The density of He II in supercritical, superfluid flow has been measured by a sensitive capacitance technique in the temperature range from 1.3 to 1.8 K. The average density of the liquid decreased with increasing superfluid velocity. The largest fractional density change observed was $\frac{\Delta \overline{\rho }}{\rho }=-9\mathrm{}\times {10}^{-6\mathrm{}}$ at a velocity of 7.75 cm/sec. The results can be accounted for adequately by the two-fluid, thermohydrodynamic equations of motion utilizing the expressions for the nonlinear, superfluid dissipation force and the mutual friction force.

Polynomial approximations for the liquid structure function $S\left(k\right)\mathrm{}$ and the energy of an elementary excitation $\epsilon \mathrm{}\left(k\right)\mathrm{}$ are found to contain both odd and even powers of $k$ if the interatomic potential falls off asymptotically as $r^{-6\mathrm{}}$. The proof is given under conditions of low density and weak interaction and hence has only suggestive force in applications to the real superfluid.

We study the zeros of the partition function of a classical spin system and compute a region of the complex fugacity plane where they necessarily lie. We recover the Lee-Yang circle theorem as a special example; we also find that a spin system with finite-range interaction has no phase transition at high temperature.

We find an anomalous behavior of the upper critical field for the superconducting system ${\mathrm{Mo}}_{1-x}{\mathrm{Mn}}_x{\mathrm{Ga}}_4$. It is suggested that this is due to the compensation of the paramagnetic term in the Hamiltonian, $-g{\mu }_{\mathrm{B}}\overrightarrow{\mathrm{H}}·\overrightarrow{\sigma }$, by a negative exchange interaction $cJ\left(\overrightarrow{\mathrm{S}}·\overrightarrow{\sigma }\right)$.

The progression observed below 1000 Å in spectra of solid ${\mathrm{N}}_2$, pure and trapped in Ne and C${\mathrm{F}}_4$ matrices at 4.2 K, is ascribed to the valence transition $b{}_{}{}^1{}_{}{}^{}\Pi _{\mathrm{u}}^{}{}_{}{}^{}\leftarrow X{}_{}{}^1{}_{}{}^{}\Sigma _{\mathrm{g}}^{}{}_{}{}^{+}$ of ${\mathrm{N}}_2$. As previously observed in the similar case of NO, the regularity in intensity distribution and in vibrational spacings indicates the absence of the Rydberg transition $c{}_{}{}^1{}_{}{}^{}\Pi _{\mathrm{u}}^{}{}_{}{}^{}\leftarrow X{}_{}{}^1{}_{}{}^{}\Sigma _{\mathrm{g}}^{}{}_{}{}^{+}$ responsible for strong perturbations in the free-molecule spectrum.

A new analysis of direct exciton data is presented and applied to the $E_0$, $E_0+{\Delta }_0$ transitions in GaP. A curve-fitting procedure is used to compare both the discrete and continuum structure in the data with theoretical curves. Good agreement is obtained for the binding energies, broadening, strengths, and shapes. It is shown that the binding energy of $E_0$ is 11 ± 1 meV rather than the 5-6-meV value previously obtained from the conventional analysis.

Four emission bands related to the $A$ and $C$ excited states, $A_{T\Pi }$, $A_{T\Sigma }$, $C_{T\Sigma }$, and $C_{T\Pi }$, have been observed in KI:${\mathrm{Sn}}^{2+}$ at 4.2°K, and the degree of polarization has been studied. The results indicate that the $\Gamma _4^{}{}_{}{}^{-}$ (${}_{}{}^3{}_{}{}^{}T_{1\mathrm{u}}^{}{}_{}{}^{}$ and ${}_{}{}^1{}_{}{}^{}T_{1\mathrm{u}}^{}{}_{}{}^{}$) excited states are tetragonally distorted by a strong Jahn-Teller effect and are also perturbed by a charge-compensating vacancy located in the nearest-neighbor position.

The results of relative spin-susceptibility measurements as a function of donor concentration are reported for heavily doped $n$-type silicon. It is shown that, at liquid helium temperatures, the susceptibility decreases with impurity concentration for samples in the impurity-band region. At still higher concentrations, the susceptibility is well described by the ${\mathrm{}\left(\mathrm{concentration}\right)\mathrm{}}^{1/3}$ law expected for a degenerate electron gas.

Antiferromagnetic domains in Cr have been observed visually by the x-ray double-crystal method in the low-temperature phase which corresponds to the longitudinal spindensity-wave state. The contrast between neighboring domains is due to a slight tetragonality of the crystal lattice. The domain boundary has been found to be sharp and mobile, and its section is straight if not trapped by imperfections.

The ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ 14.4-keV-level electric-quadrupole coupling in $\alpha -\mathrm{F}\mathrm{e}$ has been detected by Mössbauer spectroscopy.

A six-parameter optical model for proton elastic scattering from ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$ has been developed in which the real part of the potential has been generated from a matter distribution that reproduces the single-particle binding energies and electron elastic-scattering cross sections. A Gaussian nucleon-nucleon force has been used and an optimum range of 4.5 ± 0.4 ${\mathrm{fm}}^2$ extracted. Improved fits are obtained with a density-dependent effective interaction.

We have observed the small-angle $j$ dependence of ($\alpha ,p$) reactions for $l=1\mathrm{}$ and $l=2\mathrm{}$ reactions. The small-angle effect appears to persist for cases where the large-angle $j$ dependence is small or absent.

A theorem is described which establishes the claim that in a certain canonical sense the Kerr metrics represent "the" (rather than merely "some possible") exterior fields of black holes with the corresponding mass and angular-momentum values.

A narrow $I=\frac{1}{2}$ enhancement at a mass of 1.462 ± 0.006 $\frac{\mathrm{GeV}}{c^2}$ with a width of 0.049 ± 0.012 $\frac{\mathrm{GeV}}{c^2}$ has been observed in $p{\pi }^0$ and $n{\pi }^{+}$ modes produced in $\mathrm{pp}$ collisions at 6.6 $\frac{\mathrm{GeV}}{c}$. The effect, having a 6-standard-deviation significance, is narrower in width than has been observed before in missing-mass-spectrometer experiments.

Measurements of polarization in ${\pi }^{+}p$ elastic scattering have been made at 1.60, 1.80, 2.11, and 2.31 $\frac{\mathrm{GeV}}{c}$. The data cover the entire angular range, with emphasis on the backward region. Comparisons have been made with both $u$-channel and $t$-channel models, as well as with predictions of phase-shift analyses. While the agreement is generally poor in all cases, the best agreement is with some $t$-channel predictions.

$\gamma n$ reactions are studied in a 40-in. bubble chamber filled with deuterium. Cross sections for channels with no neutral particles in the final state are presented up to a photon energy of 12 GeV. For channels with one neutral particle we present the cross sections at 7.5 GeV only. Comparison of $\gamma n$ reactions with their charge-symmetric $\gamma p$ reactions indicates a considerable isovector-isoscalar interference contribution.

We have examined the momentum-transfer distributions in the reactions (1) ${\pi }^{+}+p\to {\Delta }^{++}\mathrm{}\left(1236\right)+{\pi }^{+}+{\pi }^{-}$ at a ${\pi }^{+}$ beam momentum of 7 $\frac{\mathrm{GeV}}{c}$, and (2) $K^{+}+p\to {\Delta }^{++}\mathrm{}\left(1236\right)+K^{+}+{\pi }^{-}$ at an incident $K^{+}$ momentum of 12.7 $\frac{\mathrm{GeV}}{c}$. In both instances the $t$ dependence (proton to $\Delta$) can be represented remarkably well by the simple form ${(t+{\mu }^2)}^{-2\mathrm{}}$, where ${\mu }^2\approx m_{\pi }^{}{}_{}{}^2$.

The existence of a stable NaLi molecule has been demonstrated for the first time. Spectra of the ${\mathrm{Na}}^7$Li molecule were produced by laser excitation. With the aid of the technique of collision-induced energy transfer the molecular constants have been determined and the excited level assigned to a ${}_{}{}^1{}_{}{}^{}\mathrm{II}$ electronic state. The measured molecular constants are within 10% of the ${\mathrm{Na}}^7$Li constants predicted by ab initio calculations.

It is found that the angular distribution of photoelectrons from the argon $4d$ subshell oscillates as a function of energy for the first 150 eV above threshold.

The Schrödinger equation for a two-state quantum-mechanical system with sinusoidal perturbation is numerically integrated with respect to time. From these results a general formula for the induced transition probability (as a function of time, perturbation frequency, and perturbation strength) is extracted.