It is shown that the electron-phonon interaction in covalent semiconductors drives an instability in the electron-hole plasma at high temperature. The instability might be the reason for melting of tetrahedrally bound semiconductors. If this is true, the intrinsic carrier concentration of silicon at the melting temperature would be of the order of ${10}^{21}$ ${\mathrm{cm}}^{-3\mathrm{}}$ instead of the commonly accepted value of 2×${10}^{19}$ ${\mathrm{cm}}^{-3\mathrm{}}$.

A unified description of solitons in both charge-density-wave (CDW) and spin-density-wave (SDW) systems is presented within Hartree-Fock theory. Spin-carrying solitons correspond to localized SDW region within a CDW ground state or vice versa. Solitons can have a fractional spin component when the CDW and SDW coexist in the ground state. In particular, a SDW in an odd-order commensurate system must coexist with a CDW and solitons have an irrational spin component.

NMR data on Ti${\mathrm{Be}}_2$ have been taken from 1.3 to 270 K in magnetic fields up to 60 kG. The linear dependence of the Knight shift and the NMR linewidth with $\chi =\frac{M}{H}$ show that the increase of $\chi$ versus applied field at low temperatures is due to a homogeneous property of the electron gas. The $d$-electron contribution to the spin-lattice relaxation rate is found to scale linearly with $\chi T$ in a wide temperature range spanning the spin-fluctuation temperature.

The ferroelectric-paraelectric phase transformation in NaN${\mathrm{O}}_2$ microcrystals (size about 5 nm) has been investigated by differential thermal analysis. Single-domain particles exhibit spontaneous polarization irrespective of their microsize. The transformation temperature, $T_c$, of the interior of the particles remains unaffected by their dimension, whereas $T_c$ of the surface monolayer is lowered by a few degrees.

A simple way has been discovered to put model pseudopotentials, $V\left(\overrightarrow{\mathrm{r}}\right)={\Sigma }_{\mathrm{lm}}\left|Y_{\mathrm{lm}}〉V_l\mathrm{}\right(r)\mathrm{}\times 〈Y_{\mathrm{lm}}|$, into a form which reduces the number of integrals of $V\left(\overrightarrow{\mathrm{r}}\right)$ required for an energyband calculation from $\frac{\mathrm{mn}(n+1\mathrm{})\mathrm{}}{2}$ to $\mathrm{mn}$ for each $l$ in the sum (where $n$ is the number of plane waves used in the expansion and $m$ the number of points in the Brillouin zone at which the calculation is performed). The new form may be chosen to improve the accuracy of the pseudopotential when used in other chemical environments.

The mechanism for near-threshold electron- and photon-stimulated desorption of protons from transition-metal surfaces has been investigated by using configuration-interaction techniques on finite clusters. The calculations indicate that the protons arise from repulsive states involving double excitations from the metal-hydrogen bonding orbitals and suggest that the process may occur via a surface predissociation mechanism. The calculations are compared to experimental electron-stimulated-desorption results for $\mathrm{Ni}〈111〉$.

Pair-tunneling measurements have been carried out on Er${\mathrm{Rh}}_4$${\mathrm{B}}_4$-${\mathrm{Lu}}_x{\mathrm{O}}_y$-In thin-film junctions. The Fraunhofer-like pattern of the magnetic-field dependence of the dc Josephson current has been found to split at a temperature just above the reentrant transition of Er${\mathrm{Rh}}_4$${\mathrm{B}}_4$. This suggests the onset of a magnetic structure of semimacroscopic scale which does not break up the superconductivity of the Er${\mathrm{Rh}}_4$${\mathrm{B}}_4$ electrode into disconnected domains, at least within the surface region probed (∼ 150 Å).

A numerical analysis of the evolution of the Higgs expectation value and the temperature of the universe during the symmetry-breaking phase transition in an SU(5) theory with radiatively induced symmetry breaking is presented. It is shown that there is sufficient inflation (exponential expansion) to explain the cosmological homogeneity, isotropy, flatness, and monopole puzzles, and also that the universe reheats to a temperature $O\left({10}^{14} \mathrm{GeV}\right)$ so that the usual scheme for baryogenesis can proceed.

A model of spin-½ statistics that explains the observed frequencies on the basis of the validity of the principle of locality is proposed. The model is based on the observation that certain density conditions on the unit sphere correspond with the observed frequencies while the resulting expectation values violate Bell's inequality.

The gravitino must be either light enough so that ambient gravitinos would not produce too large a cosmic deceleration, or heavy enough so that almost all gravitinos would have decayed before the time of helium synthesis. The second alternative is shown to allow supersymmetry-breaking scales above a model-dependent lower bound of ${10}^{11}$ to ${10}^{16}$ GeV.

$K^0$ regeneration data in the energy range 30-110 GeV have been analyzed to determine the values of the $K_L-K_S$ mass difference, $\Delta m$, $K_S$ lifetime, ${\tau }_S$, and $\mathrm{CP}$-nonconservation parameter ${\eta }_{+-}$. We find $\Delta m=0.482\mathrm{}\left(14\right)\mathrm{}\times {10}^{10}\hslash$ ${\mathrm{s}}^{-1\mathrm{}}$, ${\tau }_S=0.905\mathrm{}\left(7\right)\mathrm{}\times {10}^{-10\mathrm{}}$ s, $\left|{\eta }_{+-}\mathrm{}\right|=2.09\left(2\mathrm{}\right)\mathrm{}\times {10}^{-3\mathrm{}}$, and $tan{\varphi }_{+-}=0.709\mathrm{}\left(102\right)\mathrm{}$, corresponding to ${\varphi }_{+-}=35\mathrm{}\left(4\right)\mathrm{}°$. The data suggest that these parameters may have an anomalous energy dependence.

The Liouville theory is quantized with use of Fock-space methods, an infinite set of charges $L_n$, $n=0, \pm 1, \dots$, is constructed which represents the conformal algebra in two dimensions, and consequences of this algebra are discussed. It is then argued, with use of variational methods in Fock space, that the spectrum of the Liouville Hamiltonian is continuous, and that there exist energy eigenstates obeying the constraints $L_n|E〉=0\mathrm{}$, $n>\mathrm{}0\mathrm{}$.

The effective coupling for muonium-antimuonium transitions is calculated from analogous couplings obtained in recent analyses of nuclear double-beta decay. An effective four-fermion coupling as large as $10G_{\mathrm{F}}$ is found, which should be experimentally accessible with use of muonium in vacuum.

An amplitude analysis of ${\pi }^{+}{\pi }^{-}\to {\pi }^0{\pi }^0$ scattering, from the reaction ${\pi }^{+}p\to {\Delta }^{++}{\pi }^0{\pi }^0$, has been carried out between 1.0 and 2.3 GeV. The data are consistent with only one of the four $\pi -\pi$ phase-shift solutions determined from ${\pi }^{+}{\pi }^{-}\to {\pi }^{+}{\pi }^{-}$ data. A new $J^{\mathrm{PC}}=2^{++}$ state at 1800 MeV with a width of 280 MeV is observed.

A model has been constructed for nucleon-antinucleon annihilation which is of short range but is state (energy, spin, isospin,...) dependent as dictated by the calculation of annihilation diagrams. This model fulfills general theoretical requirements and, at the same time, provides a good fit of the presently available $p\overline{p}$ experimental data, better than the existing models which are state independent but effectively long ranged. The present results contradict the generally accepted claim that fitting the $p\overline{p}$ data requires an effective long-ranged annihilation potential.

A sensitive method is demonstrated for determining ($v^{\prime \prime }, J^{\prime \prime }$) populations of ${\mathrm{H}}_2$. Single rotational levels of the $E$, $F{{}_{}{}^1{}_{}{}^{}\Sigma _g^{}{}_{}{}^{}}^{+}$ state are prepared by two-photon excitation; absorption of a third quantum causes ionization. Detection of $J^{\prime \prime }$ levels of $v^{\prime \prime }=0, 1, \mathrm{and} 2$ is possible while scanning the dye laser fundamental over just 5 nm. Measured rotational populations at different temperatures (295-750 K) agree well with the corresponding Boltzmann distributions. Rotational line strengths show almost no $J^{\prime \prime }$ dependence for this process.

This new technique in optogalvanic spectroscopy yields Doppler-free resonances when degeneracies in atomic energy levels are removed by a magnetic field, and also when the interval between levels matches the mode spacing of the laser. $g_J$ factors and magnetic hyperfine interaction constants were determined for lower and upper levels of the transition at 619.2 nm in YI. The value $A=(+)89.6\left(9\right)\mathrm{}$ MHz for the upper level $4d5s\left(a{}_{}{}^3{}_{}{}^{}D\right)5p{{}_{}{}^2{}_{}{}^{}D_{\frac{3}{2}}^{}{}_{}{}^{}}^0$ is new; the remaining determinations agree with published values.

A relativistic many-body perturbation-theoretic investigation has been carried out of the hyperfine field in the francium atom which when combined with recent measurements of hyperfine constants leads to $(3.39\mathrm{}\pm 0.09){\mu }_N$ for the magnetic moment of the neutron magic isotope ${}_{}{}^{213}{}_{}{}^{}\mathrm{Fr}$ and equally accurate values for the other isotopes ${}_{}{}^{208-212}{}_{}{}^{}\mathrm{Fr}$.

The $1{}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}\to 2{}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}$ transition in positronium has been observed using two-photon Doppler-free excitation. Transitions were induced by a pulsed dye laser, and detected by photoionization of the $2{}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}$ state. The resonance signal has a linewidth of 1.5 GHz and a signal-to-noise ratio of 20:1. The frequency agrees with theory to 0.5 GHz. The resonance signal has an amplitude proportional to the square of the laser intensity and displays the expected Stark shift to higher frequency under an applied electric field.

The first experimental observation of van der Waals molecules by means of Raman scattering is presented. Ground-state rotational and vibrational spectra of dimers of argon were observed in a supersonic expansion. A dipole-induced-dipole model for the dimer polarizability could accurately reproduce the spectra. With use of a seeded expansion the value of the controversial dimer-to-monomer ratio was directly determined as 0.020(4).