The full potential curves for ${\mathrm{Mo}}_2$ (and ${\mathrm{Cr}}_2$) have been examined by using highly correlated wave functions. A competition is found between $5s-5s$ bonding and $4d-4d$ bonding, so that the lower states lead to two minima, with the inner minimum involving $d\sigma$, $d\pi$, and $d\delta$ bonds. The results disagree dramatically with those from local-density calculations, and the present analysis suggests that this approximation (in its current formulation) is unsuitable for describing multiple bonds in transition-metal systems.

The cyclotron resonance is considered upon action of strong electromagnetic quasiresonant wave. It is shown that even a very weak relativistic (${\beta }^2\ll 1$) mass effect of the electron can result in large hysteretic jumps of its steady-state kinetic energy, if the wave intensity or frequency is varied.

First results are presented of a free-electron-laser experiment that utilizes a tapered wiggler for efficiency enhancement. The spontaneous spectrum and the electron-beam energy loss due to the free-electron-laser interaction are measured and compared with theory. With a 2.25% magnetic field taper, 12% of the electrons were decelerated by 0.6%, corresponding to a gain of 2.7% and an efficiency of 0.07%, which is 10 times higher than that calculated for a zero taper and otherwise identical conditions.

A negative potential barrier on the low-potential side of a moving double layer gives rise to a current limitation in a collisionless plasma terminated by a positively biased cold collector plate in a $Q$ machine. The double layer is produced in front of the plasma source and moves toward the collector during the limitation. When the double layer arrives at the collector, the barrier dissolves and the current increases, causing a rise in potential along the whole plasma column. The double layer then reforms at the source.

The space-charge coupling between the electrons and ions composing an intense, charge-neutralized ion beam is shown theoretically and experimentally to result in both species being brought to a common focus by a solenoidal magnetic lens. The collective focal length is the geometric mean of the focal lengths of the two species calculated independently. Upper and lower limits on the electron density allowed with the lens are derived.

Experimental verification of lower-hybrid rf current drive in the Versator II tokamak is presented. The experiments show that efficient current drive exists only in low-density discharges (${\overline{n}}_e\lesssim 6\times {10}^{12}$ ${\mathrm{cm}}^{-3\mathrm{}}$) in the "slide-away" regime where a preformed suprathermal electron tail exists prior to the application of the rf power.

With use of ac calorimetry and neutron diffraction, the lithium sublattice melting transition in ${\mathrm{C}}_6$Li, which belongs to the universality class of the discrete three-dimensional, three-state Potts model, has been discovered and studied. Strong precursive effects surround the (weak) first-order transition at 715±2 K. Fitting power laws away from the discontinuous region yields $\alpha =0.50\mathrm{}\pm 0.05$ and $\beta =0.22\mathrm{}\pm 0.03$. These values are consistent with three-dimensional tricritical exponents and may reflect conjectured tricritical behavior.

The third-order invariants in the Landau free-energy expansion for an elastic phase transition reached by group theory are determined from third-order elastic constants measured from the effects of hydrostatic and uniaxial pressure on ultrasonic wave velocities in In-Tl alloys near the fcc-to-fct transition. Results extend application of the Landau theory to a first-order transition in which the atomic displacements are small.

A practical method is described for the phase determination of x-ray reflections from single crystals. Considerations on the dynamical interaction in multiple diffraction and on the relative rotation of the crystal lattice with respect to the Ewald sphere reveal both experimentally and theoretically the phase dependence of the reflected intensity. Applications can be made for a direct experimental determination of phases without carrying out the complicated dynamical calculation.

Brillouin-scattering measurements show an anomalous dip in the elastic constants of Nb/Cu superlattices for superlattice wavelengths of ≲ 100 Å. This dip is correlated with changes in the electrical resistivity of the samples giving the first evidence that electronic effects are responsible for phonon softening in these materials.

Direct evidence of alloy clustering in the ${\mathrm{Ga}}_{1-x}{\mathrm{Al}}_x\mathrm{As}$ alloy system is presented. Clustering is observed only on the nonpolar surface of GaAs. An exchange reaction model is proposed to account for the existence of a surface-orientation-dependent miscibility gap for this alloy system.

Shear mechanical properties have been measured for free-standing films of the liquid crystal $n$-hexyl 4′-$n$-pentyloxybiphenyl-4-carboxylate (65OBC) in its stacked-hexatic $B$ phase. The smectic-$I$ phases of (±)-4-(2′-methylbutyl)phenyl 4′-$n$-nonyloxybiphenyl-4-carboxylate and (-1)-2 methylbutyl 4-(4′-$n$-decyloxybenzylideneamino) cinnamate (DOBAMBC) were also studied. Unlike the crystal $B$ phase, none of these phases exhibited an in-plane shear modulus. Enhanced crystalline surface ordering was observed in 65OBC.

Low-energy electron diffraction has been used to study the orientational ordering of the molecular axes of nitrogen molecules physisorbed on graphite single crystals. A 2 × 1 superlattice structure with in-plane orientations is inferred for the monolayer where the molecular centers have the commensurate ($\sqrt{3}\times \sqrt{3}$) 30° structure. The superlattice intensity decreases significantly near 30 K but persists up to 40 K, consistent with a first-order transition with finite-size effects.

Monte Carlo simulations are used to show that the orientational ordering transition of planar quadrupoles on a triangular lattice is first order. The orientational ground state is a herringbone structure, and the transition lies in the universality class of a ($n=3\mathrm{}$, $d=2\mathrm{}$) Heisenberg model with face-oriented cubic anisotropy. The Monte Carlo results are related to recent low-energy electron-diffraction studies of ${\mathrm{N}}_2$ on graphite. Domainwall motion and the applicability of this model to herringbone phases of liquid crystals are also discussed.

In certain binary solutions the lower of the two liquid phases forms a layer which intrudes between the upper liquid phase and the vapor. The intruding layer's thickness (measured by ellipsometry) was between 70 and 400 Å. It varied approximately as $L^{-\frac{1}{3}}$ where $L$ is the height spanned by the upper liquid phase. This behavior was predicted by de Gennes using the idea that the long-ranged part of the intermolecular potential governs the layer's thickness. Deviations from $L^{-\frac{1}{3}}$ behavior occur near consolute points.

Elastic compliance data are reported for thin plates of ${\mathrm{SrTiO}}_3$. An analysis is presented in terms of two spatially separated phases, the one of low symmetry residing near the surface. From the thickness of the surface layers, a static correlation length $\xi ={\xi }_0{(\frac{T}{T_c}-1)}^{\nu }$, with ${\xi }_0=(0.35\mathrm{}\pm 0.02)$ μm and $\nu =-0.68\mathrm{}\pm 0.03$, is deduced.

The phase diagram of chiral smectic $p$-decyloxybenzilidene-$p^{\prime }$-amino-2-methylbutyl cinnamate in an external magnetic field parallel to the smectic layers has been determined by dielectric and optical measurements up to 14.5 T. The data indicate the existence of a Lifshitz point between the disordered smectic-$A$, the modulated smectic-$C^{*}$, and the homogeneously ordered smectic-$C$ phases. A reentrant $C^{*}$ phase has been found above 8.5 T. The critical field for the unwinding of the helix becomes extremely large as the $\lambda$ line is approached.

Conductance has been studied in metal-oxide-silicon field-effect transistor accumulation-layer samples in which it is possible to constrict the channel to small dimensions both perpendicular to the surface and perpendicular to the channel. A temperature-dependent conductance $\sigma ={\sigma }_0\exp [-{\left(\frac{T_0}{T}\right)}^n]$ is observed, where $n=\frac{1}{2}$ for small channel widths and $n=\frac{1}{3}$ for larger channel widths. It is believed that this behavior arises from a transition from one-dimensional to two-dimensional variable-range hopping in the sample.

High-resolution electron-energy-loss studies of ${\mathrm{H}}_2$ on Ag at ∼ 10 K reveal both rotational and rotational-vibrational excitations of molecularly adsorbed and condensed ${\mathrm{H}}_2$. The rotational losses for submonolayers of adsorbed ${\mathrm{H}}_2$ indicate unhindered rotational motion as well as a H-H internuclear separation which is within ∼2% of that of condensed ${\mathrm{H}}_2$. Adsorbed ${\mathrm{H}}_2$ is also converted from predominantly orthohydrogen to parahydrogen.

The first observations of magnetic energy fluctuations by light scattering are reported. The spectra observed in antiferromagnetic ${\mathrm{KNiF}}_3$ are strongly polarized, mildly $q$ dependent, but strongly temperature dependent near $T_{\mathrm{N}}=248.5\mathrm{}$ K. The observed line shapes exhibit two characteristic frequencies, one less than 0.6 GHz and the other between 5 and 15 GHz, depending on temperature.