Strongly enhanced hole-phonon coupling in the metallic state of the dilute two-dimensional hole gas

We have studied the temperature dependent phonon emission rate P(T) of a strongly interacting (r(s) > or =22) dilute 2D GaAs hole system using a standard carrier heating technique. In the still poorly understood metallic state, we observe that P(T) changes from P(T) approximately T5 to P(T) approximately T7 above 100 mK, indicating a crossover from screened piezoelectric (PZ) coupling to screened deformation potential (DP) coupling for hole-phonon scattering. Quantitative comparison with theory shows that the long range PZ coupling between holes and phonons has the expected magnitude; however, in the metallic state, the short range DP coupling between holes and phonons is almost 20 times stronger than expected from theory. The density dependence of P(T) shows that it is easier to cool low-density 2D holes in GaAs than higher density 2D hole systems.     

Shape resonances in slow electron scattering by aromatic molecules. I. Anthraquinone derivatives

A series of anthraquinone (C(14)O(2)H(8)) derivatives has been studied by means of electron capture negative ion mass spectrometry (ECNI-MS), photoelectron spectroscopy (PES), and AM1 quantum chemical calculations. Mean lifetimes of molecular negative ions M(-.) (MNI) have been measured. The mechanism of long-lived MNI formation in the epithermal energy region of incident electrons has been investigated. A simple model of a molecule (a spherical potential well with the repulsive centrifugal term) has been applied for the analysis of the energy dependence of cross sections at the first stage of the electron capture process. It has been shown that a temporary resonance of MNI at the energy approximately 0.5 eV corresponds to a shape resonance with lifetime 1-2.10(-13) s in the f-partial wave (l = 3) of the incident electron. The next resonant state of MNI at the energy approximately 1.7 eV has been associated with the electron excited Feshbach resonance (whose parent state is a triplet npi* transition). In all cases the initial electron state of the MNI relaxes into the ground state by means of a radiationless transition, and the final state of the MNI is a nuclear excited resonance with a lifetime measurable on the mass spectrometry timescale. Copyright 1999 John Wiley & Sons, Ltd.     

Observation of magnetic breakdown in double quantum wells

We report on our studies of magnetic breakdown (MB) in coupled GaAs/Al_0.3Ga_0.7As double quantum wells (DQWs) subject to crossed magnetic fields. MB is a failure of semiclassical theory that occurs when a magnetic field causes electrons to tunnel across a gap ink-space from one Fermi surface (FS) branch to another. We study MB in a two-branch FS created by subjecting a DQW to an in-plane magnetic field (B_∥). The principal effect ofB_∥is a distortion in the dispersion curve of the system, yielding a FS consisting of two components, a lens-shaped inner orbit and an hour-glass-shaped outer orbit. The perpendicular field (B_⊥) causes Landau level formation and Shubnikov–de Haas (SdH) oscillations for each branch of the FS. At higher perpendicular fields MB occurs and electrons tunnel throughk-space from one FS orbit to the other. MB is observed by noting which peaks are present in the Fourier power spectrum of the magnetoresistance versus 1/B_⊥at constantB_∥. We observe MB in two DQW samples over a range ofB_∥.     

Possible field-tuned superconductor-insulator transition in high-Tc superconductors: implications for pairing at high magnetic fields

The behavior of some high temperature superconductors (HTSC), such as La(2-x)Sr(x)CuO(4) and Bi(2)Sr(2-x)La(x)CuO(6 + delta), at very high magnetic fields, is similar to that of thin films of amorphous InOx near the magnetic-field-tuned superconductor-insulator transition. Analyzing the InOx data at high fields in terms of persisting local pairing amplitude, we argue by analogy that the local pairing amplitude also persists well into the dissipative state of the HTSCs, the regime commonly denoted as the "normal state" in very high magnetic field experiments.