A conductivity peak has been found at zero bias in electron-tunneling measurements on single-barrier GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As/GaAs junctions at 4 K in the dark. The spectral dependence of the quenching of the peak has been measured to extract a photoioization cross section which indicates that the peak is associated with the DX defect. Since zero-bias conductance peaks are a signature of tunneling via paramagnetic defects, the data presented here are consistent with the existence of paramagnetic DX defects.

Pronounced oscillations have been observed in linewidth, amplitude, and mass of electron cyclotron resonance in InAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$Sb quantum wells when holes coexist with electrons. The strength of the oscillations increases sensitively with electron-hole pair density; the oscillations are absent for semiconducting samples (x≳0.3) in which there are no holes. Results are interpreted in terms of a filling-factor-dependent electron-hole interaction.

X-ray standing-wave measurements and tunneling microscopy have been combined to solve the atomic geometry of the √3×√3R30° honeycomb phase of Bi on Si(111). The standing-wave measurements utilize three different diffracting planes to triangulate the surface position of Bi atoms. The unoccupied surface sites required to completely determine the structure can be deduced from Rutherford-backscattering coverage and low-energy electron-diffraction symmetry arguments. These arguments are completely confirmed by a tunneling-microscope study, which is free of the ambiguities of previous studies. The final result is a 2/3-ML √3×√3R30° structure with Bi atoms in the ${\mathit{T}}_1$ sites directly above first-layer Si atoms.

GaAs layers have been grown by metal-organic molecular-beam epitaxy from trimethylgallium and cracked arsenic (${\mathrm{As}}_2$) on (001)-oriented substrates held at 450 or 500°C, and the directions of the (2×4) surface reconstruction observed by high-energy electron diffraction were marked. Postgrowth infrared measurements using polarized light demonstrated that a carbon-hydrogen complex giving vibrational absorption at 2688 ${\mathrm{cm}}^{\mathrm{-}1}$ was aligned along the [110] direction, perpendicular to the [1¯10] direction of the surface arsenic dimer bond. After hydrogen passivation in a plasma, this absorption was lost but was regenerated by annealing samples at 450°C in a vacuum. The observations confirm a previous assignment of the 2688-${\mathrm{cm}}^{\mathrm{-}1}$ defect to a H-(${\mathrm{C}}_{\mathrm{As}}$${)}_2$ complex.

We present an exact quantum-mechanical calculation of the conductance for a large but finite two-dimensional antidot lattice. Rich structure is predicted for the mesoscopic system and is characterized by two kinds of conductance fluctuations: slow fluctuations as seen in the thermally averaged conductance and rapid fluctuations appearing at low temperature on the top of the slow ones. The former are due to electron wave interferences in a form of Bragg reflections, while the latter can be attributed to resonant tunneling via electron states of the system.

It is conclusively demonstrated using surface infrared spectroscopy, in combination with selective isotopic labeling, that methoxy coordinates with ${\mathit{C}}_{3\mathit{v}}$ symmetry on Mo(110), at coverages below 0.17 ML. The existence of intense Fermi resonances in the C-H stretch region of adsorbed ${\mathrm{CH}}_3$O is firmly established. By application of these observations to previous studies of methoxy on the low-index planes of copper and nickel, the disparity between the adsorbate geometry predicted by infrared spectroscopy and that deduced from photoelectron studies is removed.

We have settled a controversy that surface dimers in the Si(001) surface are intrinsically symmetric or asymmetric. Atom-resolved scanning-tunneling-microscopy (STM) images of a low-defect-density (∼1%) surface at 144 K give definite evidence for the asymmetric-dimer model. We have identified a local influence of surface defects on buckling of dimers at low temperatures, which was not clarified in previous STM observations at 120 K.

The crystal structure of ${\mathrm{Fe}}_{16}$${\mathrm{N}}_2$ was optimized by calculating atomic forces on the basis of an augmented-plane-wave force method. We found interesting cooperative structural relaxations of the iron atoms around the nitrogen, though the optimized structure is not far from the experimental proposal. Calculated hyperfine fields for each iron site for the optimized ${\mathrm{Fe}}_{16}$${\mathrm{N}}_2$ show an interesting systematic feature when compared with experimental results.

The critical behavior near a surface that breaks the order parameter symmetry is analyzed with field-theoretic renormalization-group methods. We study the short-distance expansions of the order parameter and the energy density about the surface and investigate various density profiles and correlation functions in the half-space and film geometries. Our results confirm Cardy's conjecture that the stress-energy tensor appears as the leading surface density. Its universal prefactor $C$ in the expansions determines several physical properties of interest in the critical adsorption of fluids: In a film at the bulk critical temperature $T_c$, $C$ determines the amplitude ratio of the Fisher-de Gennes distant-wall correction to the order-parameter profile and the Casimir correction to the free energy. In the half-space close to $T_c$, $C$ determines the amplitude ratio of the ${\left(\frac{z}{\xi }\right)}^d$ term in the order parameter profile to the Ferer-Stauffer-Wortis term in the bulk free energy. Here $z$ is the distance from the surface and $\xi$ the bulk correlation length. In the half-space at $T_c$, $C$ also determines the universal short-distance amplitudes of layer susceptibilities, which should be observable in computer experiments. We calculate $C$ to second order in $\epsilon =4-d$ and estimate its value for $d=3\mathrm{}$. $C$ is closely related to the stress-stress correlation in a half-space, for which we obtain the bulk to surface crossover. We present a simple argument showing why this crossover depends, in general, on the surface universality class except in two dimensions.

The micromagnetic theory is a simple and powerful method to calculate the spin-wave dispersion relation in the long-wavelength region. The calculation has been performed within the framework of the two-sublattice model which is the adequate micromagnetic description for rare-earth–transition-metal compounds. A magnetocrystalline anisotropy of the easy-axis type is assumed and the cases of ferromagnetic and ferrimagnetic coupling of the two sublattices are considered. The theoretical results are compared to neutron-scattering data obtained from R${\mathrm{Fe}}_2$, ${\mathit{R}}_2$${\mathrm{Fe}}_{17}$, ${\mathit{R}}_2$${\mathrm{Co}}_{17}$, and ${\mathit{R}}_2$${\mathrm{Fe}}_{14}$B, with R representing different rare-earth atoms.

We obtain an analytical form for the muon relaxation function in a dynamical magnetic environment and an external logitudinal magnetic field using perturbation theory. The new relaxation function is compared with the semiclassical Kubo-Toyabe function and the result of Monte Carlo simulation. We also demonstrate the application of our result to the case of dilute spin glasses.

We present a nonperturbative method for deriving a quasiparticle description of the low-energy excitations in the t-J model for strongly correlated electrons. Using the exact diagonalization technique we evaluated exactly the spectral functions of composite operators, which describe an electron or hole dressed by antiferromagnetic spin fluctuations as expected in the string or spin bag picture. For hole doping up to 1/8, use of the composite operators leads to a drastic simplification of the single-particle spectral function: at half-filling it takes free-particle form, for the doped case it resembles a system of weakly interacting fermions corresponding to the doped holes. We conclude that for all doping levels studied, the elementary electronic excitations next to the Fermi level are adequately described by the antiferromagnetic spin-fluctuation picture.

Exact spectra of periodic samples are computed up to N=36. Evidence of an extensive set of low-lying levels, lower than the softest magnons, is exhibited. These low-lying quantum states are degenerated in the thermodynamic limit; their symmetries and dynamics as well as their finite-size scaling are strong arguments in favor of Néel order. It is shown that the Néel order parameter agrees with first-order spin-wave calculations. A simple explanation of the low-energy dynamics is given as well as the numerical determinations of the energies, order parameter, and spin susceptibilities of the studied samples. It is shown how suitable boundary conditions, which do not frustrate Néel order, allow the study of samples with N=3p+1 spins. A thorough study of these situations is done in parallel with the more conventional case N=3p.

We report on resistance and magnetoresistance measurements in multilayer Mo/Si samples at temperatures of 1.5–300 K and magnetic fields up to 6 T. The temperature dependence of the conductivity can be entirely described by quantum interference effects. The experimental results are compared with calculations of the quantum corrections due to weak localization, electron-electron interactions, and supeconducting fluctuations. The resistance shows a crossover from two-dimensional (2D) to 3D behavior when the temperature is increased.

In the framework of the reconnecting vortex-tangle model, flow properties of He II were studied numerically. The Crank-Nicolson method was used to calculate the motion of quantized ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ vortices, assuming smooth walls in infinitely long channels, applying periodic boundary conditions in the direction of the flow, and with friction coefficients α=0.1 and α’=0. Flow channels with square and circular cross sections and infinitely wide slits were studied. The superfluid velocity profile was assumed to be flat; the normal-component velocity profile was either flat or parabolic. Special interest was paid to the time-averaged line-length density and the average mutual-friction force density as functions of the difference between the average normal-fluid velocity and the externally applied superfluid velocity. The influence of the periodicity length on the calculated flow properties was determined. The results were compared with experiment.

This work reports a systematic study of polycrystalline samples of ${\mathrm{Sm}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Ce}}_{\mathit{x}}$${\mathrm{CuO}}_{4\mathrm{-}\mathit{y}}$ (0.15≤x≤0.18) obtained from a sol-gel precursor and subjected to different cooling rates after reduction. A double resistive superconducting transition is a common feature of all samples studied, suggesting that this is an intrinsic property of these polycrystalline compounds. At an upper temperature ${\mathit{T}}_{\mathit{c}\mathit{i}}$, there is a fairly sharp drop in the magnitude of the electrical resistivity, which is followed by a well-defined plateau down to a lower temperature ${\mathit{T}}_{\mathit{c}\mathit{j}}$. At this temperature, Josephson coupling between superconducting islands is believed to complete the transition to the zero resistance state. From the compositional dependence of electrical resistivity, we infer that ${\mathit{T}}_{\mathit{c}\mathit{i}}$ decreases slightly from x=0.15 through x=0.18, while ${\mathit{T}}_{\mathit{c}\mathit{j}}$ shows a maximum for x=0.16. With increasing excitation current, no significant changes in the behavior of the electrical resistivity between ${\mathit{T}}_{\mathit{c}\mathit{i}}$ and ${\mathit{T}}_{\mathit{c}\mathit{j}}$ are observed, while a dramatic broadening and a shift of the transition at ${\mathit{T}}_{\mathit{c}\mathit{j}}$ towards lower temperatures are found. Magnetic-susceptibility measurements reveal appreciable diamagnetism just below the coupling temperature ${\mathit{T}}_{\mathit{c}\mathit{j}}$ suggesting that superconducting properties are really confined to small regions, with size comparable to the London penetration depth.

The average size of these regions are estimated to be between ∼6 and 300 Å, in good agreement with recent estimates obtained from magnetoresistance measurements on polycrystalline ${\mathrm{Sm}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Ce}}_{\mathit{x}}$${\mathrm{CuO}}_{4\mathrm{-}\mathit{y}}$ (0.14≤x≤0.17) samples and with both neutron-diffraction studies and Mössbauer spectroscopy measurements in the isomorphic compound ${\mathrm{Nd}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Ce}}_{\mathit{x}}$${\mathrm{CuO}}_{4\mathrm{-}\mathit{y}}$ (0.0≤x≤0.18). All the observed macroscopic properties, as well as the absence of a peak in the specific heat at ${\mathit{T}}_{\mathit{c}\mathit{i}}$, are discussed within the framework of a granular superconductor model. In addition, we give qualitative arguments suggesting the importance of charging effects in the macroscopic properties of these polycrystalline samples.

The quantum state of vortices in a model of two-dimensional superconducting arrays is discussed. It is argued that in the appropriate limit vortices may be regarded as bosons with hard cores and described by the Laughlin-type wave functions with the induced charge density taking the role of the magnetic field. This in turn leads to the quantization of the Hall conductivity. The possibility of voltage oscillations with multiple Aharonov-Casher periods in an annulus geometry is also pointed out.

A two-dimensional array of Josephson junctions in a magnetic field is considered. It is shown that the dynamics of the vortices in the array resembles that of electrons on a two-dimensional lattice put in a magnetic field perpendicular to the lattice. Under appropriate conditions, this resemblance could result in the formation of a quantum Hall fluid of vortices. The bosonic nature of vortices and their long-range logarithmic interaction make some of the properties of the vortices’ quantum Hall fluid different from those of the electronic one. Some of these differences are studied in detail, and it is shown that a quantum Hall fluid of vortices manifests itself in a quantized Hall electronic transport in the array.

The magnetization curves and their corresponding phase, field, and current-density profiles for symmetric 0-π Josephson junctions (JJ’s) with length 2L are calculated from the sine-Gordon equation. The results show that there is a remanent magnetization ${\mathit{M}}_{\mathit{r}}$ after switching off the field and a coercive field ${\mathit{H}}_{\mathit{c}}$ which is required for magnetization reversal. With increasing l (≡L/${\lambda }_{\mathit{J}}$, ${\lambda }_{\mathit{J}}$ being the Josephson penetration depth), ${\mathit{M}}_{\mathit{r}}$ increases from 0 to a value corresponding to ${\mathrm{\Phi }}_0$/2 flux within the junction; and ${\mathit{H}}_{\mathit{c}}$ increases from 0 and is proportional to the tunneling current density ${\mathit{J}}_0$ and √${\mathit{J}}_0$ at l<0.7 and >6, respectively. Field-cooled magnetization ${\mathit{M}}_{\mathrm{FC}}$ is paramagnetic if the flux in the JJ produced by the applied field H is less than ${\mathrm{\Phi }}_0$/2. The hysteresis loop of very long (l>13) 0-π JJ’s is, similar to that of 0 JJ’s, of a surface-barrier type; for shorter (l<2.378) JJ’s, it is of anti-surface-barrier type. The magnetization of 0-π JJ’s can be explained by the coexistence of ${\mathrm{\Phi }}_0$ vortices, whose entry and exit are through the edges, and ${\mathrm{\Phi }}_0$/2 vortex, which is created at the center.

We calculate the local density of states for a thin metallic film deposited on a superconducting substrate. We use a tight-binding model with a BCS interaction in the substrate. We show that for energies lower than the superconducting gap of the substrate, the local density of states in the normal metal film presents a structure due to the proximity effect. In very thin normal films, we obtain a structure that was missed in previous calculations. We interpret recent experiments in Au-${\mathrm{NbSe}}_2$ systems in terms of this structure.