We have proposed an approach to solve the eigenvalue problem of the isotropic S=1/2 Heisenberg chain. Many interesting properties of the antiferromagnetic ground state have been reanalyzed by our technique. These results show our method is an appropriate technique.

We have studied finite and infinite ferromagnetic/nonmagnetic superlattices in which the surface layer has a different magnetization. The magnetostatic limit is used, and the magnetic potentials in any region are obtained by solving recurrence relations. The equation giving the frequency for all the spin waves for a finite system is derived. It is shown that in the infinite limit, in addition to the usual Damon and Eshbach mode, two additional surface modes appear when the surface magnetization is larger than the host one. The finite results for eight layers are also shown.

The spin stiffness of frustrated spin-1/2 Heisenberg models in one and two dimensions is computed by exact diagonalizations on small clusters that implement spin-dependent twisted boundary conditions. Finite-size extrapolation to the thermodynamic limit yields a value of 0.14±0.01 for the spin stiffness of the unfrustrated planar antiferromagnet. We also present a general discussion of the linear-response theory for spin twists, which ultimately leads to the moment sum rule.

A chemical-ordering-induced peak in the polar magneto-optical Kerr effect spectra of FePt compounds is found to be in quantitative agreement with predictions based on ab initio magneto-optical calculations. Experimental spectra of epitaxial molecular-beam-epitaxy grown FePt(001) films reveal a more than twofold enhancement of the Kerr rotation if full chemical ordering is present. A spectral feature at 2-eV photon energy shows a peak Kerr rotation of up to ∼0.8°, which is strongly correlated with the presence of perpendicular magnetic anisotropy in ordered compound films.

The temperature dependence of the giant magnetoresistance (GMR) and magnetic properties have been measured for granular solid ${\mathrm{Co}}_{20}$${\mathrm{Ag}}_{80}$. From the measured GMR, we have derived the temperature dependence of the effective spin S’ of the magnetic scatterers experienced by the conduction electrons. It is found that S’ shares the same temperature dependence as the spontaneous magnetization ${\mathit{M}}_{\mathit{s}}$(T) over the whole temperature range (5–310 K) of our measurement. Our results suggest that the origin of the temperature dependence of GMR is the reduced magnetization as a result of spin-wave excitations in magnetic granules.

A quadratic diffusion Monte Carlo method has been used to obtain the equation of state of liquid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ including the negative pressure region down to the spinodal point. The atomic interaction used is a revised version of the Aziz potential, which reproduces quite accurately the features of the experimental equation of state. The spinodal pressure has been calculated and the behavior of the sound velocity around the spinodal density has been analyzed.

The structural and electrical properties of ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{Ca}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Pr}}_{\mathit{x}}$${\mathrm{Cu}}_2$${\mathrm{O}}_{8+\mathrm{\delta }}$ have been investigated as a function of x. Powder-x-ray-diffraction patterns indicate that this system forms a continuous solid solution. It is found that phase separation exists in this system. Two phases with different modulated periods appear as the Pr doping changes. At low doping levels, the short-period phase dominates, while at high doping levels the long-period phase dominates. The short-period phase and the long-period phase are thought to correspond to the poor-Pr- and rich-Pr-concentration phases, respectively. ${\mathit{T}}_{\mathit{c}}$ was found to increase slightly for x≤0.1, decrease linearly for 0.1≤x≤0.4, then to be depressed rapidly for 0.4≤x≤0.5, and finally to disappear at x=0.6. The valence of Pr was inferred to be close to +3 from the measurement of the oxygen content and the comparison with other rare-earth-doped Bi-2212 systems. The variation of ${\mathit{T}}_{\mathit{c}}$ was explained in terms of hole filling and phase separation.

The rotation of a superconducting sample in a fixed field H, starting in a hysteretic state, is known to produce a bifurcation of the vortex-flux density into a trapped rotational component ${\mathbf{B}}_{\mathit{R}}$, which turns rigidly with the sample, and a frictional component ${\mathbf{B}}_{\mathit{F}}$, which stays fixed relative to H, reflecting the effects of vortex pinning and unpinning. Our rotational magnetic measurements on polycrystalline (Ba,K)${\mathrm{BiO}}_3$ at 4.2 K now show that both ${\mathit{B}}_{\mathit{R}}$ and ${\mathit{B}}_{\mathit{F}}$ and their changes with H are the same for starting states on different branches of a major hysteresis loop. Moreover, for starting states after zero-field cooling, we find that ${\mathit{B}}_{\mathit{R}}$ is zero for all H but that ${\mathit{B}}_{\mathit{F}}$ rises at ${\mathit{H}}_{\mathit{c}1}$ and rapidly joins the ${\mathit{B}}_{\mathit{F}}$ values for the hysteretic starting states at higher H. Hence, regardless of the magnetic history before sample rotation, the frictional component ${\mathit{B}}_{\mathit{F}}$ depends essentially only on H (at fixed temperature) and thus equilibrates with the magnetic environment.

We report the results of temperature dependence of S-parameter and lifetime (τ) measurements on the normal and superconducting states of ${\mathrm{Bi}}_2$${\mathrm{CaSr}}_2$${\mathrm{Cu}}_2$${\mathrm{O}}_{\mathit{y}}$ (Bi-2:1:2:2) and ${\mathrm{Bi}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Pb}}_{\mathit{x}}$${\mathrm{Ca}}_2$${\mathrm{Sr}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{y}}$ (Bi-2:2:2:3) superconductors. A decrease of the S parameter as well as τ below ${\mathit{T}}_{\mathit{c}}$ is seen for both compounds. These observations are interpreted qualitatively considering a reported positron-density-distribution calculation and invoking the local charge-transfer process from ${\mathrm{CuO}}_2$ to Bi-O planes at the onset of the superconducting transition. The possible influence of sample preparation conditions and structural changes on the temperature dependencies of the positron annihilation parameters across ${\mathit{T}}_{\mathit{c}}$ is also discussed.

The local susceptibility around Zn atoms in Gd-doped ${\mathrm{YBa}}_2$(${\mathrm{Cu}}_{0.97}$${\mathrm{Zn}}_{0.03}$${)}_3$${\mathrm{O}}_{\mathit{y}}$ has been measured by the electron-spin-resonance Knight shift of ${\mathrm{Gd}}^{3+}$ at 245 GHz. The magnitude and distribution of the shifts are compared with the corresponding data for compounds with no Zn. For both y=7.0 and 6.76 the full density of states (corresponding to the high-temperature normal state of the pure y=7.0 compound) is restored at the first-neighbor sites of Zn atoms irrespective of whether the material is superconducting or not. This shows that both the superconducting gap and the spin-fluctuation gap are locally suppressed by Zn substitution.

We have studied the effects of the oxygen stoichiometry on the magnetic irreversibility line and on the decay of the zero-field-cooled (ZFC) magnetization in ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{8+\mathrm{\delta }}$ epitaxial films. We have found that the irreversibility line is shifted to lower temperatures and fields (relative to the as-grown sample) by reducing thermal treatments, while it is slightly affected by oxidizing treatments. The decay of the ZFC magnetization is also strongly affected by the oxygen content: although at low temperatures (T/${\mathit{T}}_{\mathit{c}0}$≤0.1) the decay rate of the ZFC magnetization does not depend on the oxygen content, as temperature increases, the relaxation becomes faster with decreasing the oxygen content (above T/${\mathit{T}}_{\mathit{c}0}$∼0.3 for the reduced sample the time decay is so rapid that it cannot be measured). Such effects are explained taking into account the strong dependence of the effective anisotropy in ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{8+\mathrm{\delta }}$ on the oxygen content. The effective anisotropy is higher in reduced samples, thus shifting the vortex-lattice melting temperature toward lower values. From the analysis of the magnetic irreversibility lines, values of the effective anisotropy factor γ have been obtained for different degrees of oxidation. These results are in good agreement with other estimates obtained by different techniques.

The nonlinear resistivity in the superconducting vortex glass regime varies with current density J as ρ=${\mathrm{\rho }}_0$exp[-(${\mathit{J}}_{\mathit{T}}$/J${)}^{\mathrm{\mu }}$] as J→0. It is argued here that for the gauge glass model in d dimensions the exponent μ=ψ/(d-1-θ) where θ is the scaling exponent for the free energy of droplets at length scale L, ΔF(L)∼${\mathit{L}}^{\mathrm{\theta }}$, and ψ is the barrier height exponent. A real-space renormalization-group calculation for the gauge glass gives ψ=θ≊0.26 and hence μ≊0.15, a result in reasonable agreement with some experiments on ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$.

Oxygen isotope effects have been investigated in the electron-doped ${\mathrm{Pr}}_{1.85}$${\mathrm{Ce}}_{0.15}$${\mathrm{CuO}}_{4\mathrm{-}\mathit{y}}$. We find the exponent of oxygen isotope effect ${\mathrm{\alpha }}_{\mathrm{O}}$=0.08±0.01. The sizable oxygen isotope effect in this electron-doped system indicates that the electron-phonon interaction plays an important role in the pairing mechanism.

The temperature dependence of the electrical resistivity along the a and b axes of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6.9}$ has been measured in high-purity fully detwinned single crystals. Below 300 K, ${\mathrm{\rho }}_{\mathit{a}}$(T) was found to be perfectly linear, while ${\mathrm{\rho }}_{\mathit{b}}$(T) revealed a distinct upward curvature. Viewing the b-axis conductivity as the sum of conductivities in the ${\mathrm{CuO}}_2$ planes and along the Cu-O chains, a ${\mathit{T}}^2$ dependence for the chain resistivity is obtained. Such a dependence, also found in quasi-one-dimensional conductors, explains a number of hitherto ill-identified deviations from linearity reported recently.

We report on the electrical resistivity of ${\mathrm{K}}_3$${\mathrm{C}}_{60}$ and ${\mathrm{Rb}}_3$${\mathrm{C}}_{60}$ thin films. These films, grown at elevated temperatures, are highly textured and consist of large single-crystal grains. The films exhibit metallic behavior up to 500 K, with residual resistivities of 1.2 mΩ cm. The low-temperature (T) resistivity exhibits a ${\mathit{T}}^2$ dependence. We suggest that it is dominated by electron-electron interactions, and we find quantitative agreement not only with estimates based on the electron density and the bandwidth, but also with other narrow band systems. The electron-phonon interactions only become important for the resistivity above room temperature, because these phonons correspond to high-frequency vibrations of the ${\mathrm{C}}_{60}$ molecule. Nevertheless, virtual excitations of these same high-energy phonons are responsible for the superconductivity. Using similar criteria of conventional organic superconductors, we find that these materials are dominated at all temperatures by electron-phonon interactions.

The pressure dependence of the superconducting transition temperature ${\mathit{T}}_{\mathit{c}}$ of simple-cubic ${\mathrm{Na}}_2$${\mathrm{CsC}}_{60}$ is studied by an ac susceptibility method with a Swenson-type piston-cylinder apparatus. The observed value of ${\mathit{dT}}_{\mathit{c}}$/dP is -12.5±0.2 K/GPa, significantly larger than the values of -7.8 and -9.7 K/GPa reported earlier for ${\mathrm{K}}_3$${\mathrm{C}}_{60}$ and ${\mathrm{Rb}}_3$${\mathrm{C}}_{60}$, respectively. Using the compressibility available (that of ${\mathrm{K}}_3$${\mathrm{C}}_{60}$), we find that the evolution of ${\mathit{T}}_{\mathit{c}}$ with interfullerene spacing in ${\mathrm{Na}}_2$${\mathrm{CsC}}_{60}$ is significantly different from that established for merohedrally disordered face-centered-cubic superconductors ${\mathit{M}}_3$${\mathrm{C}}_{60}$ (M=alkali metal), but in excellent agreement with the ambient pressure data for the simple cubic superconductors ${\mathrm{Na}}_2$${\mathrm{RbC}}_{60}$ and ${\mathrm{Na}}_2$${\mathrm{CsC}}_{60}$. The present results provide an additional dimension to the existing features of fulleride superconductors, establishing the importance of the relative orientation of ${\mathrm{C}}_{60}^{3\mathrm{-}}$ ions in affecting the electronic and conducting properties of these materials.

The effect of a Gaussian-distributed random field on the Sherrington-Kirkpatrick spin glass is analzyed by means of the replica method. It is shown, within replica symmetry, that the free energy and equations of state are the same as those obtained in the pair-approximation approach by Ma, Gong, and Li. A phase diagram is presented which differs essentially in the concerns of their main conclusion, i.e., the reentrant transition, which is weakened by the presence of the random field, contrary to what they claim.

The exact harmonic-generation (HG) spectrum is obtained analytically for a one-band one-dimensional conductor for both ac and ac + dc external fields. It is shown that even and odd harmonics can be created and controlled depending on the initial-state conditions and the field amplitudes E. The HG spectrum forms a single plateau with a cutoff defined by a maximum HG order ${\mathit{N}}_{\mathit{m}}$ corresponding to the maximum energy acquired by the electron in the laser field. This maximum energy corresponds to single transfer between neighboring sites, i.e., ${\mathit{N}}_{\mathit{m}}$=eEa/ħω, where a is the lattice constant and ω the field frequency.

We have investigated the kinetics of domain growth in the Ising model of a two-dimensional alloy comprising two kinds of atom plus a single vacancy, where the atoms are allowed to move only by changing places with the vacancy. Not only is this model more realistic than the more usual Kawasaki dynamics in describing the decomposition process in real metal alloys, but it can also be up to 30 times faster in terms of computing time. The domains have shapes similar to those observed in the analogous model with Kawasaki dynamics, but the asymptotic regime of domain size growth (R ∝ ${\mathit{t}}^{1/3}$) is approached much faster particularly in the case of 1:1 alloy composition at low temperatures. The data also show that interface diffusion cannot fully explain the slow approach towards the asymptotic regime in the case of Kawasaki dynamics.

We have studied the effect of 500-keV ${\mathrm{Xe}}^{+}$ irradiation on the structure, magnetotransport, and magnetic properties of sputtered Fe/Cr multilayers. Initially, with increasing dosage the giant magnetoresistance (GMR) increases with a concomitant increase in the interfacial roughness as indicated by changes in the low-angle x-ray-diffraction spectra. At doses higher than 1×${10}^{13}$ ions/${\mathrm{cm}}^2$, the value of the GMR progressively decreases, corresponding to progressively increasing regions of the film which are ferromagnetically coupled. The resulting structural disorder increases the overall electronic scattering rate and hence the electrical resistance at these higher doses. These results demonstrate that increased interfacial scattering enhances the GMR and thus plays an important role in the mechanism determining the GMR in Fe/Cr superlattices.