Large negative magnetoresistance, ΔR(B)/R(0), of weak Kondo-like systems, ${\mathrm{CeCu}}_2$${\mathrm{Sn}}_2$ and ${\mathrm{CeNi}}_2$${\mathrm{Sn}}_2$, was observed in the vicinity of 2 K. At 8.3 T, it reached 7% and 23% for the former and latter materials, respectively. The measurements were taken up to 28 K where the magnetoresistance has diminished even at the highest magnetic fields used in our study. The results were interpreted in terms of relatively weak molecular fields which correlate the spin sublattice of these materials. Following the approach proposed by Abrikosov for calculating resistivity in weakly spin-correlated Kondo-like systems, we were able to fit successfully all our results. The fitting produced an estimate of the spin exchange integral for electron-impurity interaction J, which was found to be of the order of 0.36 meV for both systems.

We report on giant magnetoresistance (GMR) effects in a type of magnetic nanostructure including both layers and clusters. Our structures are prepared by sputtering and include both continuous permalloy layers and cobalt layers broken into small clusters (islands). We obtain magnetoresistance ratios as large as 35% in less than 10 Oe, with slopes of about 6.5% per Oe in the steepest part of the curve. The advantage of such hybrid structures including layers and clusters, and the origin of the observed GMR effects are the following: (a) lateral decoupling of the thin Co layers into islands prevents ferromagnetic bridging by pinholes or other defects from extending to a significant part of the sample; (b) the field dependence of the GMR is controlled by the magnetization reversal in the thick and soft permalloy layers; (c) the antiparallel magnetic ordering is better as the contrast between the hard magnetic properties of the clusters and the soft ones of the layers is higher. The drawback of these hybrid systems is the crossover to superparamagnetic behavior as one goes to room temperature [clearly shown by superconducting quantum interference device (SQUID) measurements] and the resulting deterioration of the GMR slopes.

Magnetic-field–temperature phase diagrams of the axial next-nearest-neighbor Ising model are calculated within the framework of a Landau-type expansion of the free energy derived from molecular field theory. Good qualitative agreement is found with recently reported results on body-centered-tetragonal ${\mathrm{UPd}}_2$${\mathrm{Si}}_2$. This work is expected to also be relevant for related compounds.

Nuclear relaxation in half-integer spin chains at low temperatures (T≪J, the antiferromagnetic exchange constant) is dominated by dissipation from a gas of thermally excited, overdamped, spinons. The universal low-temperature dependence of the relaxation rates 1/${\mathit{T}}_1$ and 1/${\mathit{T}}_{2\mathit{G}}$ is computed.

We report results of an investigation of the low-temperature magnetic properties of the ${\mathrm{Gd}}_2$${\mathrm{CuO}}_4$ compound via the ac-susceptibility method. The measurements reveal complex magnetic behavior with two characteristic susceptibility maxima at ${\mathit{T}}_{\mathit{a}}$=8 K and ${\mathit{T}}_{\mathit{b}}$=9.5 K, i.e., in the temperature range between the Néel temperature of the ${\mathrm{Gd}}^{3+}$ sublattice (${\mathit{T}}_{\mathit{N}}^{\mathrm{Gd}}$≊7 K) and the temperature of a significant magnetic anomaly at ${\mathit{T}}_{\mathit{L}}$≊20 K. The maxima are strongly dependent on the amplitude of the excitation field. The corresponding anomalies in the out-of-phase part of the ac susceptibility indicate the presence of relaxation processes associated with magnetic-structure changes. Susceptibility data collected at very low values of excitation field amplitude reveal a double-peak structure of the 20-K anomaly.

We investigate the existence of a T=0 quantum melting transition in the vortex system of a type-II superconductor. We find that homogeneous high-resistance thin films are good candidates for the observation of this effect. We calculate both the continuous dislocation-mediated melting line ${\mathit{B}}_{\mathit{m}}^{\mathit{d}}$(T) as well as the first-order melting line ${\mathit{B}}_{\mathit{m}}^{\mathit{G}}$(T) driven by Gaussian fluctuations and present the resulting H-T phase diagram. At low temperatures the melting line ${\mathit{B}}_{\mathit{m}}^{\mathit{G}}$ extrapolates to a value below the upper critical field, resulting in a quantum liquid state in the region ${\mathit{B}}_{\mathit{m}}^{\mathit{G}}$<B<${\mathit{H}}_{\mathit{c}2}$.

We use a theory developed in a previous paper [T. Kopp and F. Mila, Phys. Rev. B 43, 12 980 (1991)] to interpret recent Monte Carlo data relative to the doping dependence of the zero-temperature susceptibility of the Hubbard model on a square lattice. According to this theory, the susceptibility does not decrease, like the density of states, but increases upon doping, the essential reason being that the hopping processes that contribute to the susceptibility are suppressed close to half-filling because the wave functions on different sublattices have a vanishing overlap in that limit. We show that, for U/t=4, this theory is in very good agreement with recent Monte Carlo data.

The effect of a magnetic (S=1/2) impurity coupled to a two-dimensional system of correlated electrons (described by the t-J model) is studied by exact diagonalizations. It is found that, if the exchange coupling of the impurity with the neighboring spins is ferromagnetic or weakly antiferromagnetic, an extra hole can form bound states of different spatial symmetries with the impurity extending to a few lattice spacings. The binding energy is maximum when the impurity is completely decoupled (vacancy) and vanishes for an antiferromagnetic coupling exceeding ∼0.3J. Several peaks appear in the single-hole spectral function below the lower edge of the quasiparticle band as signatures of the d-, s-, and p-wave bound states.

We have measured the magnetic hysteresis in melt-textured ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ of different specimen shapes: a platelet and a slab, both in the H∥c orientation. Significant differences in the magnetization especially at low field have been found. The low-field peak in the magnetization on the reverse leg appears to occur at a significantly higher field in the slab than in the platelet. This is tentatively explained by the effect of the geometry on the local magnetic field, which is turned from H∥c (as applied) in the slab towards H⊥c in the platelet.

We describe magnetic measurements on ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_8$ (BSCCO) fibers irradiated by neutrons up to a fluence of 2.2×${10}^{15}$ ${\mathrm{cm}}^{\mathrm{-}2}$. The irradiation effects are similar to those reported for BSCCO crystals irradiated by fast neutrons. In particular, the critical current ${\mathit{J}}_{\mathit{C}}$ increases with the increase fluence of neutrons and the irreversibility line is shifted gradually with irradiation to higher fields and temperatures. Results obtained from relaxation measurements show only a slight increase of the effective barriers as a consequence of the irradiation.

The C 1s near-edge absorption spectrum of ${\mathrm{C}}_{60}$ has been studied experimentally and theoretically. An assignment of the C 1s transitions to unoccupied states below the continuum threshold is suggested on the basis of multiple-scattering Xα local density and semiempirical intermediate-neglect-of-differential-overlap self-consistent-field calculations, including effects of screening and relaxation of the symmetry-breaking core hole. The present work shows that modeling the core hole is necessary for understanding the absorption spectrum of ${\mathrm{C}}_{60}$. Thus excitonic interactions between the excited electron and the core hole left behind give a final state that is significantly different from the ground state.

We describe a method for calculating the interatomic force constants in crystalline insulators, from first principles, with explicit inclusion of the long-ranged anisotropic dipole-dipole interaction. Using this technique, the dynamics of α-quartz, a model for tetrahedrally bonded silica, is investigated: we examine the range of interatomic forces, their anisotropy, their longitudinal and transverse character, and the importance of the dipole-dipole contribution. These force constants provide an extensive database for testing semiempirical interatomic potentials used in silica molecular-dynamics simulations.

Birefringence and dielectric measurements were performed on a triangular lattice antiferromagnet ${\mathrm{RbMnBr}}_3$. Successive structural phase transitions at ${\mathit{T}}_4$ (∼220 K), ${\mathit{T}}_3$ (=230 K), and ${\mathit{T}}_2$ (=444 K) were found. Through D-E hysteresis loop observations, the phases IV (${\mathit{T}}_4$<T<${\mathit{T}}_3$), III (${\mathit{T}}_3$<T<${\mathit{T}}_2$), and II (${\mathit{T}}_2$<T<${\mathit{T}}_{\mathrm{melt}}$) were determined to be ferroelectric. The spontaneous polarization is along the c axis, which disappears abruptly at ${\mathit{T}}_4$ with decreasing temperature. Below ${\mathit{T}}_4$ (phase V) the crystal is proposed to be antiferroelectric. The dielectric behavior of ${\mathrm{RbMnBr}}_3$ is very similar to that of ${\mathrm{KNiCl}}_3$; however, a higher-temperature phase transition at ${\mathit{T}}_1$ assigned for ${\mathrm{KNiCl}}_3$ was not detected in differential-scanning-calorimetry data on ${\mathrm{RbMnBr}}_3$.

We report the existence of a reversible pressure-induced first-order structural transition between the metastable Si-III phase (body-centered cubic with 16 atoms per conventional cell or 8 atoms per primitive cell known as BC8) and a topologically distinct rhombohedral structure (Si-XII) which also contains 8 atoms. The observed transition pressure of 20 kbar indicates that Si-III exists over a far smaller pressure range than was previously believed. It also implies, contrary to previous reports, that Si-II (β-Sn structure) transforms first to Si-XII before converting to the BC8 structure of Si-III. The Si-XII structure has a unique feature among metastable phases of silicon in that it contains both five- and six-membered rings, the presence of which may have important consequences for the electronic properties of the material. The existence of odd-membered rings also serves to explain why diatomic analogs of the BC8 structure are not found in compound semiconductors.

The structural pressure dependence of GaSb has been determined to 35 GPa. The well-known phase transition at 7 GPa is found to be to a disordered orthorhombic structure with space group Imma, rather than to the β-tin structure previously reported. We find the Imma phase to be stable to at least 35 GPa, and do not observe the transition to a simple-hexagonal structure previously reported at 28 GPa. Striking similarities with the Imma phase of silicon are found in the pressure dependence of the lattice parameters and the variable atomic coordinate.

The time decay of the isothermal magnetization of tetrakis-dimethylaminoethylene (TDAE)-${\mathrm{C}}_{60}$ after switching off a small external magnetic field has been studied between 20 and 4 K. Below ${\mathit{T}}_{\mathit{c}}$ a remanent magnetization ${\mathit{M}}_0$—representing the fraction of the frozen out spins—that becomes unobservably small above 12 K has been found. The relaxing magnetization fraction component ${\mathit{M}}^{\mathrm{*}}$, on the other hand, increases as ${\mathit{T}}_{\mathit{c}}$ is approached from below. The time decay of the magnetization is rather slow below ${\mathit{T}}_{\mathit{c}}$ and can be described by a stretched exponential form M(t)=${\mathit{M}}_0$+${\mathit{M}}^{\mathrm{*}}$exp[-(t/τ${)}^{\mathrm{\alpha }}$]. The exponent α decreases from 0.14 at 4 K to 0.08 at 12 K. This shows that the spectrum of relaxation times slowly changes with temperature and becomes very wide as ${\mathit{T}}_{\mathit{c}}$ is approached from below. No slowly relaxing component is found above ${\mathit{T}}_{\mathit{c}}$. The above features are characteristic of a spin-glass-like freezing process.

We present an experiment in which the attenuation of a spin-polarized free electron beam is measured by direct transmission through an ultrathin ferromagnetic layer. The self-supported metal target consists of a 1-nm-thick cobalt film sandwiched between 21- and 2-nm-thick gold layers. Measurements are performed throughout a wide energy range (incident electron energies from 4 eV to 50 eV above the Fermi level). At low energy, close to the clean surface vacuum level, we find that the transmission coefficient of minority spin electrons is about 0.7 times that of majority spin electrons.

The oscillatory interlayer exchange coupling and its dependence on magnetic-layer thickness have been studied from first principles for (100) and (110) oriented Co/Cu/Co sandwiches. It is found that with increasing magnetic-layer thickness there is generally an important redistribution of weights among the different characteristic Ruderman-Kittel-Kasuya-Yosida-like oscillations. This effect is discussed in terms of the corresponding change in the electronic structure of the magnetic layers. The calculated exchange coupling energies for thicker cobalt layers are in excellent agreement with existing experiments.

We have measured both the longitudinal and transverse spin diffusion coefficients, ${\mathit{D}}_{\mathrm{\parallel }}$ and ${\mathit{D}}_{\mathrm{\perp }}$, in a saturated mixture of ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ in ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ ${(}^3$He concentration ${\mathit{x}}_3$=6.4%) in a magnetic field of 8.8 T in the temperature range 11≤T≤500 mK. Consequently we are able to provide direct evidence for the anisotropy in spin diffusion predicted by Meyerovich. Below ∼30 mK, ${\mathit{D}}_{\mathrm{\parallel }}$ varies as ${\mathit{T}}^{\mathrm{-}2}$ in accordance with the predictions of Landau Fermi-liquid theory; but ${\mathit{D}}_{\mathrm{\perp }}$ becomes smaller than ${\mathit{D}}_{\mathrm{\parallel }}$ at the lowest temperatures resulting in an anisotropy factor ${\mathit{D}}_{\mathrm{\parallel }}$/${\mathit{D}}_{\mathrm{\perp }}$ of 4 at T=11 mK. We believe that this large anisotropy is due to a breakdown of the standard Landau Fermi-liquid theory in concentrated mixtures of ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ in ${}_{}{}^4{}_{}{}^{}\mathrm{He}$. Finally we use our measurements of the spin rotation parameter, μ${\mathit{M}}_0$, along with results from previous experiments at various ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ concentrations to deduce a value for the quasiparticle scattering length a=-0.88±0.05 Å.

The superresistive phase of thin granular films near the superconductor-insulator transition is believed to result from the suppression of Josephson tunneling by the large charging energy, ${\mathit{E}}_{\mathit{c}}$, of the grains. Our ac I-V studies of granular Al films suggest that a small dc bias voltage can be used to overcome this Coulomb barrier and to recover Josephson tunneling. This bias, in fact, drives a critically disordered film from the superresistive phase to a quasisuperconducting phase.