The critical behavior of (111) surfaces in an fcc Ising ferromagnet is studied by means of the cluster variation method in the tetrahedron approximation. The critical value of the surface interaction ${\mathrm{J}}_{\mathrm{sc}}$ leading to a surface transition temperature larger than in the bulk is equal to 1.778${\mathrm{J}}_{\mathrm{b}}$. We find that the phase transition at the surface is of first order for a finite range of ${\mathrm{J}}_{\mathrm{s}}$>${\mathrm{J}}_{\mathrm{sc}}$. Results for the phase diagram and the temperature dependence of the magnetization and of the magnetic susceptibility are presented.

First-principles total-energy pseudopotential and all-electron calculations predict (001) (GaAs${)}_1$(AlAs${)}_1$ and (CdTe${)}_1$(HgTe${)}_1$ superlattices to be intrinsically unstable towards disproportionation into compounds. This instability is traced to unfavorable charge redistribution in the system.

Valence-band discontinuities, Δ${\mathrm{E}}_{\mathrm{v}}$, of the heterojunctions formed by combination of CdTe, HgTe, and ZnTe, grown in situ by molecular-beam epitaxy, are investigated by core-level x-ray photoemission spectroscopy. They are found to be insensitive to interface Fermi-level position, commutative, and transitive. Values measured for Δ${\mathrm{E}}_{\mathrm{v}}$ are 0.36±0.05 eV, 0.10±0.06 eV, and 0.25±0.05 eV for HgTe-CdTe(1¯ 1¯ 1¯), ZnTe-CdTe(1¯ 1¯ 1¯), and HgTe-ZnTe(1¯ 1¯ 1¯) heterojunctions, respectively. They are discussed in terms of linear-theory predictions.

Composite In/${\mathrm{InO}}_{\mathrm{x}}$ films are reproducibly fabricated with two classes of microstructure, islanded at low oxide content and amorphous-composite at high oxide content. The islanded films, near the percolation threshold, exhibit a two-dimensional depression of the mean-field transition temperature proportional to normal-state sheet resistance. The corresponding behavior in the amorphous composite films with similar resistivities is three-dimensional with a quadratic dependence on normal-state resistivity. Consistency with respective theory for each case is discussed.

Two-photon photoemission is observed from unoccupied and occupied states of atomic oxygen adsorbed on Cu(111). The unoccupied state is found as an intermediate in the two-photon process with an energy of 2.79±0.07 eV above the Fermi level. The occupied state is excited by two photons and has a binding energy of 2.13±0.04 eV below the Fermi level.

The first temporal analysis of spontaneous pulsing of dc biased, p-i-n diodes in RC circuits is reported. Time intervals ${\mathrm{T}}_{\mathrm{n}}$ between successive pulses in spike trains with durations up to 30 h are analyzed as a function of n. Temporal patterns depend on bias voltage and are characterized by symmetric sets of Farey-fraction frequencies associated with remarkably narrow peaks in power spectra. Resolution permitted identification of 115 of the 129 Farey fractions with period ≤40. A rule involving fractions appearing in this system and other nonlinear systems is proposed.

Susceptibility and magnetic-moment measurements from 1.9 to 35 K in magnetic fields up to 1.5 T in powder samples of ${\mathrm{La}}_2$${\mathrm{CuO}}_{4\mathrm{-}\mathrm{y}}$:Ba are reported. The diamagnetism observed in the zero-field–cooled state is considerably larger than under field cooling. The former is metastable like the magnetic moment induced after switching the field off. These observations indicate the existence of a superconductive glass state.

The enhancement of superconductivity by a small concentration of excitonic negative-U centers is studied by a new method which combines diagrammatic perturbation theory and Monte Carlo simulation. Results are given which show how the impurity parameters should be selected in order to obtain the maximum increase in the superconducting transition temperature ${\mathrm{T}}_{\mathrm{c}}$.

Uniaxial stress induces striking effects on the polarized emission from the 2D Fermi sea in modulation-doped GaAs quantum well structures. These are interpreted within the effective-mass theory in terms of changes in valence-band mixing caused by modifications to the subband spacing and departure from tetragonal symmetry. Calculation of the electron-hole recombination together with a phenomenological approximation for the Fermi-sea shake-up processes successfully accounts for the luminescence spectra and their stress dependence.

Electromagnetic fields with photon-number fluctuation reduced below the standard quantum limit have been generated in a constant-current–driven semiconductor laser. The generation is based on a new principle of high impedance suppression for pump-amplitude fluctuation in a highly saturated laser oscillator. The observed noise level is 7.3% (31% after correction for detection quantum efficiency) in power below the standard quantum limit in the entire frequency range between 350 and 450 MHz.

Neutral-beam heating of plasmas in the Tokamak Fusion Test Reactor at low preinjection densities [$n_e$(0)≃${10}^{19}$ ${\mathrm{m}}^{\mathrm{-}3}$] were characterized by $T_e$(0)=6.5 keV, $T_i$(0)=20 keV, $n_e$(0)=7×${10}^{19}$ ${\mathrm{m}}^{\mathrm{-}3}$, ${\tau }_E$=170 msec, ${\beta }_{\mathrm{theta}}$=2, and a d(d,n${)}^3$He neutron emission rate of ${10}^{16}$ ${\sec }^{\mathrm{-}1}$. The ion temperature and the deuterium-fusion neutron yields were significantly higher than for previous tokamak experiments. The low initial densities were achieved by operation of the Tokamak Fusion Test Reactor with low plasma currents (≤1 MA) and by extensive limiter conditioning.

The phase separation of a binary liquid mixture contained in a rigid porous medium has been investigated by study of the light scattered at 90° from a porous glass rod containing a critical mixture of n-${\mathrm{C}}_6$${\mathrm{H}}_{14}$+n-${\mathrm{O}}_6$${\mathrm{F}}_{14}$. Evidence is found for phase separation at a temperature markedly below the critical temperature of the bulk mixture. Temporal concentration fluctuations in the rod are not observed. The separated phases remain metastable at temperatures well above the bulk critical temperature. These phenomena are in part consistent with a description in terms of a random field.

In situ observation of the commensuration process of the long-period superlattice in an Ag–24-at.%-Mg alloy annealed at 219 °C for 39 d was made by transmission electron microscopy at 325 °C. The commensuration process takes place in the long-period superlattice by the elimination of discommensuration lines in the form of the motion of jellyfish patterns just as in 2H-${\mathrm{TaSe}}_2$.

By means of inelastic neutron scattering we investigated the localized H vibrations in β-${\mathrm{V}}_2$H (octahedral H sites). Using a unique single-domain single crystal, for the first time we determined the directions of the fundamental excitations of a hydrogen atom in a metal. Overtones of these vibrations were observed up to the fourteenth order. They establish a well-defined H potential up to more than 1 eV. The sequence of the excitations could be described in terms of an empirical potential.

A 2D Ni-Cr quasicrystal displaying a twelvefold symmetry consists of a dodecagonal quasiunit cell (QUC) which is an intergrowth of ${\mathrm{Zr}}_4$${\mathrm{Al}}_3$ and ${\mathrm{Cr}}_3$Si structures. If the QUC is represented by a geometric point, then this quasicrystal can be described by a quasilattice derived by a projection method.

${\mathrm{La}}_{1.85}$${\mathrm{Ba}}_{0.15}$${\mathrm{CuO}}_4$ exhibits high-${\mathrm{T}}_{\mathrm{c}}$ superconductivity with an onset temperature near 33 K as measured by resistivity and magnetic susceptibility. The nonsuperconducting end-member compound, ${\mathrm{La}}_2$${\mathrm{CuO}}_4$, which has an orthorhombic, Cmca structure, exhibits semiconducting behavior. The orthorhombic distortion is proposed to result from a Peierls 2${\mathrm{k}}_{\mathrm{F}}$ instability or a soft zone-boundary phonon mode. The role of Ba is to suppress the instability and stabilize a higher-symmetry tetragonal, I4/mmm structure which is metallic and superconducting.

The results of electronic-structure calculations for tetragonal ${\mathrm{La}}_2$${\mathrm{CuO}}_4$ provide insight concerning the origin of high-temperature superconductivity in the ${\mathrm{La}}_{2\mathrm{-}\mathrm{y}}$${\mathrm{X}}_{\mathrm{y}}$${\mathrm{CuO}}_4$ alloys. A half-filled Cu(3d)-O(2p) band with two-dimensional character and a nearly square Fermi surface produces a Peierls instability for y=0 that opens a semiconductor gap over the Fermi surface. Alloying with divalent or tetravalent atoms should spoil the nesting features while maintaining the strong coupling of O phonons to the conduction electrons.

We describe a model for the ejection of large biomolecules from surfaces by impact of high-energy (∼1 MeV/nucleon) heavy ions. The model involves the rapid vibrational excitation of the biomolecule by low-energy electron irradiation arising from the incident particle track. The excitation breaks most hydrogen bonds binding the molecule to the surface and causes the molecule to expand on a subpicosecond timescale; the expanding molecule pushes against the substrate and generates sufficient momentum to escape.

Body-centered tetragonal ${\mathrm{La}}_2$${\mathrm{CuO}}_4$ is shown to have its electronic structure and properties dominated by the layered in-plane Cu-3d–O-2p interactions. A strong Fermi surface instability along [110] with ‖q‖=2$k_{\mathrm{F}}$ leads, via a soft-phonon mode, to the observed orthorhombic phase and accounts for its semiconducting properties. The addition of divalent metals, i.e., Ba, Sr, suppresses the instability and stabilizes the tetragonal phase where the same soft-phonon branch apparently contributes to a large electron-phonon interaction and a high $T_c$.

When a can containing one large ball and a number of smaller ones is shaken, the large ball rises to the top, even when the larger ball is more dense than the others. Similarly, a mixture of different sized particles will segregate by size when shaken. An adaptation of the Monte Carlo method is used to study this size segregation. The results show the local, geometric mechanism by which the segregation is produced. Segregation by size is to be distinguished from the more obvious sifting process which occurs when tiny grains filter down through the interstices between large particles.