Mean-square displacements in superconducting R${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ (R=Y, Ho) were studied as function of temperature by neutron diffraction on single crystals. From a comparison of the results with the pure dynamical displacements calculated from the phonon dispersion relations of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ we estimate the static contributions to the displacements. In particular we find static contributions to the displacements parallel to the (a,b) plane of the chain and the apical oxygen atoms. There is no evidence of an anomalous displacement of the apical oxygen in the c direction, in contrast to extended x-ray-absorption fine-structure studies suggesting a double-well potential. We also report on a structural anomaly near ${\mathit{T}}_{\mathit{c}}$, which is observed in the intensity of specific Bragg reflections.

We study the bulk properties of d-wave superconductors relevant to high-${\mathit{T}}_{\mathit{c}}$ copper oxides within weak-coupling theory. The superfluid density, the tunneling conductance, and the frequency-dependent electric conductivity appear to be quite consistent with some of the experimental data from single crystals of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ and ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCuO}}_8$. We also predict strong anisotropy in the ultrasonic attenuation when a sound wave is propagated within the a-b plane.

High-pressure in situ x-ray-diffraction studies on GaN and InN have been carried out using an imaging-plate technique and a diamond-anvil cell up to about 60 GPa. The two compounds crystallize in the wurtzite-type structure at ambient conditions. The axial ratio c/a of GaN remains unchanged from the ambient value of 1.626 whereas c/a of InN is considerably decreased from 1.613 to 1.597 with increasing pressure to about 15 GPa. Equation-of-state data obtained for the wurtzite phase have yielded the bulk modulus of GaN to be 237(31) GPa and that of InN to be 125.5(4.6) GPa. Structural phase transition into the rocksalt-type structure takes place in GaN at 52.2 GPa and in InN at 12.1 GPa. The trend in the transition pressures of the III-V nitrides is discussed in terms of various ionicity scales.

The quantum system associated with a resistor-shunted weak-link junction circuit may be described in terms of a single Lagrangian. In this representation the degrees of freedom internal to the dissipative energy of the resistor can be systematically separated from the Hamiltonian formulation of the circuit. This allows us to solve the quantum states of the junction while the effects of the resistor on the circuit are included as a Lagrangian multiplier. In this manner the effects of the measuring apparatus or resistor on the quantum state of the junction may be calculable. The calculations are in very good agreement with experiments.

In this paper we will present measurements, using various kinds of spectroscopy, on the hole-doped compound ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{Ca}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Y}}_{\mathit{x}}$${\mathrm{Cu}}_2$${\mathrm{O}}_{8+\mathrm{\delta }}$, with x ranging from 0.0 to 1.0. Cu 2p x-ray photoelectron spectra are compared with cluster calculations using nonlocal screening effects. The consequences of hole doping and multiplet interaction are also studied. The photoemission and inverse-photoemission data indicate that the appearance of intensity at the Fermi level is a result of the shift of the chemical potential to the top of the valence band combined with a shift of spectral weight from high- to low-energy states.

We have performed ac susceptibility, dc magnetization, and critical-current measurements on a series of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ samples with low levels of Co and Zn doping. The anisotropic superconducting properties were studied by magnetically aligning the powders in wax. By combining previous results of the magnetic penetration depth λ with the thermodynamic critical field ${\mathit{H}}_{\mathit{c}}$ data, obtained from previous specific-heat measurements, we were able to estimate the coherence length ξ, the lower and upper critical fields ${\mathit{H}}_{\mathit{c}1}$ and ${\mathit{H}}_{\mathit{c}2}$, and the Ginzbug-Landau parameter κ of these samples. We have compared these estimates with values determined from dc magnetization curves near ${\mathit{T}}_{\mathit{c}}$. Both intra- and intergrain critical current densities, ${\mathit{J}}_{\mathit{c}\mathrm{}\left(\mathrm{intra}\right)\mathrm{}}$ and ${\mathit{J}}_{\mathit{c}\mathrm{}\left(\mathrm{inter}\right)\mathrm{}}$, were measured magnetically on bulk samples and found to decrease with Co and Zn doping. In particular, the ratio of ${\mathit{J}}_{\mathit{c}\mathrm{}\left(\mathrm{intra}\right)\mathrm{}}$ and ${\mathit{J}}_{\mathit{c}\mathrm{}\left(\mathrm{inter}\right)\mathrm{}}$ was found to be insensitive to doping, although the coherence length was increased in the doped samples. Moreover, ${\mathit{J}}_{\mathit{c}\mathrm{}\left(\mathrm{intra}\right)\mathrm{}}$ correlates well with the condensation energies of the samples. This indicates that doping with Co or Zn weakens the energy barrier of the existing pinning sites and has a negative overall effect on the ability of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ to carry loss-free currents. Finally, the results of irreversibility line measurements show that the Co substitution, occurring in the CuO chain sites, has a more severe effect than Zn substitution, occurring in the ${\mathrm{CuO}}_2$ plane sites, because of the increased anisotropy.

The electronic structure of Ca-doped ${\mathrm{C}}_{60}$ has been studied by electron-energy-loss spectroscopy. A consistent picture of the electronic structure is obtained by comparing electron diffraction, valence-band excitation, and core-level excitation data. In ${\mathrm{Ca}}_3$${\mathrm{C}}_{60}$, the measurements indicate that the bands derived from the lowest unoccupied molecular orbitals (LUMO) of ${\mathrm{C}}_{60}$ are completely filled leading to insulating behavior, and that, upon further doping, the LUMO +1 derived bands are partially filled.

Raman spectra of α-${\mathrm{NaVO}}_3$ were recorded under pressures up to 170 kbar at room temperature. A crystalline-amorphous phase transition was found at ∼61 kbar, much lower than the pressures of other ionic materials. This transition is marked by the sudden appearance of very broad bands at the 800- and 350-${\mathrm{cm}}^{\mathrm{-}1}$ regions. The amorphization involves the complete breaking up of the infinite chains of corner-linked tetrahedral ${\mathrm{VO}}_4$, most likely into ${\mathrm{VO}}_3^{\mathrm{-}}$. On decompression, the amorphous phase transforms to another phase, probably also amorphous at ∼40 kbar. It reverts to the stable ambient-condition α phase upon heating.

The formula giving the probability of electron escape is generalized to the case of electron excitation involving two conduction bands in the vicinity of the Brillouin-zone boundary when the gap between the bands is of critical importance. The escape probability in this case can be significantly larger than that found for excitation from the occupied states lying well within the first Brillouin zone. It is also shown that the Fowler law (commonly used to describe photoemission in the threshold region) is not restricted to the usually accepted assumption of indirect, momentum nonconserving, electron transitions but is a consequence of the linear dependence of the escape function on the electron energy near threshold and is valid for both direct and indirect electron transitions.

We present a universal theory that describes the low-temperature properties of real structural glasses. The key ingredient is a mapping of real and possibly complex glasses on Lennard-Jones glasses. A dimensional analysis allows us to find a general explicit expression that relates the tunneling parameters [density of tunneling systems (TS’s), coupling constant between TS’s and heat bath] to material constants. Experimental evidence is presented, and the relation to recent numerical calculations on the NiP system is discussed.

The lattice dynamics of noble metals has been reexamined by incorporating a short-range three-body interaction in the transition-metal-model potential method of Animalu. The theoretically calculated phonon dispersion curves of the noble metals, which include the repulsive contribution also, are found to be in good agreement with the experimental data.

Low-temperature calorimetric measurements show an antiferromagnetic ordering of ${\mathrm{Yb}}^{+3\mathrm{}}$ in ytterbium sesquioxide. The well-defined Néel temperature of 2.3 K is corroborated by magnetic susceptibility data. Entropy associated with this ordering process is close to Rln2 as expected for a ground-state doublet, even though there are two nonequivalent cation sites in the cubic crystal.

The ferromagnetic exchange-interaction model on the simple-cubic lattice is studied by considering the effective Hamiltonian of a cluster of eight spins which form a cubic unit cell. Through the use of the Bogoliubov inequality and a group-theoretical method, thermodynamic properties of the model are examined. Critical parameters are then determined for general spins.

We show that the properties of the heavy-electron superconducting state induced by the interorbital kinetic exchange scale with the effective-mass renormalization ${\mathit{m}}^{\mathrm{*}}$/${\mathit{m}}_0$∼1/${\mathit{T}}_{\mathit{K}}$. Explicitly, the pairing potential J̃∼J(${\mathit{m}}_0$/${\mathit{m}}^{\mathrm{*}}$)${\ln }^2$(${\mathit{m}}_0$/${\mathit{m}}^{\mathrm{*}}$), where J is the magnitude of the bare Kondo coupling; the coherence length ξ∼${\mathit{T}}_{\mathit{K}}$/${\mathit{T}}_{\mathit{c}}$, where ${\mathit{T}}_{\mathit{c}}$ is the transition temperature, whereas the penetration depth λ∼(${\mathit{m}}^{\mathrm{*}}$/${\mathit{m}}_0$${)}^{1/2}$ so that λ/ξ≫1. We also determine the scaling of magnetic critical fields.

Anomalies are found in the resistivity ρ and NMR rate 1/${\mathit{T}}_1$ of the infinite-dimensional Hubbard model using quantum Monte Carlo calculations and the noncrossing approximation. For temperatures greater than the ‘‘Kondo scale’’ ${\mathit{T}}_0$, we obtain 1/${\mathit{T}}_1$∼a+bT and ρ∼c+dT (a, b, c, d constants). For temperatures T≪${\mathit{T}}_0$ we infer from a saturation of the effective mass of the carriers that the ground state of the system is a Fermi liquid.

We investigated the structural and physical properties of spinel compounds ${\mathrm{Mg}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Ti}}_{1+\mathit{x}}$${\mathrm{O}}_4$ in the Mg[${\mathrm{Ti}}_2$]${\mathrm{O}}_{4\mathrm{-}}$Mg[MgTi]${\mathrm{O}}_4$ system. Polycrystalline samples of ${\mathrm{Mg}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Ti}}_{1+\mathit{x}}$${\mathrm{O}}_{4\mathrm{-}\mathrm{\delta }}$ (0≤x≤1.0) were synthesized by the conventional solid-state reaction method. Powder x-ray-diffraction analysis indicated that the samples with 0.33≤x≤0.67 were nearly single phase of the cubic magnesium titanate spinel. For all the samples, electrical resistivity (ρ) exhibited semiconducting behavior at temperatures below 300 K and the magnitude of ρ at 77 K was about 1×${10}^6$ Ω cm. For all the single-phase samples, the activation energies (${\mathit{E}}_{\mathit{a}}$) were estimated at ${\mathit{E}}_{\mathit{a}}$∼0.34 eV from the Arrhenius plots in the temperature range from 100 to 300 K and, moreover, the magnetic susceptibility versus temperature curves exhibited Curie-Weiss-type behavior at temperatures below 300 K.

The response to an alternating field of a slab of flux-permeated superconducting material with a local electrodynamic response law E∝‖J${\mathrm{\Vert }}^{\mathrm{\sigma }}$J is obtained numerically. Fundamental and third-harmonic components of the susceptibility are reported as functions of σ and slab thickness. The third-harmonic component is always smaller than in the critical-state model, but not much smaller if σ≫1. If the slab is thick enough, and σ>0 the field is totally extinguished at a finite depth. In that case it gradually adopts a self-replicating, nonsinusoidal wave form whose amplitude diminishes according to a power law while the phase lag diverges logarithmically.

We consider the electron dynamics and transport properties of one-dimensional continuous models with random, short-range correlated impurities. We develop a generalized Poincaré map formalism to cast the Schrödinger equation for any potential into a discrete set of equations, illustrating its application by means of a specific example. We then concentrate on the case of a Kronig-Penney model with dimer impurities. The previous technique allows us to show that this model presents infinitely many resonances (zeroes of the reflection coefficient at a single dimer) that give rise to a band of extended states, in contradiction with the general viewpoint that all one-dimensional models with random potentials support only localized states. We report on exact transfer-matrix numerical calculations of the transmission coefficient, density of states, and localization length for various strengths of disorder. The most important conclusion so obtained is that this kind of system has a very large number of extended states. Multifractal analysis of very long systems clearly demonstrates the extended character of such states in the thermodynamic limit. In closing, we briefly discuss the relevance of these results in several physical contexts.

Superconducting quasiparticle lifetimes associated with spin-fluctuation scattering are calculated. A Berk-Schrieffer interaction with an irreducible susceptibility given by a BCS form is used to model the quasiparticle damping due to spin fluctuations. Results are presented for both s-wave and d-wave gaps. Also, quasiparticle lifetimes due to impurity scattering are calculated for a d-wave superconductor.

An exact description is given of the long-distance behavior of the correlation functions for the one-dimensional anisotropic t-J model. We employ the Bethe-ansatz method and the finite-size scaling technique in conformal-field theory. The behavior of correlations indicates that the singlet electron pairs of the Cooper type are formed. This conclusion is also confirmed by the calculation of the conductivity and the effective transport mass.