We use a hydrodynamics theory to discuss the response of a viscous flux-line liquid to an ac perturbation applied at the surface of the sample. The theory incorporates viscoelastic effects and describes the crossover between liquidlike and solidlike response of the vortex array as the frequency of the perturbation increases. A large viscosity from flux-line interactions and entanglement leads to viscous screening of surface fields. As a result, two frequency-dependent length scales are needed to describe the penetration of an ac field. For large viscosities the imaginary part of the ac permeability can exhibit, in addition to the well-known peak associated with flux diffusion across the sample, a low-frequency peak corresponding to the transition from solidlike to liquidlike behavior.

We present a theoretical description of the near-specular (dipole) electron-energy-loss spectrum of a superlattice constructed from anisotropic dielectric layers, as a model of the superconducting copper-oxide-based materials. The theory is applied to a description of recent data taken on the Bi 2:2:1:2 compound. We find that parameters deduced from the electron loss spectrum are compatible with infrared reflectivity data. Manifestations of the superconducting energy gap are not evident in the new data.

The preparation of polycrystalline ${\mathrm{TlBa}}_{1+\mathit{x}}$${\mathrm{La}}_{1\mathrm{-}\mathit{x}}$${\mathrm{CuO}}_5$ (x=0.0, 0.1, 0.2, 0.3, 0.4, and 0.5) is investigated using Tl-free precursors. Single-phase samples are obtained except for x=0.5, in which a small amount of impurity phase is detected. Compositional and structural analyses confirm good homogeneity of the Ba and La distribution in each sample. In this system, hole-carrier doping increases with increasing x. The sample with x=0, ${\mathrm{TlBaLaCuO}}_5$, shows semiconductive resistivity (ρ) and a T-independent Hall coefficient (${\mathit{R}}_{\mathit{H}}$), indicating two-dimensional (2D) variable range hopping. As x increases, the resistivity becomes metallic and superconductivity is observed for 0.1≤x≤0.4, with a maximum ${\mathit{T}}_{\mathit{c}}$ value of 45 K at x=0.2. For x=0.5, the sample is metallic but not superconducting. In all metallic samples (0.1≤x≤0.5), ${\mathit{R}}_{\mathit{H}}$ has a broad maximum at about 100 K, as seen in most high-${\mathit{T}}_{\mathit{c}}$ cuprates. The inverse Hall mobility, ${\mathrm{\mu }}_{\mathit{H}}^{\mathrm{-}1}$≡ρ/${\mathit{R}}_{\mathit{H}}$, roughly obeys a ${\mathit{T}}^2$ dependence regardless of the ${\mathit{T}}_{\mathit{c}}$ value, which is widely observed in other oxide superconductors.

We present and discuss theoretical and experimental a-axis-projected two-dimensional angular correlation of annihilation radiation (2D-ACAR) spectra from ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$. The experiment involved an untwinned metallic single crystal of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6.9}$ at room temperature. The calculations have been carried out within the conventional first-principles band-theory framework based on the local-density approximation; the positron state is treated in the independent-particle model. After the as-observed 2D-ACAR spectrum is corrected for an isotropic background, a good accord is found between not only the overall shape but also the fine structure in the measured and computed spectra, some discrepancies notwithstanding. Extensive analysis shows that the experimental spectrum clearly contains the signature of the electron ridge Fermi surface (FS) arising from Cu-O chains in the first Brillouin zone. Also the first and second Umklapp images of the ridge FS at higher momenta are in substantial accord with theoretical predictions. These results suggest that the electron states in Y123 are not localized, at least not strongly, along the c direction.

We present results of ${}_{}{}^{17}{}_{}{}^{}\mathrm{O}$ and ${}_{}{}^{63,65}{}_{}{}^{}\mathrm{Cu}$ nuclear magnetic resonance (NMR) and nuclear quadrupolar resonance (NQR) studies in the normal and superconducting state of the 82-K superconductor ${\mathrm{YBa}}_2$${\mathrm{Cu}}_4$${\mathrm{O}}_8$. The various components of the Cu and O Knight-shift tensors show strong but similar temperature dependences over the temperature range from 8.5 to 300 K in both the ${\mathrm{CuO}}_2$ planes and the chains, supporting the picture that there is only one spin component in the planes and the chains, although with different susceptibilities. The oxygen data obey the Korringa relation. This may be interpreted as Fermi-liquid behavior of the electronic system far away from the antiferromagnetic wave vector. The temperature dependence of both the planar Cu and O shift tensors and the planar Cu spin-lattice relaxation rate suggest the opening of a pseudo-spin-gap well above ${\mathit{T}}_{\mathit{c}}$. The very different temperature dependence of 1/${\mathit{T}}_1$ at the planar O and Cu sites points to the reduced role of the antiferromagnetic correlated spin fluctuations at the O site. The data favor the conclusion that ${\mathrm{YBa}}_2$${\mathrm{Cu}}_4$${\mathrm{O}}_8$ is a d-wave superconductor. Evidence is provided by three data sets: the chain Knight shifts, the ratio of the planar copper and oxygen relaxation rates, and the individual low-temperature behavior of these rates.

The electron-phonon coupling of photoexcited carriers is investigated in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ (YBCO) with δ=0.8 and 0.1 by measuring nonequilibrium phonon occupation numbers ${\mathit{n}}_{\mathrm{ph}}$ on the ultrashort timescale with picosecond Stokes and anti-Stokes resonant Raman spectroscopy. In both insulating and metallic materials significant nonequilibrium ${\mathit{n}}_{\mathrm{ph}}$ are observed due to the interaction of carriers with lattice vibrations. Increasing the photoexcited carrier density to beyond ∼${10}^{19}$ ${\mathrm{cm}}^{\mathrm{-}3}$ in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6.2}$, an abrupt change of both the electron-phonon coupling and reflectivity is observed, as a consequence of a photoinduced insulator-to-metal (I-M) transition. Whereas ${\mathit{n}}_{\mathrm{ph}}$ of the apical O vibrations is found to increase with increasing excited carrier density with a change of slope at the I-M transition, ${\mathit{n}}_{\mathrm{ph}}$ for the planar 340 ${\mathrm{cm}}^{\mathrm{-}1}$ vibration shows a marked decrease of coupling in the metallic state. The clear implication is that whereas the out-of-plane carrier interaction with apical O vibrations remains quite strong in both the insulating and metallic phases, the in-plane coupling is significantly reduced in the metallic phase of the material.

The spectral function and the momentum distribution for holes in an antiferromagnet are calculated on the basis of the t-J model in a slave-fermion representation. The self-consistent Born approximaton for the two-time Green’s function is used to study the temperature and doping (δ) dependence of the self-energy operator. The numerical calculations show a weak concentration and temperature dependence of the spectral function (quasiparticle hole spectrum) while the momentum-distribution function changes dramatically with increasing temperature for T>${\mathit{T}}_{\mathit{d}}$ with ${\mathit{T}}_{\mathit{d}}$≃Jδ.

General potential linearized-augmented-plane-wave computations using the generalized-gradient approximation (GGA) of Perdew and Wang (PW-II) show excellent agreement with the measured equations of state of bcc and hcp phases to over 300 GPa, the bcc to hcp phase transition pressure, and the bcc magnetic moment. The results are a significant improvement over the local-spin-density approximation and the earlier PW-I GGA functional. The bcc phase is mechanically unstable at high pressure with respect to a tetragonal strain, and is thus unlikely to exist in the earth’s inner core.

Mössbauer spectroscopy and x-ray diffraction were employed to investigate the magnetic and structural properties of the high-pressure monoclinic phase of magnetite. Measurements were performed to 66 GPa at 300 K using diamond anvil cells. Based primarily upon the Mössbauer parameters, the following features of the high-pressure phase were deduced. With increasing pressure the high-pressure phase evolves at P≥25 GPa and its abundance increases monotonically at the expense of the low-pressure cubic phase. The high-pressure phase is not magnetic at 300 K and its monoclinic structural features resemble that of the low-pressure phase. The tetrahedral and octahedral sites characteristic of the inverse spinel structure, albeit distorted, remain the building blocks of the high-pressure phase. The fast electron hopping between ${\mathrm{Fe}}^{3+}$ and ${\mathrm{Fe}}^{2+}$ at the octahedral sites prevails to the highest pressure. It is suggested that the cubic→monoclinic→cubic hysteretic cycle involves a mild displacive phase transition not affecting the coordination number of any of the iron cations.

Self-diffusion along the [112] Shockley parital dislocation pair (self-pipe-diffusion) in Au is studied with Nosé-Hoover molecular dynamics (MD) using the many-atom Ackland-Tichy-Vitek-Finnis model. We find the following formation energy difference between interstitials (i) and vacancies (v) at partial dislocations: Δ${\mathit{E}}_{\mathit{f}}$=${\mathit{E}}_{\mathit{f}}$(i)-${\mathit{E}}_{\mathit{f}}$(v)=1.3 eV. Thermal disorder in the stacking fault region makes it difficult (or even impossible) to follow the migration of single vacancies or interstitials sufficiently long times in the temperature range 0.78${\mathit{T}}_{\mathit{m}}$. . . ${\mathit{T}}_{\mathit{m}}$ (${\mathit{T}}_{\mathit{m}}$=1475 K is the melting temperature). The diffusion induced by one vacancy and one interstitial calculated at the temperature range 1150–1400 K gives the following migration energies: ${\mathit{E}}_{\mathit{m}}$(v)=0.75 eV and ${\mathit{E}}_{\mathit{m}}$(i)≊0.0 eV. Since the activation energy for interstitials is about 0.5 eV larger than that for vacancies, self-pipe-diffusion is more vacancy type than interstitial type. The components of the induced diffusion constants indicate the spread of diffusion into the whole stacking fault region.

Optical absorption in a random one-dimensional lattice in the presence of paired correlated disorder is studied. The absorption line shape is evaluated by solving the microscopic equations of motion of the Frenkel-exciton problem in the lattice. We find that paired correlation causes the occurrence of well-defined characteristic lines in the absorption spectra which clearly differ from the case of unpaired correlation. The behavior of the absorption lines as a function of defect concentration is studied in detail. We also show how exciton dynamics can be inferred from experimental data by deriving an analytical expression relating the energy and intensity of lines to the model parameters.

The temperature-dependent magnetic properties of thick Gd(0001)/W(110) films were investigated with respect to the annealing temperature as well as the film thickness (d=10–100 nm). After annealing to sufficiently high temperatures, thick films (d≥40 nm) show evidence for a canted magnetization, accompanied by a strong anomaly of the coercive field just below the Curie temperature ${\mathit{T}}_{\mathit{C}}$. These effects can be attributed to the temperature-dependent magnetic anisotropy of bulk Gd. For thinner films, the magnetization behavior implies the existence of residual strain, which cannot be fully annealed by thermal treatment. The implications of our experimental results for surface magnetic order are discussed.

A rigorous analysis of the curling mode for an infinite ferromagnetic hollow cylinder with exchange and uniaxial anisotropy and external field applied along its axis is made using Brown’s equation. Curling instability occurs only in a hollow cylinder, but never in a solid one. Moreover, the energy of the curling mode with large-angle deviations in the nonlinear region is always higher than that of the uniform mode. This suggests that at the nucleation field the initial saturated state cannot reverse through the curling mode to a new stable state.

Nanocrystalline samples were obtained by subsequent annealing of the ${\mathrm{Fe}}_{93\mathrm{-}\mathit{x}\mathrm{-}\mathit{y}}$${\mathrm{Zr}}_7$${\mathrm{B}}_{\mathit{x}}$${\mathrm{Cu}}_{\mathit{y}}$ metallic glass ribbons either in a conventional furnace or by electric current flowing through the ribbon. The coercive field of the slightly crystallized material shows a drastic increase at temperatures about the Curie point of the amorphous matrix, whereas for samples with high content of the crystalline phase the coercivity increases slowly in the temperature range well above ${\mathit{T}}_{\mathit{C}}$. Temperature dependencies of the coercive field show that at elevated temperatures the magnetic interactions between ferromagnetic particles, most probably of exchange origin, take place through the paramagnetic intergranular matrix.

The ground state of closed Heisenberg spin chains with an odd number of sites has a chiral degeneracy, in addition to a twofold Kramers degeneracy. A nonzero chirality implies that the spins are not coplanar, and is a measure of handedness. The chirality operator, which can be treated as a spin-1/2 operator, is explicitly constructed in terms of the spin operators, and is given as commutator of permutation operators.

The static fluxon solutions of a Josephson window junction have been studied numerically. We show that the effect of the idle region surrounding the junction is to ‘‘dress’’ the fluxon causing its energy to increase. This effect can be predicted accurately by a simple model.

We study the effects of the Coulomb electron-electron interaction on the Aslamazov-Larkin fluctuation correction to the conductivity in impure two-dimensional metals. We show that unlike the correction to the superconducting transition temperature, the above correction to the conductivity is sensitive to the form of the Coulomb interaction. In particular we studied the case of a thin metallic film separated from a massive metallic substrate by an insulator, when the electron-electron interaction is modified by image forces.

One model of granular superconductors is based on a Josephson-junction array with capacitive, or charging, terms. As a result of having these noncommuting terms in the Hamiltonian, energy transport is possible. We have obtained an expression for the heat current for this model and have employed it in calculating the thermal conductivity κ. We find that as the temperature is lowered through the system’s order-disorder phase transition, κ increases by more than one order of magnitude. Effects of having a distribution of grain sizes are also calculated.

It is shown that the Eliashberg’s equations (EE) have two kinds of solutions for the critical temperature ${\mathit{T}}_{\mathit{c}}$. The solution of the first kind leads to the traditional smooth dependence of ${\mathit{T}}_{\mathit{c}}$ on the value of the Coulomb pseudopotential ${\mathrm{\mu }}^{\mathrm{*}}$. This dependence has been extensively studied in literature. The second kind of solutions manifests as quantized critical temperatures appearing at finite values of ${\mathrm{\mu }}^{\mathrm{*}}$ but not depending on its magnitude. Such solutions are only possible for strong-coupled superconductors. Thus, the dependence ${\mathit{T}}_{\mathit{c}}$(${\mathrm{\mu }}^{\mathrm{*}}$) in the mean-field approximation is a many-valued function. In the vicinity where the traditional solution approaches a quantized solution the mean-field approximation fails and the system fluctuationally switches to another stable mean-field branch. This process results in appearance of flat regions in the ${\mathit{T}}_{\mathit{c}}$(${\mathrm{\mu }}^{\mathrm{*}}$) dependence.

Local-density-approximation electronic-structure calculations for the recently discovered ${\mathrm{LaPt}}_2$${\mathrm{B}}_2$C superconductor are reported. The calculations show, as in ${\mathrm{LuNi}}_2$${\mathrm{B}}_2$C, that despite the quasi-two-dimensional crystal structure, the electronic structure in the vicinity of the Fermi energy (${\mathit{E}}_{\mathit{F}}$) is quite three dimensional, with substantial contributions from all atoms. In this compound, ${\mathit{E}}_{\mathit{F}}$ falls on the low-energy side of a peak in the electronic density of states, suggesting that ${\mathit{T}}_{\mathit{c}}$ may be increased by doping to a higher electron count. The value of the density of states at ${\mathit{E}}_{\mathit{F}}$ is found to be substantially smaller than that of ${\mathrm{LuNi}}_2$${\mathrm{B}}_2$C in agreement with experiment.