Pressure-dependent measurements of the electrical resistivity, ρ(T,ρ), of ${\mathrm{YbCu}}_4$Ag show a noticeable decrease of the resistivity maximum and a strong increase of the coefficient A in the ${\mathit{T}}^2$ term of ρ(T) when the pressure rises. This indicates that the Kondo temperature ${\mathit{T}}_{\mathit{K}}$ of this compound decreases with increasing pressure, stabilizing the 4${\mathit{f}}^{13}$ configuration of the Yb ion. In the scope of the compressible Kondo model, a value for the coupling constant times the electronic density of states at the Fermi energy has been deduced.

This work studies the appearance of a Haldane gap in quasi-one-dimensional antiferromagnets in the long-wavelength limit, via the nonlinear σ model. The mapping from the three-dimensional, integer-spin Heisenberg model to the nonlinear σ model is explained, taking into account two antiferromagnetic couplings: one along the chain axis (J) and one along the perpendicular planes (${\mathit{J}}_{\mathrm{\perp }}$) of a cubic lattice. An implicit equation for the Haldane gap is derived, as a function of temperature and coupling ratio ${\mathit{J}}_{\mathrm{\perp }}$/J. Solutions to these equations show the existence of a critical coupling ratio beyond which a gap exists only above a transition temperature ${\mathit{T}}_{\mathit{N}}$. The cutoff dependence of these results is discussed.

The interface magnetocrystalline anisotropy (MCA) of a Pd/Co/Pd sandwich is calculated by employing the recently proposed state-tracking approach and the full-potential linearized augmented-plane-wave energy band structure. The strong positive (perpendicular) Co-Pd interface MCA arises from the hybridization between the out-of-plane Co bonding xz(yz) states with the interface Pd atom: the large d-d bonding strength and high energy of the Pd d states shifts a substantial component of these Co states (bands) above the Fermi energy. Comparisons with a previous study on the Co-Cu interface makes clear that the difference between the effects of two groups of substrates (Cu, Ag versus Pd, Au, etc.) depends on the interface bonding strength and the position of the d states of the interface atoms—as is qualitatively revealed by an analysis based on the use of an effective-ligand-interaction model.

We have obtained an analytic expression for the reflectivity ${\mathit{R}}^{\pm }$(θ,λ) of polarized neutrons from a superconductor in the Meissner state, where λ is the magnetic-field penetration depth and θ is the incident angle of the neutron beam. The result is derived as an exact solution of the 1D Schrödinger equation in the continuum limit, with an interaction potential ${\mathit{V}}^{\pm }$(x)=${\mathit{V}}_{\mathit{N}}$±μH[exp(-x/λ)-1], where ${\mathit{V}}_{\mathit{N}}$ is a constant representing the nuclear interaction, μ is the neutron magnetic moment, and H is the applied magnetic field. The solution for ${\mathit{R}}^{\pm }$(θ,λ) reveals surprising features in its λ dependence that have not been discovered in previous numerical studies. In particular, ${\mathit{R}}^{\mathrm{-}}$(θ,λ) displays an oscillatory dependence on λ within a narrow angular range immediately above the total reflection angle (θ≳${\mathrm{\theta }}_{\mathit{c}}^{\mathrm{-}}$), instead of a monotonic dependence as inferred from earlier numerical calculations. The solution also reveals that complete transmission for the down-spin state [${\mathit{R}}^{\mathrm{-}}$(θ,λ)=0] may occur when λ and H satisfy certain conditions. In addition to the analytic expression of the reflectivity for a semi-infinite sample, we have also obtained the reflectivity R̃ ${}_{}{}^{\pm }{}_{}{}^{}$(θ,λ) from a thick superconducting film where the magnetic field can penetrate from both sides. In the case of a free-standing film of which the two surfaces are identical, we have simply R̃ ${}_{}{}^{\pm }{}_{}{}^{}$=2${\mathit{R}}^{\pm }$/(1+${\mathit{R}}^{\pm }$), and therefore all the interesting features of the exact solution ${\mathit{R}}^{\pm }$ persist in R̃ ${}_{}{}^{\pm }{}_{}{}^{}$. Finally, the exact solution can also be applied to other systems where the scattering potential has an exponential dependence with distance, such as for the magnetization at the surface of a ferromagnet.

By means of a variational scheme of the Pekar type, a system of strongly coupled ripplonic polarons on the outer surface of liquid-helium film under the influence of a magnetic field of arbitrary strength is studied. Analytical expressions for the ground-state energy, the effective mass, the average number of virtual ripplons around an electron, and the spatial extension of the electron are obtained as functions of the magnetic-field strength and the electron-ripplon coupling constant and compared with previous work.

We have calculated the ground-state energy and the fluctuation contribution to the free-energy for the Hubbard model at half filling. We have used a gauge that leads to the Hartree-Fock solution in the saddle-point approximation. The mean-field ground-state energy calculated here agrees with the results obtained by Dagotto et al. [Phys. Rev. B 45, 10 741 (1992)] with use of the exact-diagonalization techniques on a 4×4 site cluster. In the large-U limit, the ground-state energy per site is obtained as ${\mathit{E}}_{\mathit{g}}$/N=-U/4-J+O(${\mathit{J}}^2$/U). The fluctuation contribution to the free-energy was found to have a stronger temperature and U dependence than the mean-field contribution, and to play a dominant role for strongly correlated systems.

The elastic moduli of the vortex lattice in uniaxial strong type-II superconductors are calculated for magnetic inductions ${\mathit{H}}_{\mathit{c}1}$≪B<0.2${\mathit{H}}_{\mathit{c}2}$ arbitrarily tilted with respect to the crystal axes. The derivation of the elastic moduli for the anisotropic situation is based on the mapping of the anisotropic problem to the corresponding isotropic situation. In performing this transformation we use the scaling rules connecting the anisotropic quantities with their known isotropic counterparts. The scaling approach is valid in the dispersive region which accounts for the major part of the Brillouin zone for fields B≫${\mathit{H}}_{\mathit{c}1}$. The advantage of this method lies in its simplicity and in its general applicability. The resulting moduli agree with the elastic moduli derived previously by means of the traditional approach based on the expansion of the anisotropic London functional. In addition to the usual moduli, we obtain a new mixed shear-tilt modulus.

We study a ring of N Josephson junctions with gauge charges induced by external sources and in the presence of a magnetic field. At zero temperature the system is mapped into a standard two-dimensional classical XY model, thus shown to exhibit the Kosterlitz-Thouless transition between the superconducting and the insulating phases, which are characterized by the gauge-invariant phase and charge correlation functions. In particular, the persistent current and voltage in the two phases are obtained, and the duality relation between the strong-coupling limit and the weak-coupling limit is discussed.

Following the procedure recently proposed by Wiegmann, we improve his analysis and derive a complete effective Lagrangian describing long-wavelength fluctuations around hypothetical three-dimensional (3D) and 2D chiral states of Heisenberg spin systems. We study realizations of the previously proposed high symmetry group of the continuous theory [SO(3,1)×SU(2)×U(1) in 3D and SO(2,1)×${\mathrm{U}}_{\mathit{c}}$(1)×${\mathrm{U}}_{\mathit{s}}$(1) in 2D] and observe that, in general, Lorentz symmetry is broken already in the bare Lagrangian, although it could be restored after a renormalization. In contrast to the conjectures made by Wiegmann, an additional non-Abelian SU(2) gauge symmetry expected in 3D (and its Abelian counterpart in 2D) are, in fact destroyed due to a spontaneous parity violation in chiral ground states.

We study the excess conductance due to superconductance fluctuations of a metallic two-dimensional (2D) channel at temperatures slightly above the superconductor transition temperature. The conductance of a straight channel with arbitrary constant width d is computed. We find that the characteristic length scale ξ depends on both temperature and frequency. The crossover from 1D (at d≪ξ) to 2D (at d≫ξ) behavior is characterized by a continuously varying effective dimensionality: 1≤${\mathit{D}}_{\mathit{e}}$(d/ξ)≤2. For a channel with hyperbolic edges the results are similar, only the length of the channel is replaced by an effective length, which is logarithmic in the actual length. When a perpendicular magnetic field is applied, the excess conductance follows a scaling law with the number of vortices that fit in the area occupied by a fluctuating Cooper pair. The applied field induces a change in the effective dimensionality of the fluctuations. In a wide channel there is a direct crossover from 2D to 0D character. For a narrow channel, there is an intermediate regime, due to the formation of edge states, where the fluctuations are 1D. The length scale for this crossover depends on the temperature and the shift in ${\mathit{T}}_{\mathit{c}}$ due to the field. The dependence of the transition temperature on the magnetic field is computed for arbitrary channel width. A dimensional crossover from 1D to 2D, as a function of the channel width, is observed. The characteristic length scale in this case is the radius of a vortex.

As an example of a system with competing (frustrated) interactions, we study a model of the spatial arrangement of flux lines in a layered superconductor, with the magnetic field parallel with the layers, and in the limit of strong pinning to the layers. In this approach, we approximate the energy-minimization problem by a discrete nonlinear dynamical system, in which a vortex structure is built up one layer at a time. We find a complex variety of metastable states nearly degenerate with the ground state, including lattices with unit cells of many layer spacings, and disordered structures which correspond to chaotic trajectories of the dynamical map. The latter case provides an example of (metastable) quenched disorder in a model with no intrinsic randomness.

There is a great deal of interest in attributing the high critical temperatures of the cuprates to either the proximity of the Fermi level to a Van Hove singularity or to structure of the superconducting pairing potential in momentum space far from the Fermi surface; the latter is particularly important for spin-fluctuation-mediated pairing mechanisms. We examine these ideas by calculating the critical temperature ${\mathit{T}}_{\mathit{c}}$ for model Einstein-phonon- and spin-fluctuation-mediated superconductors within both the standard, Fermi-surface-restricted Eliashberg theory and the exact Eliashberg theory, which accounts for the full momentum structure of the pairing potential and the energy dependence of the density of states. Our computations employ band structures chosen to model both the ${\mathrm{La}}_2$${\mathrm{Sr}}_{2\mathrm{-}\mathit{x}}$${\mathrm{CuO}}_4$ and ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ families. For our spin fluctuation calculations, we take the dynamical susceptibility to be the pairing potential and examine two models of this susceptibility in the cuprates. We compare and contrast these models with available magnetic neutron-scattering data, since these data provide the most direct constraints on the susceptibility. We conclude that a model constrained by neutron-scattering measurements will not yield the observed 90-K ${\mathit{T}}_{\mathit{c}}$’s regardless of the strength of the electron-spin fluctuation coupling, even when the Van Hove singularity and momentum-space structure are accounted for; moreover, when transport constraints are applied to this type of model, we expect ${\mathit{T}}_{\mathit{c}}$≊10 K, as was found in an earlier paper. We also find that the Van Hove singularity enhances ${\mathit{T}}_{\mathit{c}}$ much less effectively than weak-coupling calculations would suggest.

The model of a layered structure constituted by two, periodically arranged, London superconducting layers is considered in a linear steady-current regime. The model admits exact solutions in a quite general situation. The use of these solutions allows one to derive differential equations describing the steady-current electrodynamics in the limit when characteristic penetration lengths essentially exceed the period of the structure. The intuitive idea that the structure in this limit can be described as an anisotropic medium is only partially confirmed. The London equation has the appropriate anisotropic form, but Maxwell’s equations bear additional features. A magnetic field, divided into components: normal to layers and two parallel, longitudinal and transverse, is not affected as it should be for a uniaxial medium. The normal and longitudinal components are not influenced essentially by the structure, while the transverse component is essentially screened. It is argued that this screening effect is a common feature of layered structures not related to the model.

The effects of the charging energy in the superconducting transition of granular materials or Josephson-junction arrays are investigated using a pseudo-spin-one model. Within a mean-field renormalization-group approach, we obtain the phase diagram as a function of temperature and charging energy. In contrast to earlier treatments, we find no sign of a reentrant transition in agreement with more recent studies. A crossover line is identified in the nonsuperconducting side of the phase diagram and along which we expect to observe anomalies in the transport and thermodynamic properties. We also study a charge ordering phase, which can appear for large nearest-neighbor Coulomb interaction, and show that it leads to first-order transitions at low temperatures. We argue that, in the presence of charge ordering, a nonmonotonic behavior with decreasing temperature is possible with a maximum in the resistance just before entering the superconducting phase.

The phase boundary between normal and superconducting phases of the negative-U Hubbard model is obtained within a self-consistent weak-coupling approach known as the ‘‘pseudopotential parquet’’ approximation. The shortcomings of simpler self-consistent approaches, including the Hartree-Fock and fluctuation-exchange approximations, are outlined. For intermediate values of U, pseudopotential parquet predicts (i) a maximum in the superconducting transition temperature ${\mathit{T}}_{\mathit{c}}$ at an electron filling 〈n〉 of approximately 0.92; and (ii) a sharp decrease in ${\mathit{T}}_{\mathit{c}}$ for values of 〈n〉 closer to 1. The scale of transition temperatures is consistent with results from quantum simulations.

We report systematic measurements of the resistivity and of the Hall coefficient ${\mathit{R}}_{\mathit{H}}$ of bulk ${\mathrm{YBa}}_2$${\mathrm{Cu}}_{3\mathrm{-}\mathit{x}}$${\mathrm{Fe}}_{\mathit{x}}$${\mathrm{O}}_{\mathit{y}}$ by varying both the iron doping and the oxygen concentration and for temperatures ranging from 100 to 250 K. The Hall number ${\mathit{n}}_{\mathrm{cell}}$ (at 100 and 200 K) decreases linearly with decreasing y for all x and exponentially upon Fe doping. The temperature dependence of ${\mathit{n}}_{\mathrm{cell}}$ is strongly suppressed upon removing holes from the system either by increasing the amount of iron doping or by de-oxygenation. The dependence of ${\mathit{n}}_{\mathrm{cell}}$ on temperature and on doping as well as the relation between ${\mathit{dn}}_{\mathrm{cell}}$/dT-${\mathit{T}}_{\mathit{c}}$ and ${\mathit{T}}_{\mathit{c}}$-${\mathit{n}}_{\mathrm{cell}}$ are also discussed. Finally, we discuss our results in the context of the cot${\mathrm{\theta }}_{\mathit{H}}$ vs ${\mathit{T}}^2$ variation.

Detailed studies on the bi-Tl layer family of cuprate superconductors for the first three members have been carried out by valence-level and core-level photoemission. Ca 2p and Ba 3d core levels show a variation in broadening with an increasing number of ${\mathrm{CuO}}_2$ planes. The Tl 4f core level shows a mixed valent character [Tl(I) and Tl(III)] in all the three samples. The Cu 2p core-level spectrum also shows a change in the satellite to main line intensity and the energy difference between them. Simplified configuration interaction calculations are carried out to explain the observed changes. A rigid shift of the valence band is observed with the increase in the number of ${\mathrm{CuO}}_2$ planes. Near-Fermi-level features are identified as due to a Kondo-type resonance and resulting heavy-electron band states.

Dielectric functions of ${\mathrm{BaPb}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Bi}}_{\mathit{x}}$${\mathrm{O}}_3$ (0<x<0.4) are investigated by ellipsometry. A broad excitation, which was observed in the visible photon energy region, is quantitatively analyzed. As the material becomes a normal metal with doping of x=0, the oscillator strength of the excitation decreases to almost zero. In the Hubbard-band model, this indicates that in the normal metallic (${\mathit{T}}_{\mathit{c}}$<4 K) doping regime, the density of the upper Hubbard state is reduced dramatically due to the screening of electron-electron Coulomb interaction. Based on that assumption, it is suggested that there might be a connection between the ‘‘upper Hubbard band’’ and the superconductivity.

We use a combination of quantum Monte Carlo (QMC) methods and analytic continuation techniques to study the formation of charge-density-wave (CDW) gaps at half-filling in the two-dimensional Holstein model. From QMC data for the dressed unequal-time Green’s function G(τ,p) we calculate the real frequency spectral function A(ω,p) for different temperatures T, electron-phonon coupling constants g, phonon frequencies ${\mathrm{\omega }}_0$, and spatial lattice sizes N. We also compute the spectral functions by approximate diagrammatic methods and find these results to be in good qualitative agreement with the QMC results. We discuss attempts to get quantitative agreement for a range of T and ${\mathrm{\omega }}_0$ by using a single renormalized vertex g¯ in place of g, an approach that proved useful in previous studies of the Hubbard model. Finally, comparisons are made with previous estimates of the CDW transition temperature based on measurements of static correlation functions and susceptibilities.

We have measured the photoresponse of current-biased bridge structures of epitaxial ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ thin films on ${\mathrm{LaAlO}}_3$ using 100 ps, 532 nm laser pulses. Voltage transients with fast and slow components were observed below ${\mathit{T}}_{\mathit{c}}$. The amplitude of the slow component agrees with a resistive bolometric response where the laser pulse heats the bridge into the resistive transition region. The decay time of the slow component is consistent with a thermal time constant for heat loss out of the film governed by the thermal boundary resistance at the interface between the film and the substrate. We show that the fast component can be explained by changes in the kinetic inductance of the bridge due to temperature-induced changes in the superfluid density from heating of the bridge by the laser pulse. Our interpretation of the origin of the fast component, therefore, is bolometric in nature over the time scale of the laser pulse contrary to some of the nonequilibrium or nonbolometric mechanisms that have been proposed. A simulation of this kinetic inductive bolometric effect provides reasonable agreement with the temperature dependence of the observed photoresponse, and a comparison is made between the various postulated temperature dependencies of the superfluid fraction in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$. The simulation further shows that the speed of the kinetic inductive bolometric photoresponse is not limited by the thermal escape time out of the film, but is determined by the rate at which the incident laser pulse initially heats the film and is therefore the same duration in time as the laser pulse. Evidence for negative voltage transients observed in the photoresponse data consistent with the simulation results is also presented.