Metastable Resistance Anisotropy Orientation of Two-Dimensional Electrons in High Landau Levels

In half-filled high Landau levels, two-dimensional electron systems possess collective phases which exhibit a strongly anisotropic resistivity tensor. A weak, but as yet unknown, rotational symmetry-breaking potential native to the host semiconductor structure is necessary to orient these phases in macroscopic samples. Making use of the known external symmetry-breaking effect of an in-plane magnetic field, we find that the native potential can have two orthogonal local minima. It is possible to initialize the system in the higher minimum and then observe its relaxation toward equilibrium.Comment: 5 pages, 3 figures. Figure references corrected. Version accepted for publication in Physical Review Letter

Quantum Hall Phase Diagram of Second Landau-level Half-filled Bilayers: Abelian versus Non-Abelian States

The quantum Hall phase diagram of the half-filled bilayer system in the second Landau level is studied as a function of tunneling and layer separation using exact diagonalization. We make the striking prediction that bilayer structures would manifest two distinct branches of incompressible fractional quantum Hall effect (FQHE) corresponding to the Abelian 331 state (at moderate to low tunneling and large layer separation) and the non-Abelian Pfaffian state (at large tunneling and small layer separation). The observation of these two FQHE branches and the quantum phase transition between them will be compelling evidence supporting the existence of the non-Abelian Pfaffian state in the second Landau level.Comment: 4 pages, 3 figure

Evidence for a Goldstone Mode in a Double Layer Quantum Hall System

The tunneling conductance between two parallel 2D electron systems has been measured in a regime of strong interlayer Coulomb correlations. At total Landau level filling $\nu_T=1$ the tunnel spectrum changes qualitatively when the boundary separating the compressible phase from the ferromagnetic quantized Hall state is crossed. A huge resonant enhancement replaces the strongly suppressed equilibrium tunneling characteristic of weakly coupled layers. The possible relationship of this enhancement to the Goldstone mode of the broken symmetry ground state is discussed.Comment: 4 pages, 3 figures, 2 minor typeos fixe

Observation of narrow-band noise accompanying the breakdown of insulating states in high Landau levels

Recent magnetotransport experiments on high mobility two-dimensional electron systems have revealed many-body electron states unique to high Landau levels. Among these are re-entrant integer quantum Hall states which undergo sharp transitions to conduction above some threshold field. Here we report that these transitions are often accompanied by narrow- and broad-band noise with frequencies which are strongly dependent on the magnitude of the applied dc current.Comment: 4 pages, 3 figure

New Physics in High Landau Levels

Recent magneto-transport experiments on ultra-high mobility 2D electron systems in GaAs/AlGaAs heterostructures have revealed the existence of whole new classes of correlated many-electron states in highly excited Landau levels. These new states, which appear only at extremely low temperatures, are distinctly different from the familiar fractional quantum Hall liquids of the lowest Landau level. Prominent among the recent findings are the discoveries of giant anisotropies in the resistivity near half filling of the third and higher Landau levels and the observation of re- entrant integer quantum Hall states in the flanks of these same levels. This contribution will survey the present status of this emerging field.Comment: 8 pages, 9 figures. To be published in the Proceedings of the 13th International Conference on the Electronic Properties of Two-Dimensional System

Reply to Simon's Comment on "Evidence for an Anisotropic State of Two-Dimensional Electrons in High Landau Levels"

We recently reported [PRL 82, 394 (1999)] large transport anisotropies in a two-dimensional electron gas in high Landau levels. These observations were made utilizing both square and Hall bar sample geometries. Simon recently commented [cond-mat/9903086] that a classical calculation of the current flow in the sample shows a magnification of an underlying anisotropy when using a square sample. In this reply we present more recent data obtained with a very high mobility sample, and reiterate that, with or without magnification, an anisotropic state develops in high Landau levels at very low temperatures.Comment: 1 page, 1 figur

Improving Cosmological Distance Measurements by Reconstruction of the Baryon Acoustic Peak

The baryon acoustic oscillations are a promising route to the precision measure of the cosmological distance scale and hence the measurement of the time evolution of dark energy. We show that the non-linear degradation of the acoustic signature in the correlations of low-redshift galaxies is a correctable process. By suitable reconstruction of the linear density field, one can sharpen the acoustic peak in the correlation function or, equivalently, restore the higher harmonics of the oscillations in the power spectrum. With this, one can achieve better measurements of the acoustic scale for a given survey volume. Reconstruction is particularly effective at low redshift, where the non-linearities are worse but where the dark energy density is highest. At z=0.3, we find that one can reduce the sample variance error bar on the acoustic scale by at least a factor of 2 and in principle by nearly a factor of 4. We discuss the significant implications our results have for the design of galaxy surveys aimed at measuring the distance scale through the acoustic peak.Comment: 5 pages, LaTeX. Submitted to the Astrophysical Journa

Tunneling-driven breakdown of the 331 state and the emergent Pfaffian and composite Fermi liquid phases

We examine the possibility of creating the Moore-Read Pfaffian in the lowest Landau level when the multicomponent Halperin 331 state (believed to describe quantum Hall bilayers and wide quantum wells at the filling factor $\nu=1/2$) is destroyed by the increase of tunneling. Using exact diagonalization of the bilayer Hamiltonian with short-range and long-range (Coulomb) interactions in spherical and periodic rectangular geometries, we establish that tunneling is a perturbation that drives the 331 state into a compressible composite Fermi liquid, with the possibility for an intermediate critical state that possesses some properties of the Moore-Read Pfaffian. These results are interpreted in the two-component BCS model for Cauchy pairing with a tunneling constraint. We comment on the conditions to be imposed on a system with fluctuating density in order to achieve the stable Pfaffian phase.Comment: 10 pages, 7 figure

Non-parametric Lagrangian biasing from the insights of neural nets

We present a Lagrangian model of galaxy clustering bias in which we train a neural net using the local properties of the smoothed initial density field to predict the late-time mass-weighted halo field. By fitting the mass-weighted halo field in the AbacusSummit simulations at z=0.5, we find that including three coarsely spaced smoothing scales gives the best recovery of the halo power spectrum. Adding more smoothing scales may lead to 2-5% underestimation of the large-scale power and can cause the neural net to overfit. We find that the fitted halo-to-mass ratio can be well described by two directions in the original high-dimension feature space. Projecting the original features into these two principal components and re-training the neural net either reproduces the original training result, or outperforms it with a better match of the halo power spectrum. The elements of the principal components are unlikely to be assigned physical meanings, partly owing to the features being highly correlated between different smoothing scales. Our work illustrates a potential need to include multiple smoothing scales when studying galaxy bias, and this can be done easily with machine-learning methods that can take in high dimensional input feature space.Comment: submitted to JCA