Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session A45: Quantum Hall Effect: Bilayers and Microwave Induced Resistance Oscillations |
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Sponsoring Units: FIAP Chair: Michael Zudov, University of Minnesota Room: Mile High Ballroom 4D |
Monday, March 3, 2014 8:00AM - 8:12AM |
A45.00001: Tunneling at $\nu_T=1$ in a bilayer quantum Hall exciton condensate D. Nandi, T. Khaire, A.D.K. Finck, J.P. Eisenstein, L.N. Pfeiffer, K.W. West Closely-spaced bilayer quantum Hall systems at total filling factor $\nu_T=1$ exhibit spontaneous interlayer phase coherence. This phase coherence, which is tantamount to excitonic Bose condensation, is most dramatically revealed via interlayer tunneling measurements.In the condensed phase the tunneling current-voltage ($IV$) characteristic of this semiconductor system strongly resembles the dc Josephson effect observed in superconducting tunnel junctions. Here we report on a detailed study of this phenomenon. We find the maximum, or critical tunneling current $I_c$ to be a well-defined global property of the macroscopic tunnel junction, insensitive to external circuit elements and the precise contact configuration used to observe it. Interestingly, the temperature dependence of $I_c$ displays an unexpected scaling behavior. At the lowest temperatures the slope of the ``supercurrent'' branch of the tunneling $IV$ curve, while extremely large, remains finite. Careful measurements in this regime suggest that dissipative processes arising from in-plane exciton transport limit the maximum tunneling conductance. Finally, comparisons of the experimentally observed $IV$ with recent theoretical predictions will be discussed. [Preview Abstract] |
Monday, March 3, 2014 8:12AM - 8:24AM |
A45.00002: Effect of perpendicular electric fields on quantum Hall stripe phases J. Pollanen, J.P. Eisenstein, L.N. Pfeiffer, K.W. West High quality two dimensional electron systems (2DES) in GaAs can exhibit large transport anisotropies near half filling of excited Landau levels [1,2] associated with the emergence of collective electron states possessing broken rotational symmetry in the plane of the 2DES. These states, known as the stripe phases, appear to be among the first known examples of purely electronic nematic liquid crystals. Experiments show that the orientation of the stripes is keyed to the crystallographic axes of the GaAs host lattice. Identification of the native symmetry-breaking potential remains an active area of interest, with strain and spin-orbit mechanisms recently proposed [3,4] as being responsible. Noting that both strain and spin-orbit effects can be altered by the application of a perpendicular electric field, we have performed magneto-transport experiments on narrow (20nm) GaAs quantum wells equipped with front and backside electrostatic gates. These gates allow us to study the effect, at constant 2D electron density, of perpendicular electric fields on the various quantum Hall stripe phases. [1]M.P. Lilly et al. PRL 82, 394 (1999) [2]R.R. Du et al. Solid State Commun. 109, 389 (1999) [3]S.P. Kovudayur et al. PRL. 106, 016804 (2011) [4]I. Sodemann and A.H. MacDonald, arXiv:1302.3896 [Preview Abstract] |
Monday, March 3, 2014 8:24AM - 8:36AM |
A45.00003: Bilayer mapping of the paired quantum Hall state in the half-filled second Landau level Jae-Seung Jeong, Kwon Park The fractional quantum Hall effect observed in the half-filled second Landau level is one of the most fascinating phenomena in condensed matter physics due to the possibility of emergent pairing with quasiparticle excitations satisfying non-Abelian statistics. The leading theory for the paired quantum Hall state in the half-filled second Landau level is based on the Moore-Read Pfaffian wave function, which is intimately connected with the Halperin (331) wave function for the bilayer quantum Hall system in the sense that the former is obtained via antisymmetrization of the spatial part of the latter. Motivated by this intriguing connection, we investigate a generalized mapping between the bilayer and the paired quantum Hall state at half filling via exact diagonalization in the torus geometry. [Preview Abstract] |
Monday, March 3, 2014 8:36AM - 8:48AM |
A45.00004: Observation of microwave-induced resistance oscillations in high-mobility 2D hole gas in sGe/SiGe quantum wells Q.A. Ebner, P.D. Martin, Q. Shi, M.A. Zudov, O.A. Mironov, R.J.H. Morris, D.R. Leadley Microwave-induced resistance oscillations (MIRO) have been extensively studied for more than a decade but, until now, have remained unique to GaAs/AlGaAs-based 2D electron systems. In this talk we report on the observation of MIRO in a very different setting, a 2D hole gas hosted in strained Ge/SiGe quantum wells. These findings demonstrate that MIRO is a universal phenomenon and that microwave photoresistance can be utilized to probe the energy spectrum and the correlation effects of 2D holes in Ge/SiGe quantum wells. [Preview Abstract] |
Monday, March 3, 2014 8:48AM - 9:00AM |
A45.00005: Effect of direct current on giant negative magnetoresistance in two-dimensional electron systems Q. Shi, P.D. Martin, Q.A. Ebner, M.A. Zudov, L.N. Pfeiffer, K.W. West We report on a giant negative magnetoresistance in a 200 micron-wide Hall bar fabricated from GaAs/AlGaAs quantum well. Comparison with theory shows that magnetoresistance is much stronger than one could expect from either electron-electron interaction or classical memory effects due to sharp and smooth disorder. In this talk we systematically examine the effect of direct current and compare our findings with temperature dependence. [Preview Abstract] |
Monday, March 3, 2014 9:00AM - 9:12AM |
A45.00006: Photoresistance of two-dimensional electron gas at sub-Terahertz frequencies P.D. Martin, M.A. Zudov, J.D. Watson, M.J. Manfra, L.N. Pfeiffer, K.W. West Extending experiments on photoresistance of ultra-high mobility 2DES to higher radiation frequencies allows to enter the regime of strong Shubnikov-de Haas oscillations (SdHO), which remains largely unexplored. This talk reports on low-temperature photoresistance measurements using frequencies from 0.2 to 0.4 THz. At higher radiation intensity, we observe a series of very strong and narrow peaks which occur near the cyclotron resonance. At lower intensities, strong peaks disappear and the data reveal a suppression of SdHO near the cyclotron resonance, and to a lesser extent, near its harmonics. These findings will be compared to existing theoretical predictions. [Preview Abstract] |
Monday, March 3, 2014 9:12AM - 9:24AM |
A45.00007: Subharmonics of microwave induced resistance oscillations in the presence of high electric fields S. Chakraborty, A.T. Hatke, L.W. Engel, M. Manfra, J. Watson, M.P. Lilly, J. Reno We investigate the photoresistance of a two-dimensional electron system to high power microwave radiation, using a Hall bar within the slot of a coplanar waveguide (CPW) capable of electric fields in excess of 100 V/cm. The contacts of the Hall bar were screened within the ground plane of the CPW. Our measurements focus on longitudinal transport at magnetic fields larger than that of the cyclotron resonance at the frequency applied to the CPW. We observe a series of subharmonic resonances that can have amplitude in excess of the cyclotron resonance photoresistance as well as different power and temperature dependence. [Preview Abstract] |
Monday, March 3, 2014 9:24AM - 9:36AM |
A45.00008: Giant negative magnetoresistance in irradiated two-dimensional electron systems M.A. Zudov, Q. Shi, P.D. Martin, Q.A. Ebner, J.L. Reno, L.N. Pfeiffer, K.W. West Several recent magnetotransport studies in high-mobility two-dimensional electron systems reported very strong negative magnetoresistance whose origin remains unclear. In an attempt to advance our understanding of this phenomenon, we have performed measurements on microwave-irradiated GaAs/AlGaAs heterostructures and quantum wells exhibiting giant magnetoresistance. We have found that microwave photoresistance is usually positive over a wide range of magnetic fields indicating that negative magnetoresistance is suppressed by microwave radiation. This suppression, however, is too strong to be attributed solely to radiation-induced heating of electrons. [Preview Abstract] |
Monday, March 3, 2014 9:36AM - 9:48AM |
A45.00009: Imaging 2D Electron Density Variations in High Mobility AlGaAs/GaAs Systems Jerry Lee, Ken West, Kirk Baldwin, Loren Pfeiffer, Lara Fernandes Lavelli, Aron Pinczuk We demonstrate two different techniques to measure the local 2D electron densities of high mobility AlGaAs/GaAs systems on the micron scale. We used micro-photoluminescence imaging to look for 2D density variations on the 50 micron scale, as well as local magneto-transport measurements to look for 2D density and mobility variations on the scale of several hundred microns. Our results suggest that the 2D electron systems indeed have local 2D densities and mobilities that vary from their corresponding mean values. Spatial maps suggest that the origin of these variations is likely due to variations in the MBE layer thicknesses across the wafer, as well as by fixed charge sites that we believe are located within the GaAs substrate or at the substrate-MBE interface. Our findings suggest that the current limits on the 2D electron mobility could be raised by devising methods to negate the effects of these fixed charges. [Preview Abstract] |
Monday, March 3, 2014 9:48AM - 10:00AM |
A45.00010: Combined study of microwave-power-dependence and linear-polarization-dependence of the microwave-radiation-induced magnetoresistance oscillations Tianyu Ye, Han-Chun Liu, Ramesh Mani, Werner Wegscheider Microwave radiation induced magnetoresistance oscillations (MRIMOs) represent an interesting electrical property of the high mobility two dimensional electron gas (2DEG) at low temperatures in a perpendicular magnetic field and under microwave excitation. Some questions under discussion in this topic include: (a) whether MRIMOs' amplitudes grow linearly with the microwave power and (b) how the MRIMO amplitudes change with the rotation of the microwave polarization with respect to the sample. In this study, we utilize swept microwave power and continuously changed linear polarized microwave polarization angle as two variables in four-terminal low-frequency lock-in magnetoresistance measurements of the 2DEG samples. The results show that amplitude of MRIMOs varies non-linearly with the microwave power. Also, the microwave polarization dependence measurements show that MRIMOs depend sensitively on the polarization angle of the linearly polarized microwaves, while the oscillatory magnetoresistance follows a cosine square function of the polarization angle. We provide a simple model that conveys our understanding of our observations. [Preview Abstract] |
Monday, March 3, 2014 10:00AM - 10:12AM |
A45.00011: Size-dependent giant-magnetoresistance in millimeter scale GaAs/AlGaAs 2D electron devices R.G. Mani, A. Kriisa, W. Wegscheider This study examines a ``bell-shape'' negative Giant Magneto-Resistance (GMR) that grows in magnitude with decreasing temperatures in $mm$-wide devices fabricated from the high-mobility GaAs/AlGaAs 2-Dimensional Electron System (2DES). Experiments show that the span of this magnetoresistance on the magnetic-field-axis increases with decreasing device width, $W$, while there is no concurrent Hall resistance, $R_{xy}$, correction. A multi-conduction model, including negative diagonal-conductivity, and non-vanishing off-diagonal conductivity, reproduces experimental observations. The results suggest that boundary scattering in the $mm$-wide 2DES with $mm$-scale electron mean-free-paths might be responsible for the observed ``non-ohmic'' size-dependent negative GMR [1]\\[4pt] [1] R. G. Mani, A. Kriisa, and W. Wegscheider, Sci. Rep. 3, 2747 (2013). [Preview Abstract] |
Monday, March 3, 2014 10:12AM - 10:24AM |
A45.00012: Study of the phase-shift in the linear-polarization-angle-dependence of the microwave radiation-induced magnetoresistance oscillations in the GaAs/AlGaAs system Han-Chun Liu, Tianyu Ye, R.G. Mani, W. Wegscheiger Transport studies of microwave- and terahertz-induced magneto-resistance oscillations (MTIMRO) identified novel photo-excited zero-resistance states in the GaAs/AlGaAs two-dimensional electron system system. Some theories based on the premise of linear-polarization-insensitivity have been developed for the MRIMRO. Some studies have shown, however, a strong linear polarization sensitivity of MTIMRO [1,2] using new experimental methods. In addition, Ramanayaka \textit{et al.} [2] has observed that using fitting formula, R$_{\mathrm{xx}}(\theta )=$A $\pm$ Ccos$^{2}(\theta $-$\theta_{0})$, to sinusoidal variation of diagonal resistance, R$_{\mathrm{xx}}$, with polarization angle $\theta $, the extracted phase shift, $\theta_{0}$, depends on radiation frequency, magnetic field $B$, sign of $B$ [2]. Here, in addition to those mentioned factors, we investigate the dependence of the phase shift $\theta_{0}$ in the linear-polarization-angle-dependence upon sample geometry. \\[4pt] [1] R. G. Mani \textit{et al.}, Phys. Rev. B 84, 085308 (2011). \\[0pt] [2] A. N. Ramanayaka \textit{et al.}, Phys. Rev. B 85, 205315 (2012). [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 10:36AM |
A45.00013: Degeneracy and Effective Mass in the Valence Band of Two-Dimensional (100)-GaAs Quantum Well Systems Vinicio Tarquini, Talbot Knighton, Zhe Wu, Jian Huang, Loren Pfeifer, Ken West Quantum Hall measurement of two-dimensional high-mobility ($\mu\sim 2\times$ 10$^6$ cm$^2$/(V$\cdot$s)) hole systems confined in a 20 nm wide (100)-GaAs quantum well have been performed for charge densities between $4-5\times$ 10$^{10}$ cm$^{-2}$ in a temperature range of 10-160 mK. The Fourier analysis of the Shubnikov-de Haas (SdH) oscillations of the magnetoresistance vs. the inverse of the magnetic field $1/B_{\bot}$ reveals a single peak, indicating a degenerate heavy hole (HH) band. The corresponding hole density $p=(e/h)\cdot f$ agrees with the Hall measurement result within $3\%$. The HH degeneracy is understood through the diminishing Rashba spin-orbit interaction due to the low charge density and the nearly symmetric confinement. SdH oscillations fitted for 0.1 T $\leq B_{\bot} \leq 0.25 $ T to the Dingle parameters yield an effective mass ($m^*$) between 0.39 $m_e$ and 0.51 $m_e$ that increases moderately with increasing magnetic field and charge density, in very good agreement with previous cyclotron resonance measurements. [Preview Abstract] |
Monday, March 3, 2014 10:36AM - 10:48AM |
A45.00014: Cyclotron mass and g-factor of high mobility holes in SiGe/Ge/SiGe in tilted magnetic field A. Suslov, I. Drichko, V. Malysh, I. Smirnov, L. Golub, S. Tarasenko, O. Mironov, M. Kummer, H. von Kanel Complex ac conductivity of a high quality single quantum well p-GeSi/Ge/GeSi (p=6$\cdot$10$^{11}$cm$^{-2}$) is measured using the surface acoustic wave technique at frequencies 30 and 85 MHz in magnetic fields of up to 18 T in the temperature range 0.3 - 5.8 K. In minima of the conductivity oscillations with small filling factors in integer quantum hall regime the ac conductivity is of the hopping nature and is described within the ``two-site'' model. In tilted fields for odd filling factors 3 and 5 increase of conductivity in the minima of the oscillations is due to effect of the in-plane field component on the g-factor. The same in-plane component causes rising of the cyclotron effective mass and damping of the oscillation magnitudes at even filling factors larger than 8. [Preview Abstract] |
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