Temperature- and pressure-dependent measurements of the electrical resistivity ρ(T,p) of the ternary compound ${\mathrm{YbCu}}_4$Au have been performed. These investigations yield an increase of the antiferromagnetic transition temperature ${\mathit{T}}_{\mathit{N}}$ from 0.6 K at ambient pressure up to more than 2 K at 50 kbar. The bulk modulus has been determined (${\mathit{B}}_0$≊1.4 Mbar) at room temperature which together with ∂${\mathit{T}}_{\mathit{N}}$/∂p≊0.044 K/kbar derived from ρ(T,p) allows an estimation of the Grüneisen constant Ω(${\mathit{T}}_{\mathit{N}}$)≊60. A study of the field-dependent behavior of the specific heat justifies an antiferromagnetic ground state. In the scope of an empirical model, the Kondo temperature ${\mathit{T}}_{\mathit{K}}$ of ${\mathrm{YbCu}}_4$Au was obtained to be 1.65 K, which is significantly smaller than the energy of the first excited crystal-field level above the ground-state doublet.

Magnetization as a function of temperature has been measured for amorphous (a-) (${\mathrm{Fe}}_{\mathit{p}}$${\mathrm{Ni}}_{1\mathrm{-}\mathit{p}}$${)}_{80}$${\mathrm{B}}_{19}$${\mathrm{Si}}_1$ (0.0625≤p≤0.2), (${\mathrm{Fe}}_{\mathit{p}}$${\mathrm{Ni}}_{1\mathrm{-}\mathit{p}}$${)}_{80}$${\mathrm{B}}_{20}$ (0.25≤p≤1.0), and (${\mathrm{Fe}}_{\mathit{p}}$${\mathrm{Ni}}_{1\mathrm{-}\mathit{p}}$${)}_{80}$${\mathrm{P}}_{14}$${\mathrm{B}}_6$ (0.1125≤p≤1.0) alloys in the temperature range 3.8–300 K at various constant applied magnetic-field values in the interval 5≤H≤15 kOe. An elaborate data analysis reveals the following. (i) For all p in the a-(${\mathrm{Fe}}_{\mathit{p}}$${\mathrm{Ni}}_{1\mathrm{-}\mathit{p}}$${)}_{80}$(B,Si${)}_{20}$ alloy series, the contribution to the thermal demagnetization due to single-particle (SP) excitations of the weak itinerant type, though present at all temperatures, is completely dominated by that arising from spin-wave (SW) excitations in the temperature range 0≲t (=T/${\mathit{T}}_{\mathit{C}}$)≲${\mathit{t}}^{\mathrm{*}}$(p) but the reverse is true for t>${\mathit{t}}^{\mathrm{*}}$(p). However, in a-(${\mathrm{Fe}}_{\mathit{p}}$${\mathrm{Ni}}_{1\mathrm{-}\mathit{p}}$${)}_{80}$${\mathrm{P}}_{14}$${\mathrm{B}}_6$ alloys, SP excitations of the weak itinerant type give a feeble contribution, which is masked by the SW contribution, for temperatures up to 0.9${\mathit{T}}_{\mathit{C}}$ (300 K) in the concentration range 0.1125≤p≤0.25 (0.375≤p≤0.625) but for T≲300 K and p≳0.75, a significant SP contribution of the strong itinerant type accompanies a dominant SW contribution.

(ii) The spin-wave stiffness coefficient D in both the alloy series, as in fcc ${\mathrm{Fe}}_{\mathit{x}}$${\mathrm{Ni}}_{100\mathrm{-}\mathit{x}}$ alloys, varies with temperature as D(T)∼${\mathit{T}}^{5/2}$ and D(T)∼${\mathit{T}}^2$ for Fe concentrations below and above x=80p=60 at. %, respectively. (iii) The direct exchange interactions extend beyond the second nearest-neighbor distance for compositions close to, but above, the critical concentration for the appearance of long-range ferromagnetic order whereas the competing interactions in the alloy with p>0.75 confine the direct exchange to the nearest neighbors only. The observation (i) above is shown to imply that all the compositions in the a-(${\mathrm{Fe}}_{\mathit{p}}$${\mathrm{Ni}}_{1\mathrm{-}\mathit{p}}$${)}_{80}$ (B,Si${)}_{20}$ alloy series behave as weak itinerant ferromagnets while a transition from weak itinerant to strong itinerant ferromagnetism occurs at p≃0.75 in the a-(${\mathrm{Fe}}_{\mathit{p}}$${\mathrm{Ni}}_{1\mathrm{-}\mathit{p}}$${)}_{80}$${\mathrm{P}}_{14}$${\mathrm{B}}_6$ alloy series. Arguments are presented to show that the property ${\mathit{D}}_{\mathit{n}}$ (inelastic neutron scattering) ≫${\mathit{D}}_{\mathit{m}}$ (magnetization) of the alloys with p>0.75 in both the alloy series studied is a consequence of the fact that the longitudinal spin fluctuations make as significant a contribution to the ${\mathit{T}}^{3/2}$ decrease of magnetization as the transverse spin fluctuations (spin waves) do, but leave ${\mathit{D}}_{\mathit{n}}$ unaltered from its ‘‘spin-wave-only’’ value.

We report the experimental determination of the crossover exponent (φ) and the percolation critical exponents for magnetization (${\mathrm{\beta }}_{\mathit{p}}$) and spin-wave stiffness (θ) for quench-disordered (amorphous) three-dimensional (d=3) dilute Heisenberg ferromagnets. The values of φ, θ, and ${\mathrm{\beta }}_{\mathit{p}}$, so obtained, as well as those of the percolation correlation-length critical exponent ${\nu }_{\mathit{p}}$ and the conductivity exponent σ, deduced from the exponent equalities ${\nu }_{\mathit{p}}$=φ-(θ/2) and σ=(d-2)${\nu }_{\mathit{p}}$+φ, conform very well with the most accurate theoretical estimates published recently. A comparison of the presently determined exponent values with those theoretically predicted for site or bond percolation on a d=3 crystalline lattice asserts that the critical behavior of percolation on a regular d=3 lattice does not get altered in the presence of quenched randomness if the specific-heat exponent of the regular system is negative. Consistent with the Alexander-Orbach conjecture (Golden inquality), the fracton dimensionality d¯¯ of the percolating cluster at threshold (the conductivity exponent σ) turns out to be d¯¯≃4/3 (σ≤2). The present results vindicate the universality hypothesis.

We study the ferromagnetic phase transitions of single crystals from the Invar system ${\mathrm{Fe}}_{100\mathrm{-}\mathit{x}}$${\mathrm{Pt}}_{\mathit{x}}$ for different concentrations x and metallurgical order parameters S by measurements of the ac susceptibility and the magnetization. Well-defined phase transitions only exist within a limited range of the (S-x) plane—outside of this range the phase transitions are smeared. The ferromagnetic critical behavior of ordered ${\mathrm{Fe}}_{100\mathrm{-}\mathit{x}}$${\mathrm{Pt}}_{\mathit{x}}$ is described by a universal fixed point for a three-dimensional magnet with long-range magnetic interactions with the critical exponents β=0.46, γ=1.22, and δ=3.6 for the spontanous magnetization, the susceptibility, and the isothermal magnetization exponent, respectively. Random metallurgical disorder changes the numerical values of the exponents, especially close to the stoichiometric composition ${\mathrm{Fe}}_{75}$${\mathrm{Pt}}_{25}$.

One of the important but little studied characteristics of spin-polarized hydrogen is the enormous energy release due to surface recombination into molecular ${\mathrm{H}}_2$. We have studied the relaxation and distribution of this energy throughout a sample. A phenomenological parameter, F, defined as the fraction of total recombination energy released locally at the point of recombination has been measured. Our upper bound of F<0.04 demonstrates that very little energy is directly coupled to the surface at this point.

The quantum kinetic-equation method is applied to calculate the response of a metal to a temperature gradient above the superconducting transition temperature ${\mathit{T}}_{\mathit{c}}$. It is shown that the fluctuation correction to the thermoelectric coefficient of a three-dimensional superconductor is nonsingular near ${\mathit{T}}_{\mathit{c}}$. The fluctuation correction is more important in low-dimensional systems: it diverges as ln(T-${\mathit{T}}_{\mathit{c}}$) in a thin film and as (T-${\mathit{T}}_{\mathit{c}}$${)}^{1/2}$ in a filament. Applying the linear-response method, we confirm these results and prove microscopically the Onsager relation for this problem. We also demonstrate how more singular terms obtained in earlier papers are canceled out by other terms representing the corrections to the heat current operator. The universal relation, independent on the electron mean free path, between the electric and heat current operators for the fluctuating order parameter is obtained.

We present a system of equations of motion for an array of fluxons trapped in a stack of parallel tunnel-coupled long Josephson junctions driven by a uniformly distributed ac bias current. Detailed numerical simulations yield the dependence of the critical amplitude of the drive required to destroy the fluxon string on the driving frequency. In the strongly damped system, this dependence is rather trivial, but in the absence of dissipation it exhibits deep minima, which can be associated with nonlinear resonances. Near the resonance points, the critical amplitude of the dynamical (ac) drive may be lower than the amplitude of the static (dc) drive for the same system. The problem considered here may find applications for detecting trapped fluxon arrays in high-temperature superconductors.

We calculate hysteretic magnetization curves and ac-susceptibility components ${\mathrm{\chi }}_{\mathrm{n}}^{\prime }$ and ${\mathrm{\chi }}_{\mathrm{n}}^{\mathrm{″}}$ for n=1, 3, and 5 for a type-II superconducting thin circular disk subjected to a low-frequency oscillating applied field, extending previous approaches for calculating hysteretic current-density and magnetic flux-density profiles in the critical state. We also present limiting expressions for χ′=${\mathrm{\chi }}_1^{\prime }$ and χ″=${\mathrm{\chi }}_1^{\mathrm{″}}$ for both small and large values of the ac field amplitude, and we show how χ″ is simply related to the ac loss per cycle per unit volume. Finally, a comparison is made between our results and those obtained for slab and strip geometries.

We have observed a ‘‘cross-talk’’ effect in structures composed of two metal films, at least one of which is a superconductor, S, separated by a thin insulating layer. A current was passed through one film and the voltage induced in the other, V, was monitored. While V was negligibly small when film S was either normal or superconducting, V exhibited a peak in the immediate vicinity of the normal-to-superconducting transition. This effect was found to be nonreciprocal, as the sign of V depended on which film carried the current.

The influence of the electromagnetic environment on the dc Josephson current is studied allowing for nonadiabatic motion of the phase difference across a junction. The critical current is evaluated nonperturbatively within mean-field theory, fully taking into account the nonlocal kernels in the Ambegaokar-Eckern-Schön effective action. It turns out that the adiabatic approximation, which can be justified when the superconducting energy gap exceeds the charging energy, yields qualitatively correct results even for the opposite case, although quantitative deviations from the adiabatic approximation are found to be substantial in some experimentally accessible regions.

We investigate the dynamical behavior of chaotic states in two-dimensional capacitive Josephson-junction arrays (JJA) driven by uniform external ac and dc currents via numerical simulations of the resistively-shunted-junction (RSJ) model. In chaotic states the distribution functions for the fluctuating voltages in individual transverse junctions have exponential tails similar to findings for the temperature in fluid turbulence. We present a mechanism for the origin of this behavior in arrays based on the random generation and motion of vortices. We also provide evidence to argue that collectively chaotic behavior does not occur.

We discuss numerical studies of the properties of underdamped Josephson junction arrays driven with a constant current, concentrating on the effect of self-induced magnetic fields and the dynamics of row-switched states. In the rwo-switched state, junction dynamics are ordered both along rows and columns of the array. Junctions in a switched row coherently oscillate, and ordered vortex patterns distinct from ground-state configurations are observed to form along array columns. Strong self-field effects can perturb the ordered vortex states in the row-switched regime and reduce the tendency of the junction to coherently oscillate.

Using numerical simulation, we study the effect of self-induced magnetic fields on the properties of Josephson-junction arrays driven with combined ac and dc currents. We show that inductive effects in strongly overdamped arrays can induce subharmonic giant Shapiro steps in zero applied magnetic field. Inductive effects can also reduce the range of dc current biases over which the array can lock to the ac current. Calculation of the correct current-voltage characteristics of the array requires considering mutual inductance interactions between all cells in the array. We also discuss efficient numerical schemes for including these induced fields in dynamic simulation of Josephson arrays.

We measured the in-plane thermoelectric-power coefficients (S) of c-axis-oriented ${\mathrm{Sr}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Nd}}_{\mathit{x}}$${\mathrm{CuO}}_{2\mathrm{-}\mathrm{\delta }}$ (0≤x≤0.125) thin films deposited by means of a pulsed-laser deposition technique. The sign of S was negative for all the samples deposited on ${\mathrm{SrTiO}}_3$ substrates and its absolute value decreased with increasing x. The value of S for ${\mathrm{Sr}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Nd}}_{\mathit{x}}$${\mathrm{CuO}}_{2\mathrm{-}\mathrm{\delta }}$ with x=0.125 was demonstrated to be strongly dependent on the substrate material on which the a-axis length depended. On the other hand, the value of S for ${\mathrm{Sr}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Nd}}_{\mathit{x}}$${\mathrm{CuO}}_{2\mathrm{-}\mathrm{\delta }}$ with x=0.05 was found to be rather weakly dependent on the a-axis length. These observations were discussed based on bond-valence-sum calculations. The variation in S of the x=0.125 sample was attributed to the change in the electronic structure.

We calculated the critical temperature of a two-dimensional superconductor taking into consideration the effect of disorder on the Coulomb pseudopotential and on the magnetic correlations. The results are in agreement with the experimental data for high-temperature superconductors.

Half-integral constant voltage steps were observed in many high-${\mathit{T}}_{\mathit{c}}$ grain-boundary Josephson junctions of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ when a microwave field was applied. Five distinct observed behaviors of the widths of both integral and half-integral steps as a function of microwave amplitude, Δ${\mathit{I}}_{\mathrm{dc}}$(${\mathit{I}}_{\mathrm{ac}}$), are reproduced by simulations of two or three junctions in parallel. This provides quantitative evidence that a single high-${\mathit{T}}_{\mathit{c}}$ grain-boundary junction is composed of several junctions in parallel. These junctions are formed by the overlap of superconducting filaments on either side of the grain boundary, and the spacing between ones with relatively large critical currents is ∼20 μm.

The cuprate superconductor, ${\mathrm{La}}_{1.68}$${\mathrm{Nd}}_{0.14}$${\mathrm{Na}}_{0.10}$${\mathrm{K}}_{0.082}$${\mathrm{CuO}}_4$, was prepared by selective precipitation from a NaOH/KOH eutectic melt at 350 °C. Synchrotron x-ray-diffraction data indicate that the structure is related to the tetragonal T-structure type of doped ${\mathrm{La}}_2$${\mathrm{CuO}}_4$, but the presence of superlattice reflections shows the unit cell to be tripled along the c direction [a=3.763(1) Å and c=39.63(2) Å]. The existence of this supercell is confirmed by electron microscopy and electron diffraction, which show that the tripling is due to cation ordering. Careful examination of the positions and widths of the superlattice reflections in the synchrotron data indicates that the dopant cations are in fact only partially ordered, and a simple Hendricks-Teller model using an ordering scheme suggested by the electron microscopy data agrees well with the diffraction data. There is no structural change (orthorhombic distortion) between room temperature and 28 K, just above the onset temperature for superconductivity determined from magnetic-susceptibility measurements. The Meissner fraction, estimated to be 80% at 5 K, indicates that this material is a bulk superconductor.

Within the framework of an electronic density response approach, recently worked out to describe screening, lattice dynamics, and the electron-phonon interaction in the high-temperature superconductors, it is shown, using ${\mathrm{La}}_2$${\mathrm{CuO}}_4$ as an example, that the already large (nonlocal) electron-phonon coupling as calculated in the adiabatic approximation is further strongly enhanced for certain phonons in the high-temperature superconductors (HTSC’s) in the nonadiabatic regime where dynamical screening comes into play. At the same time, a low-lying plasma mode of the electronic system appears for wave vectors in the (0,0,1) direction which mixes strongly with the phonons of appropriate symmetry and leads in addition to the phonons to very large changes of the crystal potential. Such a coexistence of phononlike and plasmonlike modes in the same frequency range is argued to be a specific feature of the HTSC’s related to their layered structure and weak screening of the strong ionic forces in the c direction. It leads to a significant increase of the retarded attractive interaction and provides, at least in part, an understanding of the reduction of the oxygen isotope effect in these materials, which is difficult to explain for a pure (harmonic) electron-phonon superconductor.

The Hall coefficient ${\mathit{R}}_{\mathit{H}}$(T) has been measured for ${\mathrm{Nd}}_{1.85}$${\mathrm{Ce}}_{0.15}$${\mathrm{CuO}}_{4\mathrm{-}\mathrm{\delta }}$ c-axis-oriented thin films at various oxygen deficiency levels produced by successive reduction treatments. The results definitely reflect the change in ${\mathit{R}}_{\mathit{H}}$(T) brought about by a decrease in the oxygen content with the Ce concentration precisely fixed. It is found that with decreasing oxygen content ${\mathit{R}}_{\mathit{H}}$ increases monotonically and changes sign from negative to positive. Regardless of the sign, ${\mathit{R}}_{\mathit{H}}$ shows significant temperature dependence at low temperatures. Superconductivity is observed in the oxygen compositional range in which ${\mathit{R}}_{\mathit{H}}$ changes sign. The sign reversal is interpreted in terms of the band-structure effect rather than the two-band model.

Time-logarithmic magnetic-relaxation measurements have been obtained in an epitaxial ${\mathrm{Tl}}_2$${\mathrm{Ba}}_2$${\mathrm{Ca}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{x}}$ thin film for the zero-field cooled and remanent magnetization, with low and moderate applied magnetic fields. With low fields (${\mathit{H}}_{\mathit{a}}$=50 Oe), the normalized magnetic relaxation rates, S≡‖d(M/${\mathit{M}}_{\mathit{i}}$)/d lnt‖, are temperature independent below T≃3.4 K, and the transition from the thermal to the quantum regime appears to be quite sharp. With moderate fields (${\mathit{H}}_{\mathit{a}}$=500 Oe), the transition becomes smooth and takes place at temperatures higher than with low fields, T≃7 K. This seems to indicate that the mechanism responsible for low-temperature relaxation would be nondissipative Hall tunneling of two-dimensional vortices, rather than dissipative vortex motion.