High-resolution specific-heat data as a function of temperature and applied magnetic field have been obtained on an untwinned ${\mathrm{LuBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{y}}$ single crystal near its normal-superconducting transition. The magnitude of the specific-heat anomaly has been found to depend on the magnetic-field strength and the orientation of magnetic field relative to all three crystallographic axes. The implications of this anisotropy, specifically along the a and b axes, will be discussed.

The dielectric response functions parallel and perpendicular to the c axis of single-crystal ${\mathrm{La}}_2$${\mathrm{CuO}}_4$ are determined at room temperature from 0.7 to 4.9 eV by analysis of ellipsometric data. Strong anisotropy is observed; for E∥c the optical properties are nearly featureless in this energy range, while for E⊥c there is a sharp peak in Im(ɛ) near 2 eV associated with the O(2p) to Cu(3d) charge-transfer excitation. Additional features are observed on the high-energy side of this peak, and the origin of these is discussed in the light of recent theoretical predictions.

The optical conductivity of the ${\mathrm{CuO}}_3$ chains, a subsystem of the 1:2:3 materials, is dominated by a broad peak in the midinfrared (ω≊0.2 eV), and a slowly falling high-frequency tail. The one-dimensional (1D) t-J model is proposed as the relevant low-energy Hamiltonian describing the intrinsic electronic structure of the ${\mathrm{CuO}}_3$ chains. However, due to charge-spin decoupling, this model alone cannot reproduce the observed σ(ω). We consider two additional scattering mechanisms: (i) Disregarding the not so crucial spin degrees of freedom, the inclusion of strong potential disorder yields excellent agreement with experiment, but suffers from the unreasonable value of the disorder strength necessary for the fit. (ii) Moderately strong polaronic electron-phonon coupling to the mode involving Cu(1)-O(4) stretching, can be modeled within a 1D Holstein Hamiltonian of spinless fermions. Using a variational approximation for the phonon Hilbert space, we diagonalize the Hamiltonian exactly on finite lattices. As a result of the experimental hole density ≊1/2, the chains can exhibit strong charge-density-wave (CDW) correlations, driven by phonon-mediated polaron-polaron interactions. In the vicinity of half filling, charge motion is identified as arising from moving domain walls, i.e., defects in the CDW. Incorporating the effect of vacancy disorder by choosing open boundary conditions, good agreement with the experimental spectra is found. In particular, a high-frequency tail arises as a consequence of the polaron-polaron interactions.

The Knight shift and the nuclear magnetic relaxation rates ${\mathit{W}}_{\mathrm{\perp }}^{63}$, ${\mathit{W}}_{\mathrm{\parallel }}^{63}$, and ${\mathit{W}}_{\mathrm{\parallel }}^{17}$ are calculated for temperatures below the superconducting transition temperature of the high-temperature cuprate superconductors; the superscripts refer to the ${}_{}{}^{63}{}_{}{}^{}\mathrm{Cu}$ and the ${}_{}{}^{17}{}_{}{}^{}\mathrm{O}$ nuclei while the subscripts refer to the direction of the magnetic field with respect to the CuO layers. The calculations are based on a recently proposed interlayer tunneling mechanism of superconductivity in these materials. In general, the results are consistent with experiments even though the superconducting gap respects the full point-group symmetry of the lattice (anisotropic s wave); the anisotropy of the magnitude of the gap is an essential element.

The temperature dependence between 4 and 294 K of the positron lifetime is measured in the organic superconductor κ-(BEDT-TTF${)}_2$Cu(NCS${)}_2$. The lifetime decreases rapidly with temperature from 350 to 343 ps below ${\mathit{T}}_{\mathit{c}}$, and increases monotonically above ${\mathit{T}}_{\mathit{c}}$ to 372 ps. These experimental results are interpreted in terms of a transfer in the local charge density of organic superconductors associated with the superconducting transition. Theoretical positron density and positron-electron overlap distributions are calculated to support this interpretation.

Electric-field gradients (EFG’s), due to substitutional transition-metal impurities in vanadium have been evaluated using a discrete-lattice static method. The valence and size EFG’s are generated simultaneously with the help of perfect and imperfect crystal potentials. The EFG’s at the first- and second-nearest neighbors (1NN, 2NN) of the impurity are calculated for the dilute alloys VNb, VMo, VTa, and VW. The d-band effects of both the host and impurity are included through the crystal potential. The calculated results are in agreement with the experimental values. The valence EFG is larger than the size EFG at the 1NN while the size EFG starts dominating at the 2NN sites.

We have calculated the kinetic energies of liquid and solid neon in the temperature range 9.2–35 K. We have used the Wigner-Kirkwood expansion technique for the calculation of quantum corrections to the kinetic energy up to the order of ${\mathrm{ħ}}^6$ in the expansion. All the averages are calculated by molecular dynamics computer simulation. Our results are in agreement with the experimental findings of Peek et al. Both the Lennard-Jones and Aziz pair potentials have been used in calculations with comparable results.

We present oblique-incidence frequency scans of an antiferromagnetic-resonance reflectivity spectrum in the presence of an applied magnetic field ${\mathit{H}}_0$. Results are shown for the c axis of an ${\mathrm{FeF}}_2$ single crystal both parallel and perpendicular to ${\mathit{H}}_0$, which is normal to the plane of incidence. In both cases the s-polarization spectra show nonreciprocal behavior with respect to ${\mathit{H}}_0$.

The magnetoelastic anomaly observed in cubic antiferromagnetic materials is theoretically investigated. The antiferromagnetism of doubly degenerate narrow d bands is described using a two-band Hubbard Hamiltonian and the nesting condition of the band structure. It is shown that the electron redistribution between the antiferromagnetic states introduces a large elastic softening just below the Néel temperature. The theoretical results satisfactorily account for the elastic anomaly observed in chromium and its alloys and, also, in antiferromagnetic Invar.

We present results on magnetic ordering in a Dy(60 Å)/Er(60 Å) superlattice made up of two magnetic rare earths exhibiting in-plane and perpendicular anisotropies. Neutron diffraction reveals that the helical magnetic coherence of dysprosium propagates through paramagnetic erbium layers, without inducing spin ordering in erbium. Below the ordering temperature of dysprosium individual ferromagnetic dysprosium layers show antiparallel stacking through magnetic erbium. This antiferromagnetic phase of dysprosium is irreversibly destroyed by a 5.5-kOe external magnetic field and replaced by a ferromagnetic order between dysprosium layers. Magnetization measurements indicate that this irreversibility could arise from an energy barrier between parallel and antiparallel stacking of ferromagnetic dysprosium layers.

Cerium ${\mathit{L}}_3$ XANES (x-ray-absorption near-edge-structure) spectra were analyzed to separate Ce moment contributions and mixed valence (MV) in complex magnetic silicides ${\mathrm{CePd}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$${\mathrm{Si}}_2$ (0≤x≤2). The Ce valence mixing does not vary linearly with x, but increases rapidly for x≥1.5. The associated moment collapse correlates with pronounced deviations of the unit-cell volume from Vegard law and the onset of structural instability. Reorientation of [001] Mn 3d antiferromagnetic order for x<2 appears to rapidly suppress the weak Ce valence mixing coexisting with antiferromagnetic order in ${\mathrm{CeMn}}_2$${\mathrm{Si}}_2$.

The scaling behavior of the dynamical structure factor S(q,ω) of percolating Heisenberg antiferromagnets is investigated in terms of a dynamic scaling argument and numerical calculations. It is found, based on the single-length-scaling postulate, that the asymptotic behavior of S(q,ω) can be characterized by only two exponents: the dynamic exponent ${\mathit{z}}_{\mathit{a}}$ and a new exponent y. This theoretical prediction is confirmed by direct numerical calculations. The values of the exponents ${\mathit{z}}_{\mathit{a}}$ and y are determined for the case of d=2 percolating Heisenberg antiferromagnets.

A rigorous upper bound for the spontaneous magnetization per site of a general Hubbard model is obtained. The bound is independent of the on-site Coulomb potential ${\mathit{U}}_{\mathbf{r}}$ of the model, and is valid for arbitrary band fillings. From it one may find that the magnetic (ferromagnetic or antiferromagnetic) long-range order disappears in the atomic limit at finite temperatures.

We have considered a classical system, consisting of three-component unit vectors, associated with a three-dimensional simple cubic lattice {${\mathbf{u}}_{\mathit{j}}$‖j∈${\mathit{Z}}^3$}, and interacting via a nearest-neighbor pair potential of the form W=${\mathit{W}}_{\mathit{i}\mathit{j}}$=ε[-3(${\mathbf{u}}_{\mathit{i}}$⋅r^)(${\mathbf{u}}_{\mathit{j}}$⋅r^) +${\mathbf{u}}_{\mathit{i}}$⋅${\mathbf{u}}_{\mathit{j}}$], where r=${\mathbf{x}}_{\mathit{i}}$-${\mathbf{x}}_{\mathit{j}}$, r=‖r‖, r^=r/r; here ${\mathbf{x}}_{\mathit{j}}$ denote the dimensionless lattice site coordinates, and ε is a positive quantity setting energy and temperature scales (i.e., ${\mathit{T}}^{\mathrm{*}}$=${\mathit{k}}_{\mathit{B}}$T/ε). For this potential model the existence of an ordering transition at finite temperature was proved rigorously by Fröhlich and Spencer. We carried out Monte Carlo simulations, and estimated the transition temperature and critical exponents by finite-size-scaling theory; we found ${\mathit{T}}_{\mathit{c}}^{\mathrm{*}}$=1.867±0.009, and values of β and ν consistent with the ones reported in the literature for the classical Heisenberg model in three dimensions.

Using an effective Heisenberg model for clusters the spin-wave spectrum is calculated by direct diagonalization. The inclusion of a nonuniform magnetization profile at zero temperature, nonuniform exchange interactions as a result of structural relaxations, longer-range interactions, and of magnetic surface anisotropy have been studied. Very small effects are found on the thermal magnetic properties relative to those predicted for the simple nearest-neighbor Heisenberg model by Hendriksen, Linderoth, and Lindgård [J. Phys. C 31, 5675 (1993); Phys. Rev. B 48, 7259 (1993)].

We present the results of a study of magnetism in rhodium, ruthenium, and palladium clusters. We report values for the magnetic moments of rhodium clusters which are free from a systematic error present in our previous measurements [Phys. Rev. Lett. 71, 923 (1993)] and show that their magnetic moments decrease to the bulk value of zero as the cluster sizes increase. Ruthenium and palladium clusters are nonmagnetic.

We have measured the specific-heat critical behavior of the Ising stacked triangular lattice antiferromagnet ${\mathrm{CsCoBr}}_3$, an excellent example of a fully frustrated three-dimensional second-order phase transition. For the transition near T=28 K, α=-0.025±0.004 and ${\mathit{A}}^{+}$/${\mathit{A}}^{\mathrm{-}}$=1.07±0.02 are consistent both with predicted d=3 XY criticality and Monte Carlo simulations which indicate critical behavior close to, but distinct from XY universality. No latent heat is observed, consistent with a second-order transition. No specific-heat peak is observed at the transitions reported at lower temperatures.

Working in second quantization, the ground-state energy and eigenfunction of an interacting aggregate of N spin-1 deuterons confined to a volume Ω in a charge-neutralizing background and immersed in a uniform magnetic field B is estimated. The analysis is appropriate to the low-density limit in which magnetic-field energy is large compared to particle-particle and spin-spin interaction energies. The resulting ground-state energy is given by ${\mathit{E}}_{\mathit{G}}$≊Nħ${\mathrm{\omega }}_{\mathit{c}}$/2+N(N-1) (${\mathit{e}}^2$${\mathit{b}}_{\mathit{c}}$/${\mathrm{\Omega }}^{2/3}$)Γ+NΦ-${\mathrm{\mu }}_{\mathit{D}}$BN -γf${\mathrm{\mu }}_{\mathit{D}}^2$[N(N-1)/2], where ${\mathrm{\omega }}_{\mathit{c}}$ is deuteron cyclotron frequency, ${\mathit{b}}_{\mathit{c}}$ is a characteristic magnetic-field length, Φ is background potential, ${\mathrm{\mu }}_{\mathit{D}}$ is the deuteron magnetic moment, Γ represents a nondimensional interaction integral and the constants f,γ model spin-spin correlation and deuteron-deuteron overlap, respectively. The coordinate component of the ground-state wave function is a product of Gaussian forms whereas the spin component is the product of single-particle spin functions corresponding to spins aligned with the imposed magnetic field.

The ternary system fullerides ${\mathrm{Na}}_2$A${\mathrm{C}}_{60}$ (A=Cs, Rb, and K) are studied by differential scanning calorimetry in the temperature range of 100 to 450 K. All show reproducible phase transitions at the transition temperatures of 299 K (ΔH=2.5±0.5 J/g) for ${\mathrm{Na}}_2$${\mathrm{CsC}}_{60}$, 313 K (ΔH=2.7±0.5 J/g) for ${\mathrm{Na}}_2$${\mathrm{RbC}}_{60}$, and 305 K (ΔH=3.1±0.5 J/g) for ${\mathrm{Na}}_2$${\mathrm{KC}}_{60}$. Orientational ordering of the fullerene unit is responsible for the observed phase transitions (${\mathit{T}}_{\mathrm{sc}\mathrm{-}\mathrm{fcc}}$) from a high-temperature face-centered-cubic (fcc) to a low-temperature simple-cubic (sc) phase. The observed ${\mathit{T}}_{\mathrm{sc}\mathrm{-}\mathrm{fcc}}$ and enthalpy can be compared to those for pristine ${\mathrm{C}}_{60}$ which has ${\mathit{T}}_{\mathrm{sc}\mathrm{-}\mathrm{fcc}}$=256.5 K (ΔH=8±0.5 J/g). The observed behavior is discussed in terms of the balance between the ${\mathrm{C}}_{60}^{3\mathrm{-}}$-${\mathrm{C}}_{60}^{3\mathrm{-}}$ intermolecular interaction and the ${\mathrm{C}}_{60}^{3\mathrm{-}}$-${\mathit{A}}^{+}$ Coulombic interaction as a function of lattice parameter.

We study the low-field diamagnetic response of granular superconductors at finite temperatures by means of a simple two-dimensional Josephson-junction array. The temperature effects are taken into account by inserting white-noise current sources in parallel to the resistively shunted junction circuit models of the Josephson junctions of the network. By this analysis we argue that a simplified one-dimensional description of the equivalent circuit, proposed by the authors for cylindrical granular superconductors, is still valid even in the presence of thermally activated flux jumps. A flux-creep picture for intergranular flux motion follows.