In the present study, dense pellets of polycrystalline ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ were made by dynamic powder compaction. The shock wave, which passes through the initially loose powder, generates multiple defects. Its hydrostatic component suppresses the brittle nature of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$, and so plastic deformation such as dislocation generation and glide is expected. This paper reports on the characterization of structure defects observed in shock loaded samples, compacted at E/M ratios ranging from 0.8 to 2.3. The microstructure of shock compacted samples is compared to that of the initial, noncompacted powders. Apart from the well-established 〈100〉(001) glide system, two additional glide systems have been identified: [110](11¯0) and [010](100). They are characterized both by diffraction contrast and high-resolution transmission-electron-microscopy techniques. The interaction between the different types of dislocations and other structure defects, as well as their role in cleavage and stacking fault generation is investigated. Finally, the possible role in flux pinning of the dislocation types is briefly discussed.

We have investigated systematically the particle formation on the surface of high-quality ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ (YBCO) epitaxial films grown by pulsed laser deposition. Scanning-Auger-electron spectroscopy identified two types of precipitates, i.e., ${\mathrm{BaCuO}}_2$ and CuO, on the films deposited from a stoichiometric YBCO target. The coexistence of these two phases with a YBCO matrix film is well explained by the pseudoternary phase diagram of ${\mathrm{YO}}_{1.5}$-BaO-CuO, assuming a slight Y deficiency in the film. The precipitate formation was reduced when the films were grown under conditions slightly deviated from the optimum for the formation of well-equilibrated, highly crystalline, and high-${\mathit{T}}_{\mathit{c}}$ and high-${\mathit{J}}_{\mathit{c}}$ films. Under such thermodynamically off-equilibrated conditions, the phase separation into YBCO and secondary phases is presumed to be kinetically prevented to give solid-soluted films and off-stoichiometric compositions and inferior crystallinity and superconductivity. The film-thickness dependences of the precipitate density and size were quantitatively analyzed by scanning electron microscopy to indicate that the nucleation of precipitates occurred by the initial 50 nm growth of the film to be followed by the increase of the precipitate’s size. The use of off-axis polished substrates could successfully suppress the nucleation of the precipitates. A possible mechanism of precipitate formation is proposed.

Magnetic measurements on vacuum-annealed orthorhombic ${\mathrm{Pb}}_2$${\mathrm{Sr}}_2$${\mathrm{PrCu}}_3$${\mathrm{O}}_{8+\mathit{y}}$ (y≊0) reveal an anomalous Pr antiferromagnetic ordering at ${\mathit{T}}_{\mathit{N}}$(Pr)≃6 K with transition onset up to 11 K. This Pr-2213 compound shares common structural features with the tetragonal Pr-123 compound ${\mathrm{PrBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_6$, which also undergoes a Pr ordering at 10 K. However, oxygenated pseudotetragonal ${\mathrm{Pb}}_2$${\mathrm{Sr}}_2$${\mathrm{PrCu}}_3$${\mathrm{O}}_{8+\mathit{y}}$ (y≊1) exhibits only low transition onset around 3 K, while orthorhombic ${\mathrm{PrBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ has a higher ${\mathit{T}}_{\mathit{N}}$(Pr) value of 17 K. Complementary studies by adiabatic calorimetry on ${\mathrm{Pb}}_2$${\mathrm{Sr}}_2$${\mathrm{PrCu}}_3$${\mathrm{O}}_8$ yield a specific-heat anomaly with a peak near the magnetically derived ${\mathit{T}}_{\mathit{N}}$(Pr), with onset around 11 K. A large heavy-fermion-like linear term in the specific heat prevails as often observed in other Pr-containing high-${\mathit{T}}_{\mathit{c}}$ systems. The magnetic entropy Δ${\mathit{S}}_{\mathit{m}}$ below 11 K onset is relatively small, only 13% of R ln3 for ${\mathrm{Pr}}^{3+}$ with a quasitriplet ground state, implying a quasi-2D nature of the magnetic ordering with a strong 2D short-range magnetic correlation which persists to higher temperatures in this extremely anisotropic structure.

We describe a method for treating fluctuations in two-dimensional superconducting films in zero magnetic field. The method involves expanding the order parameter ψ(x,y) in empty-lattice Wannier functions of a fictitious square lattice. Despite the discrete basis, the order parameter is continuous and has no unphysical pinning. The thermodynamics of the model is a function of a single variable analogous to the Josephson coupling in granular superconductors. We estimate the Kosterlitz-Thouless (KT) transition temperature ${\mathit{T}}_{\mathit{c}}$ of the model by Monte Carlo techniques. If amplitude fluctuations are neglected, the model reduces to a partially frustrated XY Hamiltonian, even in zero magnetic field. With amplitude fluctuations, ${\mathit{T}}_{\mathit{c}}$ is further reduced, the Coulomb-gas scaling hypothesis of Minnhagen is automatically satisfied, and the jump in superfluid density at the transition may possibly be nonuniversal. Snapshots of ψ(x,y) near ${\mathit{T}}_{\mathit{c}}$ reveal the rapid development of pairs of oppositely charged vortices, accompanied by zeros of the order parameter, and, above ${\mathit{T}}_{\mathit{c}}$, by unpaired vortices, in agreement with the original KT picture. The extension of this approach to layered three-dimensional superconductors is briefly discussed.

The distribution of structural phases formed in polycrystalline ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{x}}$ by quenching has been measured by means of x-ray diffraction. Disk-shaped samples were annealed in a temperature range of 680–800 °C in air, and subsequently quenched in liquid nitrogen. The x-ray diffraction patterns revealed that the tetragonal phase is formed near the surface although the orthorhombic phase is distributed entirely in the inside. The depth to the phase boundary increased with increasing sample thickness and annealing temperature. The conclusion is that the tetragonal phase is not a preserved phase from high temperature but a newly produced phase by deoxidation during quenching. The lower concentration of oxygen near the surface was directly confirmed by x-ray photoelectron spectroscopy. It was also confirmed that the transition from superconductor to normal conductor takes place in the orthorhombic phase.

We report a determination of the irreversibility line of epitaxial c-axis-oriented ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ thin films with different thicknesses obtained by irreversible magnetization measurements. While close to ${\mathit{T}}_{\mathit{c}}$ the irreversibility line is obtained by direct measurements, a universal relation is found at lower temperatures which allows a determination of the irreversibility line down to 4.2 K. This line is found to shift towards larger values when the film thickness d is increased and saturates for d larger than the penetration depth ${\lambda }_{\mathit{a}\mathit{b}}$(0). The results are discussed in the framework of flux creep in quasi-two-dimensional flux-line-lattice and vortex-lattice melting models.

The anisotropy of the high-temperature superconductors has strong impact on their transport properties in the mixed state. We have performed a comparative study of the Nernst, Seebeck, and Hall effects of the anisotropic and extremely anisotropic high-temperature superconductors ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ and ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{8+\mathit{x}}$, respectively. High-quality, c-axis-oriented epitaxial thin films have been used for our study. The temperature and the magnetic-field dependence of the Nernst and Seebeck electric fields were measured both in the mixed state and in the fluctuation regime above the thermodynamic critical temperature. The Seebeck-effect data can be explained well by an extended two-fluid counterflow picture in analogy to the fountain effect in superfluids. From the Nernst-effect data the temperature and magnetic-field dependence of the transport entropy of magnetic-flux lines has been derived. For the magnetic field applied parallel to the c axis of the films, approximately the same value of the transport entropy was found for ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ and ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{8+\mathit{x}}$, showing that the different anisotropy of these materials has little influence on this quantity.

From our experimental data in the mixed state and the fluctuation regime well above the mean-field critical temperature, the upper-critical-field slope was derived. For both regimes, ${\mathit{dH}}_{\mathit{c}2}$/dT=-2.4±0.2 and -2.5±0.2 T/K for ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ and ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{8+\mathit{x}}$, corresponding to a Ginzburg-Landau coherence length ${\xi }_{\mathit{a}\mathit{b}}$=14.5 and 14.2 Å, respectively. The Hall resistivity ${\mathrm{\rho }}_{\mathit{x}\mathit{y}}$ of both materials showed a sign anomaly in the mixed state and scaled with the longitudinal resistivity ${\mathrm{\rho }}_{\mathit{x}\mathit{x}}$ as ${\mathrm{\rho }}_{\mathit{x}\mathit{y}}$(T)=${\mathit{K}}^{\mathrm{-}1}$${\mathrm{\rho }}_{\mathit{x}\mathit{x}}^{\mathrm{\beta }}$(T) with β≃2 and a magnetic-field-independent coefficient K.

Magnetic studies on polycrystalline ${\mathrm{HgCa}}_2$${\mathrm{CuO}}_{4+\mathrm{\delta }}$ superconductors reveal pronounced pinning by the grain surfaces at low temperatures over a wide range of magnetic fields. The response to small ac fields shows that up to 80% of the width of the dc magnetic hysteresis loop in high fields originates from shielding supercurrent flowing in the surface sheath, a vortex-free region with thickness of ∼10 nm. At 10 K and 15 kG, the observed persistent surface current has a density of ∼5×${10}^7$ A/${\mathrm{cm}}^2$ when averaged over the barrier thickness. The study suggests that a high current density may be achieved in very thin films by surface pinning.

The ab-plane penetration depth λ(T) in twinned ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ films exhibits a crossover in its T dependence at 25 K. Below 25 K, δλ≡λ(T)-λ(0)∝${\mathit{T}}^2$. This differs from recent measurements on a crystal which find δλ∝T below 40 K. Above 25 K, the normalized data λ(T/${\mathit{T}}_{\mathit{c}}$)/λ(0) vs T/${\mathit{T}}_{\mathit{c}}$ for our films agree extremely well with the crystal data, including quasilinear behavior between 25 and 40 K. Differences and similarities of films and crystals agree well with a theory of slightly disordered d-wave superconductors.

Measurements of the tunnel current as a function of applied voltage on the layered high-${\mathit{T}}_{\mathit{c}}$ material ${\mathrm{TlBa}}_2$(${\mathrm{Ca}}_{0.8}$${\mathrm{Y}}_{0.2}$)${\mathrm{Cu}}_2$${\mathrm{O}}_7$ have been shown to exhibit single-electron tunneling behavior. It has been proposed that the ultrasmall capacitance required to explain the single-electron tunneling spectra obtained with a low-temperature scanning tunneling microscope arises naturally from the planar crystal structure of this high-${\mathit{T}}_{\mathit{c}}$ material. This hypothesis is further examined using a single-electron tunneling model that incorporates three tunnel capacitors, a condition that is required by atomic-force-microscope studies of the topography of the film’s surface. An improved fit to the tunneling conductance is obtained, providing further evidence that a key to understanding Coulomb blockade in layered materials like ${\mathrm{TlBa}}_2$(${\mathrm{Ca}}_{0.8}$${\mathrm{Y}}_{0.2}$)${\mathrm{Cu}}_2$${\mathrm{O}}_7$ is the tunnel capacitors that form between multilayer CuO planes. This conclusion has important implications for the fabrication of nanoelectronc devices based on single-electron effects.

We present an exact diagonalization study of the doping dependence of the single-particle Green’s function in 16-, 18-, and 20-site clusters of the t-J model. We find evidence for the validity of the rigid-band picture starting from the half-filled case: upon doping, the topmost states of the quasiparticle band observed in the photoemission spectrum at half-filling cross the chemical potential and reappear as the lowermost states of the inverse photoemission spectrum. Features in the inverse photoemission spectra, which are inconsistent with the rigid-band picture, are shown to originate from the nontrivial point-group symmetry of the ground state with two holes, which enforces different selection rules than at half-filling. Deviations from rigid-band behavior which lead to the formation of a large Fermi surface in the momentum distribution are found to occur at energies far from the chemical potential. A Luttinger Fermi surface and a nearest-neighbor hopping band do not exist.

Local distortions of Tl and O(3) atoms from the high-symmetry crystallographic sites in ${\mathrm{Tl}}_2$${\mathrm{Ba}}_2$${\mathrm{CuO}}_6$ and ${\mathrm{Tl}}_2$${\mathrm{Ba}}_2$${\mathrm{Ca}}_2$${\mathrm{CuO}}_8$ have been observed by x-ray-absorption fine structure at the Tl ${\mathit{L}}_{\mathrm{III}}$ edge. The interlayer Tl-Tl distance around R=3.50 Å remains, but the intralayer Tl-Tl distance around R=3.87 Å is not found. The Tl-O(3) distances (in the Tl-O layers) are R=2.31 Å in Tl-2201 and R=2.35 Å in Tl-2212. These results show that both Tl and O atoms are shifted from their ideal crystallographic positions. The differences in the near-edge features of the x-ray-absorption spectra of these materials are consistent with this result. A negative edge shift of 1.2 eV at the Tl ${\mathit{L}}_{\mathrm{III}}$ edge compared to that of ${\mathrm{Tl}}_2$${\mathrm{O}}_3$ indicates that the valence state of the Tl ion is less than 3+, which is consistent with charge-transfer models.

Current-voltage (I-V) characteristics and the increase of the superconducting transition temperature ${\mathit{T}}_{\mathit{c}}$ as a function of the number of half-unit-cell layers n are studied for ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{8+\mathrm{\delta }}$ thin films grown by molecular beam epitaxy. These observations are interpreted in terms of Kosterlitz-Thouless (KT) transitions associated with vortex string pairs threading the film. Considering the change in the types of the lowest free-energy excitations as a function of the size of vortex pair, we have formulated KT renormalization-group (RG) equations. They are formulated with respect to independent vortex and antivortex pairs for a distance r less than the n-dependent characteristic length ${\mathit{r}}_{\mathit{n}}$, and then in terms of vortex-string and antivortex-string pairs for r>${\mathit{r}}_{\mathit{n}}$. RG equations of these two regions are connected by the continuity of superconducting carrier density and vortex density per area. Using experimental data obtained with ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{8+\mathrm{\delta }}$ thin films, we have solved the RG equations. It is shown that the KT transition occurs in the vortex-string region with an interstring lnr interaction only. Our calculation successfully explains KT scaling in I-V characteristics and the relation between ${\mathit{T}}_{\mathit{c}}$ and n. These results indicate that the interlayer interaction plays an important role in ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{8+\mathrm{\delta }}$ thin films.

We have performed a theoretical study of superconductor–normal-metal layered structures using a microscopic method. Results are given for the single-particle density of states, the order parameter, and the energy gap as a function of the coherence length, temperature, pair-breaking impurity mean-free path, and depth or probe range. The results are found to be in good agreement with those obtained from tunneling experiments on a Au-${\mathrm{NbSe}}_2$ bilayer and on the high-temperature superconductor ${\mathrm{Nd}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Ce}}_{\mathit{x}}$${\mathrm{CuO}}_{4\mathrm{-}\mathit{y}}$. The spatial variation of all quantities has been taken into account self-consistently. The roles of all relevant lengths and their experimental implications are discussed and emphasis is put on the case of short-coherence-length materials.

Utilizing an exact inversion scheme, we determine the complex linear conductivity σ(ω) of a ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ film from the linear magnetic ac susceptibility which has been measured from 3 mHz to 50 MHz in fields between 0.4 and 4 T applied parallel to the c axis of a 250-nm thin disk. The frequency derivative of the phase σ’’/σ’ and the dynamical scaling of σ(ω) above and below ${\mathit{T}}_{\mathit{g}}$(B) provide clear evidence for a continuous phase transition at ${\mathit{T}}_{\mathit{g}}$ to a generic superconducting state. Based on the vortex-glass scaling model, the resulting critical exponents ν and z are close to those frequently obtained on films by other means and associated with an isotropic vortex glass. The field effect on σ(ω) can be related to the increase of the glass coherence length, ${\xi }_{\mathit{g}}$∼B.

We answer the question of how many independent experimental measurements (or theoretical conditions) are needed to determine a projector.

We study two different scalar wave equations. One of them exhibits the main gross features of the simple scalar and elastic wave propagation in periodic composite media. The other behaves similarly to the electromagnetic waves in preferring the network topology and the higher volume fractions for developing spectral gaps.

We present an analysis correlating ab initio methods with those based on model Hamiltonians, e.g., the Pariser-Paar-Pople method. We propose a method to derive the model Hamiltonian parameters from first principles, thus making it possible to calculate properties with this approach for geometries other than equilibrium.

The diffusion of polystyrene in solution in the pore space of Vycor porous glass has been studied by the method of dynamic light scattering. In contrast to diffusion rates reported by others, we find results that are completely consistent with our previous measurements in other porous glass systems and that are in agreement with predictions of hydrodynamic theories and a model of chaotically connected pores. The anomalously slow diffusion of azobenzene in Vycor reported by others is possibly attributable to the residual effects of surface adsorption, which we have carefully eliminated in our experiment.

Detailed measurements of the temperature and magnetic field dependences of the magnetization of hydrothermally grown ${\mathrm{Bi}}_2$${\mathrm{CuO}}_4$ single crystals have been performed. It is concluded from an analysis of the experimental data that ${\mathrm{Bi}}_2$${\mathrm{CuO}}_4$ is a weak ferromagnet with a net magnetization in the basal plane. The application of a magnetic field in the basal plane induces a phase transition from a weak ferromagnetic to an antiferromagnetic structure.