Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session Q37: Magnetotransport in 2DEGs |
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Sponsoring Units: FIAP Chair: Mansour Shayegan, Princeton University Room: Morial Convention Center 229 |
Wednesday, March 12, 2008 11:15AM - 11:27AM |
Q37.00001: Effective mass suppression in interacting, fully spin-polarized 2D electron systems in wide AlAs quantum wells Medini Padmanabhan, T. Gokmen, N.C. Bishop, M. Shayegan We report effective mass measurements, via analyzing the temperature dependence of the Shubnikov-de Haas oscillations, in dilute two-dimensional electron systems (2DESs) confined to wide AlAs quantum wells. In this system electrons have an anisotropic in-plane Fermi contour. When the 2DES is partially spin-polarized, the effective mass is larger than its band value and increases as the density is reduced, consistent with previous results in various 2DESs. An unexpected trend emerges as we fully spin-polarize the 2DES by subjecting it to a strong parallel magnetic field: the mass falls below the band value and tends to decrease with decreasing density. [Preview Abstract] |
Wednesday, March 12, 2008 11:27AM - 11:39AM |
Q37.00002: Effective mass suppression in interacting, fully spin-polarized 2D electron systems in narrow AlAs quantum wells Tayfun Gokmen, Medini Padmanabhan, K. Vakili, M. Shayegan Similar to the study described by M. Padmanabhan et al. (previous abstract), we perform effective mass measurements in a two-dimensional electron system (2DES) confined to a narrow (45{\AA}-wide) AlAs quantum well. In contrast to the 2DESs confined to wide AlAs quantum wells, in this system the electrons occupy a single out-of-plane valley with an isotropic in-plane Fermi contour. We confirm that the effective mass for the fully spin-polarized 2DES is suppressed compared to the partially spin-polarized value. [Preview Abstract] |
Wednesday, March 12, 2008 11:39AM - 11:51AM |
Q37.00003: Effects of mass anisotropy, thickness and disorder on the spin susceptibility of the 2DEG in AlAs QWs Mariapia Marchi, Stefania De Palo, Gaetano Senatore, Saverio Moroni We present predictions of the spin susceptibility $\chi _{s}$, obtained from extensive DMC simulations, for a two dimensional (2D) electron gas (EG) with mass anisotropy appropriate to AlAs QWs[1], both in the strictly 2D limit and with thickness included. We demonstrate that in the one-valley case anisotropy suppresses $\chi _{s}$ substantially at all densities and in particular at those relevant to experiments[1], the effect being larger at lower density. This suppression adds onto the one due to thickness[2,1]. The comparison of our results for a model EG including both thickness and anisotropy with experiments for AlAs QW's[1] reveals the role of disorder in determining the measured spin susceptibility. In the two-valley case we find an interesting interplay of anisotropy and valley degree of freedom in determining the EG properties and in particular the spin susceptibility. [1] T. Gokmen et. al., Phys. Rev. B in press and cond-mat 0711.1294. [2] S. De Palo et. al., Phys. Rev. Lett. 94, 226405 (2005); and to be published [Preview Abstract] |
Wednesday, March 12, 2008 11:51AM - 12:03PM |
Q37.00004: Heat Transport Measurements on 2D Electron Systems at Zero Magnetic Field using Quantum Point Contacts D.A. Nichols, G. Granger, J.P. Eisenstein, J.L. Reno, L.N. Pfeiffer, K.W. West Three adjacent quantum point contacts (QPCs) separated by 20 micrometers are fabricated along the edge of a GaAs/AlGaAs two-dimensional electron gas (2DEG). The 2DEG is heated locally by passing an electrical current through the middle QPC, which is tuned so only a few channels propagate. A thermovoltage develops across a detector QPC on either side of the heater, and its gate voltage dependence is related to the derivative of the conductance of the detector QPC as expected from Mott's formula. The thermovoltage dependences on power and temperature are also investigated. These experiments illustrate the feasibility of using mesoscopic devices to study heat transport in 2D electron gases with various geometries. [Preview Abstract] |
Wednesday, March 12, 2008 12:03PM - 12:15PM |
Q37.00005: Observation of Chiral Heat Transport in the Quantum Hall Regime G. Granger, D.A. Nichols, J.P. Eisenstein, J.L. Reno, L.N. Pfeiffer, K.W. West The nature and properties of heat transport at the edge of a quantum Hall state are investigated using three adjacent quantum point contacts (QPCs) separated by 20 micrometers fabricated along the edge of a GaAs/AlGaAs two-dimensional electron gas (2DEG). With the bulk of the device at filling factor $\nu$=1, a thermovoltage signal appears across a detector QPC only on one side of the heater QPC depending on the direction of the magnetic field. This behavior indicates that heat transport is chiral at this filling factor. Raising the temperature decreases the thermovoltage, as the electrons carrying the heat find more ways to cool off at higher temperatures. When the distance between the heater and the detector is doubled, the thermovoltage is reduced, meaning that the electrons cool significantly over distances on the order of tens of micrometers. These findings are qualitatively insensitive to the exact magnetic field over the field range corresponding to the $\nu$=1 minimum. [Preview Abstract] |
Wednesday, March 12, 2008 12:15PM - 12:27PM |
Q37.00006: Momentum resolved transport spectroscopy of quantum Hall edges in a bent quantum well Lucia Steinke, A. Fontcuberta i Morral, M. Bichler, G. Abstreiter, M. Grayson A new magnetic field orientation is introduced for a bent quantum well, such that momentum-resolved edge-state spectroscopy is possible up to fractional filling factors. A bent quantum well (BQW) provides a unique way of coupling to quantum Hall edges when the junction length is reduced to the mean free path $\sim 10 \mu$m, constituting a weak link between the two facets. With a magnetic field B perpendicular to one facet, the other facet can probe momentum-resolved transport spectroscopy of the edge states even though no barrier is present. We measure the differential conductance dI/dV across the BQW junction as a function of the dc bias voltage Vdc at magnetic fields between 0 and 18 T ($\nu < 1/3$). Above 1.5 T a conductance-suppression gap evolves around zero bias, and resonance peaks are observed at the gap edges, reminiscent of previous momentum-resolved work. The gap size increases and at 6 T becomes asymmetric, with one of the flanking resonances becoming stationary and extremely sharp. These main features can be interpreted in terms of momentum resolved coupling, and the positions of these features are in good quantitative agreement with Hartree calculations of this system. [Preview Abstract] |
Wednesday, March 12, 2008 12:27PM - 12:39PM |
Q37.00007: From insulating behavior to quantum Hall liquid at low magnetic fields T.-Y. Huang, Chi-Te Liang, Gil-Ho Kim, Chun Feng Huang, C.-P. Huang, J.-Y. Lin, H.-S. Goan, D.A. Ritchie It is an interesting, but unsettled issue whether a direct transition from an insulating (I) state to a $\nu>2$ quantum Hall (QH) liquid is a genuine phase transition where $\nu$ denotes the filling factor [1]. It is argued that the observed low-field direct transition is not a quantum phase transition, but can be ascribed to a crossover from weak localization to Landau quantization (LQ) [1]. We shall show that between the insulating region and the QH regime, multiple temperature ($T$)-independent points in the longitudinal resistance can be observed in a moderate-mobility two-dimensional electron system containing InAs quantum dots. Interestingly, the amplitudes of the accompanying resistance oscillations can be well approximated by the conventional Shubnikov-de Haas theory, suggesting metallic behavior. Moreover, our data show that LQ can modulate the density of states without causing the formation of a QH liquid, demonstrating that the crossover from insulating behavior to Landau quantization can occur over a wide range of magnetic field. We suggest that to obtain a correct insight into the low-field I-QH transition, the argument raised by Huckestein [1] ought to be modified. Ref: [1] B. Huckestein, PRL 84, 3141 (2000) and references therein. [Preview Abstract] |
Wednesday, March 12, 2008 12:39PM - 12:51PM |
Q37.00008: In-plane field induced anisotropy in the microwave/rf resonances of 2D electrons at the second excited Landau level Han Zhu, G. Sambandamurthy, L. Engel, D. C. Tsui, L. Pfeiffer, K. West We report measurements of the microwave/rf diagonal conductivity of ultrahigh mobility 2D electron systems in GaAs/AlGaAs quantum wells, at the second excited Landau level (LL), with in-plane magnetic field $B_{ip}$. Previous measurements [1] at $B_{ip}=0$ have found an essentially isotropic pinning mode resonance of the bubble phase near LL filling 4.15 to 4.4. As $B_{ip}$ is applied, with the rf electric field $E$ either parallel or perpendicular to $B_{ip} $, the peak frequencies $f_{pk}$ and resonance widths almost identically increase, probably due to stronger pinning as the electron wavefunction is being pushed closer to the GaAs/AlGaAs interface. However, $B_{ip}$ is found to induce anisotropy in the ratio of the integrated intensity $S$ over $f_{pk}$, which is thought to be proportional to the participating carrier density. As $B_{ip}$ is applied, $S/f_{pk}$ increases with $E$ perpendicular to $B_{ip}$ and decreases with them parallel. Similar behavior is found in the resonances from the Wigner crystal phase formed within the same LL. [1] R. Lewis et al., PRL 89,136804 (2002). [Preview Abstract] |
Wednesday, March 12, 2008 12:51PM - 1:03PM |
Q37.00009: Microwave modes of two dimensional electron systems near macroscopic ferromagnets Brenden Magill, L. W. Engel, M. P. Lilly, J. A. Simmons, J. L. Reno We report on microwave measurements of a high mobility two dimensional electron system (2DES) in a homogenous external field B$_{0}$, and with cylindrical ferromagnets of radii r$_{m}$ placed on the surface of the sample with the long axis perpendicular to the 2DES. The magnet materials are Dy and permalloy, and r$_{m}$ varies from 0.5 mm to 0.125 mm. Microwave spectra measured for transmission between two ohmic contacts show resonant absorption at peak frequency, f$_{pk}$, decreasing as B$_{0 }$or r$_{m }$increase. We will interpret the data in terms of plasma excitations similar to edge magnetoplasmons [1] confined under the edges of the magnets by the large magnetic field gradients there. [1] See, for example, V. A. Volkov and S. A. Mikhailov, Sov. Phys.-JETP \textbf{67}, 1639(1988). [Preview Abstract] |
Wednesday, March 12, 2008 1:03PM - 1:15PM |
Q37.00010: Radiation-induced decay of Shubnikov-de Haas oscillations in the regime of the radiation-induced zero-resistance states in GaAs/AlGaAs devices Ramesh Mani The observation of zero-resistance states upon photo-exciting a quasi two-dimensional electron system has increased the experimental and theoretical interest in the photoexcited steady states of the low dimensional electron system.[see 1] We examine here the experimentally observed decay of Shubnikov-de Haas (SdH) oscillations under photoexcitation in the regime of the radiation-induced zero- resistance states with resolved Landau levels, in order to obtain further insight into the zero-resistance states themselves.[2,3] A covariation study indicates that the SdH amplitude scales simply with the average background resistance in the vicinity of the radiation-induced resistance minima and maxima,[3,4] as the SdH amplitude vanishes in proportion to the background resistance at the centers of the zero-resistance states. The results suggest that the dissipative current is suppressed in proportion to the background resistance, at the centers of the radiation-induced zero-resistance states. [1] R. G. Mani, Sol. St. Comm. 144, 409 (2007). [2] R. G. Mani, Appl. Phys. Lett. 85, 4962 (2004). [3] R. G. Mani, Appl. Phys. Lett. 91, 132109 (2007). [4] R. G. Mani, unpublished. [Preview Abstract] |
Wednesday, March 12, 2008 1:15PM - 1:27PM |
Q37.00011: Floquet Theory of Magneto-Resistivity Oscillations in Microwave Irradiated 2DEGs Assa Auerbach, G. Venketeswara Pai Some remarkable phenomena have been recently observed in semiconductor heterostructures: microwave induced resistivity oscillations (MIRO), Hall induced resistivity oscillations (HIRO), and zero resistance states (ZRS). These effects were seen at weak magnetc fields and high temperatures, where Shubnikov de-Hass oscillations are thermally smeared and the transport is expected to be classical Drude-like. However microwave radiation, or large Hall currents expose the underlying Landau quantization and result in MIRO and HIRO. Theoretically, it is essential to get the full {\bf nonlinear current-field response} in the presence of strong radiation fields and disorder to handle these effects. Here we generalize the Floquet operator approach to incorporate arbitrary large electric fields into the zeroth order evolution operator. We construct the {\em disordered Floquet evolution operator} which allows us to systematically calculate the nonlinear photocurrent to second order in short range disorder. We derive the magnitude of MIRO from the microscopic parameters. We deduce the optimal conditions of disorder for obse4rving large MIRO and ZRS effects. We determine the characteristic Hall fields detemining the HIRO, and the magnitude of spontaneous ZRS fields. Reference: A. Auerbach and G. V. Pai, Phys. Rev. B {\bf 76}, 205318 (2007). [Preview Abstract] |
Wednesday, March 12, 2008 1:27PM - 1:39PM |
Q37.00012: The origin of the stripes observed in scanning single-electron transistor and in mesoscopic transport measurements of quantum Hall samples Chenggang Zhou, Mona Berciu We analyze two seemingly unrelated types of experiments on quantum Hall samples. When the measured quantities (local compressibility and resistances) are plotted as a function of magnetic field($B$) and electron density($n_e$), both experiments exhibit stripes parallel to lines of integer filling factors on the $B$-$n_e$ plane. Unlike the popular belief in Coulomb blockade physics, we explain this within the framework of non-interacting electron theory. Our numerical simulations and theoretical analysis demonstrate that new electronic states appear predominantly at the center of Landau levels, when the magnetic field increases. This leads to a certain ``spectral ordering" of the localized states that is sufficient to explain the main features observed in the experiments. [Preview Abstract] |
Wednesday, March 12, 2008 1:39PM - 1:51PM |
Q37.00013: External Field Effecting Excitonic Superfluid in Drag Geometry Jung-Jung Su, Allan H. MacDonald We will report on transport properties of excitonic superfluid under the influence of external applied field in generalized drag geometries. In such geometry, voltages are applied to the driven layer and a tunnable load resistance is added across the drag layer, which enable current flows in the drag layer when an excitonic condensate is established. The physics of the excitonic superfluid is described macroscopically by classical model and justified by microscopic mean-field plus non-equilibrium Green's function(NEGF) approach. We found that the excitonic superfluid only exist in the voltage configurations in which charge conservation law is satisfied, given a zero tunneling system. This system is then well described by a set of circuit, on the premise that the quasiparticle current can not flow through the system, and the current thus obtained is in great consistent with our NEGF calculation. Finally, we proposed a method of detecting tunneling strength by the non-conservation of charge in the presence of bare tunneling. [Preview Abstract] |
Wednesday, March 12, 2008 1:51PM - 2:03PM |
Q37.00014: Supersolid of indirect excitons in electron-hole quantum Hall systems Chang-hua Zhang, Yogesh N. Joglekar We investigate the ground state of a balanced electron-hole system in the quantum Hall regime using mean-field theory and obtain its phase diagram as a function of interlayer distance $d$ and the filling factor within a layer. We identify an excitonic condensate phase, a supersolid phase, as well as uncorrelated Wigner crystal states. We find that balanced electron-hole system exhibits a supersolid ground state over a wide range of filling factors. We obtain the ground state stiffness in the the excitonic phases and show that the phase transitions from a uniform condensate to a supersolid is accompanied by a marked change in the stiffness. Our results provide the first semi-quantitative determination and analysis of the supersolid of excitons. [Preview Abstract] |
Wednesday, March 12, 2008 2:03PM - 2:15PM |
Q37.00015: Insulating, Metallic, and Superconducting Transport regimes of 2D Amorphous Superconducting Films in B-T-Disorder Space Yize Li, Jongsoo Yoon Amorphous tantalum thin films are known to exhibit a superconductor-metal-insulator transition in the zero temperature limit with increasing magnetic fields. The metallic phase intervening the superconducting and insulating phase is unexpected. Each phase is known to exhibit unique nonlinear transport properties [1] with intrinsic origins [2]. In order to study how the mechanism behind each phase is influenced by B, T, and disorder, we have measured the evolution of nonlinear transport properties by changing B, T, and disorder. The resulting ``phase diagram'' of a sample with normal state sheet resistivity of 2.3 kilo-ohm indicates that the superconducting phase is completely surrounded by metallic phase, and a direct superconductor-insulator transition is not allowed. Recently, we extend our study on other samples with different disorder that is controlled by film thickness. By combining these results, we can map out the 3D phase diagram in B-T-disorder space. [1] Y.Qin et al., Phys. Rev. B 73, 100505(R) (2006). [2] Y. Seo et al., Phys. Rev. Lett. 97, 057005 (2006). [Preview Abstract] |
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