Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session U19: Fractional QHE/Composite Fermions |
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Sponsoring Units: DCMP Chair: Kun Yang, Florida State University Room: LACC 406B |
Thursday, March 24, 2005 8:00AM - 8:12AM |
U19.00001: Korringa-Like Nuclear Spin-Lattice Relaxation in a 2DES at $\nu = 1/2$ L.A. Tracy, I.B. Spielman, J.P. Eisenstein, L.N. Pfeiffer, K.W. West Via a resistively-detected NMR technique, the nuclear spin
lattice relaxation time $T_{1}$ of $^{71}$Ga at low temperatures
has been measured in a GaAs/AlGaAs heterostructure containing
two weakly-coupled 2D electron systems (2DES), each at Landau
level filling $\nu = 1/2$. Incomplete electronic spin
polarization, which has been reported previously [1,2] for low
density 2DESs at $\nu = 1/2$, should facilitate hyperfine-
coupled nuclear spin relaxation owing to the presence of both
electron spin states at the Fermi level. Within composite
fermion theory, a Korringa law temperature dependence: $T_{1}T
= constant$, is expected for temperatures $T<1$ K. Our
measurements made at temperatures in the range 35 mK $ |
Thursday, March 24, 2005 8:12AM - 8:24AM |
U19.00002: Evidence of Composite Fermion interactions in the Fermi sea at $\nu\rightarrow 1/2$ Irene Dujovne, A. Pinczuk, B. S. Dennis, L. N. Pfeiffer, K. W. West Interactions between composite fermions with two attached flux quanta ($^2$CFs) are explored at filling factors of the fractional quantum Hall effect $\nu\rightarrow 1/2$. Low-lying ($\omega < $1meV) spin flip excitations modes, in which spin orientation and Landau level index of composite fermions change simultaneously, are measured in resonant inelastic light scattering experiments. The measurements uncover a delicate balance between spin reversal and Fermi energies in the Fermi sea of composite fermions that emerges in the limit of $\nu\rightarrow$1/2. A collapse of the spin-flip excitation gap as $\nu\rightarrow$1/2 is linked with vanishing quasiparticle energy level spacings and loss of full spin polarization. This work is supported by the NSF under Award Number DMR-03-52738 and by the DoE award DE-AIO2-04ER46133. It is also supported by a research grant of the W. M. Keck Foundation. [Preview Abstract] |
Thursday, March 24, 2005 8:24AM - 8:36AM |
U19.00003: Fermi Liquid Behavior and the Ground State at Half-Filled Landau Levels E.H. Rezayi, Mike Gaston, F.D.M. Haldane, Xin Wan, Kun Yang Experiments show an unquantized quantum Hall state at $\nu=1/2$---widely believed to be a Fermi-liquid state of composite fermions. In contrast, at $\nu=5/2$ a Hall liquid is seen, which numerical calculations have shown to be a p-wave paired state of composite fermions.In the third and higher Landau levels transport becomes anisotropic indicating the development of stripe-type order. All three states may be associated with the existence of a Fermi surface. We construct many-particle wavefunctions with periodic boundary conditions and cast them as Slater-determinants of composite fermion plane waves. We obtain the variational energies of the ground state and a number of low lying excited states near the Fermi surface by quantum Monte Carlo simulations for up to 100 electrons. From this and the K-space translational invariance of the Fermi states we extract the effective mass and the first few Fermi liquid parameters. We compare the Fermi liquid parameters, discuss the stability of the isotropic Fermi surfaces, and explore anisotropic Fermi surfaces in these Landau levels. [Preview Abstract] |
Thursday, March 24, 2005 8:36AM - 8:48AM |
U19.00004: Resonant Enhancement of Inelastic Light Scattering in the Fractional Quantum Hall Regime at $\nu=1/3$ C.F. Hirjibehedin, Irene Dujovne, I. Bar-Joseph, A. Pinczuk, B.S. Dennis, L.N. Pfeiffer, K.W. West Strong resonant enhancement of the inelastic light scattering cross-section is essential in obtaining the sensitivity required to observe inter-Landau level and the intra-Landau level charge and spin density excitations in fractional quantum Hall liquids. We find that at $\nu = 1/3$ the energies of the sharp peaks in the resonant enhancement profiles of inelastic light scattering intensities coincide with the energies of optical excitations measured in photoluminescence, which recently have been assigned to negatively charged excitons. To interpret the observed enhancement profiles, we propose light scattering mechanisms in which the intermediate resonant transitions are to states with charged excitonic excitations. This work is supported by the NSF under Award Number DMR-03- 52738 and by the DoE award DE-AIO2-04ER46133. It is also supported by a research grant of the W. M. Keck Foundation. *Present address: IBM-Almaden Research Center [Preview Abstract] |
Thursday, March 24, 2005 8:48AM - 9:00AM |
U19.00005: Insulating phases in the limit of strong Landau level mixing Gabor Csathy, Daniel C. Tsui, Loren Pfeiffer, Ken West We have explored the electronic phases of 2D holes subjected to perpendicular magnetic fields in the new regime of very low densities. The sample density is $1.6 \times 10^{10}$cm$^{-2}$ and it is tunable with a backgate. At the highest densities, beside the $\nu=1/3$, 2/5, and 2/3 fractional quantum Hall states, we observe both of the previously reported high field insulating and reentrant insulating phases. Similarly to the result in higher density samples, the reentrant insulating phase strengthens as the density is lowered. With a further decrease in density, however, the reentrant insulator unexpectedly weakens then it completely disappears. Since both of the insulating phases have been interpreted as electronic solids, the behavior observed can be regarded as a melting of the solid with decreasing density. Such a melting is at odds with expectations and we think that it reflects the influence of the strong Landau level mixing on the quantum fluctuations of the nodes of the solid. [Preview Abstract] |
Thursday, March 24, 2005 9:00AM - 9:12AM |
U19.00006: Observation of fractional statistics V.J. Goldman, F.E. Camino, W. Zhou Our present experiment utilizes a novel Laughlin quasiparticle interferometer, where a quasiparticle with charge $e/3$ of the $f=1/3$ FQH fluid executes a closed path around an island of the $f=2/5$ fluid. The interference fringes are observed as peaks in conductance as a function of the magnetic flux $\Phi $ through the $f=2/5$ island, in a kind of the Aharonov-Bohm effect. A similar situation of resonant tunneling in an FQH fluid at filling $f_1 $ surrounding an FHQ island at a different filling $f_2 $ was considered theoretically by Jain et. al.. We observe the interference pattern shift by one fringe upon introduction of five magnetic flux quanta into the $f=2/5$ island, i.e., the Aharonov-Bohm period $\Delta \Phi =5h/e,$ corresponding to excitation of ten $q=e/5$ quasiparticles of the $f=2/5$ fluid. Such ``superperiod'' of $\Delta \Phi >h/e$ has never been reported before. This $\Delta Q=2e$ charge period is directly confirmed in calibrated backgate experiments. These observations imply \textit{relative} statistics of $\Theta _{2/5}^{1/3} =-1/15$, when a charge $e/3$, statistics $\Theta _{1/3}^ =2/3$ Laughlin quasiparticle encircles one $e/5$, $\Theta _{2/5}^ =2/5$ quasiparticle of the $f=2/5$ fluid. [Preview Abstract] |
Thursday, March 24, 2005 9:12AM - 9:24AM |
U19.00007: Spin excitations at $\nu >1/3$: a probe of composite fermions interactions Yann Gallais, Thomas Kirschenmann, Irene Dujovne, Cyrus Hirjibehedin, Brian Dennis, Aron Pinczuk, Ken West, Loren Pfeiffer We present a resonant inelastic light scattering study of spin excitations at filling factors of the fractional quantum Hall effect (FQHE) with $\nu >1/3$, in which composite fermions may condense into higher order quasiparticles. Observations of low lying spin excitations enable us to study the composite fermions (CF) Landau level configuration and the impact of CF residual interactions at filling factors away from the major FQHE sequences. A very low energy spin mode ($\omega\leq0.1~meV$), which displays a marked temperature dependence above T$\sim$100~mK, emerges at filling factors close to $\nu=4/11$ and remains with small changes in energy in the filling factor range $4/11\leq\nu\leq5/13$. The spectral intensity of this spin excitation becomes negligible at temperatures above 300~mK. The marked temperature dependence of the intensity suggest the existence of even lower excitation modes at energies well below 0.1~meV. We discuss the implication of these experiments on the possibility of higher order CF suggested by transport measurements at $\nu=$4/11. \\ This work is supported by the NSF under Award Number DMR-03-52738 and by the DoE award DE-AIO2-04ER46133. It is also supported by a research grant of the W. M. Keck Foundation. [Preview Abstract] |
Thursday, March 24, 2005 9:24AM - 9:36AM |
U19.00008: Generalized Clustered Quantum Hall States Steven H. Simon, Nigel R. Cooper, Ed Rezayi The Read-Rezayi (parafermion) quantum Hall states[1] for bosons can be defined as states where the wavefunction does not vanish when $g$ bosons come together to the same point, but does vanish as $z^2$ as a $g+1$st particle approaches that point. These states can equivalently be defined as the unique ground state of a point contact $g+1$ particle interaction Hamiltonian. Interestingly, the series of Read-Rezayi states appears to describe well the groundstates of rotating Bose condensates with point-contact two body interactions at a series of filling fractions [2]. If one attaches a Jastrow factor to such bose wavefunctions, one obtains fermion wavefunctions that may occur in electronic quantum Hall systems including the ($g=2$) Pfaffian [3] and the ($g=3$) $\nu=13/5$ Read-Rezayi state [1]. In this work, we consider generalized cluster wavefunctions defined by the algebraic manner in which a wavefunction vanishes as $g+1$ particles coalesce. We find Hamiltonians that generate these wavefunctions as their exact ground state. Among this series of states is the previously studied Haffnian wavefunction[4] and a host of states not previously discussed. We catalogue and study the new states and discuss whether any of them might occur in actual physical systems. [1] N. Read and E. Rezayi, PRB{\bf 59}, 8084 (1999). [2] N. R. Cooper, N. K. Wilkin, and J. M. F. Gunn, PRL{\bf 87}, 120405 (2001) [3] G. Moore and N. Read, Nuc. Phys. B{\bf 360}, 362 (1991). [4] D. Green, PhD Thesis. [Preview Abstract] |
Thursday, March 24, 2005 9:36AM - 9:48AM |
U19.00009: Off-diagonal long-range order in the fractional quantum Hall effect G.S. Jeon, M.R. Peterson, J.K. Jain It is generally accepted that the fundamental physics of the fractional quantum Hall effect lies in the topological binding of quantized vortices and electrons. From a microscopic point of view, however, the non-Pauli vortices are not strictly bound to electrons in realistic ground state wave functions. We study the Girvin-MacDonald off-diagonal long-range order at Landau level fillings $\nu=1/m$ ($m$ odd) for bosonic wave functions obtained from fermionic fractional Hall wave functions by a singular gauge transformation. In order to test the robustness of the concept, we work with accurate representations of the Coulomb ground state, constructed using the framework of the composite-fermion theory, and find strong evidence that the exponent describing its long-distance algebraic decay has a universal value $m/2$ independent of the form of the wave function. We interpret this to mean that the topological notion of electron-vortex binding remains generally well defined as a long-distance property. [Preview Abstract] |
Thursday, March 24, 2005 9:48AM - 10:00AM |
U19.00010: Re-emergence of the electron in a fractional quantum Hall fluid Michael R. Peterson, Jainendra K. Jain The low energy physics of the fractional quantum Hall fluid is described in terms of fermionic quasiparticles called composite fermions, which are distinct from electrons. We show that a long lived electron- like quasiparticle also exists in the excitation spectrum. Specifically, we find, using wave functions that are demonstrably very accurate for small systems, that there is a non-zero overlap (in the thermodynamic limit) between the state obtained by application of an electron creation operator to a fractional quantum Hall ground state and a high energy bound state complex containing an odd number of composite-fermion quasiparticles. The electron annihilation operator similarly couples to a bound complex of composite-fermion holes. We predict that these bound states can be observed through a conductance resonance in experiments involving a tunneling of an electron into a quantum Hall fluid. [Preview Abstract] |
Thursday, March 24, 2005 10:00AM - 10:12AM |
U19.00011: Collapse of Composite Fermion Spin-Flip Roton at Filling Factor Below $\nu=2/5$ Thomas Kirschenmann, Yann Gallais, Irene Dujovne, Cyrus Hirjibehedin, Brian Dennis, Aron Pinczuk, Loren Pfeiffer, Ken West Composite fermions (CF) consisting of an electron with attached even numbers of magnetic flux quanta are formed in the fractional quantum Hall regime. In this work resonant inelastic light scattering methods are used to study low energy excitations at electron Landau level filling factors close to $\nu=2/5$ and at cold finger temperatures below 40mK. Our measurements focus on low lying spin excitations in which quasiparticles change CF Landau level and reverse spin orientation. The evolution of the roton in the dispersion curve of spin-flip modes is measured as a function of changes in filling factors near $\nu<2/5$. These experiments probe CF properties in the range of filling factor close to $\nu=5/13$ in which the population of the excited CF Landau level is close to 2/3 and CF's that are composed of electrons with two attached flux quanta are expected to condense into higher order quasiparticles. This work is supported by the NSF under Award Number DMR-03- 52738 and by the DoE award DE-AIO2-04ER46133. It is also supported by a research grant of the W. M. Keck Foundation. [Preview Abstract] |
Thursday, March 24, 2005 10:12AM - 10:24AM |
U19.00012: Measuring Fractional Charge and Statistics in fractional quantum Hall Jain States Eun-Ah Kim, Smitha Vishveshwara, Michael Lawler, Eduardo Fradkin We study quantum noise in a multiple lead setup of a 2DEG in the FQH regime as a means of capturing clear signatures of fractional statistics and fractional charge in Jain states. Quasiparticles in FQH systems are predicted to have fractional charge and statistics. While the fractional charge for Laughlin states was observed recently through shot noise measurements, and found to be consistent with predictions, a clear signature of fractional statistics has yet to be seen. Here we propose an experiment involving the tunneling of quasiparticles in a multiple lead setup, that can unmistakably capture fractional statistics . Of particular interest are non-Laughlin states since in these states charge $Q$ and statistical angle $\theta$ are different fractions (e.g., $\nu=2/5$, $Q=e/5$, $\theta=3\pi/5$) and in principle, one should be able to tell how each contributes to measured correlations of tunneling currents between different leads. By analyzing finite-temperature cross correlations between tunneling currents, we find more boson-like (bunching) or more fermion-like (anti-bunching) statistical behavior of quasi particles depending on their filling fraction $\nu$ and related statistical angle $\theta$. Furthermore we are able to distinguish contributions coming from the fractional charge and fractional statistics. [Preview Abstract] |
Thursday, March 24, 2005 10:24AM - 10:36AM |
U19.00013: Coulomb Correlations in Landau Levels: Novel Squeezed States and Optical Transitions Alexander Todd, Lionel Cuggia, Alexander Dzyubenko We report on a novel theoretical formalism developed for dealing with Coulomb interparticle interactions in composite charged complexes in Landau levels. The formalism is based on a unitary transformation of the exact interacting Hamiltonian and describes condensation of an infinite number of degrees of freedom into new squeezed vacuum and excited states. The squeezed states are characterized by built-in correlations between particles and form a complete basis of states, which is compatible with axial rotations and magnetic translations. We introduce and discuss novel and surprisingly simple exact optical selection rules that govern inter- and intra-band transitions of charged composite complexes in magnetic fields. We report on applications of the developed formalism to studying optical transitions and charged collective excitations in a magnetically quantized 2D electron gas. [Preview Abstract] |
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