Superconducting and magnetic properties of ${\mathrm{La}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Tm}}_{\mathit{x}}$${\mathrm{Ru}}_3$${\mathrm{Si}}_2$ (x=0, 0.08, and 0.16) have been investigated through measurements of both electric and magnetic properties. Tm is in a trivalent state and carries a magnetic moment ≊8${\mathrm{\mu }}_{\mathit{B}}$. We find that the magnetic measurements are not suited to the determination of the critical line ${\mathit{H}}_{\mathit{c}}$(T) for the onset of type-II superconductivity in materials where one has at the same time superconducting diamagnetic shielding and paramagnetic contribution due to the localized spins. On the other hand, transport experiments are much less ambiguous, and allow us to determine accurately the critical temperature in a given applied magnetic field. In particular, they allow us to suggest a more appropriate interpretation of the maximum in the magnetic-susceptibility curve already observed in materials of the same family: this maximum results from a competition between the incomplete diamagnetic shielding and the paramagnetic contribution from the magnetic ions in the mixed phase, rather than from a spin-glass freezing of the paramagnetic ions. In zero field, the rate of depression of the critical temperature upon substituting nonmagnetic La ions by magnetic Tm ions is dT/dx=-8±1 K/at. % Tm.

The single-rung t-J ladder is analyzed in a mean-field theory using Gutzwiller renormalization of the matrix elements to account for strong correlation. The spin-liquid (resonance valence bond) state at half-filling evolves into a superconducting state upon doping. The order parameter has a modified d-wave character. A lattice of weakly coupled ladders should show a superconducting phase transition.

We present the results of experiments to study cavitation in liquid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ at negative pressures. A focused sound wave is used to generate a negative pressure in a small region of the liquid, and the cavitation is detected by light scattering. We have studied the cavitation as a function of the temperature and static pressure applied to the liquid. We are able to observe the statistical nature of the cavitation process, and to determine the nucleation barrier and the attempt frequency. The results for the attempt frequency are lower than expected for homogeneous nucleation. We discuss the possibility that the nucleation is occurring on quantized vortices. The tensile strength at constant pressure decreases slowly with increasing temperature in the range 0.8 to 1.5 K, drops rapidly as the temperature approaches the λ point, and then decreases slowly above ${\mathit{T}}_{\lambda }$.

Using the x-y model and a nonlocal updating scheme called cluster Monte Carlo, we calculate the superfluid density of a two-dimensional superfluid on large-size square lattices L×L up to 400×400. This technique allows us to approach temperatures close to the critical point, and by studying a wide range of L values and applying finite-size scaling theory we are able to extract the critical properties of the system. We calculate the superfluid density and from that we extract the renormalization-group beta function. We derive finite-size scaling expressions using the Kosterlitz-Thouless-Nelson renormalization group equations and show that they are in very good agreement with our numerical results. This allows us to extrapolate our results to the infinite-size limit. We also find that the universal discontinuity of the superfluid density at the critical temperature is in very good agreement with the Kosterlitz-Thouless-Nelson calculation and experiments.

In the framework of a finite-range density-functional theory, we compute the response of ${}_{}{}^4{}_{}{}^{}\mathrm{He}_{\mathit{N}}^{}{}_{}{}^{}$ clusters doped with a rare-gas molecule. For this purpose, the mean field for the ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ atoms, their wave functions and effective quasiparticle interaction, are self-consistently calculated for a variety of particle numbers in the cluster. The response function is then evaluated for several multipolarities in each drop and the collective states are consequently located from the peaks of the strength function. The spectra of pure droplets approach those previously extracted with a similar algorithm resorting to a zero-range density functional. The spectra of doped clusters are sensitive to the presence of the impurity and are worth a future systematic investigation.

The stability of uncharged superflow is studied within the Ginzburg-Landau theory. The linear-stability problem for flow in the presence of a flat wall is solved analytically. We prove that the critical velocity is not modified from its value for homogeneous superflow if the order parameter is required to vanish at the wall. We further show that surface roughness on the scale of the coherence length does have a strong effect on the stability of the flow. The critical velocity in the vicinity of a two-dimensional surface ‘‘bump’’ is well described by a power law for the parameter values considered.

The dielectric properties of 〈100〉-oriented epitaxial ${\mathrm{SrTiO}}_3$ films as used in high-${\mathit{T}}_{\mathit{c}}$ heterostructures have been studied as a function of temperature and applied electric field by using in situ grown heterostructures (Mg/${\mathrm{SrTiO}}_3$/${\mathrm{SrTiO}}_3$:Nb and Au/${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$/${\mathrm{SrTiO}}_3$/${\mathrm{SrTiO}}_3$:Nb). The dielectric behavior of these films is characterized by a rather low sample capacitance as compared to bulk single-crystal values and by occurrences of maxima in the capacitance-voltage and capacitance-temperature curves. The maximum attainable polarization was found to be 80 mC/${\mathrm{m}}^2$ at 4.2 K. Complementary measurements on single crystals of ${\mathrm{SrTiO}}_3$ and ${\mathrm{SrTiO}}_3$:Nb reveal that the behavior of these ${\mathrm{SrTiO}}_3$ thin films can be understood from the bulk properties with additional strong charge trapping at the film-substrate interface.

Anharmonic contributions to the Eliashberg function due to a low-energy local boson mode of finite width are considered for a harmonic metal. Excitation of the multiple-frequency modes leads to redistribution of spectral weight in the generalized Eliashberg function from the low- to the high-energy region, which reduces the electron-boson coupling. This is seen in the calculated temperature dependence of the interaction constants ${\lambda }_{\mathit{n}}$ entering the Eliashberg equations. The rapid decrease with temperature of ${\lambda }_{\mathit{n}>0\mathrm{}}$ can be used to derive a simple analytical expression, which reproduces almost exactly the temperature dependence, ${\lambda }_{\mathit{n}>0\mathrm{}}$(T), calculated from the exact expression. Normal-state properties of alkaline-doped ${\mathrm{C}}_{60}$ are investigated within the two-peak model (${\mathrm{\omega }}_0$∼50 K and ${\mathrm{\omega }}_1$∼900 K) for the possibility to extract information about the local modes themselves and their coupling to the conduction electrons. The electronic specific heat shows peculiarities only in the case of an extremely strong coupling. Due to the low-energy peak the temperature dependence of the electrical resistivity deviates more and more from the linear law with increasing adiabaticity parameter A=(m/M)(${\mathrm{\epsilon }}_{\mathit{F}}$/ħ${\mathrm{\omega }}_0$). This leads, however, to a negative-curvature dependence. The experimental curve for ${\mathrm{K}}_3$${\mathrm{C}}_{60}$ has a positive curvature and can be fitted only in the case of a very strong coupling of electrons to the high-energy mode. The thermal electronic conductivity displays a minimum at T∼ħ${\mathrm{\omega }}_0$/${\mathit{k}}_{\mathit{B}}$. The curvature as a function of temperature is determined by the magnitude of A.

Universal properties of the zero-temperature superconductor-insulator transition in two-dimensional amorphous films are studied by extensive Monte Carlo simulations of bosons in a disordered medium. We report results for both short-range and long-range Coulomb interactions for several different points in parameter space. In all cases we observe a transition from a superconducting phase to an insulating Bose glass phase. From finite-size scaling of our Monte Carlo data we determine the universal conductivity ${\mathrm{\sigma }}^{\mathrm{*}}$ and the critical exponents at the transition. The result ${\mathrm{\sigma }}^{\mathrm{*}}$=(0.55±0.66)(2e${)}^2$/h for bosons with long-range Coulomb interaction is roughly consistent with experiments reported so far. We also find ${\mathrm{\sigma }}^{\mathrm{*}}$=(0.14±0.03)(2e${)}^2$/h for bosons with short-range interactions.

The Bloch waves, the energy bands, and the local and global densities of states are computed for superconducting multilayers in which the Fermi energy in the superconducting (S) layers, ${\mathrm{\varepsilon }}_{\mathrm{FS}}$, may exceed the one in the normal (N) layers, ${\mathrm{\varepsilon }}_{\mathrm{FN}}$. Self-consistent pair potentials and their equivalent square-well representations are considered. If the S-layer thickness exceeds several coherence lengths the densities of states exhibit the subgap peak (besides the BCS peak) and the Tomasch-McMillan-Anderson oscillaitons known from SNS junctions. These features, due to off-diagonal (Andreev) scattering, decrease with increasing ratio ${\mathrm{\varepsilon }}_{\mathrm{FS}}$/${\mathrm{\varepsilon }}_{\mathrm{FN}}$ because of the competition from diagonal scattering. The temperature dependence of the energetic position of the subgap peak is weaker than that of the BCS peak. In multilayers with S-layer thickness of the order of magnitude of one coherence length and ${\mathrm{\varepsilon }}_{\mathrm{FS}}$/${\mathrm{\varepsilon }}_{\mathrm{FN}}$≫1, the local density of states is BCS-like in the S layers and normal-metal-like in the N layers. This is due to electron localization in the S layers and consistent with recent scanning tunneling microscope conductance measurements in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$.

The polarized Raman scattering of the ${\mathit{R}}_{0.5}$${\mathrm{Pr}}_{0.5}$${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ series (R=La, Pr, Nd, Sm, Eu, Gd, Dy, Y, Ho, Er, Tm, Yb, Lu; δ∼0) which undergoes a superconductor-to-nonsuperconductor transition between R=Gd and Dy was studied. It was found that the appearance of superconductivity correlates with observation of a two-mode behavior of the R-ion-size-sensitive ${\mathit{B}}_{1\mathit{g}}$-like mode of O(2),O(3) out-of-phase vibrations. It was also established that the Ba (xx,${\mathit{A}}_{\mathit{g}}$) phonon line is of asymmetrical (Fano) shape for all R. The origin of the Fano effect usually attributed to interference with an electronic continuum is discussed.

The electron-electron correlation has been considered by using the local-spin-density approximation (LSDA) with on-site Coulomb interaction in the first-principles discrete variational cluster method. Based on the correction to LSDA, we have carried out self-consistent electronic-structure calculations for the strongly correlated electronic systems of ${\mathrm{La}}_2$${\mathrm{CuO}}_4$ and ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_6$. In contrast to LSDA, the LSDA+U calculations reveal the experimentally observed antiferromagnetic and insulating ground state. The energy gap for these compounds is of a charge-transfer character, and the values of the energy gap and Cu magnetic moment are in good agreement with experiments. The orbital character of the states near the Fermi level in ${\mathrm{La}}_2$${\mathrm{CuO}}_4$ and ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_6$ is discussed and a comparison with experiments is made.

The ab-plane optical conductivity of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6.95}$ has been studied by infrared reflectance spectroscopy in the presence of disorder introduced by ion irradiation. The frequency dependence of the conductivity in the superconducting state suggests that ion irradiation has caused carrier localization. Localization explains both the suppression of ${\mathit{T}}_{\mathit{c}}$ and the reduction of the superfluid fraction. The temperature dependence of the penetration depth in pure and disordered ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6.95}$ supports the hypothesis of non-s-wave superconductivity.

Anomalous dispersive x-ray-diffraction measurements have determined the dopant-site distribution in Ni-doped single crystals of the high-temperature superconductor ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$. The data from a ${\mathrm{YBa}}_2$(${\mathrm{Cu}}_{0.83}$${\mathrm{Ni}}_{0.17}$${)}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ crystal show that over 95% of the Ni dopants occupy the Cu(2) site in the copper oxide planes, with negligible occupation of the Cu(1) copper oxide chain sites. General guidelines are presented for selecting the most effective hkl diffraction planes for measuring dopant concentration profiles. Direct measurement of the energy dependence of the dopant atomic scattering forces is also demonstrated.

The damping of spin waves due to spin-phonon interaction in a spin-S Heisenberg antiferromagnet on a square lattice is calculated. The theory applies to the regimes where the sound velocities ${\mathit{v}}_{\mathit{s}}$ are small compared with magnon velocities ${\mathit{v}}_{\mathit{m}}$, which is the case in antiferromagnetic high-${\mathit{T}}_{\mathit{c}}$ superconductor basis materials like ${\mathrm{YaBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_6$, and kξ≫1, where k and ξ are wave vector and correlation length, respectively. It is concluded that the broad spectral feature of the two-magnon line observed in layered perovskites can be explained qualitatively as a consequence of spin-phonon interaction.

We study the superconducting properties of the system described by the correlated-hopping model in the limit of intermediate and strong correlations. The main effect of the Hubbard term U is reflected in the appearance of the separated Hubbard subbands. The other interaction term ${\mathit{K}}_{\mathit{i}\mathit{j}}$ in the Hamiltonian introduces the correlation between quasiparticles and for small number of holes in the system leads to the appearance of superconductivity. The superconducting transition temperature hardly changes with U in the approximation we use. We have found the dependence of the superconducting energy gap on temperature T and energy ${\mathrm{\varepsilon }}_{\mathit{k}}$. It turns out to be a nearly linear function of ${\mathrm{\varepsilon }}_{\mathit{k}}$. The temperature dependence of the two components of the gap slightly differ from the BCS Δ(T) function. The effective gap to the ${\mathit{T}}_{\mathit{c}}$ ratio strongly depends on the carrier concentration n taking on large values for small n (n<0.1) and being close to the BCS value of 1.76 at larger n.

Josephson- (intergrain-) flux depinning was studied in a Josephson-junction network of granular YBCO and YBCO/Ag superconductors. We measured temperatures and magnetic fields at which the Josephson-flux depinning occurs and analyzed the corresponding relaxation processes. A direct relationship between depinning of the Josephson vortices and dissipation of the transport (Josephson) critical current was found. Special experimental techniques were applied to achieve those objectives, allowing (a) detection of the Josephson-flux motion without the influence of the intragrain (Abrikosov) flux; and (b) direct measurement of changes in the effective activation energy for the Josephson-flux motion and corresponding changes in the Josephson critical current. The results were interpreted in the framework of the models of granular superconductivity developed by Tinkham and Lobb [Solid State Phys. 43, 91 (1989)], and by Clem [Physica C 153-155, 50 (1988)]. The results support the idea given in those models, that the pinning barrier for the Josephson-flux motion originates from the variation of the Josephson coupling energy within the junction network.

Neutron-scattering measurements have been performed to study the magnetic ordering of Pr and Cu in semiconducting ${\mathrm{TlBa}}_2$${\mathrm{PrCu}}_2$${\mathrm{O}}_{7\mathrm{-}\mathit{y}}$. This compound has a nuclear structure which is similar to that of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ but with the CuO-chain layers fully replaced by TlO layers. At low temperatures the magnetic diffraction pattern reveals magnetic Bragg peaks of the {1/21/21/2} type, which are associated with the antiferromagnetic ordering of the Pr moments. The magnetic structure consists of nearest-neighbor spins along all three crystallographic directions being antiparallel. The spin direction is along the c axis, with a saturated moment of 1.05${\mathrm{\mu }}_{\mathit{B}}$. The temperature dependence of the intensity of these reflections indicates a Néel temperature ${\mathit{T}}_{\mathit{N}}$≊8 K, which is about half that observed in the related ${\mathrm{PrBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ system. A second set of magnetic reflections, of the {1/21/21} type, are also observed, and these are associated with the Cu magnetic sublattice. This magnetic structure is identical to the ‘‘plane ordering’’ observed in the related 1:2:3 system, where the nearest-neighbor Cu spins are again antiparallel. The spin direction is in the tetragonal ab plane, with a saturated ordered moment of 0.59${\mathrm{\mu }}_{\mathit{B}}$. The temperature dependence indicates a Néel temperature of ≊370 K for the Cu spins.

To explain the behavior of thermal fluctuations, we have measured the electrical resistivity of five samples: thin-film ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$, polycrystalline ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$, and ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{Ca}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Y}}_{\mathit{x}}$${\mathrm{Cu}}_2$${\mathrm{O}}_{8+\mathrm{\delta }}$ with x=0, 0.05, and 0.20. The whole temperature range of fluctuations consists of three regions. Within the mean-field region, ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ and ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{Ca}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Y}}_{\mathit{x}}$${\mathrm{Cu}}_2$${\mathrm{O}}_{8+\mathrm{\delta }}$ samples show predominantly three-dimensional (3D) and 2D behaviors with the exponents 1/2 and 1, in the critical region the two systems display a behavior analogous to 3D-XY and 2D-XY models, respectively. Above the mean-field region [ln(ε)≥-1.0] exponents have values larger than those predicted by Reggiani et al. Both the systems favor strong pair-breaking effects and integral dimensions for the thermal fluctuations.

Using local-density-functional theory in the full-potential linear-muffin-tin-orbital implementation we have calculated the potential curves for q=0 phonons, and the couplings of the latter to electrons near the Fermi surface in ordered cubic ${\mathrm{Ba}}_{0.5}$${\mathrm{K}}_{0.5}$${\mathrm{BiO}}_3$. We find that one of the two rotational modes of ${\mathit{T}}_1$ symmetry is described by an extremely flat, strongly anharmonic potential with a very shallow minimum far away from the cubic equilibrium position. All the other investigated phonons are only moderately anharmonic. Assuming the coupling functions q independent the largest branch contributions of q=0 phonons to the total electron-phonon coupling constant λ are ${\lambda }_{\mathit{T}1}$=0.115, ${\lambda }_{\mathit{A}1}$=0.039, and ${\lambda }_{\mathit{E}}$=0.041, with degeneracy taken into account. Using the values of all the 12 considered q=0 phonons we obtain the rough estimate λ∼0.5.