We report luminescence threshold experiments on electron-hole drops (EHD) done in conditions where an optical hysteresis can be observed, and a nucleation model is fitted to the data. The agreement between experiment and theory is good if the EHD surface energy is 20 ergs/${\mathrm{m}}^2$ and if we introduce in the calculations a sticking coefficient (4.6%) at the EHD surface. We have also studied the time required for hysteresis to vanish. We think that this effect is due to the phonon wind which pushes EHD inside the sample.

Calculations of the electron-phonon interaction, $\lambda$, and $T_c$ are presented for ten different $A15\mathrm{}$ compounds, ${\mathrm{V}}_{3}X$ and ${\mathrm{Nb}}_{3}X$, $X=\mathrm{A}\mathrm{l},\mathrm{G}\mathrm{a},\mathrm{S}\mathrm{i},\mathrm{G}\mathrm{e},\mathrm{a}\mathrm{n}\mathrm{d}\mathrm{S}\mathrm{n}$. The calculated temperature-dependent magnetic susceptibility of ${\mathrm{Nb}}_3$Sn is also shown to be in good agreement with experiment. We argue that the low-temperature anomalies in many $A15\mathrm{}$ materials are related to the very sharp structure in the density of states near the Fermi energy found in our ab initio band-structure results.

By an exact calculation for an Ising chain it is established that single-ion (electronic) energy levels are temperature dependent in anisotropic paramagnets. The effect is produced by developing anisotropic spin-spin correlations and, in a more general context, is investigated using the correlated-effective-field statistical approximation. It should be directly observable by spectroscopic means in strongly anisotropic paramagnetic systems.

The critical behavior of Cr near the first-order antiferromagnetic transition at 38.5°C has been studied by neutron scattering. This study reveals the existence of a sharp central peak at the satellite position in a narrow temperature range just above $T_{\mathrm{N}}$. It exhibits a very singular temperature dependence such as the two-dimensional planar model predicts. Such unusual behavior supports the conjecture that the two-dimensional planar model is realized in the critical region of Cr.

An analysis of the charge-density and energy-level separations for the ${\mathrm{Si}}_2$ molecule is made using local semiempirical and nonlocal first-principles pseudopotentials. The results are compared with Hartree-Fock-Slater all-electron calculations. It is demonstrated that pseudopotential calculations for molecules are reliable, and that recent objections based on charge-density topology are unwarranted.

The ambiguity in the Hamiltonian for systems with constraints is discussed, and a counter example is provided to a conjecture of Dirac, which has led previously to the identification of the system Hamiltonian as the extended Hamiltonian, $H_E$. The conjecture fails and $H_E$ also generates the wrong canonical equations; Dirac's test for identifying the Hamiltonian, however, leads to correct results for the example.

I present a class of vacuum metrics with a pair of commuting Killing vectors which do not possess the property of orthogonal transitivity. The solution is Petrov type II with expansion-, twist-, and shear-free geodesic rays and includes the van Stockum exterior metric as a limiting case.

A theory of gravity incorporating the concept of spontaneous symmetry breaking is proposed. It is suggested that the same symmetry-breaking mechanism is responsible for breaking a unified gauge theory into strong, weak, and electromagnetic interactions.

The formula ${{tan}^2\theta }_{\mathrm{C}}\simeq \frac{m_d}{m_s}$ is derived from a continuous symmetry. Cabibbo universality is guaranteed in a natural way. All weak mixing angles are determined in terms of quark masses. The $b$ quark is predicted to decay mainly to the $u$ quark, and the lifetime of the associated mesons is ∼${10}^{-10\mathrm{}}$-${10}^{-11\mathrm{}}$ sec. We argue that this additional symmetry is (essentially) the only one which can be added to the standard $\mathrm{S}{\mathrm{U}}_c\mathrm{}\left(3\right)\mathrm{}\otimes \mathrm{SU}\mathrm{}\left(2\right)\mathrm{}\otimes \mathrm{U}\mathrm{}\left(1\right)\mathrm{}$ model without generating anomalies.

The electromagnetic decay rates of mirror states are used to decompose the multipole electromagnetic matrix elements into isoscalar and isovector or equivalently proton and neutron matrix elements. This method is sensitive to the neutron matrix element because it is measured directly, and it is accurate since it depends only on electromagnetic interactions; it therefore can be used to test methods using inelastic hadron scattering. There is agreement with the results for ($\alpha$,${\alpha }^{\prime }$) on ${}_{}{}^{26}{}_{}{}^{}\mathrm{Mg}$ and ${}_{}{}^{42}{}_{}{}^{}\mathrm{Ca}$ but disagreement with the recent ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$($\pi$,${\pi }^{\prime }$) experiments.

Backward-angle cross sections for the reactions ${}_{}{}^{24}{}_{}{}^{}\mathrm{Mg}$(${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$, ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$)${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$(g.s.; 1.78 MeV) and ${}_{}{}^{24}{}_{}{}^{}\mathrm{Mg}$(${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$, ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$)${}_{}{}^{24}{}_{}{}^{}\mathrm{Mg}$(g.s.; 1.37 MeV) were obtained for $17 \mathrm{MeV}<~E_{\mathrm{c}.\mathrm{m}.\mathrm{}}<~31 \mathrm{MeV}$, ${\overline{\theta }}_{\mathrm{c}.\mathrm{m}.\mathrm{}}\simeq 176°$. The first reaction shows gross and fine resonant structures; the second shows mainly gross structures. Angular distribution measurements give $L$ values of 15 and 21, respectively, at $E_{\mathrm{c}.\mathrm{m}.\mathrm{}}=21.6\mathrm{} \mathrm{and} 28.0$ MeV. The resonant gross structures for ${}_{}{}^{24}{}_{}{}^{}\mathrm{Mg}$(${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$, ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$)${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$(g.s.) are governed by the entrance channel and are discussed from the shaperesonance point of view.

A ${10}^{+}$ resonance with a width of about 700 keV (c.m.) for ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ + ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ has been observed in the reaction ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$(${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$, ${}_{}{}^8{}_{}{}^{}\mathrm{Be}_{\mathrm{g}.\mathrm{s}.\mathrm{}}^{}{}_{}{}^{}$)${}_{}{}^{20}{}_{}{}^{}\mathrm{Ne}$ at $E_{\mathrm{c}.\mathrm{m}.\mathrm{}}=18.8\mathrm{}$ MeV leading to the ground state and several excited states in ${}_{}{}^{20}{}_{}{}^{}\mathrm{Ne}$. The spin value of $I=10\mathrm{}$ is four units of angular momentum below the grazing value of $l=14\mathrm{}$ obtained from elastic scattering, and is still two units below the strong-absorption $l$ value obtained from ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ + ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ total fusion cross sections.

We describe a derivation of a microscopic theory of the wobbling motion of a triaxial nuclear rotor at high angular momentum as a possible phase for the yrast line. The solution of a rotationally invariant approximation to the Heisenberg equations of motion (the Kerman-Klein equations) is outlined as a systematic expansion in reciprocal powers of the total angular momentum and of the number of particles.

The fraction of metastable $\mathrm{He}\left(2\mathrm{}{}_{}{}^3{}_{}{}^{}S\right)$ atoms formed by electron-capture neutralization of (25-90)-keV ${\mathrm{He}}^{+}$ ions in ${\mathrm{H}}_2$ gas or sodium vapor has been measured by using photon-particle coincidence techniques. The true metastable-atom populations measured by such techniques differ from apparent metastable-atom populations measured by the beam-attenuation technique. The consequences to existing experimental collision cross sections involving $\mathrm{He}\left(2\mathrm{}{}_{}{}^3{}_{}{}^{}S\right)$ are discussed.

A theory of quantum beats in superfluorescence is given which explains some of the recent experimental observations of Vrehen, Hickspoors, and Gibbs. Variations of this experiment and some new experiments are suggested which will throw more light on chirping in superfluorescence and will confirm the change in sign of Lamb shift due to cooperative effects in a highly excited assembly of atoms.

We found that the sheath potential plays a dominant role in the plasma-wall interaction in the Macrotor tokamak. An increase in the sheath potential, produced by removing electrons from the plasma, results in the enhancement of the metal influx. When the sheath potential is reduced by electron injection, the metal influx is suppressed. This mechanism may be used as a tool in controlling metallic impurities in tokamaks.

We report observations of the decay of persistent currents of thin superfluid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ films for which the superfluid velocity deviates strongly from the previously observed $logt$ behavior. We present data which document the evolution of this behavior as a function of film thickness. The Iordanskii-Langer-Fisher theory does not accurately describe our observations.

Using general expressions for the velocity and ultrasonic attenuation coefficient we have analyzed ultrasonic data for the 208-K phase transition in Rb${\mathrm{Ag}}_4$${\mathrm{I}}_5$. From fitting the velocity in the high-temperature phase we obtain values for the strain and pseudospin coupling constants. We extract the intrinsic pseudospin relaxation rate from an analysis of the attenuation. The Jahn-Teller model is found to provide a good description of the ultrasonic behavior associated with the phase transition.

Resistivity, annealing, and neutron-diffraction studies of $\mathrm{Pd}{\mathrm{H}}_x$ and $\mathrm{Pd}{\mathrm{D}}_x$ provide evidence for ordering of the H and D atoms on the octhaderal interstitial sublattice. Neutron diffraction results on a Pd${\mathrm{D}}_{0.76}$ single crystal indicate that the D atoms order on the fcc interstitial lattice with every fifth (4,2,0) plane vacant and all others filled, corresponding to the ${\mathrm{Ni}}_4$Mo structure.

Strong pretransitional divergence of shear viscosity was observed for the first time near a smectic-$A$—smectic-$B$ phase transition by capillary shear flow with the layer normal perpendicular to both the flow velocity and velocity gradient. The results are inconsistent with a single power-law type of critical exponent and are suggestive of an essential singularity. Possible implications of the results for recently proposed models of the smectic-$B$ phase and mechanisms for smectic-$A$—smectic-$B$ phase transitions are discussed.