${\mathrm{N}}_2$ chemisorbed on Fe(111) is identified by photoelectron spectroscopy and high-resolution electron energy-loss spectroscopy as a $\pi$-bonded surface complex in which both nitrogen atoms interact with the metal. This species forms the immediate precursor to dissociation.

The Suzuki-Trotter formula has been used to get the $m$ th approximant to the classical representation of the partition function of the $N$-spin $S=\frac{1}{2}$ $\mathrm{XY}$ chain. The equivalent two-dimensional ($N\times 2m$) Ising model with four-spin interactions has been studied in detail by use of the numerically exact transfer-matrix method for $T>~0.05$ and $m<~8$. The limiting $m\to \infty$ behavior has been estimated from the extrapolation formula of the form $E\left(m\right)=E\left(\infty \right)+\frac{a}{m^2}$. The limiting value of the energy agrees with the exact solution of Katsura.

The velocity autocorrelation of the one-dimensional stochastic Lorentz model is analyzed both numerically and theoretically. It is found that the tail becomes proportional to $-t^{-[\frac{\mathrm{}(1-D)\mathrm{}}{2}+1]}$ in the case that impurities distribute fractally with the dimension $D$. The usual analyses are included for the special case $D=0\mathrm{}$.

The lattice $C{P}^{N-1\mathrm{}}$ model at nonzero $\theta$ is analyzed by a strong-coupling expansion. At $\theta =\pi$, $\mathrm{CP}$ is spontaneously broken and the theory ceases to confine. The $\theta$ dependence of several quantities is computed and the correlation length is shown to increase with $\theta$. Most of the results apply not only to the $C{P}^{N-1\mathrm{}}$ model (for all values of $N$) but to other two-dimensional nonlinear $\sigma$ models as well.

Anomalous missing-$p_T$ events observed at the CERN $p\bar{p}$ collider can be plausibly explained by the photino decay mode $\tilde{q}\to q\tilde{\gamma }$ of scalar quarks with mass ∼ 100 GeV. This gives a Jacobian peak in the jet $p_T$ distribution suggested by the data of Arnison et al. Two-photon decays of ${\pi }^0$, $\eta$, ${\eta }^{\prime }$ may account for the "photon" events. To account for the reported rates and to avoid gluino mass limits, the gluino and photino must be light and nearly degenerate.

SU(3) lattice gauge theory with four light quark species is studied at finite temperature by microcanonical computer simulation techniques. An abrupt transition is found separating the hadronic-matter and the quark-gluon-plasma phases. Measurements of the critical temperature, latent heat, $〈\bar{\psi }\psi 〉$, Wilson line, and Wilson-line correlation function are presented and are compared to earlier calculations which ignored fermion vacuum polarization.

Discrete $\gamma$-ray transitions have been observed above spin 40 in ${}_{}{}^{158}{}_{}{}^{}\mathrm{Er}$ following the ${}_{}{}^{114}{}_{}{}^{}\mathrm{Cd}({}_{}{}^{48}{}_{}{}^{}\mathrm{Ca},4n)$ reaction. Structure characteristic of the collective rotation of a prolate nucleuś ends at $I^{\pi }={38}^{+}$ in the yrast band and probably at ${41}^{-}$ in the lowest-energy negative-parity band corresponding in both cases to a rotational energy of ∼ 0.53 MeV. It is suggested that the observed phase transition is the prolate-to-oblate shape change which is predicted to occur at higher spin.

With the same relativistic three-body theory which reproduces the pion-deuteron elastic and breakup differential cross sections, it is also possible to explain the energy and angular dependence of the reaction parameters $i{T}_{11}$ and $t_{20}$ throughout the region of the 3,3 resonance. This theory differs from other models in that it treats relativistically not only the space variables but also the spin variables and it has a much smaller contribution from pion absorption.

We report self-consistent local-spin-density calculations which provide, for the first time, the equilibrium geometries of the sodium clusters ${\mathrm{Na}}_n$ and ${{\mathrm{Na}}_n}^{+}$ with $n<~8$ and $n=13\mathrm{}$, without making any a priori assumptions. Our results are in excellent agreement with recently obtained photoionization appearance potentials and electron-spin-resonance spectra. We find rapid formation of the metallic bond with a dominance of closely packed structures in two and three dimensions, and propose a simple model to account for it.

We have performed a photon-echo experiment on the 555.6-nm intercombination line of atomic Yb vapor which demonstrates that an intense laser field acts to inhibit the dephasing resulting from velocity-changing collisions. This finding implies that the optical Bloch equations, which assume that relaxation is independent of the strength of the driving field, cannot correctly describe the behavior of gas-phase samples in all regimes.

Photon echoes are generated by excitation pulses of chaotic (thermal) light. Echo signals are large and display a modulation with pulse separation due to quantum beating of hyperfine levels.

A new kind of fluctuation is observed near the edge of plasmas in the PDX tokamak operating in the "$H$-mode" (improved-confinement) regime. These fluctuations are evidenced as vacuum-uv and density-fluctuation bursts at well-defined frequencies ($\frac{\Delta \omega }{\omega }\lesssim 0.1$) in the range between 50 and 180 kHz. The bursts are correlated, both in space and in time, with changes in the temperature-density product near the plasma edge where large density and temperature gradients develop during the $H$ mode.

The kinetic energies of neutrals desorbed by electron impact [CO from chemisorbed CO on Ru(001), and ${\mathrm{N}}_2$O and ${\mathrm{N}}_2$ from chemisorbed and physisorbed ${\mathrm{N}}_2$O on Ru(001)] have been measured by a time-of-flight technique. Wide variations are found for peak energies and low-energy behavior. Theoretical calculations are presented for physisorbed ${\mathrm{N}}_2$O which exclude a desorption mechanism via electronically induced vibrational predissociation of the surface bond, and support desorption from the repulsive region of the ground state which is reached via a strongly bound ionic state.

Multilayer relaxation of the W (001) surface is explored by the all-electron local-density-functional total-energy approach. In agreement with recent experiments, we find a contraction of the topmost interlayer spacing by 5.7%. Surprisingly, the amount of this contraction is independent of the relaxation of the inner layers, which are predicted to be expanded by 2.4% and 1.2% for the second and third interlayer spacings, respectively. Thus, the driving mechanism for the relaxation process appears to be of a local nature because of short-range screening effects at the transition-metal surface.

We have extended the Monte Carlo renormalization-group method to study the isotropic Baxter model with random, quenched, site impurities. The results suggest that the critical behavior can be characterized by the pure Baxter fixed line for negative four-spin coupling terminating at the Ising fixed point.

First-principles pseudopotential total-energy and force calculations have been used to study the $\mathrm{Si}\mathrm{}\left(111\right)\mathrm{}\sqrt{3}\times \sqrt{3}$-Al surface. A new adatom model of this reconstruction is proposed wherein each Al adatom sits in a threefold-symmetric site with three first-layer Si neighbors and one second-layer Si neighbor directly below. For this model, the calculated dispersion of the adatom-induced surface states is in good agreement with experiment. A theory for the observed momentum distribution of adatom-induced surface states is proposed.

Diffuse-neutron-scattering and resistivity measurements have been performed on ${\mathrm{Fe}}_{1-c}-{\mathrm{Cr}}_c$ alloys ($c<\mathrm{}0.15\mathrm{}$). A change of sign of the short-range-order parameters near $c=0.1\mathrm{}$ indicates that the alloys exhibit short-range clustering at $c>\mathrm{}0.1\mathrm{}$ and a tendency to form an ordered compound at $c<\mathrm{}0.1\mathrm{}$. Concurrently, the slope of the resistivity versus temperature changes its sign. This inversion of short-range ordering has been predicted by a recent calculation. These experiments are considered to be an important confirmation of the theory.

A model is presented for the Kosterlitz-Thouless superfluid transition of helium films adsorbed on the surface of a sphere. As the sphere diameter is reduced to grain sizes used in experiments the sharp "jump" to zero of the areal superfluid density is broadened and becomes continuous. The model is applied to third-sound measurements in packed alumina powders, and values of the core energy and core radius of the paired vortices are extracted.

It is shown by electron-energy-loss spectroscopy with spin analysis that electrons inelastically scattered from a ferromagnet (${\mathrm{Fe}}_{82}$${\mathrm{B}}_{12}$${\mathrm{Si}}_6$) can have a high spin polarization as a result of exchange scattering. A maximum of the spin polarization occurs around 2.2-eV energy loss corresponding to the ferromagnetic exchange splitting. This experiment offers new possibilities for studying spin-flip Stoner excitations in ferromagnets.

We report on the first experimental determination of the spectrum of Stoner excitations in Ni at $q\approx 0$. The spectrum consists of a broad distribution covering the range from ∼ 0.1 to ∼ 0.5 eV.