The structure of (√3 × √3 ) Ag/Si(111) is investigated using first-principles total-energy calculations. The lowest-energy configuration consists of a top layer of Ag atoms arranged as ‘‘honeycomb chained trimers’’ lying above a distorted ‘‘missing top layer’’ Si(111) substrate. We find that the honeycomb structure observed in scanning-tunneling-microscope images arises from the electronic charge density on an empty surface band near the Fermi level.

A short-range ±J Heisenberg spin glass with and without anisotropy on a simple cubic lattice is studied by using a hybrid Monte Carlo spin-dynamics method. The anisotropy leads to a cusplike peak of the susceptibility χ and a very slow relaxation of the spin configuration at low temperatures. From a finite-size-scaling analysis of the spin-glass susceptibility ${\mathrm{\chi }}_{\mathrm{sg}}$, we find, for the first time, that the anisotropy induces a phase transition.

A smooth model for anyons is proposed, based on the mathematical structure of the moduli space of vortices in the Abelian Higgs model.

The electrophoretic mobility μ of a flexible N-segment probe polymer has been measured in cross-linked gels of concentration c. Assuming μ∼${\mathit{N}}^{\mathrm{-}\mathrm{\alpha }}$${\mathit{c}}^{\mathrm{-}\gamma }$, α initially increases with N, reaches a maximum of ∼2 in a crossover regime, and finally decreases to 1, the exponent of reptation. In accord with a semidilute-solution scaling argument, an analogous maximum in γ is observed. Behavior at intermediate N and c is consistent with ‘‘entropic-barrier’’-mediated transport, while larger N or c leads to reptation.

We describe a new type of soliton-impurity interaction and demonstrate that the soliton can be totally reflected by an attractive impurity if its initial velocity lies in certain resonance ‘‘windows.’’ This effect has an analogy with the resonance phenomena in kink-antikink collisions [Campbell, Schonfeld, and Wingate, Physica (Amsterdam) 9D, 1 (1983)], and it can be explained by a resonant energy exchange between the soliton and the impurity mode. Taking the sine-Gordon and ${\mathrm{\phi }}^4$ models as examples, we find a number of resonance windows by numerical simulations and develop a collective-coordinate approach to describe the effect analytically.

A dissipative Fermi system is defined and the low-temperature thermodynamics is examined. Below a critical density the system behaves as a Boltzmann gas with Curie behavior of the susceptibility. Above the critical density, conventional Fermi behavior reappears, with Pauli behavior of the susceptibility. The diamagnetic susceptibility is smooth through the transition region which thus represents a type of metal-to-metal transition. In contrast, a dissipative ideal Bose system is Boltzmann-like for any density at low temperature.

We show that regular and irregular spectral statistics have direct, distinctive, and observable time-dependent manifestations in the behavior of the survival probability P(t)=‖〈ψ(0)‖ψ(t)〉${\mathrm{\Vert }}^2$, averaged over Hamiltonian ensembles and initial conditions. Specifically, systems exhibiting energy-level repulsion display characteristically strong decorrelations at short times. The proof relies solely on Liouville spectral properties of ensembles of bound quantum systems.

We show that the hadronic structure of the photon, along with the beamstrahlung phenomenon, leads to a very large rate of γγ→jets at ${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{-}}$ supercolliders. At √s =1 TeV, for round, dense bunches, we expect ∼5–50 ‘‘minijet’’ events per bunch crossing, giving rise to an ‘‘underlying event.’’ Thus ${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{-}}$ supercolliders will be ‘‘messier’’ than expected, unless beamstrahlung can be kept under control.

The polarization ${\mathit{P}}_{\mathrm{\Xi }}^{\mathrm{-}}$ of ${\mathrm{\Xi }}^{\mathrm{-}}$ hyperons produced by 800-GeV protons has been measured for ${\mathit{x}}_{\mathit{F}}$ from 0.3 to 0.7 and ${\mathit{p}}_{\mathit{T}}$ from 0.5 to 1.5 GeV/c. ${\mathit{P}}_{\mathrm{\Xi }}^{\mathrm{-}}$ has a ${\mathit{p}}_{\mathit{T}}$ dependence similar to that of the Λ but has a different ${\mathit{x}}_{\mathit{F}}$ behavior. Also, an energy dependence of ${\mathit{P}}_{\mathrm{\Xi }}^{\mathrm{-}}$ has been observed.

We examine the implication of the unitarity bounds for the differential cross section to the break structure of the forward peak of elastic pp and p¯p scattering observed at the squared momentum transfer ‖t‖=0.1–0.15 (GeV/c${)}^2$ at high energies. The break is suggested to occur at around 8.8/${\mathit{R}}_0^2$<‖t‖<14.7/${\mathit{R}}_0^2$, ${\mathit{R}}_0$ being the interaction radius given in terms of ${\mathrm{\sigma }}_{\mathit{t}}^2$/${\mathrm{\sigma }}_{\mathrm{el}}$. This fits well with the experimental data of pp scattering in the energy region √s =10–60 GeV and p¯p in 10–546 GeV. Scaling with the variable t${\mathrm{\sigma }}_{\mathit{t}}^2$/${\mathrm{\sigma }}_{\mathrm{el}}$ is discussed.

Target-fragment, shower-particle (meson), interacting-proton (36–400 MeV), and total multiplicities in ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$+Ag collisions from 5A MeV to 2×${10}^5$A MeV are studied. The target disintegration exhibits energy independence above 100A MeV even in the most central collisions. Above 5A GeV the energy dependences of the three different categories of particles are similar to those from proton-nucleus interactions. The absolute yield of target fragments is also the same while the yield of protons is twice as large and of shower particles 6–7 times larger in ${}_{}{}^{16}{}_{}{}^{}\mathrm{induced}$ central collisions as compared to p-nucleus reactions.

New precision angular distribution data for the D(γ,p)n reaction are presented for incident photon energies of 191 and 222 MeV. The difference of cross sections measured with orthogonal states of linear polarization is sensitive to the short-range behavior of the tensor interaction in models of the nuclear force. Comparisons with the Bonn and Paris potentials illustrate the sensitivity.

Recent data on central Si+Au collisions at 15A GeV are shown to imply an expectedly high degree of nuclear transparency. The paucity of observed midrapidlity protons and pions suggests that up to one half of the projectile nucleons may lose less than one unit of rapidity after traversing 5–10 fm of nuclear matter.

We have studied multifragment decays of Au projectiles after collisions with C, Al, and Cu targets at a bombarding energy of 600 MeV nucleon. We find that with increasing violence of the collision, measured via the multiplicity of light particles, the mean multiplicity of intermediate-mass fragments originating from the projectile first increases to a maximum 〈${\mathit{M}}_{\mathrm{IMF}}$〉≃3 and then decreases again. Calculations using the Boltzmann-Uehling-Uhlenbeck model suggest that the fragmentation is governed by the energy ${\mathit{E}}_{\mathrm{dep}}$ deposited into the projectile spectator and that 〈${\mathit{M}}_{\mathrm{IMF}}$〉 reaches its maximum around ${\mathit{E}}_{\mathrm{dep}}$≃8 MeV/nucleon.

A xenon time projection chamber with an active volume of 207 L has been built to study neutrinoless double β decay in ${}_{}{}^{136}{}_{}{}^{}\mathrm{Xe}$. Data were taken in the Gotthard Underground Laboratory, with 5 atm of xenon enriched to 62.5% in ${}_{}{}^{136}{}_{}{}^{}\mathrm{Xe}$. From 3380 h of data, no evidence has been found for the 0ν $0^{+}$→$0^{+}$ transition. Half-life limits of ${\mathit{T}}_{1/2}^{0\nu }$>2.5(4.9)×${10}^{23}$ yr in the mass-mechanism mode and ${\mathit{T}}_{1/2}^{0\nu }$>1.7(3.2)×${10}^{23}$ yr in the right-handed-current mode, at the 90(68)% C.L., were derived. An upper limit for the Majorana neutrino mass parameter was deduced.

The photoionization of antimony clusters has been studied in the energy range 20–120 eV, probing the inner-valence 4d-np tranistions as well as the shape resonance 4d-ɛf. When the cluster size is varied the photoionization efficiency presents a quite surprising behavior: For all clusters, except the ${\mathrm{Sb}}_{4\mathit{p}}$ group with p>1, the photoionization pattern is similar to that which is known for the bulk. On the other hand, the shape resonance collapses completely for ${\mathrm{Sb}}_{4\mathit{p}}$. It is further shown that this dramatic change of the resonance intensity is itself dependent on the nucleation conditions, which could indicate the influence of the molecular architecture.

The interaction of a strong nearly resonant cw pump laser and a weak cw probe laser in a medium consisting of two-level atoms was studied by numerical integration of the Maxwell-Bloch equations. With propagation into the optically thick medium, the numerically calculated probe gain spectrum deviates substantially from one calculated by a four-wave-mixing formulation of coupled-wave theory with an undepleted pump. For probe frequencies inside the modified gain region, the laser field, initially in an unstable steady-state condition, evolves to a stable solution of the sine-Gordon equation.

The intensity dependence of the multiphoton photoelectron spectrum of ${\mathrm{N}}_2$ has been obtained with subpicosecond laser pulses up to intensities of ${10}^{15}$ W/${\mathrm{cm}}^2$. Both Rydberg and valence states are observed to be ac Stark shifted into intermediate resonance during the photoionization process, although the valence states are seen only at low intensities. We show that the vibrational structure of the molecular Rydberg states cancels out in the photoelectron spectrum leading to sharp atomiclike series, even at high intensities. Photoelectrons from the ionization of each of the 3${\mathrm{\sigma }}_{\mathit{g}}$, 1${\mathrm{\pi }}_{\mathit{u}}$, and 2${\mathrm{\sigma }}_{\mathit{u}}$ electrons are clearly distinguished. Implications of these results for the interpretation of multiphoton dissociation experiments are discussed.

The response of molecular hydrogen to multiphoton excitation with intense short laser pulses is probed using photoelectron spectroscopy. At wavelengths and laser intensities for which molecular Rydberg states are ac Stark shifted into resonance, the small quantum defects of molecular hydrogen and the loss of the ponderomotive energy to the electromagnetic field give rise to very simple photoelectron spectra that coincide with those of excited atomic hydrogen.

The interaction of laser-induced and intrabeam forces has been studied in a dense stored beam of 100-keV ${}_{}{}^7{}_{}{}^{}\mathrm{Li}_{}^{+}{}_{}{}^{}$ ions. A fraction of the ions (∼${10}^{\mathrm{-}4}$) exist in the metastable 1s2s ${}_{}{}^3{}_{}{}^{}$S state. Using this state as a laser spectroscopic probe, we observe fast longitudinal heating in the injected, nonequilibrium distribution due to Coulomb scattering. Laser cooling of the metastable ions is ineffective during this heating period. The metastable fraction of the equilibrated beam is subsequently laser cooled to a longitudinal temperature of ∼1 mK, the lowest temperature ever reported in a stored ion beam.