additional non-dipolar (exchange) contribution to the Weiss temperature that differs in the two materials because of the difference in ligand molecules. A similar comparison will also be reported for Mn12-toluate. [Preview Abstract] |
Wednesday, March 23, 2011 4:30PM - 4:42PM |
T18.00009: Asymmetric Berry-Phase Interference Patterns in a Mn$_{4}$ Single-Molecule Magnet H.M. Quddusi, J. Liu, S. Singh, K. Heroux, E. del Barco, S. Hill, D. Hendrickson We present a low temperature magnetometry study of the quantum interference effect in a Mn$_{4}$ single-molecule magnet. Asymmetric modulations of the Berry phase interference patterns upon application of a transverse field are observed for $k$~$>$~0 resonances (i.e. non-zero longitudinal field), contrary to the symmetric patterns obtained at $k$~=~0. These asymmetries can be reversed by a full inversion of the total applied field. The observation of a fascinating motion of the Berry-phase minima as a function of both the magnitude and direction of the transverse field can be understood as an outcome of a competition between different intramolecular magnetic interactions. A multi-spin description using non-collinear zero-field splitting tensors and intra molecular dipolar interactions between the manganese ions is employed to explain the results. [Preview Abstract] |
Wednesday, March 23, 2011 4:42PM - 4:54PM |
T18.00010: Observation of double hysteresis in a MnMn$_6$(CH$_3$H$_5$O$_3$)$_3$ single-molecule magnet Klaus Gieb, Wolfgang Kroener, Paul M\"uller, Carl-Georg Freiherr von Richthofen, Thorsten Glaser We report on high field and low temperature magnetization measurements of a novel {MnMn$_6$}(CH$_3$H$_5$O$_3$)$_3$ complex. A home-made micro-Hall-probe magnetometer was used to perform the characterization at mK temperatures and fields up to 17\,T. Most 3d-ion based single-molecule magnets, known up to now, have a spin ground state well separated from the first excited state, leading to the formation of giant spin at low temperatures. In contrast to this situation, the ground state (S$=$6) of the present complex can already be exited at moderate magnetic fields. Surprisingly, magnetic hysteresis was observed for both the ground state and the first exited state leading to a double hysteresis in the low temperature magnetization measurements. The blocking temperature was found to be T$_B \approx$1.3\,K. Origin and possible consequences of this unusual behavior will be discussed. [Preview Abstract] |
Wednesday, March 23, 2011 4:54PM - 5:06PM |
T18.00011: A Muon Spin Relaxation Study of NiTCNQ2 Adam Berlie, Ian Terry, Marek Szablewski, Sean Giblin NiTCNQ$_2$ (TCNQ = 7,7,8,8-tetracyanoquinodimethane) is a novel metal-organic material that exhibits a magnetic transition at approximately 20~K. The material was first synthesized by Dunbar \textit{et al} (Chem. Mater. 15, 1840) who identified the low temperature magnetic phase as a glassy ferromagnet. We have investigated this magnetic transition with muon spin relaxation ($\mu$SR). We synthesised the deuterated form of the material to minimise hyperfine coupling between the muon and H nuclear moments on TCNQ. Using $\mu$SR we probed the transition region to determine whether there was a local or long range coherence of the ferromagnetism. We found that zero field measurements yielded a Lorentzian relaxation along with a nuclear component from the nuclear spins of the nitrogen atoms. A longitudinal field of 0.5~mT decoupled the nuclear component revealing a dynamical Kubo-Toyabe relaxation suggesting that there may be a coexisting static and dynamic field distribution. This may imply a long range static ordering below the transition temperature. [Preview Abstract] |
Wednesday, March 23, 2011 5:06PM - 5:18PM |
T18.00012: Magnetization measurements of a two-leg spin-1/2 ladder with strong leg interactions K. Ninios, Tao Hong, S.N. Herringer, M.M. Turnbull, C.P. Landee, Y. Takano, H.B. Chan We have measured the magnetization of a single crystal of the spin-1/2 ladder compound bis(2,3-dimethylpyridinium) tetrabromocuprate (DIMPY), using micromechanical force magnetometry down to 30mK. DIMPY undergoes a quantum phase transition from a gapped phase with no long--range magnetic order to a gapless phase, at critical field Hc. Recent specific heat results by Hong et al. show that the gapless phase is a Tomonaga-Luttinger liquid (TLL). Low temperature magnetic susceptibility as a function of field exhibits a maximum near Hc, in consistence with the divergence of the zero temperature susceptibility at Hc expected for an ideal 1-D system. A clear minimum in the magnetization of the gapless phase as a function of temperature is observed, a minimum that marks the limit below which the TLL exists. In addition the field dependence of the TLL parameter K is studied. [Preview Abstract] |
Wednesday, March 23, 2011 5:18PM - 5:30PM |
T18.00013: Field-induced Tomonaga-Luttinger liquid phase of a two-leg spin-1/2 ladder with strong leg interactions Tao Hong, Y.H. Kim, C. Hotta, Y. Takano, G. Tremelling, M.M. Turnbull, C.P. Landee, H.-J. Kang, N.B. Christensen, K. Lefmann, K.P. Schmidt, G.S. Uhrig, C. Broholm We study the magnetic-field-induced quantum phase transition from a gapped quantum phase that has no magnetic long-range order into a gapless phase in the spin-1/2 ladder compound bis(2,3-dimethylpyridinium) tetrabromocuprate (DIMPY) [1]. At temperatures below about 1~K, the specific heat in the gapless phase attains an asymptotic linear temperature dependence, characteristic of a Tomonaga-Luttinger liquid. Inelastic neutron scattering and the specific heat measurements in both phases are in good agreement with theoretical calculations, demonstrating that DIMPY is the first model material for an $S=1/2$ two-leg spin ladder in the strong-leg regime. \\[4pt] [1] T. Hong et al., Phys. Rev. Lett. 105, 137207 (2010) [Preview Abstract] |
Session T19: Focus Session: Spin Transport & Magnetization Dynamics in Metals VII
Sponsoring Units: GMAG DMPChair: Frances Hellman, University of California, Berkeley
Room: D170
Wednesday, March 23, 2011 2:30PM - 2:42PM |
T19.00001: Direct observation of strain-induced magnetic domain evolution in Heusler shape memory compounds C.A. Jenkins, A. Scholl, A. Doran, T. Omori The magnetic domain structure in single crystals of a novel Heusler magnetic shape memory (MSM) compound Fe$_2$MnGa was observed to undergo strain-induced evolution by synchrotron-based photoelectron emission microscopy (PEEM) at Beamline 11.0.1 of the Advanced Light Source. PEEM can produce high resolution, surface-sensitive images reflecting the spatial distribution of magnetism, and a custom flexure rig has been designed for in situ actuation and simultaneous observation of MSM. System energy of an MSM compound is lowered when the volume fraction of the favorably oriented martensitic variant is increased: intermartensitic twin boundary motion is energetically easier than magnetic rotation so magnetic domains evolve not by domain wall motion but by propagation of planar structural defects called twin boundaries. In this ternary MSM intermetallic the direction of tetragonal distortion, which is coupled to the magnetic easy axis, will lie favorably along the axis of compression as defined by the action of the custom rig. The disappearance of unfavorably oriented orthogonal magnetic domains with strain is then observed as expected. [Preview Abstract] |
Wednesday, March 23, 2011 2:42PM - 2:54PM |
T19.00002: Thin film growth and characterization of full Heusler alloys Rh$_{2-x}$Co$_{x}$FeSn and RhCoMnSn Li Gao, Mingyang Li, Mahesh G. Samant, Brian P. Hughes, Kevin P. Roche, Claudia Felser, Stuart S.P. Parkin Heusler alloys can be designed and prepared with high spin polarization, high Curie temperature, very low magnetization damping, as well as, tunable magnetic magnetization and anisotropy. Therefore, this family of compounds has great potential for applications such as spin-transfer-torque magnetic random access memory. The growth and characterization of epitaxial thin films of the Rh-based full Heusler compounds, Rh$_{2-x}$Co$_{x}$FeSn and RhCoMnSn, are presented. The magnetization, Curie temperature and crystal structure of these compounds have been investigated and are compared with bulk materials. An important known property of many Heusler alloys is low magnetization damping. Ferromagnetic resonance (FMR) studies using a strip line transmission technique reveals Gilbert damping values of $\sim $0.015 at room temperature in films of RhCoMnSn. These films have Curie temperatures well above room temperature whereas the Curie temperature of Rh$_{2}$FeSn is $\sim $350 K. [Preview Abstract] |
Wednesday, March 23, 2011 2:54PM - 3:06PM |
T19.00003: Electronic structure modifications in Cu doped Ni$_{2}$MnGa Sujoy Roy, R. Qiao, P.-A. Glans, A. Pathak, I.S. Dubenko, N. Ali, E. Blackburn, W. Yang Ni$_{2}$MnGa Heusler alloy is a multifunctional ferromagnetic alloy that exhibits various interesting properties. The compounds typically exhibit a high temperature magnetic and a low temperature martensitic transition. Stoichiometry changes or elemental substitution makes it possible to merge the two transitions to a unique magnetostructural one. We have used resonant inelastic x-ray scattering to study the effects of $d-d$ interactions and charge transfer effects in 25{\%} Cu doped Ni$_{2}$MnGa. We find distinct charge transfer effects in the Ni absorption and inelastic x-ray spectrum that are significantly modified by Cu doping. Mn on the other hand shows $d-d$ interaction effects but no charge transfer. Multiplet calculations have been performed and will be compared to the experimental data. These results provide an insight into the origin of multifunctional properties of Ni based Heusler alloys. Work at LBNL is supported by U.S DOE. [Preview Abstract] |
Wednesday, March 23, 2011 3:06PM - 3:18PM |
T19.00004: Approach towards full Heusler alloy based CPP-GMR: from Ag and non-magnetic Heusler to binary intermetallic spacers Oleg Mryasov, Sergey Faleev, S.V. Karthik Recently, it has been demonstrated that GMR response can be significantly enhanced by incorporating high spin polarization ferromagnetic (FM) full Heulser alloy into spin valve nano-structures. Experimental results for two types of non-magnetic spacers (i) elemental metal [1] and (ii) non-magnetic Heusler alloy spacers [2] deserve careful comparison. More practical (110) textured combination of Co$_{2}$MnGe (CMG) and non-magnetic Heusler alloy Rh$_{2}$CuSn (RCS) [2] have been used to build test hard disk drive [3]. In this work, we investigate the mechanism of spin dependent interface scattering for (001) CMG/Ag/CMG (Case1) and (110) CMG/RCS/CMG (Case2) models on the basis of ab-initio electronic structure calculations. We find that in both cases GMR has significant contribution from the spin dependent interface scattering. We propose new binary intermetallic spacer materials Al$_{2}$Au and Cu$_{3}$Sn as an alternative to Ag and RCS spacers. \\[4pt] [1] T.Iwase et.al. Appl.Phys.Express, \textbf{2}, 063003 (2009).\\[0pt] [2] K. Nikolaev et.al. App.Phys. Lett., \textbf{94}, 222501 (2009) [Preview Abstract] |
Wednesday, March 23, 2011 3:18PM - 3:30PM |
T19.00005: Spin fluctuations and doping trends in the itinerant magnet Pd$_{2}$TiIn$_{x}$ Jessica Missaghian, Andrew LaForge, David Matthews, Garrett Rogren, Zack Schlesinger, Arthur Ramirez The intermetallic compound Pd$_{2}$TiIn is one of several Heusler-type materials which are remarkable for possessing a significant magnetic moment despite being composed of elements which have no spontaneous local moment. We investigate the nature of the magnetic order and the role of spin fluctuations by studying the magnetic, transport and heat capacity properties of two series of polycrystalline samples derived from Pd$_{2}$TiIn$_{x}$: one in which the indium content is varied from $x$ = 0.87 to 1.22, and another in which 3d and 4d metals are substituted in small quantities for Pd and Ti. [Preview Abstract] |
Wednesday, March 23, 2011 3:30PM - 3:42PM |
T19.00006: Thermal effects on magnetic resonance in single crystal $Co_{1-x}Fe_xS_2$ B. Kaster, M. Pechan, M. Manno, A. Baruth, C. Leighton Many spintronic applications require spin injection from spin polarized ferromagnetic materials. A promising model system for fundamental studies of such processes is $CoS_2$, which has -56\% spin polarization at the Fermi level and is tunable with Fe doping in $Co_{1-x}Fe_xS_2$ to over 85\%. Single crystals of $Co_{1-x}Fe_xS_2$ have been successfully prepared with close to ideal sulfur stoichiometry using chemical vapor transport methods. We have employed variable temperature ferromagnetic resonance (FMR) at 9.2 GHz to investigate the magnetodynamic properties of this system for x=0, 0.05, 0.12 and 0.17. Resonance signals are observed upon cooling to 160 K, well above the Curie temperature, suggesting short-range order enhancements to the susceptibility lead to observable resonances. All concentrations exhibit increasing resonance position, and decreasing damping, with increasing temperature. Both the resonance position and the damping decrease with increasing Fe concentration - the former revealing Fe concentration effects on the moment and anisotropy, while the latter reflects the spin polarization influence on the damping. [Preview Abstract] |
Wednesday, March 23, 2011 3:42PM - 3:54PM |
T19.00007: Spin-dependent intergranular hopping transport in very thin highly spin-polarized CoS$_{2}$ thin films M. Manno, A. Gunawan, A. Baruth, R. Frakie, A. Mkhoyan, C. Leighton The Co$_{1-x}$Fe$_{x}$S$_{2}$ alloy system has been shown to exhibit high, composition tunable, spin polarization (-56 {\%} $< \quad P <$ +85 {\%}) in bulk, demonstrating great promise for fundamental studies in spintronics. Incorporation in heterostructures requires reliable thin film deposition routes, which have recently been developed. We present here a detailed study of the thickness ($t)$ dependence of the structural, magnetic, and electronic properties of polycrystalline CoS$_{2}$ thin films (70 -- 1600 {\AA}). As $t$ is decreased, we observe a suppression in magnetic properties accompanied by a metal-insulator transition. A distinct 3D to 2D crossover is evident in the conductance-voltage curves and intergranular tunneling magnetoresistance. At $t$ of order 70 {\AA} we observe granular metal conduction, in the presence of a Coulomb charging penalty. We demonstrate quantitative agreement between experiment and proposed models. The very thin film data are understood in terms of enhanced grain boundary resistance, due to S accumulation, which is evidenced via several modes of structural characterization. [Preview Abstract] |
Wednesday, March 23, 2011 3:54PM - 4:06PM |
T19.00008: Synthesis and basic characterization of the itinerant ferromagnet Cr$_{11}$Ge$_{19}$ Nirmal Ghimire, Michael McGuire, David Mandrus Cr$_{11}$Ge$_{19}$ is a member of the tetragonal, but structurally complex family of materials known as Nowotny chimney-ladder phases.~These materials have composition T$_{n}$X$_{m}$~where 2$>m$/$n>$1.25 (T=transition~element, X=Si, Ge, Sn or Ga). Although Cr$_{11}$Ge$_{19}$~was reported to be an itinerant ferromagnet, its basic properties have not been well characterized. Here we present resistivity, magnetization, and heat capacity results on polycrystalline Cr$_{11}$Ge$_{19}$ . [Preview Abstract] |
Wednesday, March 23, 2011 4:06PM - 4:18PM |
T19.00009: Unusual magnetism of Gd$_{5}$Ge$_{4}$ with non magnetic rare earths substitutions Durga Paudyal, Y. Mudryk, V.K. Pecharsky, K.A. Gschneidner, Jr. We present first principles and experimental studies on the small substitutions of Gd atoms by Lu, La, Y, and Sc atoms in Gd$_{5}$Ge$_{4}$. While replacing the Gd atoms located inside the slabs with the Lu or Y atoms leads to a substantial loss of ferromagnetism, the identical substitutions of other Gd locations preferred by La atoms have essentially no effect on the magnetostructural transitions. On the other hand, the Sc atoms prefer the same locations as the Lu and Y atoms. This substitution, however, changes the crystal structure of Gd$_{5}$Ge$_{4}$ from Sm$_{5}$Ge$_{4}$ type to Pu$_{5}$Rh$_{4 }$type at 25{\%} of Sc which was not observed with the former substitutions. The Pu$_{5}$Rh$_{4}$-type has structural parameters that are intermediate between the Gd$_{5}$Si$_{4}$ and Sm$_{5}$Ge$_{4}$ types. The exchange interactions in this substitution are positive which indicates that the Sc substituted Gd$_{5}$Ge$_{4}$ has the ferromagnetic ground state. [Preview Abstract] |
Wednesday, March 23, 2011 4:18PM - 4:30PM |
T19.00010: Examining the AF$>$FM transition in Fe-Rh thin films through specific heat, photoemission, and Mossbauer spectrometry measurements David Cooke, Catherine Bordel, Frances Hellman Iron-rhodium alloys near equiatomic composition undergo a metamagnetic antiferromagnetic-to-ferromagnetic (AF$>$FM) transition at just above room temperature. This material has been proposed as an exchange layer in thermally-assisted magnetic recording, using the ferromagnetic phase to reduce the switching field of a high-anisotropy storage layer, so clearly being able to control this transition is crucial to implementation. However, theoretically there is still much debate as to the precise mechanism of the AF$>$FM transition, primarily centered on the contributions of electronic and magnetic entropy differences in the two phases. Through thermodynamic measurements on epitaxially- grown ferromagnetic and antiferromagnetic Fe-Rh alloy films, we test two different thermal fluctuation models of the transition. We also discuss complementary photoemission and Mossbauer spectrometry data above and below the transition to examine the magnetic behavior and electronic densities of state in the two phases and compare these to theoretical calculations. [Preview Abstract] |
Wednesday, March 23, 2011 4:30PM - 4:42PM |
T19.00011: Transport Spin Polarization of High-Curie Temperature MnBi Films Pushkal Thapa, Parashu Kharel, Pavel Lukashev, Renat Sabirianov, Evgeny Tsymbal, David Sellmyer, Boris Nadgorny We report on the study of the structural, magnetic and transport properties of highly textured MnBi films with the Curie temperature of 628 K. In addition to detailed measurements of resistivity and magnetization, we measure transport spin polarization of MnBi by Andreev reflection spectroscopy and perform first-principles electronic structure calculations. A transport spin polarization of up to 63{\%} is observed, consistent with the calculations and with a recent observation of a large magnetoresistance in MnBi contacts. The band structure calculations indicate that, in spite of almost identical densities of states at the Fermi energy, the large disparity in the Fermi velocities leads to high transport spin polarization of MnBi. The correlation between the values of magnetization and spin polarization observed in MnBi will be discussed. [Preview Abstract] |
Wednesday, March 23, 2011 4:42PM - 4:54PM |
T19.00012: Anomalous electron transport in ferromagnetic MnBi films Parashu Kharel, D.J. Sellmyer Materials having high spin polarization, large perpendicular magnetic anisotropy and high Curie temperature hold great potential for a range of spintronic applications.\footnote{S. Mangin, D. Ravelosona, J. A. Katine, M. J. Carey, B. D. Terris and Eric E. Fullerton, Nature Mater. \textbf{5}, 210 (2006).} MnBi has the hexagonal NiAs structure and possesses strong permanent magnet and magneto-optical properties. Our recent research shows that MnBi exhibits a high transport spin polarization of 63{\%}, so it is useful to investigate the electron transport properties of this material. We have found that MnBi is a metallic conductor but the resistivity shows an anomalous temperature dependence at low temperature. Analysis of the Hall data for various samples shows that the extraordinary Hall effect is the dominant part in the transverse Hall effect and a Hall angle of 2.8{\%} has been measured. An experimental investigation on the origin of the observed large extraordinary Hall effect in MnBi thin films will be discussed. This research is supported by NSF-MRSEC Grant DMR-0820521, the DOE Grant DE-FG02-04ER46152 and NCMN. [Preview Abstract] |
Wednesday, March 23, 2011 4:54PM - 5:06PM |
T19.00013: SrMnBi$_2$, a new transition metal compound with metallic spacer layer Jiakui J. Wang, Liang L. Zhao, Q. Yin, G. Kotliar, Emilia Morosan To explore the correlation between superconductivity and crystal structure in transition metal-pnictides systems, we investigate the band structure and physical properties of SrMnBi$_2$ single crystals. This compound is isostructural with the superconducting Fe-pnictides. In this talk, magnetization, resistivity and specific heat data will be compared with band structure calculations. Both the experimental results and the density functional theory (DFT) calculation are consistent with this material being a bad metal with large residual resistivity, similar to the well-studied Fe-pnictides. The key difference is that the Sr-Bi blocking layer in SrMnBi$_2$ is metallic, which may be more favorable to the occurrence of superconductivity upon doping, likely with a higher transition temperature, commensurate with the high T$_N$ $\approx$ 280 K. [Preview Abstract] |
Wednesday, March 23, 2011 5:06PM - 5:18PM |
T19.00014: Doping - induced Quantum Phase Transition in Sc$_{3.1}$In Eteri Svanidze, Emilia Morosan Sc$_3$In is a known itinerant ferromagnet with a reported T$_{\rm{c}}~\approx~$6 K. In this talk we will show that Lu doping induces a quantum phase transition in this compound. Temperature and field - dependent magnetization measurements on (Sc$_{1-\rm{x}}$Lu$_{\rm{x}}$)$_{3.1}$In polycrystalline samples were performed, where 0 $\leq$ x $\leq$ 0.08. The 3.1 : 1 stoichiometry was chosen because it showed the highest T$_{\rm{c}}~\approx$ 10 K for x = 0 when Arrott plots were employed to determine the Curie temperature. In this study we use modified Arrott plots M$^2$ vs. (H/M)$^{1/\alpha}$. For $\alpha = 1.5$, the corresponding isotherms were linear over larger field ranges, and, for the critical composition x$_c~\approx$ 0.02, the isotherm was linear down to (M,H) = (0,0). The Curie temperature determined using this method was close to 6 K. [Preview Abstract] |
Wednesday, March 23, 2011 5:18PM - 5:30PM |
T19.00015: On the origin of the magnetic susceptibility anomaly in nearly ferromagnetic alloys Romeo de Coss, Aar\'on Aguayo, Filiberto Ortiz-Chi The magnetic susceptibility of the Ni-Rh and Ni-Cu alloys shows an anomaly near the transition from ferromagnetism to paramagnetism. In order to contribute to understand this phenomenon, we have studied the electronic and magnetic properties of the Ni$_{1-x}$Cu$_{x}$ alloy by means of first principles calculations. The ground state properties were obtained using the Full-Potential Linear Augmented Plane Waves method. The alloying was modeled using the self-consistent virtual crystal approximation. The spin magnetic susceptibility is calculated from the total energy as a function of the spin moment, obtained using the Fixed Spin Moment methodology. We found that the calculations predict correctly the reduction of the magnetic moment with the Cu concentration and that the critical concentration where the magnetic moment goes to zero is x$_{c}$ = 0.5, in excellent agreement with the experimental data. The calculated magnetic susceptibility is in good agreement with the experimental data in the whole range of concentrations for the Ni$_{1-x}$Cu$_{x}$ alloy, in particular the anomaly present at x $\approx $ 0.4 is reproduced by the calculations. This research was supported by Conacyt-M\'{e}xico under Grant No. 83604. [Preview Abstract] |
Session T20: Focus Session: Thermoelectric Materials: LAST/TAGS, Heusler, and Silicides
Sponsoring Units: DMP GERA FIAPChair: Joseph Heremans, Ohio State University
Room: D168
Wednesday, March 23, 2011 2:30PM - 2:42PM |
T20.00001: Charge Neutral Yukawa Lattice Gas on a FCC Lattice He Huang, S.D. Mahanti Structural phase transitions associated with the ordering of $Ag$ and $Sb$ ions in the quaternary systems, $(AgSbTe_{\rm 2}) _x (PbTe)_{2(1 - x)}$ (of current thermoelectric interest) has been investigated using an anti-ferromagnetic 3-state Ising model on a FCC lattice with screened Coulomb interaction (Yukawa lattice gas (YLG) model). We have carried out Monte Carlo simulations (MCs) to study phase transitions (PT) in YLG. The nature and the strength of PT depend on the screening parameter $\kappa$. The transition is $1^{st}$ order and the transition temperature $T_c$ is a weak function of the concentration x (excepting when $x\sim 0$ or 1), in agreement with earlier work for $\kappa =0$. We find $T_c(x, \kappa)=f(x)g (\kappa)$, where $g(\kappa)\to const$ when $\kappa\to 0$ and $g (\kappa)\to 0$ when $\kappa\to \infty$. For $x=0.5$, there are two special structures, layered and tubular which have the same ground state energy, independent of $\kappa$. This is understood by looking at the connectivity and ordering of ions. Above but near $T_c$, the generation rates of different micro structures have been analyzed using a simple surface energy density picture. MCs results agree with this analysis and show that the energy barriers decide the generation rates of different micro structures. [Preview Abstract] |
Wednesday, March 23, 2011 2:42PM - 2:54PM |
T20.00002: Effect of doping Ag$_{y}$Sb$_{y}$Ge$_{50-2y}$Te$_{50}$ thermoelectric materials with rare earths E.M. Levin, S.L. Bud'ko, K. Schmidt-Rohr The Ag$_{y}$Sb$_{y}$Ge$_{50-2y}$Te$_{50}$ system represents some of the most efficient thermoelectrics, the so-called TAGS materials. In order to understand the effect of doping of Ag$_{6.52}$Sb$_{6.52}$Ge$_{36.96}$Te$_{50}$ (``TAGS-85'') with rare earth atoms on the Ge and Te sites, Ag$_{6.52}$Sb$_{6.52}$Ge$_{36.96-x}$R$_{x}$Te$_{50}$ and Ag$_{6.52}$Sb$_{6.52}$Ge$_{36.96}$R$_{x}$Te$_{50-x}$ materials with R = Gd and Dy (rare earth atoms with large magnetic moments) have been studied by measuring X-ray diffraction (XRD) and $^{125}$Te nuclear magnetic resonance (NMR) at 300 K, thermopower and resistivity at 300-760 K, and the magnetization at 1.8-350 K and in magnetic field 0-55 kOe. XRD and $^{125}$Te NMR show that some rare earth atoms are incorporated into the lattice and enhance the thermopower by $\sim $10{\%}. At 700 K, this yields a power factor of up to 36 $\mu $W$\cdot $cm$^{-1}\cdot $K$^{-2}$, which is $\sim $20{\%} higher than in TAGS-85. All materials studied can be considered as degenerate magnetic semiconductors with non-interacting localized magnetic moments formed by rare earth atoms, with a different effect of rare earths on the Ge and Te sites. Reasons for the thermopower enhancement due to doping with rare earths including magnetic and non-magnetic phenomena are discussed. [Preview Abstract] |
Wednesday, March 23, 2011 2:54PM - 3:06PM |
T20.00003: Nanodopant Induced Band Modulations and Electronic Transport Properties in AgPb$_m$SbTe$_{2+m}$-type Thermoelectric Nanocomposites Yi Zhang, Changfeng Chen, Xuezhi Ke, Jihui Yang, Paul Kent The remarkable performance of many novel thermoelectric materials is attributed to their nanosized inclusions. By extensive first-principles calculations we show the distinct band structure modulation in AgPb$_m$SbTe$_{2+m}$ (LAST)-type nanocomposites. A band gap widening and conduction band minimum splitting process resulting from the nanodopants is discovered for a series of nanocomposites. Boltzmann transport calculations demonstrate that this process leads to a pronounced change in the high temperature electronic transport. The effects of different substitutional elements and atomistic orderings are discussed. Our results provide new understanding of nanosized doping in thermoelectric materials and narrow gap semiconductors. [Preview Abstract] |
Wednesday, March 23, 2011 3:06PM - 3:18PM |
T20.00004: Strong off-stoichiometry and large Gr\"{u}neisen parameter in AgSbTe$_2$: a first principles study Sergey V. Barabash, Vidvuds Ozolins, Michele D. Nielsen, Joseph P. Heremans We use first-principles density-functional theory calculations to study the dynamical and compositional instabilities in AgSbTe$_2$, and compare the theoretical predictions to the results of an experimental investigation. For pure AgSbTe$_2$, some native defects, particularly Ag vacancies, have negative formation energies for a wide range of experimental conditions, thus forming in high concentrations even at low $T$. This leads to large deviations from the formal stoichiometry, in agreement with experimental results. Substantial deviations of the AgSbTe$_2$ phase field away from the isoplethal Ag$_2$Te-Sb$_2$Te$_3$ section may be expected, potentially explaining the contradictions in the low-temperature regions of the previously published phase diagrams. We estimate the defect concentrations and the resulting intrinsic doping levels under various experimental conditions. Finally, we demonstrate that the stoichiometric AgSbTe$_2$ is at the verge of a dynamical instability: the energies of acoustic phonons near the L point depend strongly on volume, changing sign at nearly the experimental volume. This leads to an unusually large value of the Gr\"{u}neisen parameter, in agreement with experiment. [Preview Abstract] |
Wednesday, March 23, 2011 3:18PM - 3:30PM |
T20.00005: Effect of electron correlation on thermoelectric properties of the full-Heusler compound Fe$_2$VAl Dat Do, Mal-Soon Lee, S.D. Mahanti Heusler-type alloys have been studied extensively since they were first discovered by Heusler in 1903. Among those Fe$_2$VAl became interesting when Nishino {\it et. al.}[1] suggested that it might be a heavy fermion system. LDA/GGA calculations have shown that Fe$_2$VAl is a pseudo-gap system with sharp edges in the density of state near the Fermi level. This feature makes it a promising thermoelectric material. Since then electronic properties of nominally pure and doped Fe$_2$VAl have been studied extensively. However the exact nature of the ground state of this system is still not well understood. Since it contains d-electrons one expects electron correlation effects to be important. We have carried out band structure calculations using GGA+U method with several values of the on-site Coulomb interaction parameter U. Using the obtained band structure, transport coefficients were calculated within Boltzmann approach. Electronic structure and thermoelectric properties were studied for different values of U and compared to available experiments.\\[4pt] [1] Y. Nishino {\it et. al., Phys. Rev. Lett. {\bf 79} (10), 1909} (1997). [Preview Abstract] |
Wednesday, March 23, 2011 3:30PM - 3:42PM |
T20.00006: Phase-separated-induced changes in the transport properties of Heusler compounds for thermoelectric applications Tanja Graf, Michael Schwall, Peter Klaer, Hans-Joachim Elmers, Benjamin Balke, Claudia Felser The solid solution Co$_{2}$Mn$_{1-x}$Ti$_{x}$Sn shows a phase separation into two Heusler compounds, Co$_{2}$MnSn and Co$_{2}$TiSn. Only at the edges of the composition range a slight admixture of Mn and Ti to the respective other phase is observed. This phase separation leads to a distinct microstructure which can be altered by the composition of the material. Due to the formed phase and grain boundaries, pronounced changes in the magnetic and electronic properties take place with varying composition. The observed reduction of the thermal lattice conductivity is of particular interest for an optimization of Heusler compounds for thermoelectric applications. Thus, the concept of phase separated materials is transferred to Half-Heusler compounds with an improved thermoelectric performance. [Preview Abstract] |
Wednesday, March 23, 2011 3:42PM - 4:18PM |
T20.00007: Strategies to Bulk Half-Heusler Nanocomposites with Simultaneously Enhanced Power Factor and Reduced Thermal Conductivity Invited Speaker: Among promising thermoelectric materials for power generation, half-Heusler (HH) phases with general compositions TNiSn and TCoSb (T = Ti, Zr, Hf) have attracted tremendous attention not only because they involve abundant and environmentally friendly elements, but also due to their combination of large Seebeck coefficients with moderately low electrical resistivities. However, the ability to synthesize HH based materials with decent figures of merit (ZT$>$1) has been jeopardized by their very large thermal conductivities. Strategies to reduce the thermal conductivity of HH phases focusing on mass fluctuation phonon scattering via solid solution alloying or phonon scattering at grain boundaries and interfaces in HH phases with embedded pre-synthesized nanoparticles have failed to generate materials with high figures of merit due to simultaneous reductions in the power factor. Here, we introduce innovative approaches to revolutionary increases in the figure of merit of HH based materials through simultaneous large enhancement of the power factor and drastic reduction in the thermal conductivity. Our strategy is focused on atomic-scale structural engineering of the HH matrix through the confinement of full-Heusler (FH) inclusion phases on the lattice constant length-scale. Emphasis will be placed on the n-type Zr$_{0.25}$Hf$_{0.75}$Ni$_{1+x}$Sn$_{1-y}$Pn$_{y}$ and Ti$_{0.5}$Zr$_{0.5}$Ni$_{1+x}$Sn$_{1-y}$Pn$_{y}$ as well as the p-type Ti$_{0.5}$Zr$_{0.5}$Co$_{1+x}$Pn$_{1-y}$Sn$_{y}$, (Pn = Sb, Bi) nanocomposites. We will discuss the underlying mechanism for the formation of half-Heusler/full-Heusler (HH/FH) nanocomposites with coherent matrix/inclusion interfaces. The role of synthetic and processing methods; and size, dispersion and mole fraction of the FH inclusions on the thermoelectric performance of bulk HH/FH nanocomposites will be assessed by combining transmission electron microscopy studies with thermal and electronic charge transport data. [Preview Abstract] |
Wednesday, March 23, 2011 4:18PM - 4:30PM |
T20.00008: Thermoelectric Properties of Boron-doped CoSi Bo Yu, Hui Wang, Hongli Gao, Weishu Liu, Xinbing Zhao, Gang Chen, Zhifeng Ren Engineering in density of states $D(E)$ has been found effective in improving the transport properties of thermoelectric materials. As one example, intermetallic CoSi, when doped with boron or other suitable elements, exhibits a good combination of high electrical conductivity (\textit{$\sigma $}) and Seebeck coefficients ($S)$ due to possible sharp structures in $D(E) $near Fermi level. However, despite of its high power factor ($S^{2}$\textit{$\sigma $}), the high thermal conductivity ($\kappa )$ becomes the obstacle for the performance. Here, we present that mechanical alloying and hot press which had been proved successful in many thermoelectric materials, could also reduce the thermal conductivity of boron doped CoSi while keeping its high power factor. [Preview Abstract] |
Wednesday, March 23, 2011 4:30PM - 4:42PM |
T20.00009: The Electrical Contact for Higher Manganese Silicide Thermoelectric Material Xinghua Shi, Zahra Zamanipour, Daryoosh Vashaee The Electrical Contact for Higher Manganese Silicide Thermoelectric Material Xinghua Shi, Zahra Zamanipour, Daryoosh Vashaee Several electrical contact materials for Higher Manganese Silicide (HMS) are introduced. HMS is useful thermoelectric material for medium to high temperature applications. We have investigated several materials including Co, Ni, Cr, Ti, Mo, MnSi, MoSi2, and TiSi2 in search of the best contact material to HMS. The low electrical resistivity and reliability of the contact are two important elements to make a high efficient TE device. Moreover, the contact must maintain its chemical, mechanical, thermal, and electrical properties over a broad range of temperature (20C-700C). The investigated elemental metals failed to make reliable contact in terms of mechanical and chemical stability at high temperature. In contrast, the investigated metal silicides showed superior stability over extended operation at high temperature. The thermal stability and strong mechanical bonding of TiSi2 C54 phase and MnSi were specially observed. Their ohmic contact resistance was also within the range of interest over the whole range of temperature (10$^{-5}$-10$^{-4}\Omega $cm$^{2})$. [Preview Abstract] |
Wednesday, March 23, 2011 4:42PM - 4:54PM |
T20.00010: High Thermoelectric Power Factor in CoSi$_{1-x}$B$_{x}$ Alloys Hui Sun, Donald Morelli CoSi$_{1-x}$B$_{x}$ alloys with x ranging from 0 to 0.1 have been prepared by an arc melting and annealing procedure. X-ray diffraction studies suggest the occurrence of minor CoB phase when x$\ge $0.05. The thermoelectric (TE) properties were measured from 80 to 300K. The samples with x$\le $0.02 showed much lower electrical resistivity than CoSi. The Seebeck coefficient was negative for all samples over the investigated temperature range, indicating dominant transport by electrons in this temperature range. A high TE power factor (70 $\mu $W/K$^{2}$cm at room temperature) was obtained in CoSi$_{0.98}$B$_{0.02}$, which we ascribe to the appropriate tuning of the Fermi level near the pseudogap in the density of states. In optimized samples the dimensionless figure of merit is in excess of 0.13 due to this enhanced power factor. We will also report on our efforts to further increase the figure of merit by thermal conductivity reduction methods. [Preview Abstract] |
Wednesday, March 23, 2011 4:54PM - 5:06PM |
T20.00011: Thermoelectric properties of FeSi and Related Alloys: Evidence for Strong Electron-Phonon Coupling Brian Sales, Olivier Delaire, Michael McGuire, Andrew May The effects of various transition metal dopants on the electrical and thermal transport properties of Fe$_{1-x}$M$_{x}$Si alloys (M= Co, Ir, Os) are reported. The thermoelectric figure of merit ZT is improved from 0.007 at 60 K for pure FeSi to ZT = 0.08 at 100 K for 4{\%} Ir doping. A comparison of the thermal conductivity data among Os, Ir and Co doped alloys indicates strong electron-phonon coupling in this compound. The common approximation of dividing the total thermal conductivity into electronic and lattice components ($\kappa _{Total}=\kappa _{electronic}+\kappa _{lattice})$ fails spectacularly for these alloys. The effects of nanostructuring on thermoelectric properties of Fe$_{0.96}$Ir$_{0.04}$Si alloys are also reported. The thermal conductivity can be lowered by about 50{\%} with little or no effect on the electrical resistivity or Seebeck coefficient. This results in ZT$_{max}$ = 0.125 at 100 K, still about a factor of five too low for solid-state refrigeration applications. Research sponsored by the Materials Science and Engineering Division, Office of Basic Energy Sciences, U.S. DOE. [Preview Abstract] |
Wednesday, March 23, 2011 5:06PM - 5:18PM |
T20.00012: Electron-phonon coupling in FeSi thermoelectrics: inelastic neutron scattering and first-principles simulations Olivier Delaire, Jie Ma, Brian Sales, Paul Kent, Matthew Stone, Karol Marty, Matthew Lucas, Douglas Abernathy, David Mandrus FeSi is a promising thermoelectric material for refrigeration applications, with a Seebeck coefficient over 500$\mu$V/K at 40K. FeSi is a narrow band-gap semiconductor at low temperature (B20 structure), and undergoes a semiconductor-to-metal transition around room temperature. Using inelastic neutron scattering, phonons were measured on both single crystals and powders as a function of composition and temperature. We report a strong coupling between the phonons and the semiconductor-to-metal transition, upon increasing temperature and carrier concentration. Using first-principles electronic structure calculations and ab-initio molecular dynamics, we show that the band gap and the sharp features around the band edges are strongly affected by the thermal disordering induced by phonon excitations. We also report on the effect of heavy impurities (Ir, Os) on the phonons. [Preview Abstract] |
Wednesday, March 23, 2011 5:18PM - 5:30PM |
T20.00013: Carrier concentration optimization and electrical and thermal transport properties of the defect manganese silicide MnSi$_{\delta }(\delta \sim $ 1.74)$^{ \ast }$ V. Ponnambalam, Gloria Lehr, D.T. Morelli Defect manganese silicide MnSi$_{\delta }$ ($\delta \sim $ 1.74) is known for unusually low thermal conductivity. In addition, it also exhibits promising thermoelectric properties. We have substituted MnSi$_{\delta}$ with various elements to optimize the carrier concentration. Electrical and thermal transport properties of the resulting alloys have been studied over a temperature (T) range of 80-300 K. Both resistivity and Seebeck coefficient vary with substitution. Hall measurements suggest that the carrier concentration indeed varies in these alloys. Interestingly, thermal conductivity either remains constant or weakly increases with T in the temperature range 80- 300 K, eventually reaching values $\sim $ 3 W/m K at 300 K. The results will be presented and discussed. *This work was supported as part of the Center for Revolutionary Materials for Solid State Energy Conversion, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001054. [Preview Abstract] |
Session T21: Classical and Quantum Monte Carlo
Sponsoring Units: DCOMPChair: Bruce Miller, Texas Christian University
Room: D161
Wednesday, March 23, 2011 2:30PM - 2:42PM |
T21.00001: Steps beyond the fixed-phase approximation in diffusion Monte Carlo Fernando Reboredo The self-healing diffusion Monte Carlo algorithm (SHDMC) [Reboredo, Hood and Kent, Phys. Rev. B {\bf 79}, 195117 (2009); Reboredo, {\it ibid.} {\bf 80}, 125110 (2009)] is extended to study the ground and excited states of magnetic and periodic systems. The method converges to exact eigenstates as the statistical data collected increases if the wave function is sufficiently flexible. A recursive optimization algorithm is derived from the time evolution of the mixed probability density, which is given by an ensemble of electronic configurations (walkers) with complex weight. This complex weight allows the amplitude of the fixed-node wave function to move away from the trial wave function phase. This novel approach is both a generalization of SHDMC and the fixed-phase approximation [Ortiz, Ceperley and Martin, Phys Rev. Lett. {\bf 71}, 2777 (1993)]. The algorithm is demonstrated to converge to nearly exact solutions of model systems with periodic boundary conditions or applied magnetic fields for the ground state and low energy excitations. The computational cost is proportional to the number of independent degrees of freedom of the phase. [Preview Abstract] |
Wednesday, March 23, 2011 2:42PM - 2:54PM |
T21.00002: Understanding framework flexibility of periodic structures by Monte Carlo simulation An Ghysels, Veronique Van Speybroeck, Michel Waroquier, Berend Smit Metal Organic Frameworks (MOFs) are a new class of porous materials synthesized from metal clusters connected by organic linkers. Most crystalline solids are fairly rigid, and undergo small changes in volume when stress is applied. Although most MOFs are rigid, some have an unexpectedly high flexibility, and swell under pressure, temperature or adsorption changes. A well-known structure showing volume changes of over 50\% is MIL-53. A better understanding of the process will allow to design materials with improved properties for carbon capture, i.e. the framework captures CO2 from fuel gasses. In this presentation, we explore framework flexibility effects induced by gas adsorption using Monte Carlo techniques. For instance, when MIL-53 is brought into contact with a gas at increasing pressure, the framework's pores constrict, while at even higher pressures, the pores return to their original geometry. To study this phenomenon, it is essential to incorporate framework flexibility into the Monte Carlo free energy calculation. [Preview Abstract] |
Wednesday, March 23, 2011 2:54PM - 3:06PM |
T21.00003: Quantum Monte Carlo studies of solvated systems Kathleen Schwarz, Kendra Letchworth Weaver, T.A. Arias, Richard G. Hennig Solvation qualitatively alters the energetics of diverse processes from protein folding to reactions on catalytic surfaces. An explicit description of the solvent in quantum-mechanical calculations requires both a large number of electrons and exploration of a large number of configurations in the phase space of the solvent. These problems can be circumvented by including the effects of solvent through a rigorous classical density-functional description of the liquid environment, thereby yielding free energies and thermodynamic averages directly, while eliminating the need for explicit consideration of the solvent electrons. We have implemented and tested this approach within the CASINO Quantum Monte Carlo code. Our method is suitable for calculations in any basis within CASINO, including b-spline and plane wave trial wavefunctions, and is equally applicable to molecules, surfaces, and crystals. For our preliminary test calculations, we use a simplified description of the solvent in terms of an isodensity continuum dielectric solvation approach, though the method is fully compatible with more reliable descriptions of the solvent we shall employ in the future. [Preview Abstract] |
Wednesday, March 23, 2011 3:06PM - 3:18PM |
T21.00004: Fast evaluation of multideterminant wavefunctions in quantum Monte Carlo Miguel A. Morales, Bryan K. Clark, Jeremy McMinis, Jeongnim Kim, Gustavo Scuseria Quantum Monte Carlo (QMC) methods such as variational and diffusion Monte Carlo depend heavily on the quality of the trial wave function. Although Slater-Jastrow wave functions are the most commonly used variational ansatz, more sophisticated wave functions are critical to ascertaining new physics. One such wave function is the multislater- Jastrow wave function which consists of a Jastrow function multiplied by the sum of slater determinants. In this talk we describe a method for working with these wave functions in QMC codes that is easy to implement, efficient, and easily parallelized. The algorithm computes the multi determinant ratios of a series of particle hole excitations in time O(n$^{2})$+O(n$_{s}$n)+O(n$_{e})$ where n, n$_{s}$ and n$_{e}$ are the number of particles, single particle excitations, and total number of excitations, respectively. This is accomplished by producing a (relatively) compact table that contains all the information required to read off the excitation ratios. In addition we describe how to compute the gradients and laplacians of these multi determinant terms. [Preview Abstract] |
Wednesday, March 23, 2011 3:18PM - 3:30PM |
T21.00005: ABSTRACT WITHDRAWN |
Wednesday, March 23, 2011 3:30PM - 3:42PM |
T21.00006: Wave Function Optimization in QMCPACK Jeremy McMinis, Miguel Morales, Jeongnim Kim, David Ceperley Wave function optimization is essential for both the accuracy and efficiency of diffusion, reptation, and variational quantum Monte Carlo (QMC). In this talk we outline the wave function optimization strategy used in the QMC software package QMCPACK developed at the University of Illinois. We use an extension of the linear optimization method originally developed by Umrigar et. al.[1] to optimize parameters in Slater-Jastrow, multi-determinant Slater-Jastrow, and Backflow-Jastrow trial wave functions. The efficiency and accuracy of this method is presented for bulk Silicon, Jellium, and the Nitrogen dimer.\\[4pt] [1] Umrigar et al. PRL 98, 110201 (2007) [Preview Abstract] |
Wednesday, March 23, 2011 3:42PM - 3:54PM |
T21.00007: Testing of He-core pseudopotentials for 3d elements in quantum Monte Carlo Minyi Zhu, Lubos Mitas We construct He-core pseudopotentials for several elements in the 3rd row such as for V, Cr, Mn, Fe and Zn, with the goal of using these in high-accuracy quantum Monte Carlo (QMC) calculations. We study the accuracy of constructed pseudopotentials on MnO molecular states with different spin polarizations. We compare these results also with Density Functional Theory and Hartree-Fock approaches since we previously found noticeable differences between Ne-core pseudopotential and relativistic all-electron calculations in high-spin vs. low-spin state comparisons. The result indicates that these discrepancies stem from method biases related to the presence of core states, as we conjectured earlier. Additionally, we also discuss the computational cost of the He-core pseudopotentials in QMC calculations. [Preview Abstract] |
Wednesday, March 23, 2011 3:54PM - 4:06PM |
T21.00008: Spin-orbit interaction in quantum Monte Carlo Lubos Mitas, Rene Derian, Shi Guo For spinless Hamiltonians (ie, no explicit spin operators), real space quantum Monte Carlo (QMC) methods such as variational and fixed-node diffusion Monte Carlo are well established. In these cases the electron spins and their components commute with the Hamiltonian and therefore are conserved quantities. This implies that spins can be fixed as given by the symmetry of a considered state and one solves only for the spatial part of the corresponding wave function. Indeed, this is a common practice in electronic structure QMC and also in most quantum chemical calculations. However, many systems require treating spins as quantum dynamical variables. We will present progress of our studies in this direction both in variational and diffusion Monte Carlo for heavy atoms with spin-orbit operators. One possibility is to use possibilities offered by various formulations of Hubbard-Stratonovitch transformation as realized in QMC for nuclear systems. We explore also other options both in variational and diffusion Monte Carlo framework. In particular, we define new representation for spinors which enable to formulate the diffusion Monte Carlo along the lines of fixed-phase approximation. We compare the results for the considered approaches also from the point of computational efficiency. [Preview Abstract] |
Wednesday, March 23, 2011 4:06PM - 4:18PM |
T21.00009: A quantum Monte Carlo study of molecular systems with heavy elements Shi Guo, Kevin Rasch, Lubos Mitas, Enrique Batista, Richard Martin We use quantum Monte Carlo method to study the bis-cyclopentadienyl Hafnium dichloride molecule $Cp_2HfCl_2$. There are two Cl dissociation channels for $Cp_2HfCl_2$: one is to break into neutral fragments, the other one into charged fragments. We employ the Stuttgart pseudopotential to represent the Hf atom and optimized Slater-Jastrow trial wave function at the variational Monte Carlo level. The calculations of the dissociation energies are carried out by the fixed-node diffusion Monte Carlo. We observe that for the heavy elements the low valence density in the core region can generate large energy fluctuations and we address this by improvements of the correlation factor. Alternatively, we construct Hf pseudopotentials with different core sizes and test for the accuracy of such pseudopotential Hamiltonians. We compare the QMC results also with DFT calculations with hybrid functionals. [Preview Abstract] |
Wednesday, March 23, 2011 4:18PM - 4:30PM |
T21.00010: Fixed-Node Errors in Diffusion Monte Carlo Study of Li Molecular and Solid Systems Kevin Rasch, Lubos Mitas We study the fixed-node bias in the Diffusion Monte Carlo calculations of Li systems such as Li dimer, Li clusters, and Li body-centered cubic crystal at the equilibrium lattice constant. The calculations include both core and valence electrons in order to avoid any possible impact by pseudopotentials. We examine the fixed-node errors for different types of orbitals and wave-function forms. We use estimations of exact total energies from alternative approaches such as correlated basis set methods or from experiment. The results suggest that for Li systems it is possible to construct accurate wave-functions which recover correlation energy at 97-99 \% of correlation energy in the full many-body framework. [Preview Abstract] |
Wednesday, March 23, 2011 4:30PM - 4:42PM |
T21.00011: Comparison of the Angular Dependence of Monte Carlo Particle Transport Modeling Software Jeff Chancellor, Stephen Guetersloh Modeling nuclear interactions is relevant to cancer radiotherapy, space mission dosimetry and the use of heavy ion research beams. In heavy ion radiotherapy, fragmentation of the primary ions has the unwanted effect of reducing dose localization, contributing to a non-negligible dose outside the volume of tissue being treated. Fragmentation in spaceship walls, hardware and human tissue can lead to large uncertainties in estimates of radiation risk inside the crew habitat. Radiation protection mandates very conservative dose estimations, and reduction of uncertainties is critical to avoid limitations on allowed mission duration and maximize shielding design. Though fragment production as a function of scattering angle has not been well characterized, experimental simulation with Monte Carlo particle transport models have shown good agreement with data obtained from on-axis detectors with large acceptance angles. However, agreement worsens with decreasing acceptance angle, attributable in part to incorrect transverse momentum assumptions in the models. We will show there is an unacceptable angular discrepancy in modeling off-axis fragments produced by inelastic nuclear interaction of the primary ion. The results will be compared to published measurements of 400 MeV/nucleon carbon beams interacting in C, CH2, Al, Cu, Sn, and Pb targets. [Preview Abstract] |
Wednesday, March 23, 2011 4:42PM - 4:54PM |
T21.00012: A Quantum Monte Carlo study of Hydrogen Adsorption on Carbon and Transition Metal Systems Todd D. Beaudet, Jeongnim Kim, Richard M. Martin We present a quantum Monte Carlo study of many molecular structures of Ti-ethylene with up to 5 H$_{2}$ molecules. These structures have been of recent interest due to energetics favorable for reversibly storing hydrogen.\footnote{E. Durgun \textit{et al}., Phys. Rev. Lett. \textbf{97}, 226102 (2006).} Diffusion Monte Carlo is employed with the fixed node approximation and pseudopotentials that have been tested for H$_{2}$ adsorbed on benzene and calculations on TiH$_{2}$ molecules.\footnote{T. D. Beaudet, Doctoral Dissertation, University of Illinois at Urbana-Champaign (2010).} Many low energy configurations were studied by calculation of ground and excited states energy surfaces. The formation energies are comparable to other work\footnote{Y. Y. Sun \textit{et al}., Phys. Rev. B \textbf{82}, 073401 (2010).} and indicate that at least 3 hydrogen molecules can be adsorbed with energies in the range considered relevant for practical hydrogen storage. [Preview Abstract] |
Wednesday, March 23, 2011 4:54PM - 5:06PM |
T21.00013: Release-Node quantum Monte Carlo studies for molecules Norm Tubman, Jonathan Dubois, Randolph Hood, Berni Alder Release-Node quantum Monte Carlo (RN-QMC) is a method that calculates unbiased ground-state energies of fermionic systems. However, while RN-QMC has been successfully applied to the homogeneous electron gas with more than one hundred electrons, obtaining converged results for molecular systems has proven to be problematic for all but the smallest systems. A promising route to extending the method's success to a wider class of physically interesting Hamiltonians lies in the application of projection techniques such as Maximum Entropy (MaxEnt) which, in principle, allows for extrapolation to the converged ground-state energy. Direct application of MaxEnt to higher Z elements is, however, not entirely straightforward. We propose strategies for optimizing MaxEnt analysis of short time RN-QMC data and demonstrate their effectiveness in obtaining ground state energies for the first row dimers. Attention is given to the determination of statistical errors in the resulting extrapolations as well as an attempt to characterize the minimum decay time required for unbiased results. [Preview Abstract] |
Wednesday, March 23, 2011 5:06PM - 5:18PM |
T21.00014: Melting transition of Lennard-Jones particles in two dimensions Keola Wierschem, Efstratios Manousakis The melting transition of Lennard-Jones particles in two dimensions is investigated along a single isochore using classical Monte Carlo methods. A finite-size scaling analysis is conducted for the second moments of the translational and bond-orientational order parameters, and their critical exponents are determined. The behavior of these exponents is consistent with the predictions of the two-stage Kosterlitz-Thouless-Halperin-Nelson-Young (KTHNY) theory of melting in two dimensions. The translational and bond-orientational correlation lengths are also studied, with evidence of a divergence in the bond-orientational correlation length while the translational correlation length remains finite. This provides further support for the KTHNY melting scenario, although we cannot rule out possible phase co-existence due to a first order phase transition. [Preview Abstract] |
Wednesday, March 23, 2011 5:18PM - 5:30PM |
T21.00015: Micro-canonical Monte Carlo study of spin wave excitations in 2D XY model Smita Ota We have carried out micro-canonical Monte Carlo simulation of 2D XY model in a 30x30 lattice using periodic boundary conditions. In this micro-canonical Monte Carlo simulation, the energy is the input quantity and the temperature of the system is obtained from the simulations. Spin waves and bound vortex excitations dominate in the 2D XY model below the topological vortex unbinding transition. We have studied the spin waves from the energy distribution of an individual spin in the 2D XY model. The most probable spin wave energy corresponds to the maximum in the energy distribution. The probability of the spin wave excitation is found to be reduce exponentially, by an order of magnitude as the temperature increases to the topological transition. [Preview Abstract] |
Session T22: Correlated Electrons and Magnetic Phase Transitions
Sponsoring Units: DCMPChair: Harold Baranger, Duke University
Room: D163
Wednesday, March 23, 2011 2:30PM - 2:42PM |
T22.00001: Quantum spin metal state on a decorated honeycomb lattice Konstantin Tikhonov, Mikhail Feigel'man We present a modification of exactly solvable spin-(1/2) Kitaev model on the decorated honeycomb lattice, with a ground state of ``spin metal'' type. The model is diagonalized in terms of Majorana fermions; the latter form a 2D gapless state with a Fermi-circle those size depends on the ratio of exchange couplings. Low-temperature heat capacity $C(T)$ and dynamic spin susceptibility $\chi(\omega,T)$ are calculated in the case of small Fermi-circle. Whereas $C(T)\sim T$ at low temperatures as it is expected for a Fermi-liquid, spin excitations are gapful and $\chi(\omega,T)$ demonstrate unusual behavior with a power-law peak near the resonance frequency. The corresponding exponent as well as the peak shape are calculated. [Preview Abstract] |
Wednesday, March 23, 2011 2:42PM - 2:54PM |
T22.00002: Quantum Order by Disorder Driven Phase Reconstruction at Itinerant Electron Quantum Critical Points Una Karahasanovic, Andrew Green, Gareth Conduit Phase reconstruction at itinerant electron quantum critical points is driven by quantum fluctuations lowering the energy of certain deformations of the Fermi surface through second and higher order perturbation theory, i.e. quantum order by disorder. This approach was previously shown to predict a fluctuation-driven spatially modulated phase near to the ferromagnet to paramagnet quantum critical point [1]; a phase that had previously been predicted from diagrammatic evaluation of non-analytic corrections to Moriya-Hertz-Millis theory. We extend our analysis to include several other phases which may be stabilized at the ferromagnetic quantum critical point, including nematic order and superconductivity. The itinerant quantum critical point is unstable to the formation of multiple phases. \\[4pt] [1] G. J. Conduit, A. G. Green and B. D. Simons, Phys. Rev. Lett. {\bf 103}, 207201 (2009) [Preview Abstract] |
Wednesday, March 23, 2011 2:54PM - 3:06PM |
T22.00003: Fixed Spin Moment Study of Quantum Critical Fe$_3$Mo$_3$N Brian Neal, Warren E. Pickett Quantum critical behavior and weak magnetism occurs in a handful of intermetallic transition metal compounds, with a recent example being Fe$_3$Mo$_3$N with the geometrically frustrated {\it stella quadrangula} lattice. Neutron scattering reveals antiferromagnetic ordering, but a 14 T magnetic field induces a ferromagnetic state as does substitution of 5\% Co on the Fe site [1]. We present the energetics of a transition between these states with density functional based fixed spin moment studies. Our (mean field) ground state occurs with nearly equal Fe1 and Fe2 moments of 1.8 $\mu_B$. As the total moment is reduced, a crossover occurs until at zero total moment the Fe1 moment is -1.1 $\mu_B$ (antialigned with the strong Fe2 moment). We use these results to construct scenarios for discussing the observations. \\[4pt] [1] T. Waki et al., J. Phys. Soc. Japan {\bf 79}, 043701 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 3:06PM - 3:18PM |
T22.00004: Stability of Quantum Critical Points in the Presence of Competing Orders Jian-Huang She, Jan Zaanen, Alan Bishop, Alexander Balatsky We investigate the stability of Quantum Critical Points (QCPs) in the presence of two competing phases. These phases near QCPs are assumed to be either classical or quantum and assumed to repulsively interact via square- square interaction. We find that for any dynamical exponents and for any dimensionality strong enough interaction renders QCPs unstable, and drive transitions to become first order. We propose that this instability and the onset of first order transition leads to spatially inhomogeneous states in practical materials near putative QCPs. [Preview Abstract] |
Wednesday, March 23, 2011 3:18PM - 3:30PM |
T22.00005: Field-Induced Orbital Antiferromagnetism in Mott Insulators K.A. Al-Hassanieh, C.D. Batista, G. Ortiz, L.N. Bulaevskii We report on a new electromagnetic phenomenon that emerges in Mott insulators. The phenomenon manifests as antiferromagnetic ordering due to orbital electric currents which are spontaneously generated from the coupling between spin currents and an external homogenous magnetic field. This novel spin-charge-current effect provides the mechanism to measure the so-far elusive spin currents by means of unpolarized neutron scattering, nuclear magnetic resonance or muon spectroscopy. We illustrate this mechanism by solving a half-filled Hubbard model on a frustrated ladder. [Preview Abstract] |
Wednesday, March 23, 2011 3:30PM - 3:42PM |
T22.00006: Global phase diagram of heavy fermions and the Kondo destroyed quantum critical points of Anderson models with a transverse field Jedediah Pixley, Stefan Kirchner, Qimiao Si Recent studies in quantum critical heavy fermion metals have pointed towards a global phase diagram [1]. The zero-temperature phase diagram involves a combination of phases, featuring Kondo screening/breakdown and antiferromagnetic order/disorder as the quantum fluctuations of the local moments are tuned relative to their effective interaction with the spins of the conduction electrons. In the case of Ising-anisotropic Kondo lattice systems, the fluctuations among the local moments can be generated by coupling them to a transverse magnetic field. With these effects in mind, we study the Kondo-destroyed quantum critical behavior of the Anderson impurity model in the presence of a bosonic bath or a transverse field. We extend our recent studies of the low-temperature quantum critical behavior [2,3] based on the continuous time quantum Monte Carlo, and obtain the dynamical scaling functions of the local spin susceptibility and single-electron Green's function. \\[4pt] [1] Q. Si and F. Steglich, Science 329, 1161 (2010). \\[0pt] [2] M. T. Glossop, S. Kirchner, J. H. Pixley and Q. Si, arXiv:0912.4521 to be published (2009). \\[0pt] [3] J. H. Pixley, S. Kirchner and Q.Si, arXiv:1010.3024 to be published (2010). [Preview Abstract] |
Wednesday, March 23, 2011 3:42PM - 3:54PM |
T22.00007: Electron paring instabilities in 8-site Betts lattice: exact result Kun Fang, Gayanath Fernando, Armen Kocharian We use numerical methods (exact diagonalization and Lanczos method) to study single-orbital and multi-orbital Hubbard models (off half filling). The whole lattice is divided into identical 8-site square clusters immersed in a thermal bath. The electron pairing instabilities, order parameters and quantum critical points are evaluated by monitoring the charge and spin gaps in a wide range of parameters including the on-site interaction $U$. Calculations show level crossing behaviors at zero and finite temperature. The corresponding pairing instabilities are remarkably similar to electronic inhomogeneities observed in correlated systems such as the high temperature superconductors and Fe pnictides. The next nearest hopping is also introduced. We find that it can shift quantum crossover point and gap magnitude, but for reasonable hopping amplitudes, it will not eliminate characteristics of electron paring instabilities. [Preview Abstract] |
Wednesday, March 23, 2011 3:54PM - 4:06PM |
T22.00008: Functional RG for the Single Impurity Anderson Model Michael Kinza, Carsten Honerkamp, Jutta Ortloff We present a functional Renormalization Group (fRG) approach to the Single Impurity Anderson Model at finite temperatures. Starting with the exact spectral function and interaction vertex of a small system (``core'') containing a correlated site, we switch on the hybridization with a non-interacting bath in the RG-flow and calculate spectra of the correlated site. Different truncations of the RG-flow-equations and choices of the core are compared and discussed. Furthermore we calculate the linear conductance as function of temperature and interaction strength. [Preview Abstract] |
Wednesday, March 23, 2011 4:06PM - 4:18PM |
T22.00009: Mesoscopic Anderson Box: Connecting Weak to Strong Coupling Dong E. Liu, Sebastien Burdin, Harold U. Baranger, Denis Ullmo Both the weakly coupled and strong coupling Anderson impurity problem are characterized by a Fermi-liquid theory with weakly interacting quasiparticles. In an Anderson box, mesoscopic fluctuations of the effective single particle properties will be large. We study how the statistical fluctuations in these two problems are connected. We use random matrix theory and the slave boson mean field approximation (SBMF, at low temperature) to address this question, obtaining the following results. First, for a resonant level model such as results from the SBMF approximation, we find the joint distribution of energy levels with and without the resonant level present. Second, if only energy levels within the Kondo resonance are considered, the distribution of perturbed levels collapse to one universal form for both GOE and GUE for all values of the coupling V. Finally, a purely Fermi liquid method is developed for calculating the perturbed levels within the Kondo resonance. Comparing the levels that result to those of the SBMF, we find remarkable agreement. [Preview Abstract] |
Wednesday, March 23, 2011 4:18PM - 4:30PM |
T22.00010: Superconducting pairing of interacting electrons: implications from the two-impurity Anderson model Lijun Zhu, Jian-Xin Zhu We study the non-local superconducting pairing of two interacting Anderson impurities, which has an instability near the quantum critical point from the competition between the Kondo effect and an antiferromagnetic inter-impurity spin exchange interaction. As revealed by the dynamics over the whole energy range, the superconducting pairing fluctuations acquire considerable strength from an energy scale much higher than the characteristic spin fluctuation scale while the low energy behaviors follow those of the staggered spin susceptibility. We argue that the superconducting pairing might not need the spin fluctuations as the glue, but rather originated from the effective Coulomb interaction. On the other hand, critical spin fluctuations in the vicinity of quantum criticality are also crucial to a superconducting pairing instability, by preventing a Fermi liquid fixed point being reached to keep the superconducting pairing fluctuations finite at low energies. A superconducting order, to reduce the accumulated entropy carried by the critical degrees of freedom, may arise favorably from this instability. [Preview Abstract] |
Wednesday, March 23, 2011 4:30PM - 4:42PM |
T22.00011: Pressure Tuning of the Shastry-Sutherland Quantum Phase Transition S. Haravifard, A. Banerjee, T.F. Rosenbaum, G. Srajer, J.C. Lang, Y. Feng, B.D. Gaulin, H.A. Dabkowska SrCu2(BO3)2 is a quasi-2D quantum spin system known to possess a collective singlet ground state. It serves as an experimental realization of the Shastry-Sutherland model for interacting S=1/2 dimers. The ratio of the intra and inter-dimer exchange in this compound is close to a quantum critical point, where the ground state transforms from a gapped, non-magnetic state to a gapless long-range ordered antiferromagnetic state as a function of the ratio of the strength of the magnetic interactions. We use synchrotron x-ray diffraction in a diamond anvil cell to investigate the pressure-driven quantum phase transition in high-quality single crystals of SrCu2(BO3). We will present the evolution of both the magnetic and structural properties up to pressures of 5 GPa. [Preview Abstract] |
Wednesday, March 23, 2011 4:42PM - 4:54PM |
T22.00012: Quantum phase transitions in generalized J-Q models Arnab Sen, Anders Sandvik The ``J-Q'' model is an extension of the Heisenberg model which contains multi-spin interactions that suppress N\'eel order and lead to a valence-bond-solid (VBS) ground state. It is free from quantum Monte Carlo (QMC) sign problems. There is now good evidence from QMC studies for a continuous N\'eel--VBS transition with non-trivial features like a large anomalous exponent $\eta$ and an emergent U(1) VBS symmetry at the quantum-critical point in this model. We study various generalizations of the J-Q model, with both SU(2) and U(1) symmetric interactions, to further elucidate unusual aspects of the N\'eel-VBS transition. In the SU(2) case, we construct a model which stabilizes a staggered VBS instead of the columnar pattern obtained in previous studies. This type of VBS does not harbor an emergent U(1) symmetry near the transition. We find that the transition is strongly first-order, unlike in the original J-Q model. This illustrates the importance of the emergent U(1) symmetry for the possibly exotic transition in the standard J-Q model. We also investigate a new U(1)-symmetric generalization of the J-Q model to explore such unconventional transitions in the easy plane case. [Preview Abstract] |
Wednesday, March 23, 2011 4:54PM - 5:06PM |
T22.00013: Criticality of compact and noncompact $(1+1)D$ quantum dissipative $Z_4$-models Einar Stiansen, Iver Sperstad, Asle Sudbo We study two versions of a $(1+1)D$ $Z_4$-symmetric model with Ohmic bond dissipation. In one version the phase variable is restricted to the interval $[0,2\pi\rangle$, while the domain is unrestricted in the other. The compact model features a completely ordered phase with a broken $Z_4$-symmetry and a disordered phase, separated by a critical line. The non-compact model features three phases. In addition to the two phases exhibited by the compact model, there is also an intermediate phase, characterized by isotropic power-law phase correlations. We calculate the dynamical critical exponent $z$ along the critical lines of both models to see if the compactness of the variable is relevant to the critical scaling between space and imaginary time. We find $z\approx1$ for the single phase transition in the compact model as well as for both transitions in the non- compact model. [Preview Abstract] |
Wednesday, March 23, 2011 5:06PM - 5:18PM |
T22.00014: Kondo and spin-Peierls phases and Berry phase effects in Heisenberg-Kondo chain Pallab Goswami, Qimiao Si Recent theoretical and experimental results on heavy fermion systems have motivated a global phase diagram, as a function of the Kondo coupling and the strength of quantum fluctuations of the local moments. Correspondingly, there has been growing interest in understanding the phase transition from a small Fermi surface antiferromagnet to large or small Fermi surface paramagnets with or without Kondo screening respectively. Because a perturbative nonlinear sigma model analysis only accesses the small Fermi surface antiferromagnetic phase, the transition into the paramagnetic phases must involve non-perturbative effects. We consider here the effect of the instanton configurations of the nonlinear sigma model and the associated Berry's phase for the Kondo singlet formation, and for concreteness focus on the one dimensional Heisenberg Kondo lattice model. Using semiclassical nonlinear sigma model and bosonization techniques both at and away from half-filling, we demonstrate how the competition between the Kondo singlet and spin Peierls phases are manifested through the effects of such a Berry phase. Based on these results we comment upon similar effects that may be realized in higher dimensional Kondo lattice models. [Preview Abstract] |
Wednesday, March 23, 2011 5:18PM - 5:30PM |
T22.00015: Complex Critical Exponents in Diluted Systems of Quantum Rotors Rafael Fernandes, J\"org Schmalian In this work, we investigate the effects of the Berry phase $2 \pi \rho$ on the critical properties of $XY$ quantum-rotors that undergo a percolation transition. This model describes a variety of randomly-diluted quantum systems, such as interacting bosons coupled to a particle reservoir, quantum planar antiferromagnets under a perpendicular magnetic field, and Josephson-junction arrays with an external bias-voltage. Focusing on the quantum critical point at the percolation threshold, we find that, for rational $\rho$, one recovers the power-law behavior with the same critical exponents as in the case with no Berry phase. However, for irrational $\rho$, the low-energy excitations change completely and are given by emergent spinless fermions with fractal spectrum. As a result, critical properties that cannot be described by the usual Ginzburg-Landau-Wilson theory of phase transitions emerge, such as complex critical exponents, log-periodic oscillations, and dynamically-broken scale invariance. [Preview Abstract] |
Session T23: Focus Session: Search for New Superconductors III: Reduced Dimensionality
Sponsoring Units: DMPChair: Ray Baughman, University of Texas at Dallas
Room: D165
Wednesday, March 23, 2011 2:30PM - 3:06PM |
T23.00001: Understanding anisotropy to develop superconducting design principles Invited Speaker: Superconductivity is often found in families of compounds which share a common building block (e.g. CuO$_{2}$ planes in cuprates, FeAs planes in pnictides, and CeIn$_{3}$ planes in a subset of heavy fermion superconductors). This fact provides a rationale to search for new superconductors, and subsequently a means to try and understand the origin of superconductivity by examining trends in superconducting behavior within a family of superconductors which hopefully transcends any one particular family of compounds. The notion of common building blocks has led us to the recent discovery of superconductivity at 2.1 K in CePt$_{2}$In$_{7}$, coexisting magnetism and superconductivity in PuCoIn$_{5}$, and a correlated paramagnet in PuPt$_{2}$In$_{7}$. I will discuss our attempts to understand the role of reduced dimensionality and increased bandwidth within the ``115'' class of heavy fermion superconductors by examining trends in the charge and spin degrees of freedom that are correlated with superconductivity. In this way, we aim to lay the foundation for a modern, microscopic version of Matthias' rules for unconventional superconductivity from which superconducting design principles can be developed. In collaboration with Eric Bauer, Jianxin Zhu, Paul Tobash, Moaz Altarawneh, HB Rhee, Hironori Sakai, Kris Gofryk, Neil Harrison, and Joe Thompson. [Preview Abstract] |
Wednesday, March 23, 2011 3:06PM - 3:18PM |
T23.00002: PuCoIn$_{5}$: A New Magnetic Superconductor Eric D. Bauer, J.N. Mitchell, P.H. Tobash, F. Ronning, J.-X. Zhu, B.L. Scott, J.D. Thompson There is renewed interest in actinide research following the discovery of superconductivity at T$_{c}$=18.5 K in PuCoGa$_{5}$ and at T$_{c}$=8.7 K in PuRhGa$_{5}$. These materials appear to be unconventional superconductors with a moderate effective mass enhancement and are similar to the more well characterized CeMIn$_{5}$ (M=Co, Rh, Ir) superconductors. We have discovered a new member of this ``115'' family of superconductors, PuCoIn$_{5}$. This material superconducts at T$_{c}$=2.7 K and exhibits another phase transition at T$_{N}$=15 K, likely due to antiferromagnetic order. The Sommerfeld coefficient $\gamma $ = 200 mJ/mol K$^{2}$ and the large initial slope of the upper critical field indicate a large enhancement of the effective mass. The physical properties of PuCoIn$_{5}$ will be discussed. [Preview Abstract] |
Wednesday, March 23, 2011 3:18PM - 3:30PM |
T23.00003: Search for Superconductivity in Carbon Nanotubes Doped by Boron Ion Implantation Nicholas Cornell, Alex Kutsenov, Austin Howard, Nathaniel Mayo, Eduard Galstayan, Wei Kan Chu, Herbert Freyhardt, Anvar Zakhidov, Xuemei Wang The boron doping of single wall carbon nanotubes(CNT) by laser ablation synthesis has been reported to create superconducting B-CNTs with Tc's ranging from 12-19 Kelvin, depending on CNT inter-tube connection strength. We attempt to create boron doped multiwall CNT by ion implantation doping. Ion doping of boron(B) was performed at 60keV and 20keV, and low temperature transport combined with SQUID and ESR/LFMA was used in searching for SC. We have found that R(T) strongly depends on the metallic contact geometry. With thin film contacts on CNT sheets the R(T) shows no SC signatures, while when an Ag or Au paste penetrates the highly porous network of B doped multiwall CNT then R(T) drops and curvature changes are observed resembling SC transitions with Tc depending on B concentration and metallic electrode distances. We discuss these results in terms of possible SC in hybride ``metal-CNT'' system in which metal was predicted to supress phase fluctuation in one dimensional CNT network [1]. \\[4pt] [1] Erez Berg, Dror Orgad, and Steven A. Kivelson, Phys. Rev. B 78, 094509(2008) [Preview Abstract] |
Wednesday, March 23, 2011 3:30PM - 3:42PM |
T23.00004: Low Field Microwave Absorption Studies of Carbon Nanotubes Doped by Chemical and Ion Implantation Techniques Austin Howard, Alexander Kuznetsov, Nicholas Cornell, Myron Salamon, Eduard Galstayan, Wei Kan Chu, Herbert Freyhardt, Ray Baughman, Junji Haruyama, Jason Reppert, Apparao Rao, Anvar Zakhidov The motivation of this study is to develop a highly sensitive method of microwave absorption in low magnetic fields (LFMA), combined with SQUID magnetometry and resistivity, for searching for superconducting phases in in-situ doped nanomaterials; either chemically (by alkali metals or metalloids) or through Boron ion implantation. These methods have been applied to both MWNTs grown by CVD, as well as SWNTs which have been separated into metallic and semiconducting chiralities. Regardless of the doping technique or element, we have found a much higher rate of doping in the semiconducting SWNTs. Additionally, in the Boron doped SWNTs, we see two transitions at $\sim$8 K and $\sim$30 K, but the nature of the transition is not clear at the moment: it depends on the type of measurement. While SQUID and resistivity indicate a superconducting type transition, LFMA/ESR reveals that there is a clear magnetic transition at 30 K. Resolution of these differing results will be discussed. [Preview Abstract] |
Wednesday, March 23, 2011 3:42PM - 3:54PM |
T23.00005: Electric Field Induced Superconductivity in Layered Materials J.T. Ye, M.F. Craciun, S. Russo, M.F. Morpurgo, Y. Kasahara, H.T. Yuan, H. Shimotani, Y. Iwasa Using electric double layer (EDL) gating, large amount of carriers can be accumulated on a broad range of materials, which provides new opportunities in effectively manipulating their electronic properties in complementary with the chemical doping. In searching for novel transport phenomena, layered materials are advantageous because atomically flat surface can be easily fabricated using the graphene techniques. We used layered material: ZrNCl and graphite to act as the channel of EDL transistors. For both ZrNCl and graphene, we achieved high carrier density up to 10$^{14}$ cm$^{-2}$, electrostatically. For graphene, we studied the high carrier density transport for graphene of 1-3 layers. Transport properties at the high carrier density exhibit clear layer dependence governed by the intrinsic band structures of graphene and its multi-layers. For ZrNCl EDL transistor, we observed metallic states at gate voltage higher than 3.5 V followed by gate-induced superconductivity after metal-insulator transition when the transistor was cooled down to about 15 K. [Preview Abstract] |
Wednesday, March 23, 2011 3:54PM - 4:06PM |
T23.00006: Electric Double Layer Charging on Graphene Feng Chen, Bing Lv, Yuyi Xue, C.W. Chu, Howard Wang Electric Double Layer (EDL) charging as a new charging method has attracted wide interests recently. We have employed this method to graphene and obtained an estimated surface charge density of 4$\times 10^{15}$ electrons/cm$^2$. The resistance dropped significantly upon charging and the physical properties under various charging conditions were studied. We will present these along with results of the EDL charging on other superconducting candidates. [Preview Abstract] |
Wednesday, March 23, 2011 4:06PM - 4:18PM |
T23.00007: Search for new superconductors in the La-Si-C system Jose De la Venta, Ali C. Basaran, Ted Grant, Antonio Jefferson S. Machado, Zachary Fisk, Ivan K. Schuller We have searched for the presence of superconductivity in the La-Si-C system in bulk and thin film samples. This system has some of the common features that are present in high T$_{C}$ superconducting materials. It is a multi-element compound and also incorporates a light element, Carbon. Furthermore, one of the binary phases, La$_{5}$Si$_{3}$ exhibits a tetragonal layered structure. This system exhibits the presence of a possible new superconducting compound with T$_{C}$'s ranging from 6.1 K to 8.5 K. In the binary La-Si system there are five inter-metallic phases. Among these phases, those that exhibit superconductivity are: LaSi$_{2}$ with T$_{C}$ of 2.3 K, La$_{3}$Si$_{2}$ with a T$_{C}$ of 2.1 K and La$_{5}$Si$_{3}$ with a T$_{C}$ of 1.6 K. A careful analysis of several physical properties (SQUID, Modulated Microwave Absorption) and x-ray powder diffraction, (using Rietveld refinement) shows that superconductivity in this system could be ascribed to intermediate binary (La$_{2}$C$_{3})$ and single (La-\textit{$\beta $}) phases of the system. [Preview Abstract] |
Wednesday, March 23, 2011 4:18PM - 4:30PM |
T23.00008: Superconducting properties of quasi-one dimensional graphene George Karakonstantakis, Steve Kivelson We study quasi-one dimensional graphene (polyacene) two leg ladders modeled by repulsive U Hubbard model using DMRG. The strong repulsive interactions along with the high density of states at the Fermi energy enhance the conducting properties of the ladder (which is a conductor) and give rise to enhancement of the pairing energy scales, having to do with the superconducting properties of the ladder. The presence of phonons in this system has been known to give rise to Peierls ans superconducting instabilities as well. [Preview Abstract] |
Wednesday, March 23, 2011 4:30PM - 4:42PM |
T23.00009: Unusual Superconductivity in the Homologous Series (Cu$_{0.75}$Mo$_{0.25})$Sr$_{2}$(Ce,Y)$_{s}$Cu$_{2}$O$_{5+2s+\delta }$ Omar Chmaissem, Inga Grigoraviciute, Maarit Karppinen, Hisao Yamauchi, Massimo Marezio The structures and bulk superconductivity ($>$30{\%} Meissner volume fraction) of the first four members of the high-$T_{c}$ series (Cu$_{0.75}$Mo$_{0.25})$Sr$_{2}$(Ce,Y)$_{s}$Cu$_{2}$O$_{5+2s+\delta }$have been successfully determined. Partial Mo substitution for Cu in the square-chains enhances $T_{c}$ to 87 K (for $s $= 1) and leads to significant oxygen loading capabilities well beyond the levels achieved in typical YSr$_{2}$Cu$_{3}$O$_{6+\delta }$, YBa$_{2}$Cu$_{3}$O$_{6+\delta }$, and other similar cuprates. Higher members of the series have their adjacent superconducting CuO$_{2}$ layers separated by increasingly thicker fluorite-like (Ce,Y)$_{2}$O$_{2}$ insulating blocks. Insertion of two or more of these blocks must drastically affect the CuO$_{2}$ interlayer coupling and causes $T_{c}$ to immediately drop and saturate at $\sim $57 K ($s$ = 2-4). The infinite chains of Cu-centered squares in YBCO change to alternate chains of mixed Cu squares and Cu and Mo octahedra. Neutron diffraction confirms the formation of reservoir blocks with a new structure and stoichiometry and of a surprisingly large Cu oxidation state of $\sim $2.5+, suggesting the possibility of an unusual superconducting pairing mechanism. [Preview Abstract] |
Wednesday, March 23, 2011 4:42PM - 4:54PM |
T23.00010: Electrical Transport Properties of Boron-Based Nanostructures Joel D. Chudow, Daniel F. Santavicca, Luigi Frunzio, Daniel E. Prober, Michael Rooks, Eswaramoorthi Iyyamperumal, Gayatri Keskar, Fang Fang, Lisa Pfefferle Many metal boride materials exhibit interesting electrical properties. Bulk MgB$_{2}$ has a superconducting transition temperature of 39 K. Boron-based nanostructures are predicted to possess special properties superior to those of other one-dimensional nanomaterials. Recent progress in material fabrication has enabled the successful synthesis of boron-based nanomaterials [J. Phys. Chem. C 113, 17661 (2009)]. Magnetization measurements of magnesium-boride nanostructures show evidence of a diamagnetic transition at high temperature, about 80 K. We describe electrical transport measurements of individual single nanowires composed of these and related materials. [Preview Abstract] |
Wednesday, March 23, 2011 4:54PM - 5:06PM |
T23.00011: Structure and electronic properties of the La$_{4}$Ni$_{3}$O$_{8}$ Konstantin Lokshin, Takeshi Egami The Ni$^{1+}$/Ni$^{2+}$ states of nickelates have the identical (3$d^{9}$/3$d^{8})$ electronic configuration as Cu$^{2+}$/Cu$^{3+}$ in the high temperature superconducting cuprates, and are expected to show interesting properties. However, La$_{4}$Ni$_{3}$O$_{8}$, has infinite NiO$_{2}$ layers with Ni valence 1.33 and demonstrate a magnetic transition at 105 K, which has not been explained unambiguously yet. Here we report X-rays and Neutron diffraction evidences clarifying the nature of the transition. The observed structural changes around 105 K suggest that the magnetic transision in La$_{4}$Ni$_{3}$O$_{8}$ originates from Yahn Teller effect that accompanies with high spin -- low spin transition. Thus, at low temperature the structural motive, electronic configuration and the spin state of Ni$^{1+}$/Ni$^{2+}$ nickelates are identical to Cu$^{2+}$/Cu$^{3+}$ cuprates. [Preview Abstract] |
Wednesday, March 23, 2011 5:06PM - 5:18PM |
T23.00012: Transport and Spectroscopy Studies of Ultrathin Doped Nickelate Films Yaron Segal, Joseph Ngai, Divine Kumah, Ankit Disa, James Reiner, Jarret Moyer, Dario Arena, Fred Walker, Charles Ahn The notion of a rational, first-principles based design of a novel superconducting material has intrigued physicists for decades. Recently it was suggested that by enforcing a two- dimensional confinement and tensile strain on LaNiO$_{3}$ films, their electronic structure can be made sufficiently similar to that of the Mott-Hubbard system in the cuprates, possibly inducing an antiferromagnetic insulator-superconductor transition [PRL 100, 016404]. We adopt this approach through the synthesis of ultrathin La$_{x}$Nd$_{1-x}$NiO$_{3}$ and hole-doped La$_{x}$Ba$_{1- x}$NiO$_{3}$, Nd$_{x}$Ba$_{1-x}$NiO$_{3}$ films using molecular beam epitaxy. High structural quality is demonstrated by RHEED oscillations and synchrotron x-ray diffraction. Transport measurement show a transition from metallic behavior to localization for films less than 8 uc thick. Tuning of the La/Nd ratio allows the film to be driven into the antiferromagnetic insulating regime. Surprisingly, Ba incorporation increases the localization in the films, which is in contrast to the metallicity-promoting effect of hole doping in bulk nickelates. X-ray absorption measurements allow us to follow the evolution of the Ni and O orbitals and relate it to the observed transport properties. [Preview Abstract] |
Wednesday, March 23, 2011 5:18PM - 5:30PM |
T23.00013: Superconductivity at 7.7 K in new hexagonal bronze Hg$_{x}$ReO$_{3}$ Kenya Ohgushi, Ayako Yamamoto, Yoko Kiuchi, Chandreyee Ganguli, Kazuyuki Matsubayashi, Yoshiya Uwatoko, Hidenori Takagi We have successfully synthesized a new rhenium-based hexagonal bronze material, Hg$_{x}$ReO$_{3}$, which exhibits superconductivity with the transition temperature $T_c$ = 7.7 K at ambient pressure and 11.1 K at 4 GPa. This compound is a superconductor with the highest $T_c$ among hexagonal bronzes. Moreover, it presents the novel crystallographic feature that $($Hg$_{2})^{2+}$ polycations, in contrast to monatomic cations in known hexagonal bronzes, are incorporated into open channels. There is evidence that conducting electrons tightly couple with Hg-related phonons. Our results inspire detailed studies on the role of the rattling phonon in the occurrence of superconductivity in the hexagonal bronzes. [Preview Abstract] |
Session T24: Focus Session: Quantum Transport Simulations and Computational Electronics -- Disorder
Sponsoring Units: DCOMPChair: Massimo Fischetti, University of Texas at Dallas
Room: D167
Wednesday, March 23, 2011 2:30PM - 3:06PM |
T24.00001: Recent progress in computational electronics: disorder effects in nonequilibrium quantum transport Invited Speaker: Realistic nanoelectronic devices inevitably have some disorder which affect device operation. Unintentional impurities sit at unpredictable locations and any predicted quantum transport property should be averaged over the impurity configurations. Impurity atoms are also intentionally doped into device material where the average is also necessary. One may generate many impurity configurations, calculate all and average the results. Such a brute force approach is not practical for first principles analysis as it is computationally too costly. I shall present the theory of nonequilibrium vertex correction (NVC) [1] where the configurational average is analytically done resulting to a NVC self-energy contributing to the nonequilibrium density matrix. NVC accounts for multiple impurity scattering at nonequilibrium. By integrating NVC with the density functional theory (DFT) and Keldysh nonequilibrium Green's functions (NEGF), nonequilibrium quantum transport in nanoelectronic systems having atomistic disorder can be carried out. By further integrating a recently proposed semi-local exchange potential that accurately determines band gaps [2], semiconductor nanoelectronics can now be analyzed from atomic first principles. Several examples will be presented including disorder scattering in Fe/MgO/Fe magnetic tunnel junction, electronic structure of In$_{x}$Ga$_{1-x}$N with varying concentrations x for solar cells, and quantum transport properties of doped Si membrane. \\[4pt] [1] Youqi Ke, Ke Xia and Hong Guo, Phys. Rev. Lett. 100, 166805 (2008); \textit{ibid} (in print, 2010). \\[0pt] [2] Fabien Tran and Peter Blaha, Phys. Rev. Lett. 102, 226401 (2009). [Preview Abstract] |
Wednesday, March 23, 2011 3:06PM - 3:18PM |
T24.00002: Seebeck Coefficients in Nanoscale Junctions: Effects of Electron-Vibration Scattering and Local Heating Bailey C. Hsu, Yu-Shen Liu, Shen Hsien Lin, Yu-Chang Chen We report first-principles calculations of inelastic Seebeck coefficients in an aluminum monatomic junction. We compare the elastic and inelastic Seebeck coefficients with and without local heating. In the low temperature regime, the signature of normal modes in the profiles of the inelastic Seebeck effects is salient. The inelastic Seebeck effects are enhanced by the normal modes, and further magnified by local heating. In the high temperature regime, the inelastic Seebeck effects are weakly suppressed due to the quasi-ballistic transport. [Preview Abstract] |
Wednesday, March 23, 2011 3:18PM - 3:30PM |
T24.00003: Critical current noise in rough Josephson junctions Pierre-Luc Dallaire-Demers, Frank Wilhelm While dissipationless, Josephson junctions as elements in superconducting nanocircuits are plagued by intrinsic noise mechanisms that will limit the coherence time of future high-precision quantum devices. Important sources of noise may arise from the non-cristallinity and disorder of the oxide layer sandwiched between the two superconducting leads. This work presents a microscopic calculation of the spectral density of noise of a rough superconducting tunnel junction. As for disordered conductors, a Josephson junction is modeled as a set of pinholes with a universal bimodal distribution of transmission eigenvalues that add their noise power incoherently. Each of these pinholes is treated as a ballistic point contact with an intrinsic thin barrier that modulates the transmission coefficient. The noise spectrum is computed using the quasiclassical Green's function method for superconductivity. This formalism allows us to investigate high and low transmission limits at finite temperature for any relevant frequency. As suggested by experiments, low transmission pinholes are expected to generate shot noise while fast switching between the subgap states of high transmission channels should create a strong non-poissonian low-frequency noise yet to be measured. [Preview Abstract] |
Wednesday, March 23, 2011 3:30PM - 3:42PM |
T24.00004: Computing Transport coefficients from the Microscopic Response Method Mingliang Zhang, David A. Drabold If an external perturbation to a system may be expressed as additional terms in the Hamiltonian, the microscopic response is determined by the the wave function of the system. To obtain the macroscopic response, an ensemble average can be carried out at the final stage. With the help of a systematic diagrammatic expansion, one is able to consistently compute the corresponding transport coefficient. If the spatial fluctuation of the carrier distribution is small, the microscopic response method reduces to the usual Kubo-Greenwood formula (KGF). We illustrate with the conductivity and Hall mobility of amorphous semiconductors. Because the direction of the Lorentz force is determined by the line connecting the initial and final localized states, the sign of Hall mobility in a-Si:H can be anomalous. The method is being implemented in an \textit{ab initio} code, and it is applicable to any temperature. Thus it significantly improves upon the usual method which averages KGF over a trajectory of classical molecular dynamics. \\[4pt] M.-L. Zhang and D. A. Drabold, Phys. Rev. Lett. \textbf{105}, 186602 (2010); Eur. Phys. J. B. \textbf{77}, 7-23, (2010); arXiv: 1008.1067. [Preview Abstract] |
Wednesday, March 23, 2011 3:42PM - 3:54PM |
T24.00005: Counting Statistics in Nanoscale Junctions from First Principles Yu-Chang Chen, Yu-Shen Liu We present first-principle calculations for moments of the current up to the third-order atomic-scale junctions. The quantum correlations of the current calculated in terms of wave functions obtained self-consistently within the static density functional theory are also demonstrated herein. Relationships between the conductance, the second, and the third moment of the current for carbon atom chains of various lengths bridging two metal electrodes in the linear and nonlinear regimes are investigated. The conductance, the second-, and the third-order Fano factors exhibit odd-even oscillation with the number of carbon atoms. The third-order Fano factor is positively correlates with conductance. [Preview Abstract] |
Wednesday, March 23, 2011 3:54PM - 4:06PM |
T24.00006: The Escape Problem in a Classical Field Theory With Two Coupled Fields and its Application to Monovalent Metallic Nanowires Lan Gong, Daniel Stein We introduced and analyzed a system of two coupled partial differential equations with external noise. The equations are constructed to model transitions of monovalent metallic nanowires with non- axisymmetric intermediate or end states, but also have more general applicability. They provide a rare example of a system for which an exact solution of nonuniform stationary states can be found. We have also explored the escape dynamics numerically, using the String Method, a relaxational technique. We find two kinds of transitions in activation behavior as we tune different parameters in our model, such as the interval length on which the fields are defined, and the bending coefficients of the fields. We discuss how these results apply to real nanowires. [Preview Abstract] |
Wednesday, March 23, 2011 4:06PM - 4:18PM |
T24.00007: Non-linear canonical transformations and Kondo physics Johan Nilsson We study the Kondo problem and the Kondo lattice using non-linear canonical transformations starting from the underlying Anderson model, generalizing the work of Ostlund in PRB 76, 153101 (2007). One such transformation, which is suitable to describe Fermi-liquid physics, provides an adiabatic connection between the quasi-particles of the interacting model and the electron- and hole-excitations in the non-interacting system as a function of the interaction parameter. We will also discuss other more unconventional transformations. [Preview Abstract] |
Wednesday, March 23, 2011 4:18PM - 4:30PM |
T24.00008: Spatial correlations in chaotic nanoscale systems with spin-orbit coupling Anh Ngo, Eugene Kim, Sergio Ulloa We investigate the properties of wave functions in chaotic nanostructures with spin-orbit (SO) interactions, focusing, in particular, on the evolution of the wave function statistics as the SO interaction is varied. We compare results obtained via random matrix theory for one- and two-point distribution functions with numerical results obtained from microscopic calculations on a stadium billiard, both with and without magnetic fields. We discuss how SO interactions weaken correlations in the system, as it evolves from the gaussian orthogonal (GOE) to the symplectic ensemble (GSE). In the presence of magnetic fields, a weak SO coupling decorrelates the two spin components, resulting in decoupled gaussian unitary ensembles (dGUE). We discuss experimental consequences of these (weakened) correlations, particularly for spin-dependent phenomena. [Preview Abstract] |
Wednesday, March 23, 2011 4:30PM - 4:42PM |
T24.00009: Test of Lattice Constant with Correction from Zero-Point Energy Pan Hao, Yuan Fan, John Perdew In Born-Oppenheimer system, the total energy doesn't include the kinetic energy of the nucleus. The zero point energy of the crystal can influence the lattice constant found by minimizing the total energy. The zero point anharmonic expansion (ZPAE) can be estimated in the Debye model. We can also use a direct way to get the zero point energy by calculating the phonon frequency using density-functional perturbation theory. Some solids were tested using different functionals to get the total energy and using DFPT to get the zero-point energy. We also expanded the vibration frequency as the compression ratio, which can give us the trends of the zero-point energy. For those different correction ways, the phonon frequency correction should be the most precise method in theory. The Debye model gives a reasonable approximation in most of those solids, but for Diamond structure and the Zinc-Blende structures, the Debye model may overestimate the correction. The expansion frequency way also overestimates the corrections compared to the phonon frequency correction. [Preview Abstract] |
Wednesday, March 23, 2011 4:42PM - 4:54PM |
T24.00010: ABSTRACT HAS BEEN MOVED TO W12.00014 |
Wednesday, March 23, 2011 4:54PM - 5:06PM |
T24.00011: ABSTRACT HAS BEEN MOVED TO T34.00016 |
Session T25: Superconductivity: Theory, Mainly Vortices
Sponsoring Units: DCMPChair: Maxim Korshunov, University of Florida
Room: D166
Wednesday, March 23, 2011 2:30PM - 2:42PM |
T25.00001: Microscopic investigation of vortex-vortex interaction in conventional and unconventional superconductors Masaru Kato, Yuhei Niwa Recently, we have investigated the vortex structures in nano-sized superconductors. We found the interference of vortex bound states around multiple vortices. And their interaction is affected by such quasi-particle interference. Vortex structures becomes different from which phenomenological theory predicts [1]. Therefore we clarify how such quasi-particle structure changes the vortex-vortex interaction. In order for this, we investigate the quasi-particle structures around a pair of vortices, using the Bogoliubov-de Gennes equation in the elliptical coordinates, where two vortices sit at two foci We expand quasi-particle wave functions by the (modified) Mathieu function. From the numerical results, we discuss the distance dependence of interference of the quasi-particle bound states and free energies. We will extend our method to unconventional superconductors. \\[4pt] [1] H. Suematsu, T. Ishida, T. Koyama, M. Machida, M. Kato, J. Phys. Soc. Jpn. 79, no.12 (2010) in press. [Preview Abstract] |
Wednesday, March 23, 2011 2:42PM - 2:54PM |
T25.00002: Type-1.5 superconductivity in multiband systems: the effects of interband couplings Egor Babaev, Johan Carlstrom, Martin Speight Two-component superconductors can posses a ``type-1.5" state which falls outside the usual type-I/type-II dichotomy. In this regime two vortices attract one another at long range but repel at shorter ranges. Multiple vortices thus should form clusters in cases where their interaction could be approximately described by a superposition of such nonmonotonic two-body forces and one can define a negative interface energy inside a cluster and at the same there one can define a positive interface energy associated with the cluster's boundary. We describe the appearance of type-1.5 regimes in the case of two bands with various kinds of substantial interband couplings such as Josephson coupling, mixed gradient coupling and density-density interactions. We show that in these cases the system supports type-1.5 superconductivity with fundamental length scales being associated with the mass of the gauge field and two masses of normal modes represented by mixed combinations of the density fields. Talk based on arXiv:1009.2196 and Phys. Rev. Lett. 105, 067003 (2010) [Preview Abstract] |
Wednesday, March 23, 2011 2:54PM - 3:06PM |
T25.00003: Proposed Aharonov-Casher interferometry of non-Abelian vortices in chiral p-wave superconductors Eytan Grosfeld, Babak Seradjeh, Smitha Vishveshwara We propose a two-path vortex interferometry experiment based on the Aharonov- Casher effect for detecting the non-Abelian nature of vortices in a chiral p-wave superconductor. The effect is based on observing vortex interference patterns upon enclosing a finite charge of externally controllable magnitude within the interference path. We predict that when the interfering vortices enclose an odd number of identical vortices in their path, the interference pattern disappears only for non-Abelian vortices. When pairing involves two distinct spin species, we derive the mutual statistics between half quantum and full quantum vortices and show that, remarkably, our predictions still hold for the situation of a full quantum vortex enclosing a half quantum vortex in its path. We discuss the experimentally relevant conditions under which these effects can be observed. [Preview Abstract] |
Wednesday, March 23, 2011 3:06PM - 3:18PM |
T25.00004: Microscopic theory of vortex interaction in two-band superconductors and type-1.5 superconductivity Mihail Silaev, Egor Babaev In the framework of self-consistent microscopic theory we study the structure and interaction of vortices in two-gap superconductor taking into account the interband Josephson coupling. The asymptotical behavior of order parameter densities and magnetic field is studied analytically within the microscopic theory at low temperature. At higher temperatures, results consistent with Ginzburg-Landau theory are obtained. It is shown that under quite general conditions and in a wide temperature ranges (in particular outside the validity of the Ginzburg-Landau theory) there can exist an additional characteristic length scale of the order parameter density variation which exceeds the London penetration length of magnetic field due to the multi-component nature of superconducting state. Such behavior of order parameter density variation leads to the attractive long-range and repulsive short-range interaction between vortices. [Preview Abstract] |
Wednesday, March 23, 2011 3:18PM - 3:30PM |
T25.00005: Hairy balls and flux lines in superconductors Mark Laver, Ted Forgan Many physical phenomena originate from geometrical effects rather than from local physics. For example, the hairy ball theorem --- a hairy sphere cannot be combed --- is fulfilled by the atmospheric circulation with the existence of stratospheric polar vortices, and the fact that there is always at least one place on Earth where the horizontal wind is still. We examine the consequences of the hairy ball theorem for the flux line lattice (FLL). We find that discontinuities must exist in lattice shape as a function of field direction relative to the crystal. The remarkable ways in which the hairy ball theorem is fulfilled are demonstrated for FLL's in superconducting niobium. We show that extraordinary, unconventional flux line lattice shapes that spontaneously break the underlying crystal symmetry are surprisingly likely across all Type-II superconductors, both conventional and unconventional. [Preview Abstract] |
Wednesday, March 23, 2011 3:30PM - 3:42PM |
T25.00006: Boundary Wess-Zumino-Novikov-Witten model, BCS superconductivity, and Maxwell-Bloch theory Tigran Sedrakyan, Victor Galitski We establish an exact correspondence between the discrete-state pairing Hamiltonian (Richardson model) and the Wess-Zumino-Novikov-Witten (WZNW) model modified by an additional boundary operator. We solve this boundary WZNW model exactly and from this solution re-derive the Richardson equations of the pairing Hamiltonian. As an example of practical applications of the boundary WZNW model we use the obtained results to derive solution to the Maxwell Bloch theory of a two-level laser with damping and pumping. We use the results to calculate various observable characteristics of a laser: (i) the complex electrical field amplitude, (ii) the polarization of the laser medium, (iii) the population inversion. We discuss the relation of our results to recent experimental data. [Preview Abstract] |
Wednesday, March 23, 2011 3:42PM - 3:54PM |
T25.00007: Geometric phases of d-wave vortices in a model of lattice fermions Zhenyu Zhou, Alexander Seidel, Oskar Vafek We study the local and topological features of Berry phases associated with the adiabatic transport of vortices in a d-wave superconductor of lattice fermions. At half filling, where the local Berry curvature must vanish due to symmetries, the phase associated with the exchange of two vortices is found to vanish as well, implying that vortices behave as bosons. Away from half filling, and in the limit where the magnetic length is large compared to the lattice constant, the local Berry curvature gives rise to an intricate flux pattern within the large magnetic unit cell. This renders the Berry phase associated with an exchange of two vortices highly path dependent. However, it is shown that ``statistical'' fluxes attached to the vortex positions are still absent. Despite the complicated profile of the Berry curvature away from half filling, we show that the average flux density associated with this curvature is tied to the average particle density. This is familiar from dual theories of bosonic systems, even though in the present case, the underlying particles are fermions. [Preview Abstract] |
Wednesday, March 23, 2011 3:54PM - 4:06PM |
T25.00008: Quasiparticle scattering from vortices in d-wave superconductors: Superflow and Berry phase contributions Adam C. Durst, Sriram Ganeshan, Manas Kulkarni In the vortex state of a d-wave superconductor, massless Dirac quasiparticles are scattered from magnetic vortices via a combination of two basic mechanisms: effective potential scattering due to the superflow swirling about the vortices and Aharonov-Bohm scattering due to the Berry phase acquired by a quasiparticle upon circling a vortex. First, we consider the superflow contribution by calculating the differential cross section for a quasiparticle scattering from the effective non-central potential of a single vortex. Next, we consider the Berry phase contribution, which results in branch cuts between neighboring vortices across which the quasiparticle wave function changes sign. Here, the simplest problem that captures the physics is that of scattering from a single finite branch cut that stretches between two vortices. Elliptical coordinates are natural for this two-center problem and we proceed by separating the massless Dirac equation in elliptical coordinates. The separated equations take the form of the Whittaker-Hill equations, which we solve to obtain radial and angular eigenfunctions. With these eigenfunctions in hand, we construct the scattering cross section via partial wave analysis. We discuss the scattering effect of each mechanism, superflow and Berry phase, leaving the important issue of interference between the two mechanisms to future work. [Preview Abstract] |
Wednesday, March 23, 2011 4:06PM - 4:18PM |
T25.00009: Ground states of multi-band type-I and type-1.5 superconductors and interlaced type-I/type-II layered superconducting structures in external magnetic field Julien Garaud, Johan Carlstrom, Egor Babaev We report a numerical study of magnetic field-induced structures in multiband/multi-component superconductors and type-I/type-II multilayers. The magnetic ground state in these different regimes shows very rich structure formation. In particular we report vortex cluster formation in the cases of strong interband Josephson coupling. The results in particular can be applied to layered structures manufactured from interlaced layers of type-I and type-II superconductors yielding effectively the type-1.5 superconducting behavior with tunable intercomponent couplings. [Preview Abstract] |
Wednesday, March 23, 2011 4:18PM - 4:30PM |
T25.00010: Properties of vortex clusters and intercluster interaction in type-II and type-1.5 two-band superconductors and type-I/type-II superconducting bilayers Johan Carlstrom, Julien Garaud, Egor Babaev We discuss magnetic flux-carrying vortex states in multiband type-II and type-1.5 superconductors and interlaced type-I/type-II superconducting multilayers. Especially we focus on the case where there is a substantial disparity in characteristic variations of superfluid densities in bands or superconducting layers. We discuss the properties of vortex clusters in the type-1.5 regime both in the cases of strong and weak interband Josephson coupling, including interaction between vortex clusters with different numbers of vortices. [Preview Abstract] |
Wednesday, March 23, 2011 4:30PM - 4:42PM |
T25.00011: Novel time-dependent Ginzburg-Landau simulation of extreme type-II superconductors with a focus on the Nernst signal and its fluctuations Sangwoo Chung, Paata Kakashvili, Carlos Bolech Recently, different transport coefficients have been measured in High-$T_{c}$ superconductors to pinpoint the nature of the pseudogap phase. In particular, the thermoelectric coefficients received a considerable attention both theoretically and experimentally. We numerically simulate the Nernst effect in extreme type-II superconductors using the time-dependent Ginzburg-Landau equations. We report the sign reversal of the thermoelectric coefficient, $\alpha_{xy}$, at temperatures close to the mean-field transition temperature $T^{MF}_{c}(H)$, which qualitatively agrees with recent experiments on high-$T_{c}$ materials. We also discuss the noise power spectrum of $\alpha_{xy}$, which shows $1/f^{\beta}$ behavior. Based on this observation, we propose an experiment to determine different regimes of vortex dynamics by measuring the noise correlations of the Nernst signal. [Preview Abstract] |
Wednesday, March 23, 2011 4:42PM - 4:54PM |
T25.00012: Temperature dependence of the superheating field in type-II superconductors Mark Transtrum, Gianluigi Catelani, James Sethna The expulsion of an applied magnetic field is a hallmark characteristic of superconductivity. For a sufficiently large external field, the superconducting state transitions to a normal metal (type-I) or a flux-lattice state (type-II) at a field $H_{c1}$. The superconducting state is metastable and persists up to a field above $H_{c1}$, the so-called superheating field. We numerically solve the semi-classical equations of Eilenberger for the anomalous Green's functions, order parameter, and vector potential for a clean superconductor in an external magnetic field. We use a linear stability analysis to explore the local stability of the free energy to two-dimensional fluctuations, mapping the stability onto an eigenvalue problem of a linear operator. We systematically calculate the dependence of the superheating field on both temperature and the Ginzburg-Landau parameter $\kappa$. We compare our results with the analogous calculation for Ginzburg-Landau theory, which is valid only near the critical temperature, and to experimental measurements. [Preview Abstract] |
Wednesday, March 23, 2011 4:54PM - 5:06PM |
T25.00013: Molecular dynamic simulation on the rule of the defect size on critical current at low temperature Abdalla Obeidat, Hadeel Abulahim Molecular dynamics have been used to study the effect of the pinning center sizes on the critical current density of driven vortex lattices interacting with periodic arrays of pinning sites in two dimensions. In our study, we assumed that the radii of the pinning centers are much larger than the coherence length of the vortices. The critical current density has been studied at different temperatures for several values of pinning strengths. The overdamped equation of vortex motion has been solved taking into account the vortex-vortex repulsion, the thermal force, the attractive vortex-pinning interaction, and the driving Lorentz force. We found that the critical current density is independent of pinning size at low temperatures. [Preview Abstract] |
Session T26: Focus Session: Iron Based Superconductors -- Magnetic Properties & Phase Diagrams
Sponsoring Units: DMP DCOMPChair: Stephen Hayden, Bristol University
Room: D162/164
Wednesday, March 23, 2011 2:30PM - 2:42PM |
T26.00001: Magnetic and Structural Phase Diagram of Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ Sevda Avci, Duck-Young Chung, Stephan Rosenkranz, John-Paul Castellan, Ray Osborn, Omar Chmaissem, Mercouri Kanatzidis, Eugene Goremychkin, Aziz Daoud-Aladine It is well known that the partial substitution of Ba by K in Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ causes a steep suppression of both the antiferromagnetic and tetragonal-orthorhombic transitions, leading to the onset of superconductivity over a large substitution range peaking at 38 K for x = 0.4. We report high resolution neutron powder diffraction results, which show that the magnetic and structural transitions are coincident over the entire phase diagram, in contrast to Ba(Fe$_{1-x}$,Co$_{x})_{2}$As$_{2}$. Volume discontinuities show that the combined transitions are first-order. The superconducting phase diagram has been refined with greater precision and a narrow region of phase coexistence have been delineated. [Preview Abstract] |
Wednesday, March 23, 2011 2:42PM - 2:54PM |
T26.00002: Coexistence of Superconductivity and Magnetism in EuFe$_{2}$(As$_{0.7}$P$_{0.3}$ )$_{2}$ A.A. Aczel, T.J. Williams, T. Goko, F.L. Ning, Y.J. Uemura, C. Arguello, W. Yu, G.F. Chen, G.M. Luke We have performed resistivity, magnetization, and $\mu $SR studies on single crystalline EuFe$_{2}$(As$_{0.7}$P$_{0.3}$)$_{2}$. These measurements provide clear evidence for bulk superconductivity in this system, with the sample resistance dropping to zero around 12 K. This work has also revealed ferromagnetic ordering of the S = 7/2 Eu$^{2+}$ moments along the c-axis (T$_{curie} \quad \sim $ 19 K). Finally, our $\mu $SR results indicate that the Eu magnetism is very homogeneous and occupies the full-volume fraction, pointing to real-space coexistence of magnetism and superconductivity in this material. [Preview Abstract] |
Wednesday, March 23, 2011 2:54PM - 3:06PM |
T26.00003: Physical and magnetic properties of Ba(Fe$_{1-x}$Mn$_x$)$_2$As$_2$ single crystals Alexander Thaler, Sheng Ran, Alfred Kracher, Warren Straszheim, Jiaqiang Yan, Sergey Bud'ko, Paul Canfield Single crystals of Ba(Fe$_{1-x}$Mn$_x$)$_2$As$_2$, $0 |
Wednesday, March 23, 2011 3:06PM - 3:18PM |
T26.00004: The magnetic form factor of SrFe2As2 William Ratcliff, P.A. Kienzle, J.W. Lynn, S. Li, P. Dai, G.F. Chen, N.L. Wang The Fe-pnictide based superconductors have recently been the subject of great interest. In this talk, we discuss recent neutron diffraction measurements of the magnetic form factor of SrFe2As2. These measurements reveal that while the form factor is primarily isotropic, a maximum entropy reconstruction reveals that there is evidence of hybridization between the Fe and As orbitals. [Preview Abstract] |
Wednesday, March 23, 2011 3:18PM - 3:30PM |
T26.00005: Temperature-pressure-composition phase diagram for Ba(Fe$_{1-x}$Ru$_{x})_{2}$As$_{2}$ (x$\le $0.285 and P$\le $84kbar) S.K. Kim, M.S. Torikachvili, A. Thaler, E.C. Colombier, S.L. Bud'ko, P.C. Canfield BaFe$_{2}$As$_{2}$ shows a structural/magnetic (SM) phase transition near T$_{SM}$=134K, which is suppressed upon partial substitutions at the Ba (K), Fe (Cr, Mn, Co, Ni, Cu, Ru, Rh, Pd, Ir) or As (P) sites, and also by adding pressure (P), most times leading to the emergence of superconductivity (SC) at low T. Here, we report on the P-dependence of the electrical resistivity in under- to near-optimally doped Ba(Fe$_{1-x}$Ru$_{x})_{2}$As$_{2}$ (0$\le $x$\le $0.285) for P$\le $84kbar. Pressure suppresses the SM transition at a rate that increases with Ru content, e.g. for x=0.092, 0.161, and 0.210, dT$_{SM}$/dP $\sim $-0.2, -0.4, and -0.6K/kbar, respectively. Although the x=0.092 and x=0.161 compositions are not SC at P=0, SC is seen when T$_{SM}$ is sufficiently reduced by P. For x=0.161, T$_{c}$ at first increases to a maximum near 25K at $\sim $20kbar, after which it decreases at a rate of $\sim $-0.4K/kbar. Likewise, the T$_{c}$ values for the higher dopings peak $\sim $20kbar and decrease at higher P at similar rates. We find that the application of pressure on Ba(Fe$_{1-x}$Ru$_{x})_{2}$As$_{2}$ enhances T$_{c}$ beyond that which was achieved with only doping, seen before in Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$. -- Supported by U.S. DOE (DE-AC02-07CH11358), AFOSR-MURI (FA9550-09-1-0603), and NSF (DMR-0805335). [Preview Abstract] |
Wednesday, March 23, 2011 3:30PM - 3:42PM |
T26.00006: Phase diagram of superconductivity and antiferromagnetism in single crystals of Sr(Fe1-xCox)2As2 and Sr1-yEuy (Fe0.88Co0.12)2As2 Rongwei Hu, Sergey Bud'ko, Paul Canfield We report magnetic susceptibility, resistivity and heat capacity measurements on single crystals of Sr(Fe$_{1-x}$Co$_{x}$)$_{2} $As$_{2}$ and Sr$_{1-x}$Eu$_{x}$(Fe$_{0.88}$Co$_{0.12}$)$_{2} $As$_{2}$ series. The optimal Co concentration for superconductivity in Sr(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2} $ is determined to be $x=0.117$. Based on this we grew members of the Sr$_{1-y}$Eu$_{y}$ series to examine the effects of well defined local moment scattering on the superconducting state. We show the evolution of superconductivity and development of antiferromagnetism across the whole doping range. The suppression of superconductivity within Abrikosov-Gor'kov's theory and de Gennes scaling as well as the antiferromagnetic transition temperature will be discussed. [Preview Abstract] |
Wednesday, March 23, 2011 3:42PM - 4:18PM |
T26.00007: Lattice distortion and magnetic quantum phase transition in CeFeAs$_{1-x}$P$_{x}$O Invited Speaker: With the advent of Fe-based superconductivity initially discovered in the prototypical electron doped Fe-pnictide LaFeAsO$x$F1$-x$, came a surge of renewed interest in high temperature superconductivity. The discovery of ubiquitous antiferromagnetic (AFM) order in the parent compounds of iron arsenide superconductors has brought attention to the understanding of the interplay between magnetism and high-transition temperature (high-$Tc)$ superconductivity in these materials. Although superconductivity in iron arsenides arises from charge carrier doping of their semimetal parent compounds, the resulting electronic phase diagrams are dramatically materials dependent, ranging from first-order-like AFM to superconductivity phase transition for LaFeAsO1-$x$F$x$, to the gradual suppression of the AFM order before superconductivity for CeFeAsO1-$x$F$x$, and finally to the co-existing AFM order with superconductivity in SmFeAsO1-$x$F$x$. A feature of the parent compounds is the structural distortion that occurs in the vicinity of the onset of long range magnetic order of the Fe-spins. In the RFeAsO(R=rare earth) family, the magneto-structural transition is suppressed in favor of superconductivity upon doping charge carriers into the system, which alters the system electronically and crystallographically as well. To understand the lattice effect on the suppression of the AFM ground state itself by quantum fluctuations, it is important to isoelectronically tune the crystal lattice structure without the influence of charge carrier doping and superconductivity. Here we use neutron scattering to show that replacing the larger arsenic with smaller phosphorus in CeFeAs1-$x$P$x$O simultaneously suppresses the AF order and orthorhombic distortion near $x $= 0.4, providing evidence for a magnetic quantum critical point. Furthermore, we find that the pnictogen height in iron arsenide is an important controlling parameter for their electronic and magnetic properties, and may play an important role in electron pairing and superconductivity. Preliminary work on systematic phosphorous doping in LaFeAs1-$x$P$x$O was also done to possibly identify characteristic changes in the lattice that may be correlated with the phosphorous doping induced superconductivity in the La system and in turn give insights as to the absence of superconductivity in the Ce system. [Preview Abstract] |
Wednesday, March 23, 2011 4:18PM - 4:30PM |
T26.00008: Rare earth substitution in AFe2As2 single crystals Shanta Saha, Nicholas Butch, Tyler Drye, Jeff Mcgill, Johnpierre Paglione, Peter Zavalij, Jeffrey Lynn Synthesis and characterization of aliovalent light rare earth substitutions for alkaline earth atoms are studied in single crystals of FeAs-based compounds with the ThCr2Si2 structure. Electrical resistivity, magnetic susceptibility and structural parameters determined via x-ray and neutron scattering techniques are investigated as a function of chemical pressure and charge doping induced by substitution. Measured physical properties are compared to the effects of external applied pressure on CaFe2As2, known to induce a collapse of the tetragonal unit cell. [Preview Abstract] |
Wednesday, March 23, 2011 4:30PM - 4:42PM |
T26.00009: Magneto-elastic Coupling in Single-crystal CeFeAsO H.-F. Li, J.-Q. Yan, J.W. Kim, R.W. McCallum, T.A. Lograsso, D. Vaknin Single-crystal synchrotron X-ray diffraction studies of CeFeAsO reveal strong anisotropy in the charge correlation lengths along or perpendicular to the in-plane antiferromagnetic (AFM) wave-vector at low temperatures. The high-resolution setup allows to distinctly monitor each of the twin domains by virtue of a finite misfit angle between them that follows the order parameter. We find that the in-plane correlations, above the orthorhombic (O)-to-tetragonal (T) transition, are shorter than those in each of the domains in the AFM phase, indicating a distribution of the in-plane lattice constants. This strongly suggests that the phase above the structural transition is virtually T with strong O-T fluctuations that are induced by magnetic fluctuations. [Preview Abstract] |
Wednesday, March 23, 2011 4:42PM - 4:54PM |
T26.00010: Competing magnetic ground states in Ba(Fe$_{1-x}$Cr$_x$)$_2$As$_2$ K. Marty, M.D. Lumsden, A.D. Christianson, C. Wang, M. Matsuda, H. Cao, L. VanBebber, J.L. Zarestky, D.J. Singh, A.S. Sefat Understanding the origin of unconventional superconductivity is a great challenge of condensed matter physics. In the so called 122 family, doping in the conductive layer (i.e. on the Fe site) of the BaFe$_2$As$_2$ iron pnictide parent compound leads to superconductivity for almost any transition metal, except for Cr and Mn. The absence of superconductivity in these cases remains an unresolved issue. We report here neutron diffraction measurements of Ba(Fe$_{1-x}$Cr$_x$)$_2$As$_2$ for concentrations up to x=0.47. The results show that Cr doping stabilizes magnetism across the phase diagram with a competing magnetic order favoured at high Cr-doping, in contrast to the other superconducting Ba(Fe$_{1-x}$TM$_x$)$_2$As$_2$. [Preview Abstract] |
Wednesday, March 23, 2011 4:54PM - 5:06PM |
T26.00011: Superconductivity and magnetism in Eu$_{1-x}$K$_{x}$Fe$_{2}$(As$_{1-y}$P$_y$)$_2$ H.S. Jeevan, J. Maiwald, Philip Gegenwart, Deepa Kasinathan, Helge Rosner We report detail investigation of superconductivity and magnetism in EuFe$_{2}$As$_{2}$ by doping of K in Eu site and P in As site. In this new class of FeAs-based superconductors, it is found that superconductivity appears close to a magnetic instability, suggesting a possible unconventional pairing mechanism. We have synthesized single crystals of both doped and undoped samples and investigated their physical properties by means of heat capacity, resistivity, magnetization and thermal conductivity measurements. The parent compound shows an antiferromagnetic spin-density-wave (T$_{SDW}$) at $\approx190$K related to the Fe$_{2}$As$_{2}$ layers and magnetic ordering of Eu$^{2+}$ (T$_N$) moments at $\approx20$K. Upon doping Eu with K $>$30$\%$, T$_{SDW}$ gets suppressed and superconductivity (SC) appears at $\approx32$K and also Eu$^{2+}$ ordering suppressed to the low temperature. On the other hand, P doping to the As site suppresses the SDW transition and results in SC, but Eu ordering remains undisturbed. Further increasing the P doping, Eu order transitions from AFM to FM phase which leads to disappearance of SC. We will compare our experimental findings with density functional theory based calculations. [Preview Abstract] |
Wednesday, March 23, 2011 5:06PM - 5:18PM |
T26.00012: Effects of annealing time and temperature on the transition temperature and low temperature state of AFe2As2 materials Sheng Ran, Sergey Bud'ko, Alex Thaler, Dominic Ryan, Yuji Furukawa, Beas Roy, Andreas Kreyssig, Robert McQueeney, Daniel Pratt, Alan Goldman, Paul Canfield Over the past couple of years the AFe2As2 (A = Ca, Sr, Ba) family of compounds has become a model system for the study of FeAs-based superconductivity. Superconductivity can be stabilized by hole and electron doping (on A and Fe sites) as well as hydrostatic pressure and isoelectronic substitutions on both the Fe and As sites. In all cases the adequate suppression of the structural / antiferromagnetic phase transition appears to be a necessary condition for the appearance of superconductivity. In this talk we will review the effects of annealing time and temperature on the structural / antiferromagnetic phases of AFe2As2. Transition temperature / time and transition temperature / annealing temperature plots will be presented and discussed. [Preview Abstract] |
Wednesday, March 23, 2011 5:18PM - 5:30PM |
T26.00013: Miscibility gap between BaMn$_2$As$_2$ and BaFe$_2$As$_2$ Abhishek Pandey, David C. Johnston ${\rm BaMn_2As_2}$ and ${\rm BaFe_2As_2}$ both crystallize at room temperature in the same tetragonal ThCr$_2$Si$_2$-type structure but with divergent unit cell volumes of 234.12 and 204.38~\AA$^3$, respectively, suggesting that the Mn$^{+2}$ is in a high-spin state while Fe$^{+2}$ is in a low-spin state. The physical properties of the two compounds are therefore also highly divergent; \emph{e.g.}, ${\rm BaMn_2As_2}$ is an insulating local moment antiferromagnet with a high N\'eel temperature $T_{\rm N} = 625$~K whereas BaFe$_2$As$_2$ is a metallic itinerant antiferromagnet with a much lower $T_{\rm N} = 137$~K\@.\footnote{D. C. Johnston, Adv. Phys. {\bf 59}, 803--1061 (2010).} We have discovered a miscibility gap in the pseudobinary phase diagram between these two isostructural compounds, probably arising from their divergent chemistry. Our investigations of the miscibility gap and of the structural, magnetic, electronic transport and thermal properties of various compositions in this system will be discussed. [Preview Abstract] |
Session T27: Open Quantum Systems and Decoherence
Sponsoring Units: GQI DAMOPChair: Michael Zwolak, Los Alamos National Laboratory
Room: C155
Wednesday, March 23, 2011 2:30PM - 2:42PM |
T27.00001: Landau-Zener-St\"uckelberg interference in the presence of quantum noise Yang Yu, Lingjie Du, Minjie Wang We investigated the Landau-Zener-St\"uckelberg (LZS) interference in strongly driven two-level systems subjected to quantum noise. The transition rate induced by consecutive LZ transitions is obtained, from which LZS interference can be analytically calculated based on rate equation. In the presence of significant frequency dependent noise, the evolving paths of LZS interference is going to be detoured. Therefore, the position of the resonant peaks is shifted and a stationary population inversion in TLS without involving the third qubit state is generated. The LZS interferometry can be used to investigate the noise property hence the decoherence source of the system. In addition, the stationary population inversion may find application in lasing and microwave cooling. [Preview Abstract] |
Wednesday, March 23, 2011 2:42PM - 2:54PM |
T27.00002: Exact master equations for linearly coupled Bosons or Fermions Shao-Wen Chen, Ren-Bao Liu Using the coherent-state representation (P-representation), we derived the exact master equation for a quantum system in an environment, which has linear but otherwise arbitrary couplings. This method works for both Boson and Fermion systems, since the coherent states of Bosons and Fermions have the similar algebra structure. The new derivation reproduces the previous works on photon dynamics in coupled cavities or quantum transport through double quantum dots, but it provides a more general theoretical framework for studying quantum dynamics in photonic, mechanical, and photomechanical systems, and quantum transport in nanostructures. [Preview Abstract] |
Wednesday, March 23, 2011 2:54PM - 3:06PM |
T27.00003: Engineering inverse power law decoherence of a qubit Francesco Petruccione, Filippo Giraldi The exact dynamics of a Jaynes-Cummings model for a qubit interacting with a bath of bosons, characterized by a special form of the spectral density, is evaluated analytically. The special reservoirs are sub-ohmic at low frequencies and inverse power law at high frequencies. The exact dynamics of the qubit is described analytically through Fox H-functions. Over estimated long time scales, decoherence results in inverse power laws with powers decreasing continuously to unity, according to the particular choice of the engineered reservoir. If compared to the exponential-like relaxation obtained from the original Jaynes-Cummings model for Lorentzian-type spectral density functions, decoherence is considerably hindered. [Preview Abstract] |
Wednesday, March 23, 2011 3:06PM - 3:18PM |
T27.00004: Decoherence of high-$\ell $ Rydberg wave packets by collisions and electrical noise Brendan Wyker, S. Ye, T. McKinney, F.B. Dunning, S. Yoshida, C.O. Reinhold, J. Burgd{\"o}rfer Quantum revivals in very-high-$n$ ($n\sim $300) high-$\ell $ Rydberg wave packets generated from parent $n$p states are used to examine decoherence induced by collisions and by the application of ``colored'' noise from a random pulse generator. In the absence of external perturbations, the high-$\ell $ wave packets maintain their coherence for periods $\sim $ 1 $\mu $s, i.e., for many hundreds of orbits. However, their coherence can be destroyed on sub-microsecond timescales by the presence of even small amounts of electrical noise at a rate that depends markedly on the spectral characteristics of the noise. In contrast, measurements over similar timescales with CO$_{2}$ target gas at densities of $\sim $ 10$^{11}$ cm$^{-3}$ provide no evidence of collisional dephasing. [Preview Abstract] |
Wednesday, March 23, 2011 3:18PM - 3:30PM |
T27.00005: Non-Markovian trajectory approach of three-level quantum systems Ting Yu, Jun Jing The non-Markovian dynamics of a three-level quantum system coupled to a bosonic environment is a difficult problem due to the lack of an exact dynamic equation such as a master equation. We present for the first time an exact quantum trajectory approach to a dissipative three-level model. We have established a convolutionless stochastic Schr\"{o}dinger equation called time-local quantum state diffusion (QSD) equation without any approximations, in particular, without Markov approximation. Our exact time-local QSD equation opens a new avenue for exploring quantum dynamics for a higher dimensional quantum system coupled to a non-Markovian environment. [Preview Abstract] |
Wednesday, March 23, 2011 3:30PM - 3:42PM |
T27.00006: Manipulating decoherence of a single solid-state spin by quantum control of its spin bath environment Gijs de Lange, Toeno van der Sar, Machiel Blok, Zhihui Wang, Viatcheslav Dobrovitski, Ronald Hanson The coherence of solid-state spins is limited by uncontrolled interactions with their spin environment. High-fidelity single-spin control can be used to prolong the coherence by dynamically decoupling the spin from the environment [see De Lange et al., Science 330, 60 (2010)]. Here, we demonstrate a new approach towards decoherence control based on coherent manipulation of the spin bath environment itself. Our system consists of a single NV center spin in diamond, surrounded by a bath of electronic spins belonging to nitrogen impurities. By driving the bath spins resonantly and using the NV spin as a sensor, we are able to detect all transitions of the bath spins and demonstrate independent quantum control of each of them. This newly gained control opens the door to a number of exciting experiments such as measurement of the spin bath dynamics, manipulation of the spin bath correlation time, decoherence editing, and protection of NV spin coherence by suppressing the dynamics in its spin environment. In this talk we will present our latest results towards these goals. [Preview Abstract] |
Wednesday, March 23, 2011 3:42PM - 3:54PM |
T27.00007: Optical cooling of a 122 kHz mechanical resonator Evan Jeffrey, Petro Sonin, Dustin Kleckner, Brian Pepper, Dirk Bouwmeester We demonstrate radiation pressure cooling of a 122-kHz, 60-ng mechanical resonator in an optical cavity. We use a dilution refrigerator to achieve a low base temperature ($<$ 100 mK). The resonators consist of high reflectivity mirrors suspended on a stressed silicon nitride cross resonator. Due to their low frequency, high Q ($>$40000) and high finesse ($>$ 10000) these devices are excellent candidates for demonstrating quantum behavior of macroscopic systems, with the possibility of achieving quantum superpositions, entanglement with external degrees of freedom, and studying exotic decoherence mechanisms. [Preview Abstract] |
Wednesday, March 23, 2011 3:54PM - 4:06PM |
T27.00008: Long-lived Fermionic Fock-space coherence in quantum dots Eduardo Vaz, Jordan Kyriakidis The Fock-space coherence between quantum states with different particle numbers naturally arising in an open quantum system, qualitatively differs from the more common Hilbert-space coherence between states with the same particle number. For a quantum dot with multiple channels available for transport, we have found specific energy and coupling regimes where a long-lived resonance in the fermionic Fock-space coherence of the system is realized, even where no resonances are found either in the population probabilities or Hilbert-space coherence of the system. We discuss how this resonance in the Fock-space coherence remains robust even in the presence of both boson-mediated relaxation and transport through the quantum dot, as well as its physical origin. [Preview Abstract] |
Wednesday, March 23, 2011 4:06PM - 4:18PM |
T27.00009: Excitation induced dephasing from indistinguishability Peter Bryant In a variety of experiments, Rabi oscillations suffer dephasing that often and perhaps generally depends on the Rabi frequency. This unexpected result has been called excitation induced dephasing. Explanations specific to experiments have been suggested, but here we describe a new approach to the treatment of decoherence and open systems, that allows one to address the indistinguishability of quantum systems. When physical systems are indistinguishable, excitation induced dephasing is a general phenomenon, and a preliminary model shows good agreement with a wide range of experimental results. [Preview Abstract] |
Wednesday, March 23, 2011 4:18PM - 4:30PM |
T27.00010: Noise in a Josephson junction qubit due to two-level-systems coupled to a quantum EM field Victor Galitski, So Takei We theoretically study loss of the dielectric film located within an LC resonator circuit due to two-level defects (TLDs). We present a fully quantum mechanical treatment of the full system in which the TLDs couple to a quantized harmonic oscillator, which models the resonator, and to quantized bosonic fields that describe the feedline used to pump and probe the resonator. We focus on the forward transmission as a function of the microwave pump frequency, and investigate how the fluctuating defects affect the noise spectrum of the transmitted voltage signal. Our quantum mechanical treatment makes connections to experiments conducted down to energies where a single photon is stored in the resonator. [Preview Abstract] |
Wednesday, March 23, 2011 4:30PM - 4:42PM |
T27.00011: Suppression of Decoherence and Disentanglement in Qubits via the Exchange Interaction Amrit De, Dong Zhou, Robert Joynt We show that the decoherence and disentanglement for a pair of interacting qubits can be suppressed by the exchange interaction in the presence of one or more uncorrelated random telegraphic noise sources. The suppression of the dissipative dynamics is more apparent for the maximally entangled Bell states, particularly if the noise is non-Markovian. Hence, the entangled singlet-triplet superposition state of two qubits can be protected by the interaction, while for the triplet-triplet state, it is less effective. This makes the former more suitable for encoding quantum information. Our calculations are done using a recently developed quasi Hamiltonian formalism that is suitable for describing non-unitary temporal dynamics in an open quantum system subjected to classical stochastic noise processes. Exact and approximate solutions are obtained for a number of cases. [Preview Abstract] |
Wednesday, March 23, 2011 4:42PM - 4:54PM |
T27.00012: Concatenated Stabilizer Dynamical Decoupling Gerardo Paz Silva, Daniel Lidar We show how to integrate concatenated dynamical decoupling (CDD) techniques with quantum error correction (QEC) codes: the two main strategies to protect quantum information from the decoherence induced by unwanted interaction with the environment. It has been shown that CDD can be used as a lower level protection layer against decoherence and improves the effective error rate of a physical gate, provided one assumes certain locality conditions (local bath assumption) [Ng, Lidar, Preskill, arXiv:0911.3202]. The typical CDD protocol uses pulses from a group of non-commuting operators to decouple to arbitrary order, in the sense of Magnus expansion, the state one wants to protect from the environment. Here, in the same spirit as [Lidar, Phys. Rev. Lett. 100, 160506 (2008)], we show how to decouple a state encoded in some stabilizer QEC code to arbitrary order by applying pulses from the stabilizer group of the QEC code. We demonstrate the technique for concatenated and non-concatenated QEC codes and show that, in contrast to the CDD case, (i) one can omit the local bath assumption, and (ii) has the freedom of simultaneously introducing evolution for the protected state. [Preview Abstract] |
Wednesday, March 23, 2011 4:54PM - 5:06PM |
T27.00013: Decoherence and entanglement of a pair of coupled qubits Mohammad Sahrapour, Nancy Makri We analyze the quantum dynamics of a pair of qubits coupled via Ising-type coupling under the influence of a common dissipative bath. We present results of simulations for a range of system biases and spin-spin couplings at two values of bath temperature ($\beta =1,5)$. We also discuss the dynamics of entanglement when starting with fully entangled states and find that for some values of the system parameters, steady-state entanglement is observed. These simulations are carried out via the iterative path integral methodology developed earlier in our group which delivers efficient, numerically exact long time quantum dynamics. [Preview Abstract] |
Wednesday, March 23, 2011 5:06PM - 5:18PM |
T27.00014: Entanglement and Coherence: Differences and Similarities Robert O'Connell Entanglement and coherence both decay due to environmental (heat bath) effects. Apart from the well-known fact that decoherence occurs exponentially and disentanglement occurs with a sudden death, there are many other differences. Here, we concentrate on the effects of temperature T along in the absence of dissipation. Thus, whereas the effect of T on decoherence increases exponentially with time [1], the effect of T on disentanglement is constant for all times [2], reflecting a fundamental difference between the two phenomena. Also, the possibility of disentanglement at a particular T increases with decreasing initial entanglement. Supported in part by NSF under Grant No. ECCS-0757204. \\[4pt] [1] G. W. Ford and R. F. O'Connell, Phys. Lett. A 286, 87 (2001). \\[0pt] [2] G. W. Ford and R. F. O'Connell, Phys. Scr. 82, 038112 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 5:18PM - 5:30PM |
T27.00015: Quantum signatures of chaos in quantum tomography Carlos Riofrio, Vaibhav Madhok, Ivan Deutsch We study the connection between quantum chaos and information gain in the time series of a measurement record used for quantum tomography. The record that is obtained as a sequence of expectation values of a Hermitian operator evolving under repeated application of the Floquet operator of the quantum kicked top on a large ensemble of identical systems. We find that, in the limit of vanishing noise, the fidelities of reconstruction are independent of the underlying chaos of the Floquet map. In the presence of noise, however, the fidelities on an average increase with the chaoticity of the map. Moreover, the number of time steps required to achieve a given fidelity decreases with the increase in the chaoticity, suggesting a connection between the rate of information gain and classical Lyapunov exponents. [Preview Abstract] |
Session T28: Graphene: Optical and Transport Properties
Sponsoring Units: DCMPChair: Tony Heinz, Columbia University
Room: C156
Wednesday, March 23, 2011 2:30PM - 2:42PM |
T28.00001: Selection rule for Raman spectroscopy at graphene edge Ken-ichi Sasaki, Katsunori Wakabayashi, Toshiaki Enoki The optical matrix element may depend on position in graphene since an electronic wave function is position dependent. In particular, the matrix element near the edges of graphene can differ greatly from that in the bulk. We are pursuing our studies on this point in relation to Raman spectroscopy. We found a selection rule for the G band near the edges of graphene: the intensity is enhanced when the polarization of incident laser is parallel (perpendicular) to the armchair (zigzag) edge [1]. This asymmetry between the armchair and zigzag edges is useful in identifying the orientation of the edge of graphene. Some application of the selection rule is mentioned. We have extended our study to the polarization dependences of the D and 2D (G') bands [2]. The D and 2D bands have different selection rules at bulk and edge. At bulk, the 2D band intensity is maximum when the polarization of the scattered light is parallel to that of incident light, whereas the D band intensity does not have a polarization dependence. At edge, the 2D and D bands exhibit a selection rule similar to that of the G band.\\[4pt] [1] Sasaki et al., J. Phys. Soc. Jpn. 79, 044603 (2010).\\[0pt] [2] Sasaki et al., Phys. Rev. B 82, 205407 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 2:42PM - 2:54PM |
T28.00002: Photoconductive response study on a dual-gated bilayer graphene M.-H. Kim, J. Yan, G.S. Jenkins, A.B. Sushkov, D.C. Schmadel, M.S. Fuhrer, J. Melngailis, H.D. Drew A continuously tunable bandgap as high as 100 meV is produced in a gated bilayer graphene (BLG) by applying an electric field perpendicular to the layers (J. Yan, Nano Lett. 2010). The bandgap and the Fermi energy of BLG are tuned by top and bottom gate potentials. We measure the infrared photoconductive response from the dual-gated BLG from far infrared 30 cm$^{-1}$ to mid-infrared 5000 cm$^{-1}$ by broadband spectroscopy and with a CO$_2$ laser near 10.6 $\mu$m. We report the photoresponse and the measured band gap as a function of an applied electric field perpendicular to the BLG layers. This work is supported by IARPA grant \#W911NF1010443. [Preview Abstract] |
Wednesday, March 23, 2011 2:54PM - 3:06PM |
T28.00003: Ultrafast carrier dynamics in pristine and FeCl$_{3}$-intercalated bilayer graphene Xingquan Zou, Da Zhan, Xiaofeng Fan, Dongwook Lee, Saritha K. Nair, Li Sun, Zhenhua Ni, Zhiqiang Luo, Lei Liu, Ting Yu, Zexiang Shen, Elbert E.M. Chia Ultrafast carrier dynamics of pristine bilayer graphene (BLG) and bilayer graphene intercalated with FeCl$_{3}$ (FeCl$_{3}$-G), were studied using time-resolved transient differential reflection ($\Delta R/R)$. Compared to BLG, the FeCl$_{3}$-G data showed an opposite sign of $\Delta R/R$, a slower rise time, and a single (instead of double) exponential relaxation. We attribute these differences in dynamics to the down-shifting of the Fermi level in FeCl$_{3}$-G, as well as the formation of numerous horizontal bands arising from the $d$-orbitals of Fe. Our work shows that intercalation can dramatically change the electronic structure of graphene, and its associated carrier dynamics. Appl. Phys. Lett. \textbf{97, 141910 }(2010) [Preview Abstract] |
Wednesday, March 23, 2011 3:06PM - 3:18PM |
T28.00004: Microwave microscopy of graphene and graphite Vladimir Talanov, Christopher Del Barga, Lee Wickey, Irakli Kalichava, Edward Gonzales, Eric Shaner, Aaron Gin, Nikolai Kalugin Graphene has emerged as a promising material for high speed nano-electronics applications due to the relatively high carrier mobility that can be achieved. To further investigate electronic transport in graphene and reveal its potential for microwave applications [1,2], a near-field scanning microwave microscope with the probe formed by an electrically open end of a 4 GHz half-lambda parallel-strip transmission line resonator has been employed [3]. We find that the microwave response of mono- and few-layer graphene flakes is determined by the local sheet impedance, which is found to be predominantly active. From fitting a quantitative electrodynamic model (relating the probe resonant frequency shift to 2D conductivity of single- and few-layer graphene) to the experimental data we evaluate graphene sheet resistance as a function of thickness. Near-field scanning microwave microscopy can simultaneously image location, geometry, thickness, and distribution of electrical properties of graphene without a need for device fabrication. [Preview Abstract] |
Wednesday, March 23, 2011 3:18PM - 3:30PM |
T28.00005: Laser Scanning Microscopy of Few-Layer Graphene: Optical Reflectivity Contrast Behnood Ghamsari, Alexander Zhuravel, Daniel Lenski, Michael Fuhrer, Steven Anlage We report laser scanning microscopy (LSM) of few-layer graphene, where a laser beam is raster scanned over the samples and the local reflectivity of the structure is directly measured through a silicon photodiode. The samples are grown by ambient-pressure chemical vapor deposition on copper foils, and transferred to SiO2/Si substrates, and consist of regions of single- and multi-layer graphene (D. R. Lenski, and M. S. Fuhrer, e-print arXiv: 1011.1683). While the local reflectivity of the structure depends on the thickness of the graphene layer, the LSM data is used to construct a two-dimensional reflectivity image of the sample which, in turn, enables identifying the local distribution of different graphene multilayers and local microscopic properties of the graphene sample. [Preview Abstract] |
Wednesday, March 23, 2011 3:30PM - 3:42PM |
T28.00006: Terahertz Imaging and Spectroscopy of Large-Area Single-Layer Graphene Ethan Minot, Joe Tomaino, Andrew Jameson, Joshua Kevek, Michael Paul, Arend van der Zande, Robert Barton, Paul McEuen, Yun-Shik Lee The high electron mobility of graphene points to potential for high-speed electronic and opto-electronic devices operating at terahertz (THz) switching rates. Therefore, there is great interest in probing the electronic properties of large-area graphene at ultrafast time scales. We have demonstrated THz imaging and spectroscopy of a 15x15-mm$^{2}$ single-layer graphene film using broadband THz pulses. The THz images clearly map out the THz carrier dynamics of the graphene-on-Si sample, allowing us to measure sheet conductivity with sub-mm resolution without fabricating electrodes. The THz carrier dynamics are dominated by intraband transitions and the THz-induced electron motion is characterized by a flat spectral response. A theoretical analysis based on the Fresnel coefficients for a metallic thin film shows that the local sheet conductivity varies across the sample from 1.7 -- 2.4 x 10$^{-3}$ Ohm$^{-1}$ (sheet resistance 420 - 590 Ohm/sq). [Preview Abstract] |
Wednesday, March 23, 2011 3:42PM - 3:54PM |
T28.00007: Aromatic molecule-like fluorescence from Graphene Oxide Charudatta Galande, Aditya Mohite, Anton Naumov, Wei Gao, Lijie Ci, Anakha Ajayan, Hui Gao, Anchal Srivastava, R. Bruce Weisman, Pulickel M. Ajayan Graphene Oxide (GO) is a functionalized derivative of graphene, obtained by chemical exfoliation and chemical oxidation of graphite. Recent NMR studies on GO have revealed presence of hydroxyl, epoxy, carbonyl, carboxyl and lactols. Although there have been several studies on electronic and optical properties of GO, the role of functional groups in determining the electronic density of states is still unclear. Here we report pH dependent fluorescence and excitation spectra of GO, with spectroscopic signatures indicating the presence of molecule-like fluorophores in GO. In acidic medium, a single, broad emission peak is observed at ca. 660nm. In contrast, relatively sharp emission at lower wavelengths (480nm-515nm) appears in a short pH range between 7.6 and 8.0, while the broad peak is completely quenched in basic conditions. The fluorescence and excitation spectra have pH-dependence strikingly similar to several aromatic carboxylic acids. The observed spectral features are proposed to arise from quasi-molecular fluorophores, similar to polycyclic aromatic compounds that are formed by the electronic coupling of carboxylic acid groups with nearby carbon atoms of the graphene. [Preview Abstract] |
Wednesday, March 23, 2011 3:54PM - 4:06PM |
T28.00008: Broadband spatial self-phase modulation of few-layer graphene sheet in solution Rui Wu, Yingli Zhang, Fei Bian, Shichao Yan, Rui Wang, Wenlong Wang, Xuedong Bai, Xinghua Lu, Jimin Zhao Spatial self-phase modulation (SPM) was found for a suspension of few-layer graphene flakes. Multiple concentric conical diffraction rings was observed as a 532nm cw laser beam passes through the nearly transparent suspension, of which self-focusing occurred. The dependence of ring numbers and ring diameters on the laser intensity was recorded, from which we obtained the third order optical nonlinearity $n_{2 }$of the sample. In our case $n_{2}$=10$^{-9}$ m$^{2}$/W, which is the one of the largest among the reported carbon materials including carbon nanotubes and C$_{60}$ samples. We also found that the intensity threshold for observing the diffraction rings is as low as about 0.6W/cm$^{2}$, which is the smallest compared with most of the reported sample having spatial SPM, including nematic liquid crystals. Furthermore we found that both 267nm and 800nm ultrashort laser pulses can also easily generate spatial SPM. This large and broadband optical nonlinearity is a manifestation of the few-layer graphene's conical-shaped band structure, which is true for a relatively large energy scale. [Preview Abstract] |
Wednesday, March 23, 2011 4:06PM - 4:18PM |
T28.00009: Optical transitions between Landau levels: AA-stacked bilayer graphene Yen-Hung Ho, Rong-Bin Chen, Jhao-Ying Wu, Yu-Huang Chiu, Ming-Fa Lin The magneto-absorption spectra are calculated for the AA- stacked bilayer graphene. Two groups of Landau levels with different symmetry in wave function are found to coexist in the low energy region. The optical transitions between the two groups give rise to two kinds of absorption peaks. The wave- function distribution can clearly characterize individual Landau levels, and further determine the optical selection rules and absorption rates. The AA bilayer has quite different spectral features compared to the AB bilayer and monolayer, as a result from the interlayer interactions and stacking symmetry. Only a single absorption survives below certain critical frequency, while other peaks are paired together and sequentially emerged above this critical energy. With a continuous change in field strength, the excitation channels are switched, associated with the abrupt changes in their frequency. [Preview Abstract] |
Wednesday, March 23, 2011 4:18PM - 4:30PM |
T28.00010: High temperature Graphene-based Quantum Hall Effect Infrared photodetector Nikolai G. Kalugin, Lei Jing, Wenzhong Bao, Lee Wickey, Christopher Del Barga, Mekan Ovezmyradov, Eric A. Shaner, Chun Ning Lau We demonstrate successful operation of quantum Hall effect (QHE) graphene-based detectors at 70K, a temperature achievable using simple pumped liquid Nitrogen cryostats, and in magnetic field of 7.35T. Because of graphene's unique band structure, the first few Landau levels are well-separated energetically, thus allowing observation and manipulation of QHE at unprecedentedly high temperatures [1]. Our results overcome the obstacle of low operating temperature of traditional semiconductor systems-based QHE photodetectors [2], and open the door for wide arrays of applications. \\[4pt] [1] K.S. Novoselov \textit{et al}. \textit{Science} \textbf{315}, 1379 (2007).\\[0pt] [2] N. G. Kalugin \textit{et.al.} \textit{Phys.Rev.B} 66, 085308 (2002). [Preview Abstract] |
Wednesday, March 23, 2011 4:30PM - 4:42PM |
T28.00011: Phonon scattering in intrinsic graphene using tight-binding Bloch waves Nishant Sule, Irena Knezevic The overall interest in graphene as a material for devices has led to tremendous advances in the understanding of transport in graphene. However, there are still questions about the intrinsic limit to electron mobility. Recent experiments have demonstrated mobility greater than $10^{7}$ cm$^{2}/$Vs at temperatures close to $50$ K, exceeding previous theoretical predictions of the limit for intrinsic mobility. Here, we present a simple model for phonon scattering rates in intrinsic graphene using tight-binding Bloch wave functions for electrons. The tight binding approximation produces an accurate band structure near the Dirac points, as opposed to the nearly free electron model; thus, it is reasonable to assume that the electron wave functions are localized near the atomic centers. These tight-binding Bloch wave functions are calculated by linear combination of the carbon $p_{z}$ orbitals. We show that the scattering matrix is anisotropic and the small overlap of the Bloch functions results in scattering rates that are lower in comparison to those calculated by assuming plane-wave wave functions. Electron mobility calculated in the relaxation time approximation is compared for scattering rates with Bloch functions and as well as plane waves. [Preview Abstract] |
Wednesday, March 23, 2011 4:42PM - 4:54PM |
T28.00012: Chirality-dependent phonon-limited resistivity in graphenes Hongki Min, Euyheon Hwang, Sankar Das Sarma We develop a theory for the temperature and density dependence of phonon-limited resistivity $\rho(T)$ in bilayer and multilayer graphene, and compare with the corresponding monolayer result. For the unscreened case, we find $\rho \approx C T$ with $C \propto v_{\rm F}^{-2}$ in the high-temperature limit, and $\rho \approx A T^4$ with $A \propto v_{\rm F}^{-2} k_{\rm F}^{-3}$ in the low-temperature Bloch-Gr\"uneisen limit, where $v_{\rm F}$ and $k_{\rm F}$ are Fermi velocity and Fermi wavevector, respectively. If screening effects are taken into account, $\rho \approx C T$ in the high-temperature limit with a renormalized $C$ which is a function of the screening length, and $\rho \approx A T^6$ in the low-temperature limit with $A \propto k_{\rm F}^{-5}$ but independent of $v_{\rm F}$. These relations hold in general with $v_{\rm F}$ and a chiral factor in $C$ determined by the specific chiral band structure for a given density.\\[4pt] Reference: Hongki Min, E. H. Hwang, and S. Das Sarma, arXiv:1011.0741 (unpublished). [Preview Abstract] |
Wednesday, March 23, 2011 4:54PM - 5:06PM |
T28.00013: Boundary properties between monolayer and bilayer graphene and valley filter Takeshi Nakanishi, Mikito Koshino, Tsuneya Ando Graphene consists of a two-dimensional hexagonal crystal of carbon atoms, in which electron dynamics is governed by the Dirac equation. The purpose of this paper is to study the boundary between monolayer and bilayer graphenes and show a valley polarization in transmission probability through the boundary [1]. We consider the boundary of monolayer and bilayer graphene, in which lower layer in bilayer graphene is continuously connected to the monolayer graphene and upper layer is terminated along a straight edge having zigzag or armchair structures. Boundary conditions between monolayer and bilayer graphene are derived in an effective-mass scheme. The transmission probability vanishes at the Dirac point and increases roughly in proportional to the electron density. The transmission probability varies strongly as a function of the incident angle and its maximum appears at an angle deviating from the vertical direction. This asymmetry is opposite between the K and K' points, showing that strong valley polarization can be induced across the interface of monolayer and bilayer graphenes.\\[4pt] [1] T. Nakanishi et al., PRB 82 (2010) 125428. [Preview Abstract] |
Wednesday, March 23, 2011 5:06PM - 5:18PM |
T28.00014: Anomalous tunneling of dressed Dirac electrons through potential barrier Andrii Iurov, Oleksiy Roslyak, Godfrey Gumbs It has been shown that when a potential barrier is placed on a layer of graphene, electrons incident on the barrier head-on can be transmitted without any reflection, regardless of how high the barrier is made to become. This anomalous scattering has also been investigated in the case of bilayer graphene. The energy gap between the valence and conduction bands for bilayer graphene leads to perfect reflection for head-on collisions for all barrier heights. We report on results for reflection and transmission coefficients for dressed Dirac electrons when circularly polarized light is applied to graphene and an energy gap in the energy bands is opened up. Since this gap depends on the frequency and intensity, we investigate how the electron and hole scattering off a fixed barrier is modified by varying the energy gap produced by light. We also present results for the transmission for the perpendicular incidentce. Both numerical and analytical results are obtained. [Preview Abstract] |
Wednesday, March 23, 2011 5:18PM - 5:30PM |
T28.00015: In-gap transport in random-gap grapheme: metallic and insulating phases Vagharsh Mkhitaryan, Mikhail Raikh 1D- like counter-propagating states at a gap center of graphene with random gap constitute two chiral networks. In the absence of intervalley scattering, transport over each network is either metallic or insulating, depending on the gap randomness. We demonstrate that properties of both phases as well as transitions between them are accurately captured within a simple real-space renormalization group approach. The most striking feature of this network transport is that it can be metallic even when the neighboring plaquettes are weakly coupled. We show that randomness in local gap signs reflected in randomness in signs of local transmission coefficients, gives rise to resonant transmission of the RG superblock. Delocalization occurs by proliferation of these resonances to larger scales. As the disorder exceeds a critical value, the RG flow towards insulator switches to a flow towards metallic fixed point. Evolution of the conductance distribution to metallic fixed point is synchronized with evolution of transmission coefficient signs, so that delocalization is accompanied with sign percolation. [Preview Abstract] |
Session T29: Focus Session: Superconducting Qubits - Coherence and Materials I
Sponsoring Units: GQIChair: Andrew Houck, Princeton University
Room: C148
Wednesday, March 23, 2011 2:30PM - 2:42PM |
T29.00001: Feedback suppression of the low-frequency noise in qubits by the low-frequency quantum measurements Qiang Deng, Dmitri Averin The problem of the low-frequency noise has dominated the development of the solid-state qubits. Up to now, the main approach to solve this was to employ the qubit structures with the basis states having the same values of the main qubit coordinate (e.g., electric charge or magnetic flux) that are, as a result, not decohered by the noise. The goal of this work is to suggest an alternative approach based on direct suppression of the low-frequency noise in a qubit through the measurement/feedback loop. Continuous quantum measurement required for this loop should also be ``low-frequency'' so that it does not affect the quantum dynamics of the qubit. We calculate the minimal noise induced in the qubit by such a feedback loop when the measurement is the quantum-limited. [Preview Abstract] |
Wednesday, March 23, 2011 2:42PM - 2:54PM |
T29.00002: Tunable quantum beam splitters for quantum manipulation of a hybrid tripartite qubit system G.Z. Sun, J. Chen, P.H. Wu, X.D. Wen, Y. Yu, B. Mao, S.Y. Han We demonstrated coherent control of quantum states in a tripartite system consisting of a superconducting qubit and two microscopic two-level states (TLS). An initially prepared qubit state was swept through qubit-TLS avoided crossings in the energy-level spectrum. The avoided crossings act as tunable quantum beam splitters of wave function. In an analogy to optics, the transmission coefficient of the beam splitters can be varied from zero to unity or any value in between by adjusting the rate of energy sweep. When performed within the decoherence time, consecutive crossings through the beam splitters lead to coherent quantum oscillations between the quantum states of the tripartite qubit-TLS system. This Landau-Zener-Stueckelberg interference controlled by the sweeping rate provides an alternative means to manipulate multiple qubits and demonstrates macroscopic quantum coherence. [Preview Abstract] |
Wednesday, March 23, 2011 2:54PM - 3:06PM |
T29.00003: Surface and Interface Defects in Linear and Nonlinear Superconducting Resonators Steven Weber, Kater Murch, Allison Dove, Gustaf Olson, Zack Yoscovits, R. Vijay, Eli Levenson-Falk, James Eckstein, Irfan Siddiqi We report on progress to identify and mitigate noise mechanisms in both linear superconducting resonators and devices embedded with Josephson junctions. Defects, either microscopic fluctuators or remnant residue layers associated with nanofabrication, can exist on metal surfaces, at the metal-dielectric interface, within the dielectric, or within the Josephson junctions themselves. We have investigated the quality factor and phase noise of lumped element and distributed element resonators at low temperature and photon number- the operating regime of superconducting qubits. In particular, we compare the performance of poly-crystalline and epitaxial films, silicon and sapphire substrates, and weak link and tunnel type Josephson junctions. [Preview Abstract] |
Wednesday, March 23, 2011 3:06PM - 3:18PM |
T29.00004: Quantum model for superconducting resonator loss via a two-level system Mishkatul Bhattacharya, K. Osborn, A. Mizel Clarifying the mechanisms of dissipation in superconducting resonators is crucial for advancing superconducting quantum computation. The models currently employed to study dielectric loss due to two level charge fluctuators have been based largely on a classical treatment of the problem. In contrast, we carry out a quantum mechanical investigation using a dissipative Jaynes-Cummings model in which the resonator is coupled to a two-level system that is in turn coupled to a bath. We present an analysis of the dynamics of energy decay in the system, comparing its predictions to those of well-known classical models, which agree with our results in the limit of high oscillator excitation. [Preview Abstract] |
Wednesday, March 23, 2011 3:18PM - 3:30PM |
T29.00005: Possible interactions between two-level system defects in SiNx films Sergiy Gladchenko, Moe Khalil, C.J. Lobb, F.C. Wellstood, Kevin D. Osborn Low-temperature properties of PECVD SiNx dielectric films are measured within the capacitor of superconducting LC resonators. Experiments are made at temperatures from 30 to 300 mK, and at storage energies from 1 to 10$^{6}$ photons in a resonant cavity. While the power and temperature dependence of the loss agrees with two-level system (TLS) theory above 60 mK, below this temperature we observe significant deviations. In this regime we observe a reduction in loss upon lowering dielectric temperature, in direct contrast with the independent TLS model of defects within our film. This new phenomena may indicate interactions between two-level systems. We can also spectroscopically resolve the loss from dominant defects in our capacitors, which have a volume of $\sim $2000 $\mu $m$^{3}$. [Preview Abstract] |
Wednesday, March 23, 2011 3:30PM - 3:42PM |
T29.00006: A study of glassy behavior in amorphous dielectrics using GHz frequency superconducting resonators Moe Khalil, M.J.A. Stoutimore, Aaron Holder, Charles Musgrave, C.J. Lobb, F.C. Wellstood, K.D. Osborn It has been shown that the dielectric constant of certain glassy materials can be changed with a dc electric field bias. Here we extend those studies to higher frequencies where both the real and imaginary part of the dielectric constant can be studied. We have designed a dc electric-field tunable LC resonator built from superconducting thin-film aluminum to test this effect in SiN$_{x}$ at GHz frequencies. We will report progress on measuring these devices down to single photon storage energies at temperatures of approximately 30 mK, where the dynamics are dominated by the tunneling of two-level defects. [Preview Abstract] |
Wednesday, March 23, 2011 3:42PM - 3:54PM |
T29.00007: Microwave loss of novel epitaxial superconductor-insulator-superconductor (SIS) trilayers U. Patel, K.H. Cho, L. Maurer, S. Sendelbach, D. Hover, C.B. Eom, R. McDermott The performance of superconducting phase qubits is currently limited by spurious coupling of the qubit to two-level state (TLS) defects in the amorphous dielectric materials of the circuits. Thus, it is highly desirable to develop defect free epitaxial dielectric materials for improved junction barriers and capacitor dielectrics. We have characterized the dielectric loss of several candidate SIS trilayers including Re/MgO/Al and Re/LaAlO$_{3}$/Al grown on c-sapphire substrates. We describe our multiplexed microstrip resonator device layout and present data on the intrinsic quality factors of the MBE-grown dielectrics. [Preview Abstract] |
Wednesday, March 23, 2011 3:54PM - 4:06PM |
T29.00008: Growth of epitaxial superconductor/dielectric heterostructures using a sputtering PLD hybrid system with in-situ RHEED Kwang Hwan Cho, Jacob Podkaminer, Chad Folkman, Chang-Beom Eom One of limiting factors in superconducting qubits is decoherence caused by microscopic defects in dielectric layer such as nanocrystalline regions and grain boundaries in a shunted capacitor. We have grown epitaxial Re thin films on a c-plane sapphire substrate using RF magnetron sputtering, then transferred \textit{ex-situ} to a pulsed laser deposition (PLD) system where dielectrics thin film layer is deposited. One drawback of this fabrication approach is the necessity to expose the sample to air when the sample is transferred to different deposition chambers. In order to avoid these drawbacks, we have employed a hybrid PLD-sputtering deposition that will allow us to grow the oxide dielectric/Re heterostructures in an \textit{in-situ} environment without breaking vacuum. The system is also equipped with a reflection high energy electron diffraction (RHEED) which will allow us to perform \textit{in-situ} characterization of the structure and growth dynamics. We will discuss our strategy of epitaxial growth of various single crystal dielectrics on superconducting thin films in this system and their structural and electrical properties of the heterostructures [Preview Abstract] |
Wednesday, March 23, 2011 4:06PM - 4:18PM |
T29.00009: Barrier defect analysis using Josephson junction resonators M.J.A. Stoutimore, Bahman Sarabi, Moe Khalil, C.J. Lobb, K.D. Osborn We have designed Josephson junction (JJ) resonators by adding an Al/AlO$_{x}$/Al Josephson junction in parallel with a coplanar sapphire capacitor and an inductor so that the total loss will be dominated by the junction barrier. JJ resonators couple to individual defects in the junction barrier, causing splittings in the spectroscopy of the resonator when it is excited near single-photon energies, similar to phase and other qubits. Measurements are performed in a dilution refrigerator at 30mK with a drive frequency of approximately 7GHz. By applying a dc flux bias, we can tune the resonance frequency by as much as 1GHz. Analysis of the frequency of splittings as a function of junction area and barrier growth process provides a method for determining the source of the defects. We will use these devices to study~amorphous aluminum oxide barriers and~will report~our progress towards studying novel barrier dielectrics. [Preview Abstract] |
Wednesday, March 23, 2011 4:18PM - 4:30PM |
T29.00010: Losses in Josephson junction resonators Martin Weides, Jiansong Gao, Jeffrey Kline, Michael Vissers, David Wisbey, David Pappas Josephson junctions for superconducting circuits such as SQUIDs and qubits are conventionally based on Al-AlO$_x$-Al multilayer technology, which was shown to have a low quality factor and two-level-fluctuators in the dielectric AlO$_x$ as limiting decoherence source. By replacing the amorphous Al-rich tunnel oxide with nearly stoichiometric Al$_2$O$_3$ we aim to increase the qubit coherence times by reducing the number of dangling bonds in the Josephson tunnel junction. In this talk a test platform for loss determination in high-Q tunnel oxides based on junction resonators will be presented. We will show alternative tunnel junctions based on high temperature grown tunnel oxides. [Preview Abstract] |
Wednesday, March 23, 2011 4:30PM - 4:42PM |
T29.00011: Growth and Properties of Epitaxial Dielectrics/Superconducting Thin Film Heterostructures Chang-Beom Eom, Kwang-Hwan Cho, Jacob Podkaminer, Chad Folkman Our objective is the growth of epitaxial dielectrics on crystalline superconducting underlayers to improve the performance of superconducting Qbits. A major challenge is heteroepitaxial growth of single crystal dielectric layers with high crystalline quality and atomically sharp interfaces between the dielectric and superconducting electrodes. First, we have grown high quality epitaxial rhenium (Re) thin films on c-plane sapphire substrates by DC magnetron sputtering. The full width at half maximum (FWHM) of Re 0002 rocking curve is less than 0.5 degrees. The RMS surface roughness determined by AFM is less than 1 nm. We have also grown epitaxially various dielectric thin films on top of the single crystal Re bottom electrode by pulsed laser deposition with in situ high pressure reflection high energy electron diffraction (RHEED). In this talk, we will discuss our strategy of epitaxial growth of various single crystal dielectrics on superconducting thin films and their structural and electrical properties of the heterostructures. [Preview Abstract] |
Wednesday, March 23, 2011 4:42PM - 4:54PM |
T29.00012: Microwave Response of Superconducting Resonant Circuits based on 3D Aluminum Nanobridge Josephson Junctions Eli Levenson-Falk, R. Vijay, Kater Murch, Irfan Siddiqi Metallic weak links are attractive candidates for low loss superconducting circuits as they offer a route to realize Josephson junctions without the need for an amorphous tunnel barrier-a potential source of both low and high frequency noise. We discuss microwave measurements of high quality factor resonators incorporating both single nanobridges and nanobridge-based SQUIDs. Our results indicate low loss and strong nonlinearity, suggesting the future utility of these devices in qubit and amplifier circuits. Our data are in quantitative agreement with numerically computed nanobridge current-phase relations and dc transport measurements. We show preliminary results on nanobridge-based qubits and parametric amplifiers. [Preview Abstract] |
Wednesday, March 23, 2011 4:54PM - 5:06PM |
T29.00013: Josephson junctions formed from superconducting nanowires B. Xiao, H.Y. Chen, I. Nsanzineza, C. Song, B.L.T. Plourde We are investigating the possibility of forming Josephson junctions from thin-film superconducting nanowires. The Josephson coupling through such a constriction can provide the necessary nonlinearity, for example, for forming a qubit, while avoiding the influence of defects in the amorphous tunnel barriers used in conventional Josephson junctions that can contribute to qubit decoherence. We have developed a fabrication process based on high-resolution electron-beam lithography with a negative-tone resist combined with ion-beam etching to pattern nanowires from 10 nm-thick, sputter-deposited, amorphous MoGe thin films. We have studied nanowires with widths between 20 - 100 nm and lengths between 50 - 200 nm. A Nb wiring layer provides electrical connections to the nanowires. Low-temperature transport measurements allow us to study the nanowire critical current and the influence of microwave irradiation on the current-voltage characteristics. [Preview Abstract] |
Wednesday, March 23, 2011 5:06PM - 5:18PM |
T29.00014: Via fabrication process for epitaxial superconducting qubits Jeffrey Kline, Fabio da Silva, Michael Vissers, David Wisbey, Martin Weides, David Pappas Reducing the density of spurious two level systems (TLS) in the dielectric layers of superconducting qubits has been shown to improve performance. We aim to reduce TLS density in the Josephson junction tunnel barrier through the use of epitaxial materials. The investigation of some new material systems using a trilayer process wherein the base electrode, tunnel barrier, and top electrode are grown and subsequently patterned is problematic due to sidewall damage during the mesa etch. We apply the via fabrication process wherein the base electrode and wiring insulator layers are grown and patterned prior to tunnel barrier growth. The via process is compatible with a different set of electrode materials than the trilayer process and allows us to investigate the suitability of these materials for qubit applications. We present room temperature and low temperature data for Re/Al$_{2}$O$_{3}$/Re Josephson junctions fabricated using the via process. [Preview Abstract] |
Wednesday, March 23, 2011 5:18PM - 5:30PM |
T29.00015: Introduction of a DC Bias into a High-Q Superconducting Microwave Cavity Fei Chen, Juliang Li, M.P. Blencowe, A.J. Rimberg, Adam Sirois, Raymond Simmonds The circuit quantum electrodynamics (QED) architecture has been demonstrated to allow study of cavity QED physics in a high-Q on-chip microwave cavity[1]. Here we develop a technique to apply a DC current or voltage bias to nanostructures embedded in the microwave cavity without significantly degrading the Q at high frequencies. Experimental results show good agreement with theoretical predictions. New highly non-linear fully quantum mechanical devices can be developed by embedding Josephson junction devices such as single electron transistors (SETs) in the high-Q microwave cavity. The interplay between the SET and the microwave cavity offers an interesting system for studying nonlinear quantum dynamics and the quantum-to-classical transition. Recent experimental results will be discussed. \\[4pt] [1] A. Wallraff et al, Nature, 431, 162 (2004). [Preview Abstract] |
Session T30: Materials: Synthesis, Growth and Processing (Bulk & Films)
Sponsoring Units: DCMPChair: Yunfeng Shi, Rensselaer Polytechnic Institute
Room: C147/154
Wednesday, March 23, 2011 2:30PM - 2:42PM |
T30.00001: ABSTRACT WITHDRAWN |
Wednesday, March 23, 2011 2:42PM - 2:54PM |
T30.00002: UHV-Compatible Aerosol-Molecule Beam Deposition Source for Organic Electronic Films Levan Tskipuri, Qian Shao, Janice Reutt-Robey The rapidly advancing technologies of flexible electronics and organic photovoltaics have triggered strong interest in new methods for electronic materials deposition from the solution phase. Materials deposition based upon direct aerosol flow offers advantages both for fundamental studies of film growth and analysis and for industrial preparation of high-performance materials. We describe an aerosol deposition source based upon a spray-jet molecular beam technique. The source produces a supersonic molecular beam of solutes (such as C$_{60 }$PCBM ([6,6]-phenyl-C$_{61}$-butyric acid methyl ester) or graphene flakes) in $\sim $10 $\mu $ solvent droplets entrained in a nitrogen carrier gas. The twice-differentially pumped source is then mated to a UHV-STM system, for fundamental studies. The source has been used to generate films of C$_{60 }$PCBM, CNT's, and graphene/PPV composites on SiO$_{2}$ and mica substrates. The spray-jet deposition provides much greater control of the growth kinetics, relative to conventional spin-coating, permitting films to be grown with submonolayer control, as we demonstrate in the growth of C$_{60 }$PCBM films. [Preview Abstract] |
Wednesday, March 23, 2011 2:54PM - 3:06PM |
T30.00003: Preparation and Characterization of $\alpha $-Fe$_{2}$O$_{3}$ Nanoparticles by a Solution-Phase Auto-Combustion Method Marc Doyle, Michael Lattanzi, Brian Kelly, Karl Unruh The effects of the reaction conditions on the structural and magnetic properties of $\alpha $-Fe$_{2}$O$_{3}$ nanoparticles prepared from the combustion products of Fe(III)-nitrate/citric acid/NaOH solutions have been systematically studied in order to gain insights that might be useful in the preparation of more complex oxides from other solution-phase auto-combustion precursors. This work has focused on the effects of the initial solution pH and fuel/oxidant ratio. In particular, precursor powders have been prepared from solutions with pH values between 1 and 12 and with fuel/oxidant ratios between 0.5 and 1.5. Increasing the solution pH and/or the fuel/oxidant ratio lead to significantly less dense and more porous precursor powders due to the greater amount of gaseous reaction products produced under these conditions. X-ray diffraction measurements indicated that under these reaction conditions a higher annealing temperature was required to obtain a phase pure $\alpha $-Fe$_{2}$O$_{3}$ product. [Preview Abstract] |
Wednesday, March 23, 2011 3:06PM - 3:18PM |
T30.00004: Synthesis, Microstructure and Bulk Properties of Complex Nasicon-Type Ceramics Kristina Lipinska, Oliver Hemmers, Julien Romann, Stanislav Sinogeikin, Patricia Kalita, Shekar Balagopal, Anthony Nickens Fast ion-conductors from the NASICON family (Na$_{1+x}$Zr$_{2}$Si$_{x}$P$_{3-x}$O$_{12})$ have been the subject of extensive research due to their use in electrochemical devices such as batteries, fuel cells, thermoelectric generators and chemical sensors A fabrication challenge for these materials is to maintain long term chemical and physical stability in harsh environments We apply a multi-technique approach to show how partial substitutions with tetravalent and pentavalent cations produce NASICONs with specific morphology and modify the primary to secondary crystalline phase ratios. We use in situ synchrotron x-ray diffraction to investigate pressure-induced structural modifications and compressibility. [Preview Abstract] |
Wednesday, March 23, 2011 3:18PM - 3:30PM |
T30.00005: Crystal structure and multiferroic properties of the 0.7(BaTiO$_{3})$ -- 0.3(Bi$_{0.45}$Dy$_{0.55}$FeO$_{3})$ ceramic composite Ricardo Martinez, Nora Ortega, Ashok Kumar, Ratnakar Palai, Ram S. Katiyar Magnetoelectric multiferroics are a novel class of next generation multifunctional materials. Intensive research is being pursued towards the development of new room temperature multiferroics with strong magnetoelectric (ME) coupling. BaTiO$_{3 }$(BT) is well known ferroelectric and Bi$_{0.45}$Dy$_{0.55}$FeO$_{3 }$(BDFO) is multiferroic in nature with weak ferroelectric properties. We have synthesized lead free ceramic composite consisting of 0.7BT--0.3BDFO (BT-BDFO) by conventional solid state. X-ray diffraction and Raman analysis revealed two sets of peaks which belong to BT and BDFO suggesting that the individual phases are retained in the composite ceramic, no additional peaks were observed. The presence of ferromagnetic and ferroelectric hysteresis loops at room temperature (M$_{s}$=14.3 emu/cm$^{3}$, M$_{r}$=1.7 emu/cm$^{3}$, P$_{r}$=3.6 $\mu $C/cm$^{2}$ and E$_{c}$=1.7 kV/cm) showed multiferroic nature of the BT-BDFO ceramic. Although the polarization and magnetization values obtained were lower compared with pure BaTiO$_{3 }$and BDFO respectively, magneto-dielectric measurements reveled a shifting of the dielectric constant peak from 2 MHz to 4 MHz with increase of magnetic field from 0 T to 2 T. [Preview Abstract] |
Wednesday, March 23, 2011 3:30PM - 3:42PM |
T30.00006: Cation Ordering within the Perovskite Block of a Six-layer Ruddlesden-Popper Oxide from Layer-by-layer Growth Lei Yan, H.J. Niu, M.J. Rosseinsky The (AO)(ABO$_{3})_{n}$ Ruddlesden-Popper structure is an archetypal complex oxide consisting of two distinct structural units, an (AO) rock salt layer separating an n-octahedra thick perovskite block. Conventional high-temperature oxide synthesis methods cannot access members with n $>$ 3, but low temperature layer-by-layer thin film methods allow the preparation of materials with thicker perovskite blocks, exploiting high surface mobility and lattice matching with the substrate. This presentation describes the growth of an n = 6 member CaO/(ABO$_{3})_{n}$ (ABO$_{3}$: CaMnO$_{3}$, La$_{0.67}$Ca$_{0.33}$MnO$_{3}$ or Ca$_{0.85}$Sm$_{0.15}$MnO$_{3})$ epitaxial single crystal films on the (001) SrTiO$_{3}$ substrates by pulsed laser deposition with the assistance of a reflection high energy electron diffraction (RHEED). [Preview Abstract] |
Wednesday, March 23, 2011 3:42PM - 3:54PM |
T30.00007: Morphological Transformation of Copper Catalysts within Helically Coiled Carbon Nanofibers Lifeng Dong, Liyan Yu, Qian Zhang With tailoring synthesis parameters, different carbon nanostructures including carbon nanotubes, carbon nanofibers, and graphene, can be synthesized using copper (Cu) as catalysts and acetylene as carbon source. Some remarkable progress has been made in synthesis techniques; however, pioneering breakthroughs have not been made yet in terms of growth mechanism, especially interactions between catalyst particles and acetylene molecules. In this study, the growth mechanism of helically coiled carbon nanofibers and morphological changes of Cu catalysts were investigated using a number of electron microscopy and microanalysis techniques, such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), and electron energy loss spectroscopy (EELS). Following the synthesis, single-crystal Cu particles deformed to small nanoparticles of cuprous oxide (Cu2O) due to internal strain, and Cu2O nanoparticles migrated within carbon nanofibers. [Preview Abstract] |
Wednesday, March 23, 2011 3:54PM - 4:06PM |
T30.00008: ABSTRACT WITHDRAWN |
Wednesday, March 23, 2011 4:06PM - 4:18PM |
T30.00009: Templated Electrodeposition of Highly Porous Nanostructured Materials Han-Chang Yang, Stephanie Lim, Jiabin Liu, Qian Wu, X.M. Cheng The fabrication of nanoporous materials has been of great interest for applications such as biosensors, photonic materials and energy storage. Compared to many other methods, the templated electrodeposition method is low cost, fast, and compatible with large-scale production. In this work, we developed a templated electrochemical deposition technique for fabricating highly ordered and highly porous nanostructured materials. The fabrication involves the following steps: self-assembly of monodispersed polystyrene spheres, electrochemical deposition of the desired materials, and sphere removal by a dissolution process. Deposition of Au and Ni layered metallic nanoporous structures were studied using different electrolytes at appropriate potentials. The pore size of the materials was tuned by using different sizes of template polystyrene spheres ranging from 50nm to 1000nm. Scanning electron microscopy images confirmed the highly ordered 3-dimensional hexagonal closed pack (hcp) structures in the samples. The templated electrochemical deposition technique provides a promising alternative approach to preparing highly porous anode materials for battery applications. [Preview Abstract] |
Wednesday, March 23, 2011 4:18PM - 4:30PM |
T30.00010: Entropic inflation of ideal zeolitic frameworks Vitaliy Kapko, Colby Dawson, Michael Treacy Ideal zeolites can be viewed as flexible networks of rigid, corner-sharing tetrahedra. Recent studies have shown that such systems can exist at a range of densities (termed the ``flexibility window'') without breaking topology or deforming the comprising tetrahedra. They also have shown that densities of real zeolites almost always correspond to the lowest densities within this range. This anomalous behavior is usually attributed to coulombic repulsion between oxygen atoms in framework cavities and channels. In this paper we show that the inflation of ideal zeolites can be driven by entropy. This effect is closely related to displacive phase transitions often observed in zeolites and related materials like quartz, which cannot be explained by potential energy minimization alone. We show that periodicity and high symmetry in ideal zeolites is a result of entropy maximization. An estimation of entropy using a harmonic oscillator model with a realistic force field is given. [Preview Abstract] |
Wednesday, March 23, 2011 4:30PM - 4:42PM |
T30.00011: Topological defects in model nanoporous carbon: population, structural characterizations and adsorption properties Xi Mi, Jeremy Palmer, Jorge Pikunic, Keith Gubbins, Yunfeng Shi Nanoporous carbon materials have drawn substantial research interests because of their unique capabilities to mediate the mass-transport, uptake through adsorptions and catalyze chemical reactions of the guest species. All aforementioned properties depend sensitively on the structural characteristics of the nanoporous carbon, including hybridization state of carbon, presence of functional groups, topology of carbon rings and curvature/connectivity of graphene sheets. Among all these defects, carbon pentagons are of particular interests since they possess 108$^{\circ}$ C-C-C bond angles which are dramatically different from the 120$^{\circ}$ angles typical for carbon hexagons and larger carbon rings. However, the pentagon concentration in the bulk has not been determined. Here we use a realistic nanoporous carbon model proposed recently to systematically synthesize a large number of samples with different concentrations of carbon pentagons using molecular simulation methods. We evaluate the different porous structures in terms of their deviations in s(q) from experiments so as to estimate pentagon concentrations. Moreover, the porous structures are related to the adsorption properties through simulated argon adsorption tests. [Preview Abstract] |
Wednesday, March 23, 2011 4:42PM - 4:54PM |
T30.00012: The stability of capillary climbing flows in porous media opposed by gravity force B. Markicevic, H.K. Navaz The experimental and numerical data reveal that the capillary climb opposed by gravity force starts as a stable flow for low climbing height, which is followed by unstable flow and multiphase pattern for higher climbing heights. For the stable flow, a sharp interface between the liquid and gas phase exists, which transforms later into a flow front of increasing thickness as climbing progresses. The flow front thickness is calculated from the difference between instantaneous climbing height and maximum stable climbing height. We carry out the analysis of capillary climb flow stability using the capillary and the Bond number, as well as using the generalized Bond number. The critical generalized Bond number defines the condition for which the interface transforms to the flow front. For three distinct porous media consisting of glass beads, and having a thin layer of low permeable material at the bottom of the glass beads columns, the values of critical generalized Bond number and the exponents in the power law of flow front thickness as a function of the generalized Bond number are compared. Furthermore, the flow stability analysis is extended to two additional cases in which a low-permeable layer is removed from the column bottom, and the case of pure capillary flow (without gravity). The corresponding critical generalized Bond numbers and the power law exponents are compared. [Preview Abstract] |
Wednesday, March 23, 2011 4:54PM - 5:06PM |
T30.00013: Soaring to New Heights in Natural Materials Sara Bodde, James Kiang, Joanna McKittrick Feathers are the most distinguishable feature of all modern Aves. Flight feathers exemplify several materials science phenomena. The most obvious attribute is the branching or hierarchical structure at macroscale to mesoscale. The primary shaft, or rachis from which secondary features project, of the flight feather is a sandwich structured composite. The thin brittle cortex of the rachis and barbules encloses a relatively thick, low-density medullary core or cellular solid. The cortex of the rachis is constructed as a fiber-reinforcement composite, and structural variations along the length of the feather invoke the comparison to functionally graded materials. We have studied microstructure and mechanical properties of the feather rachis in a piecewise fashion, and we will present results of investigations of the mechanical behavior and failure of the composite and parts thereof in tension, compression, and flexure. [Preview Abstract] |
Wednesday, March 23, 2011 5:06PM - 5:18PM |
T30.00014: Three-dimensional video imaging of drainage and imbibition processes in model porous medium Prerna Sharma, P Aswathi, Anit Sane, Shankar Ghosh, Sabyasachi Bhattacharya We report experimental results where we have performed three dimensional video imaging of the displacement of an oil phase by an aqueous phase and vice versa in a model porous medium. The stability of the oil water interface was studied as a function of their viscosity ratios, the wettability of the porous medium and the variation in the pore size distribution. Our experiments captures the pore scale information of the displacement process and its role in determining the long time structure of the interface. [Preview Abstract] |
Session T31: Novel Structural Chemistry & Materials
Sponsoring Units: DCMPChair: Daniel Finkenstadt, U.S. Naval Academy
Room: C145
Wednesday, March 23, 2011 2:30PM - 2:42PM |
T31.00001: Theoretical studies of the caged hydrocarbon, octahedrane (C$_{12}$H$_{12}$, D$_{3d}$) Steven Richardson, Daniel Finkenstadt, Michael Mehl, Mark Pederson Polyhedral hydrocarbon cages provide an interesting class of molecules for experimental and theoretical study because of their unique shapes. One such molecule is octahedrane (C$_{12}$H$_{12}$, D$_{3d}$) which was first synthesized by Lee {\it et al.} in 1993.\footnote{C-H Lee, S. Liang, T. Haumann, R. Boese, and A. de Meijere, {\it Angew. Chem. Int. Ed. Engl.} {\bf 1993,} 32, 559.} Octahedrane contains two fused three-membered rings and six five-membered rings. Theoretical work by de Meijere {\it et al.}\footnote{A. de Meijere, C-H Lee, M. A. Kuznetsov, D. V. Gusev, S. I. Kozhushkov, A. A. Fokin, and P. R. Schreiner, {\it Chem. Eur. J.} {\bf 2005,} 11, 6175.} using density-functional theory (DFT) has shown that octahedrane is more strained than the structurally related molecules decahedrane (C$_{16}$H$_{16}$, D$_{4d}$), which has two fused four-membered rings and eight five-membered rings, and dodecahedrane (C$_{20}$H$_{20}$, I$_{h}$), which has two fused-membered rings and ten five- membered rings. In this work we report the first DFT calculations of the infrared and Raman vibrational spectra for octahedrane which will be of importance for future efforts in characterizing this unique caged hydrocarbon. [Preview Abstract] |
Wednesday, March 23, 2011 2:42PM - 2:54PM |
T31.00002: Golcondane (C20H24): Theoretical studies of a novel strained, caged hydrocarbon molecule Daniel Finkenstadt, Michael Mehl, Mark Pederson, Steven Richardson In 1993 Mehta and Reddy reported the synthesis of a new C$_{20}$ polyhedrane, which they named golcondane (C$_{20}$H$_{24}$, D$_{2d}$) in honor of the 400$^{th}$ anniversary of the founding of the Indian city of Hyderabad, whose original name was Golconda. Golcondane is a caged, nonacyclic hydrocarbon that has two fused four-membered rings, four fused five-membered rings, and two fused seven-membered rings. Its chemical structure was determined by $^{13}$C NMR spectroscopy and unpublished X-ray crystal diffraction studies. Motivated by our previous success in using density-functional theory (DFT) to compute the structural, electronic, and vibrational properties of other hydrocarbons such as cubane, octanitrocubane, the medium-sized diamondoid molecule cyclohexamantane, as well as the novel class of materials known as {\it sila-}diamondoids, we have used DFT and tight- binding molecular dynamics (TBMD) to compute from first- principles similar properties for golcondane. Our work is especially significant with the lack of other theoretical or experimental studies on this interesting molecule in the published literature. [Preview Abstract] |
Wednesday, March 23, 2011 2:54PM - 3:06PM |
T31.00003: Spectroscopic identification of bond strain and pi interactions in a series of saturated carbon-cage molecules: adamantane, twistane, octahedrane, and cubane Trevor M. Willey, J.R.I. Lee, T. van Buuren, L. Landt, P.R. Schreiner, A.A. Fokin, B.A. Tkachenko, N.A. Fokina, D. Brehmer Novel nanocarbons such as fullerenes, nanotubes, graphene, and nanodiamond reside at the cutting edge of nanoscience and technology. Along with chemical functionalization, geometrical constraints such as extreme curvature or defects in crystallites can modify the electronic properties. This paper presents a fundamental study of how bond strain affects electronic structure in a benchmark series of novel saturated carbon cage compounds. Adamantane, the smallest diamondoid, has carbon atoms commensurate with the diamond lattice. Twistane has the same stoichiometry (C$_{10}$H$_{16})$, but introduces some bond strain into the cage. Octahedrane (C$_{12}$H$_{12})$ and cubane (C$_{8}$H$_{8})$ have increasing strain, culminating in cubane where C-C bonds lie either parallel, or orthogonal to one another. Using gas-phase NEXAFS spectroscopy, we observe the broad C-C $\sigma $* splits into two more narrow and intense resonances with increasing strain. Also, LUMO states associated with tertiary C-H $\sigma $* broaden and shift to lower energy, and are 3X more intense in cubane than octadedrane. The differences are entirely due to the shape rather than stoichiometry, and indicate, we believe, some $\pi $ interaction between parallel C-C bonds in the cubane. [Preview Abstract] |
Wednesday, March 23, 2011 3:06PM - 3:18PM |
T31.00004: Spectroscopy and Structure of Diamondoid-Fullerene Hybrid Molecules at the Single-Molecule Level Jason C. Randel, Georges Ndabashimiye, Hari C. Manoharan Diamondoids---a nanometer-scale form of carbon sharing the \textit{sp}$^{3}$ bonding structure of bulk diamond---are promising new electronic and mechanical device elements and have recently become accessible to experiments. While new fields of research have also sprouted from carbon's \textit{sp}$^{2}$ forms (such as graphene, fullerenes, and carbon nanotubes), materials representing the intersection of \textit{sp}$^{2}$ and \textit{sp}$^{3}$ bonding structures are an exciting new arena for nanoscale science and technology. In this study, we investigate hybrid molecules fusing \textit{sp}$^{2}$ and \textit{sp}$^{3}$ allotropes of carbon (in the form of C$_{60}$ fullerenes and molecular diamondoids, respectively) into one well-defined system. We use low-temperature scanning tunneling microscopy to characterize monolayers and single molecules with sub-molecular resolution. We show the degree to which the electronic properties of the hybrid molecules differ from their single-allotrope components, and highlight the intriguing electronic features that emerge which have no analog in either of the separate molecular constituents. [Preview Abstract] |
Wednesday, March 23, 2011 3:18PM - 3:30PM |
T31.00005: The Study of Hypothetical Carbon Allotropes Using Hartree Fock and Density Functional Computational Methods P.A. Ecton, C.J. Morris, J.M. Perez, S.G. Srivilliputhur, G.F. Verbeck We have investigated the possibility of hypothetical alternative carbon allotropes using computational methodologies using Gaussian and VASP molecular simulation programs. We investigate the possible existence of carbon based balls, nanotubes and sheets composed of hexagonal rings, cyclobutane rings or pentagonal rings. The possibility of the existence of a hypothetical allotrope is determined by the convergence of the given allotrope under geometric optimization. The theories used to compute such convergence are Hartree-Fock theory and density functional theory. The theoretical Raman spectra of each allotrope can also be computed using Gaussian. The results concerning the reality of the substances under investigation are inconclusive except for a C$_{24}$ ball, which has been shown to converge to graphene and is therefore an unstable molecule. [Preview Abstract] |
Wednesday, March 23, 2011 3:30PM - 3:42PM |
T31.00006: Electron Beam Stimulated Molecular Motions of C$_{60}$s inside Single-Walled Carbon Nanotubes Ke Ran Electron beam irradiation stimulated motions of carbon nanostructures from single C$_{60}$ to C$_{60}$s chain inside single-walled carbon nanotubes (CNTs) were investigated by low voltage and high resolution TEM. Single C$_{60}$'s jump in a defective zigzag C$_{60}$s molecular chain inside host CNT was observed. A cluster of C$_{60}$s inside an isolated partially filled CNT can translate back and forth within the hollow space for several times. Intermediate states of these translations were recorded as well, together with pickup of additional C$_ {60}$s when the moving cluster reached either end of the hollow space. Continuous rotation of a zigzag C$_{60}$ molecular chain inside an overloaded CNT resulted in alternate expansion and contraction of the projected width of the host CNT in the TEM images. The maximum expansion was up to 29\%. Potential calculation for the molecular motion was performed based on the van der Waals interaction among C$_{60}$s and CNT. Activation energies ranging from 0.3 eV to 0.7 eV were estimated. The molecular motion was attributed to momentum transfer during elastic scattering of electrons by the molecules, instead of thermal energy or thermal gradient. Our study demonstrates the potential of driving molecular motion by electron irradiation. [Preview Abstract] |
Wednesday, March 23, 2011 3:42PM - 3:54PM |
T31.00007: First-Principles Calculations of Graphene Nanomesh William Oswald, Zhigang Wu Graphene has recently attracted intensive attentions owing to its remarkable structural and electronic properties and its significant potential for applications in electronic and optoelectronic devices for size miniaturization and fast electron transportation. However, bulk graphene is a semi-metal with zero bandgap $E_{g}$, and opening a sizable $E_{g}$ is critical for building operational graphene-based transistors. Recently, a new scheme of opening bandgap through punching nanoscale holes in graphene sheet, the graphene nanomesh, was proposed and verified experimentally [1]. However, the mechanism leading to the bandgap opening remains unknown. We have carried out first-principles calculations based on density functional theory (DFT) to study the bandgap opening mechanism and $E_{g}$ as functions of structural parameters, including the hole size, the hole shape, and the hole-hole distances. Our results suggest that the bandgap opening is a result of quantum confinement at nanomesh necks, while the value of $E_{g}$ depends not only on the width of nanomesh necks, but also on the chirality of the hole edge. This work was supported by the start-up research funds from Colorado School of Mines. \\[4pt] [1] J. Bai, X. Zhong, S. Jiang, Y. Huang, and X. Duan, Nature Nanotech. \textbf{5}, 190 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 3:54PM - 4:06PM |
T31.00008: ABSTRACT WITHDRAWN |
Wednesday, March 23, 2011 4:06PM - 4:18PM |
T31.00009: Effects of shape and edge-passivation on magnetic moments in graphene nanomesh by first-principles investigation Hongxin Yang, Mairbek Chshiev First-principles calculations of electronic and magnetic properties of pure and H-terminated graphene nanomesh (GNM) are presented. We found stable antiferromagnetic and non-magnetic ground state for GNM with balanced zigzag and armchair-type edge structures, respectively. At the same time, a band gap opening in the balanced zigzag edge GNMs which can reach up to 0.40 eV is also found. Interestingly, GNM with unbalanced edge structure shows stable ferrimagnetic state giving rise to a net moment up to 4 Bohr magnetons per unit cell, and the exchange energy between ferrimagnetic state and paramagnetic state is larger than 1 eV per unit cell providing potential for high Curie temperature in this material. Furthermore, we found that the ground states for H-terminated GNM strongly depend on the hole symmetry: large spin polarization ground state is found for GNMs with triangle and pentagon hole shapes, while for GNMs with parallelogram and hexagon shaped holes the ground states are paramagnetic. Finally, we found that the magnetization of the GNM structure is strongly affected by the hole size: the larger hole size attains large moments, while small one may even kill all the moments. [Preview Abstract] |
Wednesday, March 23, 2011 4:18PM - 4:30PM |
T31.00010: Sensing gas molecules using graphitic nanoribbon films and networks Yanbin An, Jason L. Johnson, Ashkan Behnam, S.J. Pearton, Ant Ural We fabricate and study the gas sensing properties of graphitic nanoribbon (GNR) films and networks consisting of multi-layer graphene nanoribbons with an average width of 7 nm. We experimentally demonstrate the high sensitivity of these films and networks for sensing gas molecules at the parts-per-million (ppm) level, in particular hydrogen and ammonia. The sensing response exhibits excellent repeatability and full recovery in air. Furthermore, our results show that functionalization by metal nanoparticles could significantly improve the sensitivity. We characterize the sensing response at various temperatures, gas concentrations, recovery ambients, and film thicknesses. We find that the relative resistance response of the GNR films shows a power-law dependence on the gas concentration, which can be explained by the Freundlich isotherm. The activation energy obtained from the sensing experiments is consistent with the theoretical calculations of the adsorption energies of gas molecules on graphene sheets and nanoribbons. Their simple and low-cost fabrication process and good sensing response open up the possibility of using graphitic nanoribbon films and networks for large-scale sensing applications. [Preview Abstract] |
Wednesday, March 23, 2011 4:30PM - 4:42PM |
T31.00011: Opening of slit-shaped pores from bending of graphene walls Matthew Connolly, Carlos Wexler Graphene has gained particular interest in many areas of research including adsorption. Recent studies have shown deformations in graphene resulting from the pressure of intercalants or edge bonds. In this talk, the opening of slit shaped pores from uniaxial bending of the graphene walls of the pore is examined. The energy functional associated with the deformation from equilibrium shape is minimized to obtain an optimal shape. The minimization is done analytically for a simple model and numerically for various graphene-graphene interaction potentials. The strain induced from bending has been shown to effect the hybridization of carbon bonds within the graphene sheet. The effect of any increase in the number of binding sites due to bending as well as hybridization effects on excess adsorption are studied by Molecular Dynamics simulations. [Preview Abstract] |
Wednesday, March 23, 2011 4:42PM - 4:54PM |
T31.00012: Magneto-reflectance studies of graphite in intense magnetic field Li-Chun Tung, Paul Cadden-Zimansky, Jinbo Qi, Zhigang Jiang, Dmitry Smirnov Magnetic subbands of Kish graphite have been investigated by the magneto-infrared reflectance spectroscopy at 4K up to 31T. Both of the Schr\"{o}dinger- (K-point) and Dirac-like (H-point) Landau level transitions have been observed. The intense magnetic field resolves the transitions caused by the symmetry breaking of the doubly degenerate E3 band near the charge-neutrality point and the splitting of interband transitions due to electron-hole asymmetry. These transitions were not evident in the recent magneto-transmittance studies at high magnetic fields and are important in understanding electron-hole asymmetry and the opening of the energy gap between electron and hole bands. From the SWMC model, we derived a new formula to describe the magnetic field dispersion of the K-point transitions and a good agreement is achieved with a set of band parameters consistent with the ones reported in the literature. [Preview Abstract] |
Wednesday, March 23, 2011 4:54PM - 5:06PM |
T31.00013: Size-dependence of electronic and optical properties of armchair graphene nanoislands Jonathan Moussa, James Chelikowsky Atomicly precise armchair graphene nanoislands (benzenoid polycyclic aromatic hydrocarbons) have been produced by organic synthesis and to-date have attained sizes up to 222 carbon atoms. The electronic and optical properties of these nanoislands are studied using a combination of semi-empirical methods, time-dependent density functional theory, and the GW/Bethe-Salpeter formalism. Comparisons are made with experimental measurements where available. For this class of materials, theory is able to predict the necessary nanoisland sizes required for potential photovoltaic and light-emitting applications. The study of large nanoislands is focused on parallelogram-shaped islands, which should be particularly amenable to synthesis over a wide range of sizes. [Preview Abstract] |
Wednesday, March 23, 2011 5:06PM - 5:18PM |
T31.00014: Modifying Electronic and Magnetic Properties of BCN sheets by Boron Nitride Domain Size and Tensile Strain Chun Tang, Changfeng Chen Recent experiments have successfully synthesized atomic thin BCN hybrid composites with controllable BN domain concentration. Using first principles calculations, we report the scaling law of electronic and magnetic properties of this novel structures with respect to BN domain size and geometry. We find due to the BN domain induced internal electric field, the magnetic moment can be effectively modified. External tensile strain engineering can also be applied as an efficient tool to modify the electronic and magnetic properties. Our results may have important applications in semiconducting devices. [Preview Abstract] |
Wednesday, March 23, 2011 5:18PM - 5:30PM |
T31.00015: Functionalized 2D atomic sheets with new properties Qiang Sun, Jian Zhou, Qian Wang, Puru Jena Due to the unique atomic structure and novel physical and chemical properties, graphene has sparked tremendous theoretical and experimental efforts to explore other 2D atomic sheets like B-N, Al-N, and Zn-O, where the two components offer much more complexities and flexibilities in surface modifications. Using First principles calculations based on density functional theory, we have systematically studied the semi- and fully-decorated 2D sheets with H and F and Cl. We have found that the electronic structures and magnetic properties can be effectively tuned, and the system can be a direct or an indirect semiconductor or even a half-metal, and the system can be made ferromagnetic, antiferromagnetic, or magnetically degenerate depending upon how the surface is functionalized. Discussions are made for the possible device applications. [Preview Abstract] |
Session T32: Semiconducting Devices & Applications
Sponsoring Units: FIAPChair: Linda Olafsen, Baylor University
Room: C144
Wednesday, March 23, 2011 2:30PM - 2:42PM |
T32.00001: Tunable nonadiabatic excitation in a single-electron quantum dot J.D. Fletcher, Masaya Kataoka, P. See, S.P. Giblin, T.J.B.M. Janssen, J.P. Griffiths, G.A.C. Jones, I. Farrer, D.A. Ritchie We report observation of nonadiabatic excitation of single electrons in a quantum dot. We have developed a way of measuring the excitation spectrum of the quantum dot formed in a tunable-barrier single-electron pump. When the confinement potential is deformed at sub-nanosecond timescales, electrons are excited to states with higher back-tunneling rates leading to a measureable reduction in the pumped current. In the presence of a perpendicular magnetic field we have observed that these states follow a Fock-Darwin spectrum. Our experiments demonstrate a simple model system to study nonadiabatic processes of quantum particles. [Preview Abstract] |
Wednesday, March 23, 2011 2:42PM - 2:54PM |
T32.00002: The dynamic quantum dot as an accurate electron pump Stephen Giblin, Samuel Wright, Jonathan D. Fletcher, Masaya Kataoka, Michael Pepper, J.T. Janssen, David Ritchie, Christine Nicoll, Dave Anderson, Geb Jones We have developed an accurate single electron pump based on a dynamic quantum dot realised in a GaAs two-dimensional electron system. We report an accurate comparison between the pump current and a reference current derived from quantum standards of voltage and resistance: the Josephson effect and quantum Hall effect. We find that, at a clock frequency of several hundred Megahertz, the pump can transfer one electron per clock cycle with an accuracy approaching 10 parts per million. We discuss the significance of this result in relation to the proposed re- definition of the SI base unit Ampere. Theoretical estimations of the pump error rate indicate that an accuracy of one part in a hundred million is attainable, making our type of pump a candidate for a metrological current standard. [Preview Abstract] |
Wednesday, March 23, 2011 2:54PM - 3:06PM |
T32.00003: Measurement of Tunneling Conductance of Two-Dimensional Electrons in a Si MOSFET Nanostructure Hong Pan, Matthew House, Ming Xiao, Hongwen Jiang The properties of strongly correlated two-dimensional electrons in semiconductor heterostructure continue to be of a fundamental interest of condensed matter physics [1]. A collection of transport studies have revealed a wealthy of interesting effects in the low-electron density limit, particularly in Si MOSFET structures [2]. In this talk, we present an alternative, tunneling conductance measurement of the 2D electrons in a Si MOSFET nanostructure. In our device, a global gate is used to control the 2D electron density. In addition, a set of small gates, as small as 50nm, forms a lateral tunneling barrier for the measurements. We find that there is a strong correlation between the still puzzling metal-insulator transition observed in transport [2] and our tunneling characteristics. The tunneling conductance is studied under different carrier density and in-plane Magnetic field. The project is supported by the NSF under Grant No. DMR-0804794. \\[4pt] [1] B. Spivak, S. V. Kravchenko, S. Kivelson, and X.P.A. Gao, Rev. Mod. Phys. 82, 1743 (2010). \newline [2] E. Abrahams, S. V. Kravchenko, M. P. Sarachik, Rev. Mod. Phys. 73, 251 (2001) [Preview Abstract] |
Wednesday, March 23, 2011 3:06PM - 3:18PM |
T32.00004: The magnetized quantum wire: a potential candidate to act as an {\em active} laser medium Manvir Kushwaha The fundamental issues associated with the magnetoplasmon excitations are investigated in a quantum wire characterized by a confining harmonic potential and subjected to a perpendicular magnetic field. We embark on the charge-density excitations in a two-subband model within the framework of Bohm-Pines' random-phase approximation. Essentially, the focus of our study is the intersubband (magnetoroton) collective excitation which changes the sign of its group velocity twice before merging with the respective single-particle continuum. The computation of the gain coefficient suggests an interesting and important application: the electronic device based on such magnetoroton modes can act as an {\it active} laser medium. The situation is analogous to the (quasi-two dimensional) superlattices where the crystal can exhibit a negative resistance: it can refrain from consuming energy like a resistor and instead feed energy into an oscillating circuit. [Preview Abstract] |
Wednesday, March 23, 2011 3:18PM - 3:30PM |
T32.00005: Gain-Induced Refractive Index Changes in Resonantly Pumped Optical Pumping Injection Cavity Lasers Linda Olafsen, Lauren Bain, Lauren Ice, Ben Ball An optical pumping injection cavity (OPIC) laser contains a type-II W active region enclosed between two GaSb/AlAsSb distributed Bragg reflector mirrors, where the thickness of the etalon cavity surrounded by the mirrors is tuned to the desired pump wavelength. Multiple reflections of the pump photons result in more efficient absorption of the pump beam and consequently higher efficiencies and lower lasing thresholds. An optical parametric oscillator is used to pump the OPIC lasers at resonance, where the threshold pump intensities are minimized and output efficiencies are maximized. The resonant pump wavelength is found to vary quadratically with temperature, not linearly as would be expected from temperature-dependence of the lattice constant and refractive indices. Possible sources of this nonlinearity are lattice heating and gain-induced changes in the refractive indices resulting from the increase of optical pumping intensity with temperature. Through spectral measurements using step-scan Fourier Transform Infrared spectroscopy and multilayer reflectivity modeling, the relative contributions of these possible sources of parabolic temperature dependence of resonant wavelength are investigated. [Preview Abstract] |
Wednesday, March 23, 2011 3:30PM - 3:42PM |
T32.00006: ABSTRACT WITHDRAWN |
Wednesday, March 23, 2011 3:42PM - 3:54PM |
T32.00007: Mapping of Strain and Induced Polarization in GaMnAs/GaAs nanowires Edwin Fohtung, A. Minkevich, A.A. Matyshev, M. Riotte, D. Grigoriev, T. Slobodskyy, V. Holy, O.G. Shpyrko, T. Baumbach The effects of surface energy and non-localized interactions are the two major physical mechanisms that guarantees size dependent of elastic properties at the nanoscale. With the limit for linear elasticity defined in the vicinity of the lattice parameter for most materials, non-localized interaction can only arise due to the discrete nature of matter and fluctuations in interatomic forces averaged out within the elastic tensors. Using an extended elasticity theory that introduces higher order perturbations to the classical energy density state of a crystalline material, we demonstrate the possibility of mapping the strain and polarization in device nanostructures with the aid of synchrotron radiation coherent diffraction imaging. We provide experimentally confirmed 2D mapping of the strain field and polarization in etched GaMnAs on GaAs periodic wires. [Preview Abstract] |
Wednesday, March 23, 2011 3:54PM - 4:06PM |
T32.00008: Reflectance Spectra of Plasmon Waveguide Interband Cascade Lasers Robert Hinkey, Zhaobing Tian, Rui Yang, Tetsuya Mishima, Michael Santos Non-invasive reflectivity measurements have been explored as a method for measuring the carrier concentrations of the Si-doped cladding layers of Plasmon-Waveguide Interband Cascade (IC) Lasers. We present measurements and modeling done both on the IC laser structures, as well as highly doped InAs films grown on GaAs substrates that were used to calibrate the Molecular Beam Epitaxy growth. We have found that there is a sharp drop in the signal of the reflectance spectrum for p- polarized light oscillating near the plasma frequency, which falls in the mid-infrared for the cladding layers of the laser structure. This feature in the spectrum is caused by the interaction of the incident light with collective plasmon modes, and is distinct from the plasma edge feature seen in the reflectance spectrum of semi-infinite samples. A similar ``plasma absorption'' effect has been observed in thin metal films in the ultraviolet. The doping concentration and layer thicknesses of the structure were obtained by fitting a modeled curve to the measured spectrum. We were able to obtain measurements of the cladding layer doping concentrations (in a range from 10$^{18}$ to 10$^{19}$ cm$^{-3}$) with values that were in good agreement with those found using Hall effect measurements. We will discuss how these results can aid in improving the design of mid-infrared plasmon waveguide lasers. [Preview Abstract] |
Wednesday, March 23, 2011 4:06PM - 4:18PM |
T32.00009: ABSTRACT WITHDRAWN |
Wednesday, March 23, 2011 4:18PM - 4:30PM |
T32.00010: Electrical and Optical Characterization of $\alpha $-Silicon Thin Films Kiran Shrestha, Tanweer Mirza Beig, Pradeep Gali, Prathyusha Nukala, Chris Littler, Vincent Lopes, Usha Philipose, Nigel Shepherd, A. J. Syllaios We report on progress in the characterization of amorphous silicon thin-films utilized in infrared detectors. Specifically, we have observed changes in the Raman spectra, resistivity, and activation energy in protocrystalline Si films grown by PECVD as substrate temperature, dopant type and concentration, and hydrogen dilution of the reactants are varied. Both n- and p-type films exhibit four Raman spectral peaks [1]. The TO Raman peak becomes better defined and shifts towards the crystalline TO energy for increasing substrate temperature or H dilution, or for decreasing dopant concentration. Hall and resistivity measurements as a function of both magnetic field and temperature on the same material have been conducted to better understand the relationships between specific growth parameters and key electrical properties.\\[4pt] [1] A. J. Syllaios, et al, ``Raman Characterization of Protocrystalline Silicon Films'', MRS Symp. Proc.Vol.1153, A16-04, 2009. [Preview Abstract] |
Wednesday, March 23, 2011 4:30PM - 4:42PM |
T32.00011: Single-Crystalline Germanium Nanowire Heterostructure for High-Performance Transistors and Spintronics Jianshi Tang, Kang L. Wang, Chiu-Yen Wang, Lih-Juann Chen The formation of single-crystalline Ni$_{2}$Ge/Ge/Ni$_{2}$Ge nanowire heterostructure and its field effect characteristics by controlled reaction between a Ge nanowire and Ni contacts were studied. Transmission electron microscopy (TEM) studies reveal a wide temperature range to convert the Ge nanowire to single-crystalline Ni$_{2}$Ge by a thermal diffusion process. The \textit{in-situ} reaction examined by TEM shows atomically sharp interfaces for the Ni$_{2}$Ge/Ge/Ni$_{2}$Ge heterostructure with good epitaxial matches of Ge[-110]//Ni$_{2}$Ge[0-11] and Ge(111)//Ni$_{2}$Ge(100). Field effect transistors (FETs) built on this nanowire heterostructure show a high-performance $p$-type FET behavior with an on/off ratio over 10$^{5}$ and a field-effect hole mobility of 210 cm$^{2}$/Vs. This nanowire heterostructure with atomically sharp interfaces opens an opportunity to achieve high-performance nanowire transistors and explore promising application in spintronics. [Preview Abstract] |
Wednesday, March 23, 2011 4:42PM - 4:54PM |
T32.00012: Schottky Barrier Heights in low-k dielectric/Cu Interconnects as Determined by X-ray Photoelectron Spectroscopy Marc French, Milt Jaehnig, Markus Kuhn, Benjamin French, Sean King In order to understand the various possible leakage mechanisms in low-k/Cu interconnects, a knowledge of the basic band alignment between Cu and low-k dielectric materials is needed but has gone largely unreported. In this regard, we have utilized X-ray Photoelectron Spectroscopy (XPS) to measure the Schottky Barrier at interfaces of importance to Cu/low-k interconnects. Specifically, we have utilized XPS to determine the Schottky Barrier at the interface between Cu and low-k SiCN capping layers deposited on Cu via Plasma Enhanced Chemical Vapor Deposition (PECVD). We have also utilized XPS to determine the Schottky Barrier at interfaces between Ta barrier layer materials and low-k SiOC:H ILD materials and the valence band alignment at low-k SiCN:H/SiOC:H interfaces. Lastly, the impact of various plasma surface treatments on the band alignment at these interfaces was also investigated. The cumulative results indicate that electron transport along the SiCN:H/SiOC:H may represent the lowest energy barrier path for line-line Schottky emission based leakage. [Preview Abstract] |
Wednesday, March 23, 2011 4:54PM - 5:06PM |
T32.00013: Plasma Process to Simultaneously Clean ILD and CMP Cu Surfaces Xin Liu, Sandeep Gill, Fu Tang, Sean King, R.J. Nemanich Low-k inter-layer dielectrics (ILD) with copper interconnects display advantages for reducing energy consumption in silicon technology. However, the processing induced degradation of the ILD low-k properties has become a challenge. In this work, we have employed remote N$_{2}$/H$_{2}$ plasma processes to simultaneously clean both low-k ILD (k=2.5) and chemical-mechanical polished (CMP) Cu surfaces. FTIR and C-V results indicate that N$_{2}$ plasma cleaning processes show low carbon abstraction as well as a relatively small increase in the dielectric constant (k=2.6). A carboxamide layer is formed which apparently inhibits further etching. In contrast, the k value increases to 3.5 after an H$_{2}$ plasma treatment. For the CMP-Cu surfaces, an N$_{2}$/H$_{2}$ plasma process at 380C effectively removes the oxide and carbon contamination. In addition, the affects of plasma-induced UV light has been studied, and the results indicate enhanced carbon depletion in the ILD. Degradation of the low-k properties is attributed to carbon abstraction which is enhanced by the plasma induced UV and hydrophilic character. The results establish a range of N$_{2}$/H$_{2}$ plasma processes for simultaneous cleaning of CMP Cu and low-k ILD surfaces. [Preview Abstract] |
Wednesday, March 23, 2011 5:06PM - 5:18PM |
T32.00014: Elastic properties of high porosity low-k thin films measured by Brillouin light scattering S. Bailey, R. Sooryakumar, S. King, G. Xu, E. Mays, C. Ege, J. Bielefeld The continued scale down of material components present increasing challenges to technology development in the semiconductor industry. In particular, with the introduction of more porous materials and air gaps to further reduce permittivity, one significant issue is that low-k dielectrics in interconnects have sufficient mechanical strength. Since nano-indentation methods are questionable at these ultra-small thicknesses there is a need for non-invasive methods to characterize the mechanical properties of such highly compact porous structures. In this talk results of Brillouin scattering to measure elastic constants of thin ($<$ 200 nm) low-k SiOC:H films with porosities up to 35{\%} will be presented. Discrete longitudinal and transverse acoustic standing modes and their transformation to propagating excitations are investigated. The resulting mode dispersions provide for the Poisson's ratio ($\nu$) and Young's modulus (E) and confirm that, for highest porosity, the reduction in dielectric constant does not result in severe degradation in $\nu$ and E. [Preview Abstract] |
Wednesday, March 23, 2011 5:18PM - 5:30PM |
T32.00015: Effective Materials Properties of Interconnections in Industrial Microprocessor Designs Mary Lanzerotti, Giovanni Fiorenza, Rick Rand This talk presents a methodology to evaluate tradeoffs between technology and design to obtain the highest performance in industrial VLSI designs [1]. It is well known that the most significant circuitry constraint is that signals must arrive on time. Since the design cycle is time-consuming and complex, there is a need to migrate designs to future technology nodes to amortize design cost. However, models do not exist [1] to guide designers in their evaluation of whether migrated designs will operate successfully in a future technology or whether migrated designs will cause chip failure. There is therefore a need to evaluate the impact of design changes on performance. This talk evaluates this impact and describes it as an effective change in material properties of the design interconnections. Model estimates are compared with industrial microprocessor design data [1]. References [1] M. Y. Lanzerotti, G. Fiorenza, R. Rand, ``Impact of interconnect length changes on effective materials properties (dielectric constant),'' \textit{Proc. Ninth International ACM Workshop on System-Level Interconnect Prediction (SLIP 2007)}, Austin, TX, USA, March 17-18, 2007. Online: http://www.informatik.uni-trier.de/$\sim$ley/db/conf/slip/slip2007.html, current as of 11-16-2010. [Preview Abstract] |
Session T33: Focus Session: Dielectric, Ferroelectric, and Piezoelectric Oxides: Pb-based and novel materials
Sponsoring Units: DMP DCOMPChair: Xiaoqing Pan, University of Michigan
Room: C143/149
Wednesday, March 23, 2011 2:30PM - 3:06PM |
T33.00001: Origin of diffuse scattering in relaxor ferroelectrics Invited Speaker: High-pressure and variable temperature single crystal synchrotron X-ray measurements combined with first-principles based molecular dynamics simulations study diffuse scattering in the relaxor ferroelectric system PSN (PbSc1/2Nb1/2O3). Constant temperature experiments show pressure induced transition to the relaxor phase at different temperatures characterized by butterfly and rod shaped diffuse scattering around the {\{}h00{\}} and {\{}hh0{\}} Bragg spots, respectively. The simulations [1] reproduce the observed diffuse scattering features as well as their pressure-temperature behavior, and show that they arise from polarization correlations between chemically-ordered regions, which in previous simulations were shown to behave as polar nanoregions. Simulations also exhibit radial diffuse scattering (elongated towards and away from Q=(000)), that persists even in the paraelectric phase, consistent with previous neutron experiments on (PbMg1/3Nb2/3O3) (PMN). DFPT calculations to elucidate origin of Raman peaks in the relaxor phase will also be presented. \\[4pt] [1] P. Ganesh, E. Cockayne, M. Ahart, R. E. Cohen, B. Burton, Russell J. Hemley, Yang Ren, Wange Yang and Z.-G. Ye, Phys. Rev. B \textbf{81}, 144102 (2010) [Preview Abstract] |
Wednesday, March 23, 2011 3:06PM - 3:18PM |
T33.00002: Linear dichroism dependence on ferroelectric polarization Srinivas Polisetty, Jinling Zhou, Mikel Holcomb, Andreas Scholl, Lane W. Martin X-ray absorption spectroscopy (XAS) and photoemission electron microscopy (PEEM) have been used to determine the magnetic properties of magnetoelectrics, possessing both ferroelectric and magnetic order; however, the additional sensitivity to the ferroelectricity in these films complicates the analysis. Nevertheless, an exclusive knowledge of ferroelectric order is important due to its vital role in manipulating magnetic properties of the magnetoelectrics. To shed light on ferroelectric order and polarization near surface region, we employed mainly PEEM and XAS on ferroelectric PbZr$_{0.2}$Ti$_{0.8}$O$_{3}$ (PZT) film deposited on LaAlO$_{3}$ substrate. Both out-of-plane and in-plane ferroelectric contributions at Ti $L_{3,2}$ and O-absorption edges have been measured over multiple poled regions on the sample to test various potential mechanisms causing this ferroelectric dichroism, such as surface charge. The ferroelectric order in PZT determined to be systematically evolving as a function of incident x-ray polarization for different rotated angles of the sample revealing a similar angular dependence to that of magnetic samples, allowing a formula for linear dichroism in complex ferroelectrics. This development allows this dynamic approach to be used to study the effect of ferroelectricity on interface coupling in a various materials. [Preview Abstract] |
Wednesday, March 23, 2011 3:18PM - 3:30PM |
T33.00003: Second harmonic generation and high pressure ferroelectricity in PbTiO$_{3 }$(II) Muhtar Ahart, Alexander F. Goncharov, Maddury Somayazulu, Russell J. Hemley We employed the Raman scattering, x-ray diffraction, and second harmonic generation (SHG) experiments to investigate the behavior of PbTiO$_{3}$ under pressure up to 100 GPa at 300 K. The experimental results reveal that lead titanate undergoes a second order phase transition from a tetragonal to a cubic at 12 GPa and continuously to a non-cubic structure at 30 GPa. The integral intensity of SHG in the material decreases monotonically with pressure below 12 GPa, and does not depend on pressure above 12 GPa. The results provide no-evidence for high pressure ferroelectricity in PbTiO$_{3}$ at high pressure and room temperature. [Preview Abstract] |
Wednesday, March 23, 2011 3:30PM - 3:42PM |
T33.00004: First-principles study to identify heterostructures of (Pb, Sr)TiO$_{3}$ with enhanced ferroelectric polarisation Zhe Liu A first-principle cluster expansion method is employed to study the ferroelectric (FE) polarisation properties of Pb$_{x}$Sr$_{1-x}$TiO$_{3}$ perovskite oxide grown on SrTiO$_{3}$ (001)-substrate. Our results indicate that some heterostructures can significantly enhance polarisation in comparison with the (001) superlattice and the random alloy structures. At $x=0.5$, an (110) A/B mono-layer superlattice is identified as the structure with the most enhanced polarisation, and at $x=0.25$, the optimal structure is determined to be a body-centred superstructure. Detailed structural analysis reveals the atomic configurational patterns in the (001) plane that benefit the off-centering of Pb and Sr cations. Explanation is provided in terms of dipole interactions. Our results should apply to other multicomponent FE perovskites as well and they could have a significant impact in the design of FE materials. [Preview Abstract] |
Wednesday, March 23, 2011 3:42PM - 3:54PM |
T33.00005: Interatomic force constants and effective Hamiltonians for structural phase transitions Anil Kumar, Karin M. Rabe Expansion of the total energy of a crystal around a high-symmetry reference structure provides information about material properties including the phonon dispersion, responses to applied fields, magnetostructural coupling, and structural transitions. For complex oxides, parameterization of the structural energetics by real-space interatomic force constants (IFCs) provides a computationally convenient and physically transparent way of analyzing these properties. By projection into a subspace containing the relevant degrees of freedom, one can construct an effective Hamiltonian to study properties that are not readily accessible with DFT based calculations, including properties at finite temperature or long length scales. It is well known that first-principles density-functional-theory (DFT) based-calculations can be systematically used to determine real-space IFCs of materials; this is part of several first-principles packages including ABINIT and Quantum Espresso. Here, we discuss a simple and efficient approach for construction of first-principles effective Hamiltonians which uses this computational capability to generate and compute the quadratic inter-cell parameters in a single step. We illustrate the method through the application to systems for which effective Hamiltonians have previously been constructed, and show how this approach facilitates the construction of effective Hamiltonians for new classes of crystal structures. [Preview Abstract] |
Wednesday, March 23, 2011 3:54PM - 4:06PM |
T33.00006: All-atom effective models for first-principles simulations of the temperature-dependent behavior of complex ferroelectric oxides Jorge Iniguez, Jacek C. Wojdel, Patrick Hermet, Philippe Ghosez, Zeila Zanolli Since its introduction in the 90's, the first-principles effective-Hamiltonian method has been successfully used to simulate temperature-driven phenomena in increasingly complex ferroelectrics, from classic compound BaTiO$_3$ to multiferroic BiFeO$_3$. Currently, the emergence of nano-structured materials -- e.g., in the form of ultra-thin films or short-period superlattices -- poses new challenges to the simulations, and the development of predictive models seems to require a reconsideration of the traditional approach. Of particular interest are cases in which novel interfacial effects determine the behavior, as in the PbTiO$_3$-SrTiO$_3$ superlattices of Bousquet {\sl et al}. [Nature {\bf 452}, 7188 (2008)]. In such situations a large number of structural distortions may become active, and it may be difficult to decide which ones need to be included in the model. In order to tackle these difficulties, we are extending the first-principles effective-Hamiltonian method so as to retain a full atomistic description of the material, thus removing the so-called {\em local mode} approximation. I will describe our new approach and show preliminary results for PbTiO$_3$. [Preview Abstract] |
Wednesday, March 23, 2011 4:06PM - 4:18PM |
T33.00007: Prediction of a Low Band Gap Oxide Ferroelectric David Singh, Bo Xu, Valentino R. Cooper, Yuan Ping Feng We report a first principles study of Bi$_6$Ti$_4$O$_{17}$ which is an alternate stacking of ferroelectric Bi$_4$Ti$_3$O$_{12}$ (BiT). We the standard PBE GGA functional for the structure and polarization and a recently developed functional that yields accurate band gaps for the electronic structure. We find that this compound is ferroelectric although with a reduced polarization relative to BiT. Importantly, calculations of the electronic structure yield a band gap of approximately 1.4 eV. Therefore, we predict that this stacking is a low band gap oxide ferroelectric. [Preview Abstract] |
Wednesday, March 23, 2011 4:18PM - 4:30PM |
T33.00008: Band Gap Engineering via Local Environment in Complex Oxides Tingting Qi, Ilya Grinberg, Andrew Rappe We describe how the electronic structure energy level of the recently-developed highly polar tetragonal perovskite oxides, most prominently Bi(Zn,Ti)O$_3$, can be dramatically changed by a simple rearrangement of the $B$-cation. Using LDA+Hubbard $U$, we determine the impact of $B$-site cation ordering, lattice strain, cation identity, and oxygen octahedral cage tilts on the valence and conduction bands. We find that a combination of ultra-high tetragonality and a careful choice of $B$-cations can lead to changes of 1-2 eV in the band gap for the same composition, through a change in the $B$-cation ordering alone. We also find that a layered $B$-cation ordering exhibits high-mobility 2D hole gas (2DHG) behavior. The lower band gaps of the layered $B$-cation ordering makes these materials suitable for photovoltaic and photocatalytic applications, due to their good match with the solar spectrum. Our analysis of the crystal structure and the valence and conduction bands shows that these unusual effects can be explained in the framework of crystal field theory and underscores the crucial role that ultra-high tetragonality plays in making the band gap sensitive to the $B$-cation ordering. [Preview Abstract] |
Wednesday, March 23, 2011 4:30PM - 4:42PM |
T33.00009: Modeling the Frequency Dependence of the Complex In-plane Permittivity of Strained Ruddlesden-Popper Series Sr(n+1)Ti(n)O(3n+1) (n = 2, 3, 4, 5, 6) Phases on DyScO3 and GdScO3 N.D. Orloff, C.-H. Lee, M.D. Biegalski, Ichiro Takeuchi, D.G. Schlom, J.C. Booth We explore the in-plane complex relative permittivity as a function of frequency of epitaxial thin-films of the Ruddlesden-Popper series Srn+1TinO3n+1 (n = 2, 3, 4, 5, 6) grown on the rare-earth scandate substrates DyScO3 and GdScO3, which correspond to biaxial tensile strain of approximately 1.1{\%} and 1.7{\%}, respectively. The thin films are 50 nm on DyScO3 and 25 nm thick on GdScO3, to ensure uniform strain throughout the film. We characterize the thin films with a set of optimally designed coplanar waveguides from 45 MHz to 40 GHz and with a set of interdigitated electrodes of varying active lengths from 10 Hz to 100 MHz. We then extract the in-plane complex permittivity from 10 Hz to 40 GHz for these thin films. We report the dependence of the Curie temperature, tunability, and loss tangent on series number and strain at 1 MHz. We also present a frequency-dependent model and the corresponding fit parameters for these thin films as a function of temperature. The model assumes a distribution of domains below the Curie temperature and high frequency relaxation that we attribute to the lowest-order phonon [Preview Abstract] |
Wednesday, March 23, 2011 4:42PM - 4:54PM |
T33.00010: Study of Temperature-Graded Ferroelectrics Using First-Principle-Based Approaches Qingteng Zhang, Inna Ponomareva Temperature-graded ferroelectrics have attracted a lot of attention in the recent years owing to their many remarkable properties. Here we develop a microscopic approach based on first-principles effective Hamiltonian to simulate temperature-graded ferroelectrics. Accuracy of such approach is confirmed by comparing our computational results for (Ba$_{0.75}$Sr$_{0.25}$)TiO$_3$ alloy with available experimental data. Our computations further reveal: 1) strong anisotropy in polarization response: the polarization offset along the temperature gradient is an order of magnitude smaller than in the perpendicular direction; 2) coexistence of different phases (including low-symmetry phases) in chemically homogeneous regions; 3) rotation of polarization in response to temperature gradient in the unclamped samples. These findings could potentially lead to many novel applications such as energy converters, thermally tunable devices and efficient photovoltaics [1].\\[4pt] [1] Q. Zhang {\it et al}, Phys. Rev. Lett. {\bf 105}, 147602 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 4:54PM - 5:06PM |
T33.00011: Ferroelectric polarizations of Pb(Zr$_{0.5}$Ti$_{0.5}$)O$_3$ nanotube array R. Adhikari, Huaxiang Fu Ferroelectric polarization and structural properties are determined for the Pb(Zr$_{0.5}$Ti$_{0.5}$)O$_3$ (PZT) nanotube array embedded in matrix materials of different polarizability, by means of first-principles derived effective Hamiltonian and finite-temperature Monte Carlo simulations. The polarizability of the matrix is controlled by the on-site $\kappa_2$ quantity. We found three drastically different structural phases in PZT nanotubes, depending on the polarizability of the matrix. Microscopic insight for these structural phases will be revealed. [Preview Abstract] |
Wednesday, March 23, 2011 5:06PM - 5:18PM |
T33.00012: Raman scattering studies of the pressure- and temperature-dependent phases of the orbital ordering material KCuF$_{3}$ Shi Yuan, S. Lance Cooper We present a study of the temperature- and pressure-dependence of the prototypical orbital ordering system KCuF$_{3}$ using Raman scattering. Temperature-dependent measurements offer evidence for structural instabilities in KCuF$_{3}$ at temperatures well below the putative 800K orbital ordering temperature; this evidence includes the observation of anomalous softening of several phonon modes between 50K and 300K and an E$_{g}$ phonon mode splitting below 50K. The latter indicates a tetragonal-to-orthorhombic structural transition near 50K just preceding the 3D magnetic ordering temperature at T$_{N} \quad \approx $ 40K. Low-temperature, pressure-dependent Raman studies of KCuF$_{3}$ are also reported to clarify the pressure dependence of the low temperature structural phases in this material. [Preview Abstract] |
Wednesday, March 23, 2011 5:18PM - 5:30PM |
T33.00013: Unusually strong Stark effect in electronic ferroelectric Er$_{1-x}$Yb$_{x}$Fe$_{2}$O$_{4}$ Jimin Zhao, Rui Wang, Huanxin Yang, Jianqi Li Strong Stark splitting, which is nearly independent of the R-ions replacement, has been observed through the photoluminescence investigation of electronic ferroelectric Er$_{1-x}$Yb$_{x}$Fe$_{2}$O$_{4 }$(x=0, 0.8, 0.9, and 0.95). Initially multiple radiative decay channels have been investigated, especially the visible transition $^{4}F_{9/2}\to ^{4}I_{15/2}$, of which a quenching effect has been observed. A series of small non-Raman peaks have been observed superimposed on a broadband photoluminescence spectrum, of which we tentatively assign Stark splitting to be the cause. The splitting of the $^{4}F_{9/2}$ and $^{4}I_{15/2}$ levels is found to be 54 meV and 66 meV, respectively. This unusually large Stark splitting at visible range indicates the existence of strong local field originated from the W-layer in the charge-frustrated ErFe$_{2}$O$_{4}$. [Preview Abstract] |
Session T34: Optical and Electronic Properties of Nanocrystals and Wires
Sponsoring Units: DCMPChair: Steve Erwin, Naval Research Laboratory
Room: C141
Wednesday, March 23, 2011 2:30PM - 2:42PM |
T34.00001: Colloidal Quantum Dot Red-Shifting on Textured Metal Surfaces Christopher Ferri, Anthony Grimes, Sayantani Ghosh We have studied the influence of textured metal surfaces on the emission of an ensemble of colloidal CdSe/ZnS core-shell quantum dots (QDs). The texture was generated by sputter coating a thin film of Gold Paladium (AuPd) on a thermoplastic Polydimethylsiloxane (PDMS) sheet. We used two separate protocols to generate two types of surfaces. We constrained some substrates such that they shrunk along only one planar dimension (uniaxial) while some were allowed to shrink along both planar directions (biaxial). The uniaxial substrates forced the metal to buckle along one dimension and the biaxial substrates buckled into a pseudorandom texture. We found that the QDs deposited on the biaxial substrates had a general red shift in the emission wavelength compared to their emission in solution, which also corresponded to a change in the temporal dynamics of the emission. The QDs on the uniaxial substrates showed a change in their temporal dynamics corresponding to plasmonic coupling, but no spectral shift. We hypothesize that the effects observed on the biaxial substrates are caused by the Franz-Keldysh effect. [Preview Abstract] |
Wednesday, March 23, 2011 2:42PM - 2:54PM |
T34.00002: Single-molecule spectroscopy study of interfacial charge separation and energy transfer between quantum dots and conjugated polymers Zhihua Xu, Mircea Cotlet Blends of semiconducting quantum dots (Qdots) and conjugated polymers (CPs) are promising materials for light-emitting diodes or photovoltaic devices. Effective design of optoelectronic devices relies on further understanding of the photophysics in these hybrid materials, including charge separation (CS) and energy transfer (ET). We have studied the photophysics of the blends of two water-soluble conjugated polymers and CdSe/ZnS quantum dots with varying shell thickness, which enable us to control the competitive CS and ET processes by tuning energy-band alignment and interfacial distance. Combining single --molecule spectroscopy with ensemble-based measurements provides deeper understanding of the dynamic interactions at inhomogeneous interfaces. [Preview Abstract] |
Wednesday, March 23, 2011 2:54PM - 3:06PM |
T34.00003: Broadband ultrafast transient absorption of multiple exciton dynamics in lead sulfide nanocrystals Felice Gesuele, Chee Wei Wong, Matthew Sfeir, James Misewich, Weonkyu Koh, Christopher Murray Multiple exciton generation (MEG) is under intense investigation as potential third-generation solar photovoltaics with efficiencies beyond the Shockley-Queisser limit. We examine PbS nanocrystals, dispersed and vigorously stirred in TCE solution, by means of supercontinuum femtosecond transient absorption (TA). TA spectra show the presence of first and second order bleaches for the 1Sh-Se and 1Ph-Pe excitonic transition while photoinduced absorption for the 1Sh,e-Ph,e transitions. We found evidence of carrier multiplication (MEG for single absorbed photon) from the analysis of the first and second order bleaches, in the limit of low number of absorbed photons (Nabs$\sim $0.01), for energy three times and four times the Energy gap. The MEG efficiency, derived from the ratio between early-time to long-time TA signal, presents a strongly dispersive behavior with maximum red shifted respect the first absorption peak. Analysis of population dynamics shows that in presence of biexciton, the 1Sh-Se bleach peak is red-shifted indicating a positive binding energy. MEG efficiency estimation will be discussed with regards to spectral integration, correlated higher-order and first excitonic transitions, as well as the nanocrystal morphologies. [Preview Abstract] |
Wednesday, March 23, 2011 3:06PM - 3:18PM |
T34.00004: The effects of controlled charging on photoluminescence of individual ``giant'' core/shell nanocrystals Christophe Galland, Yagna Ghosh, Bhola Pal, Jennifer Hollingsworth, Milan Sykora, Victor Klimov, Han Htoon We report the first single-nanocrystal photoluminescence (PL) study under controlled charge injection. We investigate so-called ``giant'' nanocrystal quantum dots (g-NQDs) that comprise a CdSe core and a thick CdS shell. We use solid-state gated devices as well as electrochemical cells for charged injection. When the gate bias or electrochemical potential is tuned, we observe dramatic changes in the PL dynamics that are accompanied by spectral shifts and intensity modulations. Our initial results suggest that negatively charged excitons are at least as bright as neutral excitons in these g-NQDs for which Auger recombination has been shown to be strongly suppressed. Surprisingly, hole injection leads primarily to quenching of the NQD emission, which might explain the appearance of ``gray'' or ``dark'' periods in the single-dot PL trajectories. [Preview Abstract] |
Wednesday, March 23, 2011 3:18PM - 3:30PM |
T34.00005: Near-Unity Biexciton Quantum Yields in Individual Giant Nanocrystal Quantum Dots Young-Shin Park, Anton Malko, Javier Vela, Yongfen Chen, Yagnaseni Ghosh, Florencio Garcia-Santamaria, Jennifer Hollingsworth, Victor Klimov, Han Htoon We report quantitative studies of photoluminescence (PL) quantum yields of biexcitons ($Q_{BX})$ in individual CdSe/CdS core/shell nanocrystal quantum dots (NQDs) as a function of shell thickness. $Q_{BX}$s measured by two independent techniques show a gradual increase with increasing shell thickness, reaching a near-unity value of $\sim $0.9 for the NQDs with a 19 monolayer-thick shell. These results imply a strong suppression of Auger decay.~ However, $Q_{BX}$s show a wide variation among nominally identical NQDs implying a strong dependence of $Q_{BX}$ on subtle structural differences of the core/shell interfaces. Surprisingly, despite a wide variation in $Q_{BX}$, all thick-shell NQDs exhibit a complete suppression of PL blinking, implying that this non-blinking behavior does not result from the suppression of Auger decay and instead may simply arise from a reduced likelihood of photocharging. [Preview Abstract] |
Wednesday, March 23, 2011 3:30PM - 3:42PM |
T34.00006: Axially localized optical properties of individual nanowires Junping Zhuang, Yao Liang, S.K. Hark Nanowires are not always uniform in their properties. Studying the localized optical properties of the nanowires is thus important for their characterization and potential applications. Micro-photoluminescence spectra and fluorescence lifetime along individual ZnSe nanowires were measured and studied using laser scanning confocal microscopy. The nanowires were selected from an array that was synthesized via self catalyzed VLS mode on GaAs substrates. Through fluorescent imaging, the distribution of the deep-level emissions along the nanowires, in which a bright-tip and a dim-tail were observed, is found to be very different from the relatively uniform distribution of the near-band-edge emissions. Using fluorescence lifetime imaging, we found that the fluorescence decay behaviors are very different between the two emission bands and have position dependence. We believe that the unintended Ga diffusion during the growth of the nanowires should be responsible for the observed distributions. [Preview Abstract] |
Wednesday, March 23, 2011 3:42PM - 3:54PM |
T34.00007: Polarization dependence of ultrafast dynamics in single Si nanowires M.A. Seo, S.A. Dayeh, P.C. Upadhya, S.T. Picraux, J. Martinez, B.S. Swartzentruber, A.J. Taylor, R.P. Prasankumar Understanding how light interacts with individual nanowires (NWs), particularly depending on its polarization with respect to the NW alignment, is essential for a wide range of applications. We present the first ultrafast time-resolved, polarization-dependent experiments on both single- and ensemble-silicon nanowires using non-degenerate pump-probe spectroscopy to excite and probe carriers above the indirect band gap. Polarization sensitive pump-probe excitation and detection reveal a clear anisotropy in the ultrafast dynamics measured parallel and perpendicular to the long axis of a single nanowire. In addition, the magnitude of the photoinduced change in ensembles of NWs varies for four different sets of pump and probe polarization, without an anisotropy in relaxation time. The comparison of ultrafast dynamics between single and ensemble nanowires provides great insight into the influence of incident light polarization on different absorption and interaction mechanisms. The observed anisotropy in single NWs could enable advanced applications, such as optical switching and polarization sensitive photo detection, on the nanoscale, where directional control and high spatial resolution are much desired [Preview Abstract] |
Wednesday, March 23, 2011 3:54PM - 4:06PM |
T34.00008: Space-and-Time-Resolved Spectroscopy of Single GaN Nanowires Prashanth Upadhya, Julio Martinez, Qiming Li, George Wang, Brian Swartzentruber, Antoinette Taylor, Rohit Prasankumar Understanding the carrier relaxation pathways in individual semiconductor nanowires (NWs) is crucial, since the geometry of these nanostructures can significantly influence carrier recombination and trapping. In particular, GaN NWs are promising wide bandgap semiconductors for applications in nanophotonics, but the efficiency and lifetime of GaN-based devices are largely affected by the presence of structural and point defects. In this study we employ wavelength-tunable femtosecond optical pump-probe spectroscopy to study carrier relaxation through the defect states responsible for yellow luminescence in both a single GaN NW and NW ensembles. These are the first ultrafast optical experiments on single group III-V NWs, revealing spatially resolved carrier dynamics along the length of an individual wire. [Preview Abstract] |
Wednesday, March 23, 2011 4:06PM - 4:18PM |
T34.00009: Electronic Structure of PbSe Nanorods Adam Bartnik, Alexander L. Efros, Weon-kyu Koh, Jun Yang, Christopher Murray, Frank Wise In spherical lead-salt (PbS and PbSe) nanocrystals, their large dielectric constant and mirror-like band structure significantly weaken the Coulomb interaction, while their large exciton Bohr radii place them at the limit of strong confinement. But in a 1-dimensional structure, the Coulomb interaction can act primarily through the medium, greatly reducing screening. Thus, by controlling their length, the lead-salts can uniquely switch from strong confinement to strong Coulomb binding. To investigate this, we develop a 4-band k-p model of the electronic structure of lead-salt nanorods (NRs), which includes the Coulomb interaction through an effective 1D potential along the NR axis. Perpendicular to the cylindrical axis, confinement dominates and is the major determinant of the location of peaks in the optical spectra. Along the rod axis, the effective Coulomb potential dominates, highly correlating the electron and hole in this direction and enhancing multiparticle interactions, with the observable effect of producing isolated peaks in optical spectra. Predictions of the locations of these enhanced transitions are shown to have good agreement with measured optical spectra of recently synthesized colloidal PbSe NRs. [Preview Abstract] |
Wednesday, March 23, 2011 4:18PM - 4:30PM |
T34.00010: Atomistic calculations of the exciton-biexciton mixing and biexciton lifetime in CdSe nanocrystals Marek Korkusinski, Oleksandr Voznyy, Pawel Hawrylak We present an atomistic tight-binding theory of multi-exciton complexes in spherical CdSe nanocrystals. As shown previously [1] the properties of exciton (X) and biexciton (XX) are determined by the shell of four quasi-degenerate states at the top of the valence band, resulting in a band of correlated XX states. This XX fine structure affects the Coulomb mixing of the low-lying XX with excited X states. Here we compare different approaches to computation of the XX ground state lifetime. The Fermi's golden rule accounting only for directly coupled XX and X configurations is compared to configuration-interaction approach where XX and X with energy close to 2Eg are taken into account. We show that the expansion of the basis of single particle configurations used to describe XX leads to a significant increase of the amount of X configurations to which XX can couple. The effect of inclusion of the X configurations coupled to XX indirectly via the intermediate X states is also discussed. \\[4pt] [1] M. Korkusinski, O. Voznyy, and P. Hawrylak, arXiv:1010.0021 (Phys. Rev. B, 82, 2010, in press). [Preview Abstract] |
Wednesday, March 23, 2011 4:30PM - 4:42PM |
T34.00011: Can nanocrystals be charge-doped using surface adsorbates? Alex Kutana, Steve Erwin We study theoretically the possibility that lead selenide nanocrystals can be doped with electrons or holes by charge transfer from molecules (hydrazine, N2H4) adsorbed on their surface. Despite experiments showing that hydrazine adsorption greatly increases the conductivity of PbSe nanocrystal films, our density-functional theory results show no evidence that carriers are directly created by adsorption. Instead, we find that PbSe always remains intrinsic for a variety of hydrazine coverages and PbSe surface orientations. Moreover, analysis of the charge density shows negligible electron transfer from hydrazine to the surface. We tentatively attribute the discrepancy between experiment and theory to extrinsic factors such as surface defects, doping by surface ligands, or dopant activation by hydrazine. For example, we predict that adsorption of acetic acid will create mobile holes in PbSe, and that surface adsorption of lead atoms will result in mobile electrons. [Preview Abstract] |
Wednesday, March 23, 2011 4:42PM - 4:54PM |
T34.00012: Surface states in CdSe nanocrystals with carboxylic acid ligands: an ab initio study Oleksandr Voznyy The electronic properties of the realistic Cd-rich CdSe quantum dots with covalently bound (X-type) carboxylic ligands are investigated using density functional theory for the nanocrystal (NC) sizes sufficient to distinguish core and surface states. We find that Cd and Se atoms with only one dangling bond do not create surface traps. The amount of ligands and the crystal shape is dictated by the overall electronic balance of the NC rather than by the surface free energies of particular ligated facets. To achieve this balance more ligands are required than there are ``ideal'' adsorption sites for them, creating ligand-induced trap states near the valence band maximum. These extra ligands are mobile on surface, resulting in spectral diffusion of the trap levels, providing first atomistic example of diffusion and activated recombination centers models for blinking. [Preview Abstract] |
Wednesday, March 23, 2011 4:54PM - 5:06PM |
T34.00013: First-Principles Calculations of Lattice-Strained Core-Shell Nanocrystals K.H. Khoo, J.T. Arantes, James R. Chelikowsky, G.M. Dalpian We have studied the properties of CdS-ZnS and ZnS-CdS core-shell nanocrystals over a range of shell thicknesses using real-space pseudopotential density functional theory. The effect of structural relaxation was shown to be important as it leads to significant changes in the HOMO-LUMO gap and frontier orbital localizations. Also, strains due to lattice mismatch are predicted to be highly localized around the core-shell interface, giving rise to a thin shell regime where both confinement and strain effects are important and a thick shell regime where confinement effects dominate. This has interesting implications for the evolution of the HOMO-LUMO gap with shell thickness. [Preview Abstract] |
Wednesday, March 23, 2011 5:06PM - 5:18PM |
T34.00014: First Principles Study of Core-Shell Semiconductor Nanocrystals Igor Vasiliev Core-shell nanocrystals composed of two different semiconductors have recently attracted considerable attention. These structures provide enhanced functionality and possess more degrees of freedom than single-component semiconductor nanocrystals and quantum dots. I present the results of {\it ab initio} density functional calculations for the structures, electronic densities of states, and optical absorption gaps of core-shell nanocrystals composed of group II-VI semiconductors, such as CdSe, CdTe, ZnSe, and ZnTe. The outer surfaces of the nanocrystals are passivated using partially charged hydrogen atoms. The calculations are performed for ``traditional'' core-shell nanocrystals, in which a core a narrow gap semiconductor is covered with a shell of a wide gap material, and ``inverted'' core-shell nanocrystals, in which a wide-gap core is enclosed in a narrow-gap shell. [Preview Abstract] |
Wednesday, March 23, 2011 5:18PM - 5:30PM |
T34.00015: Jahn-Teller distortion in semiconductor nanocrystals Gustavo M. Dalpian, Aline L. Schoenhalz, Jeverson T. Arantes Semiconductor nanocrystals or quantum dots can present properties that are very different from their bulk counterparts. Quantum confinement and surface effects play important roles in determining these unusual properties. Here we report a Jahn-Teller distortion in pristine nanocrystals upon the addition of charges. Results will be presented for Si, InAs and CdSe nanocrystals. In order to observe this effect, we have performed calculations using the Density Functional Theory and hybrid functionals. We will discuss the implications of this distortion on the electronic and optical properties of these materials. [Preview Abstract] |
Wednesday, March 23, 2011 5:30PM - 5:42PM |
T34.00016: Memory, Photoconductivity, and Traps in Semiconducting Nanocrystal Arrays Jessamyn Fairfield, Lauren Willis, Tali Dadosh, Marija Drndic Nanoscale devices are extensively studied for their tunable electronic and optical properties, but the influence of impurities and defects is amplified at these length scales and can lead to poorly understood variations in material characteristics. By performing a large ensemble of photoconductivity measurements in nanogaps bridged by core-shell CdSe/ZnS semiconductor nanocrystals, we discover optoelectronic methods for affecting solid-state charge trap populations. We show that the magnitude and temperature dependence of the photocurrent depends on the illumination and electric field history on a few-hour timescale. Subband gap illumination of nanocrystals prior to measurements modifies the photocurrent more than band gap illumination. We introduce a model that unifies previous work and transforms the problem of irreproducibility in nanocrystal electronic properties into a robust photocurrent response due to trap state manipulation. Because traps dominate many physical processes, these findings may lead to improved performance and device tunability for nanoscale applications through the control and optimization of impurities and defects. [Preview Abstract] |
Session T35: Metals: Defects and Surfaces
Sponsoring Units: DCMPChair: Don Nicholson, Oak Ridge National Laboratory
Room: C140
Wednesday, March 23, 2011 2:30PM - 2:42PM |
T35.00001: Orientation effect on dislocation nucleation and related interaction during void growth simulations in Aluminum Amitava Moitra, Mehul Bhatia, Kiran N. Solanki Molecular dynamics simulations are performed to understand the void growth for fcc Aluminum. Dislocation nucleation at the void surface and growth of those dislocations in matrix, are studied for seven different crystallographic orientations: [100], [110], [111], [210], [211], [221], and [321]. A significant effect of the loading orientation on dislocation loop nucleation and configuration, and consequently the shape change of voids are found. Calculations related to the interaction of burgers vectors of the nearby leading and trailing dislocations are performed to find the reason why dislocation extremities are attached to the void surface. It has been also shown for a particular orientation that the extremities leave the void surface in order to reduce the interaction energy. Cross slip and triplanar loops are also found during the study of void growth simulations. [Preview Abstract] |
Wednesday, March 23, 2011 2:42PM - 2:54PM |
T35.00002: MD Study of the Nucleation and Growth of Deformation Twins in Polycrystalline Tantalum Luis Sandoval, David Richards Recovered samples from high strain rate experiments clearly show that twin formation serves as an important plasticity mechanism in Tantalum. Despite years of study however, the nucleation and growth mechanisms of twining are still poorly understood, especially in bcc metals. Twins are typically thought to nucleate at grain boundaries via a cooperative emission of partials after a critical value of shear stress. We have used molecular dynamics (MD) simulation to observe the nucleation and growth of twin domains from grain boundaries and grain boundary junctions in polycrystalline cells, which have been prepared as arrangements of hexagon-columnar grains. Using a Finnis-Sinclair potential, we have examined the role of strain rate, temperature and hydrostatic pressure on the kinetic phenomena, in particular the twinning threshold and twin growth rates. We discuss how kinetic parameters extracted from MD simulations help inform a multiscale strength model for Tantalum that includes both twinning and slip as deformation mechanisms in the regime of high strain rates. [Preview Abstract] |
Wednesday, March 23, 2011 2:54PM - 3:06PM |
T35.00003: Large Scale Dislocation Dyniamics Simulation of Precipitation Hardening in Ni-based Superalloys Renge Li, Zhiqiang Wang The precipitation hardening in Ni-based superalloys, which contain up to 73{\%} volume fraction of $\gamma '$, has been investigated by large scale 3D dislocation dynamics simulations. Dislocations glide under external stress across a {\{}111{\}} plane of $\gamma $/ $\gamma '$ phase, intersected by cubic $\gamma '$precipitates. The critical resolved shear stress (CRSS) has been investigated for different microstructureal parameters: $\gamma '$volume fraction, anti-phase boundary (APB) energy and channel width. It is shown that the CRSS depends on the square root of the volume fraction of $\gamma '$. The CRSS is linearly proportional to the APB energy. Stuctures with a non-uniform distribution of $\gamma '_{ }$have CRSS that is 20{\%}-30{\%} smaller than a stucture with unique $\gamma '$ size corresponding to the average size of the non-uniform disribution of $\gamma '$. The fact is that the channel width is not uniform and some channel width is larger than the average channel width of a stucture with a non-uniform distribution of $\gamma '$, which makes the dislocation line easier to bend. This reveals that the channel width plays more important role than the $\gamma '$ size. When channel width decreases to about 20nm, CRSS weakly depends on the $\gamma '$size and increases dramatically. [Preview Abstract] |
Wednesday, March 23, 2011 3:06PM - 3:18PM |
T35.00004: Thermodynamics of point defects in deformable lattices Roman Groger, Libor Smejkal, Turab Lookman We develop a mean-field model that can be used to study the evolution of microstructure and the density of point defects in irradiated materials. Within this model, the lattice is viewed as an elastic template that is distorted by point defects. The stresses that each defect exerts on its immediate neighborhood in the lattice are represented by its elastic dipole tensor. The lattice responds to these stresses by developing long-range strains that mediate interactions between spatially separated defects. Nonlocal (gradient) elasticity is used to describe the elastic strain energy of the distorted lattice. This gives rise to the gradients of strain in the free energy and ensures an accurate representation of the phonon dispersion curves. In order to demonstrate this model, we consider a cubic lattice with a given density of randomly distributed vacancies and $\langle 100 \rangle$ split interstitials (dumbbells). The occupation of each cell is described by a ``spin'' with the states \{ideal lattice, vacancy, and the three orientations of the $\langle 100 \rangle$ dumbbell\}. The evolution of this spin field is obtained by the Monte Carlo (Metropolis) method, with the free energy calculated for each state of the system as described above. Double spin-flip mechanism is adopted to conserve the total mass of the system. [Preview Abstract] |
Wednesday, March 23, 2011 3:18PM - 3:30PM |
T35.00005: Continuum dislocation dynamics: comparison between models Woosong Choi, Yong Chen, Stefanos Papanikolaou, James Sethna Many continuum theories of dislocation dynamics have been proposed to bridge the gap in between discrete microscopic simulations and macroscale phenomenology. As of yet, however, these theories had limited success in explaining or predicting the physics of microstructure formation and evolution. Recently, we have shown that a simple isotropic continuum model dynamically form walls\footnote{S. Limkumnerd and J. P. Sethna, Phys. Rev. Lett. \textbf{96}, 095503 (2006)} and exhibit complicated microstructure formation and evolution\footnote{Y. S. Chen, W. Choi, S. Papanikolaou, and J. P. Sethna, Phys. Rev. Lett. \textbf{105}, 105501 (2010)} similar to experiments. Most other continuum theories have not seen such structures emerging, and to what extent this theory explains the physics remains to be answered. We explore several variants of the current theories which have different microscopic physics as to how slip systems, cross-slip, statistically stored dislocations, explicit or effective short range interactions, etc. are treated. Comparisons among simulation results of these models will be presented, and we will discuss the relevant mechanisms and their consequences in the dynamics of microstructures. [Preview Abstract] |
Wednesday, March 23, 2011 3:30PM - 3:42PM |
T35.00006: Quantum Monte Carlo calculations of defects in aluminum Randolph Q. Hood, Paul R.C. Kent, Fernando A. Reboredo We use first-principles fixed-node diffusion quantum Monte Carlo to calculate the energetics of point defects in bulk FCC aluminum demonstrating a very high accuracy when compared to experiment. Aluminum has been well studied experimentally as a ``simple'' metal prototype for investigating the effects of radiation damage such as void formation and helium embrittlement. Often accuracies at the level of milli-electronvolts are required, which is not achieved even for the simple case of pairs of vacancies in aluminum, using common density functionals. Perhaps surprisingly, even single vacancy energies are not reliable in many simple structural materials. Also presented are results for the bulk properties of aluminum - the equilibrium lattice constant, the cohesive energy, and the bulk modulus. These calculations bring a new level of rigor to the study of defects in metals. [Preview Abstract] |
Wednesday, March 23, 2011 3:42PM - 3:54PM |
T35.00007: Molecular dynamics simulation of ablation and spallation in nickel films irradiated by ultra-short laser pulses Brian Demaske, Vasily Zhakhovsky, Nail Inogamov, Carter White, Ivan Oleynik Ablation and spallation of micron-sized Ni films irradiated by ultra-short laser pulses were investigated via large-scale molecular dynamics simulations. The interatomic interactions are described by a new embedded atom method potential that was specifically developed to accurately simulate response of Ni to strong compression and tensile waves as well as to high temperatures. It was shown that ablation results from cavitation within strongly stretched molten layer beneath the surface of the Ni film. Owing to a superposition of tensile waves, ablation threshold fluence is an increasing function of film thickness, which asymptotically approaches the experimental value for micron-sized films. Processes of wave-breaking and formation of ultra-short shock waves were also investigated in detail. Fluence threshold for onset of spallation at the rear of the film and spall strength of solid Ni subjected to ultrahigh strain rates were predicted. [Preview Abstract] |
Wednesday, March 23, 2011 3:54PM - 4:06PM |
T35.00008: The thresholds of twinning in \textit{bcc} tantalum Kyle Caspersen, Robert Rudd, Mike Surh, Luis Sandoval, David Richards The dominate stress relaxation mechanism for most crystalline materials under most conditions is dislocation motion, or slip. However, materials subjected to extreme conditions (for example, conditions that arise in laser based dynamic compression experiments) can exhibit more complex stress relaxation mechanisms. Specifically, for large stress and large strain rates there is a competition between slip and phase transformations and twinning. The conditions at which phase transformations and twinning become important are not known. Therefore, here we present a molecular dynamics study of thresholds of twinning in \textit{bcc} tantalum under various temperatures, pressures, and strain rates. [Preview Abstract] |
Wednesday, March 23, 2011 4:06PM - 4:18PM |
T35.00009: Development of Phase-Field Crystal model free energy functionals based on molecular dynamics D.M. Nicholson, J.A. Dantzig, Sarma Gorti, Bala Ranhakrishnan, D.D. Johnson The Phase-Field Crystal (PFC) model represents the density as a continuous function, whose spatial distribution evolves in time at diffusional, rather than vibrational time scales. PFC provides a tool to study defect interactions at the atomistic level but over longer time scales than those achievable with MD. We examine the behavior of the PFC model with the goal of relating the PFC parameters to physical parameters for Fe and Mo, derived from molecular dynamics (MD) simulations (using either classical force fields or on density-functioanl-theory-based Hellmann-Feynman forces). MD and PFC results for diffusion rates, energy and volumes of fusion, and melting points as a function ofvacancy concentration are used to validate free energy functionals used in the PFC model. Acknowledgments: This work was supported by the Center for Defect Physics, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences. [Preview Abstract] |
Wednesday, March 23, 2011 4:18PM - 4:30PM |
T35.00010: Terminating Surface Electromigration at the Source Kirk Bevan, Wenguang Zhu, Hong Guo, Zhenyu Zhang Through an extensive search across the periodic table utilizing first-principles density functional theory, we have established a general elemental trend for determining electromigration inhibiting impurities on the technologically important Cu(111) surface -- the dominant diffusion pathway in modern nanoelectronics interconnects. Unrecognized thus far, such inhibitors are characterized by energetically favoring (and binding strongly at) the kink sites of step edges. These properties are determined to generally reside in elements that form strong covalent bonds with substrate metal atoms. This finding sheds new light on the possibility of halting surface electromigration via kink blocking impurities. [Preview Abstract] |
Wednesday, March 23, 2011 4:30PM - 4:42PM |
T35.00011: Coverage Dependent Collective Diffusivity of Dense Pb Wetting Layer on Si(111) Li Huang, Cai-Zhuang Wang, Maozhi Li, Kai-Ming Ho The anomalous mass transport in the Pb wetting layer on Si(111) surface observed in recent experiments is studied using dynamical calculations of a generalized Frankel-Kontorova model. Instead of typical random-type diffusion, a novel collective liquid-like motion of the Pb atoms within the dense wetting layers is revealed to give rise to the ultrafast kinetics of the wetting layers even at low temperatures. With this collective spreading mechanism of the dense wetting layer, a simple kinetic Monte-Carlo simulation quantitatively reproduces the experimental observations. [Preview Abstract] |
Wednesday, March 23, 2011 4:42PM - 4:54PM |
T35.00012: Atomic layer deposition of metallic cobalt Jinhee Kwon, Mark Saly, Ravi Kanjolia, Yves Chabal Metallic cobalt has rich catalytic, electronic and magnetic properties, which makes it critical to have a better control of Co thin film deposition for various applications. This work focuses on the atomic layer deposition (ALD) of cobalt using (tertiarybutylallyl)cobalttricarbonyl ($^{t}$BuAllyl)Co(CO)$_{3}$ and dimethylhydrazine (DMHy) on H-terminated Si to uncover the growth mechanisms. The first pulse of ($^{t}$BuAllyl)Co(CO)$_{3}$ reacts with surface H--Si bonds completely, forming one monolayer of metallic silicide. In situ infrared absorption spectra show that further deposition of Co is made possible only after linear carbonyl groups which remain after the first ($^{t}$BuAllyl)Co(CO)$_{3}$ pulse as the surface ligand are removed by subsequent ALD cycles. Further ALD cycles give rise to metallic Co growth through ligand exchange after a nucleation period of 8--10 cycles. The derived growth rate of cobalt is 0.6 $\pm $ 0.1 {\AA}/cycle. The resultant Co film shows low concentration of carbon and nitrogen impurities in the bulk according to X-ray photoemission spectroscopy. [Preview Abstract] |
Wednesday, March 23, 2011 4:54PM - 5:06PM |
T35.00013: Periodic Stacking Faults in Ag Films Grown on Si(111) Decorated by Atomic Chains Aaron Gray, Manami Ogawa, Hawoong Hong, Iwao Matsuda, Tai Chaing Thin films grown on a substrate decorated by a periodic array of atomic wires can exhibit unusual properties such as stacking faults and electronic topological phase transitions due to the interfacial modulation. We report a study of Ag films grown on an array of atomic In chains on Si(111). Prior STM studies have suggested an array of stacking faults in the Ag films that allow the film lattice structure to match the interfacial modulations. STM however can only probe the surface. Our work uses x-ray diffraction to elucidate the internal 3-dimensional structure of this system. The measurements are found to be best explained by a model in which the unit cell contains a single stacking fault. [Preview Abstract] |
Wednesday, March 23, 2011 5:06PM - 5:18PM |
T35.00014: Measurement of the Spectral Distribution of Low Energy Electrons emitted as a result of NVV Auger Transitions in Ag (100) S. Kalaskar, S.L. Hulbert, Q. Dong, B.R. Bartynski, A.H. Weiss Auger Photoelectron Coincidence Spectroscopy (APECS) was used to investigate the physics of the Low Energy Tail (LET) of the Auger spectrum of Ag (100) at the National Synchrotron Light Source, Brookhaven National Lab, NY. The incident photon energy was set at 180eV. The APECS spectrum contains the contributions from electrons excited by the NVV Auger transition plus a background due to true coincidences between photoemitted valence band electrons that undergo inelastic~scattering and transfer part of their energy with other valence electrons which exit the sample. A series of coincidence measurements were made with the fixed~analyzer set at energies 150,160,171.5 and 175eV. These measurements were used to obtain an estimate of the background due to the inelastically scattered valance band electrons. The estimated background was then subtracted from the NVV APECS data to obtain the spectrum of electrons emitted solely as a result of the NVV Auger transitions, which contains implications for quantitative interpretation of the Auger spectrum. [Preview Abstract] |
Wednesday, March 23, 2011 5:18PM - 5:30PM |
T35.00015: First measurements of the Low Energy Tail (LET) down to 0 eV using Auger Photoelectron Coincidence Spectroscopy (APECS) in Ag (100) and Cu (100) K. Shastry, S. Kalaskar, S.L. Hulbert, B.R. Bartynski, A.H. Weiss We present the Auger Photoelectron Coincidence Spectroscopy (APECS) measurements of Ag (100) and Cu (100) over a full range of emitted energies from 0 eV to 81eV. The measurements were successful in separating the low energy Auger lines from a large background, due to loss processes unrelated to the Auger transition. The measurements reveal a well formed Auger peak at 60 eV for Cu and an Auger peak at 40 eV for Ag accompanied by a low energy tail (LET). The LET extends to 0 eV with a broad maximum at 6eV and 10 eV in the case of Cu and Ag respectively. The integrated intensity of the LET in Cu (100) and Ag (100) were 6 and 2 times larger than that of the Auger peak itself. The origin of this LET is discussed in terms of extrinsic mechanisms in which electrons from the peak lose energy as they propagate to the sample surface, as well as intrinsic mechanisms in which multi-electron Auger processes distribute the energy gained by the filling of the core-hole to multiple valence electrons.~ [Preview Abstract] |
Session T36: Focus Session: Graphene Growth, Characterization, and Devices: Structure, Interfaces and Transfer
Sponsoring Units: DMPChair: Joost Wintterlin, Ludwig-Maximilians-Universitaet Muenchen
Room: C142
Wednesday, March 23, 2011 2:30PM - 3:06PM |
T36.00001: Rotated graphene bilayers : from independent layers to electronic localization Invited Speaker: Graphene outstanding electronic properties rely on its pristine honeycomb lattice. Interaction with the environment - substrate, other C layers- and how it affects graphene properties will form the guiding line of the talk. While some interactions might degrade graphene properties, others can open new and interesting possibilities. We focus first on graphene on SiC. The atomic and electronic structures of the interface and of the first C-layers will be discussed on the basis of ab initio calculations (VASP) and STM experiments. At variance with the Si face, the interaction with the substrate is weak on the C face so that the first C-layer already presents graphene properties. We propose a model for the interface which explains the observed rotational disorder. We then discuss the effect of a rotation between two graphene layers to show how it can lead to an effective decoupling of these layers and a linear graphene like dispersion. To tackle very small rotation angles, we developed a tight binding scheme based on ab initio calculations. Three regimes can be defined as a function of the rotation angle. In the first one (teta$>$15$^{\circ}$) the two layers are decoupled and behave like independent graphene layers. In the second one (1$<$teta$<$15$^{\circ}$) the Dirac velocity of the bilayer is renormalised with respect to the velocity of a monolayer while in the last one (teta$\sim$1$^{\circ}$ or smaller) the velocity drops to zero which results in localisation. Theses three regimes will be discussed in the light of analytical developments. Experimental evidence or discrepancies with these three regimes will be given. [Preview Abstract] |
Wednesday, March 23, 2011 3:06PM - 3:18PM |
T36.00002: Atomic Scale Transport in Graphene on Stepped SiC(0001) Surfaces Shuaihua Ji, James B. Hannon, Ruud M. Tromp, Arthur W. Ellis, Mark C. Reuter, Frances M. Ross Thermal decomposition of SiC is a promising route to wafer-scale epitaxial graphene. However, the initial SiC surface contains steps, and graphene formation induces additional steps. Here we consider how these steps affect current transport in graphene. 1-2ML graphene was grown by annealing SiC above 1300$^{\circ}$C in disilane. Low energy electron microscopy was used to determine graphene thickness, and transport through 1ML thick regions was measured by scanning tunneling potentiometry. In this technique a bias is applied between two fixed probes while a third, scanning probe measures the local electrochemical potential as well as topography. This allows us to determine the resistivity of the graphene sheet on terraces and across substrate steps. Single steps with 0.5nm height show very weak scattering. However, multiple steps of height 1.0 and 1.5nm scatter strongly, exhibiting a potential drop equivalent to $\sim $80nm and 120nm respectively of terrace graphene. Thus, step bunching is important, and steps separated by less than a few hundred nm can dominate transport through a graphene sheet. [Preview Abstract] |
Wednesday, March 23, 2011 3:18PM - 3:30PM |
T36.00003: Twisting and Interlayer Coupling of Few Layer Graphene Minghu Pan, Xiaoting Jia, Vincent Meunier, Mildred S. Dresselhaus, Jing Kong Few layer graphene (FLG) can be synthesized by chemical vapor deposition methods. Considering a graphene bilayer with a small angle rotation between the layers---a stacking defect was observed by high resolution scanning tunneling microscopy. Low-energy Van Hove singularities in twisted graphene layers are identified as two sharp peaks in the density of states by low temperature scanning tunneling spectroscopy. Electronic instabilities at the crossing of the Fermi energy with a Van Hove singularity in the density of states often lead to new phases of matter such as charge/spin density waves. We here observe the coexistence of a charge density wave (CDW) phase and a normal phase on the top graphene layer. By analyzing the Moir\'{e} pattern in a normal region, a twisting between the two layers by a relative large angle about 3.9$^{\circ}$ is identified. This implies that the interlayer coupling for twisted layers is playing a role in the formation of different electronic phases in FLG. [Preview Abstract] |
Wednesday, March 23, 2011 3:30PM - 3:42PM |
T36.00004: Electronic structure of twisted bilayer graphene with doping and under electric fields Lede Xian, Salvador Barraza-Lopez, Mei-Yin Chou Rotational stacking faults of graphene layers in epitaxial graphene are believed to electronically decouple adjacent layers, thus single-layer graphene-like behavior can be observed. In addition, the layers close to the SiC substrate are known to be electron doped. Using density functional theory and a pi-electron, highly tuned tight-binding model, we study the modifications of the band structure in rotational stack-faulted bilayer graphene induced by doping and by external electric fields. In particular, the interlayer coupling, the magnitude of the Fermi velocity, and the possible impact on charge transport will be discussed. [Preview Abstract] |
Wednesday, March 23, 2011 3:42PM - 3:54PM |
T36.00005: Imaging the first few layers of Multilayer Epitaxial Graphene grown on SiC ($000\overline 1 )$ Jeremy Hicks, M. Sprinkle, B. Zhang, A. Tejeda, A. Taleb-Ibrahimi, P. Le F\'{e}vre, F. Bertran, W.A. de Heer, E.H. Conrad Multilayer Epitaxial Graphene (MEG) grown on the C-terminated ($000\overline 1 )$ face of SiC has been shown to behave as a series of nearly independent graphene sheets, distinguishing it from few-layer graphite. We present photoemission data from MEG films of 10 {\AA} or less, finding that the first few graphene layers on top of SiC are easily visible and are n-doped in a similar fashion to graphene grown on the Si-terminated face. Combined with the characteristic diversity of rotations in MEG films, we have obtained numerous different combinations of cone doping and rotation angles, allowing us to explore a variety of phenomena associated with the graphene-SiC interface interaction. We find that, unlike similarly-doped graphene grown on the Si-terminated face, there exists no large mismatch between the conduction and valence bands. Other potential effects of the substrate are discussed, as well as efforts in modifying the graphene-SiC interface. [Preview Abstract] |
Wednesday, March 23, 2011 3:54PM - 4:06PM |
T36.00006: ABSTRACT WITHDRAWN |
Wednesday, March 23, 2011 4:06PM - 4:18PM |
T36.00007: Epitaxial graphene on SiC(0001): More than just honeycombs L. Li, Y. Qi, R.H. Rhim, G.F. Sun, M. Weinert Combing scanning tunneling microscopy using transition-metal (Fe, Cr)-coated W tips and first-principles calculations, we show that the interface of epitaxial graphene/SiC(0001) is a warped graphene layer with periodic inclusions of hexagon-pentagon-heptagon (H$_{5,6,7})$ defects [1]. These defects break the six-fold honeycomb symmetry, thereby inducing a gap and two states below E$_{F}$ near the Dirac point. Furthermore, we show that the next graphene layer assumes the perfect honeycomb lattice, but its interaction with the warped interfacial layer modifies the linear dispersion about the Dirac point, leading to parabolic dispersion and an apparent gap of $\sim $0.25 eV. These results explain recent angle-resolved photoemission and carbon core-level shift data, and resolve the long-standing issue of the interfacial structure of epitaxial graphene on SiC(0001). \\[4pt] [1] Qi et al., Phys. Rev. Lett. \textbf{105}, 085502 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 4:18PM - 4:30PM |
T36.00008: Interfacial Structures of Graphene on 4H- SiC Substrates: SCED-LCAO Molecular Dynamics Ming Yu, Sean Fancher, Joseph H. Butera, C.S. Jayanthi, S.Y. Wu The purpose of this work is to obtain a microscopic understanding of the interface between the graphene and Si-terminated as well as C-terminated 4H-SiC substrates by studying several cases of nearly commensurate overplayed structures. Relative energies of these different structures are calculated using the SCED-LCAO method [PRB \textbf{74}, 15540; PHYSE \textbf{42},1] to gain insight into the role played by the lattice mismatch in releasing the strain and thus lowering the energy of the system. Further insight into the interfacial properties is obtained by analyzing the local strain in terms local atomic and bonding arrangements [PRB \textbf{59}, 7745] which will be correlated to the lattice mismatch. Our results will be compared with current experimental [PRL \textbf{100}, 176802; PRB \textbf{77}, 155303; J. Phys: Condens Matter \textbf{21}, 134016; PRB \textbf{78}, 205424; J. Phys: Condens Matter \textbf{20}, 323202] and theoretical [PRL \textbf{99}, 076802; PRL \textbf{99}, 126805; PRB \textbf{77}, 235412; PRL \textbf{100}, 176802] results. [Preview Abstract] |
Wednesday, March 23, 2011 4:30PM - 4:42PM |
T36.00009: Epitaxial graphene on SiC(0001): It takes a Si jump G.F. Sun, Y. Liu, S.H. Rhim, J.F. Jia, Q.K. Xue, M. Weinert, L. Li Using scanning tunneling microscopy with transition metal (Fe, Cr)-coated W tips and first-principles calculations, we have recently shown that interface of epitaxial graphene/SiC(0001) is a warped graphene layer with periodic inclusions of hexagon-pentagon-heptagon (H$_{5,6,7})$ defects that break the six-fold honeycomb symmetry [1]. Here we show that this unique structure facilitate a novel pathway for the disposal of Si during growth: the diffusion of Si vertically through the warped interfacial layer via a series of configurations that involve the dissociation and formation of C-C and Si-C bonds within the pentagon and heptagon of the H$_{5,6,7}$ complex. The calculated energy barrier for this diffusion path is 4.7 eV. These results and their implications on the self-limiting growth of epitaxial graphene on SiC(0001) will be presented at the meeting. \\[4pt] [1] Qi et al., Phys. Rev. Lett. \textbf{105}, 085502 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 4:42PM - 4:54PM |
T36.00010: Inhomogeneous strain fields in epitaxial graphene Diedrich A. Schmidt, Taisuke Ohta, Laura B. Biedermann, Thomas E. Beechem, Stephen W. Howell, Gary L. Kellogg We report a large, inhomogeneous in-plane compressive strain (up to 0.5{\%}) and its local variation at micrometer length scales in single layer graphene films on silicon carbide (SiC) (0001). The strain, due to the difference in lattice constants and thermal expansion coefficients of graphene and SiC substrate, is probed using Raman scattering and low energy electron diffraction. We show that both the growth mechanism and the relaxation along the mismatched symmetry of the graphene and underlying substrate can affect the exact amount of local strain. The large compressive strain implies that monolayer graphene is tightly grafted to the underlying interface layer and SiC substrate; otherwise it would delaminate to relieve the strain. The magnitudes of the structural strain and its local variation are significant and need to be taken into account for electronics applications of the graphene-SiC(0001) system. [Preview Abstract] |
Wednesday, March 23, 2011 4:54PM - 5:06PM |
T36.00011: Direct Printing of Graphene onto Plastic Substrates. Daniel Hines, Evgeniya Lock, Scott Walton, Mira Baraket, Matthew Laskoski, Shawn Mulvaney, Paul Sheehan, Woo Lee, Jeremy Robinson Graphene films have been synthesized on metal foils using CVD growth and have the potential to be compatible with roll-to-roll printing. To be usable in electronic devices, these films need to be removed from the metallic substrate. Currently this is accomplished by spin coating a polymer film over the graphene and chemically etching away the metal substrate. We have developed a direct printing method that allows graphene films to be printed off the metal substrate onto a polymer substrate. This printing process does not generate chemical waste, is compatible with roll-to-toll processing and renders the metal foil reusable. Adhesion of the graphene film to the polymer substrate is established by attaching perfluorophenylazides (PFPA) azide linker molecules to a plasma activated polymer surface. The transfer printing was performed by placing the PFPA treated polymer surface in contact with a graphene covered Cu foil and heating under pressure. Graphene films successfully printed onto a polystyrene substrate have been characterized by Raman spectroscopy and electrical measurements revealed the presence of Gr on the polymer surface. Details of the printing process along with characteristics of the graphene film after printing will be presented. [Preview Abstract] |
Wednesday, March 23, 2011 5:06PM - 5:18PM |
T36.00012: Improved methods of transfer of graphene from growth substrate to other surfaces and devices Yujie Ren, Huifeng Li, Weiwei Cai, Shanshan Chen, Richard Piner, Rodney Ruoff Transfer of graphene films from the growth substrate to other surfaces has turned out to be one of the important challenges to creating graphene based devices. In this talk we will review new techniques which we are developing to meet these challenges. In addition to describing the best technique we have to date, we will show data demonstrating the effectiveness of these techniques. Our analysis includes scanning micro-Raman spectroscopy, electronic measurements with FET devices created with our techniques and other microscopic techniques. FET measurements indicate a strong influence of transfer technique on the doping of the device. [Preview Abstract] |
Session T37: Focus Session: Graphene Structure, Dopants, and Defects: Nanoribbons
Sponsoring Units: DMPChair: Yong P. Chen, Purdue University
Room: C146
Wednesday, March 23, 2011 2:30PM - 3:06PM |
T37.00001: Atomically Precise Bottom-up Fabrication of Graphene Nanoribbons Invited Speaker: Graphene nanoribbons (GNRs) -- narrow stripes of graphene -- are predicted to exhibit remarkable properties making them suitable for future electronic applications. Contrary to their two-dimensional (2D) parent material graphene, which exhibits semimetallic behavior, GNRs with widths smaller than 10 nm are predicted to be semiconductors due to quantum confinement and edge effects. Despite significant advances in GNR fabrication using chemical, sonochemical and lithographic methods as well as recent reports on the successful unzipping of carbon nanotubes into GNRs, the production of sub-10 nm GNRs with chemical precision remains a major challenge. In this talk, we will present a simple GNR fabrication method that allows for the production of atomically precise GNRs of different topologies and widths [1]. Our bottom-up approach consists in the surface-assisted coupling of suitably designed molecular precursors into linear polyphenylenes and their subsequent cyclodehydrogenation, and results in GNRs whose topology, width and edge periphery are defined by the precursor monomers. By means of STM and Raman characterization, we demonstrate that this fabrication process allows for the atomically precise fabrication of complex GNR topologies. Furthermore, we have developed a reliable procedure to transfer GNRs fabricated on metal surfaces onto other substrates. It will for example be shown that millimeter sized sheets of crosslinked GNRs can be transferred onto silicon wafers, making them available for further processing, e.g. by lithography, prototype device fabrication and characterization. \\[4pt] Coauthors: Pascal Ruffieux, Rached Jaafar, Marco Bieri, Thomas Braun, and Stephan Blankenburg, Empa, Swiss Federal Laboratories for Materials Science and Technology, 3602 Thun and 8600 D\"ubendorf, Switzerland; Matthias Muoth, ETH Zurich, Department of Mechanical and Process Engineering, 8092 Zurich, Switzerland; Ari P. Seitsonen, University of Zurich, Physical Chemistry Institute, 8057 Zurich, Switzerland; Moussa Saleh, Max Planck Institute for Polymer Research, 55124 Mainz, Germany; Ivan Shorubalko, Empa, Swiss Federal Laboratories for Materials Science and Technology, 3602 Thun and 8600 D\"ubendorf, Switzerland; Shuping Pang, Xinliang Feng, and Klaus M\"ullen, Max Planck Institute for Polymer Research, 55124 Mainz, Germany; and Roman Fasel, Empa, Swiss Federal Laboratories for Materials Science and Technology, 3602 Thun and 8600 D\"ubendorf, Switzerland and University of Bern, Department of Chemistry and Biochemistry, 3012 Bern, Switzerland. \\[4pt] [1] J. Cai \textit{et.al}, Nature \textbf{466,} 470-473 (2010) [Preview Abstract] |
Wednesday, March 23, 2011 3:06PM - 3:18PM |
T37.00002: Templated graphene nanoribbon growth on SiC Ming Ruan, Mike Sprinkle, Yike Hu, John Hankinson, Miguel Rubio-Roy, Baiqian Zhang, Rui Dong, Zelei Guo, Claire Berger, Walt de Heer We demonstrate a photo-lithography fabrication method of graphene nanoribbon. Epitaxial graphene is grown selectively on SiC (1-10n) facets. For this, SiC is patterned to define 3-dimensional structures. Epitaxial graphene nanoribbons grow preferentially on the exposed sidewalls recrystallized facets that avoids post-processing lithography damage of graphene ribbons. Graphene ribbons narrower than 30nm were produced with this method and all-graphene interconnected structures are fabricated. Metal contacts are evaporated on large graphene areas seamlessly connected to nanoribbons. Transport measurement shows gap opening and high mobility. SiC crystal faceting was also explored. Low index crystal facets where found to be energetically favored. [Preview Abstract] |
Wednesday, March 23, 2011 3:18PM - 3:30PM |
T37.00003: Influence of size effects and substrate morphology on the conductance of epitaxial graphene nanoribbons Sarah Bryan, Yinxiao Yang, Raghu Murali To utilize graphene's superior electrical properties and achieve transistor operation comparable to that of silicon, the properties of graphene nanoribbons need to be better understood and optimized. Lithographically patterned nanoribbons suffer from line edge roughness which can result in a detrimental effect on the graphene conductivity. In addition to edge-induced scattering, the morphology of the silicon carbide substrate appears to have a strong effect on the line width scaling behavior. In this talk, we present experimental data that clearly shows the interplay between substrate morphology and line edge roughness in epitaxial graphene nanoribbons. Resistivity is shown to strongly increase as nanoribbon line width is reduced, although the line width at which this behavior sets in varies depending upon the substrate morphology. We also propose a model which can be used to predict the dependence of graphene nanoribbon resistivity on line width. [Preview Abstract] |
Wednesday, March 23, 2011 3:30PM - 3:42PM |
T37.00004: Large-scale production of Graphene Nanoribbons with controlled width: Electrical Properties of Graphene Nanoribbon Films Vikas Berry, Nihar Mohanty, Ashvin Nagaraja, David Moore In this talk, we will demonstrate a novel large scale production (10$^{7}$ ribbons/ sec) scheme for several microns long, smooth-edged graphene nanoribbons (GNRs) with controlled widths (from 5 -- 50 nm). We will then present detailed structural, optical and electrical properties of GNR-films $\sim $ 100 nm thick produced from 5, 15, and 45 nm wide GNRs; including their band-gap evolution and electrical transport mechanism. The high throughput method to synthesize GNR of high-quality will be a quantum leap in the graphene research. The work indents to bridge the gaps in the understanding of monodisperse-GNR film properties. [Preview Abstract] |
Wednesday, March 23, 2011 3:42PM - 3:54PM |
T37.00005: Correlated crystallographic etching of graphene and nanoribbon formation Stephen Johnson, D. Patrick Hunley, Joseph Stieha, Abhishek Sundararajan, Arunita Kar, A.T. Charlie Johnson, Douglas Strachan Catalytic etching is a promising method for constructing crystallographically defined graphene structures such as nanoribbons. Catalytic etching experiments are performed and shown to contain significant correlation yielding crystallographic graphene nanoribbons. This correlation is investigated as a function of etching conditions and compared to simulations with possible sources discussed. Supported in part by NSF Award No. DMR-0805136, the Kentucky NSF EPSCoR program, the University of Kentucky Center for Advanced Materials, and the University of Kentucky Center for Nanoscale Science and Engineering. [Preview Abstract] |
Wednesday, March 23, 2011 3:54PM - 4:06PM |
T37.00006: Aligned, ultra-long graphene nanoribbon network fabrication by nanowire etch masks Joshua Wood, Sean Sivapalan, Vincent Dorgan, Catherine Murphy, Eric Pop, Joseph Lyding Patterning semi-metallic graphene into quasi one-dimensional structures known as nanoribbons (GNRs) can open a $\sim $0.5 eV bandgap by quantum confinement [1]. To circumvent GNR lithographic difficulties, Si nanowires (NWs) were used previously as an etch mask for exfoliated graphene [2], but with no scalability or alignment control. Conversely, we transfer $\sim $1 in$^{2}$ graphene sheets off copper to silicon dioxide, giving us a template for array fabrication. We meniscus align both Au NWs ($<$w$>$=20 nm, $<$l$>$=400 nm) and Ag NWs ($<$w$>$=200 nm, $<$l$>$=10 $\mu $m), respectively, on the graphene surface. By reactive ion etch (RIE), we remove the unmasked graphene, and we etch the NWs. Based on the starting NWs, the resulting GNR arrays have lengths ranging from 200 nm to tens of microns, and widths from 10 nm to 250 nm. We create single GNRs that can span micron-separated contacts and GNR networks, similar to a graphene nanomesh. Using atomic force microscopy and Raman spectroscopy, we determine that we have monolayer GNRs with a high disorder intensity I$_{D}$/I$_{G}\sim $1, indicating rough edges and graphene grain boundaries, which are deleterious to transport. [1] K.A. Ritter and J.W. Lyding, Nat. Mat. 8, 235 (2009). [2] J. Bai et al., Nano Lett. 9, 2083 (2009). [Preview Abstract] |
Wednesday, March 23, 2011 4:06PM - 4:18PM |
T37.00007: Polymer electrolyte enhanced performance in graphene nanoribbon field-effect transistors Cheng Ling, Ming-Wei Lin, Yiyang Zhang, Xuebin Tan, Mark Ming-Cheng Cheng, Zhixian Zhou Graphene nanoribbon Field-effect transistors were fabricated from unzipped multiwall carbon nanotubes on Si/SiO2 substrate by standard electron beam lithography and metal deposition. A small drop of polymer electrolyte consisting of poly(ethylene oxide) and lithium perchlorate was applied to the graphene nanoribbon devices. Electrical transport properties of the polymer electrolyte covered devices were measured using both the Si-back-gate and polymer-electrolyte-gate configurations. We observed dramatic increase of carrier mobility, significant reduction of the peak-width of the resistance as a function of the back-gate voltage, and the shift of the charge neutrality point toward zero gate-voltage in polymer electrolyte covered graphene nanoribbon devices. These experimental results will be presented and discussed in the context of ionic and dielectric screening of charged impurities on or near the graphene nanoribbons. [Preview Abstract] |
Wednesday, March 23, 2011 4:18PM - 4:30PM |
T37.00008: Enhanced Conductance Fluctuation by Quantum Confinement Effect in Graphene Nanoribbons Guangyu Xu, Carlos Torres Jr., Jianshi Tang, Jingwei Bai, Emil Song, Yu Huang, Xiangfeng Duan, Yuegang Zhang, Wang Kang Conductance fluctuations are usually unavoidable in graphene nanoribbons (GNR) due to the presence of disorder along its edges. By measuring the low-frequency noise in GNR devices, we find that the conductance fluctuations are strongly correlated with the density-of-states of GNR [1]. In single-layer GNR, the gate-dependence of noise shows peaks whose positions quantitatively match the subband positions in the band structures of GNR. This correlation provides a robust mechanism to electrically probe the band structure of GNR, especially when the subband structures are smeared out in conductance measurement.\\[4pt] [1]. G. Xu et al. Nano Lett. 2010, 10, 4590--4594. [Preview Abstract] |
Wednesday, March 23, 2011 4:30PM - 4:42PM |
T37.00009: Doping Level Dependence of Transfer Characteristic of n-type Graphene Nanoribbon Field Effect Transistors Lu Wang, Rui Qin, Jing Zhou, Hong Li, Jiaxin Zheng, Jing Lu, Wai-Ning Mei, Shigeru Nagase By performing first principles calculations and electron transport simulations, we demonstrate that the transfer curves of graphene nanoribbon field effect transistors can be controlled by changing the concentration of potassium atoms and cobaltocene molecules doping, or nanoribbon edge carbon atoms substitution by nitrogen. We reveal that Dirac point shift downward from 0 to -12 V when the impurity concentration increase from 0 to 1.37{\%}, while the transfer curves maintain bipolar characteristics with reasonably high on/off ratios. Moreover, we observed strong charge transfer from the adsorbed atoms and molecules that facilitates n-type characteristics in graphene nanoribbons. Thus, we suggest that an effective way to achieve tunable n-type graphene nanoribbons field effects transistors is to dope them with electron donors. [Preview Abstract] |
Wednesday, March 23, 2011 4:42PM - 4:54PM |
T37.00010: The search for stable sp2 zigzag edge graphene nanoribbon termination ChengIng Chia, Vincent Crespi The zig-zag edge of a graphene ribbon has attracted much attention, since it is predicted to support a spin-polarized edge state. However, it is difficult to produce thermodynamic conditions that favor a pure sp$^2$ termination of a graphene sheet, since the edge carbons generally prefer to bond to two hydrogen atoms, in sp$^3$ hybridization, rather than one hydrogen, as sp$^2$. We examine several candidate alternative termination groups which can modify the thermodynamics of various edge configurations to favor the sp$^2$ edge termination. Ab-initio calculations demonstrate these alternative terminations can support robust edge states across a broad range of synthetic conditions. [Preview Abstract] |
Wednesday, March 23, 2011 4:54PM - 5:06PM |
T37.00011: High resolution thermal properties study of Joule self-heated graphene nanoribbon Young-Jun Yu, Melinda Y. Han, St\'ephane Berciaud, Tony F. Heinz, Louis E. Brus, Kwang S. Kim, Philip Kim We present high resolution thermal properties of Joule self-heated graphene nanoribbons (GNRs) by scanning thermal microscope (SThM) which enables local temperature survey within 100 nm spatial resolution. In order to calibrate the SThM probes, we employ the micro-Raman spectroscopy to measure the temperature distribution across a standard graphene device as a function of applied electrical power. This calibrated SThM measurement allows us to scrutinize the temperature distributions in GNRs attributed to Joule heating variation in the channel due to the locally enhanced scattering which forms hot spot formation. We also estimate the junction thermal resistance between GNR and SiO$_{2}$ substrate from the temperature distribution of the GNR devices. In addition, we will discuss simultaneous SThM and scanning Kelvin probe microscopy study of high quality exfoliated graphene on hexagonal boron nitride substrate. [Preview Abstract] |
Wednesday, March 23, 2011 5:06PM - 5:18PM |
T37.00012: Thermal transport in graphene nanoribbons: R-Matrix theory approach K.G.S.H. Gunawardana, Kieran Mullen We have developed a new theoretical tool based on R-Matrix theory to calculate phonon scattering on the atomic scale. As device sizes shrink, boundary and interface scattering have become bottlenecks to thermal transport. Therefore, calculating thermal transport considering the atomistic constitution of a device is very important. In this R-Matrix approach, we consider a finite region, which is the main scattering center of the system, connected to semi-infinite leads. We develop interior region solutions (normal modes of the finite system) and lead solutions(periodic waves with dispersion) independently that can be matched at predefined boundaries to extract the transmission probabilities of each phonon modes in the lead. In this work we demonstrate the implementation of the theory for graphene nanoribbons. [Preview Abstract] |
Wednesday, March 23, 2011 5:18PM - 5:30PM |
T37.00013: Strong suppression of thermal conductivity in edgedisordered graphene nanoribbons: Order-N methodology and thermoelectric properties Haldun Sevincli, Wu Li, Stephan Roche, Gianaurelio Cuniberti We investigate electron and phonon transport through edge disordered graphene nanoribbons. Electronic transport is calculated using Green's functions[1] while for phonons we develop an efficient linear scaling method [2-3] which is based on the Chebyshev polynomial expansion of the time evolution operator and the Lanczos tridiagonalization scheme. We show that edge disorder dramatically reduces phonon thermal transport in both armchair and zigzag ribbons, while in zigzag graphene nanoribbons edge disorder is only weakly detrimental to electronic conduction. The behavior of the electronic and phononic elastic mean free paths points to the possibility of realizing an electron-crystal coexisting with a phonon-glass. The calculated thermoelectric figure of merit (ZT) values qualify zigzag graphene nanoribbons as a promising material for thermoelectric applications. \\[4pt] [1] H. Sevin\c{c}li and G. Cuniberti Phys. Rev. B 81, 113401 (2010). [2] W. Li, H. Sevin\c{c}li, G. Cuniberti and S. Roche, Phys. Rev. B 82, 041410 (2010). [3] W. Li, H. Sevin\c{c}li, S. Roche and G. Cuniberti, arXiv:1011.1116 [Preview Abstract] |
Session T38: Focus Session: Quantum Coherence in Biology IV
Sponsoring Units: DCP DBPChair: Mohan Sarovar, University of California, Berkeley
Room: A130/131
Wednesday, March 23, 2011 2:30PM - 3:06PM |
T38.00001: Biophysics of Magnetic Orientation: Radical Pairs, Biogenic Magnetite, or both? Invited Speaker: Two major biophysical mechanisms for magnetoreception in terrestrial animals, one based on biogenic magnetite and another on radical-pair biochemical reactions, have been the subject of experiment and debate for the past 30 years. The magnetite hypothesis has stood the test of time: biogenic magnetite is synthesized biochemically in Bacteria, Protists, and numerous Animal phyla, as well as in some plants. Chains of single-domain crystals have been detected by clean-lab based SQUID magnetometry in animal tissues in all major phyla, followed by high-resolution TEM in selected model organisms, as well as by electrophysiological studies demonstrating the role of the ophthalmic branch of the trigeminal nerve in the magnetoreceptive process. Pulse-remagnetization - configured to uniquely flip the polarity of single-domain ferromagnets - has dramatic effects on the behavior of many birds, honeybees, mole rats, turtles, and bats, to cite a growing list. Magnetite-containing cells in the vicinity of these neurons in fish are now the subject of intense study by our consortium. The existence of a specialized class of magnetite-containing magnetoreceptor cells in animal tissues is no longer controversial. In contrast, less success has been achieved in gaining experimental support across a range of taxa for the radical-pair hypothesis. Although this mechanism was proposed to explain an early observation that birds would not respond to complete inversion of the magnetic vector, many organisms (even some birds) do indeed respond to the field polarity. We also note that few, if any, of these critical experiments have been done using fully double-blind methods. This is joint work with: M. M. Walker (University of Auckland, New Zealand) and M. Winklhofer (LMU Munich, Germany). [Preview Abstract] |
Wednesday, March 23, 2011 3:06PM - 3:42PM |
T38.00002: Magnetic compasses in biological systems: Does quantum physics play a role? Invited Speaker: One hypothesis of the process underlying the magnetic compass of animals surmises that the magnetic field is perceived by its effect on the coherent spin evolution within a non-equilibrium photochemical radical pair reaction. If this hypothesis were proven, it would be a dramatic demonstration of a quantum process with clear biological significance. We will review the physics of the radical pair mechanism and the current state of evidence supporting it. Experimentally, we will focus on the use radio-frequency magnetic fields to affect a radical-pair based mechanism in birds and discuss the approach and its limitations. Theoretically, we will focus on the question of how one should design a radical pair to be optimally sensitive to the direction of a weak magnetic field. Regardless of whether or not a radical pair mechanism is indeed used by birds or other animals, optimal design features could be used to manufacture biologically inspired, but man-made magnetic compass systems. [Preview Abstract] |
Wednesday, March 23, 2011 3:42PM - 3:54PM |
T38.00003: Quantum Control and Entanglement in a Chemical Compass Gian Giacomo Guerreschi, Jianming Cai, Hans J. Briegel The radical-pair mechanism is one of the two main hypotheses to explain the navigability of animals in weak magnetic fields, enabling, e.g., birds to see Earth's magnetic field. It also plays an essential role in spin chemistry. Here, we show how quantum control can be used to either enhance or reduce the performance of such a chemical compass, providing a new route to further study the radical-pair mechanism and its applications. We study the role of radical-pair entanglement in this mechanism, and demonstrate its intriguing connections with the magnetic-field sensitivity of the compass. Beyond their immediate application to the radical-pair mechanism, these results also demonstrate how state-of-the-art quantum technologies could potentially be used to probe and control biological functions. [Preview Abstract] |
Wednesday, March 23, 2011 3:54PM - 4:06PM |
T38.00004: A Biochemical Double Slit Iannis Kominis Radical-ion-pair reactions, fundamental in photosynthesis and at the basis of the avian magnetic compass mechanism, have been recently shown to offer a rich playground for applying methods and concepts from quantum measurement/quantum information science. We will demonstrate that radical-ion-pair reactions are almost the exact analog of the optical double slit experiment, i.e. Nature has already engineered biochemical reactions performing the act of quantum interference. We will further elaborate on the non-trivial quantum effects pertaining in these reactions and the recent debate on their fundamental theoretical description that these effects have sparked. [Preview Abstract] |
Session T39: Computational Molecular Biophysics
Sponsoring Units: DBPChair: Peter Hugo Nelson, Benedictine University
Room: A124/127
Wednesday, March 23, 2011 2:30PM - 2:42PM |
T39.00001: Simulation of pH-dependent Behavior of Liposome Rejwan Ali Optimized liposome for biomedical delivery applications has been a field of vigorous research for past few decades. While experimental techniques of fluorescence spectroscopy, differential scanning calorimetry and dynamic light scattering report physical suitabilities in several applications of liposomes, molecular dynamics simulation can provide more detailed feature at atomistic level for such biophysical systems. In recent times, experimental results of liposome's physical properties in different pH environment have widely been reported. The system draws interest for potential applications in several biomedical areas. We will present our molecular simulation results for such system highlighting the effect of pH on hydrogen bonding as well as correlation of dynamics to observed phase behavior. [Preview Abstract] |
Wednesday, March 23, 2011 2:42PM - 2:54PM |
T39.00002: \textit{In silico} investigation of molecular effects caused by missense mutations in creatine transporter protein Zhe Zhang, Charles Schwatz, Emil Alexov Creatine transporter (CT) protein, which is encoded by SLC6A8 gene, is essential for taking up the creatine in the cell, which in turn plays a key role in the spatial and temporal maintenance of energy in skeletal and cardiac muscle cells. It was shown that some missense mutations in CT cause mental retardation, while others are harmless non-synonymous single nucleoside polymorphism (nsSNP). Currently fifteen missense mutations in CT are known, among which twelve are disease-causing. Sequence analysis reveals that there is no clear trend distinguishing disease-causing from harmless missense mutations. Because of that, we built 3D model of the CT using highly homologous template and use the model to investigate the effects of mutations of CT stability and hydrogen bond network. It is demonstrated that disease-causing mutations affect the folding free energy and ionization states of titratable group in much greater extend as compared with harmless mutations. [Preview Abstract] |
Wednesday, March 23, 2011 2:54PM - 3:06PM |
T39.00003: Investigating the mechanism of DNA bulk hybridization with Forward Flux Sampling Daniel Hinckley DNA has become increasingly common as a building block for constructing nanomaterials. However, the mechanism by which DNA hybridization occurs is largely unknown, even in the bulk. Previous work using Transition Path Sampling and a coarse grain DNA model has shown that DNA bulk hybridization occurs via a ``slithering'' mechanism for repetitive sequences and a distinct nucleation event for random sequences. This mechanistic description remains somewhat incomplete as only configurations within the general vicinity of the transition state ensemble have been examined. In this work, we use Forward Flux Sampling and Langevin Dynamics to examine configurations along the entire transition pathway. We find that, for random sequences, barriers to hybridization arise at certain points in the hybridization pathway requiring reorientation of the two strands. Such barriers are not as pronounced for repetitive sequences where rearrangement occurs without the large scale disruption of hydrogen bonding. The formalism which we use has allowed us to calculate reaction constants for hybridization that are consistent with experiments. It has also allowed us to explain the precipitous decay of reaction rates that is observed when molecular weight is increased. [Preview Abstract] |
Wednesday, March 23, 2011 3:06PM - 3:18PM |
T39.00004: A backbone based protein model with explicit solvent Sumit Sharma, Sergey Buldyrev, Peter J. Rossky, H. Eugene Stanley, Pablo G. Debenedetti, C. Austen Angell, Sanat K. Kumar The computational expense of folding atomistically detailed protein models is prohibitive. Hence minimalist models of proteins are a popular choice. The minimalist models developed so far have excluded water, and treated the hydrophobic effect as an effective attraction between hydrophobic monomers. This simplified treatment does not capture the temperature-dependent variations in entropy and enthalpy of water molecules. Proteins have a predominantly water-screened hydrophobic core and water-exposed polar groups. This structural feature should alter the dynamics of proteins and surrounding water from that of a hydrophobic homopolymer in water. To include these features in a minimalist model, we designed heteropolymers of polar and hydrophobic monomers in explicit water-like medium. The polar monomers and water molecules were modeled with the Jagla potential, which has been shown to reproduce many water-like thermodynamic properties, and the hydrophobic monomers as hard spheres. We discuss a methodology for optimizing the sequence of these heteropolymers and how the hydrophobic collapse of these heteropolymers differs from that of a random heteropolymer. [Preview Abstract] |
Wednesday, March 23, 2011 3:18PM - 3:30PM |
T39.00005: Connecting ion channel simulations to experiment Peter Hugo Nelson A simple theoretical framework is used to connect MD simulation with ion channel permeation experiments. MD simulations of potassium channels typically exhibit at least two stable selectivity filter states, one with double occupancy and another with triple occupancy. In the association/dissociation (A/D) model, transitions between these two states occur via concerted motion of all three ions in a shunt-on shunt-off mechanism that is consistent with a large group of published MD simulations. This is the simplest model that explains the universal saturating behavior observed experimentally for many ion channels. Published permeation experiments through the MaxiK channel over a wide range of concentrations and positive voltages are shown to be remarkably consistent with the predictions of this model. Published MD simulations of the Kv1.2 potassium channel exhibit an extended shunt-on shunt-off mechanism at one end of the selectivity filter and a pop-off pop-on mechanism at the other end. This two-step mechanism is incorporated into an asymmetric variant of the A/D model that successfully explains published permeation data through the Shaker potassium channel at physiological concentrations, and successfully predicts qualitative changes in the negative current-voltage data (including a transition to super-Ohmic behavior) based solely on a fit to positive voltage data (that appear linear). Support from NSF 0836833 is gratefully acknowledged. [Preview Abstract] |
Wednesday, March 23, 2011 3:30PM - 3:42PM |
T39.00006: On the Minimum Energy Path to Membrane Pore Formation Christina Ting, Zhen-Gang Wang Several experimental methods have been developed to study the mechanical response of vesicles under an applied tension. Of particular note are the micropipette aspiration techniques and the use of a viscous solution to extend the lifetime of pores. MD simulations have also been used to study the energetic and structural properties of these transient pores on a molecular level. However, they often require extremely high tensions beyond the regime where pore formation is a thermally-activated event. We approach the nucleation problem by combining the string method with dynamic self-consistent field (DSCF) theory to obtain the full minimum energy path (MEP) to pore formation for a range of surface tensions $\gamma$. We compare our results with classical nucleation theory (CNT). Near the coexistence ($\gamma \rightarrow 0$) the rim of the pore is well-defined and the line tension is well approximated by the macroscopic definition given by CNT. However, when the free energy barrier is within $\sim 10~\rm kT$, the transition state is somewhere between a stalk-like structure and a thinned membrane leading to a hole that is partially exposed to solvents. These molecular rearrangements involved in the formation of a pore are not captured by CNT. [Preview Abstract] |
Wednesday, March 23, 2011 3:42PM - 3:54PM |
T39.00007: Capturing electrostatic interactions explicitly with the 3SPN model for DNA Gordon S. Freeman, Daniel M. Hinckley, Juan J. de Pablo The ``Three Sites Per Nucleotide'' (3SPN) model for nucleic acid simulation provides a powerful tool for computational studies of biological phenomena. Previously, this model has relied on an implicit representation of the surrounding ionic environment at the level of Debye-H\"{u}ckel theory. In this work, we eliminate this limitation and implement an explicit representation of ions, both monovalent and divalent. The coarse-grain ion-ion and ion-phosphate$_{DNA}$ potential is adapted after the model of Lenart \textit{et al.} and parameterized to reproduce the key features in the local structure and organization of ions in the bulk and in the presence of DNA. The parameters of the previous generation of 3SPN (3SPN.1) have been modified to reproduce melting temperatures observed experimentally employing a biased parallel tempering scheme. The resulting model is capable of reproducing the local structure observed in fully detailed atomistic simulations as well as the melting temperature of DNA reported experimentally for a range of DNA oligonucleotide lengths, CG-content, Na$^{+}$ concentration and Mg$^{2+}$ concentration. The usefulness of the model is demonstrated in the context of confinement of dsDNA within a viral capsid and the exploration of pathways between dehybridized and hybridized DNA. [Preview Abstract] |
Wednesday, March 23, 2011 3:54PM - 4:06PM |
T39.00008: Intrinsic noise in stochastic models of gene expression with molecular memory and bursting Tao Jia, Rahul V. Kulkarni Regulation of intrinsic noise in gene expression is essential for many cellular functions. Correspondingly, there is considerable interest in understanding how different molecular mechanisms of gene expression impact variations in protein levels across a population of cells. In this work, we analyze a stochastic model of bursty gene expression which considers general waiting-time distributions governing arrival and decay of proteins. By mapping the system to models analyzed in queueing theory, we derive analytical expressions for the noise in steady-state protein distributions. The derived results extend previous work by including the effects of arbitrary probability distributions representing the effects of molecular memory and bursting. The analytical expressions obtained provide insight into the role of transcriptional, post-transcriptional and post-translational mechanisms in controlling the noise in gene expression. [Preview Abstract] |
Wednesday, March 23, 2011 4:06PM - 4:18PM |
T39.00009: First-Principles Study of Muon Trapping in Singlet and Triplet States of Oxyhemoglobin S.R. Badu, Achana Dubey, Lee Chow, R.H. Pink, R.H. Scheicher, K. Nagamine, N. Sahoo, T.P. Das Observation of muon spin-lattice relaxation effects in Oxyhemoglobin by the muon-spin rotation ($\mu $SR) technique [1] has sparked current interest in the possibility of magnetic character in Oxyhemoglobin (OxyHb). First-Principles variational Hartree-Fock Many Body Perturbation Theory (VHFMBPT) technique investigations on the singlet and triplet states of pure (OxyHb) have shown [2] that the triplet state is considerably higher than the singlet state ruling out magnetic character. However the charge distribution obtained by the VHFMBPT procedure in both states show a number of sites that have negative charges where the trapping of muon is being investigated to examine if the energy gap in the ordering of singlet and triplet states can be reduced or reversed leading to magnetic effects. Other possible sources of magnetism in Oxyhemoglobin will also be discussed. [1] K. Nagamine et al. Proc. Japan. Acad. B-Physics 83, 120 (2007); [2] S.R. Badu et al. Reported at Third Joint HFI-NQI International Conference, CERN, Geneva, September 2010. [Preview Abstract] |
Wednesday, March 23, 2011 4:18PM - 4:30PM |
T39.00010: Binding-rebinding dynamics of proteins interacting non-specifically with a long DNA molecule Azita Parsaeian, John F. Marko, Monica Olvera de la Cruz Protein interactions with DNA chains and/or fibers regulate a large number of cell functions, and are also important in the understanding of experiments that reveal biochemical and physical cell processes. In order to determine the time range and length range of interactions between proteins and DNA, we analyze the adsorption and de-sorption of units (proteins) that bind reversibly to linear chains (DNA fibers) via non specific interactions through Monte Carlo simulations. We assume the particles are random walkers and that bind reversibly to stretched DNA fiber. In particular we determine the number of re-bindings events. We find that the number of protein re-bindings have a logarithmic dependence on DNA fiber length. [Preview Abstract] |
Wednesday, March 23, 2011 4:30PM - 4:42PM |
T39.00011: Regulation of gene expression by small RNAs via coupled stoichiometric degradation: a variational approach Thierry Platini, Tao Jia, Rahul V. Kulkarni Regulatory genes called small RNAs (sRNAs) are known to play critical roles in cellular responses to changing environments. For several bacterial sRNAs, regulation is effected by coupled stoichiometric degradation with messenger RNAs (mRNAs). The nonlinearity inherent in this regulatory scheme implies that exact analytical solutions for the corresponding stochastic models are intractable. Based on the mapping of the master equation to a quantum evolution equation, we use the variational method (introduced by Eyink) to analyze a well-studied stochastic model for regulation by sRNAs. Results from the variational ansatz are in excellent agreement with stochastic simulations for a wide range of parameters, including regions of parameter space where mean-field approaches break down. The results derived provide new insights into sRNA-based regulation and will serve as useful inputs for future studies focusing on the interplay of stochastic gene expression and regulation by sRNAs. [Preview Abstract] |
Wednesday, March 23, 2011 4:42PM - 4:54PM |
T39.00012: Spatial gradients of Ran-GTP-importin-$\beta$ complex around chromosomes in a cell of spheroidal shape Guillermo Ramirez-Santiago, Gerardo Sosa The concept of signaling gradients of a diffusible and slowly degraded chemical plays an important role in the description of cell signal transduction. It has been suggested that the generation of spatial gradients around chromosomes of the complex, Ran-GTP-importin-$\beta$, promotes microtubule nucleation and growth of the mitotic-spindle in Xenopus egg extracts. Here we solved the appropriate reaction-diffusion equation in spheroidal coordinates, and use measured values of the diffusion coefficients and activities to find out how the magnitude of the gradients depend upon the shape and geometry of the chromatin and cytoplasm. We found that the greater the eccentricity the smaller the magnitude of the stationary gradient. When the chromatin becomes spherical the magnitude of the gradient of the complex appears to be optimized. [Preview Abstract] |
Wednesday, March 23, 2011 4:54PM - 5:06PM |
T39.00013: Kinetic simulations of tension-induced DNA strand-unpeeling transition Yuanyuan Qu, Hongxia Fu, Jie Yan Sequence- and salt- dependent kinetic simulation assuming strand-unpeeling from B-DNA using the Gellispie's stochastic kinetics simulation algorism was performed for DNA fragments of a few hundred base pairs. Similar to DNA unzipping experiments, sequence-dependent energy barriers resulted stepwise extension changes were observed during the transition. The simulations were compared with recent single-molecule studies of overstretching transition of the same DNAs occurring at around 65 pN. The results quantitatively reproduced the dynamics of overstretching transition of the same DNAs under conditions when overstretching led to strand separation, and were distinct from that when the transition led to a double-stranded overstretched DNA called ``S-DNA'' through the B-S transition pathway. We conclude that the strand separation transition pathway was a strand-unpeeling transition from the two free ends of DNA. Further, our results suggest that the B-S transition pathway does not involve base-pair separation. [Preview Abstract] |
Wednesday, March 23, 2011 5:06PM - 5:18PM |
T39.00014: Determination of NMR chemical shifts for cholesterol crystals from first-principles Emine Kucukbenli, Stefano de Gironcoli Solid State Nuclear Magnetic Resonance (NMR) is a powerful tool in crystallography when combined with theoretical predictions. So far, empirical calculations of spectra have been employed for an unambiguous identification. However, many complex systems are outside the scope of these methods. Our implementation of ultrasoft and projector augmented wave pseudopotentials within \textit{ab initio} gauge including projector augmented plane wave (GIPAW) method in Quantum Espresso simulation package allows affordable calculations of NMR spectra for systems of thousands of electrons. We report here the first \textit{ab initio} determination of NMR spectra for several crystal structures of cholesterol. Cholesterol crystals, the main component of human gallstones, are of interest to medical research as their structural properties can shed light on the pathologies of gallbladder. With our application we show that \textit{ab initio} calculations can be employed to aid NMR crystallography. [Preview Abstract] |
Session T40: Physics of Proteins V: Protein-Protein Interaction, and Protein Aggregation
Sponsoring Units: DBPChair: Huan-Xiang Zhou, Florida State University
Room: A122/123
Wednesday, March 23, 2011 2:30PM - 2:42PM |
T40.00001: Early aggregation studies of diabetic amyloid in solution Sadanand Singh, Juan de Pablo Islet amyloid polypeptide (IAPP, also known as amylin) is responsible for pancreatic amyloid deposits in type II diabetes. The deposits, as well as intermediates in their assembly, are cytotoxic to pancreatic $\beta $-cells and contribute to the loss of $\beta $-cell mass associated with type II diabetes. To better understand the mechanism and cause of such aggregation, molecular simulations with explicit solvent models were used to compare monomer structure and early aggregation mechanism. Using free-energy maps generated~through~a variety of novel, enhanced sampling free-energy calculation techniques, we have found that, in water, the peptide adopts three major structures. One has a small $\alpha $-helix at the N-terminus and a small $\beta $-hairpin at the other end. The second and the most stable one, is a complete $\beta $-hairpin, and the third is a random coil structure. Transition Path Sampling simulations along with reaction coordinate analysis reveal that the peptide follows a ``zipping mechanism'' in folding from $\alpha $-helical to $\beta $-hairpin state. From studies of the dimerization of monomers in water, we have found that the early aggregation proceeds by conversion of all $\alpha $-helical configurations to $\beta $-hairpins, and by two $\beta $-hairpins coming together to form a parallel $\beta $-sheet. Several aspects of the proposed mechanism have been verified by concerted 2D IR experimental measurements, thereby adding credence to the validity of our predictions. [Preview Abstract] |
Wednesday, March 23, 2011 2:42PM - 2:54PM |
T40.00002: Applied Electric Fields and the Aggregation of Highly Charged Proteins Louis Nemzer, Bret Flanders, Christopher Sorensen The abnormal aggregation of misfolded proteins is associated with the onset of Alzheimer's disease, along with other neurodegenerative disorders, and there is increasing evidence that prefibrillar clusters, rather than fully-formed amyloid plaques, are primarily responsible. Therefore, weakly invasive methods, such as dynamic light scattering, which can probe the size distribution and structure factor of early nuclei and proto-aggregate clusters, can serve an important role in understanding this process, and may lead to insights regarding future therapeutic interventions. Here we study a highly charged model protein, lysozyme, under the influence of applied AC and DC fields in an effort to evaluate general models of protein aggregation, including the coarse-grained ``patchy protein'' method of visualizing charge heterogeneity. This anisotropy in the interprotein interaction can lead to frustrated crystalline order, resulting in low density phases. Dynamic measurements of the size distribution and structure factor can reveal local ordering, hierarchical clustering, and fractal properties of the aggregates. Early results show that applied fields affect early cluster growth by modulating local protein and counterion concentrations, in addition to their influence on protein alignment. [Preview Abstract] |
Wednesday, March 23, 2011 2:54PM - 3:06PM |
T40.00003: Ion Specificity in Protein Aggregation Predicted from Diffusivity Measurements in Stable Protein Solutions Jonathan Rubin, Adriana San Miguel, Andreas Bommarius, Sven Behrens The aggregation of therapeutic proteins in solution represents a major challenge in pharmaceutical development, as the mid- and long-term stability of these proteins is crucial for their efficacy and for compliance with FDA requirements. Monitoring slow aggregation experimentally is notoriously time-consuming, yet often unavoidable, since no theory with predictive power is currently available. In the present work, diffusion and aggregation kinetics of the globular model proteins lysozyme and BSA were studied in sodium-salt solutions of different composition and ionic strength using dynamic light scattering. We find a strong correlation between the concentration dependent protein diffusivity in stable solutions and the kinetics of protein aggregation in unstable solutions of similar composition but higher salt content. Our findings suggest a fast and convenient new way to assess a protein's specific tendency to aggregate in different types of electrolytes and buffer solutions. [Preview Abstract] |
Wednesday, March 23, 2011 3:06PM - 3:18PM |
T40.00004: ABSTRACT WITHDRAWN |
Wednesday, March 23, 2011 3:18PM - 3:30PM |
T40.00005: Controlling Protein Oligomerization with Surface Curvature on the Nanoscale Marty Kurylowicz, John Dutcher We investigate the effect of surface curvature on the conformation of beta-lactoglobulin ($\beta$LG) using Single Molecule Force Spectroscopy. $\beta$LG is a model interfacial protein which stabilizes oil droplets in milk and is known to undergo structural rearrangement when adsorbed onto a surface. We reliably control nanoscale surface curvature by creating close-packed monolayers of monodisperse polystyrene (PS) nanoparticles with diameters of 20, 40, 60, 80 and 140 nm, which are stable in aqueous buffer. By adsorbing $\beta$LG onto these hydrophobic surfaces and collecting force-extension curves in the fluid phase we can compare the conformation of $\beta$LG on 5 different surface curvatures with that on a flat PS film. We demonstrate a transition from oligomeric to monomeric $\beta$LG as the surface curvature is increased. Histograms of contour length from fits to peaks in the force-extension curves show a single maximum near 30 nm for $\beta$LG adsorbed onto nanoparticles with diameters less than 80 nm. For the larger nanoparticles, the histogram approaches that observed for $\beta$LG adsorbed onto a flat PS film, with maxima indicative of $\beta$LG dimers and trimers. [Preview Abstract] |
Wednesday, March 23, 2011 3:30PM - 3:42PM |
T40.00006: Computational Analysis of $\beta $-Peptide Self-Assembly Michael McGovern $\beta -$peptides are a class of synthetic oligomers that are capable of folding in precise patterns. The wide variety of side chains that are available for insertion into $\beta $-peptide sequences along with the stability of these folded secondary structures allow precise control over the nanoscale presentation of various chemical functional groups in three dimensional space. Some $\beta $-peptides have been shown to spontaneously fold into complex supramolecular structures, and others have been shown to be effective antimicrobial agents that are believed to act by aggregating in certain types of cell membranes. However, more work is needed to understand what drives this assembly in order to design $\beta $-peptides that assemble in particular ways. Using molecular simulations, the process of $\beta $-peptide aggregation is examined in a variety of environments that allow for direct comparison to experiment. Using new simulation techniques, the structure of the aggregates formed by several $\beta $-peptides are predicted in both bulk solutions, and at interfaces. Free energy surfaces are generated using multiple geometric parameters to directly compare the favorability of different modes of aggregation. By analyzing these results, we gain an understanding of the factors that drive self-assembly and aggregation. [Preview Abstract] |
Wednesday, March 23, 2011 3:42PM - 3:54PM |
T40.00007: Electrospun Synthetic Polypeptide Nanofibrous Biomaterials Dhan Khadka, Donald Haynie Water-insoluble nanofiber mats of synthetic polypeptides of defined composition have been prepared from fibers electrospun from aqueous solution in the absence of organic co-solvents. 20-50 kDa poly(L-glutamate, L-tyrosine) 4:1 (PLGY) but not 15-50 kDa or 50-100 kDa poly(L-glutamate) was spinnable at 20-55{\%} (w/v) polymer in water. Applied voltage and needle-collector distance were crucial for spinnability. Attractive fibers were obtained at 50{\%} polymer. Fiber diameter and mat morphology have been characterized by electron microscopy. Exposure of spun fiber mats to 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC), which reacts with carboxylate, decreased fiber solubility. Fluorescein-conjugated poly(L-lysine) (FITC-PLL) but not the fluorophore alone was able bind PLGY fiber mats electrostatically, judging by fluorescence microscopy. Key advances of this work are the avoidance of an animal source of peptides and of an inorganic co-solvent to achieve polypeptide spinnability. Polypeptide fiber mats are a promising type of nano-structured biomaterial for applications in biomedicine and biotechnology. [Preview Abstract] |
Wednesday, March 23, 2011 3:54PM - 4:06PM |
T40.00008: Exploring the dewetting transition in the hydrophobic collapse of melittin Patrick Varilly, Amish J. Patel, David Chandler We present our recent results on understanding the hydrophobic collapse of melittin dimers. Melittin dimers have large, complementary hydrophobic patches, and the dimer collapse mechanism involves a dewetting transition [Liu, Huang, Zhou and Berne, \textsl{Nature} \textbf{437}, 159--162 (2005)]. As a result, melittin has become a model system for studying dewetting transitions in proteins. We apply our recently- developed tools for probing density fluctuations in water [Patel, Varilly and Chandler, \textsl{JPCB} \textbf{114}, 1632--1637 (2010)] to understand this dewetting transition in terms of free energy surfaces, their bistability and their barrier heights. We show how the hydrophobic character of melittin's tetramerization surface results in an enhanced probability of density depletion next to that surface. When two dimers come together, the density depletion is further enhanced, so that even at large separations, there is a metastable dry phase in the region between the dimers. As the dimers come together, the dry phase is stabilized and eventually the wet phase is destabilized, leading to the collapse of the dimers. We explore how mutations that have been observed to suppress the dewetting transition affect the corresponding free energy surfaces and discuss our ongoing efforts to fully map out the reaction coordinate of melittin collapse. [Preview Abstract] |
Wednesday, March 23, 2011 4:06PM - 4:18PM |
T40.00009: Effect of the ordered interfacial water layer in protein complex formation: a non-local electrostatic approach Alexander Rubinstein, Renat Sabirianov Using a non-local electrostatic approach that incorporates the short-range structure of the contacting media, we evaluated the electrostatic contribution to the energy of the complex formation of two model proteins. In this study, we have demonstrated that the existence of an low-dielectric interfacial water layer at the protein-solvent interface [1] reduces the charging energy of the proteins in the aqueous solvent, and consequently increases the electrostatic contribution to the protein binding (change in free energy upon the complex formation of two proteins). This is in contrast with the finding of the continuum electrostatic model, which suggests that electrostatic interactions are not strong enough to compensate for the unfavorable desolvation effects [2]. \\[4pt] [1] Rubinstein and Sherman, Biophys. J. 87, 1544, 2004 \\[0pt] [2] Rubinstein et al., Phys. Rev. E 82, 021915, 2010). [Preview Abstract] |
Wednesday, March 23, 2011 4:18PM - 4:30PM |
T40.00010: Modeling virus capsids and their protein binding -- the search for weak regions within the HIV capsid Otto F. Sankey, Daryn E. Benson, C. Michael Gilbert Viruses remain a threat to the health of humans worldwide with 33 million infected with HIV. Viruses are ubiquitous, infecting animals, plants, and bacteria. Each virus infects in its own unique manner making the problem seem intractable. However, some general physical steps apply to many viruses and the application of basic physical modeling can potentially have great impact. The aim of this theoretical study is to investigate the stability of the HIV viral capsid (protein shell). The structural shell can be compromised by physical probes such as pulsed laser light [1,2]. But, what are the weakest regions of the capsid so that we can begin to understand vulnerabilities of these deadly materials? The atomic structure of HIV capsids is not precisely known and we begin by describing our work to model the capsid structure. We have constructed three representative viral capsids of different CA protein number -- HIV-900, HIV-1260 and HIV-1740. The complexity of the assembly requires a course grained model to investigate protein interactions within the capsid which we will describe. \\[0pt] [1] K-T. Tsen, WS.-D. Tsen, O.F. Sankey, J.G. Kiang, Journal of Physics -- Condensed Matter, 19 472201 (2007). \\[0pt] [2] E.C. Dykeman, D.Benson, K.-T. Tsen, and O.F. Sankey, Physical Review E 80, 041909 (2009). [Preview Abstract] |
Wednesday, March 23, 2011 4:30PM - 4:42PM |
T40.00011: Low-Frequency Raman Spectroscopy of Trialanine. Rachel M. Stephenson, Angela R. Hight Walker The effect of sample conditions on the structural conformation of trialanine has been investigated with visible Raman spectroscopy. Trialanine is used here as a simple protein-mimetic system in order to more easily isolate the backbone amide vibrational modes, low-frequency tortional modes and modes from hydrogen bonding. Crystalline trialanine is known to exist with both parallel ($p$-Ala$_{3})$ and antiparallel (\textit{ap}-Ala$_{3})$ \textit{$\beta $}-sheet crystal structures, depending on the solvent composition during crystallization. The \textit{ap}-sheet form of trialanine co-crystallizes with water, which is easily removed under vacuum, offering a further opportunity to examine the effect of solvation on the vibrational spectra, especially when also compared with trialanine in solution. By collecting Raman spectra in different sample phases, and at different concentrations, pH and temperatures, the vibrational modes most sensitive to the secondary structure can be identified. The collected data will be compared to the literature, including other vibrational spectroscopic data and high-level simulations. [Preview Abstract] |
Wednesday, March 23, 2011 4:42PM - 4:54PM |
T40.00012: Fast X-ray Photon Correlation Spectroscopy measurements from the diffusion of concentrated Alpha Crystallin suspensions Vidanage Nuwan Karunaratne, Janae Debartolo, Justin Berry, Laurence Lurio, George Thurston, Suresh Narayanan, Alec Sandy, John Weizeorick Alpha Crystallin constitute up to half of the total protein found in the mammalian eye lens. It has chaperone like behavior and may play a key role in maintaining lens transparency by preventing condensation of other lens proteins. We report here Fast X-ray Photon Correlation Spectroscopy (XPCS) measurements of protein diffusion within concentrated suspensions of Alpha Crystallin. Bovine calf eye lens cortices were homogenized, centrifuged and ultra-filtered to obtain concentrated Alpha Crystallin suspensions. Diffusion of proteins within these suspensions was measured as a function of temperature. The overall observed diffusion rates imply that the proteins exist in a glassy or gel phase, even at concentrations where equivalent hard sphere system would still be liquid. We interpret these results within the context of strongly interacting proteins, with protein-protein interactions possibly mediated by subunit exchange among Alpha Crystallin oligormers. [Preview Abstract] |
Wednesday, March 23, 2011 4:54PM - 5:06PM |
T40.00013: Ultrasound and Hypersound Speeds in Lysozyme Solutions Alfons Schulte, Christian Pruner, Emmerich Wilhelm, Augustinus Asenbaum Ultrasound velocimetry and Brillouin spectroscopy provide information on the compressibility of proteins and the surrounding hydration layer. Employing both techniques we investigate the sound speeds at GHz (hypersound) and MHz (ultrasound) frequencies in lysozyme solutions (250 mg / ml, pH 7) and pure water over the temperature range from 275 K to 335 K. Compared to water the Brillouin peaks in the lysozyme solutions are shifted by about 400 MHz towards higher frequencies. This shift reflects the change in sound speed and is attributed to the influence of the compressibility of the protein and bound water in the hydration shell. In addition, we measure a dispersion of the sound velocity in the lysozyme solution. The higher sound speed at GHz frequencies, as measured by Brillouin scattering, may indicate additional relaxation processes as compared to pure bulk water, where no sound dispersion between ultrasound speed and hypersound speed is observed. [Preview Abstract] |
Session T41: Focus Session: The Role of Water in Energy Production and Utilization I
Sponsoring Units: DCPChair: Greg Kimmel, Pacific Northwest National Laboratories
Room: A115/117
Wednesday, March 23, 2011 2:30PM - 3:06PM |
T41.00001: Deciphering the morphology of ice films on metal surfaces Invited Speaker: Although extensive research has been aimed at the structure of ice films [1], questions regarding basic processes that govern film evolution remain. Recently we discovered how ice films as many as 30 molecular layers thick can be imaged with STM [2]. The observed morphology yields new insights about water-solid interactions and how they affect the structure of ice films. This talk gives an overview of this progress for crystalline ice films on Pt(111) [2-5]. STM reveals a first molecular water layer very different from bulk ice: besides the usual hexagons it also contains pentagons and heptagons [3]. Slightly thicker films ($\sim $1nm, at T$>$120K) are comprised of $\sim $3nm-high crystallites, surrounded by the one-molecule-thick wetting layer. These crystals dewet by nucleating layers on their top facets [4]. Measurements of the nucleation rate as a function of crystal height provide estimates of the energy of the ice-Pt interface. For T$>$115K surface diffusion is fast enough that surface smoothing and 2D-island ripening is observable [5]. By quantifying the T-dependent ripening of island arrays we determined the activation energy for surface self-diffusion. The shape of these 2D islands varies strongly with film thickness. We attribute this to a transition from polarized ice at the substrate towards proton disorder at larger film thicknesses. Despite fast surface diffusion ice multilayers are often far from equilibrium. For example, ice grows between $\sim $120 and $\sim $160 K in its cubic variant rather than in its equilibrium hexagonal form. We found this to be a consequence of the mismatch in the atomic Pt-step height and the ice-bilayer separation and propose a mechanism of cubic-ice formation via growth spirals around screw dislocations [2]. \\[4pt] [1] A. Hodgson and S. Haq, Surf. Sci. Rep. 64, 381 (2009). \\[0pt] [2] K. Th\"{u}rmer and N. C. Bartelt, Phys. Rev. B 77, 195425 (2008). \\[0pt] [3] S. Nie, P. J. Feibelman, N. C. Bartelt and K. Th\"{u}rmer, Phys. Rev. Lett. 105, 026102 (2010). \\[0pt] [4] K. Th\"{u}rmer and N. C. Bartelt, Phys. Rev. Lett. 100, 186101 (2008). \\[0pt] [5] S. Nie, N. C. Bartelt, and K. Th\"{u}rmer, Phys. Rev. Lett. 102, 136101 (2009). [Preview Abstract] |
Wednesday, March 23, 2011 3:06PM - 3:18PM |
T41.00002: {\it A Priori} Method for First Principles Study of Aqueous Electrochemistry: Application to Biofuels and Catalysis Kendra Letchworth Weaver, Ravishankar Sundararaman, Tomas Arias We present a novel description of water which will allow the first {\it a priori} studies of catalysis of biofuels in aqueous electrochemical environments. Our method offers a computationally efficient alternative to the thermal sampling required by molecular dynamics yet provides a more realistic description of bulk water than including explicit frozen water or traditional continuum solvation models. Into Joint Density Functional Theory (JDFT), which joins an electron density-functional for the solute with classical density-functional theories for liquid water\footnote{R. Sundararaman et al, unpublished, to be presented at the APS March Meeting (2011)} into a single variational principle for the free energy of the combined system, we introduce the innovation of an {\it a priori} form of the coupling functional between the quantum-mechanical system and liquid water based on a local density approximation to the Hohenberg-Kohn density-only functional. Without any fits to solvation data whatsoever, this new method predicts solvation energies of small organic molecules well compared to state-of-the art empirical quantum-chemical cavity approaches. The site interaction potentials produced closely resemble the widely used TIP3P site potentials for water without requiring any empirical parameters. [Preview Abstract] |
Wednesday, March 23, 2011 3:18PM - 3:30PM |
T41.00003: \emph{A priori} classical density functionals of water: toward first principles exploration of aqueous based energy systems Ravishankar Sundararaman, Kendra Letchworth Weaver, Tomas Arias The microscopic structure of inhomogeneous water plays a critical role in the properties of a wide variety of important energy systems including fuel cells and photoelectrochemical cells. Joint density functional theory has proven to be an efficient tool for the quantum-mechanical modeling of systems such as Pt electrodes in the presence of water, but requires theories for water which go beyond semi-empirical continuum solvation models, and accurate models for the coupling between water and electronic systems {\footnote{K. L. Weaver et al, to be presented at APS March Meeting 2011}}. Toward this end, we present a new density-functional description of liquid water capable of predicting interatomic correlation functions, the linear and nonlinear dielectric response, and solvation energies without empirical fit parameters. The functional itself is built upon the site-potential representation of the ideal gas, a hard sphere reference fluid for the repulsive correlations, and an equation of state that reproduces the bulk properties of water over the entire extent of its liquid phase. Hydrogen bonding, the local tetrahedral structure and orientational correlations are captured \textit{a priori} by a density-functional reformulation of the Kirkwood model for the dielectric constant. [Preview Abstract] |
Wednesday, March 23, 2011 3:30PM - 4:06PM |
T41.00004: Quasicrystal and ice phases tiled with pentagons in confined water Invited Speaker: Bulk water is known to form a wealth of ice polymorphs and two distinct amorphous phases. Less is known of the structures that confined and interfacial water can adopt, and whether there is a correspondence between the structures and phase diagrams of water in bulk and in confinement. In this talk I will present a molecular simulations study of the phase behavior of a water bilayer confined between two non-hydrogen bonding walls and demonstrate that a water bilayer also presents rich polymorphism, including an ice crystal fully tiled by pentagons and a quasicrystal, the first ever reported for water. The water quasicrystal and the ice polymorph tiled with pentagons are not templated by the confining surfaces. This indicates that these novel phases are intrinsically favored in bilayer water and suggests that they may be formed, without confinement, on surfaces. \\[4pt] [1] J.C. Johnston, N. Kastelowitz and V. Molinero, ``Liquid to quasicrystal transition in bilayer water,'' J. Chem. Phys. 133, 154516 (2010)\\[0pt] [2] N. Kastelowitz, J. C. Johnston and V. Molinero, ``The anomalously high melting temperature of bilayer ice,'' J. Chem. Phys. 132, 124511 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 4:06PM - 4:18PM |
T41.00005: The role of interfacial water in ``nano'' Alenka Luzar, Jihang Wang, Chris Daub, Dusan Bratko To understand the role of interfacial water on nanostructred surfaces is important for materials science and biology. The talk will describe some of our recent progress in predicting and understanding the effects of nanopatterning on topologically or chemically heterogeneous surfaces on wetting via \emph{in silico} experiments. [Preview Abstract] |
Wednesday, March 23, 2011 4:18PM - 4:30PM |
T41.00006: Ordering of confined water between metallic surfaces Adrien Poissier, Maria V. Fernandez-Serra It has been pointed out (PCCP 2010, Poissier et al.) that the hydrogen bonding type interaction occuring at water/metal interface makes the two type of interfacial water orderings (hydrophobic or hydrophilic overlayers) very close in energy. The most stable, hydrophobic, overlayer has very small net dipole moment perpendicular to the surface, while the least stable (in vacuum) hydrophilic interface has a large ($\approx 1.8 D$) net dipole moment. First principles molecular dynamics simulations of liquid water confined between two Pd surfaces have been performed and structural and electronic water properties have been studied in detail. We show that water confinement in this situation results in a spontaneous symmetry breaking of the system, inducing an electric field across the liquid water slab. We discuss the origin of this spontaneous polarization and show its dependence with the confinement distance along the direction perpendicular to the planes of the surfaces. [Preview Abstract] |
Wednesday, March 23, 2011 4:30PM - 5:06PM |
T41.00007: Exploring hydration at the nanoscale Invited Speaker: It is widely appreciated that water molecules contribute a critical element to the forces governing chemical processes in an aqueous environment, and the purported differences in water structure induced by the presence of confining surfaces are correspondingly likely to play a role in interfacial chemistry. The development of a detailed understanding of the organization of fluid water at the interface with real materials is therefore of great interest. In this presentation, results obtained from fully atomistic computer simulations of water in the presence of confining interfaces will be discussed, with the goal of elucidating the molecular level influence of surface character on water structure and energetics. Further, we emphasize the extension of studies to temperatures and pressures well outside the conventional realm of the ambient solvent thermodynamic state. The interface examples to be considered in this presentation emphasize systematic studies designed to elucidate guiding principles. These include extended and nanoscale hydrophobic and hydrophilic crystalline surfaces and interfaces with systematically patterned hydrophobicity. [Preview Abstract] |
Wednesday, March 23, 2011 5:06PM - 5:18PM |
T41.00008: ABSTRACT WITHDRAWN |
Wednesday, March 23, 2011 5:18PM - 5:30PM |
T41.00009: Energy transport during sessile-water-droplet evaporation Hadi Ghasemi, Charles Ward Energy transport mechanisms for a steadily evaporating water droplet maintained on Cu or Au(111) surfaces are compared. In the absence of buoyancy-driven convection, thermal conduction and thermocapillary convection are the active modes of energy transport. The dominant mode varies along the liquid-vapor interface. Although thermal conduction is the dominant mode in regions far from the contact line, thermocapillary convection is by far the larger mode of energy transport near the three-phase contact line. The latter region is where most of the droplet evaporation occurs. Evaporation experiments on both Cu and Au(111) suggest that the thermocapillary convection provides more than 92\% of the total energy required for the evaporation. [Preview Abstract] |
Session T42: Focus Session: The Physics of Evolution I
Sponsoring Units: DCP DBPChair: Eugene Shakhnovich, Harvard University
Room: A302/303
Wednesday, March 23, 2011 2:30PM - 3:06PM |
T42.00001: Insights into protein evolution landscapes from folding models Invited Speaker: Off-lattice models of protein folding were employed to investigate the origins of the evolutionary rate distributions and fitness landscapes. For each robust folder, the network of sequences that share its native structure is identified. The fitness of a sequence is a simple function of the number of misfolded molecules produced to reach a characteristic protein abundance. Fixation probabilities of mutants are computed under a simple population dynamics model, and the fold-averaged evolution rate is computed a using a Markov chain on the fold network. The distribution of the logarithm of the evolution rates exhibits a peak with a long tail on the low rate side and resembles the universal empirical distribution of the evolutionary rates more closely than either distribution resembles the log-normal distribution. We next addressed the question of the extent of determinism in protein evolution. Limited empirical studies suggest that the fitness landscapes of protein evolution are significantly smoother, or more additive, than random landscapes. However, widespread sign epistasis seems to restrict evolution to a small fraction of available trajectories, thus making the evolutionary process substantially deterministic. Access to complete fitness landscapes within the model framework enables exhaustive analysis of evolutionary trajectories. The model landscapes were compared to a continuum of artificial landscapes of varying smoothness. In maximally smooth, fully additive landscapes, evolution cannot be predicted because all paths are accessible. However, a small amount of noise can make most paths inaccessible while preserving the overall structure of the landscape. Although the model landscapes are almost additive, most paths are non-monotonic with respect to fitness, so evolutionary trajectories can be approximately predicted. Thus, protein folding physics seems to dictate the universal distribution of the evolutionary rates of protein-coding genes and the quasi-deterministic character of evolution. [Preview Abstract] |
Wednesday, March 23, 2011 3:06PM - 3:18PM |
T42.00002: A mechanism for the evolution of allosteric control in proteins Kimberly Reynolds, Rama Ranganathan Co-evolution analysis indicates a general architecture for natural proteins in which sparse networks of physically contiguous amino acids underlie basic aspects of structure and function. These networks, termed sectors, are spatially organized such that active sites are linked to many surface sites distributed throughout the protein structure. Here, we propose that sectors represent an evolutionarily conserved ``wiring'' mechanism that effectively functionalizes a large but non-random fraction of the protein surface - that is, perturbation at sector-connected surface positions should preferentially yield coupling to the active site. To test this premise, we conducted a comprehensive ``domain insertion scan,'' and show that sector-connected surface sites are statistically significant locations for the emergence of allosteric control in vivo. This finding suggests practical guidelines for the engineering of new allosteric systems, and permits description of a plausible model for the evolution of intermolecular communication and regulation. [Preview Abstract] |
Wednesday, March 23, 2011 3:18PM - 3:30PM |
T42.00003: Biophysical Aspects of Spindle Evolution Reza Farhadifar, Charlie Baer, Daniel Needleman The continual propagation of genetic material from one generation to the next is one of the most basic characteristics of all organisms. In eukaryotes, DNA is segregated into the two daughter cells by a highly dynamic, self-organizing structure called the mitotic spindle. Mitotic spindles can show remarkable variability between tissues and organisms, but there is currently little understanding of the biophysical and evolutionary basis of this diversity. We are studying how spontaneous mutations modify cell division during nematode development. By comparing the mutational variation - the raw material of evolution - with the variation present in nature, we are investigating how the mitotic spindle is shaped over the course of evolution. This combination of quantitative genetics and cellular biophysics gives insight into how the structure and dynamics of the spindle is formed through selection, drift, and biophysical constraints. [Preview Abstract] |
Wednesday, March 23, 2011 3:30PM - 4:06PM |
T42.00004: The relationship between relative solvent accessibility and evolutionary rate in protein evolution Invited Speaker: Recent work with the yeast Saccharomyces cerevisiae shows a linear relationship between the evolutionary rate of sites and the relative solvent accessibility (RSA) of the corresponding residues in the folded protein. Here, we aim to develop a mathematical model that can reproduce this linear relationship. We first demonstrate that two models that both seem reasonable choices (a simple model in which selection strength correlates with RSA and a more complex model based on RSA-dependent amino-acid distributions) fail to reproduce the observed relationship. We then develop a model based on observed site-specific amino-acid distributions and show that this model behaves appropriately. We conclude that evolutionary rates are directly linked to the distribution of amino acids at individual sites. Because of this link, any future insight into the biophysical mechanisms that determine amino-acid distributions will improve our understanding of evolutionary rates. [Preview Abstract] |
Wednesday, March 23, 2011 4:06PM - 4:18PM |
T42.00005: Quantum Darwinian Evolution Implies Tumor Origination W. Grant Cooper Quantum uncertainty limits operating on metastable amino DNA protons drive the arrangement, keto-amino ? enol-imine, which contributes to time-dependent stochastic mutations. Product enol-imine protons participate in coupled quantum oscillations at frequencies of about 1013 s-1 until ``measured by'' an evolutionarily selected quantum reader, the transcriptase. This introduces entanglement states between coherent protons and transcriptase components, which ultimately yield an ensemble of decohered, non-reequilibrated enol and imine isomers that participate in ``molecular clock'' base substitutions at G'-C' and *G-*C sites. This introduces a quantum Darwinian evolution model which (a) simulates incidence of cancer data and (b) implies insight into quantum origins of evolutionary extinction. Data identify an inherited ``genetic space,'' s, which is initially mutation-free and satisfies the inequality, 1 = s = 0.97. When accumulated stochastic mutations cause s-values to approach their evolutionarily allowed threshold limit, s 0.97 + e, age-related degenerative disease is manifested. This implies a gain in evolutionary advantage which protects the gene pool against acquiring unsafe levels of mutation. Data requiring coherent states imply that classical duplex DNA contains an embedded microphysical subset of electron lone-pairs and hydrogen bonded protons that govern time-dependent genetic specificity in terms of quantum probability laws. [Preview Abstract] |
Session T43: Physics of Bacteria
Sponsoring Units: DBPChair: Rob Phillips, California Institute of Technology
Room: A306/307
Wednesday, March 23, 2011 2:30PM - 2:42PM |
T43.00001: Guided Motion of Individual and Collective Swimmers in Funnel Arrays Cynthia Olson Reichhardt, Thuc Mai, Charles Reichhardt We generalize a model of swimming bacteria in asymmetric arrays of obstacles [1] to include different rules of motion, including various rules for collective behvaiors. For individual noninteracting swimmers, we observe guided motion and rectification by the asymmetric barriers when the particles align with the walls they contact, but we find no rectification if the particles are reflected by the walls or bounce off the walls. For collectively interacting swimmers, it is possible for the particles to form large swimming clumps that can move against the normal rectification direction of the asymmetric barrier array. In general, the rectification by the barriers is lost when the length scale of the swarms of collectively moving particles is significantly larger than the length scale of the funnel shaped barriers. A particle swarm can become trapped inside a funnel; however, individual strings of particles that follow each other can escape from the trap and move against the funnel direction. \\[4pt] [1] M.B. Wan, C.J. Olson Reichhardt, Z. Nussinov, and C. Reichhardt, Phys. Rev. Lett. 101, 018102 (2008). [Preview Abstract] |
Wednesday, March 23, 2011 2:42PM - 2:54PM |
T43.00002: Surface motility of Myxococcus Xanthus Maxsim Gibiansky, William Hu, Fan Jin, Kun Zhao, Wenyuan Shi, Gerard Wong We examine the surface motility of Myxococcus Xanthus, a bacterium species found in soil that exhibits a broad range of self-organizing behavior, including predatory ``swarms'' and survival-enhancing ``fruiting bodies.'' To quantify the effects of exopolysaccharides (EPS) on surface adhesion and motility, we use modified versions of particle tracking algorithms from colloid physics to analyze bacterial trajectories, and compare the wild type (WT) strain to EPS knockout and EPS overproducer strains. We find that EPS deficiency leads to an increase in the number of ``standing'' bacteria oriented normal to the surface, attached by one end with minimal motility. EPS overproduction, by contrast, suppresses this phenotype. A detailed investigation of the influence of EPS on Myxococcus social motility will be presented. [Preview Abstract] |
Wednesday, March 23, 2011 2:54PM - 3:06PM |
T43.00003: Dynamics of microorganisms with autochemotactic interactions Johannes Taktikos, Vasily Zaburdaev, Holger Stark Our work aims at the description of the early stage of bacterial biofilm formation. In light of this, we model bacteria as self-propelled particles that move on a surface with constant speed and whose directions of motion diffuse on the unit circle. Individual cells communicate by autochemotaxis, so they follow the gradient of a chemical which is produced by the microorganisms themselves. We investigate how the autochemotactic coupling influences the mean squared displacement of a single particle and show that the long-time dynamics is diffusive. We present theoretical predictions for the diffusion coefficient and compare them to numerical results. To incorporate the size of bacteria, we model them as disks that experience a harmonic repulsion force when they start to overlap. Our repulsion mechanism for particles in contact assumes a linear relationship between force and velocity. For such a soft model microorganism, we present numerical results on two-particle collisions and study the cluster formation in a multi-particle system. [Preview Abstract] |
Wednesday, March 23, 2011 3:06PM - 3:18PM |
T43.00004: Shear flow influences the twitching motility of \textit{Pseudomonas Aeruginosa} Yi Shen, Sigolene Lecuyer, Albert Siryaporn, Zemer Gitai, Howard Stone Twitching motility is one of the mechanisms by which bacteria can spread on surfaces and is important in the process of biofilm formation. Flow is often involved in biofilm formation, for instance when bacteria contaminate medical devices or water systems. We have studied the twitching mobility of \textit{Pseudomonas aeruginosa }in straight microfluidic channels under laminar shear flow at low Reynolds number. We tracked all the bacteria adhering and moving on the immersed glass surface. We observed that upon applying a flow, a significant fraction of bacteria started to twitch, and that many twitched upstream, opposite to the flow direction. By measuring the displacement and residence time of the bacteria staying on the surface, we found that the flow not only tuned the direction of twitching by orienting bacteria, but also that the shear rate significantly influenced the fraction of bacteria moving upstream, with an optimal shear rate about 500s$^{-1}$. [Preview Abstract] |
Wednesday, March 23, 2011 3:18PM - 3:30PM |
T43.00005: Biofilm growth: a lattice Monte Carlo model Yuguo Tao, Gary Slater Biofilms are complex colonies of bacteria that grow in contact with a wall, often in the presence of a flow. In the current work, biofilm growth is investigated using a new two-dimensional lattice Monte Carlo algorithm based on the Bond-Fluctuation Algorithm (BFA). One of the distinguishing characteristics of biofilms, the synthesis and physical properties of the extracellular polymeric substance (EPS) in which the cells are embedded, is explicitly taken into account. Cells are modelled as autonomous closed loops with well-defined mechanical and thermodynamic properties, while the EPS is modelled as flexible polymeric chains. This BFA model allows us to add biologically relevant features such as: the uptake of nutrients; cell growth, division and death; the production of EPS; cell maintenance and hibernation; the generation of waste and the impact of toxic molecules; cell mutation and evolution; cell motility. By tuning the structural, interactional and morphologic parameters of the model, the cell shapes as well as the growth and maturation of various types of biofilm colonies can be controlled. [Preview Abstract] |
Wednesday, March 23, 2011 3:30PM - 3:42PM |
T43.00006: Transitions in biofilm formation Vernita Gordon, Travis Thatcher, Benjamin Cooley Biofilms are multicellular, dynamic communities formed by interacting unicellular organisms bound to a surface. Forming a biofilm is a developmental process, characterized by sequential changes in gene expression and behavior as bacteria and yeast progress from discrete, free-swimming cells though stages that arrive at a mature biofilm. We are developing automated metrics to identify key transitions in early biofilm formation as cells attach to a surface, populate that surface, and adhere to each other to form early microcolonies. Our metrics use high-throughput tracking and analysis of microscopy movies to localize these transitions in space and time. Each of these transitions is associated with a loss of entropy in the bacterial system and, therefore, with biological activity that drives this loss of entropy. Better understanding of these transitions will allow automated determination of the strength and turn-on of attractive cell-surface and cell-cell interactions as biofilm development progresses. [Preview Abstract] |
Wednesday, March 23, 2011 3:42PM - 3:54PM |
T43.00007: Changes in the Mechanical Properties of Pseudomonas aeruginosa Bacterial Cells Induced by Antimicrobial Peptides Shun Lu, John Dutcher In our research group, we have developed an atomic force microscopy nano-creep technique [1] to study the mechanical properties of individual Pseudomonas aeruginosa bacterial cells in a liquid environment. In the present study, we have used this technique to measure changes to the mechanical properties of the cells produced by exposing the cells to well-studied antimicrobial peptides: polymyxin B (PMB) and its derivative polymyxin B nonapeptide (PMBN). We find that the creep response of cells under a fixed applied load is very different after exposure of the cells to PMB and PMBN, which is possibly due to the disruption of its outer membrane. To describe the viscoelastic properties of the cells exposed to PMB and PMBN, we found that it was necessary to use a four element spring and dashpot model, instead of the three element standard linear solid model that describes the viscoelastic properties of cells in Millipore water [1]. We also found that PMB and PMBN have qualitatively different effects on the stiffness of the cell membrane. These measurements provide a first step towards understanding the different mechanisms of action of PMB and PMBN on bacterial cells. \\[4pt] [1] V. Vadillo-Rodriguez, T. J. Beveridge, and J. R. Dutcher, J. Bacteriol., 190, 4225-4232, 2008. [Preview Abstract] |
Wednesday, March 23, 2011 3:54PM - 4:06PM |
T43.00008: Nanofabrication and Detection of Molecular Shuttles powered by Kinesin Motor Proteins Daniel Oliveira, Kim Domyoung, Mitsuo Umetsu, Tadafumi Adschiri, Winfried Teizer The intracellular cargo delivery performed by kinesin motor proteins can be biomimetically employed to engineer tailor-made artificial nanotransport systems. Kinesin (expressed on an \textit{Escherichia coli} system) and microtubules (obtained from the polymerization of tubulin proteins) were prepared and characterized. We report recent results and explore the aim of the construction of Nanoelectromechanical Systems and their potential applications, e.g. as drug delivery systems. This work was supported by the WPI Program. [Preview Abstract] |
Wednesday, March 23, 2011 4:06PM - 4:18PM |
T43.00009: Electrodynamics of Nanosystems Samina Masood We use Electrodynamics to study the nanosystems. Quantum nature of electrodynamics has been used to describe the physics of nanosystems including carbon nanotubes as well as the cellular growth. We use bacterial cell as an example and test a part of our theory on the bacterial growth experimentally. Preliminary results of these experiments are also mentioned here. [Preview Abstract] |
Wednesday, March 23, 2011 4:18PM - 4:30PM |
T43.00010: Long range electronic transport in microbial nanowires bridging an electrode and scanned probe Joshua Veazey, Sanela Lampa-Pastirk, Kathy Walsh, Jiebing Sun, Pengpeng Zhang, Gemma Reguera, Stuart Tessmer The filament-like appendages known as pili, expressed by the bacterium \textit{Geobacter sulfurreducens}, are believed to act as electrically conductive nanowires [1]. Previously, we used scanning tunneling microscopy to study the local density of states at different positions along the wire. However, the long range electron transfer believed to occur in this protein has not been directly observed. Here we discuss a system for verifying long range transport using a scanning probe technique. Transport at distances of more than a few nanometers would require a novel biological electron transfer process. \\[4pt] [1] G. Reguera, K.D. McCarthy, T. Mehta, J.S. Nicoll, M.T. Tuominen, and D.R. Lovley, Nature 435, 1098 (2005) [Preview Abstract] |
Wednesday, March 23, 2011 4:30PM - 4:42PM |
T43.00011: Heterogeneous diversity of spacers within CRISPR Michael Deem, Jiankui He Clustered regularly interspaced short palindromic repeats (CRISPR) in bacterial and archaeal DNA have recently been shown to be a new type of anti-viral immune system in these organisms. We here study the diversity of spacers in CRISPR under selective pressure. We propose a population dynamics model that explains the biological observation that the leader-proximal end of CRISPR is more diversified and the leader-distal end of CRISPR is more conserved. This result is shown to be in agreement with recent experiments. Our results show that the CRISPR spacer structure is influenced by and provides a record of the viral challenges that bacteria face. 1) J. He and M. W. Deem, Phys. Rev. Lett. 105 (2010) 128102 [Preview Abstract] |
Wednesday, March 23, 2011 4:42PM - 4:54PM |
T43.00012: Filament depolymerization can pull a chromosome during bacterial mitosis Edward Banigan, Michael Gelbart, Zemer Gitai, Andrea Liu, Ned Wingreen Chromosome segregation is fundamental to all cells, but the force-generating mechanisms underlying chromosome translocation in bacteria remain mysterious. {\it Caulobacter crescentus} utilizes a depolymerization-driven process in which a ParA protein structure elongates from the new cell pole and binds to a ParB-decorated chromosome, and then retracts via disassembly, thus pulling the chromosome across the cell. This poses the question of how a depolymerizing structure can robustly pull the chromosome that is disassembling it. We perform Brownian dynamics simulations with a simple and physically consistent model of the ParABS system. The simulations suggest that the mechanism of translocation is ``self-diffusiophoretic'': by disassembling ParA, ParB generates a ParA concentration gradient so that the concentration of ParA is higher in front of the chromosome than behind it. Since the chromosome is attracted to ParA via ParB, it moves up the ParA gradient and across the cell. We find that translocation is controlled by the product of an effective relaxation time for the chromosome and the rate of ParA disassembly. Our results provide a physical explanation of the mechanism of depolymerization-driven translocation and suggest physical explanations for recent experimental observations. [Preview Abstract] |
Wednesday, March 23, 2011 4:54PM - 5:06PM |
T43.00013: Polymerization and oscillation stuttering in a filamentous model of the subcellular Min oscillation Andrew Rutenberg, Supratim Sengupta, Anirban Sain, Julien Derr We present a computational model of the {\em E. coli} Min oscillation that involves polymerization of MinD filaments followed by depolymerization stimulated by filament-end zones of MinE. Our stochastic model is fully three-dimensional, and tracks the diffusion and interactions of every MinD and MinE molecule. We recover self-organized Min oscillations. We investigate the experimental phenomenon of oscillation stuttering, which we relate to the disruption of MinE tip-binding at the filament scale. [Preview Abstract] |
Wednesday, March 23, 2011 5:06PM - 5:18PM |
T43.00014: Effect of Antimicrobial Agents on MinD Protein Oscillations in E. coli Bacterial Cells Corey Kelly, Megan Murphy, Maximiliano Giuliani, John Dutcher The pole-to-pole oscillation of the MinD proteins in E. coli determines the location of the division septum, and is integral to healthy cell division. It has been shown previously that the MinD oscillation period is approximately 40 s for healthy cells [1] but is strongly dependant on environmental factors such as temperature, which may place stress on the cell [2,3]. We use a strain of E. coli in which the MinD proteins are tagged with green fluorescent protein (GFP), allowing fluorescence visualization of the MinD oscillation. We use high resolution total internal reflection fluorescence (TIRF) microscopy to observe the effect of exposure to antimicrobial agents on the MinD oscillation period and, more generally, to analyze the time variation of the spatial distribution of the MinD proteins within the cells. These measurements provide insight into the mechanism of antimicrobial action. \\[4pt] [1] Raskin, D.M.; de Boer, P. (1999) Proc Natl. Acad. Sci. 96: 4971-4976.\\[0pt] [2] Colville, K.; Tompkins, N.; Rutenberg, A. D.; Jericho, M. H. (2010) Langmuir 2010:26.\\[0pt] [3] Downing, B.; Rutenberg, A.; Touhami, A.; Jericho, M. (2009) PLoS ONE 4: e7285. [Preview Abstract] |
Wednesday, March 23, 2011 5:18PM - 5:30PM |
T43.00015: Pattern Transitions in Bacterial Oscillating System under Nanofluidic Confinement Jie-Pan Shen, Chia-Fu Chou Successful binary fission in E. coli relies on remarkable oscillatory behavior of the MinCDE protein system to determine the exact division site. The most favorable models to explain this fascinating spatiotemporal regulation on dynamic MinDE pattern formation in cells are based on reaction-diffusion scenario. Although not fully understood, geometric factors caused by bacterial morphology play a crucial role in MinDE dynamics. In the present study, bacteria were cultured, confined and reshaped in various micro/nanofluidic devices, to mimic either curvature changes of cell peripherals. Fluorescence imaging was utilized to detail the mode transitions in multiple MinDE patterns. The understanding of the physics in multiple pattern formations is further complemented via in silico modeling. The study synergizes the join merits of in vivo, in vitro and in silico approaches, to grasp the insight of stochastic dynamics inherited from the noisy mesoscopic biophysics. [Preview Abstract] |
Session T44: Evolutionary and Ecological Systems
Sponsoring Units: DBPChair: Ao Ping, Shanghai Jiaotong University, China
Room: A309
Wednesday, March 23, 2011 2:30PM - 2:42PM |
T44.00001: Spatial Disorder in Cyclic Three-Species Predator-Prey Models Qian He, Mauro Mobilia, Uwe C. T\"auber By numerically studying the oscillatory dynamics of several variants of cyclic three-species predator-prey models with conserved total particle density, we investigate the effects of spatial variability of the reaction rates and site occupancy restrictions on the system's co-evolutionary dynamics. It is shown that both quenched disorder in the reaction rates and lattice site occupancy restrictions have only minor effects on the dynamics of cyclic competing systems. This result is starkly different from the finding in two-species predator-prey model where spatial disorder can greatly enhance species fitness. We also numerically compute the dependence of the mean extinction time, for small systems, on system size. \par \noindent Reference: Phys. Rev. E {\bf 82}, 051909 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 2:42PM - 2:54PM |
T44.00002: A Two-Species Social Dominance Model Swapnil Jawkar, Geoffrey Adams, Uwe C. T\"auber We study the general properties of a stochastic two-species social-domination model defined on a $d$-dimensional lattice. The introduction of spatial degrees of freedom and allowance of stochastic fluctuations surprisingly does not invalidate the deterministic mean-field picture. In the active state, where the dominant and submissive species coexist, no patch formation is observed, with correlation lengths restricted to a few lattice sites. Oscillations seen in the submissive population density are strongly damped and restricted to a small section of the parameter space. Observations are explained to be a result of the two-particle reactions being restricted to the same social group. [Preview Abstract] |
Wednesday, March 23, 2011 2:54PM - 3:06PM |
T44.00003: Cooperation and cheating in microbes Jeff Gore Understanding the cooperative and competitive dynamics within and between species is a central challenge in evolutionary biology. Microbial model systems represent a unique opportunity to experimentally test fundamental theories regarding the evolution of cooperative behaviors. In this talk I will describe our experiments probing cooperation in microbes. In particular, I will compare the cooperative growth of yeast in sucrose and the cooperative inactivation of antibiotics by bacteria. In both cases we find that cheater strains---which don't contribute to the public welfare---are able to take advantage of the cooperator strains. However, this ability of cheaters to out-compete cooperators occurs only when cheaters are present at low frequency, thus leading to steady-state coexistence. These microbial experiments provide fresh insight into the evolutionary origin of cooperation. [Preview Abstract] |
Wednesday, March 23, 2011 3:06PM - 3:18PM |
T44.00004: Cooperative Bacterial Growth Dynamics Predict the Evolution of Antibiotic Resistance Tatiana Artemova, Ylaine Gerardin, Sophia Hsin-Jung Li, Jeff Gore Since the discovery of penicillin, antibiotics have been our primary weapon against bacterial infections. Unfortunately, bacteria can gain resistance to penicillin by acquiring the gene that encodes beta-lactamase, which inactivates the antibiotic. However, mutations in this gene are necessary to degrade the modern antibiotic cefotaxime. Understanding the conditions that favor the spread of these mutations is a challenge. Here we show that bacterial growth in beta-lactam antibiotics is cooperative and that the nature of this growth determines the conditions in which resistance evolves. Quantitative analysis of the growth dynamics predicts a peak in selection at very low antibiotic concentrations; competition between strains confirms this prediction. We also find significant selection at higher antibiotic concentrations, close to the minimum inhibitory concentrations of the strains. Our results argue that an understanding of the evolutionary forces that lead to antibiotic resistance requires a quantitative understanding of the evolution of cooperation in bacteria. [Preview Abstract] |
Wednesday, March 23, 2011 3:18PM - 3:30PM |
T44.00005: Slowly switching between environments facilitates reverse evolution in small populations Longzhi Tan, Jeff Gore The rate at which a physical process occurs usually changes the behavior of a system. In thermodynamics, the reversibility of a process generally increases when it occurs at an infinitely slow rate. In biological evolution, adaptations to a new environment may be reversed by evolution in the ancestral environment. Such fluctuating environments are ubiquitous in nature, although how the rate of switching affects reverse evolution is unknown. Here we use a computational approach to quantify evolutionary reversibility as a function of the rate of switching between two environments. For small population sizes, which travel on landscapes as random walkers, we find that both genotypic and phenotypic reverse evolution increase at slow switching rates. However, slow switching of environments decreases evolutionary reversibility for a greedy walker, corresponding to large populations (extensive clonal interference). We conclude that the impact of the switching rate for biological evolution is more complicated than other common physical processes, and that a quantitative approach may yield significant insight into reverse evolution. [Preview Abstract] |
Wednesday, March 23, 2011 3:30PM - 3:42PM |
T44.00006: Bacterial cheating limits the evolution of antibiotic resistance Hui Xiao Chao, Manoshi Datta, Eugene Yurtsev, Jeff Gore The widespread use of antibiotics has led to the evolution of resistance in bacteria. Bacteria can gain resistance to the antibiotic ampicillin by acquiring a plasmid carrying the gene beta-lactamase, which inactivates the antibiotic. This inactivation may represent a cooperative behavior, as the entire bacterial population benefits from removing the antibiotic. The cooperative nature of this growth suggests that a cheater strain--which does not contribute to breaking down the antibiotic--may be able to take advantage of cells cooperatively inactivating the antibiotic. Here we experimentally find that a ``sensitive'' bacterial strain lacking the plasmid conferring resistance can invade a population of resistant bacteria, even in antibiotic concentrations that should kill the sensitive strain. We observe stable coexistence between the two strains and find that a simple model successfully explains the behavior as a function of antibiotic concentration and cell density. We anticipate that our results will provide insight into the evolutionary origin of phenotypic diversity and cooperative behaviors found in nature. [Preview Abstract] |
Wednesday, March 23, 2011 3:42PM - 3:54PM |
T44.00007: A slowly evolving host moves first in symbiotic interactions James Damore, Jeff Gore Symbiotic relationships, both parasitic and mutualistic, are ubiquitous in nature. Understanding how these symbioses evolve, from bacteria and their phages to humans and our gut microflora, is crucial in understanding how life operates. Often, symbioses consist of a slowly evolving host species with each host only interacting with its own sub-population of symbionts. The Red Queen hypothesis describes coevolutionary relationships as constant arms races with each species rushing to evolve an advantage over the other, suggesting that faster evolution is favored. Here, we use a simple game theoretic model of host- symbiont coevolution that includes population structure to show that if the symbionts evolve much faster than the host, the equilibrium distribution is the same as it would be if it were a sequential game where the host moves first against its symbionts. For the slowly evolving host, this will prove to be advantageous in mutualisms and a handicap in antagonisms. The model allows for symbiont adaptation to its host, a result that is robust to changes in the parameters and generalizes to continuous and multiplayer games. Our findings provide insight into a wide range of symbiotic phenomena and help to unify the field of coevolutionary theory. [Preview Abstract] |
Wednesday, March 23, 2011 3:54PM - 4:06PM |
T44.00008: How do the effects of mutations add up? Andrea Velenich, Mingjie Dai, Jeff Gore Genetic mutations affect the fitness of any organism and provide the variability necessary for natural selection to occur. Given the fitness of a wild type organism and the fitness of mutants A and B which differ from the wild type by a single mutation, predicting the fitness of the double mutant AB is a fundamental problem with broad implications in many fields, from evolutionary theory to medicine. Analysis of millions of double gene knockouts in yeast reveals that, on average, the fitness of AB is the product of the fitness of A and the fitness of B. However, most pairs of mutations deviate from this mean behavior in a way that challenges existing theoretical models. We propose a natural generalization of the geometric Fisher's model which accommodates the experimentally observed features and allows us to characterize the fitness landscape of yeast. [Preview Abstract] |
Wednesday, March 23, 2011 4:06PM - 4:18PM |
T44.00009: Theory of cooperation in a micro-organismal snow-drift game Zhenyu Wang, Nigel Goldenfeld We present a mean field model for the phase diagram of a community of micro-organisms, interacting through their metabolism so that they are, in effect, engaging in a cooperative social game. We show that as a function of the concentration of the nutrients glucose and histidine, the community undergoes a phase transition separating a state in which one strain is dominant to a state which is characterized by coexisting populations. Our results are in good agreement with recent experimental results, correctly predicting quantitative trends and the phase diagram. [Preview Abstract] |
Wednesday, March 23, 2011 4:18PM - 4:30PM |
T44.00010: Spatial population genetics in a petri dish Kirill Korolev, Joao Xavier, Melanie Muller, Nilay Karahan, Oskar Hallatschek, Kevin Foster, Andrew Murray, David Nelson The evolution of natural populations involves more than mutations followed by natural selection: Stochasticity and spatial migrations are also important. The effects of fluctuations and spatial structure become especially pronounced when organisms expand to new territories. The fluctuations are enhanced because the number of organisms at the front of the expansion is typically small, and the spatial structure is more pronounced due to dimensional reduction from two to one spatial dimension (because colonization occurs along the quasi-one-dimensional periphery of the population). The interplay of fluctuations and space leads to spatial segregation of different genotypes, which significantly alters the evolutionary dynamics of the population. We investigate this process by combining theory, simulations, and experiments on microbial expansions on the surface of a Petri dish. In particular, I will discuss how one can use simple microbiology experiments to measure important parameters of microbial populations such as the strength of fluctuations, migration rate, and relative fitness. [Preview Abstract] |
Wednesday, March 23, 2011 4:30PM - 4:42PM |
T44.00011: The Goldilocks Principle and Rapid Evolution of Antibiotic Resistance in Bacteria Qiucen Zhang, Robert Austin Goldilocks sampled the three bear's wares for the ``just right'' combination of taste, fit and comfort. Like Goldilocks's need for the just right parameters, evolution proceeds most rapidly when there is the just right combination of a large number of mutants and rapid fixation of the mutants. We show here using a two-dimensional micro-ecology that it is possible to fix resistance to the powerful antibiotic ciprofloxacin (Cipro) in wild-type E. coli in 10 hours through a combination of extremely high population gradients, which generate rapid fixation, convolved with the just right level of antibiotic which generates a large number of mutants and the motility of the organism. Although evolution occurs in well-stirred chemostats without such Goldilocks conditions, natural environments are rarely well stirred in nature.For complex environments such as the Galapagos Islands, spatial population gradients and movement of mutants along these population gradients can be as important as genomic heterogeneity in setting the speed of evolution. The design of our micro-ecology is unique in that it provides two overlapping gradients, one an emergent and self generated bacterial population gradient due to food restriction and the other a mutagenic antibiotic gradient. Further, it exploits the motility of the bacteria moving across these gradients to drive the rate of resistance to Cipro to extraordinarily high rates. [Preview Abstract] |
Wednesday, March 23, 2011 4:42PM - 4:54PM |
T44.00012: Evolution on a Lattice under Strong Mutation Jakub Otwinowski, Stefan Boettcher The most common approach to study biological evolution in a population considers mutations to arise one at a time, and spread to the whole population. However, recent experimental work has shown that under conditions of strong mutation and strong selection, multiple mutations may arise simultaneously. Such overlapping mutations compete with each other and make the results difficult to analyse. Theorists are working on understanding the relationships between different parameters such as population size, mutation rate, and selection coefficients, in the way they affect observables such as the speed of evolution, and the probability of fixation. We have shown with simulations that under additional spatial constraints the dynamics are very different compared to well-mixed populations. A surface in fitness space evolves, akin to surface growth phenomena, with non-trivial power-law exponents. The result is that the speed of evolution is restricted and the probability of fixation is reduced. [Preview Abstract] |
Wednesday, March 23, 2011 4:54PM - 5:06PM |
T44.00013: Modeling Political Populations with Bacteria Chris Cleveland, David Liao Results from lattice-based simulations of micro-environments with heterogeneous nutrient resources reveal that competition between wild-type and GASP rpoS819 strains of E. Coli offers mutual benefit, particularly in nutrient deprived regions. Our computational model spatially maps bacteria populations and energy sources onto a set of 3D lattices that collectively resemble the topology of North America. By implementing Wright-Fishcer re- production into a probabilistic leap-frog scheme, we observe populations of wild-type and GASP rpoS819 cells compete for resources and, yet, aid each other's long term survival. The connection to how spatial political ideologies map in a similar way is discussed. [Preview Abstract] |
Wednesday, March 23, 2011 5:06PM - 5:18PM |
T44.00014: Mutualistic Interactions and Community Structure in Biological Metacommunities Per Arne Rikvold, Elise Filotas, Martin Grant, Lael Parrott The role of space in determining species coexistence and community structure is well established. However, previous studies mainly focus on simple competition and predation systems, and the role of mutualistic interspecies interactions is not well understood. Here we use a spatially explicit metacommunity model, in which new species enter by a mutation process, to study the effect of fitness-dependent dispersal on the structure of communities with interactions comprising mutualism, competition, and exploitation [1]. We find that the diversity and interaction network undergo a nonequilibrium phase transition with increasing dispersal rate. Low dispersion rate favors spontaneous emergence of many dissimilar, strongly mutualistic and species-poor local communities. Due to the local dissimilarities, the {\it global\/} diversity is high. High dispersion rate promotes local biodiversity and supports similar, species-rich local communities with a wide range of interactions. The strong similarity between neighboring local communities leads to reduced global diversity. \\[4pt] [1]. E.~Filotas, M.~Grant, L.~Parrott, P.A.\ Rikvold, J.\ Theor.\ Biol.\ {\bf 266}, 419 (2010); Ecol.\ Modell.\ {\bf 221}, 885 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 5:18PM - 5:30PM |
T44.00015: Universal Description of Interactive Growth Carlos Condat, Lucas Barberis Although the existence of organism-organism interactions during ontogenesis is well documented, ontogenetic growth models usually focus exclusively on the organism-environment interaction. We develop a new formalism to describe the interactive growth of two or more organisms in a given environment. Using a vector formulation of the Phenomenological Universalities concept, we are able to characterize the joint growth of two or more interacting organisms and assess the direct mutual influences between them, as well as the indirect influences that operate through environment modifications. The resulting equations describe synergetic, antagonistic, and cooperative growth, and can be applied to biological and ecological problems. As an example, we examine the growth dynamics in a mixed-species plantation. [Preview Abstract] |
Wednesday, March 23, 2011 5:30PM - 5:42PM |
T44.00016: Large fluctuations and fixation in evolutionary games Michael Assaf, Mauro Mobilia One of the most striking effects of fluctuations in evolutionary game theory is the possibility for mutants to fixate (take over) an entire population. In this work we use a semi-classical theory to study fixation in evolutionary games under non-vanishing selection, and investigate the relation between selection intensity and demographic (random) fluctuations. This approach allows the accurate treatment of rare large fluctuations and yields the probability and mean time of fixation beyond the weak-selection limit, often considered in previous works. The power of the theory is demonstrated on prototypical models of cooperation dilemmas with multiple absorbing states, and we find excellent agreement between the theoretical predictions and numerical simulations. Furthermore, we show that our treatment is superior to the Fokker-Planck approximation for finite selection intensity. M. Assaf and M. Mobilia, J. Stat. Mech. P09009 (2010). M. Mobilia and M. Assaf, Euro. Phys. Lett. 91, 10002 (2010). [Preview Abstract] |
Session T45: Focus Session: Magnetic and Spin Ordering in Atomic and Optical Systems
Sponsoring Units: GMAG DAMOPChair: Erich Mueller, Cornell University
Room: A310
Wednesday, March 23, 2011 2:30PM - 2:42PM |
T45.00001: Macroscopic quantum phenomenon in a spin-orbit coupled Bose-Einstein condensate Shizhong Zhang, Tin-Lun Ho It is well-known in electron physics in semiconductors that the spin-orbit coupling gives rise to many exciting new physics, for example, the topological insulator that is now being actively studied. With the advent of artificial gauge field that can be generated using Raman lasers for neutral bosonic atoms, we can now study the corresponding effects in the boson system. In this talk, I shall discuss the structure of the spinor condensate with spin-orbit coupling. In particular, we show that the system develops stripe structure in each spin component, as a result of the fact that the ground state consists of two dressed states carrying different momentum. We also work out the phase diagram of the system, which compares well with the recent experiment. [Preview Abstract] |
Wednesday, March 23, 2011 2:42PM - 2:54PM |
T45.00002: Spin-orbit coupled spinor Bose-Einstein condensate Hui Zhai, Chunji Wang, Chao Gao, Chao-Ming Jian An effective spin-orbit coupling can be generated in cold atom system by engineering atom-light interactions. We study spin-1/2 and spin-1 Bose-Einstein condensates with Rashba spin-orbit coupling, and find that the condensate wave function will develop non-trivial structures. From numerical simulation we have identified two different phases. In one phase the ground state is a single plane wave, and often we find the system splits into domains and an array of vortices plays the role as domain wall. In this phase, time-reversal symmetry is broken. In the other phase the condensate wave function is a standing wave and it forms spin stripe. The transition between them is driven by interactions between bosons. We also provide an analytical understanding of these results and determines the transition point between the two phases. [Preview Abstract] |
Wednesday, March 23, 2011 2:54PM - 3:06PM |
T45.00003: Phase transitions of spin-orbit coupled Bose-Einstein Condensate in an external trap potential Xiangfa Zhou, Ian Mondragon-Shem, Congjun Wu Recently, the experimental realization of artificial magnetic fields using laser beams in a Rb87 Bose-Einstein condensate provides a valuable opportunity to investigate the rich physics of atomic gases in the presence of external Abelian and Non-Abelian gauge fields. We investigate the ground state properties of two-component BECs with Rashba spin-orbit coupling in the presence of external trapping potential. In the presence of density-density interaction between particles, the competitions among interaction, spin-orbit coupling and trap potential results in phase transitions of the ground states from a spiral spin-density wave state to a skyrmion type spin texture with rotational symmetry. We numerically solve the Gross-Pitaevskii equation and plot the phase diagram. The corresponding physics with asymmetrical Rashba coupling is also discussed. [Preview Abstract] |
Wednesday, March 23, 2011 3:06PM - 3:42PM |
T45.00004: Artificial Staggered Magnetic Field for Ultracold Atoms in Optical Lattices Invited Speaker: Uniform magnetic fields are ubiquitous in nature, but this is not the case for staggered magnetic fields. In this talk, I will discuss an experimental set-up for cold atoms recently proposed by us [1], which allows for the realization of a ``staggered gauge field'' in a 2D square optical lattice. If the lattice is loaded with bosons, it may be described by an effective Bose-Hubbard Hamiltonian, with complex and anisotropic hopping coefficients. A very rich phase diagram emerges: besides the usual Mott-insulator and zero-momentum condensate, a new phase with a finite momentum condensate becomes the ground-state at strong gauge fields [2]. By using the technique of Feshbach resonance, the dynamics of a coherent superposition of a vortex-carrying atomic condensate and a conventional zero-momentum molecular condensate can also be studied within the same scheme [3]. On the other hand, if the lattice is loaded with fermions, a highly tunable, graphene-like band structure can be realized, without requiring the honeycomb lattice symmetry [2]. When the system is loaded with a mixture of bosons and two-species fermions, several features of the high-Tc phase diagram can be reproduced. A dome-shaped unconventional superconducting region arises, surrounded by a non-Fermi liquid and a Fermi liquid at low and high doping, respectively [4].\\[4pt] [1] A. Hemmerich and C. Morais Smith, Phys. Rev. Lett. 99, 113002 (2007).\\[0pt] [2] Lih-King Lim, A. Hemmerich, and C. Morais Smith, Phys. Rev. Lett. 100, 130402 (2008), Phys. Rev. A 81, 023404 (2010).\\[0pt] [3] Lih-King Lim, T. Troppenz, and C. Morais Smith, arXiv:1009.1471.\\[0pt] [4] Lih-King Lim, A. Lazarides, A. Hemmerich, and C. Morais Smith, EPL 88, 36001 (2009) and Phys. Rev. A 82, 013616 (2010) [Preview Abstract] |
Wednesday, March 23, 2011 3:42PM - 3:54PM |
T45.00005: Itinerant ferromagnetism in a Fermi gas with contact interaction: Magnetic properties in a dilute Hubbard model Chia-Chen Chang, Shiwei Zhang, David M. Ceperley Motivated by recent experiments addressing the issue of itinerant ferromagnetism in a dilute ultra-cold Fermi gas with contact interaction, we examine ground state properties of the repulsive Hubbard model on a cubic lattice [1] by means of a very accurate auxiliary-field quantum Monte Carlo method [2]. We focus on low-density systems with varying on-site interaction $U/t$, in the range relevant to the experiments. Twist-averaged boundary conditions are used to eliminate open-shell effects and large lattice sizes are studied to reduce finite-size effects. The sign problem is controlled by a generalized constrained path approximation [2]. We find no ferromagnetic phase transition in this model. The ground-state correlations are consistent with those of a paramagnetic Fermi liquid. \\[4pt] [1] Chia-Chen Chang, Shiwei Zhang, and David M. Ceperley, arXiv:1009.1409\\[0pt] [2] Chia-Chen Chang and Shiwei Zhang, Phys. Rev. B 78, 165101 (2008). [Preview Abstract] |
Wednesday, March 23, 2011 3:54PM - 4:06PM |
T45.00006: Magnetic instabilities in spin imbalanced ultracold Fermi gases Inti Sodemann Villadiego, Dmytro Pesin, Allan MacDonald We study the possibility of preparing magnetic states of spin imbalanced ultracold Fermi gases near a broad Feshbach resonance by analyzing the unstable collective magnetization modes developed when the system is placed on the BEC side. Within the approximation of momentum independent interatomic scattering, transverse magnetization instabilities appear at lower critical interaction strengths than those corresponding to the longitudinal instabilities, suggesting that the former ones are primarily responsible for driving the system into a textured state with inhomogeneous magnetization direction. The critical interaction for the onset of transverse instabilities increases with polarization. However the system already has ferromagnetic character below these interaction strengths because of a change in sign of the spin stiffness which occurs close to the Stoner transition of the corresponding unpolarized gas. We also discuss the behavior expected beyond the momentum independent scattering approximation across the resonance for, both, the superfluid and ferromagnetic instabilities present in the system and the implication of these results for experiments. [Preview Abstract] |
Wednesday, March 23, 2011 4:06PM - 4:18PM |
T45.00007: Energy decay constant in sodium spinor condensates Jie Jiang, Yingmei Liu, Eduardo Gomez, D.A. Quinones , P.D. Lett Spinor condensates of F=1 sodium atoms display rich spin dynamics due to the antiferromagnetic nature of the interactions in this system. Damped spin oscillations are observed in sodium spinor condensates, which eventually lead to the mean-field ground state. In recent experiments we have been able to track and observe the time evolution of atom number fluctuations, which enables the first quantitative measure of energy dissipation in the spinor condensate. We also develop a method to extract the energy in spinor dynamics from experimental data, and characterize the energy dissipation with a decay constant. This decay constant appears to follow a power-law dependence with the energy of spinor condensates. This power-law dependence has been experimentally checked for a wide range of the spinor energy, by varying the applied magnetic field strength, the magnetization and the density of the spinor condensate. [Preview Abstract] |
Wednesday, March 23, 2011 4:18PM - 4:30PM |
T45.00008: Tunneling properties of collective spin wave excitations in the supercurrent state of a spin-1 spinor BEC Shohei Watabe, Yusuke Kato, Yoji Ohashi We theoretically investigate tunneling properties of spin wave excitations through a barrier in the supercurrent state of a spin-1 BEC. In the ferromagnetic phase, we show that the transverse spin wave always exhibits perfect transmission, when the spin- wave momentum $p$ coincides with the momentum of supercurrent $q$. This is quite different from the case of the Bogoliubov mode, where the so-called anomalous tunneling phenomenon always occurs when $p=0$, unless the system is in the critical current state ($q=q_c$). In the polar phase, spin wave modes always exhibit perfect transmission when $p=0$, as in the case of the Bogoliubov mode. However, this anomalous tunneling behaviors of spin wave modes are shown to still hold even in the critical current state, in contrast to the breakdown of the perfect transmission of the Bogoliubov mode at $q_c$. Only when the Gross-Pitaevskii equation for the spin-1 BEC is integrable, perfect transmission of the spin wave is absent at $q_c$. Using a simple $delta$-functional barrier, we also discuss similarity between the condensate wave function in the supercurrent state and the wave functions of spin wave excitations when perfect transmission occurs. [Preview Abstract] |
Wednesday, March 23, 2011 4:30PM - 4:42PM |
T45.00009: Quantum rotor theory of spinor condensates in tight traps Ryan Barnett, Hoi-Yin Hui, Chien-Hung Lin, Jay D. Sau, S. Das Sarma In this talk, we theoretically construct exact mappings of many-particle bosonic systems onto quantum rotor models. In particular, we analyze the rotor representation of spinor Bose-Einstein condensates. There is an exact mapping of a spin-one condensate of fixed particle number with quadratic Zeeman interaction onto a quantum rotor model. We use the rotor mapping to describe the different dynamical regimes recently observed in $^{23}$Na condensates. We also suggest a way to experimentally observe quantum mechanical effects (collapse and revival) in spinor condensates. We classify three distinct physical limits of the rotor model: the Rabi, Josephson, and Fock regimes. The last regime corresponds to a fragmented condensate and is thus not captured by the Bogoliubov theory. The semiclassical limit of the rotor problem is discussed and connections with the quantum wave functions are made through use of the Husimi distribution function. Finally, we describe how to extend the analysis to higher-spin systems and derive a rotor model for the spin-two condensate. This work was supported by the NSF JQI Physics Frontier Center. [Preview Abstract] |
Wednesday, March 23, 2011 4:42PM - 4:54PM |
T45.00010: Spin susceptibility of spin-1/2 fermions with dipole interactions Benjamin M. Fregoso, Eduardo Fradkin The general form of the spin susceptibility is found for spin- 1/2 fermions with dipole interactions. In the paramagnetic phase and partially magnetized phase (ferro-nematic) the spin susceptibility is explicitly computed in the Random Phase Approximation. Important modifications to the static susceptibility are discussed which are relevant for future experiments. Unconventional collective modes in the paramagnetic and ferro-nematic phases are also discussed. [Preview Abstract] |
Wednesday, March 23, 2011 4:54PM - 5:06PM |
T45.00011: Quasi-two-dimensional fermionic dipolar gases in the Hartree-Fock approximation: band renormalization, inter-subband excitons and the liquid-solid phase diagram Mehrtash Babadi, Eugene Demler We study quasi-two-dimensional systems of fermionic dipolar gases in the Hartree-Fock approximation for various trap frequencies and dipolar interactions at finite temperatures and evaluate the energy dispersions of the renormalized subbands. We also study the inter-subband excitation spectrum of the system in the Time-Dependent Hartree-Fock approximation and predict the energy absorption rates in lattice modulation spectroscopy experiments. It is shown that the spectrum consists of inter-subband particle-hole excitation continuums as well as excitonic modes, and that their observation is highly likely in current experiments. Finally, we calculate the liquid-solid phase diagram of the system and find novel features such as multiple crystalline orders and re-entrant crystallization. [Preview Abstract] |
Wednesday, March 23, 2011 5:06PM - 5:18PM |
T45.00012: Probing non-Abelian statistics of Majorana fermions in ultracold atomic superfluid Shi-Liang Zhu, L.B. Shao, Z.D. Wang, Lu-Ming Duan We propose an experiment to directly probe the non-Abelian statistics of Majorana fermions by braiding them in an s-wave superfluid of ultracold atoms. We show different orders of braiding operations give orthogonal output states that can be distinguished through Raman spectroscopy. Realization of Majorana bound states in an s-wave superfluid requires strong spin-orbital coupling and a controllable Zeeman field in the perpendicular direction. We present a simple laser configuration to generate the artificial spin-orbital coupling and the required Zeeman field in the dark state subspace. [Preview Abstract] |
Wednesday, March 23, 2011 5:18PM - 5:30PM |
T45.00013: Magnetism and Cooper pairing in one-dimensional large spin fermions with repulsive interactions Hsiang-hsuan Hung, Yupeng Wang, Congjun Wu The recent experimental realization of ultracold large-spin fermionic systems provides a new opportunity to investigate exotic magnetism and Cooper pairing physics. By means of exact diagonalizaton and the density matrix renormalization group, we systematically study the magnetic properties of the Mott-insulating state of the simplest large-spin systems with hyperfine spin $F=3/2$ in one-dimension and at quarter filling. Such a system is characterized by an exact $SO(5)$ symmetry. The ground state shows various profiles at various $\theta=\tan^{-1}J_0/J_2$, where $J_0/J_2$ is the ratio of exchange strengths of the singlet ($S_T=0$) and quintet ($S_T=2$) channels. As $\theta> 45^{\circ}$ the ground state is a gapped state with dimerization patterns whereas as $\theta \le 45^{\circ}$ it is a gapless Luttinger liquid state. Furthermore, we found that in the Luttinger liquid phase the static correlation functions show power-law decays with a four-site periodicity, which is similar to an SU(4) chain. We also study the spin-$3/2$ model with doping. In the regime of $\theta> 45^{\circ}$ and at moderate doping, the singlet pairing correlations indicate power-law decays whereas the quintet pairing correlations have exponential decays. On the other hand, in the regime of $\theta \le 45^{\circ}$ the quintet pairing correlations are more robust than the singlet pairing correlations. [Preview Abstract] |
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