Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session H35: Electronic Properties of Quantum Wells and Superlattices |
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Sponsoring Units: DCMP Chair: Joe Tischler, Naval Research Laboratory Room: Baltimore Convention Center 338 |
Tuesday, March 14, 2006 11:15AM - 11:27AM |
H35.00001: Improving optical performance of W-structured type-II superlattices by adjusting As$_2$ and In flux J.C. Kim, J.G. Tischler, I. Vurgaftman, J.R. Meyer, E.H. Aifer, L.J. Whitman, C.L. Canedy, E.M. Jackson We are investigating ``W-structured'' type-II superlattices (W-SLs) for use in IR photodiodes. A typical W-SL period consists of a hole quantum well (QW) sandwiched by two electron QWs, which are in turn bounded by barrier layers (\emph{e.g.}, InAs/InGaSb/InAs/AlGaInSb), such that the bulk band-edges form a ``W'' pattern. The mini-band straddles the GaSb valence and InAs conduction bands, with a direct gap that can be varied from mid- to very long-wave IR. Photoluminescence (PL) has proven to be a sensitive indicator of photodiode performance, and here we present results correlating the PL intensity of W-SL's grown under varying conditions with their structure. X-ray diffraction is used to measure overall strain and periodicity, and cross-sectional STM (XSTM) is used to directly image the atomic-scale structure. We have discovered that roughness at InAs-on-InGaSb interfaces can be controlled by adjusting the As$_2$:In flux ratio and thereby reducing Sb cross-incorporation in the InAs. The smoother interfaces and reduced strain correlate with stronger PL. By independently adjusting the As$_2$ and In flux, W-SLs with low strain can be fabricated across a range of wavelengths with PL up to 20\% greater than we have previously achieved. We discuss our analysis using XSTM to correlate the PL spectra with the structure of the W-SLs as a function of As$_2$ and In flux. [Preview Abstract] |
Tuesday, March 14, 2006 11:27AM - 11:39AM |
H35.00002: Equilibration of Two-dimensional Excitons in an In-Plane Harmonic Potential Zoltan Voros, David Snoke, Loren Pfeiffer, Ken West We report on the equilibration dynamics of a 2D gas of long-lifetime (10 $\mu$s), high-mobility excitons trapped in an in-plane harmonic potential produced by inhomogeneous external stress. Spatially indirect excitons are created in a GaAs/AlGaAs double quantum well structure by a quasi-continuous laser pulse. After excitation, the excitons approach equilibrium with a well-defined volume and constant temperature. When the system is far from quantum degeneracy, the effective temperature can be deduced from the spatial profile of the exciton cloud. We show that at the lowest temperatures, the disorder prevents the excitons from reaching the lattice temperature and that the external confining potential is greatly reduced by exciton-exciton repulsion. We discuss the implications of these two phenomena on the possibility of Bose-Einstein condensation of quantum well excitons. [Preview Abstract] |
Tuesday, March 14, 2006 11:39AM - 11:51AM |
H35.00003: Quantum well structure for a test of intersubband plasma instability P. Bakshi, C. Du, K. Kempa, T. Rocheleau, M. Sherwin, A. Gossard We have shown theoretically that a resonant interaction of an intersubband plasmon emission mode with an absorption mode creates a plasma instability [1]. This can be tested experimentally in an asymmetric double quantum well where the lowest three levels, with appropriate occupancy, provide near resonant emission and absorption modes. The third level is to be partially populated by THz laser pumping from the first level. An applied bias tunes the second level in relation to the other two. The two modes attract each other to form a pair of complex conjugate modes, thus creating a plasma instability. Such a structure has been designed, grown and characterized through absorption measurements without pumping. The results are in good agreement with theory calculations. Narrow Lorentzian lineshapes are observed, indicating a low level of collisions. A realistic pumping fraction can produce sufficiently strong instability to overcome the collisional damping. Characteristic instability criteria, including a modified lineshape are predicted, to be tested with pumping in the next phase of the program. [1] P. Bakshi and K. Kempa, Cond. MatterTheories,12,399 (1997); 20,45 (2005); Physica E7,63 (2000).Work supported by ARO and NSF. [Preview Abstract] |
Tuesday, March 14, 2006 11:51AM - 12:03PM |
H35.00004: Mapping the Copper energy band using the quantum well states J. Wu, J. Choi, T. Owens, Z. Q. Qiu, E. Rotenberg, N. V. Smith Quantum well states (QWS) of copper electrons in Cu/Co/Cu(100) system are investigated using Angle Resolved Photoemission Electron Spectroscopy (ARPES). The samples were grown epitaxially at room temperature and measured in situ at beamlime 7 of the Advanced Light Source (ALS). Photoemission intensity oscillates with both the electron energy and the Cu film thickness. By counting the thickness oscillation periodicity at a given energy, we can determine the out-of-plane electron momentum without the need of the phase value in the phase accumulation model. This allows the experimental determination of the E-k relation (energy band) for the Cu film. We here report the Cu energy band determined in this way at different in-plane momentum. In addition, by fitting the oscillation as a function of the Cu thickness, we also determined the phase value of the quantization condition as a function of the energy and in-plane momentum. [Preview Abstract] |
Tuesday, March 14, 2006 12:03PM - 12:15PM |
H35.00005: Coulomb drag in coupled quantum wells: effects of bandstructure and {\bf q}-dependent scattering Richard W. Deneen III, Ben Yu-Kuang Hu We study the effects of non-parabolicity of the band-structure and wavevector-dependent scattering rates on the Coulomb drag in coupled quantum wells, using a theoretical formulation that takes these effects into account [K. Flensberg and B. Y.-K. Hu, Phys.~Rev.~B {\bf 52}, 14796(1995)]. We examine the conditions in which this formulation reduces to the result given by von Oppen {\it et al.} [F. von Oppen, S. Simon and A. Stern, Phys.~Rev.~Lett. {\bf 87}, 106803 (2001)], in which the effects of band-structure are included through the terms $d\hat{\sigma} _i/dn_i$, where $\hat\sigma$ is the (single-layer) conductivity tensor and $n$ is the (single-layer) density and $i$ is the layer index. The effect of scattering that is strongly wavevector-dependent will also be discussed. [Preview Abstract] |
Tuesday, March 14, 2006 12:15PM - 12:27PM |
H35.00006: Dependence of current switching dynamics on contact conductivity in semiconductor superlattices Stephen W. Teitsworth, Huidong Xu Numerical simulation results are presented for a discrete drift-diffusion electronic transport model appropriate to weakly-coupled semiconductor superlattices [1]. Sequential resonant tunneling between adjacent quantum wells is the primary conduction mechanism for this model which also incorporates an effective contact conductivity $\sigma _{c}$. We study the dependence on $\sigma _{c}$ of time-averaged current-voltage characteristics and transient current response to abrupt steps in applied voltage. For intermediate values of $\sigma _{c}$, three qualitatively distinct transient responses -- each associated with a different mechanism for the \textit{relocation} of a static charge accumulation layer [1] - are observed for different values of voltage step $V_{step}$; these involve, respectively, 1) the motion of a single charge accumulation layer, 2) the simultaneous motion of one depletion and two accumulation layers [2], and 3) the simultaneous motion of two accumulation layers. The range of $V_{step}$ values for each mechanism and the relocation times associated with each are studied as a function of $\sigma _{c}$; a critical value of $\sigma _{c}$ is identified above which the second relocation mechanism is not observed for any value of $V_{step}$. Relocation times are found to depend sensitively on specific values of $\sigma _{c}$ and $V_{step}$. [1] L. L. Bonilla and H. T. Grahn, Rep. Prog. Phys. \textbf{68}, pp. 577-683 (2005), and refs. therein. [2] A. Amann, A. Wacker, L. L. Bonilla, and E. Schoell, Phys. Rev. E \textbf{63}, 066207 (2001). [Preview Abstract] |
Tuesday, March 14, 2006 12:27PM - 12:39PM |
H35.00007: Scanning probe study of dopant charging in a semiconductor heterostructure Irma Kuljanishvili, Cemil Kayis, James Harrison, Carlo Piermarocchi, Thomas Kaplan, Stuart Tessmer, Loren Pfeiffer, Ken West The smallest possible semiconductor nano device is a single dopant atom. The properties of such a structure are most prominent at low temperature, where multiple electrons or holes can be localized at the parent donor or acceptor. Using a scanning probe method, we study dopant atom charging in an AlGaAs/GaAs heterostructure with a delta-doped Si layer. The method is based on the Scanning Charge Accumulation (SCA) imaging technique that allows for studying a subsurface electronic system. Here we report on our results resolving the single-electron addition spectra of small numbers of dopant atoms. These highly localized capacitance measurements are contrasted to gated capacitance measurements. The SCA data are compared to a simple model based on a donor-molecule picture. [Preview Abstract] |
Tuesday, March 14, 2006 12:39PM - 12:51PM |
H35.00008: Theoretical Carrier Mobilities in $\delta$-doped AlInSb/InSb Hereterostructures Y. Shao, S. A. Solin, L. R. Ram-Mohan {Ultrathin films with thicknesses ($<$100 nm) and very high ($>$1 $m^{2} $/Vs) room temperature carrier mobility are of immense practical importance in a number of magnetic sensor applications.$\footnote [2]{S.A. Solin, and D.R. Hines, J. Magn. Magn. Mater., {\bf226,} 1976 (2000).} $ The electron transport mobilities in $\delta$-doped AlInSb/InSb heterostructures had been studied. The sub-band electron occupation and the energy levels were numerically obtained by solving the Schr\"{o}dinger and Poisson equations self-consistently as a function of spacer layer thickness, well width and temperature. The quantum energy levels were found within the quasi-classical approximation. The electron mobilities were calculated by combining ionized impurity, background impurity, deformation potential acoustic phonon and polar optic phonon scattering. The dependencies of the electron mobility on temperature, spacer layer thickness and quantum well thickness were simultaneously obtained. At 0K and room temperature, mobilities as high as 1.3x$10^{3}$ and 10 $m^{2}$/V s, respectively, were obtained at larger spacer layer (400nm) and well widths (400nm). In contrast to privious work, for the application of device design, the product of electron density and mobility was studied to maximize the transconductance. The model we used can be adaped to study other heterostructure. } [Preview Abstract] |
Tuesday, March 14, 2006 12:51PM - 1:03PM |
H35.00009: Three novel effects in nanostructures Gagik Shmavonyan To achieve the broadband characteristics, a sequence of non-identical multiple quantum well (MQWs) were designed. When designing a broadband semiconductor optical amplifier (SOA) using a non-identical MQW structure, factors such as QW transition energy, number and sequence of different QWs, the thickness of the separate confinement heterostructure (SCH) layer, the selection of the dominant carrier, the ability of the QW to trap the 2D carrier, the uniformity of the 2D carrier within the QWs, etc. must be taken into account. For SOAs fabricated on the substrate with five 6 nm InGaAsP quantum wells and two 150 nm InGaAsP quantum wells, a very broad emission spectrum is obtained. The spectral width is 400 nm, covering range from 1250 to 1650 nm. The broadest bandwidth for SOA ($\sim $ 400 nm) allows us to observe three novel effects: \begin{enumerate} \item Bi-directional guided effect of lasing mode in a bending waveguide of SOA. \item An optical switching effect in one SOA \item The SCH layer thickness effect. \end{enumerate} [Preview Abstract] |
Tuesday, March 14, 2006 1:03PM - 1:15PM |
H35.00010: Mesoscopic Anisotropic Magnetoconductance Fluctuations in Ferromagnets Shaffique Adam, Markus Kindermann, Saar Rahav, Piet W. Brouwer The conductance of a ferromagnetic particle depends on the relative orientation of the magnetization with respect to the direction of current flow. This phenomenon is known as ``anisotropic magnetoresistance.'' Quantum interference leads to an additional, random dependence of the conductance on the magnetization direction. These ``anisotropic magnetorestance fluctuations'' are caused by spin-orbit scattering, which couples the electron motion to the exchange field in the ferromagnet. We report a calculation of the dependence of the conductance autocorrelation function on the rotation angle of the magnetization direction. [Preview Abstract] |
Tuesday, March 14, 2006 1:15PM - 1:27PM |
H35.00011: Minigap plasmons in a periodically modulated two-dimensional electron gas: single-particle necktie spectra Hiroyuki Sakaki, Manvir Kushwaha We investigate the plasmon excitations in a two-dimensional electron gas (2DEG) subjected to a one-dimensional (1D) weak periodic potential. We derive and discuss the dispersion relations for both intrasubband and intersubband excitations within the framework of Bohm-Pines' random-phase approximation (RPA). For such an anisotropic system with spatially modulated charge density, we observe a splitting of the 2D plasmon dispersion. The splitting is caused by the superlattice effect of the charge-density modulation on the collective excitation spectrum. In addition, We observe how the energy-tunneling allows the intersubband single-particle excitations (SPE) to attain a finite width at the zone center and to contain the gaps inside. The resulting gaps inside the intersubband SPE give rise to an intriguing structures of {\it neckties} appearing at the zone boundaries in the excitation spectra illustrating plasmon energy versus Bloch vector. We discuss how the tunneling and the potential amplitude affect such a {\it necktie} spectrum. [Preview Abstract] |
Tuesday, March 14, 2006 1:27PM - 1:39PM |
H35.00012: Work function oscillations of atomic-layer-resolved Pb films Jinfeng Jia, Yun Qi, Xucun Ma, Shuaihua Ji, Yingshuang Fu, Qikun Xue By scanning tunneling microscopy, the local work function (LWF) dependance on thickness was studied on Pb quantum wedges with flattop grown on stepped Si(111) substrate. The oscillatory behavior of LWF was observed due to the quantum well state formation induced by the electrons confined in the [111] direction of Pb wedges. The electronic density of state, which is believed to play an important role in such physical property as LWF, was also obtained by scanning tunneling spectroscopy. It is found that the highest occupied quantum well state near Fermi level presented by dI/dV curve has great relationship with the LWF oscillating. Our results show good agreement with the theoretical predictions. This work directly proves that the local work function and the related physical properties are also modulated by quantum well state. [Preview Abstract] |
Tuesday, March 14, 2006 1:39PM - 1:51PM |
H35.00013: The classical limit of coherent transport properties Saar Rahav, Piet Brouwer We investigate the behavior of weak localization, conductance fluctuations, and shot noise of a chaotic scatterer in the semiclassical limit. Time resolved numerical results, obtained by truncating the time-evolution of a kicked quantum map after a certain number of iterations, are compared to semiclassical theory. Considering how the appearance of quantum effects is delayed as a function of the Ehrenfest time gives a new method to compare theory and numerical simulations. We find that both weak localization and shot noise agree with semiclassical theory, which predicts exponential suppression with increasing Ehrenfest time. However, conductance fluctuations exhibit different behavior, with only a slight dependence on the Ehrenfest time. [Preview Abstract] |
Tuesday, March 14, 2006 1:51PM - 2:03PM |
H35.00014: Composite-fermion antiparticle description of the hole excitation in the maximum-density droplet Alev Devrim Guclu, Gun Sang Jeon, Cyrus J. Umrigar, Jainendra K. Jain The maximum-density droplet of quantum dots is a finite-size realization of the state at filling factor one. For a sufficiently small number of electrons, it becomes unstable to the creation of a central hole as the magnetic field is increased or the strength of the confinement potential reduced. The simplest model for the hole is a vortex at the center, which, however, is renormalized by edge excitations. We show that a remarkably accurate description of the actual hole state is achieved in terms of a ``composite-fermion antiparticle,'' which is surprising in view of the fact that composite fermions are thought to be relevant only in the fractional Hall regime. The composite-fermion antiparticle description also allows us to study the effect of Landau level mixing through variational and diffusion Monte Carlo calculations in a very efficient manner. Generalizations to systems containing several holes, as well as to the quasiholes of fractional quantum Hall states are presented. [Preview Abstract] |
Tuesday, March 14, 2006 2:03PM - 2:15PM |
H35.00015: Giant piezoresistance in AlAs 2D electron systems with antidot lattice O. Gunawan, Y.P. Shkolnikov, K. Vakili, E.P.D. Poortere, M. Shayegan We report a novel giant piezoresistance effect in AlAs wide quantum well 2D electron system, patterned with an antidot lattice of about 1.0 $\mu $m period. At a low density of $\sim $ 3.5$\times $10$^{11}$/cm$^{2}$ and at T=0.3 K, the piezoresistance exhibits a strain gauge factor as large as 20,000, the largest value reported so far without magnetic field. Compared to the region without the antidot pattern, this antidot region represents $\sim $ 3.5$\times $ larger gauge factor and $\sim $ 5$\times $ wider dynamic range in piezoresistance. Such device may find important applications for super sensitive strain detection in mechanical microstructures. [Preview Abstract] |
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