We have performed a calculation on the change of the critical temperature ${\mathit{T}}_{\mathit{c}}$ of a superconducting host with respect to the change of the concentration c of two-channel Kondo impurities in the limit of vanishing c. ${\mathit{T}}_{\mathit{c}}$ suppressions are found for both spin-singlet flavor-triplet and spin-triplet flavor-singlet pairings for temperatures as low as ${\mathit{T}}_{\mathit{K}}$/${\mathit{T}}_{\mathit{c}0}$>1, where ${\mathit{T}}_{\mathit{K}}$ and ${\mathit{T}}_{\mathit{c}0}$ are the Kondo temperature and the critical temperature of the host in the absence of impurities, respectively. The suppression is much larger for the spin-singlet flavor-triplet pairing. We also found empirically that (∂${\mathit{T}}_{\mathit{c}}$/∂c${)}_{\mathit{c}=0\mathrm{}}$ can be expressed as a product of two functions g(${\mathit{T}}_{\mathit{K}}$/T) and f(${\lambda }_0$,${\mathrm{\omega }}_0$), where ${\lambda }_0$ and ${\mathrm{\omega }}_0$ are electron phonon coupling and Einstein frequency, respectively.

Measurements of the resistivity, Hall effect, Nernst effect, and thermopower in the normal and in the mixed state of ${\mathrm{La}}_{1.85}$${\mathrm{Sr}}_{0.15}$${\mathrm{CuO}}_4$ for magnetic fields between 0 and 7 T and temperatures between 20 and 300 K are reported. In the normal state the resistivity shows a linear temperature dependence, while thermopower and Hall effect are only weakly temperature dependent. The Nernst effect in the normal state is about two orders of magnitude smaller than in the mixed state and of opposite sign, with a sign change occurring at about 60 K. Resistivity and thermopower show a strong broadening of the superconducting transition in a magnetic field. The Hall coefficient is positive, i.e., there is no sign change near ${\mathit{T}}_{\mathit{c}}$ in the mixed state in the whole temperature and magnetic-field range examined.

Current-voltage characteristics of superconductor-normal-metal-superconductor junctions are calculated. Multiple Andreev reflections and the effect of normal elastic scattering at the superconductor-normal-metal interfaces are considered. The normal reflections of quasiparticles appear due to the mismatches in Fermi velocities and effective masses of superconductors and weak link. The wave-packet solution of nonequilibrium time-dependent Bogoliubov–de Gennes equations was obtained with the help of the boundary conditions on the wave function of the quasiparticle. At voltages of interest for multiple Andreev reflections (eV<2Δ), the result is applied to the rather thick weak links where the density of bound Andreev states is small.

Neutron-diffraction measurements have been performed on supercritical fluid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ in the 14–85 K temperature range along an isochore passing close to the lower λ point. The quantum nature of the particle pair distribution functions is analyzed by comparing our results with classical molecular-dynamics simulations. Deviations from classical behavior are seen to persist up to T=85 K although they are much larger at lower temperatures. A measure of these deviations is obtained through an empirical parameter describing quantum delocalization around the classical positions. Limiting low-temperature behavior of this parameter is discussed and compared with direct experimental evidence of low-temperature quantum delocalization obtained from the particle momentum distribution.

We have studied the acoustics of fluid-filled porous media by measuring the velocity and attenuation of ultrasonic (4 to 31 MHz) shear waves in several different ceramic materials. We used liquid helium as the pore fluid and made measurements down to low temperatures where the helium was superfluid. This allowed us to completely eliminate the effects of viscosity and thus to unambiguously determine the velocity changes and attenuation due to the Biot mechanism (fluid ‘‘sloshing’’ in the pores). By using ceramics with different pore sizes and a corresponding wide range of permeabilities (from 2.6×${10}^{\mathrm{-}14}$ to 3.5×${10}^{\mathrm{-}11}$ ${\mathrm{cm}}^2$) we were able to make measurements in both the low-frequency regime (where the fluid is viscously locked to the porous frame) and in the high-frequency regime (where most of the fluid is decoupled from the frame). One of the samples had an extremely high porosity (92%), allowing us to study the fluid motion in a very open geometry. In all cases, we found that the Biot model could quantitatively describe the temperature and frequency dependence of our results. This allowed us to determine the structural parameters of the porous media (pore tortuosity c, permeability ${\mathrm{\kappa }}_{\mathit{a}}$ and effective pore size Λ), something which has previously required measurements of the Biot slow wave (fourth sound and second sound in superfluid helium). The acoustically determined parameters were compared to the independently measured static permeability, ${\mathrm{\kappa }}_0$, and to previous experimental and theoretical work on model porous media. Our results indicate that, even in materials with irregular pores and a range of length scales, acoustic measurements made in either the low- or high-frequency regime can be used to estimate the permeability.

In this detailed study of inhomogeneous superfluid turbulence created in a spatially nonuniform flow, we measure the turbulent dissipation produced by thermal counterflow in a nonuniform rectangular channel. The width of the channel increases linearly with position r along the channel length to impose a known spatial nonuniformity on the velocity in the flow direction, V∝Q̇/r, where Q̇ is the heat current. Because the channel widens gradually, this divergence is weak, producing a small deviation from uniform flow. Probes embedded in the channel walls allow access to the local temperature at several positions r. We employ sensitive differential thermometry to measure the temperature difference between any two of these locations to high precision. The single turbulent state we observe is clearly the analog of the T-II state of fully developed, homogeneous turbulence present in uniform flows. Making an approximation of local uniformity, we attempt to extend the theoretical description of turbulence in uniform flows to our results by replacing the uniform velocity with the local velocity in the expression for the temperature gradient. When applied locally, the Schwarz model of homogeneous turbulence, so successful in describing the T-II state in uniform flows, has the wrong dependence on both r and Q̇ to characterize our results. The Geurst three-fluid model, which extends to situations where a gradient is present in the line density in an otherwise uniform flow, likewise does not agree with our observations when used in the local approximation.

We therefore find that even a small perturbation imposed on a uniform flow radically changes the nature of the T-II state, and the inhomogeneous T-II state we observe is markedly different from that of homogeneous turbulence. This rather surprising result points to the need for a proper theory, or direct numerical simulation, of inhomogeneous turbulence in nonuniform flow. By introducing a substantial modification to the Schwarz line density, we identity a modified line density that constitutes a three-parameter fit yielding excellent agreement with our results. Finally, we find that the transition from laminar to turbulent flow occurs at a stationary front. Observations of the location of this front as a function of Q̇ show that the critical condition for maintaining turbulence is more consistent with a minimum line density than a critical velocity.

We present evidence for and study the properties of coreless Josephson vortices (parallel to the superconducting layers) in the organic superconductor (BEDT-TTF${)}_2$Cu(NCS${)}_2$, using ac susceptibility measurements in the mixed state. We observe (1) extremely weak-pinning restoring forces for flux motion parallel to the layers, due to the absence of the vortex normal core; (2) single vortex (noncollective) pinning, due to weak interactions between Josephson vortices; (3) the ‘‘lock-in’’ of the vortices parallel to the layers when the dc field is applied at an arbitrary angle; and (4) highly nonlinear response to the ac field h in the lock-in state, above a threshold of h∼0.5 G. The behavior of tilted flux lines is dominated by the much stronger pinning and collective interactions of two-dimensional Abrikosov ‘‘pancake’’ vortices, and linear response is largely restored when the flux lines unlock from the layers. We measure the Josephson penetration depth to be ${\lambda }_{\mathrm{\perp }}$(T=5 K)∼200 μm, yielding a penetration depth anisotropy of γ=${\lambda }_{\mathrm{\perp }}$/${\lambda }_{\mathrm{\parallel }}$∼160-350.

Analytic expressions are derived for the root-mean-square (rms) radius of a pair of fermions in a BCS many-fermion state in one, two, and three dimensions, in terms of the BCS gap energy and the associated chemical potential. These expressions are valid for any coupling strength of any pair interaction model implying a momentum-independent gap energy. The latter holds, e.g., for an attractive δ pair potential examined in the one-dimensional (1D) case (whose N-fermion ground state can be determined exactly) or for the BCS (electron-phonon) model interaction in any dimension. Weak-coupling and/or high-density limits for the rms radius are identical in 1D, 2D, and 3D, and reduce to the familiar well-known Pippard result to within a factor of order unity. In contrast, strong-coupling and/or low-density limits coincide in 1D and 3D, but differ by a factor of order unity in the 2D limit, and in each case are essentially the size of a single, isolated pair. The 1D δ interaction McGuire-Yang-Gaudin many-fermion model is studied in detail. The interaction renormalization scheme of Miyake and of Randeria, Duan, and Shieh, and the BCS interaction model, both in 2D, are employed to analyze cuprate superconductor empirical results. Reasonable agreement between theoretical rms radii with experimental coherence lengths suggests that cuprates can be described moderately well as weakly coupled superconductors within the BCS-Bose formalism.

We study quantum interference effects due to electron motion on the kagome$aa— lattice in a perpendicular magnetic field. These effects arise from the interference between phase factors associated with different electron closed paths. From these we compute, analytically and numerically, the superconducting-normal phase boundary for kagome$aa— superconducting wire networks and Josephson junction arrays. We use an analytical approach to analyze the relationship between the interference and the complex structure present in the phase boundary, including the origin of the overall and fine structure. Our results are obtained by exactly summing over (∼${10}^{21}$) closed paths, each one weighted by its corresponding phase factor representing the net flux enclosed by each path. We expect our computed mean-field phase diagrams to compare well with several proposed experiments.

The dynamics of the normal/superconducting interface in type-I superconductors has recently been derived from the time-dependent Ginzburg-Landau theory of superconductivity. In a suitable limit these equations are mapped onto a ‘‘free-boundary’’ problem, in which the interfacial dynamics are determined by the diffusion of magnetic flux in the normal phase. The magnetic field at the interface satisfies a modified Gibbs-Thomson boundary condition which involves both the surface tension of the interface and a kinetic coefficient for motion of the interface. In this paper we calculate the surface tension and kinetic coefficient numerically by solving the one-dimensional equilibrium Ginzburg-Landau equations for a wide range of κ values. We compare our numerical results to asymptotic expansions valid for κ≪1, κ≊1/ √2 , and κ≫1, in order to determine the accuracy of these expansions.

We have carried out in situ resistivity measurements in a high-voltage electron microscope to determine how the superconducting transition temperature ${\mathit{T}}_{\mathit{c}}$ and the resistivity ρ(T) change with low-temperature electron irradiation. We find that point defects introduced by oxygen displacements from the ${\mathrm{CuO}}_2$ planes make the dominant contribution to the irradiation-induced suppression of ${\mathit{T}}_{\mathit{c}}$. We find that ${\mathit{dT}}_{\mathit{c}}$/dρ is ∼-0.30±0.04 K/μΩ cm. Analysis of this result indicates the effect of strongly anisotropic superconductivity, and rules out isotropic s-wave pairing. Even though our data cannot determine whether the pairing symmetry is s or d wave, they provide a test for further theoretical developments.

Assuming s-wave boson-exchange electronic-pairing superconductivity, we obtain indications on the electron-boson coupling spectral function of cuprates and fullerides from the experimental gap measurements. In cuprates, the coupling is quite strong and it is concentrated in a region of frequencies of the order of 25–50 meV. From a careful analysis of the incompatibility of the infrared and the other gap measures on these materials, we obtain evidence for the validity of our framework. If the large gap values reported by tunneling on fullerides are confirmed then at least 90% of the coupling in these materials is concentrated in a low-energy region at ≊10 meV, the rest being concentrated in a high-energy region 180–200 meV.

The field suppression of the critical current density in superconductors, ${\mathit{j}}_{\mathit{c}}$, is modeled within the critical-state theory by the modified Kim-Anderson relation, ${\mathit{j}}_{\mathit{c}}$(B,T)=α(T)/(B+h${)}^{\mathit{n}}$, with α, h, and n being phenomenological parameters. This quasistatic approach of analyzing the flux evolution in a sample under varying external field allows us to explain a few ubiquitous, experimentally known results: the occurrence of a peak in the M(H) hysteresis at low fields and the saturation of the remanent magnetization as a function of sample size for larger sample sizes. In the present work, the critical-state concept is extended to describe the flux-profile evolution under temperature changes. Within this approach, the temperature dependences of the zero-field-cooled magnetization and of the remanent magnetization are calculated and compared with measurements performed on single crystals of ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_8$, ${\mathrm{UPt}}_3$, and ${\mathrm{URu}}_2$${\mathrm{Si}}_2$. This approach has been applied for computing magnetocaloric effects in a superconductor exposed to a changing external field. Estimates on the value and temperature dependence of the field for flux jumps are given for heavy-fermion superconductors and they are compared with the known results for ${\mathrm{URu}}_2$${\mathrm{Si}}_2$.

The temperature and field dependencies of the Hall voltage in the mixed and in the normal state of supeconducting ${\mathit{L}}_{1.85}$${\mathrm{Ce}}_{0.15}$${\mathrm{CuO}}_4$ (L=Nd and Sm) single crystals have been measured. Crystals having different critical temperatures (15<${\mathit{T}}_{\mathit{c}}$<20 K) and transition widths (0.5<Δ${\mathit{T}}_{\mathit{c}}$<3 K) have been used in order to explore the effects of crystal quality on the controversial mixed-state Hall effect. In spite of the cerium doping, the normal-state Hall coefficient is found to be positive at low temperatures (T≳${\mathit{T}}_{\mathit{c}}$) for all measured crystals. At higher temperatures (T≫${\mathit{T}}_{\mathit{c}}$), the Hall coefficient becomes negative. In the mixed state, the Hall resistivity displays an anomalous ${\mathrm{\rho }}_{\mathit{x}\mathit{y}}$(H) peak. However, in contrast to what has been found in other high-${\mathit{T}}_{\mathit{c}}$ superconductors, the polarity of the anomaly can have, depending on temperature, either the same sign or opposite to the Hall effect in the normal state, just above ${\mathit{T}}_{\mathit{c}}$. The appearance of the ${\mathrm{\rho }}_{\mathit{x}\mathit{y}}$(H) anomaly is not ubiquitous but is only observed in crystals of narrow transition widths. To account for these results we explore several of the possible explanations that have been already proposed.

We have measured the ac susceptibility of several high quality twinned and detwinned ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ (YBCO) single crystals. We see multiple peaks in the χ’’(T) curves, and explain them as a manifestation of the peak effect in ${\mathit{J}}_{\mathit{c}}$(T) near ${\mathit{T}}_{\mathit{c}}$ and increased ${\mathit{J}}_{\mathit{c}}$ at low temperatures. We have investigated the effect of electron irradiation on these peaks, and found that it increases ${\mathit{J}}_{\mathit{c}}$ of the sample. We suggest that the narrow, high-temperature (just below ${\mathit{T}}_{\mathit{c}}$) peak in χ’’(T) is an indication that the sample is clean, and therefore exhibits a first-order melting transition of the flux-line lattice. We also propose that the peak effect in ${\mathit{J}}_{\mathit{c}}$ is a result of the softening of the lattice before it melts, in accordance with Pippard’s model of this phenomenon for low-temperature superconductors. Finally, we suggest a method of determining the true irreversibility line.

The crystal structure of the newly discovered 117-K superconductor, (Cu,C)${\mathrm{Ba}}_2$${\mathrm{Ca}}_3$${\mathrm{Cu}}_4$${\mathrm{O}}_{11+\mathrm{\delta }}$, has been refined from time-of-flight neutron-powder-diffraction data. The structure has ‘‘average’’ tetragonal symmetry and is similar to that of Tl(or Hg)${\mathrm{Ba}}_2$${\mathrm{Ca}}_3$${\mathrm{Cu}}_4$${\mathrm{O}}_{\mathit{y}}$. C atoms in ${\mathrm{CO}}_3$ groups substitute at the Cu site in the (Cu,C)${\mathrm{O}}_{1+\mathrm{\delta }}$ layer leading to a chemical composition of (${\mathrm{Cu}}_{0.68}$${\mathrm{C}}_{0.32}$)${\mathrm{Ba}}_2$${\mathrm{Ca}}_3$${\mathrm{Cu}}_4$${\mathrm{O}}_{11.06}$. This compound has two inequivalent kinds of ${\mathrm{CuO}}_2$ layers with pyramidal and square coordination of Cu to oxygen. The inner ${\mathrm{CuO}}_2$ layers with Cu in four coordination are less corrugated than the outer ones with Cu in five coordination, and exhibit a structure very similar to those of infinite-layer compounds.

We present a Fermi-liquid description of magnetic scaling and spin-gap behavior in strongly correlated electron systems. We show that a gap in the spin-excitation spectrum is a natural consequence of the existence of a second energy scale of magnetic origin, in systems of itinerant, but nearly localized electron spins, such as are found in the cuprate superconductors and heavy-electron systems. A second energy scale leads to a frequency-dependent restoring force ${\mathit{f}}^{\mathit{a}}$(q,ω) that becomes stronger as the frequency increases. A spin gap at Q=(π/a,π/a) is a consequence of these frequency-dependent vertex corrections; it takes on a constant value when the antiferromagnetic correlation length becomes a constant. We show that the spin-spin response function reduces to that recently introduced by Barzykin to describe the damping by particle-hole excitations of the spin-wave spectrum found in the N=∞ quantum nonlinear σ model. We discuss fully doped and underdoped cuprate superconductors, the appearance of a pseudogap of magnetic origin in the quasiparticle density of states in the underdoped system, and the constraints nuclear magnetic resonance and neutron-scattering experiments impose on the magnitude and temperature dependence of the Fermi-liquid parameters.

The magnetic properties of ${\mathrm{HoBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ are investigated by taking account of the quantization of the induced ${\mathrm{Ho}}^{3+}$ moments to superconducting vortices. Based on the London model, we show that the induced moment decreases the penetration depth, and increases the equilibrium flux density by a factor of (1+4πχ), where χ is the susceptibility of the ${\mathrm{Ho}}^{3+}$ ions. Therefore, when the induced moment is greater than the Meissner diamagnetism, flux lines move into the specimen as temperature decreases and move out of it as temperature increases. This flux motion, together with flux pinning, causes different magnetization hysteresis, particularly in flux expulsion, from that of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$.

We have measured the effect of cobalt on the superconducting transition temperature (${\mathit{T}}_{\mathit{c}}$) and the dc irreversibility line (IRL) of single-crystal ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ (YBCO). ${\mathit{T}}_{\mathit{c}}$ of a single crystal annealed in flowing oxygen at atmospheric pressure is markedly lower than that of a polycrystalline sample with the same cobalt concentration and annealing pressure. High-pressure oxygen annealing (HPA) increases ${\mathit{T}}_{\mathit{c}}$ to a value comparable to that of a polycrystalline sample. This increase implies that Co-doped YBCO single crystals low-pressure annealed (LPA) in flowing oxygen are not as fully oxygenated as polycrystalline samples. The IRL was found to have two different functional behaviors, one for low concentrations of cobalt and another for high concentrations. By comparison with the Bi-Sr-Ca-Cu-O and Tl-Ba-Ca-Cu-O systems, this change in functional behavior is attributed to a decrease in coupling between Cu-O planes.

We have synthesized overdoped metallic samples of ${\mathrm{Tl}}_2$${\mathrm{Ba}}_2$${\mathrm{CuO}}_{6+\mathrm{\delta }}$ and ${\mathrm{TlSr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ beyond the superconductor-metal phase boundary by annealing the samples in high-pressure oxygen. Oxygen incorporation is clearly demonstrated by structural refinements of neutron powder-diffraction data and reversible changes in sample weight. The same ∼${\mathit{T}}^2$ scattering mechanism in transport properties as previously observed in superconducting samples is observed in the present overdoped metallic samples. Hall coefficients do not change sign and are still positive for the overdoped metals. Although the crystal structures of both compounds change gradually and continuously with oxygen doping beyond the superconductor-metal phase boundary, the electronic structure of the ‘‘further overdoped metals’’ might still be near the supeconductor-metal phase boundary.