P. M. Platzman

LAUGHLIN-LIQUID-WIGNER-SOLID TRANSITION AT HIGH DENSITY IN WIDE QUANTUM WELLS

Assuming that the phase transition between the Wigner solid and the Laughlin liquid is first-order, we compare ground-state energies to find features of the phase diagram at fixed

Freezing of the quantum Hall liquid at nu =1/7 and 1/9.

\nu

Observation of positron bragg reflection from A1 and Cu surfaces

. Rather than use the Coulomb interaction, we calculate the effective interaction in a square quantum well, and fit the results to a model interaction with length parameter

Inelastic electron scattering in nickel

\lambda

Quantum theory of surface adsorption

roughly proportional to the width of the well. We find a transition to the Wigner solid phase at high density in very wide wells, driven by the softening of the interaction at short distances, as well as the more well-known transition to the Wigner solid at low density, driven by Landau-level mixing.

Evidence for a position mobility edge in gaseous helium

We compare the free energy computed from the ground-state energy and low-lying excitations of the two-dimensional Wigner solid and the fractional quantum Hall liquid, at magnetic filling factors [nu]=1/7 and 1/9. We find that the Wigner solid melts into the fractional quantum Hall liquid at roughly the same temperature as that of some recent luminescence experiments, while it remains a solid at the lower temperatures characteristic of the transport experiments. We propose this melting as a consistent interpretation of both sets of experiments.

Resonant Raman scattering and numerical study in the fractional quantum Hall regime

Abstract The elastic scattering of low energy positrons from single crystal surface of Cu and A1 has been observed. For (111) surfaces of Cu and A1, the (0,0) (specular) beams exhibit a single large maximum (reflected intensity ∼20%) ∼3.5 eV wide peaked at 6.7 eV and 5.0 eV near the center of the positron band gap due to (111) Bragg reflections expected at 8.2 eV and 6.9 eV respectively. The smaller Bragg peaks observed for A1 (110) and (100) and the absence of higher order peaks (

Fractional quantum hall effect: Superfluidity, magneto-rotons and fractionally charged vortices

Abstract Using a 200 keV electron spectrometer, with an energy resolution of ∼0.25 eV and a momentum resolution of ∼0.2 A -1 , we have measured the energy loss spectra for transmission of electrons through thin (∼600 A) Ni films. These results address the general question of the validity of momentum transfer estimates in electron loss scattering. Using low-energy electron backscattering, we have observed the dipole forbidden M 1 transition at 112 eV. For high-energy scattering, we have observed this transition only at high momentum transfer ( q ⩾ 2 A -1 ). These results indicates sizable contributions from high momentum transfer collisions in the low-energy experiments.

Bragg reflection and isochromat spectroscopy of electrons from a magnetic single crystal surface: Novel polarization detectors?

Abstract We review the progress in the study of quantum coherence effects observed in surface physics at very low temperatures. After a brief discussion of experiments on the scattering of 4He and ↓H atoms off 4He surfaces, as well as the scattering of Ps off metal surfaces, we review the two existing theories for surface adsorption, and their application to the experiments. We demonstrate that the two theories - based respectively on perturbation theory and on time-dependent Hartree theory — are in fundamental conflict. Finally, we describe a recent coherent-state theory of quantum adsorption which appears to resolve the conflict. The theory finds that as a function of the particle-surface coupling constant, there is a very rapid crossover — potentially a phase transition — from quantum to classical behavior. Similar singular behavior is known from the study of related phenomena such as quantum tunneling in the presence of dissipation.

Quantum Hall liquid-Wigner solid phase boundary

Abstract We show that the rather peculiar behavior of position lifetimes in cold, dense helium vapor are not inconsistent with some very simple ideas regarding the localization of carriers in random systems.