Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session T24: Focus Session: Quantum Transport Simulations and Computational Electronics -- Disorder |
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Sponsoring Units: DCOMP Chair: 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 |
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