Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session L23: Fractional Quantum Hall Effect, Bilayers |
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Sponsoring Units: DCMP FIAP Chair: Raymond Ashoori, Massachusetts Institute of Technology Room: 325 |
Tuesday, March 17, 2009 2:30PM - 2:42PM |
L23.00001: Different Signatures of the Total Filling Factor 1 State Lars Tiemann, Youngsoo Yoon, Stefan Schmult, Maik Hauser, Werner Dietsche, Klaus von Klitzing Bringing two 2-dimensional electron systems in close proximity can yield a correlated state as the electrons will experience the presence of the neighboring system. At the individual filling factors of 1/2 this leads to a new double-layer ground state as positive and negative charges from opposite layers couple to excitons. Many remarkable properties were found such as vanishing Hall and longitudinal resistances in the counterflow configuration [1], a resonantly enhanced zero bias tunneling peak [2], and more recently, a critical DC tunneling current and vanishingly small interlayer resistances in DC measurements [3]. We will show how it is possible to combine the results of these three different measurements into a consistent picture. Under certain conditions it is possible to exceed the critical currents but still observe a minimum at total filling factor 1 in the counterflow configuration.\newline [1] M. Kellogg et al. PRL 93, 036801 (2004); E. Tutuc et al. PRL 93, 036802 (2004)\newline [2] I.B. Spielman et al., PRL 87, 036803 (2001)\newline [3] L. Tiemann et al., New Journal of Physics 10, 045018 (2008) [Preview Abstract] |
Tuesday, March 17, 2009 2:42PM - 2:54PM |
L23.00002: Valley polarization and the polarization mass of composite fermions around $\nu$ = 3/2 Medini Padmanabhan, Tayfun Gokmen, Mansour Shayegan In two-dimensional electron systems confined to AlAs quantum wells, composite fermions (CFs) around $\nu $ = 3/2 are known to possess a valley degree of freedom [1]. The relative occupation of the valleys can be controlled via the application of uniaxial, in-plane strain. In this study, we measure the strain needed to completely valley-polarize the various fractional quantum Hall states around $\nu $ = 3/2 as a function of density and compare our results to the theory explaining the complete spin-polarization of CFs in GaAs [2] . While the theory predicts it to be a constant, the energy needed for complete valley-polarization in units of the Coulomb energy is experimentally found to increase with increasing density. Translating this to the language of the 'polarization mass' for the CFs [2], we find an absence of the theoretically expected $\sqrt B$ dependence for the polarization mass. [1] N. C. Bishop \textit{et al.}, Phys. Rev. Lett. 98, 266404 (2007) [2] K. Park and J. K. Jain, Phys. Rev. Lett. 80, 4237 (1998) [Preview Abstract] |
Tuesday, March 17, 2009 2:54PM - 3:06PM |
L23.00003: Clausius-Clayperon relation for onset of the coherent $\nu=1$ phase in bilayer quantum hall systems Yue Zou, Gil Refael, James Eisenstein, Ady Stern A bilayer system of two-dimensional electron gases in a perpendicular magnetic field exhibits extremely rich phenomena. At total filling factor $\nu = 1$, as one increases the layer separation, the bilayer system goes from an interlayer coherent exciton condensed state to an incoherent phase of two decoupled composite fermion Fermi liquids. Many question still remain as to the nature of the transition between these two phases. Recent experiments investigated the phase boundary as a function of both in plane magnetic field and density imbalance. We compare these experimental results, e.g., the curvature of the phase boundary, with respect to the interlayer density imbalance, with a theoretical calculation based on the assumption that there is a direct first order transition between the two phases. [Preview Abstract] |
Tuesday, March 17, 2009 3:06PM - 3:18PM |
L23.00004: Quantum Hall Bilayer in a Periodic Potential Ganpathy Murthy, Jianmin Sun, Herbert Fertig Disorder is known to be central to the $\nu=1$ bilayer[1,2]. We study the bilayer $\nu=1$ system in a periodic potential, which mimics the nonperturbative effects of disorder by creating frozen-in Hall currents. The coupling to the potential is through the Pontryagin density of the pseudospin. We find the spinwave modes and quantize the theory to account for the effects of quantum fluctuations. 1. H. A. Fertig and G. Murthy, Phys. Rev. Lett. {\bf 95}, 156802 (2005). 2. H. A. Fertig and G. Murthy, Sol. St. Commun. {\bf 140}, 83 (2006). [Preview Abstract] |
Tuesday, March 17, 2009 3:18PM - 3:30PM |
L23.00005: Piezoresistance and Metal Insulator Transition of Composite Fermions at $\nu$ = 3/2 Tayfun Gokmen, Medini Padmanabhan, Mansour Shayegan In the composite fermion (CF) picture, at the Landau level filling factor $\nu$ = 3/2 the particle-flux CF quasi-particles are analogous to electrons at zero perpendicular magnetic field. Here we report piezoresistance measurements of CFs at $\nu$ = 3/2 in AlAs quantum wells. In this system, the electrons occupy two conduction band valleys with elliptical Fermi contours, and the valley occupation of electrons can be controlled via the application of uniaxial, in-plane strain. The system's response to strain at $\nu$ = 3/2 is qualitatively very similar to that of the electrons at zero perpendicular magnetic field, and consistent with the picture of CFs with a valley degree of freedom. Temperature dependent studies also show that CFs, like their counterpart electrons, go thorough a metal-insulator transition as they become valley polarized. [Preview Abstract] |
Tuesday, March 17, 2009 3:30PM - 3:42PM |
L23.00006: Composite Fermion signature in the single particle spectrum of the fractional quantum Hall system Ray Ashoori, Oliver Dial, Loren Pfeiffer, Ken West Using time domain capacitance spectroscopy we measure the single particle spectrum of the fractional quantum Hall system. The very high energy resolution of the techique (limited ultimately only by sample temperature) allows us to uncover the existence of new spectral features. Among these is a sharp line that crosses the Fermi level at filling factor $\nu=1/2$. The structure is consistent with the composite Fermion Landau fan, allowing measurement of the composite Fermion mass. Observation of this feature allows us to study the composite Fermion mass as a function of magnetic field and filling fraction. [Preview Abstract] |
Tuesday, March 17, 2009 3:42PM - 3:54PM |
L23.00007: Quasiparticles in the tunneling spectrum of the fractional quantum Hall system Oliver Dial, Raymond Ashoori, 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. 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 and beautiful structure present in these systems far from the Fermi energy. With increased sample quality and higher magnetic fields, we see the emergence of the fractional quantum Hall effect in the TDOS along with negative compressibility and chemical potential jumps associated with several fractions. Most strikingly, we also observe a number of new, sharp quasiparticle lines far from the Fermi energy. The dependence of the quasiparticle energies on density allows us to identify different fractional quasiparticles as well as estimate emergent effective quasiparticle masses. These high energy spectral features shed new light on the highly correlated fractional ground state, as well as the nature of the state near $\nu=1$. [Preview Abstract] |
Tuesday, March 17, 2009 3:54PM - 4:06PM |
L23.00008: Quantum Hall States at filling $\nu=\frac{2}{k+2}$ Waheb Bishara, Gregory Fiete, Chetan Nayak We study the $\nu=\frac{2}{k+2}$ quantum Hall states which are particle-hole conjugates of the $\nu=\frac{k}{k+2}$ Read-Rezayi states. We find that equilibration between the different modes at the edge of such a state leads to an emergent SU(2)$_k$ algebra in the counter-propagating neutral sector. Heat flow along the edges of these states will be in the opposite direction of charge flow. In the $k=3$ case, which may be relevant to $\nu=2+\frac{2}{5}$, the thermal Hall conductance and the exponents associated with quasiparticle and electron tunneling distinguish this state from competing states such as the hierarchy/Jain state. [Preview Abstract] |
Tuesday, March 17, 2009 4:06PM - 4:18PM |
L23.00009: Phase diagram for bilayer quantum Hall effect at total filling $\nu_T=5$ Chuntai Shi, Shivakumar Jolad, Nicolas Regnault, Jainendra Jain There has been much interest in bilayer quantum Hall systems at total filling $\nu_T=1$, which exhibit excitonic superfluidity at small separations and two uncoupled composite fermion Fermi seas at large separations. We evaluate the phase diagram of the bilayer quantum Hall effect at total filling $\nu_T=5$, neglecting interlayer tunneling and spin fluctuations, which is expected to be a bilayer excitonic superfluid at small separations and two uncoupled 5/2 FQHE states at large separations. Based on a combination of variational and exact diagonalization (for up to 20 electrons) studies, we estimate that the transition between these states occurs at a layer separation of approximately one magnetic length,independent of the individual layer thickness. The composite fermion Fermi sea is not found to be relevant for any parameters. [Preview Abstract] |
Tuesday, March 17, 2009 4:18PM - 4:30PM |
L23.00010: How universal is the fractional quantum Hall edge physics? Shivakumar Jolad, Diptiman Sen, Jainendra Jain We report on study of edge excitations of fractional quantum Hall states at filling factors $\nu=1/3$ and $\nu=2/5$. By considering the restricted basis of composite fermion wave functions, which is very accurate for the low energy eigenstates, we are able to diagonalize systems with up to 54 particles which allows us to make extrapolations to the thermodynamic limit. In a model with neutralizing positive jellium background disk at a distance d, we find that edge reconstruction is generic, occurring even when the electron and the background disks coincide. We also test the postulated form for the electron field operator of the effective field theory approach for the fractional edge and find it to be inconsistent with our microscopic calculations. Implications of our results for the observed non-universality of the edge exponent will be discussed. [Preview Abstract] |
Tuesday, March 17, 2009 4:30PM - 4:42PM |
L23.00011: A universal molecular description for the spectra of bosons and fermions in the lowest Landau level Constantine Yannouleas, Uzi Landman We show that both the {\it yrast\/} and {\it excited\/} states in the LLL spectra of small systems can be expressed as linear superpositions of appropriate rovibrational molecular trial functions, akin to the rotating-electron-molecule functions introduced earlier.\footnote{C. Yannouleas and U. Landman, Phys. Rev. B {\bf 66}, 115315 (2002); Rep. Prog. Phys. {\bf 70}, 2067 (2007).} Thus the nature of strong correlations in the lowest Landau level reflects the spontaneous emergence of intrinsic point-group symmetries associated with rotations and vibrations of molecules of localized particles arranged in concentric polygonal-ring configurations. The present molecular picture is valid for both bosons and fermions. We stress its validity and superiority for {\it low\/} (as well as high) angular momenta, where ``quantum-fluid'' trial functions of a markedly different nature (including Laughlin, composite-fermion, and Pfaffian ones) have been assumed\footnote{R.B. Laughlin, Phys. Rev. Lett. 50, 1395 (1983); J.K. Jain, {\it Composite Fermions\/} (Cambridge University Press, 2007); N.R. Cooper, arXiv:0810.4398v1.} to apply. [Preview Abstract] |
Tuesday, March 17, 2009 4:42PM - 4:54PM |
L23.00012: Origin of chiral p-wave pairing in even-denominator fraction quantum Hall effect Yuan-Ming Lu, Yue Yu, Ziqiang Wang We show that gauge field fluctuations in the composite fermion field theory can be exactly integrated out using a non-unitary transformation. An instantaneous statistical interaction is induced which makes the Fermi sea unstable to chiral p-wave pairing. We show that the paired state is a Moore-Read Pfaffian and discuss the effects of Coulomb interaction in connection to even-denominator fractional quantum Hall effect. [Preview Abstract] |
Tuesday, March 17, 2009 4:54PM - 5:06PM |
L23.00013: Pinning mode of integer quantum Hall Wigner crystal of skyrmions Han Zhu, G. Sambandamurthy, Y.P. Chen, P.-H. Jiang, L.W. Engel, D.C. Tsui, L.N. Pfeiffer, K.W. West Just away from integer Landau level (LL) filling factors $\nu$, the dilute quasi-particles/holes at the partially filled LL form an integer-quantum-Hall Wigner crystal, which exhibits microwave pinning mode resonances [1]. Due to electron-electron interaction, it was predicted that the elementary excitation around $\nu = 1$ is not a single spin flip, but a larger-scale spin texture, known as a skyrmion [2]. We have compared the pinning mode resonances [1] of integer quantum Hall Wigner crystals formed in the partly filled LL just away from $\nu = 1$ and $\nu = 2$, in the presence of an in-plane magnetic field. As an in-plane field is applied, the peak frequencies of the resonances near $\nu = 1$ increase, while the peak frequencies below $\nu = 2$ show neligible dependence on in-plane field. We interpret this observation as due to a skyrmion crystal phase around $\nu = 1$ and a single-hole Wigner crystal phase below $\nu = 2$. The in-plane field increases the Zeeman gap and causes shrinking of the skyrmion size toward single spin flips. [1] Yong P. Chen et al., Phys. Rev. Lett. 91, 016801 (2003). [2] S. L. Sondhi et al., Phys. Rev. B 47, 16 419 (1993); L. Brey et al., Phys. Rev. Lett. 75, 2562 (1995). [Preview Abstract] |
Tuesday, March 17, 2009 5:06PM - 5:18PM |
L23.00014: Observation of a Fractional Quantum Hall State at $\nu=1/4$ in a Single Wide GaAs Quantum Well Dwight R. Luhman, W. Pan, D.C. Tsui, L.N. Pfeiffer, K.W. Baldwin, K.W. West We have preformed low temperature ($T\sim35$ mK) transport measurements using a 50 nm high-quality GaAs quantum well with an electron density of $n_e=2.55\times10^{11}$ cm$^{-2}$ and a mobility of $\mu \sim 10^{7}$ cm$^2$/Vs. Magnetic fields up to $B=45$ T were used to reach filling factor $\nu=1/4$. With the sample situated perpendicular to the applied magnetic field, the diagonal resistance displays a kink at $\nu=1/4$. When the sample is tilted to an angle of $\theta=20.3^{\circ}$, a clear minimum in the diagonal resistance and plateau in the Hall resistance are present at $\nu=1/4$ indicating a fractional quantum Hall state at $\nu=1/4$ in this sample. Several possibilities regarding the origin of this state will also be discussed. [Preview Abstract] |
Tuesday, March 17, 2009 5:18PM - 5:30PM |
L23.00015: Charge-density wave formation in interacting two-dimensional electronic systems with Landau level mixing Peter Smith, Malcolm Kennett Anisotropic transport in half-filled Landau levels has been explained in terms of charge-density wave (CDW) formation. We use the Hartree-Fock approximation to study the influence of electron-electron interactions and Landau level mixing on the formation of CDWs in two-dimensional electron and hole systems. For the situation of two nearly degenerate eigenstates, we construct a Landau free-energy theory appropriate for competing order parameters that allows for both striped and triangular CDW formation. We find the possibility of coexisting CDW ordering in the two states, along with possible hysteretic behaviour. This physics might be realized using an external parameter such as spin-orbit coupling to tune states into near degeneracy. [Preview Abstract] |
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