The high-temperature phase of the three-dimensional random-field Ising model is studied using the replica symmetry-breaking framework. It is found that, above the ferromagnetic transition temperature ${\mathit{T}}_{\mathit{f}}$, a glassy phase appears at intermediate temperatures ${\mathit{T}}_{\mathit{f}}$<T<${\mathit{T}}_{\mathit{b}}$ while the usual paramagnetic phase exists for T≳${\mathit{T}}_{\mathit{b}}$ only. The correlation length at ${\mathit{T}}_{\mathit{b}}$ is computed and found to be compatible with previous numerical results.

High-precision microwave measurements have recently been carried out on superconducting Nb samples. A BCS-like coherence peak has been observed below ${\mathit{T}}_{\mathit{c}}$. We find that a strong-coupling calculation of the microwave conductivity, using electron-phonon spectral functions obtained from tunneling measurements, is unable to account for the height of the peak. We also explain how, for microwave measurements at lower frequency, the height of the coherence peak can be used to provide information about the low-frequency part of the electron-phonon spectral function and the associated inelastic scattering rate.

We have solved the Bogoliubov–de Gennes equation for a clean, one-dimensional mesoscopic superconducting ring threaded by a magnetic flux Φ. We show that the superfluid velocity is driven directly by Φ while the relative motion of the pair of electrons is independent of Φ. Meanwhile, the fluxoid quantization is obtained straightforwardly. More importantly, we have also calculated the supercurrent numerically and self-consistently and find it is periodic in Φ with the period ${\mathrm{\Phi }}_{\mathit{s}}$≡hc/2e for ${\mathrm{\Phi }}_{\mathit{s}}$≤${\mathrm{\Phi }}_{\mathit{d}}$=(${\mathit{mv}}_{\mathit{dL}}$/ħπ)${\mathrm{\Phi }}_{\mathit{s}}$ and with the period ${\mathrm{\Phi }}_0$≡hc/e for ${\mathrm{\Phi }}_{\mathit{d}}$<${\mathrm{\Phi }}_{\mathit{s}}$, which arises from mesoscopic effects.

A one-dimensional (1D) parallel array of shunted Josephson junctions is one of the basic elements in the family of rapid single-flux quantum logic circuits. It was found recently that current steps always show up in the current-voltage curve of the generator junction when an additional bias current is applied to the edge junction of the array. This effect was found to be due to the self-induced magnetic field produced by the edge current. This nonuniform field divides the array into domains each spanning several unit cells and each containing the same number of flux quanta. We report on experimental results obtained by low-temperature scanning electron microscopy (LTSEM) on the 1D array. The (1–3)-μm spatial resolution achieved by LTSEM enables us to image these domains in scanned measurements where the junctions in the array are heated sequentially. Computer simulations confirm the mechanism of the obtained images and the number of observed domains corresponds to the step position as predicted numerically.

The momentum dispersion of the peak structure of the single-particle spectral weight A(p,ω) of the two-dimensional Hubbard model is discussed. Using results obtained from Monte Carlo simulations on lattices up to 12×12 in size, we determine the low-lying quasiparticle dispersion relation. This dispersion relation is anomalously flat near the (π,0) and (0,π) points of the Brillouin zone, similar to the results of recent angular resolved photoemission measurements of the hole-doped cuprates. We argue that the generic nature of the quasiparticle dispersion relation observed in these materials arises from the strong Coulomb interaction and reflects the hole-spin correlations rather than the one-electron interactions which customarily determine the band structure.

Transport measurements in the mixed state of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ crystals (H∥c) using the ‘‘dc-flux-transformer’’ configuration show that the glass-liquid phase transition at the irreversibility line, ${\mathit{T}}_{\mathit{i}}$(H), behaves as a three-dimensional vortex phase transition. These results together with resistance measurements in the c direction show that the vortex correlation across the sample is destroyed at a temperature ${\mathit{T}}_{\mathrm{th}}$(H), higher than ${\mathit{T}}_{\mathit{i}}$(H). However, the dependence of ${\mathit{T}}_{\mathrm{th}}$(H) on sample thickness suggests that, in real thermodynamic phase transitions (infinite sample), the long-range correlation in the ab plane, as well as in the c direction, is lost at the single temperature ${\mathit{T}}_{\mathit{i}}$(H).

In doped Mott-Hubbard insulators, the electron-phonon interaction and strong Coulomb repulsion can reinforce each other to stabilize small polarons, domain walls, and charge-density waves. We discuss how the electron-phonon coupling stabilizes domain-wall states in an antiferromagnetic spin background for small doping, and promotes coupled spin- and charge-density wave transitions at larger doping, and we explain why this coupling is more important in the nickelate than in the cuprate. We discuss our results in the light of recent experiments on ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{NiO}}_{4+\mathit{y}}$.

A series of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ epitaxial thin films were prepared with values of δ ranging from 0.05 to 0.65. Electric field as a function of current-density isotherms could be scaled to a universal scaling function over the full δ range and the critical parameters determined were nearly independent of δ. The vortex-glass phase-transition temperature was determined as a function of magnetic field at all of the values of δ studied and the phase boundary was observed to broaden with δ. Based on determinations of the ratio of the zero-temperature coherence length ${\xi }_{\mathit{c}0}$ to c-axis lattice parameter using normal-state fluctuation conductivity, the anisotropy was shown to increase with δ while ${\xi }_{\mathit{c}0}$ decreases from 1.6 to 0.8 Å as δ ranges from 0.05 to 0.37.

Based on a systematic experimental study of the resistivity of high-quality single crystals of (${\mathrm{La}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${)}_2$${\mathrm{CuO}}_4$, a phenomenological approach is proposed in which the dissipation for the direction perpendicular to the superconducting layers is explained by thermal fluctuations of the phase in Josephson junctions. By introducing an effective junction area A(H,T), an analytical expression for the resistivity is formulated, with which the c-axis resistivity as a function of T, H, and θ (angle between H and the basal plane) can be very well explained.

Superconducting-Hall-effect ${\mathit{R}}_{\mathit{H}}$ transitions were measured on 11 ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ epitaxial thin films grown under a range of electronic ‘‘doping’’ conditions. The data reveal that the relative magnitude of the anomalous sign reversals in ${\mathit{R}}_{\mathit{H}}$ monotonically decrease with decreasing carrier density and vanish below a threshold cutoff value. The sign reversals in ${\mathit{R}}_{\mathit{H}}$ are observed only in films having a negative normal-state Seebeck coefficient, and are correlated in magnitude. This suggests that the energy slope of the electronic density of states plays a role in the occurrence of these anomalies.

We prove that the universality of conductance fluctuations in mesoscopic, weakly disordered systems, so far demonstrated for a contact scattering potential, is preserved in the case of an arbitrary potential. This is so at least to first order in the disorder and as long as the electron lifetime and the transport time are well-defined quantities. We also prove that the conductance fluctuations induced by the motion of a single impurity in two dimensions are not as important and universal as if one changes the entire impurity distribution, in contrast to a recent claim. These conductance fluctuations explicitly depend on the degree of disorder and on the characteristics of the system.

Exact-diagonalization and quantum Monte Carlo methods are used to study the occurrence of a metal-insulator transition and the distribution of charges and magnetic moments in the ground state of a one- and two-dimensional half-filled system of correlated electrons, described by a one-band simplified Hubbard model extended with bond-charge–site-charge interactions. Numerical results for the electrical conductivity, the density of states, and the spin- and charge-density correlation functions strongly suggest that the bond-charge–site-charge interaction plays a fundamental role in the occurrence of a metal-insulator transition and of ferromagnetic order.

X-ray-photoelectron spectroscopy (XPS) and optical measurements have been performed to investigate the structure of electronic states in yttrium-stabilized zirconia. From reflectivity observations, the spectral behavior of the dielectric function, of the energy loss, and of the effective electron number in the range 3–30 eV were obtained for single crystals with an yttria content of 12 and 24 mol %. This information, combined with binding-energy spectra from XPS, was used to infer the structure of valence and conduction states and to discuss the nature of the main optical transitions, also in connection with available theoretical data.

The valley-degeneracy dependence of the local-field correction for the two-dimensional and the three-dimensional electron gas is calculated. We use a sum-rule version of the self-consistent approach of Singwi et al. to calculate the local-field correction. Results for 0.01<${\mathit{r}}_{\mathit{s}}$<40 are given and ${\mathit{r}}_{\mathit{s}}$ is the random-phase-approximation parameter. We derive expressions for the electron density dependence of the local-field correction for different valley degeneracy. We find that with increasing valley degeneracy the kinetic energy and the exchange energy decrease while the correlation energy increases. For very large valley degeneracy, the local-field correction of the electron gas approaches the local-field correction of the Bose condensate. We find that in semiconductors with large valley degeneracy the correlation effects are enhanced and that many-body effects are very important, even at high electron density.

We have calculated the two-photon absorption (TPA) spectrum in the Pariser-Parr-Pople model of conjugated polymers. Configuration interactions among all the single and double excitations are taken into account for relatively long chains. It is shown that the maximum of TPA decreases with increasing interaction strength or with increasing bond alternation. For moderate interaction strength and bond alternation, the overall TPA spectrum is characterized by two energy regions: the low-energy part due to mainly one-electron excitations (excitonic states), and the higher-energy part with contributions mainly from two-electron excited states (biexcitonic states). The high-energy TPA is about one order of magnitude stronger than the low-energy part. For stronger interaction or smaller bond alternation, the single and double excitations are fairly mixed, and the distinction between the excitonic and the biexcitonic regions is lost.

We present a method for total-energy minimizations and molecular-dynamics simulations based either on tight-binding or on Kohn-Sham Hamiltonians. The method leads to an algorithm whose computational cost scales linearly with the system size. The key features of our approach are (i) an orbital formulation with single-particle wave functions constrained to be localized in given regions of space, and (ii) an energy functional that does not require either explicit orthogonalization of the electronic orbitals, or inversion of an overlap matrix. The foundations and accuracy of the approach and the performances of the algorithm are discussed, and illustrated with several numerical examples including Kohn-Sham Hamiltonians. In particular, we present calculations with tight-binding Hamiltonians for diamond, graphite, a carbon linear chain, and liquid carbon at low pressure. Even for a complex case such as liquid carbon—a disordered metallic system with differently coordinated atoms—the agreement between standard diagonalization schemes and our approach is very good. Our results establish the accuracy and reliability of the method for a wide class of systems and show that tight-binding molecular-dynamics simulations with a few thousand atoms are feasible on small workstations.

We generalize the Nielsen-Martin stress theorem beyond the local-density approximation (LDA) and present an alternative derivation of the whole theorem. We show that the exchange-correlation stress becomes anisotropic in the most general case: its explicit form is given within a gradient-corrected (GC) scheme. As a test implementation, we use the generalized theorem to achieve fast structural optimization in crystalline Se. In this material LDA predicts a rather poor structure period. Our GC calculation is in much better agreement with the experiment.

We demonstrate, by means of first-principles calculations, that in the field-induced magnetic state of α-cerium and α-uranium the spin and orbital moments are coupled parallel rather than antiparallel as expected from Hund’s third rule. This behavior gives a magnetic form factor for α-uranium in very good agreement with experiment. The calculated form factor of α-cerium, when assuming that the f electron is itinerant, is very different from the localized ${\mathrm{Ce}}^{3+}$ form factor, and we propose that a measurement of the magnetic form factor of α-Ce can indicate whether the f state is localized or itinerant. The field-induced magnetism of plutonium is shown to be almost exclusively of orbital character and the induced spin density of plutonium is highly anomalous.

Photoenhanced electron-paramagnetic-resonance (EPR) and contactless photoconductivity measurements on Te-doped AlSb bulk crystals are reported. Low-temperature annealing studies of both photo-EPR and persistent photoconductivity strongly support a bistable DX-type model for the Te donor in AlSb and provide information on the deep DX-type state of Te.

We present a numerical approach to the solution of the Boltzmann transport equation in an arbitrary one-dimensional potential. Instead of employing the usual piecewise linear approximation to the potential in the discretization of the equation, we use a stepwise approximation. It is shown that due to absence of the electric field between mesh points, the Boltzmann transport equation can be reduced to a relatively simple equation involving only the spherical part of the distribution function. The method naturally describes situations where energy-band discontinuities take place—situations which are important in modern semiconductor heterostructures. We demonstrate the method for a system of finite length in a constant electric field. We observe a spatial periodicity in the electron drift velocity and average energy produced by interaction of electrons with optical phonons. These oscillations demonstrate one of the advantages of our method over Monte Carlo calculations, which do not predict such oscillations due to the averaging of the statistical data over distances larger than the oscillation period. We compare our results for the drift velocity in moderate electric fields (5 to 50 kV/cm) to a phenomenological model, finding the drift velocity to depend roughly on the square root of the field in this field range.