Evidence for two-dimentional quantum Wigner crystal

Observation of reentrant phases induced by short-range disorder in the lowest Landau level of Al{x}Ga{1-x}As/Al{0.32}Ga{0.68}as heterostructures

We report the observation of a reentrant quantum Hall effect in the lowest Landau level between filling factors of 2/3 and 3/5 in a Al{x}Ga{1-x}As/Al{0.32}Ga{0.68}As heterostructure sample with x=0.85%. A reentrant insulating phase is also observed between filling factors of 2/5 and 1/3, demonstrating particle-hole symmetry between these phases. A sample with x=0.21% shows much weaker reentrant features, indicating that increased short-range scattering, due to the Al alloy in the conduction channel, aids in the formation of these phases.

Correlated states of electrons in wide quantum wells at low fillings: the role of charge distribution symmetry

Magnetotransport measurements on electrons confined to a 57-nm-wide, GaAs quantum well reveal that the correlated electron states at low Landau level fillings (nu) display a remarkable dependence on the symmetry of the electron charge distribution. At a density of 1.93 x 10;{11} cm;{-2}, a developing fractional quantum Hall state is observed at the even-denominator filling nu = 1/4 when the distribution is symmetric, but it quickly vanishes when the distribution is made asymmetric. At lower densities, as we make the charge distribution asymmetric, we observe a rapid strengthening of the insulating phases that surround the nu = 1/5 fractional quantum Hall state.

Spin-incoherent transport in quantum wires

When a quantum wire is weakly confined, a conductance plateau appears at e;{2}/h with decreasing carrier density in zero magnetic field accompanied by a gradual suppression of the 2e;{2}/h plateau. Applying an in-plane magnetic field B_{ parallel} does not alter the value of this quantization; however, the e;{2}/h plateau weakens with increasing B_{ parallel} up to 9 T, and then strengthens on further increasing B_{ parallel}, which also restores the 2e;{2}/h plateau. Our results are consistent with spin-incoherent transport in a one-dimensional wire.

Astability and negative differential resistance of the Wigner solid

We report spontaneous narrow band oscillations in the high field Wigner solid. These oscillations are similar to the recently seen and yet unexplained oscillations in the reentrant integer quantum Hall states. The current-voltage characteristic has a region of negative differential resistance in the current biased setup and it is hysteretic in the voltage biased setup. As a consequence of the unusual breakdown, the oscillations in the Wigner solid are of the relaxation type.

Thermopower of interacting GaAs bilayer hole systems in the reentrant insulating phase near nu=1

We report thermopower measurements of interacting GaAs bilayer hole systems. When the carrier densities in the two layers are equal, these systems exhibit a reentrant insulating phase near the quantum Hall state at total filling factor nu=1. Our data show that, as the temperature is decreased, the thermopower diverges in the insulating phase. This behavior indicates the opening of an energy gap at low temperature, consistent with the formation of a pinned Wigner solid. We extract an energy gap and a Wigner solid melting phase diagram.

Experimental evidence for a collective insulating state in two-dimensional superconductors

We present the results of an experimental study of the current-voltage characteristics in a strong magnetic field (B) of disordered, superconducting, thin films of amorphous indium oxide. As the B strength is increased superconductivity degrades, until a critical field (B(c)) where the system is forced into an insulating state. We show that the differential conductance measured in the insulating phase vanishes abruptly below a well-defined temperature, resulting in a clear threshold for conduction. Our results indicate that a new collective state emerges in two-dimensional superconductors at high B.

Microscopic verification of topological electron-vortex binding in the lowest Landau-level crystal state

When two-dimensional electrons are subjected to a very strong magnetic field, they are believed to form a triangular crystal. By a direct comparison with the exact wave function, we demonstrate that this crystal is not a simple Hartree-Fock crystal of electrons but an inherently quantum mechanical crystal characterized by a nonperturbative binding of quantized vortices to electrons. It is suggested that this has qualitative consequences for experiment.

Possible observation of phase coexistence of the nu=1/3 fractional quantum hall liquid and a solid

We have measured the magnetoresistance of a very low density and extremely high quality two-dimensional hole system. With increasing magnetic field applied perpendicularly to the sample we observe the sequence of insulating, nu=1/3 fractional quantum Hall liquid, and insulating phases. In both of the insulating phases in the vicinity of the nu=1/3 filling the magnetoresistance has an unexpected oscillatory behavior with the magnetic field. These oscillations are not of the Shubnikov-de Haas type and cannot be explained by spin effects. They are most likely the consequence of the formation of a new electronic phase which is intermediate between the correlated Hall liquid and a disorder pinned solid.

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 reentrant 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.

Two-dimensional electron system in high magnetic fields: Wigner crystal versus composite-fermion liquid

The two-dimensional system of electrons in a high magnetic field offers an opportunity to investigate a phase transition from a quantum liquid into a Wigner solid. Recent experiments have revealed an incipient composite fermion liquid in a parameter range where theory and many experiments had previously suggested the Wigner crystal phase, thus calling into question our current understanding. This Letter shows how very small quantitative corrections (<1%) in the energy due to the weak interaction between composite fermions can cause a fundamental change in the nature of the ground state, thus providing insight into the puzzling experimental results.

Moving Wigner glasses and smectics: dynamics of disordered Wigner crystals

We examine the dynamics of driven classical Wigner solids interacting with quenched disorder from charged impurities. For strong disorder, the initial motion is plastic, in the form of crossing winding channels. For increasing drive, there is a reordering into a moving Wigner smectic with the electrons moving in separate 1D channels. These different dynamic phases can be related to the conduction noise and I(V) curves. For strong disorder, we show criticality in the voltage onset just above depinning. We obtain the dynamic phase diagram for driven Wigner solids and demonstrate a finite threshold of force for transverse sliding, recently observed experimentally.