Recently, Madsen has argued that relativistic decays of massive neutrinos into lighter fermions and bosons may lead, via thermalization, to the formation of a Bose condensate. If correct, this could generate mixed hot and cold dark matter, with important consequences for structure formation. From a detailed study of such decays, we find that mixed dark matter can indeed form, but by a new physical mechanism; for a wide range of masses and decay times, we find that stimulated emission of bosons dominates the decay. This phenomenon can best be described as a neutrino laser, pumped by quark confinement. We discuss the implications for structure formation and the dark-matter problem.

Nonperturbative effects of instantonlike solutions are studied within the framework of supergravity theories with field-dependent gauge functions. Fermionic zero modes are constructed and some typical correlation functions are evaluated. The effects of instantons are very similar to those in globally supersymmetric theories: they preserve supersymmetry while breaking a chiral U(1) symmetry. Nonperturbative amplitudes receive corrections which are suppressed at large distances.

We consider a string with uniform energy density and tension and with a number of pointlike masses attached at fixed interdistances. We evaluate the effective interaction forces between these masses induced by the quantum fluctuations of the string. For interdistances large compared to the thickness of the string and small compared to the total length, these forces are universal and attractive and fall off for large distances as 1/${\mathit{r}}^3$ and for small distances as 1/r. The attractive nature of these forces creates an instability under which masses added to the string tend to aggregate.

The partition function of the discretized superstring in a target superspace of three (Euclidean) bosonic dimensions is shown, for a fixed triangulation of the random world sheet, to be derived from the partition function of a discretized bosonic string with an external field present in the action in the form of a specific constant matrix, using first order forms of the actions. This latter partition function appears more amenable to an exact analytical treatment.

Azimuthal correlations of pions are studied with the quantum molecular dynamics model. Pions are preferentially emitted perpendicular to the reaction plane. Our analysis shows that this anisotropy is dominated by pion absorption on the spectator matter in the reaction plane. Pions emitted perpendicular to the reaction plane undergo less rescattering than those emitted in the reaction plane and might therefore be more sensitive to the early hot and dense reaction phase.

We report on the first successful attempt to apply the auxiliary-field quantum Monte Carlo technique to the calculation of ground-state properties of systems of many electrons interacting via a Coulomb potential. We have been able to substantially reduce the huge statistical fluctuations arising from the repulsive, long-range character of the interactions, which had so far hindered the application of this method to realistic Hamiltonians for atoms, molecules, and solids. Our technique is demonstrated with calculations of ground-state properties of the simplest molecular and solid-state systems, i.e., the ${\mathrm{H}}_2$ molecule and the homogeneous electron gas.

Using a shielded cylindrical space-charge-limited diode ionization detector and optical-optical double resonance, the exotic potential energy curves of the 3, 4, 5, and 6 ${}_{}{}^1{}_{}{}^{}\mathrm{\Sigma }_{\mathit{g}}^{+}{}_{}{}^{}$ ‘‘shelf’’ states and 2 and 3 ${}_{}{}^1{}_{}{}^{}\mathrm{\Pi }_{\mathit{g}}^{}{}_{}{}^{}$ states of ${\mathrm{Na}}_2$ have been extensively studied including levels near dissociation (>99.8% of well depth) with very large outer classical turning points, and long range avoided crossings giving rise to potential ‘‘shelfs.’’ The long range potential can all be understood and also provide stringently tested high quality theoretical calculations of highly excited states.

The C 1s photoelectron spectrum of CO has been measured with high resolution in the threshold region. The data exhibit hitherto unresolved vibrational structure on the C 1s main photoelectron line as well as new features in the satellite region. The vibrational energy of the ${\mathrm{CO}}^{+}$ ion is 301±3 meV. The vibrational structure changes dramatically with photon energy due to the influence of the ${\mathrm{\sigma }}^{\mathrm{*}}$ shape resonance. The new satellite lines which are not present at high photon energies are attributed to conjugate shakeup processes.

We have made the first measurements of the equilibrium charge state and cross sections for orbital electron capture and stripping of ions with Lorentz factor γ>10. We used gold ions from the AGS at Brookhaven with a mean kinetic energy of 10.8A GeV. The measurement of the cross section for electron capture is consistent with the prediction of radiative capture of Anholt and Becker, and with the Weizsäcker-Williams calculations of the vacuum capture cross section. The measurement of the cross section for stripping differs from the prediction of Anholt and Becker by a factor of 2.0. We set an upper limit for vacuum capture which constrains recent nonperturbative calculations.

We have measured the intensity correlation of pulsed squeezed light generated by a degenerate optical parametric amplifier seeding weak coherent input signal. When the signal was deamplified, the correlation showed antibunching and the probability distribution of the photon number was sub-Poissonian, both of these being nonclassical properties of light.

It is suggested that solitary wave solutions exist in the gap region of photonic band gap materials with a Kerr nonlinearity. Using a variational trial function we estimate the amplitude, size scale, and the nature of phase modulation of these nonlinear waves. In two dimensions, we predict the occurrence of a variety of finite energy solitary waves associated with the different symmetry points of the crystalline Brillouin zone. Solutions which preserve the symmetry of the crystal exist for both positive and negative Kerr coefficient whereas solutions which break the symmetry occur only for positive nonlinearity. These states are relevant to the bistable switching properties of photonic band gap materials.

Zakharov and L’vov long ago invoked a Kolmogorov cascade hypothesis to describe the modal transfer of energy for surface gravity waves in deep water. Within this context the cascade hypothesis requires the existence of dynamical time scales in the evolution of the mode amplitudes which scale with wave number. The purpose of this Letter is to study the temporal evolution of individual modes, with the aim of identifying these dynamical time scales. The main result reported here is that mode-dependent time scales τ(k)∼${\mathit{k}}^{\mathrm{-}\mathrm{\beta }}$ can indeed be identified, with exponent β related to the exponent α which describes the spectrum of the waveheight autocorrelation function.

The scanning tunneling microscope is used to study arsenic-related point defects in low-temperature-grown GaAs. Tunneling spectroscopy reveals a band of donor states located near ${\mathit{E}}_{\mathit{v}}$+0.5 eV arising from the defects. Images of this state reveal a central defect core, with two satellites located about 15 Å from the core. The structure of the defect is found to be consistent with that of an isolated arsenic antisite defect (As on a Ga site) in a tetrahedral environment.

The phason dispersion in the antiferroelectric Sm-${\mathit{C}}_{\mathit{A}}^{\mathrm{*}}$ liquid crystalline phase of 4-(1-methylheptyloxycarbonyl)phenyl 4’-octyloxybiphenyl-4-carboxylate has been studied by quasielastic light-scattering experiments. The phason is found to be gapless as predicted for a Goldstone mode recovering the broken continuous symmetry. The dispersion has a minimum at q=2${\mathit{q}}_{\mathit{c}}$, where ${\mathit{q}}_{\mathit{c}}$ is the wave vector of the unperturbed Sm-${\mathit{C}}_{\mathit{A}}^{\mathrm{*}}$ structure. These results are consistent with the alternating-tilt model of the antiferroelectric Sm-${\mathit{C}}_{\mathit{A}}^{\mathrm{*}}$ phase.

An amorphous carbon network generated by quenching high-density high-temperature liquid carbon using tight-binding molecular-dynamics simulation is found to contain a large fraction of tetrahedral bonding sites. Investigations of the structural, vibrational, and electronic properties of the theoretical model provide fresh insights into the microscopic structure of diamondlike amorphous carbon films produced in ion-beam deposition experiments.

We present the first observations of near-surface critical phenomena of a phase transition in a molecular solid showing both an order-disorder and a strong displacive component. The surface x-ray scattering experiments were performed on an ${\mathrm{NH}}_4$Br single crystal (${\mathit{T}}_{\mathit{c}}$=235 K). For the order parameter they show a crossover from a power law with the bulk critical exponent β=0.3±0.05 to a surface behavior with exponent ${\mathrm{\beta }}_1$=0.8±0.1. From the diffuse intensity we have determined the critical exponents ${\mathrm{\eta }}_{\mathrm{\parallel }}$=1.3±0.15, ν=0.5±0.15, and the correlation length ${\xi }_0^{+}$=4±0.5 Å.

The pressure dependence of the structure of deuterated ice VIII has been studied by time-of-flight neutron powder diffraction up to 10 GPa. The precision of the values of the intramolecular bond length O-D is ±0.003 Å or better at all pressures, and the rate of increase of the O-D bond length is found to be 0.0004(4) Å ${\mathrm{GPa}}^{\mathrm{-}1}$. This value is significantly smaller than previous estimates, which all incorporate the assumption that the form of the O-H potential is constant with pressure. It appears that this is not the case.

The electromagnetic levitation technique is applied to undercool melts of quasicrystal-forming alloys of Al-Cu-Co and Al-Cu-Fe. Undercooling with respect to the solidification of the decagonal phase (quasiperiodic in two dimensions) is measured in situ by contactless temperature measurements. The undercoolings are analyzed within nucleation theory. The results indicate that the activation energy for the formation of the nuclei of quasicrystalline phases is lower than that of crystalline phases. The decrease of activation energy is more pronounced for the icosahedral phase than for the decagonal phase.

An integral-equation approach combined with molecular dynamics simulations based on the Girifalco spherical intermolecular potential has been used to predict the phase diagram for rigid ${\mathrm{C}}_{60}$ molecules. The boundary of the liquid-vapor coexistence region and the location of freezing and melting lines have been sketched out. The liquid phase is only observed in a very narrow temperature range compared with atomic systems (e.g., the rare gases). Unfortunately, the dense fluid is predicted to exist above ∼1800 K, which is sufficiently high that the ${\mathrm{C}}_{60}$ molecule may be unstable.

Back reflection x-ray standing waves and surface extended x-ray absorption fine structure have determined the atomic coordinates (i.e., the perpendicular displacement and the near-neighbor bond lengths) at the In/Si(111)-√3 × √3 interface. Although the In adatoms are found to reside at a single position, 2.10±0.06 Å above the first Si bilayer, dual In-Si near-neighbor distances are found: 2.73±0.02 Å to the first- and 2.49±0.03 Å to the second-layer Si atoms, respectively. Contrary to the accepted model, our data suggest that the ${\mathit{T}}_4$ geometry is not relaxed.