The ground-state energy and the singlet-triplet energy gap of the antiferromagnetic Heisenberg model on a ladder is investigated using a mean-field theory and the density-matrix renormalization group. Spin-wave theory shows that the corrections to the local magnetization are infinite. This indicates that no long-range order occurs in this system. A flux-phase state is used to calculate the energy gap as a function of the transverse coupling, ${\mathit{J}}_{\mathrm{\perp }}$, in the ladder. It is found that the gap is linear in ${\mathit{J}}_{\mathrm{\perp }}$ for ${\mathit{J}}_{\mathrm{\perp }}$≫1 and goes to zero for ${\mathit{J}}_{\mathrm{\perp }}$→0. The mean-field theory agrees well with the numerical results.

The two-dimensional Hubbard model is extended by placing 2S orbitals at each lattice site and studied in a systematic 1/S expansion. The 1/S results for the magnetic susceptibility and the spectra of spin-wave excitations at half-filling are consistent with the large S calculations for the Heisenberg antiferromagnet. The 1/S corrections to the fermionic spectrum lift the degeneracy along the edge of the magnetic Brillouin zone yielding minima at (±π/2,±π/2). Relation to previous papers on the subject is discussed.

By using a recent result of Ueda et al. [Phys. Rev. Lett 68, 1030 (1992)], we prove rigorously that the ground state of the symmetric periodic Anderson model has short-range antiferromagnetic order.

The theory of ferromagnetic resonance (FMR) in an assembly of single-domain uniaxially anisotropic particles is being applied to suspensions. The approach used has, as its basis, the independent-grain model once proposed for the description of FMR in polycrystals. We surmise that the basic assumptions of this model are also appropriate for composites of fine magnetic particles. With regard to the specific properties of the systems in question, we extend the model taking into account (i) the thermofluctuational effects (superparamagnetism) and (ii) the orientational mobility of the particles which leads to the change of the orientational distribution of their anisotropy axes under the influence of the applied field. The joint action of these effects imparts some specific features to the FMR spectra of a magnetic suspension. For example, in a certain temperature range two peak absorption lines must occur.

Based on Hu’s histogram Monte Carlo method and the Swendsen-Wang algorithm, we calculate the geometrical factor of the q-state bond-correlated percolation model corresponding to the q-state Potts model (QPM). The free energy f, the internal energy U, and the specific heat ${\mathit{C}}_{\mathit{h}}$ of the Ising model, i.e., the QPM with q=2, calculated from this geometrical factor agree very well with exact results. This histogram-important-sampling Monte Carlo method is very simple and has many potential applications.

The properties of the two-dimensional antiferromagnet of spin 1/2 on a square lattice with nearest- and next-nearest-neighbors interactions in a strong magnetic field close to saturation are studied in terms of the equivalent Bose-gas problem. A phase with a gap in the elementary excitations spectrum, short-range-ordered ground state, and a corresponding plateau in the magnetization curve is shown to exist in the interval of fields between the first and second critical fields ${\mathit{h}}_0$<h<${\mathit{h}}_{01}$. Its existence is closely related to the two-particle attraction in the vicinity of the wave vector ${\mathbf{p}}_{0\mathit{u}}$=(π,π). In the interval between the second and third critical fields ${\mathit{h}}_{01}$<h<${\mathit{h}}_{\mathit{c}\mathit{f}}$ the system behaves as a quasi-one-dimensional magnetic phase with long-range order in the ground state and a gapless mode. The magnetization depends on field linearly, with logarithmic corrections.

It is shown that the observed anisotropy of magnetization at high magnetic fields in ${\mathrm{RbMnBr}}_3$, a quasi-one-dimensional antiferromagnet on a distorted stacked triangular lattice, is due to quantum and thermal fluctuations. These fluctuations are taken into account in the framework of linear spin-wave theory in the region of strong magnetic fields. In this region the divergent one-dimensional integrals are cut off by magnetic field and the bare easy-plane anisotropy. Logarithmical dependence on the cutoff leads to the ‘‘enhancement’’ of the anisotropy in magnetization. Comparison between magnetization data and our theory with parameters obtained from neutron scattering experiments has been done.

Effects of an uniaxial single-ion anisotropy ${\mathit{tsum}}_{\mathit{i}}$${\mathit{D}}_{\mathit{z}}$(${\mathit{S}}_{\mathit{i}}^{\mathit{z}}$${)}^2$ on the magnetic properties of an S=1 antiferromagnetic Heisenberg chain with open boundaries are studied by a quantum Monte Carlo method. In the ground stae, how the anisotropy affects the staggered magnetization 〈${\mathit{S}}_{\mathit{i}}^{\mathit{z}}$〉 and the spin correlation functions 〈${\mathit{S}}_1^{\mathit{z}}$${\mathit{S}}_{\mathit{i}}^{\mathit{z}}$〉 and 〈${\mathit{S}}_1^{\mathit{x}}$${\mathit{S}}_{\mathit{i}}^{\mathit{x}}$〉 is investigated. Amplitudes of 〈${\mathit{S}}_{\mathit{i}}^{\mathit{z}}$〉 decrease with with increase of ${\mathit{D}}_{\mathit{z}}$. The correlation length ${\xi }_{\mathit{S}\mathit{S}}^{\mathit{z}}$ of 〈${\mathit{S}}_1^{\mathit{z}}$${\mathit{S}}_{\mathit{i}}^{\mathit{z}}$〉 monotonously decreases with increase of ${\mathit{D}}_{\mathit{z}}$, while the correlation length ${\xi }_{\mathit{S}\mathit{S}}^{\mathit{x}}$ of 〈${\mathit{S}}_1^{\mathit{x}}$${\mathit{S}}_{\mathit{i}}^{\mathit{x}}$〉 has the opposite but weak ${\mathit{D}}_{\mathit{z}}$ dependence. The fourfold degeneracy of the ground state also holds with nonzero ${\mathit{D}}_{\mathit{z}}$’s in the thermodynamic limit although it is lifted up in finite chains. At finite temperatures, the magnetic susceptibilities along the z axis (${\mathrm{\chi }}^{\mathit{z}}$) and the x axis (${\mathrm{\chi }}^{\mathit{x}}$) are calculated. Temperature dependences of the bulk susceptibilites, ${\mathrm{\chi }}_{\mathrm{\infty }}^{\mathit{z}}$ and ${\mathrm{\chi }}_{\mathrm{\infty }}^{\mathit{x}}$, are consistent with the previous experimental result on Ni(${\mathrm{C}}_2$${\mathrm{H}}_8$${\mathrm{N}}_2$${)}_2$${\mathrm{NO}}_2$${\mathrm{ClO}}_4$ (NENP). Finite-size corrections, ${\mathrm{\chi }}_0^{\mathit{z}}$ and ${\mathrm{\chi }}_0^{\mathit{x}}$, and the Curie susceptibilities resulting from chain ends, ${\mathrm{\chi }}_{\mathit{C}}^{\mathit{z}}$ and ${\mathrm{\chi }}_{\mathit{C}}^{\mathit{x}}$, are also investigated in detail. Both ${\mathrm{\chi }}_0$ and ${\mathrm{\chi }}_{\mathit{C}}$ diverge as temperature goes to zero and they are combined through a relation ${\mathrm{\chi }}_{\mathit{C}}$=${\mathrm{\chi }}_0$+${\mathrm{\chi }}_{\mathrm{\infty }}$. Temperature dependences of ${\mathrm{\chi }}_{\mathit{C}}$ are dominated by the edge effect at low temperatures, while at high temperatures they are attributed to an S=1 free spin.

We study two-dimensional frustrated arrays of Josephson junctions at nonzero voltage. There exists a family of traveling-wave solutions characterized by a constant spatial phase shift. All spatially periodic vortex configurations observed in numerical simulations are of this form, and can be parametrized in terms of this single phase shift. We present the analytical form of these solutions at high voltages (or high Josephson frequencies) and compute their I-V characteristics. We also report the numerical observation of relaxational fronts separating two regions with different phase shift and frequency. These fronts move in the direction of decreasing frequency (or voltage).

We have measured the pressure broadening and shift of the excitation and emission lines of the 6${\mathit{s}}^2$ ${}_{}{}^1{}_{}{}^{}$${\mathit{S}}_0$→6s6p ${}_{}{}^1{}_{}{}^{}$${\mathit{P}}_1$ transition of Ba atoms implanted in liquid and solid helium at 1.5 K. The observed spectra are quantitatively explained within the adiabatic line-broadening theory by assuming a bubble structure of the trapping sites for the Ba atoms. The evolution of the bubble shape and size in the optical excitation-deexcitation cycle is modeled theoretically.

We report nonlinear electrical response in superconducting/insulating ${\mathrm{Mo}}_{77}$${\mathrm{Ge}}_{23}$/Ge multilayers. In low-anisotropy multilayers, we observe an abrupt change in the current-voltage characteristic as a function of temperature. This change corresponds to a sharp drop in the pinning drag force acting on moving vortices in the flux-flow regime at low temperatures. The decrease in pinning drag force may be related to an abrupt increase in the rigidity of the system of moving vortices.

We have performed microwave measurements at 60 GHz of the surface impedance (${\mathit{Z}}_{\mathit{s}}$=${\mathit{R}}_{\mathit{s}}$-${\mathit{iX}}_{\mathit{s}}$) on superconducting Nb and Pb. The temperature dependence of the complex conductivity (σ^=${\mathrm{\sigma }}_1$+i${\mathrm{\sigma }}_2$) is inferred from the data down to T/${\mathit{T}}_{\mathit{c}}$∼0.25 for both materials. The imaginary part, ${\mathrm{\sigma }}_2$(T), gives the temperature dependence of the superconducting gap and the real part, ${\mathrm{\sigma }}_1$(T), exhibits the predicted conductivity coherence peak below ${\mathit{T}}_{\mathit{c}}$. The findings are compared with the expressions calculated by Mattis and Bardeen to describe the electrodynamics of weak-coupling BCS superconductors. Furthermore, we calculate the complex conductivity within the framework of the standard Eliashberg electron-phonon theory of superconductivity. The experimental results obtained on Nb are in good agreement with the BCS weak-coupling theory while the data on Pb are analyzed in the anomalous (Pippard) limit and show deviations from BCS laws, accounted for only by the Eliashberg strong-coupling description of the electrodynamics of the superconducting state.

Ballistic charge transport through mesoscopic superconductor–normal-metal–superconductor (SNS) microbridges is investigated within the framework of the Keldysh technique. At low voltages we find a peak in the conductance G∼(${\mathit{l}}_{\mathrm{in}}$/2a)(1/${\mathit{R}}_0$), where ${\mathit{l}}_{\mathrm{in}}$ is the inelastic mean free path, 2a is the normal layer thickness, and ${\mathit{R}}_0$ is the Sharvin resistance. In this limit the dissipative current is carried by quasiparticles which are trapped in the pair potential well of height Δ, suffering multiple Andreev reflections and relaxing inside the N layer. For larger voltages eV≳(2a/${\mathit{l}}_{\mathrm{in}}$)Δ quasiparticle are accelerated out of the N layer and relax in superconducting banks. The current-voltage characteristic of the system in this regime is calculated both analytically and numerically. In the intermediate voltage range V∼Δ/e we analyze the subharmonic gap structure on the I-V curve and show that this structure is most pronounced in the temperature range T∼Δ/${\mathit{k}}_{\mathit{B}}$ and vanishes for both T→0 and T→${\mathit{T}}_{\mathit{C}}$. We explain this behavior taking into account the contribution of thermally excited quasiparticles with momenta opposite to the direction of total current flow.

Sharp peaks appear in the I-V characteristic of the Josephson junction when levels in neighboring wells are crossing. The shape and size of these peaks strongly depend on the parameters of the junctions as well as on temperature and number of resonance levels.

We report neutron diffraction studies on a series of ${\mathrm{La}}_2$${\mathrm{NiO}}_{4+\mathrm{\delta }}$ single crystals with 0.05≲δ≲0.11. At 300 K, all of the crystals have an average tetragonal structure (space group I4/mmm). On cooling below 290 K, one or more orthorhombic phases appear, characterized by incommensurate superlattice peaks at (0,k,l±Δ) with k odd, l even, and 1/4<Δ≤1/2. The positions, widths, and intensities of the superlattice peaks are quite sensitive to the cooling rate. We show that the incommensurate peaks are evidence of intercalated layers of oxygen spaced periodically along the c axis, with a one-dimensional ordering similar to the staging of intercalates in graphite. The structure of the ${\mathrm{La}}_2$${\mathrm{NiO}}_4$ lattice between the interstitial layers is of the Bmab-type; the superlattice peaks result from the ordered antiphase domain boundaries induced by the interstitial oxygens, which sit at (1/4, 1) / 4 , 1/4)-type positions. Observed orderings involve interstitial layers separated by two to four Ni-O layers. Peak shapes and positions are modelled quantitatively using the formulas of Hendricks and Teller for one-dimensional disorder in a layer lattice. Besides the one-dimensional ordering of the intercalant layers, temperature-dependent phase separations are observed. Because of the slow ordering kinetics, phase separation can be suppressed by rapid cooling.

The nature of bunched fluxons on coupled long Josephson junctions is investigated theoretically by means of a coupled set of sine-Gordon equations. For cases of inductive and capacitive coupling between the junctions, we analyze the stability of the bunched states for different velocities and perturbation parameters, damping and bias. The internal oscillation frequency is found for the bunched modes and the origin of the stability is discussed for the different types of bound states. Extensive numerical simulations are carried out in order to validate the analytical results against the dynamical behavior of the full system. Excellent agreement between the analysis and the simulations is obtained for all values of coupling parameters and velocities. Finally, we demonstrate numerically that the stability of the bound states can persist even when the two systems are not exactly identical, and we conclude that both inductive and capacitive coupling can give rise to large-locking regimes in synchronization experiments on coupled Josephson junctions.

The resistance peak, which is occasionally observed near the resistive transition in high-${\mathit{T}}_{\mathit{c}}$ superconductors, is shown by numerical analysis to arise from a slight inclination of the sample face from the ${\mathrm{CuO}}_2$ planes. This inclination combined with large conductivity anisotropy causes a significant distribution of the current density and reduces the voltage drop between voltage probes, resulting in a very small apparent resistivity and high residual resistivity ratio. In the resistive transition region, on the other hand, the anisotropy abruptly decreases, and the apparent resistance increases and approaches the true value before the resistivity vanishes, resulting in an apparent peak. The resistance peak is therefore a manifestation of the fact that the resistive transition along the c axis is much sharper than that in the ${\mathrm{CuO}}_2$ planes.

Bitter decoration observations of vortices in thin films of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ with four different orientations of the c axis are reported. For moderate angles of the c-axis tilt relative to the film normal, θ=30° and 45°, we find a greatly enhanced anisotropy, η(θ), of the vortex magnetic dimensions, in comparison with even θ=90° in single crystals, which is attributed to the thickness dependence of the anisotropic penetration depth in films. Also, in contrast to single crystals, the anisotropy of the vortex arrangement is not proportional to η(θ), in agreement with recent theoretical calculations.

We report a study of the x-ray-absorption near-edge structure of the Bi ${\mathit{L}}_3$ and O K edges in ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_8$. A comparison with the Bi ${\mathit{L}}_3$ edges of ${\mathrm{Bi}}_2$${\mathrm{O}}_3$ and ${\mathrm{KBiO}}_3$ indicates that the Bi atoms are in a state close to a formal valency of +3 in the superconducting compound. We attribute the peaks below 529 eV in the O K-edge spectra of ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_8$ to holes on the O sites. We investigate the symmetries of these holes by using polarized x rays. By comparing the experimental spectra with simulations based on self-consistent electronic-structure calculations, we determine that the hole states at the O sites in the Cu-O and Bi-O planes do indeed lie in these planes, while those associated with the apical O atoms of the O pyramids in the structure are aligned along the c axis.

The thermal conductivity K(T) of polycrystalline ${\mathrm{YBa}}_2$(${\mathrm{Cu}}_{1\mathrm{-}\mathit{x}}$${\mathit{M}}_{\mathit{x}}$${)}_3$${\mathrm{O}}_7$ with M=Zn and Al (0≤x≤0.1) and ${\mathrm{Y}}_{1\mathrm{-}\mathit{y}}$${\mathit{R}}_{\mathit{y}}$${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ with R=Pr and Gd (0≤y≤1.0) has been measured for temperatures 1.2≤T≤200 K. In the pure material, K(T) decreases slightly with decreasing temperature above ${\mathit{T}}_{\mathit{c}}$ and below ${\mathit{T}}_{\mathit{c}}$ there is a strong enhancement of K(T) reaching a maximum at ∼50 K followed by a sharp decrease. At temperatures below 10 K a ${\mathit{T}}^2$ dependence is evident similar to behavior reported for amorphous systems with glasslike phonon scattering on the tunneling states. Doping with Zn or Pr rapidly depresses ${\mathit{T}}_{\mathit{c}}$ and also depresses the maximum in K(T) even at concentrations that remain superconducting. This effect is observable to a lesser degree for the Al and Gd dopants. Comparisons between the K(T) behavior for impurities that do not depress ${\mathit{T}}_{\mathit{c}}$ with those that diversely alter the superconducting properties are presented together with an analysis of the data using the Tewordt-Wölkhausen (TW) theory.