Collisions of kiloelectronvolt ${\mathrm{Ne}}^{+}$ on Mg and ${\mathrm{Ar}}^{+}$ on Y yield scattered-ion fractions as high as 70% and 38%, respectively, and ${}_{}{}^{1,3}{}_{}{}^{}P\to {}_{}{}^1{}_{}{}^{}S$ resonance radiation from the excited neutral projectile atoms in the vacuum-ultraviolet range 30-200 nm. These data, along with those from the oxidized and hydroxylated surfaces, show that electron promotions within the molecular orbitals of the quasidiatomic molecule formed during the close encounter are a significant, if not dominating, process in kiloelectronvolt ion-surface collisions.

Low-temperature time-resolved Faraday-rotation measurements in ${\mathrm{Cd}}_{1-x}{\mathrm{Mn}}_x\mathrm{Te}$ allow direct observation of the time evolution of the interaction between an electron spin and the magnetic moments of dilute impurity ions. The experiment displays real-time formation and evolution of magnetically organized states suggestive of the bound magnetic polaron.

We report the observation of low-frequency Raman scattering by spin-density fluctuations in heavily doped $n$-type Ge. This scattering resonates at the $E_1$ gaps (∼2.2 eV). The resonant enhancement and the absolute values of the scattering efficiencies are estimated and compared with experiment.

The Hall conductivity (${\sigma }_{\mathrm{xy}}$) on a lattice with disorder has been calculated in torus geometry to test the quantum Hall effect for finite magnetic fields $H$. It is shown that the quantum Hall effect persists independent of degree of disorder even away from the quantum ($H\to \infty$) limit, and that the Hall effect characteristic of periodic systems is not destroyed by disorder. The value of ${\sigma }_{\mathrm{xy}}$ for finite systems has a deviation from quantization, which depends on the Aharonov-Bohm flux $\Phi$ and disappears as the size is increased.

Exact calculated results for the nickel density of electron states, as would be measured in valence-band photoemission, are presented. A realistic local-density-approximation one-electron structure, sampled at four wave vectors, and an intrasite electron-electron interaction of full generality are used. This new approach to the Ni photoemission problem yields accurately the experimental bandwidth, exchange splitting, satellite position, and relative intensities. Physical conclusions about the true Ni many-body states are drawn.

Striking changes are observed in the characteristic features of the quantum Hall effect when the width of the conducting channel, fabricated on high-mobility ($\mu \gtrsim 1\times {10}^5$ ${\mathrm{cm}}^2$/V·sec) $\mathrm{GaAs}/{\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ heterostructures, is reduced to several microns. The quantum oscillations in ${\rho }_{\mathrm{xx}}$ vs $B$ appear saw-toothed and the quantized Hall plateaus become highly asymmetric about complete Landau-level fillings. The ${\rho }_{\mathrm{xx}}$ peaks are shifted to lower $B$ and drastically reduced from their wide-channel values. These results suggest possible dimensional crossover and enhancement of the electron-electron interaction in the quantum Hall regime.

A general formalism is outlined for the calculation of the transport coefficients of a normal-metal network in the weak-localization regime. Simple circuits such as loops and ladders are used to illustrate our approach. A closed expression for the magnetoresistance of an infinite regular network is derived. We find that, in contrast with superconducting networks, no fine structure due to interference effects between adjacent loops is expected. Our results agree very well with the recently observed oscillations in normal-metal networks.

We report a new quantum-type photoconductivity in a forward-biased $p^{+}-n$ junction with a superlattice in the $n$ layer. This novel phenomenon is characterized by several striking features: a high photocurrent gain (≃7×${10}^3$), accompanied by a blue shift in the spectral response and a reversal in the direction of the photocurrent, when the forward bias exceeds the built-in potential. Photoconductive gain is caused by the large difference in the tunneling rates of electrons and holes through the superlattice layers, due to their large mass difference (effective-mass filtering). This is the first time that photoconductive gain is observed in a $p-n$ junction.

The complex permittivity of a three-component water-in-oil microemulsion was measured as a function of frequency, temperature, and volume fractions. A conductivity threshold, associated with a permittivity maximum, was observed and is explained in terms of percolation theory. The experimental value of the critical exponent of the frequency dependence of the permittivity, $u=0.62\mathrm{}\pm 0.02$, is in good agreement with theoretical predictions.

A systematic program of experiments designed to test the tunneling theory of charge-density-wave conduction has been carried out over a wide temperature range below three different Peierls transitions in Nb${\mathrm{Se}}_3$ and Ta${\mathrm{S}}_3$. The results support the hypothesis that charge-density waves depin by quantum tunneling, and are in detailed and consistent agreement with predictions of the model described in the companion paper.

A consequence of photon-assisted-tunneling theory of charge-density waves in quasi one-dimensional metals is that the ac conductivity $\sigma \left(\omega \right)\simeq {\sigma }_b\exp (-\frac{{\omega }_s}{\omega })$ scales with the dc current $\frac{I_{\mathrm{dc}}\mathrm{}\left(E\right)\mathrm{}}{(E-E_T)}={\sigma }_b\exp (-\frac{E_0}{E})$. In a revised theory, ${\omega }_s=\left(\frac{c_0}{v_{\mathrm{F}}}\right){\omega }_p$, where $c_0$ is the phason velocity and ${\omega }_p$ the pinning frequency. Expressions are derived for ${\omega }_s$ and $E_0$. Relevant lengths are the Lee-Rice domain length, $L_d=\frac{\pi c_0}{{\omega }_p}$, the free path, $L\sim \frac{c_0}{{\omega }_p}$, and the current-current correlation length, $L_I\sim \left(\frac{v_F}{c_0}\right)L_d$.

A randomly diluted lattice of Josephson tunnel junctions is used as a model to generate a replicated Landau-Ginzburg field theory describing the properties of diluted superconductors near the percolation threshold. The model predicts Meissner and Abrikosov phases and a spin-glass phase with frozen currents, a nonvanishing Edwards-Anderson order parameter, and power-law decay of correlations of the magnetic field.

A model is used to show that a Josephson-junction array in an irrational magnetic field undergoes a glass transition for finite cooling rate. At zero temperature the resultant glassy state possesses a nonzero critical current. The low-temperature behavior of the system can be modeled by a spin-wave theory. The relevance of these results for real experiments on arrays is discussed.

Low-temperature magnetization data for amorphous ${\left({\mathrm{Fe}}_p{\mathrm{Ni}}_{1-p}\right)}_{80}{\mathrm{P}}_{20}$ are analyzed by use of the cluster-number distribution expected for a percolating ferromagnet. The method yields the percolation probability $P\left(p\right)\mathrm{}$ and provides a natural explanation for the difficulty in saturating dilute magnets as the percolation point is approached.

A continuous-time random-walk theory of diffusion-limited aggregation yields perimeter occupancy probabilities. Scaling relates the fractal dimension $D$ to the cluster-tip occupancy probabilities. These agree with the analytic probabilities near cusps of a lattice-symmetric array of traps. On a two-dimensional square lattice $D=\frac{5}{3}$, whereas $D=2\mathrm{}$ for the Eden model, and $D=\frac{4}{3}$ for the $\eta =2\mathrm{}$ dielectric breakdown model. $D$ is not universal: $D=\frac{7}{4}$ for the two-dimensional triangular lattice. The square and triangular lattices bracket (±2.5%) Meakin's large off-lattice simulations ($D=1.71\mathrm{}$).

As the low-frequency power input to a nonlinear medium is increased, the power spectrum of wave motion converges to a universal distribution proportional to the inverse of the frequency. Experiments which can probe the transition to $\frac{1}{f}$ noise are proposed.

Canonical discrete-time operator difference equations are introduced as an alternative approach to the numerical solution of a quantum field theory. We apply these techniques to the solution of the operator Heisenberg equations of motion describing the problem of quantum-mechanical tunneling. Our numerical solutions accurately depict the time evolution of $〈q^n〉$ and of a local probability measure.

With reliance on Regge calculus and the Regge-Einstein action, models of four-dimensional Euclidean gravity are simulated numerically. The scale $a=l_0$ is set by fixing the expectation value of a length. Monte Carlo calculations support the following results: In the infinite-volume limit, canonical dimensions are realized and a finite action density is obtained.

When near-integrable Hamiltonians systems are perturbed by dissipation, then the stable orbits become simple attracting sinks, the Kolmogorov-Arnol'd-Moser tori are destroyed, and persistent chaotic motion disappears. We determine analytically the mean lifetime, the quasistatic distribution, and the fraction trapped into the various sinks for a dissipatively perturbed area-preserving twist map.

The path-integral method is used for determination of the quantum corrections to the free energy of nonlinear systems. All quantum effects of the harmonic part of the potential are considered and a variational principle is used to account for the quantum corrections due to the anharmonic part. Correct renormalized frequencies are obtained at any temperature and an effective potential to be inserted in the configurational integral is found. A new general expression for the partition function at any temperature in the low-coupling limit is obtained.