If a band is narrow and is not nearly empty nor nearly full, the Hubbard lattice is shown to be an insulator. The lattice containing about one electron per atom will be a Hubbard-type insulator with a wide energy gap but if the number of electrons decreases or increases beyond a certain range, the gap is suddenly reduced, yielding a small-gap insulator of a completely new type.

Recombination-enhanced annealing of defects in semiconductors has been observed directly for the first time. The defects were produced in GaAs by 1-MeV electron irradiation and observed by transient-junction-capacitance techniques. The data clearly relate the enhanced defect annealing rate to electron-hole recombination processes at the defect.

Brillouin scattering experiments have revealed a new structural phase transition in PrAl${\mathrm{O}}_3$ at 118 K, in which strain is the sole order parameter. This fact, together with the second-order nature of the transition, suggests that it belongs to a class of structural transitions first delineated nearly ten years ago by Anderson and Blount.

We report electronic transport properties in a GaAs-AlAs periodic structure known as a "superlattice" prepared by a molecular-beam epitaxy. Its differential conductance in the superlattice direction first gradually decreases, followed by a rapid drop to negative values, then, at high fields, exhibits an oscillatory behavior with respect to applied voltages. This observation is interpreted in terms of the formation and expansion of a high-field domain. The voltage period of the oscillation provides the energy of the first-excited band which is in good agreement with that predicted by the theory.

The polarization in the region of Coulomb-nuclear interference has been determined from the $\gamma$-ray correlation following excitation of the 856-keV $2^{+}$ state in ${}_{}{}^{56}{}_{}{}^{}\mathrm{Fe}$ by 43-MeV ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ ions. Elastic and inelastic scattering cross sections were also measured. Dramatic interference effects are observed in the transition amplitudes which can be reproduced by coupled-channel distorted-wave calculations.

Yields of 156 nuclides were measured radiochemically to delineate the mass distribution in the reaction of 605-MeV ${}_{}{}^{84}{}_{}{}^{}\mathrm{Kr}$ with thick ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$ targets. The yields are consistent with decomposition of the mass distribution into five components: transfer products (700±120 mb), "quasi-Kr" products centered at $A\approx 85$ (470±70 mb) and the products from symmetric fission of their complements (420±60 mb), products from low-energy fission of $Z\approx 92$ nuclides (200±40 mb), products from complete fusion-fission (55±15 mb), and unexplained neutron-deficient yields at $A\approx 195$ (∼40 mb).

The $\rho -\omega$ interference patterns observed in the reactions ${\pi }^{-}p\to {\pi }^{+}{\pi }^{-}n$ and ${\pi }^{+}n\to {\pi }^{+}{\pi }^{-}p$ are used to measure the relative phases between $\rho$ and $\omega$ production amplitudes. The interference is observed to be largely constructive (destructive) for the ${\pi }^{-}-({\pi }^{+}-)$ induced reaction in both natural- and unnatural-parity exchange amplitudes. This sign of the interference agrees with that predicted from exchange-degenerate $\pi -B$ trajectories, but disagrees with the corresponding prediction for the natural-parity amplitudes using exchange-degenerate $\rho -A_2$ trajectories.

Pion charge exchange within the nucleus is shown to be an important effect in antiproton absorption experiments. It must be incorporated into any analysis which attempts to use absorption data to deduce the relative abundance of neutrons and protons at the annihilation site.

The hyperfine interaction of K in KAr has been measured in several vibrational states of KAr. The results yield a value for the hyperfine shift $\alpha$ as a function of internuclear separation $R$ given by $\alpha =-115(R-R_0)$ kHz, where $R$ is in bohrs and $R_0=12.0\mathrm{}$ bohr, valid over the range $10.25 \mathrm{bohr}<R<\mathrm{}12.0\mathrm{} \mathrm{bohr}$.

We have used the dynamic Freedericksz-deformation method to measure in detail the temperature dependence of the twist viscosity ${\gamma }_1$ in N-$p$-cyanobenzylidene-$p$-octyloxyaniline near the nematic-smectic-$A$ transition. We have demonstrated that the intrinsic contribution ${{\gamma }_1}^0$ can be determined independently by extrapolating data on doped samples. Our analysis shows ${\gamma }_1$ to diverge with a critical exponent $\zeta =0.37\mathrm{}\pm 0.05$, in agreement with theoretical expectations.

Elasticity theory is applied to nematic liquid crystals with particular boundary conditions. The calculus of variations is used to derive the torque equations, the solutions of which are found by relaxation methods on a digital computer. These solutions are used to draw pictures of the molecular arrangements and with the aid of Maxwell's equations to predict the observed optical patterns, including the case of magnetic fields applied along the symmetry axis.

Conduction-electron $g$ tensors depending on electron momentum can (through the cyclotron motion of the electron) cause transitions between spin Zeeman levels which broaden and shift the conduction-electron spin resonance. Tensor quasiparticle interactions are capable of similar effects. The relaxation time and exchange parameter controlling motional and exchange narrowing of the conduction-electron spin resonance are precisely defined.

The bond-orbital model is generalized to the treatment of elastic shear constants by assuming a reduction in covalent energy $V_2$ proportional to the square of the angular misalignment of the two hybrids forming a bond. The result, proportional to $[{(1-{{\alpha }_p}^2)}^{\frac{1}{2}}-S]\times V_2$, gives a good account of the shear constants in all zinc-blende and wurtzite crystals for which they are known, except for the noble-metal halides.

A clear splitting, present only at low temperatures, is observed where the unperturbed phonon curve crosses the C${\mathrm{N}}^{-}$ $T_{1\mathrm{u}}$ and $T_{2\mathrm{u}}$ levels in a KCl crystal doped with 6×${10}^{19}$ C${\mathrm{N}}^{-}$ ${\mathrm{cm}}^{-3\mathrm{}}$. Calculated dispersion curves require coupling constants which agree within experimental uncertainty with those determined ultrasonically. The absence of the splitting at high temperatures can be explained because the phonon gain and loss processes in the interaction with the C${\mathrm{N}}^{-}$ level cancel almost completely.

We report the observation of exciton-induced photoemission resulting from electronic energy transfer from "free" exciton levels to impurity states in solid Xe and Kr. A diffusion length of $l_0\simeq 75$ Å for Wannier excitons in solid Xe was deduced, corresponding to a diffusion coefficient of $D=0.5\mathrm{}$ ${\mathrm{cm}}^2$ ${\sec }^{-1\mathrm{}}$. The energy of the bottom of the conduction band is $V_0=-0.46\mathrm{}\pm 0.1$ eV for Xe and $V_0=-0.23\mathrm{}\pm 0.1$ eV for Kr.

The conductivity of single crystals of tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ) was measured over the temperature range 40-300 K. The maximum low-temperature conductivity parallel to the $b$ axis ranged from 12-35 times the corresponding room-temperature value. The temperature dependence of the intrinsic conductivity (after residual resistivities were excluded) was the same for all crystals measured and was fitted by a $T^{-2.33\mathrm{}\pm 0.14}$ dependence for $60<~T<~300$ K.

The analysis of spin-echo spectra of V, Mn, Co, Ru, Rh, Os, and Ir in ${\mathrm{Fe}}_3$Si shows that these impurities unexpectedly enter one of the two available Fe sites in a selective manner. Those impurities to the left of Fe in the periodic table prefer the Fe site with eight first-neighbor Fe atoms. Those beneath and to the right of Fe in the periodic table enter the Fe site with four Fe and four Si first neighbors.

Small-angle quasielastic neutron scattering measurements have revealed an intense magnetic component at low temperatures in amorphous Tb${\mathrm{Fe}}_2$ and Y${\mathrm{Fe}}_2$, in addition to a weak critical divergence at $T_c$ in Tb${\mathrm{Fe}}_2$. These observations are interpreted as evidence that spatial fluctuations remain "frozen in" below the Curie point of Tb${\mathrm{Fe}}_2$, and that only short-range order is achieved in Y${\mathrm{Fe}}_2$.

The critical behavior occurring at displacive phase transitions in anisotropically stressed perovskite crystals is examined. It is shown that, under various conditions, the asymptotic critical behavior may be of Ising, $\mathrm{XY}$-model, or Heisenberg type. The existence of a "spin-flop"-like transition (at zero stress) between two differently ordered phases is predicted. An explanation of the discrepancy between the measured exponents and those predicted theoretically is proposed, and several new experiments are suggested.

Semiempirical model-independent formulas for single-particle energies of neutrons and protons in bound nuclei have been obtained as functions of nuclear parameters $A$ and $Z$ for given states specified by $\mathrm{nlj}$. These formulas are almost as convenient as the harmonic oscillator energy formulas to use. The single-particle energies computed from these formulas have been compared with the experimental data and found in good agreement for occupied states.