A morphological model of the Volmer-Weber type is developed for the reactive interface formed between Fe and a ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_8$ single-crystal surface which takes into account microscopic aspects of the growth such as clustering, metal oxidation via the superconductor substrate, and diffusion through the cluster of elemental species released in the metal-surface redox reaction. The experimental findings of a local chemical analysis by core emission spectroscopy and an atomic-scale structural characterization by scanning tunneling microscopy are in good quantitative agreement with the proposed model indicating that the basic microscopic assumptions are correct.

We have calculated the spin susceptibility, as a function of momentum, for a two-dimensional tight-binding band including second-nearest-neighbor coupling in the electron dispersion curves. The effects of charge and spin fluctuations are fully accounted for in a self-consistent way. The renormalization effects due to these interactions are found to be large and momentum dependent. They strongly change the absolute value of the resulting susceptibilities, but their shape in the Brillouin zone is not very different from that obtained in the simple unrenormalized case. The temperature dependence of the susceptibilities is also investigated.

We compute current-voltage characteristics of an array of resistively shunted Josephson junctions with screening effects treated self-consistently by introducing inductance coefficients. The flux distribution and flux motion through the system is analyzed in the presence of transport currents and of an external magnetic field. We also discuss the applicability of our model to describe inhomogeneous high-${\mathit{T}}_{\mathit{c}}$ superconductors.

A general two-center six-band Hamiltonian, including copper ${\mathit{d}}_{\mathit{x}}^2$-${\mathit{y}}^2$ and ${\mathit{d}}_{3\mathit{z}}^2$-${\mathit{r}}^2$ as well as oxygen ${\mathit{p}}_{\mathrm{\sigma }}$ and apical ${\mathit{p}}_{\mathit{z}}$ orbitals, is used to calculate the electronic structure of copper-oxide superconductors, within a mean-field approximation. Size effects as well as the role of apical oxygen have been investigated in clusters of different sizes as a function of doping. For square clusters (2n×2n) three low-energy self-consistent solutions have been found. The lowest-energy solution is of the charge- and spin-bag type, being characterized by enhanced Zhang-Rice correlations and a broad density-of-states structure in the Fermi-level (${\mathit{E}}_{\mathit{F}}$) region. Although this structure has mainly nonplanar character just at ${\mathit{E}}_{\mathit{F}}$, its composition at the maximum, just below ${\mathit{E}}_{\mathit{F}}$, is comparable with experimental results. These features may be relevant in the superconducting regime.

The magnetic behavior of the reduced ${\mathrm{Nd}}_{1.8}$${\mathrm{Sr}}_{0.2}$${\mathrm{NiO}}_{3.72}$ nickelate has been investigated by means of neutron-powder diffraction, magnetic susceptibility, and isothermal magnetization measurements. The most noticeable result is that, in spite of the large concentration of oxygen vacancies, both the Ni and Nd sublattices become antiferromagnetically ordered below ${\mathit{T}}_{\mathit{N}1}$≊320 K and ${\mathit{T}}_{\mathit{N}2}$≊13.5 K, respectively. Moreover, two spin reorientations of the Ni magnetic moments have been observed, one around ${\mathit{T}}_1$≊130 K and the other simultaneously to the cooperative ordering of the Nd sublattice. The existence of a weak ferromagnetic component below 70 K has also been clearly established from the analysis of the macroscopic magnetic measurements. Finally, a set of effective exchange parameters have been derived, and their values compared with those reported for stoichiometric ${\mathrm{Nd}}_2$${\mathrm{NiO}}_4$.

We measured magnetic susceptibility of single-crystal rare-earth-doped R${\mathrm{Ba}}_2$${\mathrm{Cu}}_4$${\mathrm{O}}_8$ (where R=Y, Dy, Gd, Ho, Er, Tm, Yb) in the normal state along the three principal axes. The anisotropy measured fits well to calculated values obtained from a crystal electric field (CEF) analysis. From our results we developed a general scaling relation for the CEF parameters across the rare-earth series.

The self-trapped carriers of large (multisite) bipolarons redistribute themselves among the sites of their molecular orbitals as their self-trapping potential wells are altered by changing atomic positions. Through this polarizability, large bipolarons reduce the phonon frequencies. The dependence of the lowering of the zero-point vibrational energy on the spatial distribution of large bipolarons produces an intermediate-ranged phonon-mediated attraction between large bipolarons that is comparable to a phonon energy. Coulomb repulsions that oppose the phonon-mediated attractions tend to be suppressed by the large static dielectric constants associated with large-bipolaron formation. However, a strong short-range repulsion between bipolarons, analogous to the hard-core repulsion between helium atoms, persists. With the dominance of the medium-range phonon-mediated attraction and the short-range repulsion, large bipolarons can condense into a liquid. The bipolaronic liquid might be the condensed fluid phase that has been suggested as a precursor to superconductivity in cuprates.

We report temperature-dependent ${\mathrm{\chi }}_{\mathrm{ac}}$ measurements performed near the superconducting transition on three polycrystalline sintered ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ samples for selected fixed magnetic fields 0.1≤H≤80.9 G and ac excitation magnetic fields 0.9≤${\mathit{H}}_{\mathrm{ac}}$≤450 mG. The nature of ${\mathrm{\chi }}_{\mathrm{ac}}$ was found to depend strongly on the magnitude of ${\mathit{H}}_{\mathrm{ac}}$ and for small ${\mathit{H}}_{\mathrm{ac}}$ we observe the breakdown of critical-state behavior. We estimate the threshold field for the critical state and interpret our results in the context of Josephson-array granular superconductivity.

A quantitative study of the electron-spin-resonance (ESR) line shapes of the ${\mathrm{AsO}}_4^{4\mathrm{-}}$ center in ${\mathrm{RbH}}_2$${\mathrm{AsO}}_4$ and ${\mathrm{RbD}}_2$${\mathrm{AsO}}_4$ is presented. Analysis of the spectra at various temperatures, using a theoretical model and electron-nuclear double resonance (ENDOR) obtained superhyperfine (shf) couplings, shows the existence of a single correlation time with which the rubidium and hydrogen shf interactions are modulated. Further analysis of the ENDOR- and ESR-deduced shf couplings is shown to disallow any large-amplitude relative motions between the heavy atoms and the ${\mathrm{H}}_4$${\mathrm{AsO}}_4$ units. It is suggested that fluctuations in the nonsymmetric distribution of the electron density are responsible for the observed ESR features in potassium dihydrogen phosphate-type systems.

A recent report suggested that pressure-induced amorphization of the T phase of ${\mathrm{Nb}}_2$${\mathrm{O}}_5$ is accompanied by a reduction of Nb from the pentavalent state with the simultaneous loss of oxygen atoms. This conjecture is examined in detail by x-ray-photoelectron and x-ray-absorption spectroscopy. No reduction was detected within the sensitivity of the measurements, which impose an upper limit for the concentration of reduced Nb, if any, to be less than 3%. The ${\mathit{L}}_{\mathrm{I}\mathrm{I},\mathrm{I}\mathrm{I}\mathrm{I}}$-edge photoabsorption spectra suggest that the recovered high-pressure phase has a more compacted oxygen environment, which is often found in densified materials.

EPR signals of two defect centers in neutron-irradiated high-purity quartz have been analyzed. The principal values and the anisotropy of the g tensors suggest an attribution to peroxy radicals and nonbridging oxygen hole centers.

In this paper we report the results of x-ray powder diffraction experiments on ${\mathrm{C}}_{60}$-n-pentane. This material is a typical ${\mathrm{C}}_{60}$ clathrate and shows orthorhombic symmetry at room temperature. At low temperature an orthorhombic to monoclinic phase transition can be observed which involves the distortion of the base plane from rectangular. A microscopic picture of the phase transition is given by molecular-dynamics calculations.

The low-temperature phase and melting of a dilute, binary, two-dimensional array of colloidal particles with dipolar interactions are studied. The system consists of a 1% concentration of larger particles in an otherwise monodispersed array of polymer spheres confined between two glass plates. The translational and bond-orientational correlations in the condensed phase are those of a quenched hexatic glass with dislocations pinned to the larger particles. Melting into the fluid phase is consistent with the disclination-pair-dissociation-mediated theory of melting.

We report a giant-magnetoresistance (GMR) phenomenon in a bulk, spinodally decomposed Cu-20Ni-20Fe alloy containing a 30 Å size, coherent compositional modulation. A negative and isotropic magnetoresistance value at 4.2 K as high as 9% has been obtained. It is interesting to note that the magnetoresistance effect varies drastically with temperature (by as much as ∼800% between room temperature and 4.2 K) while the M-H behavior changes very little in the same temperature range. As there was no noticeable dependence of magnetoresistance on the field orientation, anisotropic magnetoresistance effect is ruled out as the main cause. The nature of the observed phenomenon may be similar to that of the GMR effect typically seen in superlattice or granular films; however, this GMR effect is seen in a non-single-domain structure where both the particles and the matrix are ferromagnetic.

The discovery of high-temperature superconductivity in doped copper oxides, which in their insulating forms are nearly ideal two-dimensional spin-1/2 Heisenberg antiferromagnets, has stimulated great interest in finding additional examples of such magnetic systems. We find that ${\mathrm{Sr}}_2$${\mathrm{IrO}}_4$, which has the ${\mathrm{K}}_2$${\mathrm{NiF}}_4$ structure and should have a 5${\mathit{d}}^5$ low-spin (S=1/2) electronic configuration, develops a ferromagnetic moment near 250 K. The small size of the remanent moment (${10}^{\mathrm{-}2}$${\mathrm{\mu }}_{\mathit{B}}$), and our structural studies, however, imply that this is weak ferromagnetism which appears at the Néel temperature due to a Dzyaloshinsky-Moriya interaction, in a manner analogous to that seen in ${\mathrm{La}}_2$${\mathrm{CuO}}_4$.

We consider the Heisenberg chain of spin 1/2 with a concentration x of impurity spins S. The thermodynamic Bethe ansatz equations are stated for this model. The ground-state equations are analyzed as a function of x and magnetic field. In zero field the ground state is a singlet for finite x, but non-Fermi-liquid-like for x→0. Two energy bands play a role at T=0. The Van Hove singularities of the empty bands define two critical fields, ${\mathit{H}}_{\mathit{c}1}$(x) and ${\mathit{H}}_{\mathit{c}2}$(x), at which the susceptibility diverges. The non-Fermi-liquid behavior as x→0 arises from nonanalyticities as ${\mathit{H}}_{\mathit{c}2}$ vanishes.

We use a microcanonical simulation to obtain the phase diagram in the temperature-concentration space of the three-dimensional site-diluted Ising model, for concentrations of magnetic atoms in the range 1≥p≥0.4. The temperature and concentration dependence of the magnetization, internal energy, and specific heat are calculated on 32×32×32 lattices. The resulting phase diagram agrees well with recent Monte Carlo simulations and extends the numerical estimates up to the regime of strong dilution.

We suggest a microscopic model that provides a consistent explanation of the recent femtosecond pump-probe experiments on ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$, including the fact that the reflectivity change in the superconducting state is much larger than that in the normal state. In this model, not only the oscillatory part of the reflectivity, but also the total reflectivity change, is due to displacive excitation of coherent phonons. The microscopic reason for this excitation is that superconductivity induces small displacements in the equilibrium positions of the ions, since the pairing energy depends on the density of states at the Fermi level, which changes with the ionic positions. When superconductivity is destroyed by the femtosecond laser pulse, the ions are pulled back to their normal equilibrium positions, thus exciting coherent phonons. The relative size of the oscillatory contribution to the reflectivity depends upon the ratio of the phonon period to the time scale of the pair breaking, and when this ratio is small, the oscillations are suppressed, as observed in the experiment. Ab initio caluclations confirm this model. The model also provides an explanation for why the mangitude of the 150 ${\mathrm{cm}}^{\mathrm{-}1}$ mode below ${\mathit{T}}_{\mathit{c}}$ may be smaller than that of the 120 ${\mathrm{cm}}^{\mathrm{-}1}$ mode.

In order to extend the density-matrix renormalization-group (DMRG) method to two-dimensional systems, we formulate two alternative methods to prepare the initial states. We find that the number of states that are needed for accurate energy calculations grows exponentially with the linear system size. We also analyze how the states kept in the DMRG method manage to preserve both the intrablock and interblock Hamiltonians, which is the key to the high accuracy of the method. We also prove that the energy calculated on a finite cluster is always a variational upper bound.

The magnetization M(T,H) of a ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{8+\mathrm{\delta }}$ single crystal has been measured above and below the superconducting transition temperature in externally applied magnetic fields H up to 50 kOe for H∥c. We find that M(T,H) is independent of H at a particular temperature ${\mathit{T}}^{\mathrm{*}}$ for external fields exceeding 10 Oe, and exhibits a two-dimensional (2D) scaling behavior as a function of [T-${\mathit{T}}_{\mathit{c}}$(H)]/(HT${)}^{1/2}$. The parameters ${\xi }_{\mathit{a}\mathit{b}}$(0)≊21.4 Å and ${\xi }_{\mathit{c}}$(0)≊0.4 Å are estimated from fitting the experimental data to a full Lawrence-Doniach model calculation of the fluctuation-induced magnetic susceptibility. From the same analysis, we obtain ${\mathit{dH}}_{\mathit{c}2}$/dT≊-14 kOe/K in the range 5 kOe<H<50 kOe.