We propose a percolation model for an exciton insulator-to-conductor transition. Experimentally, we investigate (at 2°K) the first singlet exciton in a binary organic alloy of naphthalene-perdeuteronaphthalene. A third component, ${10}^{-3\mathrm{}}$-mole-fraction betamethyl-naphthalene, serves as exciton-flow monitor, and the integrated relative fluorescence is derived. The ${}_{}{}^1{}_{}{}^{}B_{2\mathrm{u}}^{}{}_{}{}^{}$ Frenkel-exciton data are bound by the theoretical site-percolation curves for square and simple-cubic lattices.

Photocurrent measurements in GaAs-GaA1As superlattices grown by molecular-beam epitaxy enable us to observe simultaneously quantum states and associated anomalous conductance. The spectrum in the photocurrent shows a series of peaks of photon energies corresponding to transitions between quantum states in the valence and conduction bands. As a function of applied voltage, the photocurrent exhibits pronounced negative differential conductance when the potential-energy difference between two adjacent wells of the superlattice exceeds the bandwidth of the quantum states.

The friction coefficient $\eta$ for a hydrogen adatom on a transition-metal surface is calculated in the spirit of a model proposed by Newns. $\eta$ shows variation over a wide range of values as the adatom approaches the surface. The implications of our results to catalysis are discussed and related to recent work on the subject.

A theoretical analysis of Raman scattering at a slab (two-interface) configuration shows that the forward-scattering efficiency of the surface polariton can be comparable to that of the volume modes, in agreement with experiments. On the other hand, the backward-scattering efficiency of the surface polaritons at single-interface and two-interface configurations is relatively very small, which accounts for the inability to observe Raman scattering by surface polaritons at single interfaces.

We report the first observations of direct transitions between the split rotational levels of nearest-neighbor pairs of ($J=1\mathrm{}$) ortho-${\mathrm{H}}_2$ in otherwise pure para-${\mathrm{H}}_2$. Since the observed absorption lines are very narrow ($\Delta \nu \sim 0.1$ GHz at $\nu \simeq 75$ GHz), and since the theory of the pair spectrum seems tractable, we can in principle derive details of the intermolecular potential and of certain solid-state effects with much higher accuracy than was previously possible.

A model for pre-equilibrium nucleon emission is formulated in terms of diffusion of heat in nuclear matter. A strong asymmetry in the angular distribution of evaporation products is predicted.

Inelastic $\alpha$ scattering to the first excited ${\mathrm{O}}^{+}$ states in ${}_{}{}^{24}{}_{}{}^{}\mathrm{Mg}$ and ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$ has been measured at $E_{\alpha }=23.5\mathrm{}$ MeV and analyzed in a microscopic model. Strong evidence for the need of including $1p_{\frac{1}{2}}$ hole components in the wave functions of the lowest ${\mathrm{O}}^{+}$ states was found. Deduced $E0\mathrm{}$ matrix elements are in surprisingly good agreement with results from electron scattering.

The problem addressed is that of determining best values and confidence limits for the amplitude and phase of an unknown harmonic amplitude vector, given a single set of directions measured on a circle. The previously known solution, for the limiting case of small fluctuation, is inapplicable to the great majority of cosmic-ray anisotropy measurements. Solutions are given that are thought to be valid for all cases likely to occur in practice.

The decay asymmetries ($\mathcal{a}$) in polarized ${}_{}{}^{12}{}_{}{}^{}\mathrm{B}$ and ${}_{}{}^{12}{}_{}{}^{}\mathrm{N}$ have been measured as a function of $\beta$-ray energy ($E$). The coefficients ${\alpha }_{\mp }$, in a form of $\mathcal{a}=\mp P\left(\frac{p}{E}\right)(1+{\alpha }_{\mp }E)$, have been determined to be ${\alpha }_{-}\left({}_{}{}^{12}{}_{}{}^{}\mathrm{B}\right)=+(0.31\mathrm{}\pm 0.06)\%/\mathrm{MeV}$ and ${\alpha }_{+}\left({}_{}{}^{12}{}_{}{}^{}\mathrm{N}\right)=-(0.21\mathrm{}\pm 0.07)\%/\mathrm{MeV}$. The experimental value, ${\alpha }_{-}-{\alpha }_{+}=(0.52\mathrm{}\pm 0.09)\%/\mathrm{MeV}$. is larger than the prediction of conserved-vector-current theory, ${({\alpha }_{-}-{\alpha }_{+})}_{\mathrm{CVC}}\simeq 0.27\%/\mathrm{MeV}$, and in favor of the existence of the second-class induced-tensor current.

The muonic helium atom, $\alpha {\mu }^{-}{e}^{-}$, has been formed by stopping polarized negative muons in He gas at 14 atm with a Xe admixture of 2%, and observed through its characteristic Larmor precession frequency of 1.4 MHz/G. In addition a nonzero residual polarization of $P=0.06\mathrm{}\pm 0.01$ for ${\mu }^{-}$ stopped in pure He gas has been measured for the first time, which corresponds to a depolarization factor of 18±3.

Distributions in angle and in energy have been measured for ions transmitted through thin solid targets bombarded by various molecular-ion beams. The results, which differ markedly from those expected from simple Coulomb explosions, indicate that for the ion clusters created inside the target, the internuclear vectors tend to align with the beam direction. An explanation is suggested in terms of a wake potential generated behind each particle as it traverses the solid.

A previously developed theory of NMR in superfluid ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ is generalized to include relaxation effects. The physical mechanism envisaged is the relaxation of nonequilibrium Cooper-pair concentrations in the different spin bands. The results should describe both linear and nonlinear phenomena for all frequency regimes. The results for continuouswave linewidths disagree with those of Combescot and Ebisawa as to temperature dependence.

It is shown that shear Alfvén waves can be destabilized by resonance with high-energy "beam" ions near the magnetic axis in a tokamak, if the beam is radially nonuniform.

An Ohmic current has been inductively driven in the dc octopole. Resulting flux surfaces are similar to those of a tokamak with poloidal divertors. Quiet and reproducible magnetic-probe traces indicate that the plasma has obtained a stable equilibrium. For the safety factor on the axis less than unity, a current-disruptive instability is observed to redistribute current density from the peak to the wings of the spatial distribution.

We have observed a peak in the conductivity-versus-gate-voltage curves for $n$-channel silicon metal-oxide-semiconductor field-effect-transistor devices containing large oxide-charge densities, $4.7\times {10}^{11} {\mathrm{cm}}^{-2\mathrm{}}<~N_{\mathrm{ox}}<~1.1\times {10}^{12} {\mathrm{cm}}^{-2\mathrm{}}$. This peak is interpreted as an oxide-charge-induced impurity level within the lowest sub-band tail. It is found that the impurity level contains one electron state for each oxide charge situated at the Si-Si${\mathrm{O}}_2$ interface.

A recent classical theory for spin glasses of Edwards and Anderson is modified for the quantum mechanical case. One obtains a cusp in the susceptibility $\chi \mathrm{}\left(T\right)\mathrm{}$ and in the specific heat $c_H\mathrm{}\left(T\right)\mathrm{}$ at a characteristic temperature $T_f$ for finite spin $S$ and zero slope for $S\to \infty$. The specific heat is proportional to $T$ for $T\to 0$, whereas it tends to the Boltzmann constant $k$ in the classical limit.

We have investigated the number, $N_d$, of displaced lattice atoms in room-temperature implantation in Si of polyatomic-carbon-ion beams (${{\mathrm{C}}_n}^{+}$ and ${\mathrm{C}}_{n-x}{{\mathrm{O}}_x}^{+}$) for several values of $n$, using the backscattering-channeling-effect technique. For each ion species the same energy (8.8 keV) per carbon and the same atomic fluence and flux were used. $N_d$ increases rapidly with increasing $n$, indicating that the deposited-energy density within the collision cascade is a key factor in determining how much damage is created and retained.

The temperature and frequency dependence of the nuclear spin-lattice relaxation in amorphous ${\mathrm{B}}_2$${\mathrm{O}}_3$ has been investigated and a $T_1$ minimum with unusual properties has been discovered at ∼300 K. The relaxation is described in detail by Raman-like processes involving the two-level disorder modes and phonons characteristic of glassy materials.

A field-dependent charge-carrier trapping process has been observed in the photoconduction mechanism of phenanthrene-pyromellitic acid dianhydride. This effect can be observed at electric field strengths exceeding ${10}^3$ V/cm. A calculation on the basis of a hopping model shows that field-induced charge-carrier trapping occurs in a three-dimensional system at critical field strengths well above ${10}^6$ V/cm. In a one-dimensional model, however, the critical field strength can be as low as 1 V/cm.

Coordinates of atoms in the first and second layers of Si(111) 2×1 have been determined which yield surface energy bands in good agreement with experiment. Two surface resonances in angular photoemission data previously assigned to dangling-bond surface states are identified as arising from the maximum in the lower dangling-bond band and from a saddle point in the upper back-bonding band.