We have studied (LuC${)}_2$(${\mathrm{Ni}}_2$${\mathrm{B}}_2$) and (LuC)(${\mathrm{Ni}}_2$${\mathrm{B}}_2$) at ambient and high pressures. Superconductivity was discovered in (LuC${)}_2$(${\mathrm{Ni}}_2$${\mathrm{B}}_2$) at 2.9 K. Pressure was found to suppress both the electrical resistivity and the superconducting transition temperature rapidly for (LuC${)}_2$(${\mathrm{Ni}}_2$${\mathrm{B}}_2$) but only slightly for (LuC)(${\mathrm{Ni}}_2$${\mathrm{B}}_2$), in spite of their similar chemical-bonding behavior. The observations can be understood in terms of the smaller density of states at the Fermi surface and the greater pressure-induced band broadening of (LuC${)}_2$(${\mathrm{Ni}}_2$${\mathrm{B}}_2$) than (LuC)(${\mathrm{Ni}}_2$${\mathrm{B}}_2$).

We give a strong-coupling real-frequency-axis formulation of the phonon-self-energy change on entering the superconducting state for a d-wave superconductor. The effect of impurity scattering in the Born (weak scattering) and unitary (T-matrix approach) limits is considered and numerical results presented for phonon shifts and changes in width.

A Hartree approximation is used to study the interplay of two kinds of scaling which arise in high-temperature superconductors, namely critical-point scaling and that due to the confinement of electron pairs to their lowest Landau level in the presence of an applied magnetic field. In the neighborhood of the zero-field critical point, thermodynamic functions scale with the scaling variable [T-${\mathit{T}}_{\mathit{c}2}$(B)]/${\mathit{B}}^{1/2\nu }$, which differs from the variable [T-${\mathit{T}}_{\mathit{c}}$(0)]/${\mathit{B}}^{1/2\nu }$ suggested by the Gaussian approximation. Lowest-Landau-level (LLL) scaling occurs in a region of high field surrounding the upper critical-field line but not in the vicinity of the zero-field transition. For ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ in particular, a field of at least 10 T is needed to observe LLL scaling. These results are consistent with a range of recent experimental measurements of the magnetization, transport properties, and, especially, the specific heat of high-${\mathit{T}}_{\mathit{c}}$ materials.

Single crystals of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ were exposed to fast neutron irradiation with fluences up to ${10}^{19}$ n/${\mathrm{cm}}^2$ (E>0.1 MeV). Ac and dc magnetization measurements were performed to study the influence of irradiation-induced defects on the intrinsic and extrinsic parameters of superconductors. It has been shown that the observed increase of the anisotropy of the lower critical field in irradiated samples is due to the suppression of the Bean-Livingston surface barrier. Drastic changes of intrinsic parameters of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ single crystals are interpreted within the framework of existing theoretical models.

Elementary excitations are discussed in the flux-binding phase [Weng, Sheng, and Ting, Phys. Rev. B 49, 607 (1994)] of the t-J model. It is shown that the density and current constraints in the gauge-theory description will dramatically modify the charge excitations. An effective Lagrangian for the normal state of the flux-binding phase is derived, and based on it the transport properties are studied. We find that the resistivity is linearly temperature dependent with a relaxation rate ħ/τ≃2${\mathit{k}}_{\mathit{B}}$T, and the Hall coefficient involves a second relaxation rate with the cotangent Hall angle following a ${\mathit{T}}^2$ law. Furthermore, the thermopower exhibits a strong doping dependence. All these features are in good agreement with the transport measurements of the high-${\mathit{T}}_{\mathit{c}}$ copper oxide compounds.

We discuss thermal depinning of flux lines from single-line defects in the presence of an external current J. Starting from Langevin dynamics the prefactor in the Arrhenius law for the escape rate scrR is derived. From this, we show the existence of a sharp depinning temperature in the limit J→0. Moreover, the effective activation energy and time scale involved in magnetic relaxation for a sample with columnar defects are calculated. The results allow a quantitative discussion of experiments in the strong-pinning limit.

We have studied the response of vibrating Y-1:2:3 crystals and thin films at different temperatures T, magnetic fields B, and angles FTHETA between the field and ${\mathrm{CuO}}_2$ planes at very small driving forces. We have observed an enhancement of the pinning force at ${\mathit{T}}_{\mathit{m}}$ that delineates a line ${\mathit{B}}_{\mathit{m}}$(T)∝(1-${\mathit{T}}_{\mathit{m}}$/${\mathit{T}}_{\mathit{c}}$${)}^2$ for the single crystals and a magnetic-field-independent line for the thin film below the thermally activated depinning line ${\mathit{B}}_{\mathit{d}}$(T)∝(1-${\mathit{T}}_{\mathit{d}}$/${\mathit{T}}_{\mathit{c}}$${)}^{3/2}$. We argue that this ‘‘peak effect’’ as well as the observed magnetic history efects can be interpreted as due to the thermal creation of defects in the flux-line lattice. Although this efect might be due to dislocation-mediated melting, we found no indication of a first-order transition. The comparsion with the published ‘‘melting lines’’ casts some doubts about the interpretation based on flux-line lattice melting including quantum flucutations.

The anisotropy of the critical current density introduced in the (a,b) plane of ${\mathrm{DyBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ single crystals by oblique irradiation with heavy ions is observed by magneto-optics. The pinning forces on vortices oriented perpendicular to the (a,b) plane are largest when directed perpendicular to the columnar defects. Two modes of vortex motion are considered. Effects of flux creep and thermal depinning on the critical current density are discussed.

The crystal structure of ${\mathrm{Sr}}_{0.875}$${\mathrm{Nd}}_{0.125}$${\mathrm{CuO}}_{2\mathrm{-}\mathrm{\delta }}$ superconducting thin film was determined by x-ray diffraction. The intensity data were collected by four-circle x-ray diffractometry and the absorption correction for a thin platelike crystal of 780 Å thick was made. The structure was found to belong to space group P4/mmm with a=3.920(1) Å, c=3.418(1) Å, and Z=1, which is almost identical with that of ${\mathrm{Ca}}_{0.84}$${\mathrm{Sr}}_{0.16}$${\mathrm{CuO}}_2$. The Cu-O coordination was found to be square planar without detectable buckling of the plane and no apical oxygen ions were observed. The occupation factors of Sr and Nd were slightly less than the nominal ones. The anisotropic thermal parameters for Sr, Nd, and Cu ions were elongated along the c direction, indicating the crystallographical two dimensionality of the infinite-layer structure.

We report the results of a calculation of the electron-phonon interaction in Y${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ based on the nonorthogonal tight-binding approach to lattice dynamics. A self-consistent local-density electronic-structure calculation was combined with a shell-model description of the phonon spectrum. The resulting interaction matrix was used to calculate the superconducting transition temperature from a solution of the Eliashberg equations in which the full $\mathbf{k}$ dependence was retained. The transition temperature was found to be about 90 K, quite close to experiment. The oxygen isotope effect was investigated. We also calculated the $\mathbf{k}$-dependent gap function at $T=0\mathrm{}$. The gap shows significant variation between different sheets of the Fermi surface as well as dependence on $\mathbf{k}$ on the individual sheets, but is nodeless. The quantity $\frac{2\Delta }{k_B{T}_c}$ varies from 6.0 to 2.5 on the Fermi surface.

The local densities of states of an extended Hubbard model describing the ${\mathrm{CuO}}_2$ planes of superconducting cuprates are calculated by means of an approximate treatment that divides the lattice into ${\mathrm{CuO}}_2$ clusters. The exact diagonalization of the Hamiltonian on these trimers is utilized to solve the lattice problem, where the hopping between different trimers is treated as a perturbation. The hole concentrations on both orbitals and the amplitude of the staggered magnetization are obtained as a function of the total number of holes. The overall shape of the band structure is in good agreement with exact diagonalization on larger clusters. The stoichiometric compound is found to be metallic in the paramagnetic phase, but becomes a charge-transfer insulator in the antiferromagnetic phase. Electron and hole doping introduce a new band at the bottom or at the top of the charge transfer gap, respectively. Magnetic order is destroyed when the antiferromagnetic phase becomes unstable against the paramagnetic phase.

In connection with numerous x-ray- and neutron-diffraction investigations of some high-temperature superconductors (${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6+\mathit{x}}$ and related compounds), a nontrivial part of the structure factor, coming from partly disordered Cu-O-...-O-Cu chain fragments situated within the ${\mathrm{CuO}}_{\mathit{x}}$ basal planes, can be a subject of theoretical interest. Closely connected to such a diffusive part of the structure factor are the correlation lengths, which are also available in x-ray- and neutron-diffraction studies and which depend on the degree of oxygen disorder in a basal plane. The quantitative measure of such disorder can be associated with the temperature of a sample anneal ${\mathit{T}}_{\mathit{q}}$ at which the oxygen in the basal plane remains frozen in a high-temperature equilibrium state after a fast quench of the sample to room or lower temperature. The theoretical approach, giving some analytical predictions, is based on numerical calculations. The main goal is to show how the structure factor changes with oxygen content x. Also illustrated are some other physical quantities, i.e., the concentration of holes leaving ${\mathrm{CuO}}_{\mathit{x}}$ planes, the concentration of differently coordinated copper atoms, the number of paramagnetically active Cu-O-... chain fragments, etc. The theoretical approach employed in the paper has been developed for the orthorhombic state of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6+\mathit{x}}$ (YBCO).

We compare the parameters of the generalized critical-state model (GCSM) used to reproduce data obtained from two independent measurements: the critical current of thin slabs and the field penetration in a hollow cylinder, both as a function of the applied magnetic field and temperature. We obtain a good correspondence between the parameters for a thin slab and those for a hollow cylinder in the case of fully oxygenated material. From critical-current measurements, we also observe that the temperature dependence of the GCSM parameters is affected by the quality of the junctions (their resistance) and the level of oxygenation at the surface of the grains. The influence of the intergrain lower critical field in the determination of ${\mathit{I}}_{\mathit{c}}$ is also considered.

Recent angle-resolved photoemission (ARPES) experiments have indicated that the electronic dispersion in some of the cuprates possesses an extended saddle point near the Fermi level which gives rise to a density of states that diverges like a power law instead of the weaker logarithmic divergence usually considered. We investigate whether this strong singularity can give rise to high transition temperatures by computing the critical temperature ${\mathit{T}}_{\mathit{c}}$ and isotope effect coefficient α within a strong-coupling Eliashberg theory which accounts for the full energy variation of the density of states. Using band structures extracted from ARPES measurements, we demonstrate that, while the weak-coupling solutions suggest a strong influence of the strength of the Van Hove singularity on ${\mathit{T}}_{\mathit{c}}$ and α, strong-coupling solutions show less sensitivity to the singularity strength and do not support the hypothesis that band-structure effects alone can account for either the large ${\mathit{T}}_{\mathit{c}}$’s or the different ${\mathit{T}}_{\mathit{c}}$’s within the copper oxide family. This conclusion is supported when our results are plotted as a function of the physically relevant self-consistent coupling constant, which shows universal behavior at very strong coupling.

We present an UV-photoemission and inverse-photoemission investigation on the valence states of ${\mathrm{Ce}}_7$${\mathrm{Rh}}_3$. The data have been compared with those of the isostructural ${\mathrm{Lu}}_7$${\mathrm{Rh}}_3$ where the 4f signal is absent in the valence region. The d-sensitive spectra show a strong similarity between the d-like states for the two compounds. This circumstance has been interpreted in terms of balancing between variations of nearest-neighbor distances and wave-function radial extensions, and exploited to extract the whole 4f spectral function from the f-sensitive ${\mathrm{Ce}}_7$${\mathrm{Rh}}_3$ spectra both below and above the Fermi energy. These results, together with results from Ce 3d core-level spectroscopy, indicate that ${\mathrm{Ce}}_7$${\mathrm{Rh}}_3$ has a small hybridization strength (similar to that of γ-Ce) and a 4f occupation (${\mathit{n}}_{\mathit{f}}$) close to 1.

Neutrons sent at grazing incidence on imperfect multilayers of polymers are scattered both out of and within the plane of reflection. In the latter geometry the scattered intensity is highly structured in two series of ridges, whose loci can be labeled in terms of the neutron momenta. Intersecting ridges show evidence of mode coupling. Similar topology is expected for x-ray scattering.

Far-infrared absorption and reflection spectra of ${\mathrm{As}}_2$${\mathrm{S}}_3$ glass have been measured in the frequency range 10–60 ${\mathrm{cm}}^{\mathrm{-}1}$ at room temperature. The absorption coefficient α(ν) has been determined as a function of frequency ν and found to deviate from the quadratic frequency dependence. A broad absorption peak corresponding to a boson peak is clearly found at about 27 ${\mathrm{cm}}^{\mathrm{-}1}$. Infrared coupling coefficient ${\mathit{C}}_{\mathrm{ir}}$(ν) is determined experimentally from α(ν) and the density of vibrational states previously reported. It is found that ${\mathit{C}}_{\mathrm{ir}}$(ν) shows the frequency dependence similar to that of the Raman coupling coefficient ${\mathit{C}}_{\mathit{r}}$(ν) obtained in the same manner for ${\mathrm{As}}_2$${\mathrm{S}}_3$ glass and is not described by the Martin-Brenig model. Both ${\mathit{C}}_{\mathrm{ir}}$(ν) and ${\mathit{C}}_{\mathit{r}}$(ν) show no peak and the frequency dependence close to that predicted by a fractal model. The far-infrared absorption peak may be interpreted as a result of the phonon-fracton crossover.

Samples of solid deuterium, cooled below 4.2 K, and irradiated by 15 MeV protons, will spontaneously flash. The flash-spectrum occurs in the near infrared with a maximum close to 920 nm. The flash intensity is two to three orders of magnitude greater than a steady-state emission observed in the same spectral region. The flash frequency is 1 every 1 to 3 min after several hours of irradiation of 10 nA/${\mathrm{cm}}^2$. Heat pulses have been detected accompanying many of the flashes and both can be stimulated by the application of external heat triggers. Following the optical flash, the steady-state emission intensity decreases by an amount that is wavelength dependent. While it is thought that these flashes are caused by atomic association in the solid, the species responsible for the optical emission is uncertain as is the transition itself.

The Coulomb interaction among conduction electrons is introduced in the Kondo-lattice Hamiltonian. According to the Perron-Frobenius theorem, we exactly show that the ground state of the Kondo lattice in one dimension with the open boundary condition is ferromagnetic and has the total spin S=(N-${\mathit{N}}_{\mathit{c}}$)/2 in the limit U→∞, where N is the number of sites and ${\mathit{N}}_{\mathit{c}}$ is the number of conduction electrons. Exact-diagonalization calculations clearly indicate that the ferromagnetic state appears for a wide range of Coulomb strength. Our theory predicts that antiferromagnetic correlations of localized spins change into ferromagnetic ones with the increase of Coulomb interactions.

Thin films of the heavy-fermion system ${\mathrm{UCu}}_{4+\mathit{x}}$${\mathrm{Al}}_{8\mathrm{-}\mathit{x}}$ have been prepared and characterized by x-ray diffraction and by microprobe analysis. The films were found to be in an amorphous state and were subsequently investigated by ac-resistivity techniques for frequencies 20≤ν≤${10}^9$ Hz and temperatures 0.1≤T≤300 K. From these measurements it was concluded that compared to the results in the crystalline bulk material, the onset of magnetic order is suppressed at low Cu concentrations x, while the onset of a coherent heavy-fermion state is suppressed at high x. All samples investigated reveal a single-ion Kondo behavior down to the lowest temperatures. Significant deviations from the behavior of dipolar Kondo systems were detected. Up to the highest frequencies no indications of a narrow Drude peak could be found.