The luminescence of ${\mathrm{C}}_{60}$ was investigated as a function of temperature between 300 and 10 K and for various oxygen concentrations. Its intensity was found to increase strongly with decreasing temperature down to about ${\mathit{T}}_1$=100 K and to decrease again for a further reduction of T. Below 100 K, new structures appeared in the spectra. The experiments are described by a self-localized polaron exciton and by a diffusing free exciton above and below ${\mathit{T}}_1$, respectively. The important difference between the two regions is the free rotation of the ${\mathrm{C}}_{60}$ above and the definite orientation below ${\mathit{T}}_1$.

We analyze transport in a uniform electric field of arbitrary strength by using a gauge-invariant formulation of the Airy representation of the Baym-Kadanoff-Keldysh theory of nonequilibrium quantum statistical mechanics. For a model electron-phonon interaction, experimentally relevant quantities can be extracted from a function of one variable; this function obeys a linear homogeneous integral equation. The quantum distribution function and the nonlinear drift velocity versus applied field characteristics are evaluated numerically. Many features of our nonlinear theory bear a formal resemblance to linear-response theory.

We study the superfluid density and the Drude weight of the Hubbard model by investigating two different limiting behaviors of the current-current correlation function. These quantities provide criteria which allow one to distinguish between superconducting, metallic, and insulating ground states of an interacting many-body system. Monte Carlo calculations are performed to study these quantities.

The electron-energy distribution in a gold film was measured with ∼700 fs time-resolved photoemission spectroscopy following laser heating by a 400 fs visible laser pulse. The measured distribution can be fitted by the Fermi-Dirac function at an elevated temperature except within 800 fs of the heating pulse (time-resolution limited), when a reproducible departure is observed. As a result, the relaxation of nonequilibrium electrons was found to be inadequately described by the standard electron-phonon coupling model.

We report the first direct measurement of carrier-carrier scattering rates in a degenerate electron system. Our results on modulation-doped quantum wells, using femtosecond four-wave-mixing (FWM) techniques, demonstrate a strong inhibition of scattering near the Fermi edge, and exhibit other behavior predicted by Landau Fermi liquid theory. Time-resolved (100 fs) FWM measurements clearly show photon echoes, demonstrating the inhomogeneous character of this intrinsic system, as well as prompt signals arising from many-body effects.

The formation of ${\mathrm{H}}^{\mathrm{-}}$ ion by grazing-angle collisions of hydrogen on an Al(111) surface is investigated with the newly developed coupled angular mode method. The capture process involves a dynamical resonant process induced by the collision velocity. All the resonance properties of the ${\mathrm{H}}^{\mathrm{-}}$ level in front of an Al(111) surface are determined: position, width, and angular distribution of ejected electrons. The results are shown to account for the recent observations on ${\mathrm{H}}^{\mathrm{-}}$ formation by Wyputta, Zimny, and Winter.

Collective vibrations of the clean and hydrogen-saturated W(110) surface have been investigated with high-resolution inelastic helium-atom scattering. In contrast to the clean W(110) surface which exhibits ‘‘normal’’ surface phonon dispersion, the hydrogen-saturated surface shows a pronounced surface phonon anomaly in terms of a sharply defined dip in the surface phonon dispersion at Q=0.9 A${\mathrm{̊}}^{\mathrm{-}1}$ along the Γ¯ H¯ ([001]) direction. The anomalous behavior extends into the neighborhood of this high-symmetry direction.

A systematic study of 3s core-level photoemission spectra of Mn and Fe dihalides shows that exchange splitting, intrashell electron correlation, and final-state screening (charge-transfer satellite) effects all contribute to their satellite structures. In naturally explains both the facts that 3s satellites in ${\mathrm{MnF}}_2$ and ${\mathrm{FeF}}_2$ can be well understood by the exchange-splitting but that 3s splittings in other compounds are not proportional to their magnetic moments in general. We find that only when the charge-transfer satellite in the 2p spectra is negligible does the 3s splitting reflect the local moment of the ground state.

It is shown that the breakdown of a global symmetry group to a discrete subgroup can lead to analogs of the Aharonov-Bohm effect. At sufficiently low momentum transfer, the cross section for scattering of a particle with nontrivial ${\mathrm{Z}}_2$ charge off a global vortex is almost equal to (but definitely different from) maximal Aharonov-Bohm scattering; the effect goes away at large momentum transfer. The scattering of a spin-1/2 particle off a magnetic vortex provides an amusing experimentally realizable example.

The wrong-helicity states of keV-mass Dirac neutrinos are emitted copiously from the cores of newly born neutron stars, including that associated with SN 1987A. If they decay into proper-helicity neutrinos—${\nu }_{\mathit{e}}$, ${\nu }_{\mathrm{\mu }}$, ν${\mathrm{¯}}_{\mathit{e}}$, or ν${\mathrm{¯}}_{\mathrm{\mu }}$—they should have produced distinctive high-energy events in the Kamiokande II and Irvine-Michigan-Brookhaven detectors. The absence of such events excludes these decay modes for a Dirac neutrino of mass between order 1 and 300 keV and lifetime ${10}^{\mathrm{-}9}$[${\mathit{m}}_{\nu }$/(1 keV )] sec ≲τ≲5×${10}^7$[${\mathit{m}}_{\nu }$/(1 keV] sec. This places severe constraints on models for Simpson’s 17-keV neutrino.

Baryon number violation and multiple production of W and Higgs bosons are described semiclassically in terms of the instanton–anti-instanton valley. We find (i) two saddle points, one describing reflection from a barrier and the other describing tunneling through it. We find (ii) a critical energy ${\mathit{E}}_{\mathit{c}}$∼35 TeV where the cross section is suppressed as exp(-const/${\mathit{g}}_{\mathit{w}}^2$), but the formulas are no longer valid; (iii) however, depending on the (still uncertain) Higgs bosson action, the cross section at this point may be large enough to be observable.

We report a value of -[3.3±1.1 (stat)±0.7(sys)]×${10}^{\mathrm{-}3}$ for the quadratic slope parameter h in the Dalitz plot of the ${\mathit{K}}_{\mathit{L}}$→3${\mathrm{\pi }}^0$ decay. This result is obtained from a sample of 5.1×${10}^6$ decays. The validity of the ΔI=1/2 rule in the quadratic term is investigated using this result. This is the first measurement of the 3${\mathrm{\pi }}^0$ slope parameter, and also the most sensitive measurement of any of the quadratic slope parameters for the charged or neutral kaons.

We report new measurements of inclusive direct photon production at high transverse momenta (${\mathit{p}}_{\mathit{T}}$) for ${\mathrm{\pi }}^{\mathrm{-}}$ and p interactions on Be at 500 GeV/c. The yields as a function of ${\mathit{p}}_{\mathit{T}}$ and rapidity (y) are in good agreement with expectations from next-to-leading-log QCD calculations employing recently extracted quark and gluon structure functions.

We show a possible large spin polarization of the nuclei produced by one-nucleon-transfer heavy-ion reactions and investigate its origin. The asymmetry of the particle-γ angular correlation is also calculated for the ejectile and γ ray emitted from the spin polarized nucleus.

We examine the stabilization regime for an atom interacting with a superintense laser field. We investigate the transition from the ground-state wave function to the formation of a stable localized wave function. The quantum mechanical wave-packet spreading plays a key role in determining the final degree of stabilization. We compare simple analytical predictions with exact numerical calculations and stress the importance of the laser pulse shape for the final ground-state probability.

The temporally growing shear layer produced by tilting a tube filled with two immiscible fluids of different density is spatially forced via modulations of one of the lateral boundaries. The competition between the forcing wave number ${\mathit{k}}_{\mathit{f}}$ and the intrinsic wave number ${\mathit{k}}_{\mathit{n}}$ leads to a mode diagram that is compatible with corresponding observations of spatial mixing layers. As the detuning is gradually increased, the interface displays stationary phase solitons that are indicative of the existence of incommensurate states.

A nonlinear, quasi-two-dimensional model for pinch spot formation in radiation-dominated, high atomic number z pinches is presented that reproduces the experimental electrical and radiation characteristics. The high line-radiation rates of such discharges produce localized, high-density pinch spots in contrast to the spindle pinches predicted for hydrogenic discharges.

We have obtained images of acoustic energy flux in anisotropic media by an ultrasonic analog to ballistic-phonon imaging. The observed flux reveals a phonon-focusing pattern modulated by interference fringes, in accord with prior theoretical predictions. While phonon imaging is largely restricted to insulators at low temperatures, the ultrasonic technique works at ambient temperatures and is also applicable to metals and even noncrystalline anisotropic materials.

A novel critical scaling behavior is shown to be present in the theory of activated barrier crossing. The critical exponents and the scaling functions are calculated for a general memory friction kernel and for nonlinear potentials of the form V(x)=-1/2M${\mathrm{\omega }}_{\mathit{b}}^2$${\mathit{x}}^2$+(${\mathit{C}}_{\mathit{n}}$/2n)${\mathit{x}}^{2\mathit{n}}$, n=2,3,4,. . .. The critical exponents are found to depend only on the shape of the potential characterized by n and the form of the memory friction characterized by a certain parameter p.

It is shown that the dynamic-scaling-hypothesis description of an epitaxial growth front leads to a time-invariant structure factor in reciprocal space. This invariant structure factor is a result of having both stationary step density and step distribution during growth. The structure factor exists in the scaling regime where both the interface width and lateral correlation length grow in time according to power laws. This invariant characteristic, which manifests itself in the short-range behavior in the multilevel crystal surface, can be readily tested in a diffraction experiment.