Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session Y28: Superlattices and Nanostructures: Electronic Properties III |
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Sponsoring Units: DCMP Chair: Xuedong Hu, University at Buffalo SUNY Room: Morial Convention Center 220 |
Friday, March 14, 2008 11:15AM - 11:27AM |
Y28.00001: Layer interdependence of transport in an undoped electron-hole bilayer Christian Morath, John Seamons, John Reno, Mike Lilly Recently interest in the layer interdependence of a bilayer's transport has emerged. To examine this dependence the layer transport properties in an undoped electron-hole bilayer (uEHBL) device were measured as a function of density, inter-layer electric field and temperature. The uEHBL device consisted of a tunable, independently-contacted two-dimensional electron gas (2DEG) and two-dimensional hole gas (2DHG) induced in distinct GaAs quantum wells separated by a 30 nm Al$_{.9}$Ga$_{.1}$ As barrier. At T = 0.3 K, the 2DHG mobility increased with increasing 2DEG density, while the opposite effect was not observed. Decreasing the inter-layer electric field also increased 2DHG mobility without affecting the 2DEG mobility. This also decreased 2DHG Coulomb drag suggesting the inter-layer separation was increased. Distinct temperature dependencies were also measured for each layer's density and resistivity. [Preview Abstract] |
Friday, March 14, 2008 11:27AM - 11:39AM |
Y28.00002: Electronic doping in heterostructures of strongly correlated materials Ivan Gonzalez, Roger G. Melko, Elbio Dagotto Heterostructures of strongly correlated materials have attracted much attention recently. One of the main points of interest is the possibility of the stabilization of new phases at the interface between two different strongly correlated materials. In this talk, we present a study of the electronic properties of a heterostructure made of strongly correlated materials. The heterostructure is built up by alternating several layers of two different materials. The layers are thin enough (about 10 units cells) so the charges can be transferred all throughout the heterostructure. Calculations are performed using the Density Matrix Renormalization Group algorithm together with a Poisson equation formalism to account for the charge redistribution produced by the interfaces. We show that for realistic values of the parameters of the model the properties of the heterostructure are greatly determined by the behaviour at the interfaces. [Preview Abstract] |
Friday, March 14, 2008 11:39AM - 11:51AM |
Y28.00003: Exciton Formation in Coulomb Drag Measurements of Electron-Hole Bilayers J.A. Seamons, C.P. Morath, J.L. Reno, M.P. Lilly Since it was predicted over two decades ago, there has been intense interest in exciton condensation in coupled-well bilayer systems. While exciton condensation effects have been evident in optically-generated indirect excitons and quantum Hall bilayers, transport experiments in electron-hole bilayers in the regime of exciton condensation have proven to be extremely difficult. Results of Coulomb drag ($\rho _{DRAG})$ measurements at zero magnetic field on new undoped electron-hole bilayer devices formed in GaAs/Al$_{0.9}$Ga$_{0.1}$As double quantum well heterostructures are presented. For devices with 30 nm barriers $\rho _{DRAG}$ demonstrates T$^{2}$ behavior consistent with two Fermi liquids. In 20 nm barrier devices a dramatic upturn in the 2DHG Coulomb drag voltage occurs below T=1K. This upturn signals an increase in inter-layer coupling consistent with exciton formation. This work has been supported by the Division of Materials Sciences and Engineering, Office of Basic Energy Sciences, U.S. Department of Energy. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under Contract No. DE-AC04-94AL85000. [Preview Abstract] |
Friday, March 14, 2008 11:51AM - 12:03PM |
Y28.00004: Electronic states in magnetic quantum dots and quantum-dot molecules: Coulomb interaction effects and spontaneous symmetry breaking Alexander Govorov, Wei Zhang We investigate theoretically few-electron states in semi-magnetic quantum dots and quantum-dot molecules [1,2]. A double quantum-dot system made of diluted magnetic semiconductor behaves unlike the usual molecules. In a semiconductor double quantum dot or in a diatomic molecule, the ground state of a single carrier is described by a symmetric orbital. In a magnetic material molecule, new ground states with broken symmetry can appear due the competition between the tunneling and magnetic polaron energy. With decreasing temperature, the ground state changes from the normal symmetric state to a state with spontaneously broken symmetry. Interestingly, the symmetry of a magnetic molecule is recovered at very low temperatures. A magnetic double quantum dot with broken-symmetry phases can be used as a voltage-controlled nanoscale memory cell. [1] A. O. Govorov, Phys. Rev. B 72, 075359 (2005). [2] W. Zhang, T. Dong, and A. O. Govorov, Phys. Rev. B 76, 075319 (2007). [Preview Abstract] |
Friday, March 14, 2008 12:03PM - 12:15PM |
Y28.00005: The Kondo Effect and Rashba Spin-Orbit Coupling Justin Malecki We present the results of a study of the influence that weak Rashba spin-orbit coupling has on the Kondo effect induced by a magnetic impurity in a two dimensional electronic system. It is shown that the Kondo effect is robust against such coupling of momentum and spin, despite the fact that the spin of the conduction electrons is no longer a conserved quantity. A proposal is made for how the spin-orbit coupling may change the value of the Kondo temperature $T_K$ in such systems. Applications to semiconductor quantum dots and magnetic atoms on metallic surfaces are discussed. [Preview Abstract] |
Friday, March 14, 2008 12:15PM - 12:27PM |
Y28.00006: Fano-Kondo effect in a two-level system with triple quantum dots Tetsufumi Tanamoto, Yoshifumi Nishi, Shinobu Fujita Quantum dot (QD) systems have been providing opportunities to probe a wide variety of many-body effects in microelectronic structures. Recently, the Fano effect, which appears as a result of quantum interference between a discrete single energy level and a major electronic system, has attracted the interests of many researches [1]. Here, we theoretically study the Fano-Kondo effect and Fano effect in a triple QD system, where two QDs constitute a two- level system and the other QD works in a detector with electrodes. When two QDs are coupled, bonding and anti-bonding states are formed. It is expected that the detector current reflects these electronic states. Indeed, we found that the Fano dip is modulated by strongly coupled QDs with a slow detector. We also compare noise properties of Fano-Kondo effect with those of Fano effect, and we found that, depending on the coupling strength among the QDs, noise and the Fano factor are greatly modulated for a slow detector. These suggest a new method of reading out qubit states [2].-- [1] A. W. Rushforth et al., Phys. Rev. B 73, 081305 (2006). [2] T. Tanamoto et al., Phys. Rev. B 76, 155319 (2007); arXiv:0710.0912. [Preview Abstract] |
Friday, March 14, 2008 12:27PM - 12:39PM |
Y28.00007: Kondo physics with ac driving in the single electron transistor subjected to finite bias Ali Goker We employ the time-dependent non-crossing approximation to study the time averaged conductance for a single electron transistor in the Kondo regime when the dot level is sinusoidally driven from its equilibrium position by means of a gate voltage in finite bias. We find that the average conductance exhibits considerable deviation from the monotonous reduction when the applied bias is equal to the driving frequency of the dot level. We attribute this behaviour to the overlap of the satellite Kondo peaks with the split Kondo resonances formed at each lead's Fermi level. We display the spectral function to put our interpretation into more rigorous footing. We also investigate the effect of the temperature and the driving frequency on the observed enhancement. [Preview Abstract] |
Friday, March 14, 2008 12:39PM - 12:51PM |
Y28.00008: Interplay of Interactions and Phase Coherence in Open Quantum Dots Ileana Rau, Michael Grobis, Ron Potok, Hadas Shtrikman, David Goldhaber-Gordon The effect of Coulomb interactions on the electronic properties of a confined quantum system greatly weakens when electrons are allowed to rapidly enter and exit the system. For electron transport through a quantum dot, increasing the coupling of the dot to nearby leads causes a transition from the Coulomb blockade regime to a regime dominated by interference phenomena. We have investigated this transition in large, micron-sized quantum dots and have found that Coulomb blockade effects persist in a regime where they had generally been assumed absent: when a dot is coupled by one fully transmitting mode to each of two leads. We discuss the interplay of these residual Coulomb interactions with phase coherent transport through a dot. We also examine how the subtle suppression of conductance by these Coulomb interactions affects the electron dephasing rate at low temperatures in open quantum dots. [Preview Abstract] |
Friday, March 14, 2008 12:51PM - 1:03PM |
Y28.00009: Numerical estimate of correlation energy for two electrons confined in 2D quantum dot Takuma Okunishi, Yuki Negishi, Masakazu Muraguchi, Kyozaburo Takeda Local density approximation (LDA) is now widely accepted to design the novel materials as well as to predict new phenomena. In practical use of LDA, estimate of correlation energy is crucial. Tanatar et al. have given the well-known comprehensive expression of LDA correlation energy for 2D electron gas [1]. However, a straightforward use of this correlation energy to two electrons confined in 2D quantum dot (QD) leads such an inconsistent result that the correlation energy can overcome the Coulomb one because their correlation expression includes only 2D confinement. Specific correlation energy should be requested in accordance with the degree of the QD confinement. So we have obtained the correlation energy for two electrons confined in 2D QD by employing multi-reference configuration interaction (MRCI) technique. Now we are trying to improve the LDA correlation expression by refining their parameters and study time dependent phenomena of electrons in 2D QD by applying the improved LDA. [1] B. Tanatar and D. M. Ceperley, Phys. Rev. B \textbf{39}, 5005 (1989). [Preview Abstract] |
Friday, March 14, 2008 1:03PM - 1:15PM |
Y28.00010: Electron Tunneling Counting Statistics of a GaAs Quantum Dot at Thermal Equilibrium Xinchang Zhang, Ming Xiao, Eli Yablonovitch, Hongwen Jiang Full counting statistics (FCS) is an innovative way to investigate current fluctuations of mesoscopic conductors which can provide additional information beyond the conventional average current measurement [1]. Suppression of the 2nd moment and the 3rd moment were observed in a many-electron quantum dot(QD)under nonequilibrium conditions [1]. Here we studied the FCS of single electron tunneling of a GaAs QD in the few electron regime at thermal equilibrium in the in-plane magnetic fields. The device consists of a multiple-surface-gates defined GaAs QD integrated with a very sensitive, high bandwidth field effect transitor (FET) channel for the QD charge state read-out. Monitoring the FET current revealed two sequences of random telegraph signals which represent the electron tunneling onto and off the QD in real time. When the QD level is aligned with Fermi level of the reservoir, the statistics shows a maximum value of both mean ($<$n$>)$ and standard deviation ({\$}$\backslash $sigma{\$}), but a minimum skewness in its distribution function. It was also found that an in-plane magnetic field suppresses both $<$n$>$ and {\$}$\backslash $sigma{\$}, but enhances the skewness. [1], S. Gustavsson et. al, PRL 96, 76695(2006). . [Preview Abstract] |
Friday, March 14, 2008 1:15PM - 1:27PM |
Y28.00011: Non-Markovian Dynamics of Charge Carriers in Quantum Dots at High Bias Eduardo Vaz, Jordan Kyriakidis We have investigated the dynamics of bound particles in multi-level current-carrying quantum dots. We look specifically in the regime of resonant tunneling transport, where several channels are available for transport. Through the non-Markovian Born-Redfield formalism, we investigate the real-time evolution of the confined particles including transport-induced decoherence and relaxation. In the case of a coherent superposition between states with different particle number, we find that coherence may be preserved even in the presence of tunneling into and out of the dot. Real-time results are presented for various asymmetries of tunnel barriers and tunneling rates into different orbitals. [Preview Abstract] |
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