4,948 research outputs found

    Quantum Hall Ferromagnetism in a Two-Dimensional Electron System

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    Experiments on a nearly spin degenerate two-dimensional electron system reveals unusual hysteretic and relaxational transport in the fractional quantum Hall effect regime. The transition between the spin-polarized (with fill fraction ν=1/3\nu = 1/3) and spin-unpolarized (ν=2/5\nu = 2/5) states is accompanied by a complicated series of hysteresis loops reminiscent of a classical ferromagnet. In correlation with the hysteresis, magnetoresistance can either grow or decay logarithmically in time with remarkable persistence and does not saturate. In contrast to the established models of relaxation, the relaxation rate exhibits an anomalous divergence as temperature is reduced. These results indicate the presence of novel two-dimensional ferromagnetism with a complicated magnetic domain dynamic.Comment: 15 pages, 5 figure

    Spin excitations in quantum hall systems and their effect on nuclear spin relaxation and photoluminescence

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    Two problems with respect to spin excitations in quantum Hall systems are studied by means of exact numerical diagonalization. The first one is related to the formation of reversed-spin quasielectrons (QER) in a two-dimensional electron gas (2DEG). The single—particle properties of QER’S as well as the pseudopotentials of their interaction with one another and with Laughlin quasielectrons (QE’s) and quasiholes (QH’s) are calculated. Based on the short-range character of the QER—QER and QER—QE repulsion, the partially unpolarized incompressible states at the filling factors 1/ = 14—1 and 1/ = T55 are postulated within Haldane’s hierarchy scheme. To describe photoluminescence, the family of bound h(QER)n states of a valence hole h and n QER’S are predicted in analogy to the found earlier fractionally charged excitons hQEn. The binding energy and optical selection rules for both families are compared. The hQER is found radiative in contrast to the dark hQE, and the h(QER)2 is found nonradiative in contrast to the bright hQEg. The second problem involves the numerical study of the relaxation rates of nuclear spins coupled through the hyperfine interaction to a 2DEG at magnetic fields corresponding to both fractional and integral Landau level fillings 1/. The spectral functions T_1(E) describing the response of the 2DEG to the reversal of an embedded localized spin are calculated. In a (locally) incompressible 1/ = 1 or 31,; state, the finite Coulomb energy of short spin waves, together with the small nuclear Zeeman energy, prevent nuclear spin relaxation even in the limit of vanishing electron Zeeman energy (EZ). However, we find that the nuclear spins can couple to the internal excitations of mobile finite-size skyrmions that appear in the 2DEG at sufficiently low EZ and at V slightly different from 1 or %. The experimentally observed dependence of nuclear spin relaxation rate on E2 and 1/ is explained in terms of the occurrence of skyrmions and anti-skyrmions of various topological charge

    Metastable phase in the quantum Hall ferromagnet

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    Time-dependent capacitance measurements reveal an unstable phase of electrons in gallium arsenide quantum well that occurs when two Landau levels with opposite spin are brought close to degeneracy by applying a gate voltage. This phase emerges below a critical temperature and displays a peculiar non-equilibrium dynamical evolution. The relaxation dynamics is found to follow a stretched exponential behavior and correlates with hysteresis loops observed by sweeping the magnetic field. These experiments indicate that metastable randomly-distributed magnetic domains are involved in the relaxation process in a way that is equivalently tunable by a change in gate voltage or temperature.Comment: 7 pages, including 4 figures Changes made to introduction Added figure Submitted to SS

    Ultraslow Electron Spin Dynamics in GaAs Quantum Wells Probed by Optically Pumped NMR

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    Optically pumped nuclear magnetic resonance (OPNMR) measurements were performed in two different electron-doped multiple quantum well samples near the fractional quantum Hall effect ground state nu=1/3. Below 0.5K, the spectra provide evidence that spin-reversed charged excitations of the nu=1/3 ground state are localized over the NMR time scale of ~40 microseconds. Furthermore, by varying NMR pulse parameters, the electron spin temperature (as measured by the Knight shift) could be driven above the lattice temperature, which shows that the value of the electron spin-lattice relaxation time lies between 100 microseconds and 500 milliseconds at nu=1/3.Comment: 6 pages (REVTEX), 6 eps figures embedded in text; published version; minor changes to match published versio

    Effect of magnetization on the tunneling anomaly in compressible quantum Hall states

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    Tunneling of electrons into a two-dimensional electron system is known to exhibit an anomaly at low bias, in which the tunneling conductance vanishes due to a many-body interaction effect. Recent experiments have measured this anomaly between two copies of the half-filled Landau level as a function of in-plane magnetic field, and they suggest that increasing spin polarization drives a deeper suppression of tunneling. Here we present a theory of the tunneling anomaly between two copies of the partially spin-polarized Halperin-Lee-Read state, and we show that the conventional description of the tunneling anomaly, based on the Coulomb self-energy of the injected charge packet, is inconsistent with the experimental observation. We propose that the experiment is operating in a different regime, not previously considered, in which the charge-spreading action is determined by the compressibility of the composite fermions.Comment: (5+1) pages, 1 figure; (v2) minor changes and added reference to our accompanying paper arXiv:1712.02357; (v3) Final published versio
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