The general role of interparticle interactions is considered for percolation problems in liquids. With the aid of a diagrammatic formulation of the Percus-Yevick approximation, we analyze the dependence of connectedness (percolation) on correlation (interaction). This idea is illustrated by the analytical solution of the Percus-Yevick equation for a model of percolation in liquids, revealing that excluded volume (for example) may either enhance or suppress percolation.

We show with both experiment and theory that a Bragg reflection from a crystal surface excited under grazing-incidence conditions is displaced from its reciprocal-lattice point. This new phenomenon allows for the first time a confirmation of the penetration depth of the scattered intensity on the scale of a lattice constant. The experiments presented show the possibility of a controlled depth probing of near-surface atomic correlations.

The zero-temperature ground-state properties of five spin-glass models have been studied as a function of the cooling rate $r=\frac{-\Delta T}{t}$. Here $\Delta T$ is the temperature decrement and $t$ is the time (in Monte Carlo steps) at each temperature $T$. For the 2D $\pm J$ and Gaussian models, $E\left(r\right)=E_0+C_1{r}^x$, where $x\cong 0.25$, while for the 3D $\pm J$, a two-layer $\pm J$, and infinite-range models, $E\left(r\right)=E_0-C_2{\left(\mathrm{inr}\right)}^{-1\mathrm{}}$. We speculate that this difference is related to the fact that the 2D models are not $\mathrm{NP}$-complete while the other three models are.

Low-energy (10-keV) recoil scattering is proposed as a tool for measurement of adsorbate positions on monocrystalline surfaces. This technique, which makes use of high-energy-recoil adsorbate atoms (energy ≥ 1 keV), supplies real-space structure information. Its potentialities are illustrated in a study of the adsorption system H on (111) sites of Pt. By exploiting surface channeling of incident 10-keV Ne ions on vicinal Pt surfaces we show unambiguously that H occupies fcc-type adsorption sites on Pt(111) terraces.

For an ideal Euler fluid in a rigid porous medium, the effective hydrodynamic interaction known as inertial drag is manifest in an acoustic scattering correction $n$ and in the induced fluid velocity $V_s={}_{\lambda }{}^{}{}_{}{}^{}V$ when the medium is accelerated from rest to a steady velocity $V$. Theoretically $\frac{n^2=1\mathrm{}}{(1-\lambda })$, but measurements with Doppler-shifted sound in an annular acoustic resonator deviate significantly from this prediction. In this paper we show that the discrepancy is due to subtle acoustic properties of an annular resonator and that the corrected Doppler-shift expression may be used to determine properties of the porous medium.

Ions dissolved in nematics screen the flexoelectric polarization induced by distortions. I show that for long-range distortions (long compared to the Debye length), the free energy associated with these effects varies as the fourth power of the distortion wave vector. This free energy has a magnitude at least ${10}^5$ times larger than elastic energy of identical order. The equilibrium equations involve, besides a modified stress tensor, a new tensor representing the force conjugated to the second gradients of the deformation.

The tunneling-current density is computed in the vacuum region between two planar metal electrodes, on each of which is an adsorbed atom. Scanning of one atom (taken as the tip) past the other (the sample) permits plotting of tip displacement versus lateral separation for constant current. The calculation shows the extent to which scanning-tunneling-microscope images of an individual atom are visible. It confirms that for low bias, the microscope images the Fermi-level local density of states of the sample at the position of the tip.

We consider densely packed assemblies of icosahedra, such that icosahedral bond-orientational order is enforced throughout the sample. The peaks in the calculated diffraction patterns may be put in a one-to-one correspondence with the electron and x-ray diffraction patterns from icosahedral Al-Mn alloys, allowing the determination of the packing-unit size. We show that these maxima are not Bragg diffraction peaks, but have an intrinsic width, and may be understood as resulting from the interference between two or more characteristic lengths, as originally discussed by Hendricks and Teller.

Exact finite-size effects for the interfacial free energy of the planar Ising ferromagnet are obtained. They are compared with a scaling function estimated by Mon and Jasnow using Monte Carlo simulation.

The interplay between disorder and effective electron-electron interaction in impure superconductors is studied within a BCS-Bogliubov model. It is shown to yield a square-root anomaly in the temperature dependence of $H_{c2\mathrm{}}$ at the critical point. The resulting reentrant phase diagram is qualitatively discussed for ${\mathrm{Nb}}_3$Ge and for granular aluminum. Though the effect is of order $\frac{T_c}{E_F}$, it should be easily visible in ${\mathrm{Nb}}_3$Ge or other $A$ 15's, while for Al a precision experiment would be necessary.

We show that interaction effects give rise to a cutoff $\frac{1}{{\tau }_{\mathrm{ph}}}$ in the "diffusion," the basic propagator of the disordered interacting electron gas. We calculate $\frac{1}{{\tau }_{\mathrm{ph}}}$ to lowest order in the disorder but to all orders in the interaction, and exhibit its relation to the scaling variables of the disordered-interacting-electron problem. This framework provides an explanation for the anomalous enhancement of inelastic processes which is observed experimentally in systems close to the metal-insulator transition.

Periodic instabilities of frequency $f_0$ in an electron-hole plasma in Ge are excited with an external perturbation $V_1$ sin ($2\pi f_{1}t$). Varying $V_1$ and $f_1$ results in quasiperiodic, mode-locked, and chaotic states comparable to those expected theoretically. Increasing $V_1$ while keeping the ratio $\frac{f_0}{f_1}$ fixed at the golden mean (1.6180...), we observe a universal transition to chaos, in quantitative agreement with theory. Driving the instability simultaneously at two incommensurate frequencies, $f_1$ and $f_2$, we observe transitions between three-frequency quasiperiodicity, two-frequency mode locking, and chaos.

The energy dependence of the self-energy correction $\Delta \mathrm{}\left(E\right)\mathrm{}$ to local-density-functional pseudopotential theory has been determined for Si and Ge from -15 to 16 eV above the Fermi energy $E_F$, and compared to recent calculations. The real part of $\Delta \mathrm{}\left(E\right)\mathrm{}$ was determined from the shift, relative to calculations, of detailed x-ray bremsstrahlung isochromat spectra, which exhibited many new higher-energy features, and x-ray photoemission spectra; the imaginary part was estimated from escape-depth data and the widths of conduction-band features. The real part changes abruptly at $E_F$ and is otherwise roughly constant, even up to the plasmon energy.

We report the first direct observation of nonthermal photoexcited carrier distributions in GaAs quantum-well structures. We present experimental studies which show that these distributions thermalize within 200 fs. In addition we are able to show that near the band edge the effect of long-range Coulomb screening on the bleaching of the two-dimensional-exciton resonances is much weaker than that of the Pauli exclusion principle.

A model of two mutally incommensurate, interacting, dynamical, many-body systems is presented and solved. Quasiperiodic forms are shown to describe both the dc properties and the complete set of linear excitations. By use of one system to represent the crystal lattice and the other a charge-density wave (CDW), all the recently discovered, and as yet unexplained, electromechanical properties of CDW conductors are shown to occur in this model. The internal degrees of freedom of the CDW are shown to be of central importance.

We present results on the transport properties of the 2D electron gas in a narrow channel formed by the split gate of a GaAs-AlGaAs heterojunction field-effect transistor. There are both quantum-interference and interaction corrections to the conductivity. We find that the temperature dependence of the phase relaxation length is in agreement with a recent theory based on scattering by electromagnetic fluctuations. Beyond the regime of quantum interference the conductivity varies with temperature as $T^2$.

A new method is developed to measure the spin-conversion time ${\tau }_{\mathrm{con}}$ in low-tunnel-frequency systems. The experiment uses field cycling and consists of three steps: saturation of the tunnel reservoir by Zeeman-tunnel resonance; waiting, away from resonance, during a variable time interval $t_W$; and measurement of the decreased tunnel temperature $T_T$, by use of a saturated Zeeman reservoir. A plot of $T_T$ vs $t_W$ yields ${\tau }_{\mathrm{con}}$. ${\tau }_{\mathrm{con}}$ of C${\mathrm{H}}_3$ rotors in copper acetate is measured at 15, 12.5, and 6 K and turns out to be extremely long, at least ${20T}_1$. This discrepancy is discussed and compared with the one found in lithium acetate.

By manipulating the temperatures at the ends of a sample we show that sliding conductivity can occur without voltage oscillations. This is because the sliding region is kept away from the contacts. By expanding the sliding region we recover oscillations when it touches the contacts. The crucial role of the contacts in noise generation is thereby demonstrated.

A dynamical meaning is given for the phase $\Gamma$ recently found in a quantum adiabatic process. A noncanonical system, specifically a spin system coupled with a proper intrinsic system, is considered by use of a path integral in a SU (2) coherent-state representation. It is shown that $\Gamma$ appears as an addition of topological nature to the conventional action function, which leads to a novel form of the semiclassical quantization rule including the phase $\Gamma$.

The absorption of energy by highly excited electrically polarized hydrogen atoms in intense microwave fields is studied experimentally, and theoretically in terms of a one-dimensional quantum model. The quantum calculations reveal a sudden onset of basis-set instability that correlates well with the threshold for classical chaos.