The density of states in unconventional superconductors, ${\mathit{N}}_{\mathit{s}}$(E), is very sensitive to impurity scattering, especially at low energies. We investigate the behavior of ${\mathit{N}}_{\mathit{s}}$(E), using both numerical calculations and analytic approximations; we discuss the Born limit, the unitarity limit, and the crossover between these two cases. We consider a two-dimensional electron gas with a d-wave order parameter of the form Δ(k^)=Δ(k${\mathrm{^}}_{\mathit{x}}^2$-k${\mathrm{^}}_{\mathit{y}}^2$). The low-energy structure in ${\mathit{N}}_{\mathit{s}}$(E) should be quite general, applying to all gaps with the same general behavior.

We have used perturbed-angular-correlation (PAC) spectroscopy to measure nuclear-electric-quadrupole interactions at the Ti site in ceramic samples of rutile ${\mathrm{TiO}}_2$. To investigate differences in the probe crystal chemistry, we used two chemically different PAC probes, ${}_{}{}^{111}{}_{}{}^{}\to _{}^{111}{}_{}{}^{}$Cd and ${}_{}{}^{181}{}_{}{}^{}\to _{}^{181}{}_{}{}^{}$Ta, and we measured the temperature dependences of the electric-field-gradient parameters, ${\mathit{V}}_{\mathit{z}\mathit{z}}$, η, and δ over a temperature range from 290 to 1300 K. The results of these PAC measurements are compared to the results of ${}_{}{}^{47,49}{}_{}{}^{}\mathrm{Ti}$ nuclear-magnetic-resonance measurements. Significant differences are found in the temperature dependences of ${\mathit{V}}_{\mathit{z}\mathit{z}}$ and η that cannot be explained in terms of simple considerations such as ionic size and charge. These effects are discussed in terms of differences in directional bonding that the PAC probes could exhibit.

Minimum total-energy calculations, which account for both electron-lattice and electron-electron interactions in conjugated polymers, are performed for chains with up to eight carbon atoms. These calculations are motivated in part by recent experimental results on the spectroscopy of polyenes and conjugated polymers and they shed light on the long-standing question of the relative importance of electron-lattice vs electron-electron interactions in determining the properties of these systems.

We investigate the behavior of the correlation function and the susceptibility of the random-field spherical model at the critical point. In particular we calculate the critical exponents η and η¯ describing the divergences of the susceptibility and its disconnected part, respectively. In the case of short-range interactions we obtain η=η¯=0. For power-law interactions ${\mathit{U}}_{\mathit{i}\mathit{j}}$∼${\mathit{r}}_{\mathit{i}\mathit{j}}^{\mathrm{-}\mathrm{\sigma }}$ we find η=1/2η¯=D+2-σ (for D<σ<D+2), where D is the spatial dimension. The Schwartz-Soffer exponent inequality η¯≤2η is satisfied and becomes an equality independent of the functional form of the interaction.

The plaquette expansion of the Lanczos recursion method is applied to the two-dimensional antiferromagnetic Heisenberg model. Connected Hamiltonian moments 〈${\mathit{H}}^{\mathit{n}}$${\mathrm{〉}}_{\mathit{c}}$ are calculated with respect to the Néel state up to n=6. The subsequent plaquette expansion of the Lanczos matrix in the number of plaquettes on the lattice, ${\mathit{N}}_{\mathit{p}}$, is determined to order 1/${\mathit{N}}_{\mathit{p}}$. Diagonalizing the Lanczos matrix in this form gives a value of the energy density of -0.664 in the limit ${\mathit{N}}_{\mathit{p}}$→∞, in good agreement with existing calculations.

We present results from a neutron-scattering study of lattice anomalies associated with the transition to the gapped spin-singlet state observed in the spin-1/2 linear-chain compound ${\mathrm{CuGeO}}_3$. Instead of the lattice dimerization along the chain direction (c axis) expected for a spin-Peierls system, we observe a remarkable softening of the longitudinal acoustic phonons in a direction (along the b axis) perpendicular to the chains. A spontaneous strain appears in the same direction below the magnetic transition temperature of 14 K.

The optical second-harmonic intensity generated by magnetic multilayers of cobalt and gold strongly depends on the magnetization. At normal incidence the results are independent of the amount of bulk cobalt and gold, but are accurately described by pure magnetic interface contributions. This represents a way to probe buried interface magnetism using magnetization-induced second-harmonic generation.

We perform a computer simulation of the flux-pinning dynamics for a single defect under the presence of both the applied current and the external magnetic field by directly solving the time-dependent Ginzburg-Landau equation coupled with the Maxwell equation in a two-dimensional rectangular region. With one vortex flow path and a single defect, we found two types of vortex motions around the defect, which are one-flux and two-flux pinning, depending on the size of the defect. The criterion of the two-flux pinning is studied by both numerical simulation and a simplified free-energy calculation. We also present the effect of a single defect on the V-I characteristics.

The line shape and Knight shift of the ${}_{}{}^{89}{}_{}{}^{}\mathrm{Y}$ NMR resonance was obtained for fully oxygenated (x=7), oxygen deficient (x=6.7), and nonsuperconducting oxygen deficient (x=6) fluorine-doped YBCO (${\mathrm{Yba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{x}}$${\mathrm{F}}_{\mathit{y}}$). The results show that while fluorine can increase the onset superconducting transition temperature ${\mathit{T}}_{\mathit{c}}$ in oxygen deficient material, the Knight shift is practically independent of the fluorine concentration. The consequences of this result as far as ${\mathit{T}}_{\mathit{c}}$ is concerned are discussed.

A scenario for a tunable dislocation-induced atomic-scale Josephson junction in intrinsically defected high-${\mathit{T}}_{\mathit{c}}$ single crystals is proposed which is based on a competition between different external forces acting upon a dislocation to stabilize its equilibrium position inside a crystal. A possible realization of the self-tuning Josephson effect in these materials is discussed as well.

With the embedded-atom method we determine the normal vibrational modes of ${\mathrm{Ni}}_{\mathit{x}}$${\mathrm{Al}}_{1\mathrm{-}\mathit{x}}$, which displays a martensitic transformation (MT) for 0.61≤x≤0.66. In the high-temperature phase the compositional disorder induces several bands of localized phonons that discontinuously delocalize at the MT. The analysis of the phonon eigenvectors allows one to identify the atomic arrangements inducing localization. We show that the modes associated with Ni-rich clusters can be used to monitor incipient lattice instabilities, and identify regions with low barriers for nucleation of the martensite.

A density-functional approximation is used to study the high-temperature phase diagram of a system of fullerene ${\mathrm{C}}_{60}$ molecules which are considered as rigid spheres interacting via the Girifalco [J. Phys. Chem. 96, 858 (1992)] potential. We discuss the theoretical predictions together with the contradictory results given by computer simulations on the existence of a conventional liquid phase for the fullerene ${\mathrm{C}}_{60}$. Theory cannot definitively settle this point but the predictions for the melting and freezing lines are in quite good quantitative agreement with simulations results.

Fractal aggregates are considered computationally using off-lattice cluster-cluster aggregation models. The aggregates are made of spherical particles of different sizes distributed according to a Gaussian-like distribution characterized by a mean ${\mathit{a}}_0$ and a standard deviation σ. The wave-vector-dependent scattered intensity I(q) is computed in order to study the influence of the particle polydispersity on the crossover between the fractal regime and the Porod regime. It is shown that, given ${\mathit{a}}_0$, the location ${\mathit{q}}_{\mathit{c}}$ of the crossover decreases as σ increases. The dependence of ${\mathit{q}}_{\mathit{c}}$ on σ can be understood from the evolution of the shape of the center-to-center interparticle-distance distribution function.

The temperature dependence of the Hall resistivity of thin epitaxial ${\mathrm{UPd}}_2$${\mathrm{Al}}_3$ films with low defect densities is investigated. It shows a minimum at 6 K for fields up to 12 T. This feature is attributed to the superposition of a coherence-derived low-temperature Hall coefficient and a skew-scattering contribution. Further implications on the Hall effect in the coherence regime of Kondo lattices are discussed.

The ground-state properties of the Heisenberg antiferromagnet on a kagomé lattice with further-neighbor interactions are discussed. The classical ground states are described as spiral phases of an equivalent square lattice with a basis, and quantum fluctuations are incorporated by means of a rotationally invariant Schwinger-boson approach. The quasiparticle dispersion relations have the correct zero-mode structure, without the pathologies present in the standard spin-wave approximation. For the nearest-neighbor S=1/2 model the theory predicts an ordered ground state, although with a largely reduced magnetization of only 18% of the classical value. Implications of the results for the low-temperature thermodynamics of the stacked S=3/2 kagomé antiferromagnet ${\mathrm{SrCr}}_{8\mathrm{-}\mathit{x}}$${\mathrm{Ga}}_{4+\mathit{x}}$${\mathrm{O}}_{19}$ are also discussed.

Strong magnetic circular dichroism (MCD) in 4f photoemission from remanently magnetized films of Tb(0001) and ${\mathrm{TbFe}}_{\mathit{x}}$ grown on W(110) is reported. Asymmetries of up to 62% have been observed for the 4${\mathit{f}}^8$${\mathrm{-}}^8$${\mathit{S}}_{7/2}$ multiplet component, in agreement with a calculated 93% effect, taking into account the incomplete circular polarization of the photon beam and the experimental geometry. This is a very strong MCD effect and provides interesting perspectives for surface and thin-film magnetism, as well as a versatile sensor for circular polarization of soft x rays over a wide photon-energy range.

Vacuum ultraviolet (vuv) reflection spectra have been measured from amorphous ${\mathrm{SiO}}_2$(a-${\mathrm{SiO}}_2$) plates of varying densities. Two bands were observed at 10.3 and 11.4 eV in the vuv reflection spectra for as-received a-${\mathrm{SiO}}_2$. Upon densifying the a-${\mathrm{SiO}}_2$ with a multianvil-type high-pressure apparatus, both the strong band at 10.3 eV and the weaker shoulder at 11.4 eV were found to shift toward lower energies and to broaden. The shift of these bands during densification can be explained either by a unification of the extended point-dipole theory and harmonic oscillator model, or by the relationship between the band-gap energy and Si-O bond length. The broadening of the bands during densification, and thus the Si-O-Si bond-angle decrease, may be due to either the distortion of ${\mathrm{SiO}}_4$ tetrahedra or a widening of the distribution of rotational arrangements between connected ${\mathrm{SiO}}_4$ tetrahedra causing a distortion of the electronic structure. A shift of the band at 10.3 eV toward higher energy is observed upon cooling in undensified a-${\mathrm{SiO}}_2$, but this shift gradually disappears with increasing densification. Although the reason for the temperature dependence of band position for the undensified a-${\mathrm{SiO}}_2$ is not yet clear, it appears that the disappearance of this dependence with densification is related to an increase of the structural randomness of the glass.

We use a hybrid Monte Carlo algorithm to simulate the thermodynamic properties of a two-dimensional periodic array of fully frustrated quantum Josephson junctions. We find a variety of metastable configurations that correspond to spatial domain boundaries that are also present in classical arrays. In previous work, due to long temporal correlations between configurations in diffusive Monte Carlo algorithms, such metastable states were interpreted as evidence for a quantum-induced low-temperature first-order transition.

We consider a superconducting double barrier and show that, under certain conditions, resonant tunneling is directly observable. We study the effect of these resonances on the conductance of the structure as a function of the system parameters and temperature, in order to understand in which regimes these effects are experimentally accessible. The proposed structure provides an excellent vehicle for investigating certain theoretically predicted phenomena which have not yet been observed. These are the tunneling of an electron through the gap of a superconducting material and the phase change of an electron wave function due to Andreev reflection.

We report on simultaneous measurements of the dc critical current and rf penetration depth of superconducting 2H-${\mathrm{NbSe}}_2$. We observed that the large peak in the dc critical current (the ‘‘peak effect’’) occurs at the same applied magnetic field H(T) as a maximum in the rf penetration depth, indicating that the anomaly in flux-line dynamics is associated with a change in flux-line pinning strength. The anomalies in the critical current and rf penetration depth disappear in a sample with a high density of columnar defects produced by Ag-ion bombardment; both superconducting and charge-density-wave phase transitions are substantially broadened by the defects.