The condensate pair fluctuations in the frequency region ω≲2Δ and the associated Raman-scattering intensity of a two-dimensional (2D) d-wave weak-coupling BCS superconductor are investigated. Our model includes a dominant ${\mathit{d}}_{\mathit{x}}^2$-${\mathit{y}}^2$ (L=2) and a weaker s-wave (L=0) pairing interaction. All response functions involving density and pair operators are evaluated (in the low-q limit). For neutral d-wave superconductors (no Coulomb interaction), the expected phononlike phase mode is obtained in the L=0 channel (which couples to density fluctuations). In contrast, excitonlike modes corresponding to excited Cooper pair states are obtained in the L=2 channel. We find an amplitude fluctuation mode with frequency √3 Δ. For charged d-wave superconductors, the L=0 phonon is renormalized into a 2D plasmon, but the L=2 excitonlike mode remains unaffected by the Coulomb interaction. At T=0, the latter is shown to be completely washed out in the Raman scattering spectrum (q=0) due to large p-h damping (which arises in the absence of a finite pair-breaking gap in d-wave superconductors). However, at finite temperatures, we find the energy of the excitonlike mode is drastically lowered [relative to 2Δ(T)] when the s-wave attraction is comparable to the d-wave pairing. This leads to a decrease in the damping and, as a result, the mode shows up as a low-frequency resonance in the Raman cross section. Due to the anisotropy of the d-wave order parameter, the quasiparticle excitation spectrum and the noninteracting two-particle spectrum are strongly dependent on the direction of q. We also find that the excitonlike mode frequency becomes anisotropic for wave vectors of the order of Δ/${\mathit{v}}_{\mathit{F}}$.

In this article we report on the identification of different pinning regimes in epitaxial ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{Ca}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{10+\mathrm{\delta }}$ thin films by means of the scaling behavior of resistive transitions in magnetic fields B∥c. While for T>95 K the transitions are dominated by fluctuation effects, which cannot be exactly distinguished from flux flow, a flux creep region is found below 95 K, extending to 60 K for B=6 T. The characteristic dependences of the transport activation energy agree with the Geshkenbein-Vinokur model for weakly pinned vortex liquids. The low-resistivity parts for all magnetic fields under consideration (31 mT to 6 T) clearly deviate from this model, exhibiting a significantly increasing apparent activation energy. It is shown that the glasslike Vogel-Fulcher description ρ∝(T-${\mathit{T}}_{\mathit{G}}$${)}^{\nu \left(\mathit{z}\mathrm{-}1\right)}$ holds in all fields with a critical exponent of 6 to 9 for B≥1 T, and up to 15 for fields below 1 T. From this we deduce a mapping of the BT plane into a vortex-glass region, a vortex-liquid region, and the critical region of the glass transition in between. The result is compared with earlier data for ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ and ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{Ca}}_1$${\mathrm{Cu}}_2$${\mathrm{O}}_{8+\mathrm{\delta }}$ single crystals and thin films, respectively.

The electronic structure of ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_8$ and ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CuO}}_6$ high-${\mathit{T}}_{\mathit{c}}$ superconductors is investigated by Cu 2p x-ray photoemission spectroscopy and valence-band resonant photoemission at the Cu ${\mathit{L}}_3$ edge. The data suggest strong correlation effects for the Cu 3d electron states due to a large on-site Coulomb interaction. By means of a simplified cluster-configuration-interaction model we derive the model Hamiltonian parameters which quantify the Cu d-d interaction energy, the O to Cu charge-transfer energy, and the degree of Cu d–O 2p hybridization. The solutions for the two compounds differ mainly in the charge-transfer energy, which in both cases is lower than the energy required for a Cu d-d charge fluctuation. An energy gap of about 1 eV for both compounds is found. This identifies them as charge-transfer insulators, in analogy with what was already found for CuO and other copper oxide based high-${\mathit{T}}_{\mathit{c}}$ superconductors. The density of states at the Fermi level is assigned to doping induced states in the semiconductor gap.

The oxygen-defect-induced phase transition from nonergodic to ergodic state in superconductors with intragrain granularity is considered within the superconductive-glass model. The model predictions are found to be in qualitative agreement with some experimental observations in deoxygenated high-${\mathit{T}}_{\mathit{c}}$ single crystals.

The presence of different low-dimensional structural units along with a short coherence length leads to the formation of a two-gap spectrum, that is to a double-peak structure in the superconducting density of states (e.g., Y-Ba-Cu-O). The presence of magnetic impurities leads to a gapless state. As opposed to the usual one-gap isotropic model, the region of gaplessness is extended, so that one can observe the effect of gaplessness without a noticeable shift in ${\mathit{T}}_{\mathit{c}}$. Strong anisotropy of the pairing interaction within the planes of a layered superconductor also leads to a similar extension of the region of gapless behavior in the presence of magnetic impurities. This gaplessness has a great impact on the spectroscopy of the cuprates, particularly for nonstoichiometric compounds. Experimental data are discussed in detail.

Interaction of the random-telegraph-noise signals with pinned Abrikosov vortices in granular high-temperature superconductors is investigated. It is shown that the low-frequency part of random-noise spectra is suppressed due to interaction of Abrikosov vortices with pinning centers at low magnetic fields and/or due to mutual interactions of vortices in an Abrikosov lattice at high magnetic fields. Values of characteristic frequencies below which spectra are suppressed are evaluated for various experimental configurations including a typical experimental thin-film strip geometry. It is shown that characteristic frequencies and the functional dependence of the low-frequency part of the noise spectra strongly depend on the external magnetic field.

We study theoretically the depinning processes in twin boundaries in high-${\mathit{T}}_{\mathit{c}}$ superconducting crystals. It is shown that due to the thermodynamic fluctuations of the vortex system pinned in a twin boundary, the critical current can drop to zero at a temperature less than the critical one. There are two contributions to such a process: one is the individual vortex depinning determined by the deformation of a potential well by the current, and the other is the multivortex depinning because of the short-range intervortex interaction.

Neutron-diffraction data collected under vacuum at 680 and 695 K have been compared with elastic energy dissipation and dynamic Young’s modulus measurements in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6+\mathit{x}}$. The orthorhombic to tetragonal phase transition has been observed in isothermal conditions. The data suggest that a phonon softening occurs at the oxygen disordering, which, in these experimental conditions, precedes the phase transition. The elastic energy dissipation presents a peak above room temperature. Its origin and behavior are discussed in terms of an oxygen atom repopulation model, with activation energy depending on the orthorhombic strain and oxygen ordering in the basal plane.

We have measured the polarized x-ray-absorption spectra at the Cu K edge of ${\mathrm{Nd}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Ce}}_{\mathit{x}}$${\mathrm{CuO}}_{4\mathrm{-}\mathrm{\delta }}$ single crystals and the unpolarized spectra of ${\mathit{R}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Ce}}_{\mathit{x}}$${\mathrm{CuO}}_{4\mathrm{-}\mathrm{\delta }}$ (R=Pr, Nd, Sm, Eu, and Gd) ceramic samples. The results indicate that up to the T ’-phase stability limit (and including the narrow region of superconductivity), Ce doping continuously donates electrons into the Cu 3d orbitals. Our results evidence a ${\mathrm{Cu}}^{1+}$ content in these T ’-phase materials which increases almost linearly with the Ce-doping level x, and an electron doping fraction from each Ce atom to Cu which is quite close to unity (except in the R=Gd series). It is found that the size variation of the rare-earth elements R causes a number of feature changes in the Cu K edge, including an about 0.5 eV upward edge shift. These effects are explained by the ligand field shift of the unoccupied Cu 4p and 3d bands. A cluster model description has been developed to correlate the spectral features of the Cu K-edge spectra to different 3d configurations. The experimental results can be well described by the following model parameters: Cu 1s-3d Coulomb interaction ${\mathit{U}}_{\mathit{c}\mathit{d}}$∼6.5 eV, Cu 1s-O 2p Coulomb interaction ${\mathit{U}}_{\mathit{c}\mathit{L}}$∼1.1 eV, d-ligand hybridization V∼2.7 eV, Cu⇆O charge-transfer energy Δ∼1.3 eV (at x=0), and Δ∼0.5 eV (at x=0.2).

We have shown, within the Ginzburg-Landau theory, that the interaction between vortices and normal-metal layers in high-${\mathit{T}}_{\mathit{c}}$ superconductor–normal-metal superlattices can cause high critical-current densities ${\mathit{j}}_{\mathit{c}}$. The interaction is primarily magnetic, except at very low temperatures T, where the core interaction is dominant. For a lattice of vortices commensurate with an array of normal-metal layers in a parallel magnetic field H, strong magnetic pinning is obtained, with a nonmonotonic critical-current dependence on H, and with ${\mathit{j}}_{\mathit{c}}$ of the order of ${10}^7$–${10}^8$ A/${\mathrm{cm}}^2$.

Computer-simulation techniques have been used to study Zn, Ce, Pr, and Th doping in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_4$${\mathrm{O}}_8$. Our results suggest that Zn energetically prefers to substitute at the Cu(2) site whereas Pr and Th prefer to substitute at the Y site. These results are in agreement with the experimental results. Our results also suggest that Ce preferentially substitutes for the Cu(2) site. The changes in interatomic distances caused by Zn, Ce, Th, and Pr doping were also presented and analyzed. Our calculated changes in lattice parameters of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_4$${\mathrm{O}}_8$ due to Th and Ce doping are in qualitative agreement with the experimental results.

A complete set of sum rules is obtained for polarization-dependent x-ray-absorption fine structure and x-ray circular magnetic dichroism (CMD), analogous to those for CMD derived by Thole et al. These sum rules relate x-ray-absorption coefficients to the ground-state expectation values of various operators. Problems with applying these sum rules are discussed.

The pressure dependence of the phase-transition temperatures for oxygenated ${\mathrm{La}}_2$${\mathrm{CuO}}_{4.035}$ single crystals was investigated in the pressure range <14 kbar. Three successive phase transitions, one at 295 K with second-order character, one around 265 K with martensitic-transition character, and the other around 222 K with first-order character, were found to be suppressed under pressure. The pressure coefficient of the superconducting temperature ${\mathit{dT}}_{\mathit{c}}$/dp turned from -0.5 to +0.7 K/kbar when the three successive phase transitions were suppressed by pressure.

The formation free energies of the major point defects in sodium are calculated from first-principles (or ab initio$)— molecular-dynamics simulations, which allow for an accurate representation of the electron density around the defect site. Values are obtained for the vacancy, divacancy, and interstitial for a range of temperatures. The vacancy is found to have the lowest free energy of formation at all temperatures except possibly close to the melting point. Vacancies are found to experience an attractive interaction and may stay bound to each other, to form a divacancy, for a significant period of time. Contrary to previous expectations it is found that at temperatures close to the melting point the interstitial may have a significant presence.

For long-range dipole-dipole interactions, the energies of various periodic dipole orientations on a complex honeycomb lattice have been calculated based on the lattice-sublattice relations between Fourier components of interaction tensors. In the ground state of the system, vortex configurations of dipoles with their orientations differing in angles by ±2π/3 have been shown to be formed in the lattice plane. Provided any adsorption potentials force polar molecules out of the surface plane, Davydov splitting occurs in the vibration spectrum of the molecular ensemble.

Electron-paramagnetic-resonance spectra of ${\mathrm{Mn}}^{2+}$ in single crystals of ${\mathrm{Cd}}_2$Zr ${\mathrm{F}}_8$⋅${6\mathrm{H}}_2$O have been recorded at room temperature. An abundance of ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$ ligand hyperfine structure has been observed for all the 30 ${\mathrm{Mn}}^{2+}$ hyperfine lines at all orientations of the crystals. From the angular-variation study of ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$ superhyperfine structure it has been concluded that the impurity ion shows a preferential occupation of ${\mathrm{ZrF}}_8^{4\mathrm{-}}$ site over that of [${\mathrm{CdF}}_4$(${\mathrm{H}}_2$O${)}_3$${]}^{2\mathrm{-}}$. This is due to the identical ionic radius of ${\mathrm{Zr}}^{4+}$ and ${\mathrm{Mn}}^{2+}$ (0.80 Å) while the ${\mathrm{Cd}}^{2+}$ ionic radius is much larger (0.99 Å). This is an instance where ${\mathrm{Mn}}^{2+}$ has replaced ${\mathrm{Zr}}^{4+}$ in a lattice where ${\mathrm{Cd}}^{2+}$ was available and an isostructural ${\mathrm{Mn}}_2$${\mathrm{ZrF}}_8$⋅${6\mathrm{H}}_2$O was known to exist. A qualitative evaluation of the spin-Hamiltonian parameters has been carried out.

${\mathrm{UCu}}_2$${\mathrm{Ge}}_2$ is a compound that exhibits both antiferromagnetic and ferromagnetic character at low temperature. However, the transition from being ferromagnetic to antiferromagnetic has not been observed in electric measurements before. Therefore, whether the antiferromagnetic phase occurs abruptly or develops gradually as the temperature lowers is not clear. We report that the second phase transition is observed in our electric measurements of ${\mathrm{UCu}}_2$${\mathrm{Ge}}_2$. Therefore, the unusual magnetic behavior in ${\mathrm{UCu}}_2$${\mathrm{Ge}}_2$ is an intrinsic property of this compound and will not relate to the magnetic domain.

The nuclear contribution to the specific heat of thulium has been investigated in the temperature range 0.02–0.6 K and in the presence of a magnetic field. The data can be well fitted to C=γT+α${\mathit{T}}^3$+${\mathit{C}}_{\mathit{N}}$, where the first two terms represent the electronic and the lattice contribution, respectively, and ${\mathit{C}}_{\mathit{N}}$ is the nuclear specific heat. Using a two-level Schottky model for ${\mathit{C}}_{\mathit{N}}$, a ground-state energy splitting of 8.2×${10}^{\mathrm{-}6}$ eV and a degeneracy ratio of 1.28 were calculated. Consistent with the estimated very large effective magnetic field (565 T), an external magnetic field up to 14 T was found to have no significant effect on ${\mathit{C}}_{\mathit{N}}$.

Based on a theoretical model proposed for quasi-one-dimensional organic polymer ferromagnets, the spin configuration of π electrons in a π-conjugated carbon chain with a single and two side radicals, which contains unpaired electrons, is investigated. The strong electron-phonon interaction and electron-electron interaction in the one-dimensional system are taken into account self-consistently. It is found that around the unpaired electrons there appears a π-electron spin-polarization cloud with alternation of the sign and the amplitude of the spin density extending over a certain distance, which reduces rapidly with the increasing of electron-phonon coupling. In order to obtain ferromagnetic order in this kind of material, the number of carbon atoms, which is intermittent between two neighbor side radicals, should be odd. It is shown that the π-orbital alternation will result in a Peierls transition which stabilizes the magnetic order.

Magnetoresistance (Δρ/ρ) behavior of the alloys ${\mathrm{CePt}}_2$${\mathrm{Ge}}_2$, ${\mathrm{CeNi}}_{\mathit{x}}$${\mathrm{Ga}}_{4\mathrm{-}\mathit{x}}$ (x=0.5 and 1.1), ${\mathrm{CeCu}}_{\mathit{x}}$${\mathrm{Ga}}_{4\mathrm{-}\mathit{x}}$ (x=0.5 and 1.25), and ${\mathrm{CeRu}}_4$${\mathrm{Sn}}_6$ has been investigated. The results support our earlier conclusion that ${\mathrm{CePt}}_2$${\mathrm{Ge}}_2$ is characterized by a low value of Kondo temperature. The data on Ce-Ni(Cu)-Ga alloys are consistent with the existence of a competition between magnetism and Kondo effect as a function of composition. With respect to ${\mathrm{CeRu}}_4$${\mathrm{Sn}}_6$, we observe negative Δρ/ρ below 7 K and there is a change in the sign above 7 K, and this behavior is similar to that noted for ${\mathrm{Ce}}_3$${\mathrm{Bi}}_4$${\mathrm{Pt}}_3$.