We have calculated the spin-exchange two-body rate constants associated with the population dynamics of the hyperfine levels of atomic deuterium as a function of magnetic field in the Boltzmann zero-temperature limit. Results indicate that a gas of low-field–seeking deuterium atoms trapped in a static magnetic field minimum decays rapidly into an ultrastable gas of doubly spin-polarized deuterium. We also discuss the temperature dependence of various effects.

The time-dependent Ginzburg-Landau model for spinodal decomposition is studied by use of both a numerical simulation of the associated Langevin equation and a new low-temperature expansion. Both methods lead to a growth law for the characteristic domain size L(t)∼${\mathrm{t}}^{1/4}$. .AE

Quasielastic light scattering of polystyrene latex spheres incorporated in polyacrylamide solutions whose cross-link content was varied through the gelation threshold was performed over a range of k values. From the initial amplitude of the correlation functions we obtain the fraction of moving particles, which decreases monotonically with 1/k and cross-link content. The measured pore size distribution of the gel is sharply peaked. The results suggest that the gel has a large interconnected space as well as many small pores.

A method of calculation of the equilibrium defect concentration for a disordered semiconductor with exponential band tails is presented. For hydrogenated amorphous silicon, the entropy-of-mixing term in the expression for the free energy is found by consideration of the dangling bonds as an impurity in a system of bonding orbitals. These calculations are compared with experimental results.

We report here the first use of the digital image analysis method to determine the probability distribution function of concentration in binary mixtures undergoing phase separation. This method has been applied to the mixture of poly(vinyl methyl ether) and polystyrene undergoing spinodal decomposition. The distribution function is initially Gaussian, but it becomes broader and turns bimodal upon decomposition. This transformation of distribution is related to the change of phase structure and, particularly, to the sharpening of the interfacial region.

We employ a picosecond–time-resolved Raman-scattering technique to investigate the dynamics of two-dimensional electron-hole plasma confined to a multiple–quantum-well structure composed of layers of GaAs and (GaAl)As. The lifetime and the intersubband scattering time of the electrons photoexcited on the lowest electronic subbands are determined separately. Calculation of the rate of intersubband scattering by longitudinal-acoustical phonons successfully accounts for the observation of relatively long-lived electrons on the second subband of a 215-Å multiple–quantum-well structure.

We have measured Raman scattering by LO phonons in GaAs in a magnetic field (≤12.8 T) at 10 K. The intensity displays Landau-level oscillations. This effect shows promise as a modulation technique for the investigation of interband magnetoabsorption. As an application we have studied the nonparabolicity of the ${\mathrm{\Gamma }}_1$ conduction band of GaAs for energies up to 0.4 eV above its bottom. Standard k⋅p expansions, including the ${\mathrm{\Gamma }}_1$ and ${\mathrm{\Gamma }}_{15}$ conduction and the ${\mathrm{\Gamma }}_{15}$ valence bands, cannot explain the observed dispersion. It can be explained by inclusion of interactions with higher bands.

We have made a study of infrared reflectivity from an epitaxial film of ${\mathrm{Y}}_1$${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{x}}$ in which the c axis is primarily oriented perpendicular to the film surface and from bulk polycrystalline samples with no orientation. In the oriented film the region of enhanced reflectivity associated with the superconducting energy gap extends to 400 ${\mathrm{cm}}^{\mathrm{-}1}$ (50 meV) compared to only 200 ${\mathrm{cm}}^{\mathrm{-}1}$ in the bulk material. We estimate the energy gap for the film to be 2Δ=(4.7±1.2)${\mathrm{kT}}_{\mathrm{c}}$, a value which is consistent with recent tunneling measurements and with strong-coupling superconductivity.

The longitudinal spin response below the spin-wave energy has been observed in ${\mathrm{CsNiF}}_3$ in an applied magnetic field of 7 T and at temperature of 25 K by polarized-neutron inelastic scattering. The experimental results are compared with quantum-mechanical and classical theories predicting longitudinal response. The classical interpretation of a field-induced spin-energy–density coupling is found to be appropriate.

The magnetic excitations of a ${\mathrm{Ho}}_{90}$${\mathrm{Tb}}_{10}$ single crystal have been studied by inelastic neutron scattering. In the commensurable helical structure the bunching of the moments about the easy directions of magnetization induces an energy gap at long wavelengths whose magnitude, compared to that in the ferromagnetic phase, implies an anisotropic two-ion coupling. The strong dipolar forces give rise to a discontinuity in the dispersion relations at q=0, and resolve the longstanding mystery of the stability of the conical structure in Ho at low temperatures.

We use an ensemble Monte Carlo technique to study the electron and phonon dynamics in a single quantum well of GaAs-AlGaAs under optical excitation. The cooling of the photoexcited quasi two-dimensional electron distribution is studied in the presence of a nonequilibrium longitudinal-optical phonon population and electron-electron interaction. It is found that the presence of hot phonons due to emission from the photoexcited carriers in the quantum well reduces the electron relaxation rate in qualitative agreement with available experimental results in such systems.

A novel effect of alloying on the electron momentum density in a disordered α-${\mathrm{Cu}}_{91}$${\mathrm{Ge}}_9$ alloy is reported. The effect, which is the appearance of a bump in the derivative spectra at low momenta, is predicted by theory employing the coherent-potential approximation and is observed in two-dimensional angular correlation experiments on the alloy. It reflects the presence of an s-p impurity band far below the Fermi level. Accurate experimental values for the Fermi-surface radii ${\mathrm{k}}_{100}$ and ${\mathrm{k}}_{110}$ and for the associated disorder-induced smearings in CuGe are also reported for the first time.

We report on coherent double-plasmon excitation in solid-state plasmas. Our results, obtained by use of a new processing routine for removal of incoherent plural scattering from energy-loss experiments on aluminum thin films, call into question previous theoretical and experimental findings as to the magnitude of the coherent double-plasmon excitation.

A simple thermodynamic theory is given of the metal-insulator transitions concurrent with a localization of electrons in a half-filled correlated band. The approach supplies a complete phase diagram for the system (${\mathrm{V}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{Cr}}_{\mathrm{x}}$${)}_2$${\mathrm{O}}_3$. We show also that for an almost half-filled band the disappearance of local moments is accompanied by a transition to a heavy Fermi liquid.

We have fabricated high-mobility, one dimensional wires in GaAs-AlGaAs heterostructures and measured the resistance as a function of magnetic field and temperature. Because of the size of the devices and the high mobility, only a few channels carry the current at 35 mK with minimal scattering. Fluctuations in the resistance as a function of magnetic field due to quantum interference are observed for 0<${\mathrm{\omega }}_{\mathrm{c}}$τ<300, where ${\mathrm{\omega }}_{\mathrm{c}}$ is the cyclotron frequency and τ is the scattering time, superimposed upon Shubnikov–de Haas oscillations and the Hall effect.

Evidence is cited that supernova 1987A involved a large explosion energy, ≃(2–3)×${10}^{51}$ ergs. Such a large explosion energy has not come from delayed shocks to date, nor is it likely to. Improved physics in the presupernova evolution, especially the inclusion of Coulomb interactions, has brought the iron-core mass down by ≳0.1$M_{\odot }$ in the 13$M_{\odot }$ star which has recently been evolved. We find that supernova explosion energies up to 3×${10}^{51}$ ergs can be obtained by the prompt-explosion mechanism, provided that a somewhat soft equation of state is used at supranuclear densities.

The full coupled gravity-string field equations are considered and they are used to show that a general local string will have an asymptotically conical structure. For the case of the Abelian Higgs model with U(1) gauge invariance, the gravitational field of a simple local string to first order in G${\mathrm{\eta }}^2$ is exhibited. Then a C-energy argument is used to suggest stability at this linearized level.

We propose that space-time at the smallest scales is in reality a causal set: a locally finite set of elements endowed with a partial order corresponding to the macroscopic relation that defines past and future. We explore how a Lorentzian manifold can approximate a causal set, noting in particular that the thereby defined effective dimensionality of a given causal set can vary with length scale. Finally, we speculate briefly on the quantum dynamics of causal sets, indicating why an appropriate choice of action can reproduce general relativity in the classical limit.

It is shown that the Fokker-Wheeler-Feynman model of a system of massive, charged particles interacting via forces of electromagnetic origin can be rewritten to yield a physically acceptable relativistic many-particle Lagrangean. Contrary to the postulates of Wheeler and Feynman, the model satisfies causality and can be generalized to include arbitrary forces. It also allows a simple description of radiation reaction and, as well, may provide the basis for any comparable modeling of other systems.

We analyze the simplest discrete symmetries of the superpotential of superstring-inspired $E_6$ models necessary to simultaneously suppress (i) proton decay, (ii) neutrino masses, and (iii) tree-level flavor-changing neutral currents in $E_6$ superstring models without any intermediate scale. We show that there are two and only two different $Z_2$ symmetries that incorporate (i)–(iii). These symmetries put restrictions on the decays of exotic particles; in particular, the exotic quarks always decay semileptonically. A $Z_3$-symmetric model satisfying (i)–(iii) and with μ→eγ forbidden is also constructed.