Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session H41: Semiconductors: Many-body and Correlated Systems |
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Sponsoring Units: FIAP Chair: Allan MacDonald, University of Texas at Austin Room: Colorado Convention Center 504 |
Tuesday, March 6, 2007 8:00AM - 8:12AM |
H41.00001: A charge droplet picture of magnetotransport in disordered delta-doped heterostructures Malcolm Kennett, Vikram Tripathi We discuss theoretically how electrons confined to two dimensions in a delta-doped heterostructure can arrange themselves in a droplet-like spatial distribution due to disorder and screening effects when their density is low. We apply this droplet picture to magnetotransport and derive the expected dependence on electron density of several quantities relevant to this transport, in the regimes of weak and moderate magnetic fields. We find good qualitative and quantitative agreement between our calculations and recent experiments on delta-doped heterostructures. In particular we show that in the regime of magnetic fields where the resistivity $\rho$ varies with magnetic field $B$ as $\rho(B)\propto\exp[\alpha B^{2}]$, that $\alpha\propto n_{e}^{-\frac{3}{2}}$, where $n_{e}$ is the electron density in the 2DEG, even though the average tunneling distance between droplets is much larger than the average inter-electron spacing. [Preview Abstract] |
Tuesday, March 6, 2007 8:12AM - 8:24AM |
H41.00002: ABSTRACT HAS BEEN MOVED TO U32.0012 |
Tuesday, March 6, 2007 8:24AM - 8:36AM |
H41.00003: Topological defects, ``magnetic charges'' and coherence in dipolar condensates Egor Babaev The current progress in fabrication technology raised the expectation for the creation of excitonic condensates in semiconductor bilayers. We discuss an effective model for such bilayers and topological defects paying special attention to dynamics of the gauge field. We also discuss different physical systems which allow generalizations of the concept of dipolar superfluidity [Preview Abstract] |
Tuesday, March 6, 2007 8:36AM - 8:48AM |
H41.00004: Wigner Supersolid of Excitons in Electron-Hole Bilayers Yogesh Joglekar, Alexander Balatsky, Sankar Das Sarma Bilayer electron-hole systems, where carriers in one layer are electrons and carriers in the other are holes, are expected to undergo Bose-Einstein condensation of excitons when the layer separation $d$ is much smaller than the interparticle distance $r_s a_B$ within each layer. We show, based on general principles, that there are two distinct ground states in this regime. The first, a uniform Bose condensate of excitons, has been studied in the literature. We predict the existence of a second state, a Wigner supersolid of excitons, that occurs in the region $\sqrt{r_s}\leq d/a_B\leq r_s$. In this phase, the excitons are phase coherent but form a Wigner crystal due to dipolar repulsion. We present a qualitative phase diagram of the bilayer system, and discuss properties and possible signatures of the Wigner supersolid phase. [Preview Abstract] |
Tuesday, March 6, 2007 8:48AM - 9:00AM |
H41.00005: Kinematics of Cold Excitons in the Laser Induced Exciton Trap A.T. Hammack, L.V. Butov, L.E. Smallwood, L. Mouchliadis, A.L. Ivanov, A.C. Gossard We have recently demonstrated laser induced trapping of indirect excitons in coupled quantum wells [1]. An important advantage of the laser induced exciton trapping is the possibility of controlling the trap in-situ by varying the laser intensity in space and time. Moreover, the excitons at the trap center are cold since they are far from the hot area of the laser excitation. Here, we report the studies of exciton kinetics in the laser induced traps. In particular, the kinetics of the trap formation and the exciton collection to the trap center when the excitation is switched on, as well as the kinetics of the exciton cloud when the excitation and trap are switched off have been measured by ultrafast imaging spectroscopy. The dynamics of the degenerate Bose gas of excitons in the trap was measured and analyzed for the first time. [1] A.T. Hammack, M. Griswold, L.V. Butov, L.E. Smallwood, A.L. Ivanov, and A.C. Gossard, Phys. Rev. Lett. 96, 227402 (2006). [Preview Abstract] |
Tuesday, March 6, 2007 9:00AM - 9:12AM |
H41.00006: Coherence Length of Cold Exciton Gases in Coupled Quantum Sen Yang, A.T. Hammack, M.M. Fogler, L.V. Butov, A.C. Gossard We report on emergence of spontaneous coherence of excitons at low temperatures. A Mach-Zehnder interferometer with spatial and spectral resolution was used to probe spontaneous coherence in cold exciton gases, which are implemented experimentally in the ring of indirect excitons in coupled quantum wells. A strong enhancement of the exciton coherence length is observed at temperatures below a few Kelvin. The increase of the coherence length is correlated with the macroscopic spatial ordering of excitons. The coherence length reaches about 2-3 microns at the lowest temperature(1.5K), this corresponds to a very narrow spread of the exciton momentum distribution, much smaller than that for a classical exciton gas. [Preview Abstract] |
Tuesday, March 6, 2007 9:12AM - 9:24AM |
H41.00007: Phase diagrams of the three-electron quantum dots Angbo Fang, Xuguang Chi, Ping Sheng We determine the rich phase diagrams for three-electron quantum dots in both the strong-correlation and the high-magnetic-field regimes, by employing an accurate and efficient non-variational approach. Through the complete separation of spatial rotation from kinematic rotation, the hidden symmetry of the zero angular momentum state is revealed, which is related to the space inversion properties of the system. We also show that although the ground state for arbitrary total angular momentum (L) forms an electronic Wigner molecule, the highest low-energy state for large L is liquid- like in character and beyond the classical description. Our results are compared to recent experimental results and other calculations. [Preview Abstract] |
Tuesday, March 6, 2007 9:24AM - 9:36AM |
H41.00008: Optical Detection of Bilayer Exciton Condensation Jung-Jung Su, Allan H. MacDonald The generation of quantum degenerate exciton clouds in semiconductor bilayers and the detection of their anticipated Bose condensation has generated both theoretical and experimental interest. Recently the angular distribution of luminescence from a coherent trapped bilayer exciton system was recently calculated based on a bosonic description of the excitions. Motivated by the expectation that the condensation temperature in these systems will be maximized when the exciton density exceeds the area per isolated exciton, we have examined how the underlying Fermi statistics, which is relevant in this regime, alters the luminescence properties. We will report on a luminescence spectrum calculation which starts from a Bogoliubov description of the condensed ground state. [Preview Abstract] |
Tuesday, March 6, 2007 9:36AM - 9:48AM |
H41.00009: Density-functional theory of interacting electrons in inhomogeneous quantum wires Saeed H. Abedinpour, Marco Polini, Gao Xianlong, Mario P. Tosi Motivated by the experimental evidence of electron localization in cleaved edge overgrowth quantum wires and by the recent interest in the development of density-functional schemes for inhomogeneous Luttinger and Luther-Emery liquids, we present a novel density-functional study of a few interacting electrons confined by power-law external potentials into a short portion of a thin quantum wire. The theory employs the quasi-one-dimensional (Q$1D$) homogeneous electron liquid as the reference system and transfers the appropriate Q$1D$ ground-state correlations to the confined inhomogeneous system through a suitable local-density approximation (LDA) to the exchange and correlation energy functional. The LDA describes accurately ``liquid-like'' phases at weak coupling but fails in describing the emergence of ``Wigner molecules'' at strong coupling. A local spin-density approximation allowing for the formation of antiferromagnetic quasi-order with increasing coupling strength is proposed as a first step to overcome this problem. [Preview Abstract] |
Tuesday, March 6, 2007 9:48AM - 10:00AM |
H41.00010: Optical Manipulation of collective spin correlations in semiconductors with polarization squeezed vacuum Eran Ginossar, Yehoshua Levinson, Shimon Levit We calculate the transfer rate of correlations from polarization entangled photons to the collective spin of a many-electron state in a two-band system. It is shown that when a semiconductor absorbs pairs of photons from a two-mode squeezed vacuum, certain fourth order electron-photon processes correlate the spins of the excited electron pairs of different quasi-momenta. Different distributions of the quantum Stokes vector of the light lead to either enhancement or reduction of the collective spin correlations, depending on the symmetry of the distribution. We find that as the squeezing of the light becomes non- classical, the spin correlations exhibit a crossover from being positive with a $\sim N^2$ ($N$ is average photon number) scaling, to being negative with $\sim N$ scaling, even when $N$ is not small. Negative spin correlations mean a preponderance of spin singlets in the optically generated state. We discuss the possibility to measure the collective spin correlations in a combined measurement of the Faraday rotation fluctuation spectrum and excitation density in a steady-state configuration. [Preview Abstract] |
Tuesday, March 6, 2007 10:00AM - 10:12AM |
H41.00011: Many-body interaction in semiconductors probed with 2D Fourier spectroscopy Mikhail Erementchouk, Michael Leuenberger, Lu Sham A particular difficulty in studying many-body interactions in a solid is the absence of an experimental technique that can directly probe their key characteristics. We show that 2D Fourier spectroscopy provides an efficient tool for the measurement of critical parameters describing the effect of many-body interactions on the optical response of semiconductors. We develop the basic microscopic theory of 2D Fourier spectroscopy of semiconductors in the framework of the three-band model (heavy holes, light holes, and electrons). The theory includes many-body correlations nonperturbatively. We show, in particular, that 2D Fourier spectrum allows one to make a distinction between the diffraction on the gratings created by the heavy- and the light-hole excitons. We apply the theory to an analysis of the available experimental data. Based on this analysis we are able to deduce the relative contributions to four-wave mixing of the interaction between the excitons with different and the same helicities. Experiments providing more detailed information are suggested. [Preview Abstract] |
Tuesday, March 6, 2007 10:12AM - 10:24AM |
H41.00012: Many-body Interactions in Semiconductors Probed by Optical Two-dimensional Fourier Transform Spectroscopy T. Zhang, X. Li, S. T. Cundiff, R. P. Mirin, I. Kuznetsova Optically excited semiconductors with dominant near band-gap features provide an ideal laboratory for the study of fundamental many-body problems in condensed matter physics. Optical two-dimensional (2D) Fourier transform spectroscopy, which correlates the phase evolution of the nonlinear polarization during the initial evolution and final emission periods, has been utilized to investigate the interactions of excitons (the quasi-particles of bound electron-hole pairs) in semiconductor quantum wells. The strength and lineshape of the excitons and continuum states in 2D spectra show the influences of various exciton interactions, including excitation-induced dephasing, excitation-induced shift, biexciton formation, and local field corrections. The 2D spectra are sensitive to experimental conditions such as tuning, excitation density and polarization. The signatures of exciton interactions under different excitation conditions, including co-linear, co-circular and cross-linear polarizations, are identified in a microscopic theory with nonlinearities up to the third order in coherent $\chi ^{(3)}$-limit beyond the Hartree-Fock level and achieve qualitative agreement with the experiments. [Preview Abstract] |
Tuesday, March 6, 2007 10:24AM - 10:36AM |
H41.00013: Crystal and Electronic Structure of Copper Sulfides Pavel Lukashev, Walter R. L. Lambrecht Because of the complexity of the crystal structure of Cu$_{2-x}$S, no electronic band structure studies have been performed in the past. These materials have S atoms on a (hcp) or at high-temperature (fcc) close packed lattice but the Cu atoms occupy various low-symmetry Wyckoff sites of which only the statistical distribution is known from X-ray diffraction experiments. Here, we constructed supercell models for the cubic and hexagonal phases with the Cu positions determined by a weighted random number generator. The electronic structure of both these models and the monoclinic structure are studied using the full-potential linearized muffin-tin orbital method in the local density approximation (LDA). Both LDA and GW quasiparticle calculations give a zero band gap for the latter. The supercell models gave small band gaps of order 0.1--0.2 eV. Adding a Cu-s shift as suggested by the antifluorite structure GW calculation and an analysis in terms of atomic orbitals, increases the gap to about 0.5 eV. [Preview Abstract] |
Tuesday, March 6, 2007 10:36AM - 10:48AM |
H41.00014: Influence of size effect and electron correlation to the energy band gap of CuFeO$_{2}$ and AgFeO$_{2}$ Khuong Ong, Kewu Bai, Peter Blaha, Ping Wu We have calculated the electronic structure of delafossite type oxides CuFeO$_{2}$ and AgFeO$_{2}$ using the Full Potential Linearlized Augmented Plane Wave (FP-LAPW) method within Perdew-Burke-Ernzerhof Generalized-Gradient Approximation (PBE-GGA). A metallic state instead of true insulating state is obtained for CuFeO$_{2}$ and AgFeO$_{2}$. The insulating state is reproduced when electron correlations have been taken into account. An effective Hubbard parameter for Fe, U$_{eff}$=7.86eV, has been derived based on an ab initio constraint calculation. This value is an over estimation for the optical band gaps of CuFeO$_{2}$ and most probably for AgFeO$_{2}$ as well. One reasonable U$_{eff}$ has been derived by comparing between computational and experimental X-Ray emission spectra. The energy band gap of CuFeO$_{2}$ and AgFeO$_{2}$ within the PBE-GGA+U is found as charge transfer gap. Theoretical optical band gaps $\Delta _{0}$=1.30eV, $\Delta _{1}$=2.06eV, and $\Delta _{2}$=3.20eV for CuFeO$_{2}$ are quite compatible with experimental data. For AgFeO$_{2}$ an optical band gap $\Delta _{0}$=1.90eV has been predicted. The size effect is considered as the origin of the increase in optical and energy band gaps of AgFeO$_{2}$ in comparison with CuFeO$_{2}$. [Preview Abstract] |
Tuesday, March 6, 2007 10:48AM - 11:00AM |
H41.00015: Electronic structure of chalcopyrite CuInSe$_2$: LDA and GW Cihan Parlak, Tanju Gurel, Resul Eryigit CuInSe$_2$ is an important ternary semiconductor for solar cell applications with the highest demonstrated conversion efficiency. We have investigated its electronic structure by using pseudopotential density functional theory at the local density approximation (LDA) as well as by Hedin's GW approximation schemes. GW calculations are performed in self- consistent form as well as at the static COHSEX approximation level. The COHSEX approach results are found to be in reasonable agreement with the experimental data. The role of Copper semicore states (3$s^2$3$p^6$) in the band structure is found to be negligible for the LDA calculations while its crucial for obtaining a correct ordering of the bands at the GW level. The overall GW band structure is found to be similar to the LDA one with an almost dispersionless scissor shift along the $Z-\Gamma-X$ direction of the Brillouin zone. [Preview Abstract] |
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