It is shown that the continuum missing-mass spectra for the (${\mathrm{\pi }}^{\mathrm{-}}$,${\mathrm{\pi }}^{+}$) and (${\mathrm{\pi }}^{\mathrm{-}}$,p) reactions leading to extremely neutron-rich exotic nuclei can be explained in terms of phase-space distributions by invoking the presence of dineutrons as one of the products of the breakup. It is suggested that this indicates the presence of the dineutron as a cluster in these neutron-rich systems during their breakup. It is noted that these observations in weakly unbound systems may be analogs of the dineutron halos for which evidence has been found in weakly bound nuclei near the neutron drip line.

We have observed the excitation of the first excited state (${\mathit{E}}_{\mathit{x}}$=0.98 MeV, ${\mathit{J}}^{\mathrm{\pi }}$=$1^{+}$) of the radioactive, neutron-rich nucleus ${}_{}{}^8{}_{}{}^{}\mathrm{Li}$ (${\mathit{J}}_{\mathrm{g}.\mathrm{s}.\mathrm{}}^{\mathrm{\pi }}$=$2^{+}$) from its inelastic scattering on ${}_{}{}^{\mathrm{nat}}{}_{}{}^{}\mathrm{Ni}$ at E${(}^8$Li)≃14.6 MeV. Cross sections measured out to large ${\mathrm{\theta }}_{\mathrm{c}.\mathrm{m}.\mathrm{}}$ agree well with Coulomb-excitation probabilities and have been used to deduce an E2↑ transition rate for the $2_{\mathrm{g}.\mathrm{s}.\mathrm{}}^{+}$→$1^{+}$ excitation in ${}_{}{}^8{}_{}{}^{}\mathrm{Li}_{}^{\mathrm{*}}{}_{}{}^{}$. The latter [B(E2↑)=55±15 ${\mathit{e}}^2$ ${\mathrm{fm}}^4$] is large relative to nearby stable nuclei, but comparable to the neighboring neutron-rich nucleus ${}_{}{}^{10}{}_{}{}^{}\mathrm{Be}$. The relevance of the data to predicted ‘‘neutron-halo’’ giant dipole resonances in unstable nuclear projectiles is considered.

A magnetic dipole spin resonance around 6–8 MeV is described for rare-earth nuclei, ranging from ${}_{}{}^{142}{}_{}{}^{}\mathrm{Ce}$ to ${}_{}{}^{198}{}_{}{}^{}\mathrm{Pt}$. Starting from an unperturbed basis of ${\mathit{K}}^{\mathrm{\pi }}$=$1^{+}$ two-quasiparticle states, constructed within the Nilsson model and applying the BCS formalism, one can quantitatively estimate the total spin M1 strength. We include Coriolis mixing and residual quadrupole and spin separable forces and study the spin resonance in more detail for a few representative examples, ${}_{}{}^{148}{}_{}{}^{}\mathrm{Nd}$,${\mathrm{}}^{154}$Sm, ${}_{}{}^{164}{}_{}{}^{}\mathrm{Dy}$ and ${}_{}{}^{174}{}_{}{}^{}\mathrm{Yd}$. A double-humped structure in the spin M1 strength around 6–8 MeV results from the calculations, and is found to be related to separate proton and neutron spin-flip excitations.

Quantum fluctuations of an optical transition moment are observed to be suppressed by strong coherent excitation of a weak auxiliary transition. The stabilization is manifest through the appearance of subnatural linewidths in the resonance fluorescence spectrum and is due to coherent mixing of atomic states. Our results are in quantitative agreement with the predictions of L. M. Narducci et al. [Phys. Rev. A 42, 1630 (1990)].

Femtosecond photon-echo experiments on sodium resorufin in dimethylsulfoxide at room temperature show that optical dephasing in solution is of non-Markovian character. A single Gauss-Markov stochastic modulation process is used to interpret both the femtosecond light-scattering results and the steady-state absorption spectrum. The optical dynamics of this system appears to fall in the intermediate modulation regime. No evidence was found for contributions of slower dynamical processes to the absorption line breadth.

It is shown that the high efficiency of a plasma-filled backward-wave oscillator can be accounted for by a self-induced distributed feedback mechanism, which is set up by a density grating formed in the plasma by the counterpropagating electromagnetic waves. The efficiency increases with the plasma density, up to a critical density, where the oscillations are quenched.

We have studied the coarsening dynamics of line defects in the uniaxial nematic liquid crystal 4-cyano-4’-n-pentylbiphenyl, subjected to a rapid pressure jump from the isotropic to the nematic phase. At late times, the density of disclination lines ρ is expected to scale with time as ρ∝${\mathit{t}}^{\mathrm{-}\nu }$ with ν=1. We have measured the scaling exponent to be ν=1.02±0.09 over the region 16<ρ<160 ${\mathrm{mm}}^{\mathrm{-}2}$. In addition, we have measured the collapse of loops consisting of type-1/2 disclination lines. The loop radius r is expected to scale with time as r∝(${\mathit{t}}_0$-t${)}^{\mathrm{\alpha }}$, where α=1/2 and ${\mathit{t}}_0$ is the time at which the loop vanishes. We found α to be 0.50±0.03.

The width of the crack interface in the fuse model after breakdown, w, scales with the size of the network, L, as w∼${\mathit{L}}^{\mathrm{\zeta }}$. When the disorder is narrow, or when it includes arbitrarily small threshold values, we find that ζ=0.7 to within 10%, indicative of this being a universal value. This is not far from 2/3, suggested by an analogy with the random directed polymer problem. When, on the other hand, the disorder is strong and includes arbitrarily large threshold values, the exponent ζ depends on the disorder. These results suggest that the random polymer problem may be relevant for brittle fracture in real materials.

A theoretical model of an order-disorder phase transition in a crystal has been developed with strain-induced coupling between Ising spins. Strain coupling leads to long-ranged renormalized interactions between the spins which result in ferroelastic-antiferroelastic phase transitions. Computer simulations show textured microstructures evolving for some cases, similar to tweed textures. In other cases the long correlation length of strain suppresses nucleation, and the rate of (dis)ordering follows mean-field theory, very different from ‘‘nucleation and growth.’’

If a long-standing selection problem in brittle fracture is to be resolved, one must begin with dynamical equations suited to comparison with experiment. Such an equation is derived here in closed form by finding the energy flux to the tip of a slowly accelerating crack in a brittle strip.

Heat-capacity and heat-release data are reported in ${\mathrm{K}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Na}}_{\mathit{x}}$${\mathrm{TaO}}_3$ for x=0, 0.08, and 0.24 at temperatures below 1 K and attributed to reorientations of the nuclear quadrupoles of ${}_{}{}^{181}{}_{}{}^{}\mathrm{Ta}$. In pure ${\mathrm{KTaO}}_3$, the quadrupoles relax exponentially, whereas doping with Na at the K site introduces relaxation described by the Kohlrausch stretched-exponential function.

We report the experimental observation of a transient oscillatory behavior of magnetoexciton absorption in GaAs/AlGaAs heterostructures. We interpret these oscillations as absorption quantum beats of a coherent superposition of two excitonic spin states. Measurements of the oscillations period may be used as a new technique for accurately measuring the Landé g factor of excitons in semiconductors.

We present a calculation of magnetic x-ray dichroism at the ${\mathit{L}}_{2,3}$ edges of Gd metal, which takes into account dipolar transitions to the spin-polarized conduction bands, as well as quadrupole transitions to the highly localized 4f states. The striking agreement with the data demonstrates the validity of the single-particle approximation in the whole range of energies, above the Fermi level, probed by the experiments. The quadrupolar dichroism is important in determining the pre-edge structure of the measured spectra. Results for the ${\mathit{L}}_{2,3}$ edges of Ho are also briefly discussed.

Fano line shapes are reported in the excitation spectrum of the total fluorescence of NO trapped in Kr matrices. They result from the homogeneous configuration mixing of the sharp valence vibrational bands ${\mathit{B}}^2$II(v’=12,15,18)-${\mathit{X}}^2$Π(v’’=0) with the broadened multiphonon Rydberg vibrational bands ${\mathit{C}}^2$Π(v’=0,1,2)-${\mathit{X}}^2$Π(v’’=0). The main features of the spectrum are reproduced by a simulation.

It is shown how the dynamic response of lattice fermions in a magnetic field (the Wannier-Azbel-Hofstadter’’ system) may be calculated. Details are given for electromagnetic perturbations δA(r,t)∼δA(Q,ω)exp(iQ⋅r-iωt). Linear-response theory is valid for periodic boundary conditions, and can be justified in general for rational filling. Analytic results are given for the various current-current responses. Applications are briefly discussed.

The frequency of a vibrating reed made of a superconducting suspension of isolated micrometer-size grains of low- and high-${\mathit{T}}_{\mathit{c}}$ superconductors in a magnetic field directly measures the elastic pinning force on the flux line. Four different types of field and temperature dependences of the attenuation caused by moving flux lines are observed and explained. In particular, the shift to lower temperatures of the dissipation peak of small high-${\mathit{T}}_{\mathit{c}}$ grains perfectly agrees with the prediction by thermally activated depinning.

Ballistic-electron-emission-microscopy (BEEM) studies of Mg, Ni, Cu, Ag, and Au films on cleaved n-type GaP(110) show uniform Schottky-barrier heights across the surfaces, which varied from 1.02 eV for Mg to 1.41 eV for Au. BEEM images reveal areas of sharp current variations (contrast) that coincide with topographic gradients on the metal surfaces. Deduced Schottky-barrier heights depend on the transport model; a 5/2 power-law dependence of the BEEM current on applied voltage results if nonclassical transmission across the interface is included.

We report on scaling properties of the angular dependence of the magnetoresistance of ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_8$ thin films below ${\mathit{T}}_{\mathit{c}}$=88 K up to 20 T. As recently suggested by Kes et al. [Phys. Rev. Lett. 64, 1063 (1990)], the dissipation is only related to the transverse field component along the c axis. Breakdown of the scaling appears between the angle when the field is parallel to the superconducting planes and a critical angle (∼1°), which reflects the finite anisotropy of the compound. We interpret the dissipation in the tilted magnetic-field orientation as resulting from the easy motion of vortex kinks along the ${\mathrm{CuO}}_2$ planes.

An improved electron-gas model is proposed; it predicts the trends in the chemical potentials, cohesive energies, surface energies, and work functions of the simple metals. The uniform electron gas is altered so that it is in mechanical equilibrium at the electron density characteristic of a given metal. This leads to a new rule for splitting the constant background charge when jellium is cleaved. The trends in the energetics of metals result.

Orientational order in weakly pinned Abrikosov flux lattices is studied, taking into account two heretofore neglected effects: dislocations and orientational couplings to the underlying lattice. Without orientational couplings, arbitrarily weak pinning destroys long-ranged orientational order for all spatial dimensions d<4. Orientational couplings stabilize long-ranged orientational order. For fields along an axis of fourfold symmetry, sixfold (hexatic) orientational order is described by a random-field Ising model, and so does not occur in d=2 (thin films) but does in d=3 (bulk).