Field-induced breakdown of the quantum Hall effect

A numerical analysis is made of the breakdown of the quantum Hall effect caused by the Hall electric field in competition with disorder. It turns out that in the regime of dense impurities, in particular, the number of localized states decreases exponentially with the Hall field, with its dependence on the magnetic and electric field summarized in a simple scaling law. The physical picture underlying the scaling law is clarified. This intra-subband process, the competition of the Hall field with disorder, leads to critical breakdown fields of magnitude of a few hundred V/cm, consistent with observations, and accounts for their magnetic-field dependence \propto B^{3/2} observed experimentally. Some testable consequences of the scaling law are discussed.Comment: 7 pages, Revtex, 3 figures, to appear in Phys. Rev.

Reorientation of Anisotropy in a Square Well Quantum Hall Sample

We have measured magnetotransport at half-filled high Landau levels in a quantum well with two occupied electric subbands. We find resistivities that are {\em isotropic} in perpendicular magnetic field but become strongly {\em anisotropic} at $\nu$ = 9/2 and 11/2 on tilting the field. The anisotropy appears at an in-plane field, $B_{ip} \sim$ 2.5T, with the easy-current direction {\em parallel} to $B_{ip}$ but rotates by 90$^{\circ}$ at $B_{ip} \sim$ 10T and points now in the same direction as in single-subband samples. This complex behavior is in quantitative agreement with theoretical calculations based on a unidirectional charge density wave state model.Comment: 4 pages, 4 figure

Magnetic-field-dependent zero-bias diffusive anomaly in Pb oxide-n-InAs structures: Coexistence of two- and three-dimensional states

The results of experimental and theoretical studies of zero-bias anomaly (ZBA) in the Pb-oxide-n-InAs tunnel structures in magnetic field up to 6T are presented. A specific feature of the structures is a coexistence of the 2D and 3D states at the Fermi energy near the semiconductor surface. The dependence of the measured ZBA amplitude on the strength and orientation of the applied magnetic field is in agreement with the proposed theoretical model. According to this model, electrons tunnel into 2D states, and move diffusively in the 2D layer, whereas the main contribution to the screening comes from 3D electrons.Comment: 8 double-column pages, REVTeX, 9 eps figures embedded with epsf, published versio

Quantum Monte-Carlo method without negative-sign problem for two-dimensional electron systems under strong magnetic fields

The quantum Monte-Carlo method is applied to two-dimensional electron systems under strong magnetic fields. The negative-sign problem involved by this method can be avoided for certain filling factors by modifying interaction parameters from those of the Coulomb interaction. Our techniques for obtaining sign-problem-free parameters are described in detail. Calculated results on static observables are also reported for Landau level filling $\nu = 1/3$.Comment: 4 pages, 3 figure

Josephson Coupling, Phase Correlations, and Josephson Plasma Resonance in Vortex Liquid Phase

Josephson plasma resonance has been introduced recently as a powerful tool to probe interlayer Josephson coupling in different regions of the vortex phase diagram in layered superconductors. In the liquid phase, the high temperature expansion with respect to the Josephson coupling connects the Josephson plasma frequency with the phase correlation function. This function, in turn, is directly related to the pair distribution function of the liquid. We develop a recipe to extract the phase and density correlation functions from the dependencies of the plasma resonance frequency $\omega_p({\bf B})$ and the $c$ axis conductivity $\sigma_c({\bf B})$ on the {\it ab}-component of the magnetic field at fixed {\it c} -component. Using Langevin dynamic simulations of two-dimensional vortex arrays we calculate density and phase correlation functions at different temperatures. Calculated phase correlations describe very well the experimental angular dependence of the plasma resonance field. We also demonstrate that in the case of weak damping in the liquid phase, broadening of the JPR line is caused mainly by random Josephson coupling arising from the density fluctuations of pancake vortices. In this case the JPR line has a universal shape, which is determined only by parameters of the superconductors and temperature.Comment: 22 pages, 6 figures, to appear in Phys. Rev. B, December

1D generalized statistics gas: A gauge theory approach

A field theory with generalized statistics in one space dimension is introduced. The statistics enters the scene through the coupling of the matter fields to a statistical gauge field, as it happens in the Chern-Simons theory in two dimensions. We study the particle-hole excitations and show that the long wave length physics of this model describes a gas obeying the Haldane generalized exclusion statistics. The statistical interaction is found to provide a way to describe the low-T critical properties of one-dimensional non-Fermi liquids.Comment: 8 pages, revte

Thermodynamic Study of Excitations in a 3D Spin Liquid

In order to characterize thermal excitations in a frustrated spin liquid, we have examined the magnetothermodynamics of a model geometrically frustrated magnet. Our data demonstrate a crossover in the nature of the spin excitations between the spin liquid phase and the high-temperature paramagnetic state. The temperature dependence of both the specific heat and magnetization in the spin liquid phase can be fit within a simple model which assumes that the spin excitations have a gapped quadratic dispersion relation.Comment: 5 figure

Electron correlation in the second Landau level; a competition between many, nearly degenerate quantum phases

At a very low temperature of 9mK, electrons in the 2nd Landau level of an extremely high mobility two-dimensional electron system exhibit a very complex electronic behavior. With varying filling factor, quantum liquids of different origins compete with several insulating phases leading to an irregular pattern in the transport parameters. We observe a fully developed $\nu=2+2/5$ state separated from the even-denominator $\nu=2+1/2$ state by an insulating phase and a $\nu=2+2/7$ and $\nu=2+1/5$ state surrounded by such phases. A developing plateau at $\nu=2+3/8$ points to the existence of other even-denominator states