Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session Y46: Bilayers,Wigner Crystals |
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Sponsoring Units: FIAP Chair: Alan Dorsey, University of Florida Room: Baltimore Convention Center 349 |
Friday, March 17, 2006 8:00AM - 8:12AM |
Y46.00001: Coulomb frustrated phase separation in bilayer electronic systems Reza Jamei, Steven Kivelson, Boris Spivak We have previously proven that in clean two dimensional electronic systems, long range Coulomb interactions convert what might otherwise have been a first order transition between a high density liquid and a low density insulating electron crystalline phase into a sequence of continuous phase transitions between various sorts of intermediate ``electronic microemulsion phases.'' Here, we consider the electronic micromemulsion phases in a bilayer system, with Coulomb interactions both within and between the layers. At mean-field level, we find infinite families of modulated phases, with a subtle interplay of the period and magnitude of the density oscillations in the two layers. [Preview Abstract] |
Friday, March 17, 2006 8:12AM - 8:24AM |
Y46.00002: Interlayer CF Pairing in a Symmetric $\nu_T=1$ Quantum Hall Bilayer Gunnar Moller, Steven H. Simon, Edward Rezayi In bilayer quantum Hall systems at filling fractions near $\nu=\frac{1}{2} + \frac{1}{2}$, as the spacing $d$ between the layers is continuously decreased, intra-layer correlations must be replaced by inter-layer correlations, and the composite fermion (CF) Fermi seas at large $d$ must eventually be replaced by a composite boson (CB) condensate at small $d$. In recent work [1], trial wave functions were constructed that can describe how composite fermion-like intra-layer correlations can be continuously replaced by 111-like inter-layer correlations using a picture of two classes of quasi particles: composite bosons and composite fermions. Here, we present how to extend this approach both qualitatively and quantitatively, introducing a new way of writing the previously introduced wave functions, which allows for the introduction of composite fermion inter-layer positive p-wave pairing. These wave functions depend on a small number of variational parameters related to the momentum distribution of $(\mathbf{k},\uparrow; -\mathbf{k},\downarrow)$ CF pairs. Comparison of energies and correlation functions to results from exact diagonalizations for small systems shows close to perfect agreement, thus giving evidence for inter layer CF pairing in QH bilayer systems [2]. This work was supported by the DOE.\newline [1] S.~H. Simon, E.~H. Rezayi, and M.~V. Milovanovic, Phys.Rev.Lett. {\bf 91}, 046803 (2003). [2] G. M\"oller, S.~H. Simon, to be published, G. M\"oller, S.~H. Simon, E.~H. Rezayi, to be published. [Preview Abstract] |
Friday, March 17, 2006 8:24AM - 8:36AM |
Y46.00003: Excitonic codensation in quantum Hall electron-hole bilayers Yogesh Joglekar, Alexander Balatsky Bose-Einstein condensation of excitons in bilayers has been explored increasingly over the past decade. We present an effective theory of excitonic condensate phase in electron-hole systems in the quantum Hall regime and its derivation from a microscopic Hamiltonian using mean-field analysis. We present results for the critical layer separation $d$ as a function of electron-layer filling factor $\nu_e$, hole-layer filling factor $\nu_h$, and interlayer bias voltage $\Delta_v$. We extend our analysis to singlet and triplet excitons. We point out the salient differences between these systems and the (electron-electron) quantum Hall bilayers near total filling factor $\nu=1$ which have been investigated experimentally in recent years. [Preview Abstract] |
Friday, March 17, 2006 8:36AM - 8:48AM |
Y46.00004: Development of the excitonic state of bilayer quantum Hall system at total filling one Daijiro Yoshioka, Naokazu Shibata Excitonic phase, namely Haplerin's $\Psi_{1,1,1}$ state, is realized in bilayer quantum Hall systems at $\nu=1$ at small layer separation $d$ as evidenced by various experiments. This phase vanishes as $d$ is increased, and at large enough separation, composite-fermion Fermi-liquid states are realized in each layer. How the excitonic state develops into independent Fermi-liquid states has not been fully understood. We investigated this development by the density matrix renormalization group method (DMRG). We calculated the ground state wave function and energy gap for various values of $d$ for systems with up to 24 electrons. From the ground state wave function, two-particle distribution function $g(r)$ and excitonic correlation function have been calculated. The results indicate that the transition between the two limits is continuous. There is a smooth crossover of the ground state at around $d/l \simeq 1.6$ from the excitonic character at small $d/l$ to independent Fermi-liquid character at large $d/l$, where $l$ is the magnetic length. [Preview Abstract] |
Friday, March 17, 2006 8:48AM - 9:00AM |
Y46.00005: Spin-pseudospin textures in a strongly correlated bilayer quantum Hall system near $\nu=1$ Rene Cote, Jerome Bourassa, Bahman Roostaei, Herb A. Fertig, Kieran Mullen Recent experiments on strongly correlated bilayer quantum Hall systems [1,2] strongly suggest that contrary to the usual assumption, the electron spin degree of freedom is not completely frozen either in the quantum Hall or the compressibles states that occur near filling factor $\nu=1$. These experiments imply that the quasiparticles near $\nu=1$ could have both spin and pseudospin textures i.e. they could be CP3 skyrmions. Assuming that these skyrmions crystallize at very low temperature, we use a microscopic unrestricted Hartree-Fock calculation to compute the energy of these excitations as well as the number of flipped spins and pseudospins for various values of the separation between the well and of potential biais. We compare our results to previous calculations based on a field-theoretical description and discuss their relevance for the experiments of Refs. [1,2].\\ (1) I. B. Spielman, L. A. Tracy, J. P. Eisenstein, L. N. Pfeiffer, and K. W. West, Phys. Rev. Lett. 94, 76803 (2005). \\ (2) N. Kumada, K. Muraki, K. Hashimoto, and Y. Hirayama, Phys. Rev. Lett. 94, 96802 (2005). [Preview Abstract] |
Friday, March 17, 2006 9:00AM - 9:12AM |
Y46.00006: Pseudospin Vortex-Antivortex States with Interwoven Spin Texture in Double Layer Quantum Hall Systems B. Roostaei, J. Bourassa, H. Fertig, K. Mullen, R. Cote Enhanced nuclear spin relaxation rates have been observed in recent experiments[1] on double layer quantum Hall systems near total filling factor $\nu_T=1$. The effect is analogous to what happens in single layer systems, where a possible explanation lies in the development of a Skyrme crystal with low energy spin wave modes as the system is doped away from integer filling. Double layer systems are thought to support bimeron excitations, analogous to skyrmions but with layer indices playing the role of spin states. We demonstrate,within the Hartree-Fock approximation, that for low interlayer tunneling and large separations the bimerons reorganize into a vortex-antivortex lattice with an interwoven real spin texture. These states are most stable at large layer separation, where the introduction of the spin degree of freedom can relax an interlayer charge imbalance at the cores of the merons. The presence of the real spin texture produces a true spontaneously broken symmetry whose Goldstone modes can explain the enhancement of the nuclear spin relaxation. [1]I.B. Spielman et al., Phys. Rev. Lett. 94, 076803 (2005). [Preview Abstract] |
Friday, March 17, 2006 9:12AM - 9:24AM |
Y46.00007: Correlated Jumps in Magnetotransport of Imbalanced Bilayer Two Dimensional Hole Systems Nathaniel C. Bishop, Shashank Misra, Emanuel Tutuc, Mansour Shayegan A pair of dilute two-dimensional hole layers can interact to produce correlated electronic states unique to the bilayer geometry. Here, we examine the nonequilibrium resistivity of one layer when the other layer is tuned to the $\nu=1$ quantum Hall state. Previous studies have shown that the longitudinal resistivity of the detuned layer jumps with a slow ($\tau \approx 100s$) relaxation.$^1$ Surprisingly, we find that both the longitudinal and Hall resistivities of the detuned layer jump at the same time for contacts located hundreds of microns apart. This suggests either that the instabilities in charge distribution of the two layers occur over domains of macroscopic size, or that some novel quantum correlation manifests itself over similarly large length scales.\\ \\ $^1$ E. Tutuc {\it et al.}, PRB {\bf 68}, 201308 (2003). [Preview Abstract] |
Friday, March 17, 2006 9:24AM - 9:36AM |
Y46.00008: Time Dependent Magnetotransport of Imbalanced Bilayer Two Dimensional Hole Systems Shashank Misra, Nathaniel C. Bishop, Emanuel Tutuc, Mansour Shayegan The magnetotransport of a two dimensional hole layer in close proximity to a second layer tuned to the $\nu=1$ quantum Hall state exhibits correlations novel to the bilayer geometry. Previous experiments have shown the resistivity of the detuned layer to be hysteretic when sweeping the magnetic field, and to exhibit jumps as a function of time when staying at a fixed field.$^1$ We find that allowing the system to relax in middle of taking a hysteresis loop results in the magnetoresistance following a curve in between the two branches of the hysteresis loop, a hallmark of a glassy system. We further find that the seemingly random resistance jumps occur at similar times in separate measurements, each performed upon resetting the bilayer state. Work performed with support from the NSF and DOE.\\ \\ $^1$ E. Tutuc {\it et al.}, PRB {\bf 68}, 201308 (2003). [Preview Abstract] |
Friday, March 17, 2006 9:36AM - 9:48AM |
Y46.00009: Electron Drag in Intermediate Magnetic Fields Sanghun An, Gokul Gopalakrishnan, Yuko Shiroyanagi, Sarah Parks, Dongkyun Ko, Thomas Gramila, Loren Pfeiffer, Ken West We report measurements of electron drag on the bilayer two-dimensional electron gas (2DEG) in intermediate magnetic fields, for which anomalous temperature-dependence and field-dependence of the drag resistivity have been previously observed. In this regime, drag resistivity shows surprisingly little temperature dependence, as well as a highly unusual field dependence which varies roughly as the cube of magnetic field. To our knowledge, no theoretical works have predicted such behavior. We have continued investigation of drag in this regime, therefore, in an attempt to understand the source of this anomalous behavior. We report detailed measurements exploring the potential role of spin in the process via application of an in-plane magnetic field, as well as examination of possible correlation effects, and a careful study of the temperature dependence. [Preview Abstract] |
Friday, March 17, 2006 9:48AM - 10:00AM |
Y46.00010: Plasmon Enhanced Drag Measurements at Low Densities Yuko Shiroyanagi, Gokul Gopalakrishnan, Sanghun An, Sarah Parks, Thomas Gramila, Loren Pfeiffer, Ken West The plasmons of the two-dimensional electron gas(2DEG) can provide valuable information about electronic many-body correlation effects, while electron drag provides a unique method to probe electron-electron scattering processes in a double layer 2DEG system. An enhancement of electron drag by plasmons was predicted[1] and subsequently observed[2,3]. Experimental studies of the strength of the plasmon enhancement as a function of relative density, however, revealed that the maximum enhancement occurred for mis-matched layer densities, in disagreement with theoretical expectation. It has been argued that this results from correlation effects, in particular the presence of multi-particle excitations. We report new measurements of plasmon enhanced drag, which examine lower overall electron densities than had been reported earlier. Since correlation effects are stronger for lower densities, these measurements provide a means to test the role of correlations in determining the dependence of plasmon enhanced drag on relative densities. [1] K.Flensberg and B.Y.K.Hu, PRL 73, 3572(1994). [2] N.P.R.Hill et.al. PRL 78, 2204(1997). [3] H.Noh, S.Zelakiwicz, X.G.Feng, and T.J.Gramila PRB 58, 12621(1998). [Preview Abstract] |
Friday, March 17, 2006 10:00AM - 10:12AM |
Y46.00011: A Physical Picture for the Negative Drag in High Landau Levels Ady Stern, Rafi Bistritzer Experimental investigation of the longitudinal drag resistivity, $\rho^D$, of a bi-layer subjected to a strong magnetic field in the regime of large filling factors found an anomalous behavior at low temperatures $T$: $\rho^D$ depends non monotonously on $T$ and becomes negative when the filling factors of the two layers differ by an odd number. A calculation of $\rho_D$ within the framework of the self consistent Born approximation was generally consistent with the experiment; nevertheless, it left the physical picture obscure. We employ the exact eigenstates method to unravel that picture. We find the oscillating sign of $\rho^D$ to originate from the effect of disorder on the relation between an adiabatic momentum transfer to an electron and the displacement of its position. For localized states a momentum transfer $\bf q$ implies a displacement of $ql_H^2$, with $l_H$ being the magnetic length. For extended states, the combined effect of a short range disorder and a rapidly oscillating wave function at high Landau levels results in an additional, potentially larger, displacement whose sign depends on the electron's energy. [Preview Abstract] |
Friday, March 17, 2006 10:12AM - 10:24AM |
Y46.00012: Pinned Bilayer Wigner Crystals with Pseudospin Magnetism Yong Chen We study a model of \textit{pinned} bilayer Wigner crystals (WC) and focus on the effects of interlayer coherence (IC) on pinning. We consider both a pseudospin ferromagnetic WC (FMWC) with IC and a pseudospin antiferromagnetic WC (AFMWC) without IC. Our central finding is that a FMWC can be pinned more strongly due to the presence of IC. One specific mechanism is through the disorder induced interlayer tunneling, which effectively manifests as an extra pinning in a FMWC. We also construct a general ``effective disorder" model and effective pinning Hamiltonian for the case of FMWC and AFMWC respectively. Under this framework, pinning in the presence of IC involves \textit{interlayer} spatial correlation of disorder in addition to intralayer correlation, leading to \textit{enhanced} pinning in the FMWC. The pinning mode frequency ($\omega_{pk}$) of a FMWC is found to decease with the effective layer separation, whereas for an AFMWC the opposite behavior is expected. An abrupt drop of $\omega_{pk}$ is predicted at a transition from a FMWC to AFMWC. Possible effects of in-plane magnetic fields and finite temperatures are addressed. Finally we discuss some other possible ramifications of the FMWC as an electronic supersolid- like phase. [1] Yong P. Chen, cond-mat/0507124 [Preview Abstract] |
Friday, March 17, 2006 10:24AM - 10:36AM |
Y46.00013: Effect of vertical confinement on pinning mode of electron solids G. Sambandamurthy, Yong P. Chen, Lloyd Engel, D.C. Tsui, L.N. Pfeiffer, K.W. West We present the results from a study of the electron solid regime that terminates the fractional quantum Hall series in very clean two-dimensional electron systems (2DES). The disorder-induced pinning mode of the electron solid appears as a resonance in the microwave conductivity spectrum. In the present study, we compare results from two quantum well samples that were grown in the same way, with the only difference being the well widths (15 and 65 nm), allowing us to observe the effect of vertical confinement of electrons on the pinning mode of electron solids. We find that the narrower well (15 nm width) has a resonance with peak frequency ($f_{pk}$) $\sim$ 6-8 GHz, whereas the wider well (65 nm width) has $f_{pk}$ $\sim$ 100-200 MHz. Moreover, $f_{pk}$ vs. $B$ for the two samples show markedly different behaviors. Our results suggest that the disorder that dominates the pinning of electron solids is from the interface roughness associated with the quantum well. [Preview Abstract] |
Friday, March 17, 2006 10:36AM - 10:48AM |
Y46.00014: A Numerical Study of 1/4 filling in the N=2 Landau Level Barry Friedman, Moses Marchante Recently, it has been proposed that anisotropic Wigner crystals of electrons, with 1 electron per lattice site, have in certain circumstances, lower energies then the 2 electron bubble state or isotropic Wigner crystals. Using direct diagonalization and the density matrix renormalization group (dmrg) we have re examined 1/4 filling in the N=2 Landau level. Preliminary results indicate that the ground state consists of the 2 electron bubble state. Implementation of the dmrg, for quantum Hall systems, will also be discussed. [Preview Abstract] |
Friday, March 17, 2006 10:48AM - 11:00AM |
Y46.00015: Remote sensing of the radiation induced zero-resistance state in the high mobility GaAs/AlGaAs system Ramesh Mani The possibility of inducing novel zero-resistance states by photo-exciting a high mobility GaAs/AlGaAs device, with radiation from the microwave and Terahertz parts of the electromagnetic wave spectrum, has recently motivated a broad theoretical examination of the photoexcited steady states of the low dimensional electron system. Present theory suggests that radiation- induced resistance oscillations originate from a field dependent scattering at impurities and/or a steady state change in the electronic distribution function. In these theoretical scenarios, the amplitude of the magnetoresistance oscillations increases with the radiation intensity such that the resistivity/conductivity is able to take on negative values at the minima of the oscillatory magnetoresistivity (or magnetoconductivity) for sufficiently large radiation intensities. The negative resistivity/conductivity state is believed to give way, however, to a zero-resistance state as a result of domain formation, providing for the experimentally observed phenomenon. Here, we report the results of experiments which sought to find signatures of the radiation-induced zero-resistance states in microwave transmission and reflection measurements of the high mobility 2DES. Remarkable changes are observed in the remotely observed photoresponse as a function of both the frequency and intensity. From the results, we deduce that the remote sensing of the radiation-induced zero-resistance state lies within the realm of possibility. [Preview Abstract] |
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