A model of local structural states (phases) in the superconducting TI phase of the nonstoichiometric alkali tungsten bronzes MxWO3 (M an alkali) is shown to be consistent with the dramatic increase of the superconducting transition temperature as x approaches the TII phase boundary value. This model also predicts the existence of (x dependent) anharmonicity and heat capacity anomalies and it is consistent with the fact that the cubic phase of NaxWO3 is nonsuperconducting.
The line following Eq.(2) should read "where P " = --i&/eq".For stochastic processes V(q) is zero." In Eq. ( 9) replace (2we) by ( 2m&)".In the eighth line preceding Eg. ( 12) read v'-Q instead of-MODEL OF THE FERROELECTRIC PHASE
Two independent studies of the electron-hole liquid in stressed Si have provided different values for the ground-state density and the critical temperature of this important Fermi fluid. One set of experiments (by Gourley and Wolfe) was performed using the strain-well configuration while the other (by Forchel et al.) employed uniform strain. These differences raise questions of whether there was some basic difference between the two experimental systems or whether the discrepancies lay in the different methods used for data analysis. In the present joint paper these differences are discussed, several aspects of the data are reanalyzed, some new results are presented, and to a large degree the previous differences are removed. In order to test for possible differences between the line-shape analyses, we use the detailed fitting procedure of Forchel et al. on the strain-well data of Gourley and Wolfe and thereby find a ground-state density of (3.7\ifmmode\pm\else\textpm\fi{}0.2)\ifmmode\times\else\texttimes\fi{}${10}^{17}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ in the high-stress limit where the effective masses no longer have a stress dependence.This ground-state density is found to be close to that originally reported for these data using a simpler analysis. We now believe that the differences in the value of ground-state density arose primarily from differences in the value of the stress used for the high-stress limit; densities derived at the same stress are in agreement. In addition, we now believe that the critical temperature which was reported previously for the strain-confined liquid was overestimated due to compression effects at high excitation levels. By analyzing the dependence of the luminescence spectra on excitation power we estimate the critical point of the strain-confined liquid to be at a lattice temperature of approximately 10.5 K. When account is taken of the possible difference between the lattice and electronic temperatures this value is consistent with the critical temperature of 14 K reported for the uniform-strain case.Finally, a previous anomaly in the data for the phase diagram of the strain-confined liquid is removed; at intermediate excitation levels the density of the liquid is found to be temperature dependent, which is consistent with theory.
We present an experimental and theoretical study of the photoluminescence spectra of individual doubly charged quantum dot molecules. The quantum dot molecules consist of two vertically stacked InAs self-assembled quantum dots in a GaAs Schottky diode structure. We study two cases: (1) the two dots are charged with two electrons coherently coupled through electron tunneling and (2) the two dots are charged with two holes and coherently coupled through hole tunneling. The optically excited states consist of the two charges along with one or two additional electron-hole pairs, i.e., a doubly charged exciton and biexciton. We determine the spin states and the corresponding spectral fine structure and show how this fine structure depends on vertical electric and magnetic fields. We find that the results are in large part qualitatively similar for the two cases. However, when magnetic fields are applied, we find a strong $g$ factor resonance and evidence of a bonding/antibonding reversal for the hole-tunneling case only. We discuss the implications for quantum information processing using spins confined in proximate dots.
A comparison is made between experimental and theoretical results for the damping of long-wavelength spin waves in weakly anisotropic antiferromagnets at temperatures between the low-temperature and the critical regimes. A high-density theory with no adjustable parameters is shown to account well for the wave-vector and temperature dependence of the widths measured by neutron scattering from spin waves in Mn${\mathrm{F}}_{2}$. The magnitudes of the theoretical widths agree with those observed to within 30% or better.
The electron{acoustic-phonon scattering process in spherical II-VI quantum dots is discussed. The quantized acoustic modes are described in terms of the Lamb's classical theory. We considered two mechanism for the interaction between electrons and acoustic modes: microscopic deformation potential, and macroscopic acoustic deformation, also called the ripple mechanism. We also discuss the in uence of the glass matrix on the electron-phonon coupling. Our calculations show that the ripple mechanism scattering rates become dominant by more than an order of magnitude, for small dot radius. In general, the total scattering rate depends on the acoustical properties of the glass matrix.
The thermoelectric properties of systems in the form of superlattices have been studied. First, the electrical and thermal conductivities, the thermopowers, and the figures of merit of superlattice structures are given in terms of the bulk parameters of the two constituent materials for conduction both parallel and perpendicular to the superlattice axis. Second, systems in which the layers of one of the materials become sufficiently thin that their electronic properties become effectively those of a two-dimensional quantum well are considered. Numerical calculations are given for such systems with ${\mathrm{Bi}}_{2}$${\mathrm{Te}}_{3}$ quantum wells separated by barriers having the parameters of bulk ${\mathrm{Pb}}_{0.75}$${\mathrm{Sn}}_{0.25}$Te. It is shown that heat and electronic conduction through the barriers has a pronounced effect on the thermoelectric properties of the superlattice and that the figure of merit is decreased substantially for finite barrier thicknesses.
