Low-energy electron-diffraction and photoemission data are presented for Si(111) surfaces with 2 × 1 and 7 × 7 superlattices, as well as for the disordered surface obtained in the transition from one to the other. Together with work-function measurements these data suggest two different models for the two superlattices.

Heat-capacity and entropy changes at the metal-insulator transitions have been measured on ${\mathrm{Ti}}_4$${\mathrm{O}}_7$ single crystals. The 130-K transition is related to a disordering of the ${\mathrm{Ti}}^{3+}$ and ${\mathrm{Ti}}^{4+}$ chains at the unit cell level. ${\mathrm{Ti}}^{3+}$ pairing occurs in this phase but without any long-range order of the bonds. It is shown from both magnetic-susceptibility and specific-heat data that for the 150-K transition, the electronic contribution seems to be of the same order of magnitude as the lattice contribution.

We have measured the resistivity of single-crystal paramagnetic ${\mathrm{Tb}}_x{\mathrm{Y}}_{1-x}\mathrm{Sb}$ with $x=0,0.05,0.20,0.40\mathrm{}$. At low temperatures a resistance anomaly develops in proportion to $x$. The observed resistance anomaly reflects the temperature-dependent probability that the conduction electrons are scattered from the crystal-field-split $4f$ levels of the Tb ions by elastic as well as inelastic processes. A calculation of this anomaly yields excellent agreement for those values of $x$ for which indirect exchange can be neglected.

The ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ magnetic form factor is calculated in the impulse approximation using a wave function obtained previously from a complete solution of the Faddeev equations with the Reid soft-core potential. The $S-D$ matrix elements contribute significantly. The convection current contribution is found to be small. The form factor has a minimum at $Q^2=7.1\mathrm{}$ ${\mathrm{fm}}^{-2\mathrm{}}$.

A partial-wave decomposition has been made of the final-state $3\pi$ system produced in the reaction ${\pi }^{-}+A\to \left({\pi }^{-}{\pi }^{-}{\pi }^{+}\right)+A$ at 23 GeV/c and for the elements C, Al, Cu, and Ag. For low $3\pi$ masses, the $1^{+}$ state is dominant and its $t^{\prime }$ distribution shows the expected combination of coherent and incoherent components. The $t^{\prime }$ distribution of the $2^{+}$ $A_2$ state also requires both coherent and incoherent components.

We report on a search for the production of quasimolecular $K$ x rays with 30- to 90- MeV Br projectiles incident on targets with $Z$ near 35. We disagree in some important aspects with previously published results from a similar experiment, and we show that the evidence for quasimolecular $K$ x-ray emission from the Br + Br system is at present inconclusive.

Total cross sections for the ionization of atomic hydrogen by electron impact are calculated using the Glauber approximation. These cross sections are compared with the results obtained by the Born approximation and with experimental results.

We report the first ESR signal of an excited crystalline state (${\Gamma }_6$) in a metal (PdEr). The intensity of the ${\Gamma }_6$ isotropic resonance compared to the $\Gamma _8^{}{}_{}{}^{\mathrm{}\left(3\right)\mathrm{}}$ resonances of the ground-state quarter allows the determination of the energy scaling parameter $W$ of the crystalline field. A more precise value is deduced from the splitting of the transitions 1 → 2 and 3 → 4 of the $\Gamma _8^{}{}_{}{}^{\mathrm{}\left(3\right)\mathrm{}}$ ground state: $W=-0.163\mathrm{}\pm 0.015$ K, in accordance with the work of Praddaude.

Beam modes in an ion-beam-plasma system are observed to execute amplitude oscillations. Our observations are consistent with the expected organized periodic energy exchanges between the wave and trapped beam particles.

Experimental and theoretical studies of a new type of parametric instability involving the excitation of ion quasimodes (due to nonlinear wave-particle scattering) have been carried out when the "pump" frequency is near the lower hybrid frequency. Plasma heating associated with this instability has also been measured.

Turbulent heating and stabilization of the ion sound instability is investigated by two-dimensional computer simulation. Quasilinear rather than nonlinear effects determine the evolution of the instability. The instability is quenched by flattening of the electron distribution and the formation of a high-energy ion tail.

An intense electromagnetic wave propagating in a collisional plasma is found to be unstable to a thermal self-focusing instability by a self-consistent solution of the hydrodynamic, heat conductivity, and wave propagation equations. The results are applied to ionospheric modifications and proposed power transmission experiments, and to laser-plasma interactions.

Observations of ionospheric striations generated by intense radio waves have been made by a new phased-array ionospheric sounder. The results are in qualitative agreement with and provide experimental evidence for a thermal self-focusing model.

The intensity of photoemission from the Ta-derived $d$ states in the layer compound $1T-\mathrm{Ta}{\mathrm{Se}}_2$ has been measured as a function of angle at the photon energy 10.2 eV. The dependence of the $d$ emission on azimuthal angle displays three pairs of major lobes and indications of three pairs of minor lobes. If it is assumed that these photoelectrons are created on Ta atoms, they propagate preferentially along directions which avoid neighboring Se atoms. The relationship with atomic orbitals and with band theory is discussed.

The neutral lattice vacancy in diamond is simulated by the clusters ${\mathrm{C}}_4$, ${\mathrm{C}}_4$${\mathrm{H}}_{12}$, and ${\mathrm{C}}_{16}$. Many-electron effects within the ground configuration are estimated using the self-consistent-field $X\alpha$ scattered-wave method. The resulting term splittings are found to decrease precipitously as the size of the cluster increases, in contrast to the conclusions of Coulson and Larkins. It is concluded that in a real crystal these effects should, therefore, be small and that simple one-electron methods represent valid first-order treatments to the problem.

Fine structure in the $N_7\mathrm{VV}$ Auger spectrum from a W(100) surface has been observed and attributed to emission from a surface state, and from three peaks in the bulk density of states.

Resonance transitions are observed between electronic levels in an accumulation layer on $n$-type (100)Si. Signals are studied at the 220-, 171-, and 118-μm lines of a watervapor (${\mathrm{H}}_2$O, ${\mathrm{D}}_2$O) molecular laser. Strong transitions from the lowest two-dimensional sub-band to the next higher band are observed. For 118-μm radiation the resonance occurs at about 0.6 × ${10}^{12}$ electrons/${\mathrm{cm}}^2$.

Ultrasonic attenuation near the ferromagnetic-spiral transition of Dy and Tb is described on the basis of magnetoelastically driven domain-wall resonance. Good agreement with the measured attenuation coefficient is obtained on the basis of a simple model, and an apparent discrepancy between zero-field observations is discussed.

Antiferromagnetic nickel sulphide has a small temperature-independent magnetic susceptibility. The ordered moment per nickel atom, measured with neutrons, decreases from ${1.45\mathrm{\mu }}_{\mathrm{B}}$ to ${1.00\mathrm{\mu }}_{\mathrm{B}}$ at 4.2 K with a decrease of 0.6% in the Ni-Ni distance. These data indicate a band model for the $n$ electrons, making NiS an itinerant-electron antiferromagnet with no local moments. The possibility of local moments in an antiferromagnetic metal is briefly discussed.

We present here the results of an experiment to study the polarization in $p-p$ elastic scattering at the incident momenta 5.15, 7.00, and 12.33 Ge V/c, at $t$ values ranging between - 0.5 and - 6.5 ${(\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^2$. At each momentum we observe a relative maximum in the polarization around $t=-1.8\mathrm{}$ ${(\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^2$. At 12.33 GeV/c the data exhibit a double zero near $t=-2.4\mathrm{}$ ${(\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^2$ and another relative maximum near $t=-2.9\mathrm{}$ ${(\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^2$. The results are discussed in terms of the Chu-Hendry optical model.