Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session V47: Spectroscopy and Transport in 2D Semiconductors |
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Sponsoring Units: FIAP Chair: Emilio Mendez, SUNY at Stony Brook Room: Baltimore Convention Center 350 |
Thursday, March 16, 2006 11:15AM - 11:27AM |
V47.00001: Magnetotunneling Spectroscopy in InGaAs Double-well Structures Y. Lin, A.K.M. Newaz, E.E. Mendez, J. Nitta We have used magnetotunneling spectroscopy to elucidate the mechanism of electron tunneling in double-well (DW) heterostructures whose current-voltage $(I-V)$ characteristics are quite different from the $\delta$-function-like shapes expected from energy- and momentum-conservation laws. Our DW tunneling structure consisted of two In$_{0.53}$Ga$_{0.47}$As wells 53 \AA\, and 82 \AA\,wide, confined by In$_{0.52}$Al$_ {0.48}$As barriers with thickness of 100 \AA\,, 53 \AA\, and 100 \AA\,. Without magnetic field, at $T = 4.2$K the $I-V$ characteristics had several distinct regions, each one quasi- triangular in shape and corresponding to the alignment of states in adjacent wells. Under a field ($H \leq14$T) parallel to the current, each region revealed strong features that shifted to higher voltages with increasing field and are related to Landau levels from states in individual wells. The field dependence of those features yielded, in addition to the band parameters, the zero-field energies of the states at representative voltages of the $I-V$ characteristics. In the physical picture that emerges from the analysis, electrons tunnel via the two wells as long as in each of them there are states below the emitter's Fermi energy, without any consideration to in-plane momentum (or Landau-level index) conservation. This conclusion is supported by remarkably similar results we have also obtained in GaAs-GaAlAs double- well structures. [Preview Abstract] |
Thursday, March 16, 2006 11:27AM - 11:39AM |
V47.00002: Shot Noise in Coherently Coupled GaAs-AlGaAs Double-Well Resonant-Tunneling Diodes A.K.M. Newaz, W. Song, B. Nielsen, E.E. Mendez, R. Hey, H. Kostial, H.T. Grahn We have found experimentally that, regarding shot-noise, there is no difference between coherent and sequential resonant tunneling, in contrast with calculations that predict that the suppression of noise in a coherent-tunneling process should be larger. Our measurements were done at 4.2 K in GaAs-AlGaAs Double-Well Resonant-Tunneling Diodes (DWRTD) grown by molecular beam epitaxy on n$^+$GaAs substrates. Each AlGaAs barrier at the two ends of each structure was 100 \AA\,, while the central barriers were 60 \AA\, (uncoupled DWRTD), 20 \AA\, and 15 \AA\, (coupled DWRTDs) thick, depending on the sample. In all cases, the two quantum wells between the barriers were 53 \AA\, and 80 \AA\, wide, each well having two confined quantum states. The current-voltage characteristics of the diodes exhibited current peaks associated with voltage-induced alignment of states in adjacent wells. In the coupled DWRTDs each peak split into two, as a result of symmetric and anti- symmetric wavefunctions being extended to both wells. Noise measurements revealed a shot noise power spectrum up to about $50\%$ smaller than that of Poissonian noise $2eI$, regardless of whether the wells were coupled or uncoupled. Our results support those calculations in multiple-barrier structures that predict that the shot-noise reduction should be independent of whether the electronic transport is sequential or coherent. [Preview Abstract] |
Thursday, March 16, 2006 11:39AM - 11:51AM |
V47.00003: Observation of a periodic structure in quantum rings using Scanning Gate Microscopy F. Martins, B. Hackens, T. Ouisse, J.F. Motte, M. Stark, H. Sellier, J. Chevrier, S. Huant, V. Bayot, S. Bollaert, X. Wallart, A. Cappy Recent advances in scanning gate microscopies (SGM) have made it possible to image electron flow in nanostructures, using the tip as a moving gate (M.A. Topinka et al., Science 289, 2323 (2000)). Our experiment focuses on quantum rings (QR), patterned from a two-dimensional electron gas (2DEG) buried close to the sample surface (25 nm). We can therefore reach a very small tip-sample distance, thereby improving the imaging resolution. Furthermore, the absence of metallic gates on the top of our sample allows to probe the electron flow directly inside the QR. The SGM images, obtained at 4.2 K, reveal striking periodic oscillations of the resistance along the QR circumference as the tip scans over the sample surface. These oscillations respect the radial symmetry of the QR, and their periodicity is much larger than the Fermi wavelength. We analyze the influence of the tip-induced perturbation on the 2DEG by changing the tip voltage (both in accumulation and depletion regimes) and the tip-sample distance. [Preview Abstract] |
Thursday, March 16, 2006 11:51AM - 12:03PM |
V47.00004: Scanning gate microscopy on Aharonov-Bohm rings in a magnetic field~: experiments and simulations B. Hackens, F. Martins, T. Ouisse, J.F. Motte, M. Stark, H. Sellier, J. Chevrier, S. Huant, V. Bayot, S. Bollaert , X. Wallart, A. Cappy We use low temperature scanning gate microscopy to study the electron dynamics inside Aharonov-Bohm (AB) rings in the phase-coherent regime. Our samples are prepared by etching from high-mobility two-dimensional electron systems 25 nm below the surface (B. Hackens et al., Phys. Rev. Lett. 94, 146802 (2005)). The bias applied on the AFM tip, as it is scanned over the AB ring, induces a local perturbation of the electric potential experienced by the electrons, thereby affecting the ring resistance. In particular, we observe in detail the effect of small variations of the perpendicular magnetic field in different regimes (up to B=6 T). As we plot these resistance variations as a function of the AFM tip position, we observe resistance modulations which have the spatial symmetry of the ring, and an amplitude equal to that of the AB oscillations. This strongly suggests that our resistance maps are closely linked to the periodic modulations of the electron probability density. Finally, we also performed simulations of the electron wavefunctions in AB rings, as well as of the electron transmission through the AB rings. Including the effect of the perturbating potential of the tip in such simulations, we get valuable informations which help to explain our experimental results. [Preview Abstract] |
Thursday, March 16, 2006 12:03PM - 12:15PM |
V47.00005: Optical Imaging for the Determination of Minority Carrier Transport David Luber, Frank Bradley, Nancy Haegel A scanning electron microscope technique is used, in combination with an optical imaging system, to measure minority carrier diffusion length in a heavily doped GaAs double heterostructure. Both diffusion and drift of charge are imaged. A diffusion length of $\sim $ 3.2 - 4 $\mu $m is measured, corresponding to a minority carrier mobility of 950-1480 cm$^{2}$/Vs in p-type material, doped $\sim $ 5 x 10$^{18}$ cm$^{-3}$, in good agreement with theoretical predictions. Drift and diffusion behavior are also studied in more heavily doped regimes where plasmon scattering is expected to play a significant role. Measurements will be presented as a function of doping level, local electric field and sample temperature. We explore the limitations on determination of the diffusion length due to effects such as the finite nature of the excitation volume and photon recycling near the point of generation and compare approaches for extracting key material parameters. The technique offers a flexible approach to direct measurement of various transport properties and is applicable to a range of luminescent materials and multilayer devices. Examples from high resolution mapping of non-uniform electric field distributions will also be presented. [Preview Abstract] |
Thursday, March 16, 2006 12:15PM - 12:27PM |
V47.00006: Transverse magnetic focusing and spin-dependent reflection experiments in InSb- and InAs-based heterostructures Hong Chen, J.J. Heremans, J.A. Peters, J.T. Beardsley, N. Goel, S.J. Chung, M.B. Santos, W. Van Roy, G. Borghs We report on mesoscopic spin-dependent ballistic transport in InSb/InAlSb and InAs/AlGaSb heterostructures, characterized by strong spin-orbit interaction. Spin-orbit interaction in two-dimensional electron systems can lead to spin-splitting in semiclassical ballistic trajectories and in spin-dependent reflection of carriers. We experimentally demonstrate the spin reflection in open and closed ballistic geometries optimizing scattering off a lithographic barrier. Further, experimentally obtained transverse magnetic focusing spectra can be interpreted as demonstrating both spin-split ballistic transport and spin-dependent reflection for higher order peaks involving carrier reflection off the barrier. NSF DMR-0094055 (JJH), DMR-0080054, DMR-0209371 (MBS). [Preview Abstract] |
Thursday, March 16, 2006 12:27PM - 12:39PM |
V47.00007: Effect of dc and ac excitations on the longitudinal resistance of a two-dimensional electron gas in highly doped GaAs quantum wells Jingqiao Zhang, Alexey Bykov, Sergey Vitkalov, A. K. Kalagin, A. K. Bakarov Linear AC (888 Hz) resistance of highly mobile two dimensional electrons in GaAs heavily doped quantum wells is studied at different magnitude of dc and ac (10 KHz to 20 GHz) excitations. In the DC excitation regime the differential resistance oscillates with the dc current and external magnetic field similar to that observed earlier in AlGaAs/GaAs heterostructures. At external ac excitations the resistance is also found to be oscillating with the magnetic field. However the form of the oscillations is considerably different from the dc case. We show that at frequency below 100 KHz the difference is result of a specific average of the dc differential resistance during the period of the external ac excitations. [Preview Abstract] |
Thursday, March 16, 2006 12:39PM - 12:51PM |
V47.00008: Thermodynamic and transport properties of two-dimensional GaAs systems near the apparent metal-insulator transition E.A. Galaktionov, G.D. Allison, A.K. Savchenko, S.S. Safonov, M.M. Fogler, M.Y. Simmons, D.A. Ritchie An extensive study of the charge compressibility and resistance of two-dimensional electron and hole systems is reported. A total of five GaAs samples were examined, two 2DEGs and three 2DHGs. All samples have high mobilities but vary in spacer, doping, and other growth parameters. The inverse compressibility is found to have a negative minimum at a sample-dependent carrier concentration followed by an upturn to large positive values as the concentration decreases. This behavior is shown to agree quantitatively with the theory of nonlinear screening of the random impurity potential, for both types of carriers, despite a ten-fold difference in their $r_s$ parameters. The region of the apparent metal-insulator transition (MIT) where $\partial R / \partial T$ changes sign, and the position of the inverse compressibility minimum can differ by as much as 50\%. Their relative positions vary with sample and cooldown. No thermodynamic anomalies at the apparent MIT are detected. In a range of $T$ where both the capacitance and the resistance of 2DHG samples were studied, the latter exhibits a scaling characteristic of the percolation transition. The corresponding percolation threshold is different from the MIT but agrees quantitatively with the percolation point deduced from the analysis of the compressibility. [Preview Abstract] |
Thursday, March 16, 2006 12:51PM - 1:03PM |
V47.00009: Evidence for coherent transport in GaAs hole open quantum dots. S. Faniel, B. Hackens, A. Vlad, C. Gustin, L. Moldovan, S. Melinte, V. Bayot, M. Shayegan We report magnetotransport measurements in GaAs hole open quantum dots. Our samples were fabricated from a $p$-type GaAs quantum well with a density of $2.2 \times 10^{15} \ \rm m^{-2}$ and a mobility of $35 \ \rm m^2/Vs$. Two different dots were patterned using e-beam lithography and wet etching. A top gate was added in order to control the dots openings and the hole density. The measurements were performed down to 30 mK with the magnetic field applied perpendicular to the plane of the two- dimensional system. We observed large, reproducible conductance fluctuations associated with the coherent transport of holes inside the dots at lowest temperatures which vanish above 500 mK. From the variance of these fluctuations and from the Random Matrix Theory, we extracted the hole dephasing time $\tau_\varphi$. The temperature dependence of the calculated $\tau_\varphi$ lies between a $T^{-1}$ and $T^{-2}$ behavior and exhibits a saturation at very low temperature which is similar with $\tau_\varphi$ measured in 2D electron systems\footnote{B. Hackens \emph{et al.}, Phys. Rev. Lett. \textbf{94}, 146802 (2005)}. [Preview Abstract] |
Thursday, March 16, 2006 1:03PM - 1:15PM |
V47.00010: Surface Gated Quantum Hall Effect in InAs Heterostructures Ian Gelfand, Sami Amasha, Dominik Zumbuhl, Marc Kastner, Christoph Kadow, Arthur Gossard We demonstrate low leakage surface gating of Indium Arsenide two dimensional electron gasses by observing a surface gated quantum Hall effect in an InAs-AlSb heterostructure. Gating is made possible by growing an aluminum oxide layer on top of the device. We find the depletion point can be changed by applying a large positive gate voltage, and we see hysteresis below depletion. Supported by ARO W911NF-05-1-0062, NSEC PHY-0117795, NSF DMR-0353209 and DMR-0213282. [Preview Abstract] |
Thursday, March 16, 2006 1:15PM - 1:27PM |
V47.00011: Anisotropic thermopower and planar Nernst effect in Ga1-xMnxAs Yong Pu, Jing Shi We have carried out the thermopower and magneto-thermopower measurements on three Ga1-xMnxAs~ (x=0.039, 0.049, and 0.059) samples above and below their~Curie temperatures. Below the Curie temperature, we have observed magnetic field dependence in both parallel (longitudinal) and perpendicular (transverse) directions to the temperature gradient.~ The magnetic field dependence does not depend on the relative orientation between the temperature gradient and crystallographic direction of the GaAs.~ The transverse thermopower or the planar Nernst effect,~shows abrupt jumps as the magnetization switches between the four-fold symmetry axes, which resembles the giant planar Hall effect found in similar materials.~ Although the longitudinal thermopower is not sensitive to the same switchings of the magnetization, it has a strong magnetic field dependence as the magnetization rotates.~ We have measured both effects vs. the magnetic field orientation with respect to the temperature gradient.~~Similar to the origin of the giant planar Hall effect, we attribute the longitudinal and transverse effects to the anistropic magneto-thermopower.~ We interpret the both effects~in the context of~the~anisotropic transport. [Preview Abstract] |
Thursday, March 16, 2006 1:27PM - 1:39PM |
V47.00012: Long-Lived Coherent Cyclotron Oscillations in a 2D Electron Gas X. Wang, R. Srivastava, A. Barkan, D. M. Mittleman, J. Kono, J. L. Reno We have used the technique of time-domain THz spectroscopy to study coherent charge dynamics in a high-mobility two-dimensional electron gas (2DEG) in quantizing magnetic fields. By analyzing the magnetic field dependence of the transmitted THz waveform, we successfully extracted the real and imaginary parts of the dynamic conductivity at various magnetic fields and temperatures for a GaAs/AlGaAs 2DEG with a 1.5 K mobility of 3.7 $\times $ 10$^{6}$ cm$^{2}$/Vs. We observed pronounced coherent cyclotron oscillations persisting more than 50 picoseconds and 20 periods. From the frequency and decay time of these oscillations, we can directly determine the cyclotron mass and phase coherence time. The basic phenomenon observed can be understood as the free induction decay of a coherent superposition between the lowest unfilled Landau level and the highest filled Landau level induced by the incident coherent THz pulse. Finally, we discuss the possibility of using this technique to overcome the saturation effect, which is known to prevent the determination of true cyclotron resonance linewidths in high-mobility ($>$ 10$^{5}$ cm$^{2}$/Vs) 2DEG measured with conventional far-infrared techniques using CW and incoherent sources. [Preview Abstract] |
Thursday, March 16, 2006 1:39PM - 1:51PM |
V47.00013: Modulation of two-dimensional electrons in Si/SiGe heterostructures using atomic-layer-deposited gate dielectric T. M. Lu, K. Lai, P. D. Ye, W. Pan, D. C. Tsui, S. Lyon, M. Muhlberger, F. Schaffler Field effect transistors (FETs) have been fabricated using Atomic-Layer-Deposited (ALD) Al$_2$O$_3$ as gate dielectric on the Si/SiGe heterostructures to modulate the density of the two-dimensional (2D) electrons in the strained silicon channel [1]. Magnetotransport measurements taken at 0.3K show that the 2D density (n) can be uniformly tuned by the gate voltage (VG) with virtually no leakage current and negligible gate hysteresis. The characteristic of modulation using ALD Al$_2$O$_3$ as gate dielectric is shown to be better than that using Pd Schottky barrier or the PECVD SiO$_2$ dielectric. We also performed a numerical simulation that solves the one-dimensional Poisson-Schrodinger equations self-consistently for different V$_G$’s. The measured n vs. V$_G$ relation can be reproduced in our calculation by assuming a density of 0.6$\times10^{13}$eV$^{-1}$cm$^{-2}$ of the surface states. [1] K. Lai $et al.$, Appl. Phys. Lett. 87, 142103 (2005) [Preview Abstract] |
Thursday, March 16, 2006 1:51PM - 2:03PM |
V47.00014: Hysteresis and saturation of intersubband absorption by electrons on helium Dmitri Ryvkine, Michael Lea, Mark Dykman We propose a mechanism, develop a theory, and provide experimental data that demonstrate hysteresis of resonant inter-subband absorption by a quasi two-dimensional electron system on the surface of helium. The electrons form a strongly correlated nondegenerate electron liquid. The absorption occurs at the frequency of the transition from the ground to the first excited state of motion normal to the helium surface. The lifetime of the excited state is extremely long, ~ 0.1 $\mu$s at 0.3 K, leading to strong absorption nonlinearity even for low radiation intensity [1]. Besides absorption saturation, radiation-induced occupation of the excited subband causes a shift of the resonant transition frequency. We show that this shift leads to absorption hysteresis with varying radiation frequency or intensity. As a result of electron correlations, the in-plane motion in all subbands is described by the same electron temperature, which is found self-consistently. The results bear on quantum computing with electrons on helium, since they demonstrate resonant transitions responsible for a single-qubit operation; the hysteresis results from the same mechanism that leads to two-qubit operations. 1. E. Collin et al., Phys. Rev. Lett. 89, 245301 (2002) [Preview Abstract] |
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