Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session Q25: QHE: Bilayers and Tunneling |
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Sponsoring Units: FIAP Chair: Joel Moore, University of California, Berkeley Room: D135 |
Wednesday, March 17, 2010 11:15AM - 11:27AM |
Q25.00001: Critical tunneling currents in disordered quantum hall bilayers at $\nu_{tot}=1$ Dmytro Pesin, Allan MacDonald Recent experiments on semiconductor Quantum Hall bilayers in the regime of $\nu_{tot}=1$ give strong support for the existence of critical currents in tunneling experiments [L. Tiemann et al., Phys. Rev. B {\bf 80}, 165120 (2009)]. However, estimates [J. J. Su, A. H. MacDonald, in preparation] based on typical experimental parameters predict critical currents several orders of magnitude larger than those observed. In this work we argue that the presence of disorder dramatically affects the magnitude of the critical current. We propose that the relevant disorder stems from the presence of charged vortex-like excitations (merons) in the ground state of the system, and even a low-density of merons can degrade the critical current by a large factor. We discuss the role of thermal and quantum fluctuations in the present context. Finally, we show that in a disordered bilayer the parallel magnetic field dependence of the critical current deviates significantly from the naively expected Fraunhofer-type pattern. We argue the underlying physics may also be relevant to experimentally observed parallel magnetic field dependence of the zero-bias tunneling conductance peak [I. B. Spielman et al., Phys. Rev. Lett.{\bf 87}, 036803 (2001)]. [Preview Abstract] |
Wednesday, March 17, 2010 11:27AM - 11:39AM |
Q25.00002: Vortex States of a Dirty Quantum Hall Bilayer Derek K.K. Lee, Paul R. Eastham, Nigel R. Cooper We study the ground state of a quantum Hall bilayer at filling $\nu$=1/2+1/2 in the presence of strong but smooth disorder. We argue that there is a characteristic disorder strength below which vortices will be rare and above which they proliferate. In the strong-disorder regime the system can be understood as an emulsion of vortex-antivortex crystals. We will discuss the implications for the observation of counterflow superfluidity and critical currents in these systems. These include a dramatic suppression of tunneling and a new disorder-induced lengthscale in the emulsion due to phase disorder. [Preview Abstract] |
Wednesday, March 17, 2010 11:39AM - 11:51AM |
Q25.00003: Quantum Hall Phase Diagram of Second and Lowest Landau-level Half-filled Bilayers: Abelian versus Non-Abelian States Michael Peterson, Sankar Das Sarma The fractional quantum Hall phase diagram of the half-filled bilayer system in the two lowest Landau levels (lowest and second) is studied as a function of tunneling and layer separation using exact diagonalization. We make the striking prediction that second Landau level bilayer structures at filling factor 5/2 would manifest two distinct branches of incompressible fractional quantum Hall effect (FQHE) corresponding to the Abelian 331 state (at moderate to low tunneling and large layer separation) and the non-Abelian Pfaffian state (at large tunneling and small layer separation). The observation of these two FQHE branches at 5/2 and the quantum phase transition between them would be compelling evidence supporting the existence of the non-Abelian Pfaffian state in the second Landau level. In the lowest Landau level at filling factor 1/2, we find that, although possible in principle, it is unlikely that the non-Abelian Pfaffian FQHE exists. [Preview Abstract] |
Wednesday, March 17, 2010 11:51AM - 12:03PM |
Q25.00004: Observation of fractional quantum Hall effect at even-denominator 1/2 and 1/4 fillings in wide quantum wells Javad Shabani, Tayfun Gokmen, YenTing Chiu, Mansour Shayegan Magneto-transport measurements on electrons confined to a wide GaAs quantum well reveal that the correlated electron states at low Landau level fillings ($\nu$) display a remarkable dependence on the symmetry of the electron charge distribution. At a density of $1.93 \times 10^{11}$ cm$^{-2}$ and well width of 57nm, developing fractional quantum Hall states are observed at the even-denominator fillings 1/2 and 1/4 when the distribution is symmetric, but they quickly vanish when the distribution is made asymmetric. At lower densities, as we make the charge distribution asymmetric, we observe a rapid strengthening of the insulating phases that surround the $\nu = 1/5$ fractional quantum Hall state. In narrower wells with width 55nm and 47nm, we observe developing quantum Hall states at $\nu = 1/2$ and 1/4 when the charge distribution is significantly asymmetric. The very large electric subband separation and the highly asymmetric charge distribution at which we observe these quantum Hall states, together with the fact that they disappear when the charge distribution is made symmetric, suggest that these are one-component states, possibly described by the Moore-Read Pfaffian wavefunction. [Preview Abstract] |
Wednesday, March 17, 2010 12:03PM - 12:15PM |
Q25.00005: Tunneling spectra for electrons in the lowest Landau level F.J. Burnell, Steven H. Simon The recently developed experimental technique of time dependent capacitance spectroscopy [1] allows for measurements of high-resolution tunneling spectra of 2DEGs in the quantum Hall regime, giving a detailed probe of the single particle spectral function (electron addition and subtraction spectra). These experiments show a number of interesting features including Landau level structure, exchange enhanced Zeeman energy, Coulomb gap physics, effects of fractional quantization, as well as several key features that remain to be explained. While there has been some prior theoretical work[2] towards explaining low energy Coulomb gap features of tunneling spectra found in much earlier tunneling experiments [3], the new experiments[1] have uncovered physics outside of the prior theoretical explanations. Building on a number of these prior theoretical works, we investigate theoretically the expected tunneling spectra for electrons in low Landau levels, including the effects of electron spin and coupling to collective modes. [1] O. E. Dial, R.C. Ashoori, L.N. Pfeiffer, and K.W. West, Nature 448, 176-179 (2007) ; O. E. Dial et al, unpublished. [2] I. Aleiner et al, Phys. Rev. Lett 74 3435; (1994) S. R. E. Yang and A. MacDonald PRL 70 4110 (1993); S. He, P.M. Platzman, and B. I. Halperin, PRL 71 777 (1993). [3] J. P. Eisenstein et al, Phy. Rev. Lett. 69, 3804 (1992). [Preview Abstract] |
Wednesday, March 17, 2010 12:15PM - 12:27PM |
Q25.00006: Mass Renormalization in 2D Measured by Tunneling Spectroscopy Oliver Dial, Raymond Ashoori, Loren Pfeiffer, Ken West We use a novel technique, time domain capacitance spectroscopy, to measure the single particle density of states of a 2D electron gas. This technique allows quantitative measurements of the energies of features in the single particle spectrum. Applying a magnetic, we observe a series of Landau levels. However, the observed Landau levels are not evenly spaced -- electron-electron interactions modify the cyclotron energy. This is in stark contrast to optical techniques, which always observe the non-interacting cyclotron energy due to Kohn's theorem. At low magnetic fields, we can interpret changes in the cyclotron energy as changes in the electron mass, and use the spectrum to map the electron mass not just as a function of density, but also as a function of energy away from the Fermi surface. [Preview Abstract] |
Wednesday, March 17, 2010 12:27PM - 12:39PM |
Q25.00007: Plasmaron structure in the single-particle spectrum of the 2D electron system Raymond Ashoori, Oliver Dial, Loren Pfeiffer, Ken West Despite the central role that the tunneling (or single-particle) particle density of states (TDOS) plays in our theories of many-body systems, it has proven a difficult quantity to access experimentally in two dimensional electron systems (2DES). We have developed a technique, time domain capacitance spectroscopy, which allows measurement of the TDOS over a range of 30 meV centered about the Fermi surface, revealing the detailed structure present in these systems far from the Fermi energy. Remarkably, we observe a long-lived excitation in the 2DES whose creation requires more energy than is needed to eject an electron from the most tightly bound state in the 2DES. At zero field, this feature appears as a low spectral-weight band below the band edge of the 2DES, while upon raising the magnetic field it smoothly evolves into a series of ``negative energy'' Landau levels. Based on its energy as a function of the 2D electron density and behavior when a magnetic field is applied, we identify this excitation as a hole in the 2DES coupled to a plasmon. Such a ``plasmaron'' has been predicted in calculations of the density of states for 3D [1] and 2D [2] electron gases, but direct observation has proven elusive. [1] L. Hedin, B.I. Lundqvist, and S. Lunqvist. Solid State Comm. 5, 237-239 (1967). [2] P. von Allmen. Phys. Rev. B 46, 13345 (1992). [Preview Abstract] |
Wednesday, March 17, 2010 12:39PM - 12:51PM |
Q25.00008: Observation of a Fractional Quantum Hall State in the second Landau level of a two-dimensional hole system A. Kumar, N. Samkharadze, M. J. Manfra, L. N. Pfeiffer, K. W. West, G. A. Csathy We report on a dc transport measurement of an exceptional two-dimensional hole system in a Carbon-doped GaAs/AlGaAs quantum well. In a setup which is similar to that of Ref.[1] our sample and the attached sintered silver heat exchangers are immersed into a He-3 bath which can be cooled to 5 mK. The most striking feature of this sample is the presence of a fractional quantum Hall state at Landau level filling factor $\nu $=2+2/3 with an energy gap of 40mK. This state has only been observed in electron samples so far. While in electron samples the $\nu $=2+2/3 state is always observed together with the $\nu $=2+1/3 state, the latter one is not present in our hole sample. We compare our results with results of previous studies in electron systems and we examine the role of the disorder and of the Landau level mixing on the activation energy gap. [1] W. Pan, J.-S. Xia, V. Shvarts, D. E. Adams, H. L. Stormer, D. C. Tsui, L. N. Pfeiffer, K. W. Baldwin, and K. W. West, Phys. Rev. Lett. \textbf{83}, 3530 (1999) [Preview Abstract] |
Wednesday, March 17, 2010 12:51PM - 1:03PM |
Q25.00009: Interlayer coherent composite Fermi liquid phase in quantum Hall bilayers Jason Alicea, Olexei Motrunich, Gil Refael, Matthew P.A. Fisher We introduce an \emph{interlayer coherent composite Fermi liquid} for $\nu = 1/2+1/2$ bilayers, in which interlayer Coulomb drives exciton condensation of composite fermions. As a result, composite fermions propagate coherently \emph{between} layers---even though electrons do not---and form bonding and antibonding Fermi seas. This phase is compressible with respect to symmetric currents but quantum Hall-like in the counterflow channel. Quantum oscillations of the composite Fermi seas generate a new series of incompressible states at $\nu = p/[2(p\pm 1)]$ per layer ($p$ an integer), which is a bilayer analogue of Jain's sequence. [Preview Abstract] |
Wednesday, March 17, 2010 1:03PM - 1:15PM |
Q25.00010: Quantum Hall Exciton Condensation at Full Spin Polarization A.D.K. Finck, J.P. Eisenstein, L.N. Pfeiffer, K.W. West Using Coulomb drag as a probe, we explore the excitonic phase transition in quantum Hall bilayers at $\nu_T=1$ as a function of Zeeman energy, $E_Z$. The critical layer separation $(d/\ell)_c$ for exciton condensation initially increases rapidly with $E_Z$, but then reaches a maximum and begins a gentle decline. At high $E_Z$, where both the excitonic phase at small $d/\ell$ and the compressible phase at large $d/\ell$ are fully spin polarized, we find that the width of the transition, as a function of $d/\ell$, is much larger than at small $E_Z$ and persists in the limit of zero temperature. We discuss these results in the context of two models in which the system contains a mixture of the two fluids. [Preview Abstract] |
Wednesday, March 17, 2010 1:15PM - 1:27PM |
Q25.00011: Dynamic Nuclear Polarization in a Quantum Point Contact in the Integer and Fractional Quantum Hall Regimes Angela Kou, Douglas McClure, Charles Marcus, Loren Pfeiffer, Ken West We present measurements of resistively detected dynamic nuclear polarization in a quantum point contact (QPC) in the integer and fractional quantum Hall regimes. After applying a finite dc bias to the QPC, we observe shifts in the locations of integer and fractional plateaus in the diagonal resistance measured across the QPC. These shifts persist on time scales of tens of minutes and disappear upon application of rf pulses at the nuclear magnetic resonance frequencies. This behavior is explained in terms of a shift in Zeeman energy caused by the nuclear hyperfine field. In the fractional quantum Hall regime, symmetric shifts about $\nu=1/2$ suggest a composite fermion-based explanation. [Preview Abstract] |
Wednesday, March 17, 2010 1:27PM - 1:39PM |
Q25.00012: Tunneling Signatures of a Majorana Chain at the Quantum Spin Hall Edge Vasudha Shivamoggi, Gil Refael, Joel Moore A 1D chain of Majorana fermions can be created using an array of ferromagnetic and superconducting regions at the quantum spin Hall edge. By mapping the system to the transverse-field Ising spin chain, it is possible to study the behavior of thermodynamic and transport quantities at the random critical point. We examine critical transport properties of the Majorana chain by using a real-space renormalization group method, with each step expressed as a unitary transformation on the Hamiltonian of the Majorana chain. Successive steps transform the weakly interacting chain to multiple copies of the two-Majorana problem. The density of states and the tunneling along the chain are calculated from the universal distribution of the formed pairs. This approach can be generalized to weakly dimerized Majorana chains, and can be equally used to obtain transport properties of the random transverse-field Ising model. [Preview Abstract] |
Wednesday, March 17, 2010 1:39PM - 1:51PM |
Q25.00013: Spin Reconstruction in Quantum Hall Strips Yafis Barlas, Gilad Barak , Yogesh Joglekar, Kun Yang, Amir Yacoby, Bertrand Halperin We study the effect of electron-electron interactions on the ground state of a Quantum Hall Strip with triangular confinement in the Hartree-Fock Approximation. We find that for infinitesimal Zeeman splitting the soft-edge undergoes a spontaneous transition from a spin-unpolarized to spin-polarized ground state as a function of the magnetic field perpendicular to the strip. This spin-polarization shows up as an eye-structure in the dispersion and has a spatial separation of the order of a magnetic length. This transition is due to the interplay of strong edge confinement and electron-electron interaction and is independent on the Zeeman energy. [Preview Abstract] |
Wednesday, March 17, 2010 1:51PM - 2:03PM |
Q25.00014: Quantum Hall effect at a tunably sharp cleaved-edge potential Chuanle Zhou, M. Grayson, L. Steinke, E. Uccelli, G. Koblmueller, M. Bichler, G. Abstreiter, S. Schmult, W. Dietsche We study magnetotransport in the quantum Hall (QH) regime of a two-dimensional electron system with an epitaxially overgrown sharp cleaved-edge. A thick insulating barrier is overgrown at the cleaved-edge followed by a doped layer, serving as a side gate which can control depletion or accumulation at the sharp edge, hence can convert a sharp edge into a soft edge by changing the gate bias. This geometry leads to a tunable edge potential with either the standard incompressible strips in the ``soft edge'' limit, or thin or vanishing incompressible strips in the ``sharp edge'' limit. DC magnetotransport measurements show evidence of a longitudinal resistance minimum whose width depends on the current direction. This experimental result is consistent with recent theory on the role of edge potentials in defining the QH in small samples [1]. Size effect and gate bias dependence are studied. We also report an unexplained magnetic field hysteresis at the high field side of filling factors $\nu =1, 2, 3, 4$ in the limit of negative side-gate bias.\\[4pt] [1] A. Siddiki, Euro. Phys. Lett. 87, 17008(2009) [Preview Abstract] |
Wednesday, March 17, 2010 2:03PM - 2:15PM |
Q25.00015: Nematic and Valley Ordering in Anisotropic Quantum Hall Systems S.A. Parameswaran, D.A. Abanin, S.A. Kivelson, S.L. Sondhi We consider a multi-valley two dimensional electron system in the quantum Hall effect (QHE) regime. We focus on QHE states that arise due to spontaneous breaking of the valley symmetry by the Coulomb interactions. We show that the anisotropy of the Fermi surface in each valley, which is generally present in such systems, favors states where all the electrons reside in one of the valleys. In a clean system, the valley ordering occurs via a finite temperature Ising-like phase transition, which, owing to the Fermi surface anisotropy, is accompanied by the onset of nematic order. In a disordered system, domains of opposite polarization are formed, and therefore long-range valley order is destroyed, however, the resulting state is still compressible. We discuss the transport properties in ordered and disordered regimes, and point out the possible relation of our results to recent experiments in AlAs [1]. \newline [1] Y. P. Shkolnikov, S. Misra, N. C. Bishop, E. P. De Poortere, and M. Shayegan, {\it Observation of Quantum Hall ``Valley Skyrmions"}, Phys. Rev. Lett. {\bf 95}, 068809 (2005)\newline [2] D.A. Abanin, S.A. Parameswaran, S.A. Kivelson and S.L. Sondhi, {\it Nematic and Valley Ordering in Anisotropic Quantum Hall Systems}, to be published. [Preview Abstract] |
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