Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session N19: Transport and Microwave Conductivity |
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Sponsoring Units: DCMP Chair: Niti Goel, University of Notre Dame Room: LACC 406B |
Wednesday, March 23, 2005 8:00AM - 8:12AM |
N19.00001: Applications of R-Matrix Theory to Solid State Devices Thushari Jayasekera, N. Goel, Michael Morrison, Kieran Mullen R-matrix theory is a computationally efficient method for solving quantum collision problems. First introduced in nuclear physics and later applied in atomic and molecular physics, R- matrix theory is also a useful tool for calculating transport properties of solid-state devices. We have improved upon the existing implementations of R-matrix theory in device physics by inroducing boundary conditions that dramatically speed convergence. Moreover, we have extended the R-matrix formalism to scattering systems with very complicated, non-spherical device geometries. This new formalism, which we call ``the R-matrix connection formula,'' can be used to calculate the transport properties of practical solid-state devices. As an application, we calculate the bend resistance of InSb-based four-terminal devices. We compare our results with experimental data from a group at University of Oklahoma. In these experiments, the bend resistance was measured in a four-terminal device with an applied perpendicular magnetic field. A negative bend resistance was measured at zero magnetic field. This work is supported by NSF PHY-0354858, NSF MRSEC DMR- 0080054, and NSF EPS-9720651. [Preview Abstract] |
Wednesday, March 23, 2005 8:12AM - 8:24AM |
N19.00002: First-principles calculation of alloy scattering of n-type carriers in SiGe Felipe Murphy-Armando, Stephen Fahy Starting from a virtual crystal approximation (VCA) for the band structure of the Si$_{1-x}$Ge$_x$ alloy host, calculated in first-principles density functional theory, we find the scattering matrix for intra-valley and inter- valley n-type carrier scattering by a Si or Ge substitutional atom in a lattice of VCA atoms. The scattering matrix is calculated from energy splitting in large supercells (containing up to 127 VCA host atoms and one Si or Ge atom) of degenerate levels corresponding to the L, X and other points in the Brillouin zone. Atomic relaxation is found to have a substantial effect on the scattering matrix elements. Using supercells containing more than one Si or Ge atom, we test the accuracy of the approximation, in which each Si or Ge atom is considered to contribute independently to the carrier scattering matrix. The carrier mobility is calculated from the scattering rate using the Boltzmann transport equation. The results are compared to experiment. [Preview Abstract] |
Wednesday, March 23, 2005 8:24AM - 8:36AM |
N19.00003: First-principles calculation of mobilities in nano-MOSFETs Matthew Evans, Xiaoguang Zhang, John Joannopoulos, Sokrates Pantelides As metal-oxide-semiconductor field-effect transistors enter the nanoscale regime, the usual approximations made in mobility calculations fail to account for observations because wave penetration in the gate oxide becomes significant and the effective-mass approximation breaks down. We introduce a novel method for first-principles calculations of carrier mobilities in ultrathin silicon-on-insulator channels. The method is based on density functional theory and Green's functions. The silicon-oxide interface is treated at the atomic-scale, so that all wave functions extend on both sides of the interface. Interface roughness is included in terms of deviations from an abrupt interface (e.g. suboxide bonds, oxide protrusions) acting as scattering centers. Scattering from impurities (e.g. dopants, nitrogen, hydrogen) is also included. A dynamical approach to optical phonon scattering has been developed, including phonon-plasmon interactions. Initial results reveal the importance of the atomic scale in controlling the effects of interface roughness. [Preview Abstract] |
Wednesday, March 23, 2005 8:36AM - 8:48AM |
N19.00004: High electric-field quantum transport for Bloch electrons in a single band scattering from a random distribution of impurities Joseph B. Krieger, Andrey A. Kiselev, ILki Kim, Gerald J. Iafrate The quantum Boltzmann equation for a Bloch electron in a single band under the influence of a homogeneous and inhomogeneous electric field subject to scattering from a random, spatially inhomogeneous distribution of impurities will be presented. The analysis assumes the use of a single band effective Hamiltonian to describe the Bloch dynamics, and makes use of the vector potential to define the homogeneous electric field. After developing an {\em interaction picture} transformed Liouville equation for the Bloch electron based on the Wigner function, and then taking the limit of slowly varying inhomogeneous electric field and slowly varying scatterer density distribution, a novel quantum generalization of the Boltzmann equation is obtained which includes a collision term with impurity-related intra-collisional field effects correct to second order in the impurity potential, and a drift term which includes the total force based on the homogeneous and inhomogeneous fields. [Preview Abstract] |
Wednesday, March 23, 2005 8:48AM - 9:00AM |
N19.00005: Quantum and transport mobilities in an AlGaAs/GaAs parabolic quantum well structure Guolin Yu, Sergei Studenikin, Anthony SpringThorpe, Geoffrey Aers, Guy Austing We study quantum and transport mobilities in a parabolic quantum well structure when one or more subbands are occupied. We developed an original analytical method to extract the quantum mobility from the multiply-occupied subband transport characteristics at low temperature. We tune the carrier density and hence the subband structure of the parabolic quantum well over a wide range by illumination with a red light-emitting diode. In order to obtain the quantum mobilities, Fourier transforms of the first differential of the experimental magnetoresistance traces are taken, and fitted by a conductivity tensor model in the same magnetic field range. We find that both the quantum and transport mobilities increase non-linearly with increasing carrier density for both the first- and second- subbands, and conclude that the intersubband scattering is predominantly large-angle. [Preview Abstract] |
Wednesday, March 23, 2005 9:00AM - 9:12AM |
N19.00006: Studies of Zero-resistance States by Dichromatic Microwaves M.A. Zudov$^{1,2}$, R.R. Du$^{1}$, L.N. Pfeiffer$^3$, K.W. West$^3$ $^1$Dept. of Physics, University of Utah, Salt Lake City, UT 84112, $^2$School of Physics and Astronomy, University of Minnesota, Minneapolis, MN 55455, $^3$Bell Laboratories, Lucent Technologies, Murray Hill, NJ 07974 --- We have explored experimentally dichromatic (frequencies $\omega _1$ and $\omega _2$) photoresistance of a two-dimensional electron system in the regimes of microwave-induced resistance oscillations and zero-resistance states. We have found that dichromatic resistance is closely replicated by a linear superposition of $\omega _1$ and $\omega _2$ components, provided that both monochromatic resistances are positive. In contrast, if a zero-resistance state is to be formed by one of the frequencies, such superposition relation becomes invalid. More specifically, dichromatic resistance is suppressed in this regime. This finding can be explained by taking into account the absolute negative resistance and the formation of domains, as suggested by current theoretical models. [Preview Abstract] |
Wednesday, March 23, 2005 9:12AM - 9:24AM |
N19.00007: Influence of a Parallel Magnetic Field on Microwave-Induced Oscillatory and Zero-Resistance States C.L. Yang, Z.Q. Yuan, R.R. Du, L.N. Pfeiffer, K.W. West We have studied experimentally the influence of a parallel magnetic field on microwave-induced resistance oscillations and the subsequent zero-resistance states (ZRS) previously discovered in a high-quality two-dimensional electron gas. Our experiments were performed in a frequency range from 25 to 150 GHz and at temperatures from 0.5 to 8 K; samples were Hall bars of GaAs/Al$_{x}$Ga$_{1-x}$As heterojunctions and quantum wells having low temperature mobility as high as 2 x 10$^{7}$ cm$^{2}$/Vs. A two-axis superconducting magnet was employed to facilitate the experiment. We have observed pronounced suppression of oscillations/ZRS by a parallel magnetic field (B$_{//})$. In contrast, resistance peaks associated with magnetoplasmon resonance become stronger in B$_{//}$. We discuss possible explanations for the observations. [Preview Abstract] |
Wednesday, March 23, 2005 9:24AM - 9:36AM |
N19.00008: Experimental examination of current instabilities in the irradiated 2DEG R.L. Willett, K.W. West, L.N. Pfeiffer RF radiation imposed on a high quality heterostructure can result in a series of oscillations periodic in $\omega $/$\omega _{c}$ with $\omega $ the radiation frequency and $\omega _{c}$ the cyclotron frequency, using bare GaAs electron mass [1]. Subsequently it was observed [2,3] that in high mobility samples the minima can form apparent zeroes. These findings are consistent with micro- and macroscopic theoretical pictures of radiation induced transport and current instabilities due to local negative resistivities.[4] Using simple dipole configurations with radiation up to 20GHz frequency we have examined the magneto-resistance oscillations in ultra-high mobility samples, focusing on indications of current instabilities and the fundamental origin of the oscillations. We find that under radiation, voltages are observed from internal to external contacts in the absence of applied driving currents, with a distinction from simple rectification at the principal oscillation minima. Further measurements of magneto-resistance have been carried out using multiple dipoles to apply different radiation frequencies simultaneously. These results are reviewed in consideration of both the origin of the magneto-resistive oscillations and the current instabilities. [1] M.A. Zudov, et al, Phys. Rev. B. 64, 201311 (2001). [2] R.G. Mani, et al; Nature 420, 646 (2002). [3] M.A. Zudov, et al, Phys. Rev. Lett. 90, 046807 (2003). [4] A.V. Andreev, et al, Phys. Rev. Lett. 91, 056803 (2003). [Preview Abstract] |
Wednesday, March 23, 2005 9:36AM - 9:48AM |
N19.00009: Microwave photoexcited magnetoresistance in the high mobility GaAs/AlGaAs system Ramesh Mani The microwave-photoexcited high mobility GaAs/AlGaAs two dimensional electron system exhibits an oscillatory magnetoresistance with vanishing resistance in the vicinity of magnetic fields B = [4/(4j+1)]B$_{f }$where B$_{f }$= 2$\pi $ f m$^{\ast }$/e and f is the radiation frequency. Here, we report experimental results examining microwave induced phenomena in regimes where (a) the Landau level spacing, which is of the order of or smaller than the photon energy, exceeds both a broadening parameter defined from the transport time and k$_{B}$T, and (b) where the Landau level spacing significantly exceeds the photon energy. [Preview Abstract] |
Wednesday, March 23, 2005 9:48AM - 10:00AM |
N19.00010: Steady States of the Inhomogenous Microwave Irradiated Quantum Hall Gas Assa Auerbach, Ilya Finkler, Bertrand Halperin, Amir Yacoby To explain the observation of Zero-Resistance states (ZRS) in Microwave irradiated Quantum Hall gases[1], it has been proposed[2] that under appropriate conditions the sample will break into domains of photogenerated fields. In the absence of disorder induced pinning, motion of domain walls results in a ZRS state. In order to treat the effects of long wavelength disorder, we construct a Lyapunov functional for systems with uniform Hall conductivity. We use it to derive stability conditions on the domain structure and to compute the conductance. We show that weak white noise disorder does not destroy the ZRS although it produces current fluctuations. In contrast, separable and correlated disorder pin the domain walls, and produce a finite conductance and a photovoltage as demonstrated by one dimensional, and simple two dimensional, potentials.\\ 1. R.G. Mani et.al. Nature, 420, 646 (2002); M.A. Zudov et.al., Phys. Rev. Lett. 90, 046807 (2003).\\ 2. A.V. Andreev, I.L. Aleiner, and A.J. Millis, PRL 91, 056803 (2003).\\ [Preview Abstract] |
Wednesday, March 23, 2005 10:00AM - 10:12AM |
N19.00011: Numerical studies of domain patterns in inhomogeneous microwave-irradiated quantum Hall gas Ilya Finkler, Assa Auerbach, Bertrand Halperin, Amir Yacoby Experimental observations of a zero conductance state in microwave-irradiated quantum Hall systems have been explained by a model which postulates the existence of domains with a non-zero dc electric field. [1] We have carried out numerical calculations to study the effects of long-range disorder on these domains and on the resulting conductivity. If the Hall conductivity is constant throughout the sample, then one can construct a Lyapunov functional, and domain wall patterns can be obtained by looking for a potential configuration which minimizes the functional. We have studied a range of examples and find that that long-range disorder can pin the domain walls, giving the state a nonzero conductance. For a spatially varying Hall conductivity, numerical calculations are more difficult, but results will be presented for simple cases. [1] A.V. Andreev, I.L. Aleiner, and A.J.Millis, PRL 91, 056803 (2003). [Preview Abstract] |
Wednesday, March 23, 2005 10:12AM - 10:24AM |
N19.00012: Photoconductivity oscillations in a 2D electron gas and the mobility threshold for zero resistance states. Manuel Torres, Alejandro Kunold We present a model for the photoconductivity of a two dimensional electron system subjected to a magnetic field. The model includes the microwave and Landau contributions in a non-perturbative exact way, impurity scattering effects are treated perturbatively. Based on this formalism, we provide a Kubo-like formula that takes into account the oscillatory Floquet structure of the problem. We study the effects of both short-range and long-range disorder on the photoconductivity. Our calculation yields a magnetoresistance oscillatory behavior with the correct period and phase. It is found that, in agreement with experiment, negative dissipation can only be induced in very high mobility samples, an expression for the mobility threshold is provided. We analyze the dependence of the results on the microwave power and polarization. For high-intensity radiation multi-photon processes take place predicting new negative-resistance states centered at $ \omega / \omega_c=1/2$, and $ \omega / \omega_c= 3/2$. \begin{itemize} \item M. Torres, A. Kunold, {\em cond-mat/0407468, \/} {\em cond-mat/0409588. \/} \end{itemize} [Preview Abstract] |
Wednesday, March 23, 2005 10:24AM - 10:36AM |
N19.00013: Transport properties of 2DEG with spin-orbit coupling in a strong magnetic field M.G. Vavilov, I.L. Aleiner We derive a Boltzmann equation for a disordered 2DEG with a spin-orbit coupling in a strong magnetic field (large Hall angle). This equation allows us to describe relations between spin magnetization and dc electric current. We also investigate effects of microwave radiation on the magnetization and conductivity. Particularly, we discuss the beats in the microwave-induced oscillatory part of the dc conductivity originating due to the spin orbit coupling. [Preview Abstract] |
Wednesday, March 23, 2005 10:36AM - 10:48AM |
N19.00014: Quantum and classical surface acoustic wave induced magnetoresistance oscillations in a 2D electron gas Malcolm Kennett, John Robinson, Nigel Cooper, Vladimir Fal'ko We study theoretically the geometrical and temporal commensurability oscillations induced in the resistivity of 2D electrons in a perpendicular magnetic field by surface acoustic waves (SAWs). We show that there is a positive anisotropic dynamical classical contribution and an isotropic non-equilibrium quantum contribution to the resistivity. We describe how the commensurability oscillations modulate the resonances in the SAW-induced resistivity at multiples of the cyclotron frequency. We study the effects of both short-range and long-range disorder on the resistivity corrections for both the classical and quantum non-equilibrium cases. We predict that the quantum correction will give rise to zero-resistance states with associated geometrical commensurability oscillations at large SAW amplitude for sufficiently large inelastic scattering times. These zero resistance states are qualitatively similar to those observed under microwave illumination, and their nature depends crucially on whether the disorder is short- or long-range. Finally, we discuss the implications of our results for current and future experiments on two dimensional electron gases. [Preview Abstract] |
Wednesday, March 23, 2005 10:48AM - 11:00AM |
N19.00015: Microwave Photoconductivity of a High-Mobility 2D Hole Gas Z.Q. Yuan, C.L. Yang, R.R. Du, L.N. PFEIFFER, K.W. West We have measured millimeter wave (frequency from 25 to 150 GHz) photoconductivity in a high-quality 2D hole gas (2DHG), and found characteristically different responses as compared to those from a 2D electron gas. The 2DHGs are provided by Si-doped, (311) GaAs/Al$_{x}$Ga$_{1-x}$As quantum wells (QWs), or C-doped, (001) GaAs/Al$_{x}$Ga$_{1-x}$As QWs, having a range of hole sheet density from 1 to 4.5 x 10$^{11}$/cm$^{2}$, and low temperature (0.3 K) mobility from 2 to 6 x 10$^{5}$ cm$^{2}$/Vs. Magnetoresistivity, photoconductivity, as well as differential photoconductivity were measured using a low-frequency lock-in method at temperatures from 0.5 K to 1 K. Typically, differential photoconductivity data show a single peak corresponding to cyclotron resonance of 2D holes. In separate experiments we have measured the cyclotron absorption, from which we determined effective mass and scattering times of the 2DHGs. Our analysis shows that it is important to take into account the band structures of 2DHGs to understand the photoconductivity. [Preview Abstract] |
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