The authors report x-ray measurements of the attenuation of integrated Bragg intensities which is attributed to the static Debye-Waller factor, $\exp \mathrm{}(-2L)\mathrm{}$, due to hydrogen in Nb single crystals in the homogeneous $\alpha$ phase. For small hydrogen concentrations, the dependence of $2L$ versus the scattering vector, $\overrightarrow{\mathrm{K}}$, can be explained by the nonlinear coupling between static and thermal displacements around point defects. Detailed information about both types of displacements of the Nb atoms nearest to the interstitial hydrogen atom has been obtained.

Mechanical simulations of a two-dimensional system of $N$ cyclic tetramers, studied in square ($N=289\mathrm{}$) and hexagonal ($N=331\mathrm{}$) boxes, demonstrate the existence of crystalline $d=\frac{V}{V_0}\lesssim 1.14$; different structures are observed), rotator ($1.18\lesssim d\lesssim 1.29$), and fluid ($d\gtrsim 1.31$) phases ($V_0$ is the area of the box with close-packed structure). Observed phase coexistences suggest first-order transitions.

The macroscopic consequences of displacive modulation on the morphology of incommensurate single crystals are confirmed. Bands of faces in the neighborhood of stable normal crystal faces have been observed on spherically shaped ${\mathrm{Rb}}_2$Zn${\mathrm{Br}}_4$ crystals. An interpretation is given in terms of classical morphological theory extended to include (four-dimensional) superspace group symmetry. This leads to the view that the formation of these bands involves, at least partly, so-called satellite faces and gives a simple explanation of why the set of bands has a lower point-group symmetry than the set of normal faces.

Raman-scattering measurements on ${\mathrm{Nb}}_3$Sn and ${\mathrm{V}}_3$Si show new peaks in the superconducting state close in energy to the superconducting gap $2\Delta$. These are attributed to direct electronic Raman scattering by pairs of superconducting quasiparticles. A theory is presented which fits the data well and provides evidence for anisotropy of the gap (∼20% in ${\mathrm{Nb}}_3$Sn) and of the matrix element for electronic Raman scattering.

The current-voltage characteristics of Josephson tunnel junctions shunted by a resistance with a substantial self-inductance exhibit stable negative-resistance regions. Very large increases in the low-frequency voltage noise with temperatures of ${10}^6$ K or more observed near these regions arise from switching between subharmonic modes. Moderate increases in the noise, with temperatures of about ${10}^3$ K, arise from chaotic behavior. These results are substantiated by analog and digital simulations.

The time dependence of the hyperfine field at ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$ nuclei following implantation into Mn${\mathrm{F}}_2$ has been studied by the perturbed angular distribution technique. The data show a partial destruction of the magnetic order in the collision cascade, attributed to electronic energy loss processes. The recovery of this magnetism defect can be accounted for by spin diffusion from the undisturbed regions of the crystal. The temperature dependence of the spin-diffusion coefficient is consistent with a critical exponent of 0.7.

It is proposed that one should admit in the path integral for the quantized gravitational field only those space-times for which the final three-geometry is located in the future of the initial one. As a consequence, and unlike the situation for the Yang-Mills field, the resulting causal amplitude is not annihilated by all the gauge (surface deformation) generators. In supergravity the causal amplitude turns out not to be annihilated by the local supersymmetry generators either.

A preliminary experimental determination by Hill, Bos, and Goode of the interior rotation of the sun leads to a nonzero value for the quadrupole moment coefficient $J_2$. This produces a deviation of 1.6% from Einstein's prediction of the precession of the perihelion of Mercury. A nonsymmetric gravitational theory can fit the measured precession with this $J_2$ and all other solar-system relativity experiments for one value of a post-Newtonian parameter in the theory. A prediction is made for the perihelion precession of Icarus.

Chaotic oscillations due to delay-induced instabilities are easily seen with a simple and familiar acoustic system. The measured bifurcation diagram to chaos, which is very different from that for a one-dimensional map model, is compared with numerical results. The bifurcation structure is dominated by the ratio of the response time to the delay time of the system.

An experimental study of the process $e\overline{e}\to Z\to H\mu \overline{\mu }$ (for a Higgs boson $H$ with a mass $m_H$ less than 40 GeV) is likely to reveal the structure of the $\mathrm{ZH}{Z}^{*}$ vertex. It is shown how this may help resolve the question of the compositeness of $H$.

A continuing search for the no-neutrino mode of the double beta decay of ${}_{}{}^{76}{}_{}{}^{}\mathrm{Ge}$ has resulted in a new lower limit ${T_{\frac{1}{2}}}^{0\nu }>~1.7\times {10}^{22}$ yr. This value corresponds to a 90% confidence level determined with a maximum-likelihood analysis of the energy interval 2041±2 keV. Combined with recent shell-model calculations, the data imply $m_{\nu }<~10$ eV and a limit on lepton nonconservation $\left|\eta \right|<~2.4\times {10}^{-5\mathrm{}}$. In the context of the shell model, the data imply that the electron neutrino is not a Majorana mass eigenstate.

Extensive sets of yrast and nonyrast states in ${}_{}{}^{99}{}_{}{}^{}\mathrm{Ru}$ have been populated by the reaction ${}_{}{}^{98}{}_{}{}^{}\mathrm{Mo}({}_{}{}^3{}_{}{}^{}\mathrm{He}, 2n\gamma ){}_{}{}^{99}{}_{}{}^{}\mathrm{Ru}$. The population of nonyrast states is attributable to the characteristics of the reaction rather than the specifics of the nuclear structure involved. Several complete particle-core multiplets have been successfully interpreted with a particle-rotor model.

The first observation of vibration-rotation transitions in HC${\mathrm{O}}^{+}$ is reported. The ${\nu }_1$ absorption band was measured with a color-center laser by modulating the drift velocity of the ion in an ac discharge and detecting the Doppler-shifted absorptions with lock-in techniques. The ${\nu }_1$ frequency is determined as 3088.727±0.003 ${\mathrm{cm}}^{-1\mathrm{}}$.

The surface-flashover anode plasma in a magnetically insulated ion diode was investigated spectroscopically. From the Stark broadening of the neutral hydrogen ${\mathrm{H}}_{\beta }$ line an average electron density of about 2×${10}^{15}$/${\mathrm{cm}}^3$ was observed in the ≲ 1-mm anode plasma, 30 nsec into the 400-475-kV diode voltage pulse. Thereafter, the plasma front advanced into the diode gap at an average rate of 2 cm/μsec. This may be explained by the ionization of neutral atoms injected into the gap during flashover.

The possibility of describing the continuous variation ${\nu }_D$ with $D$, where ${\nu }_D$ is the shape exponent of self-avoiding walks and $D$ the spatial dimensionality, is investigated. A dimensionality $d$ is associated with the increase of a walk's volume with its length. The value of $d$ is varied at will through an extension (acceleration) of walks on all scales of length. The shape exponent ${\nu }_d$ of such accelerated self-avoiding walks is studied with the help of computer simulation. The results indicate that ${\nu }_d$ reproduces ${\nu }_D$.

Self-consistent electronic structure calculations are reported for Al at two interstitial locations in silicon. On the basis of these calculations, a novel mechanism for enhanced interstitial migration is proposed in which electron capture can cause a barrier lowering appreciably greater than $E_G$, the energy gap. The model and the present numerical results are in good agreement with recent measurements by Troxell et al.

Tunneling and conductivity measurements through the metal-insulator transition in amorphous ${\mathrm{Nb}}_x{\mathrm{Si}}_{1-x}$ are reported. The authors observe the correlation gap $\Delta$ which varies with resistivity and have related this to the metal-insulator transition as observed in the conductivity. The samples were prepared by a process which allows precise control of the Nb concentration. The results as a function of voltage, temperature, and concentration are compared with current theoretical predictions.

Quantum resistance of a Cayley tree composed of one-dimensional random scatterers is considered. The strength of a scatterers is characterized by its typical resistance ${\rho }_0$. It is shown that a metal-insulator transition occurs at some critical scattering strength ${\rho }_{0c}$, which depends on the coordination number of the tree. The resistance of an infinite tree diverges for ${\rho }_0>{\rho }_{0c}$ and saturates at some finite value for ${\rho }_0<{\rho }_{0c}$.

Surface-plasmon-polariton-mediated luminescence is observed when electrons are injected into thin Al films from the conduction band of Si${\mathrm{O}}_2$. These electron-injector structures are strikingly similar to light-emitting tunnel junctions, although tunneling can be ruled out as the driving mechanism. The emission arises from the energy relaxation of the steady-state hot-electron distribution which exists in the metal under continuous current injection. The same mechanism must explain much of the luminescence from tunnel junctions.

In electron inversion layers on InSb the cylotron resonance energies and linewidths are found to exhibit a marked discontinuity in the vicinity of the bulk longitudinal-optical-phonon energy. These features are attributed to resonant electron-phonon interaction in the two-dimensional electron system. The experimentally observed mass discontinuity indicates that the electron-optical-phonon interaction in a two-dimensionally confined electron system is enhanced in comparison to the three-dimensional case.