The temperature dependences of both the electrical resistivity and magnetic susceptibility were measured for the pseudobinary alloy U(${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Sn}}_{\mathit{x}}$${)}_3$. Moreover, the temperature variation of the specific heat was determined for ${\mathrm{UGa}}_3$. The results show a clear evolution in magnetic behavior of the system studied from a weakly temperature-dependent paramagnetism with a long-range antiferromagnetic ordering at 67 K in ${\mathrm{UGa}}_3$ to a strongly temperature-dependent paramagnetism, but without any magnetic order, in ${\mathrm{USn}}_3$. The properties of ${\mathrm{UGa}}_3$ are discussed in terms of itinerant 5f-electron magnetism. In contrast, some arguments are given for local 5f-electrons in ${\mathrm{USn}}_3$, which behaves as a nonmagnetic ‘‘Kondo-lattice’’ system with ${\mathit{T}}_{\mathit{K}}$≃60 K.

The optical and magneto-optical properties of as-deposited and annealed ${\mathrm{Fe}}_{51}$${\mathrm{Pt}}_{49}$ films were investigated in the photon energy range 1.55–10.5 eV. The magneto-optical Kerr rotation (${\mathrm{\theta }}_{\mathrm{K}}$) spectra of both films were found to show a dispersion-type structure in the 4.6–7.8 eV region: they have large negative and broad positive peaks at around 4.8 and 7.8 eV, respectively. The ${\mathrm{\theta }}_{\mathrm{K}}$ spectra showed a tendency that is essentially quite similar to that of bulk Fe. The Kerr ellipticity (${\mathrm{\eta }}_{\mathrm{K}}$) spectra are bell shaped, exhibiting a large negative peak at 6.3 eV. Plasma edges were observed at 6.9 eV for the as-deposited film and at 7.3 eV for the annealed film. The absolute value of the real part of the off-diagonal dielectric element (${\mathrm{\varepsilon }}_{\mathrm{XY}}^{\prime }$) of both films at ∼4.8 eV are almost the same as that of bulk Fe. This leads to the conclusion that the ${\mathrm{\theta }}_{\mathrm{K}}$ enhancement at 4.8 eV is mainly due to a decrease in ${\mathrm{\varepsilon }}_{\mathrm{XX}}^{\mathrm{″}}$ and a peak shift in ${\mathrm{\varepsilon }}_{\mathrm{XY}}^{\prime }$ towards lower energy, but not to a plasma-resonance effect.

The cationic distribution of the solid solution ${\mathrm{BaFe}}_{4\mathrm{-}2\mathit{x}}$${\mathrm{Sn}}_{2+\mathit{x}}$${\mathrm{Co}}_{\mathit{x}}$${\mathrm{O}}_{11}$ (0≤x≤2) has been determined by using combined neutron and x-ray diffraction. A very strong cation site selectivity has been found: Sn atoms do not enter 4f(1/2) sites at all, but this site is occupied by both Fe and Co atoms. On the other hand Sn atoms exhibit a strong preference for the 4e sites where Co atoms do not enter in any case. 6g sites are occupied by the three cations without any preference. No long-range magnetic ordering has been detected in any of the samples, but short-range three-dimensional ferrimagnetic ordering occurs below a temperature ${\mathit{T}}_{\mathit{N}}$ that decreases as x increases. Below ${\mathit{T}}_{\mathit{N}}$ down to ${\mathit{T}}_{\mathit{F}}$, the system is composed of a set of ferrimagnetic clusters of finite size. A second magnetic transition to a spin-glass-like phase is observed in all the samples at ${\mathit{T}}_{\mathit{F}}$, and this transition is attributed to the intrinsic frustration of the 6g sites arranged in a kagome$aa— lattice.

All-electron, self-consistent, Hartree-Fock cluster calculations have been carried out to derive electron densities at the zinc nucleus in the series of compounds ${\mathrm{ZnF}}_2$ (rutile-type structure), ZnO (rocksalt structure), ZnO (wurtzite structure), and the compounds ZnS, ZnSe, and ZnTe (all with the sphalerite structure). The derived density differences show a very good linear correlation with the experimental isomer shifts. The isomer shifts and results for the densities at the zinc nucleus have been combined to calculate a value for the change of the mean-square nuclear charge radius for the Mössbauer transition in ${}_{}{}^{67}{}_{}{}^{}\mathrm{Zn}$ of Δ〈${\mathit{r}}^2$〉=+(13.9±1.4)×${10}^{\mathrm{-}3}$ ${\mathrm{fm}}^2$. Our calculations clearly show the importance of the covalency of the Zn-ligand bond for the origin of the isomer shift and fully corroborate the experimental linear correlation between decreasing isomer shift values and increasing electronegativity of the ligands. The most important contribution to the electron-density differences at the zinc nucleus comes from the Zn(4s) electrons with a smaller but significant contribution from the Zn(3s) electrons appearing to arise primarily from the repulsive influence of the ligand-ion orbitals.

Fourier-transform NMR spectra and spin-lattice relaxation rates were studied for ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$ and ${}_{}{}^{31}{}_{}{}^{}\mathrm{P}$ in a monoclinic modification of the ${\mathrm{Li}}_3$${\mathrm{In}}_2$(${\mathrm{PO}}_4$${)}_3$ superionic conductor over a wide temperature range of 240–575 K. The static magnetic susceptibility of the lithium subsystem of the compound and the ${}_{}{}^{31}{}_{}{}^{}\mathrm{P}$ NMR line shape exhibit anomalous changes with increasing temperature. Quadrupole structure of the ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$ NMR spectrum is observed under fast motion of the lithium ions in the superionic phase. In this phase the temperature behavior of the spin-lattice relaxation rates is different for ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$ and ${}_{}{}^{31}{}_{}{}^{}\mathrm{P}$ nuclei. The results are discussed taking into account the preceding NMR data on monoclinic ${\mathrm{Li}}_3$${\mathrm{Sc}}_2$(${\mathrm{PO}}_4$${)}_3$ and rhombohedral ${\mathrm{Li}}_3$${\mathrm{In}}_2$(${\mathrm{PO}}_4$${)}_3$ compounds.

In our previous studies of the antiferromagnetic Potts and Q=6 clock models in three dimensions, we have argued the existence of an incompletely ordered phase (IOP) which is characterized by soft rigidity with a nonintegral stiffness exponent and an incomplete order (though long ranged) such that some of the spin states are exclusively dominant. To investigate the IOP’s further, we study Q-state general clock models with one varying energy parameter which can include the ferromagnetic Potts models, by means of our Monte Carlo twist method and analytically in the pair approximation. The twist method gives the following results for Q=6. There are two kinds of IOP’s (IOP1 and IOP2) on the opposite side of the ferromagnetic Potts model. Their stiffness exponents are about 1.2 less than 2 (for the highest rigidity) in good agreement with those previously obtained. A pair of adjacent clock states are dominantly well mixed in the IOP1, as are a set of three adjacent ones in the IOP2. Thermal fluctuations in the IOP’s can be characterized by the spin configurations of the soft structures with buffers that prevent spins in different states by more than the least angle from getting close with other, which is consistent with the nonintegral stiffness exponent. Large entropy contributions to the IOP’s are revealed in both approaches. The phase boundary, which extends from the ferromagnetic Potts point, is clearly of first order, while among those relevant to the IOP’s one is discontinuous and the other is continuous but the last two are not evident. It is strongly suggested that there is a transition without symmetry breaking between the IOP2 and the rigid phase. In the pair approximation applied for Q=4∼12, various properties for Q=6 are consistent with those obtained by the simulation, except some other properties. Peculiar Q dependence of the highest critical temperature of the IOP in the extreme case is also found.

We study the diluted site-bond-correlated (SBC) Ising model within the scheme of the mean-field renormalization group. The SBC Ising model has been proposed to describe the phase diagram for ${\mathrm{KNi}}_{\mathit{p}}$${\mathrm{Mg}}_{1\mathrm{-}\mathit{p}}$${\mathrm{F}}_3$. One treats the square (Z=4) and simple cubic (Z=6) lattices by using two different exchange interactions (denoted by models A and B). In the present paper, the phase diagram for some values of the correlation parameter α, which range from zero to unity, is studied. We obtain two critical concentrations ${\mathit{p}}_{\mathit{c}}$(α=0) and ${\mathit{p}}_{\mathit{c}}$(α≠0) in agreement with results from Monte Carlo methods. In addition, an interesting universal relation between the initial slope ${\mathrm{\Delta }}^{\left(\mathrm{\mu }\right)}$(α=1) and ${\mathrm{\Delta }}^{\left(\mathrm{\mu }\right)}$(α=0) is obtained (μ denotes model A or B). From this relation, we can estimate the possible exact values to the initial slope on the completely correlated case. Our approach is valid for lattices with any coordination number Z.

The energy levels (EL) and the zero-field-splitting (ZFS) parameters ${\mathrm{b}}_2^0$, ${\mathrm{b}}_4^0$, and ${\mathrm{b}}_4^4$ for a ${3\mathrm{d}}^4$ and ${3\mathrm{d}}^6$ ion at tetragonal sites in crystals are considered. Results are presented in the form of useful diagrams depicting the variation of the EL and the ZFS parameters with the crystal-field (CF) parameters. To determine the EL and the ZFS parameters ${\mathrm{b}}_{\mathrm{k}}^{\mathrm{q}}$, the Hamiltonian matrix including the electrostatic, tetragonal CF, spin-orbit (SO), and spin-spin (SS) interactions as well as Trees' correction is introduced. By separating the cubic CF parameter ${\mathrm{D}}_{\mathrm{q}}$ from the axial parameters ${\mathrm{B}}_{20}$ and ${\mathrm{B}}_{40}^{\prime }$, the nature of the tetragonal distortion is related to the crystal structure, and the variation of the EL and the ZFS parameters ${\mathrm{b}}_{\mathrm{k}}^{\mathrm{q}}$ with ${\mathrm{D}}_{\mathrm{q}}$, ${\mathrm{B}}_{20}$, and ${\mathrm{B}}_{40}^{\prime }$ are studied. Numerical calculations of the ZFS parameters are carried out for the ${\mathrm{Cr}}^{2+}$ ion in GaAs:${\mathrm{Cr}}^{2+}$ and ${\mathrm{CrSO}}_4$⋅${5\mathrm{H}}_2$O and for the ${\mathrm{Fe}}^{2+}$ ion in ${\mathrm{K}}_2$${\mathrm{FeF}}_4$, ${\mathrm{Cu}}_2$${\mathrm{FeGeS}}_4$, and ${\mathrm{FeV}}_2$${\mathrm{O}}_4$. The results show that the spin-triplet contributions to the ZFS are important; however, most of previously theoretical study of the ZFS for ${3\mathrm{d}}^4$ and ${3\mathrm{d}}^6$ ions in crystals were based on the spin quintet ${\mathrm{D}}^5$ approximation. The present results may be of interest to researchers in other fields relating to the fine-structure splitting of the ground state for these ions.

Elastic neutron-scattering and magnetization measurements have been performed on a single crystal of ${\mathrm{UPd}}_2$${\mathrm{Si}}_2$ in fields along the c axis. In zero field there is a simple body-centered antiferromagnetic structure for temperatures up to 108 K. Between 108 K and the Néel temperature there is an incommensurate longitudinal spin-density-wave phase. For fields above 1.9 T, a new phase appears that separates the two zero-field phases. The new phase corresponds to a commensurate spin density wave of wave vector (0,0,2/3). In the (0,0,2/3) phase the neutron-scattering data show that the amplitude of the spin-density-wave moment is (3.0±0.3)${\mathrm{\mu }}_{\mathit{B}}$, while magnetization measurements show a net ferromagnetic moment of (0.30±0.05)${\mathrm{\mu }}_{\mathit{B}}$ at 3 T.

The frequency dependencies of the uniform transverse and longitudinal susceptibilities of Heisenberg ferromagnets with a dipole-dipole interaction are studied in different temperature regions. Within a broad range of parameters the major contribution to the longitudinal component of the susceptibility, as well as to the dispersion of the fluctuational corrections to the transverse one is found to be the process containing a couple of spin waves in the intermediate state. The ferromagnetic-resonance (FMR) linewidth, which at zero temperature exists due to the quantum fluctuations is calculated. Deviations of the FMR lineshape from a Lorentzian are shown to be rather significant at sufficiently high temperatures. The results obtained within the framework of the developed perturbation theory based on the approach of Vaks, Larkin, and Pikin are generalized for the critical dynamics region. The range of validity of the employed approach is discussed. It is shown that dipole-dipole forces cannot be considered as a perturbation not only in the close vicinity of the Curie point but also at any temperature at low enough frequencies. In particular, the longitudinal susceptibility reveals the infrared discontinuity in the zero-frequency limit.

Previous linear spin-wave calculations as well as Green's-function theories predict disorder at $T>\mathrm{}0\mathrm{}$ for certain Heisenberg models in disagreement with the in-principle exact Monte Carlo simulations. In order to resolve this contradiction we extended the spin-wave theory beyond the usual Bloch's nonlinear approximation, including leading off-diagonal terms. We investigate the classical Heisenberg model on an fcc lattice for three pairs of coupling constants ($J_1$, $J_2$) which result in an infinite degeneracy for the magnetic ordering vector in the mean-field (MF) theory. We study in detail how the interaction between spin waves selects the ordering vector at the critical field between antiferromagnetic and paramagnetic phases. We find that the usually ignored nonlinear terms generate soft modes which are consistent with the order found by Monte Carlo simulations at zero field. We demonstrate the importance of these nonlinear effects by investigating antiferromagnetic models with a one- or two-dimensional degenerate MF manifold for the ordering vector. The model with the two-dimensional manifold seems ideal for a Monte Carlo study of domain growth in a first-order phase transition. Finally, we show that even when Bloch's nonlinear spin-wave theory gives qualitatively correct results, it can lead to a factor-of-three overestimate for the critical temperature of ferromagnetism. We find that this failure can be understood by considering the higher-order effects included in our extended spin-wave theory.

Aging of two-dimensional (2D) Ising spin glasses with short-range interactions ${\mathrm{Rb}}_2$${\mathrm{Cu}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Co}}_{\mathit{x}}$${\mathrm{F}}_4$ (${\mathit{T}}_{\mathit{c}}$=0) is studied in the critical regime via the low-frequency ac susceptibility and via the dc magnetization for x=0.22 and 0.33. After quenching to low temperatures, slow logarithmic relaxation is observed for both the in-phase and out-of-phase ac susceptibilities. Using the droplet scaling model, the spin-glass correlation length R is concluded to increase after a temperature quench as R∝(lnt${)}^{1/\mathrm{\psi }}$, with t in units of some microscopic time. The exponent ψ=1.0±0.1 compares with recent theoretical results and numerical simulations for 2D short-range Ising spin glasses. The waiting-time dependence found for the field-cooled and zero-field-cooled dc magnetizations points to a markedly slower crossover from the quasiequilibrium to the nonequilibrium regime than in 3D. From experiments with different cooling histories, finally, the equilibrium spin-glass state is inferred to be strongly dependent on the temperature.

We present real-space renormalization-group (RG) calculations of the critical properties of the random-field Ising model on a cubic lattice in three dimensions. We calculate the RG flows in a two-parameter truncation of the Hamiltonian space. As predicted, the transition at finite randomness is controlled by a zero-temperature, disordered critical fixed point, and we exhibit the universal crossover trajectory from the pure Ising critical point. We extract scaling fields and critical exponents, and study the distribution of barrier heights between states as a function of length scale.

We analyze the ground-state properties of the U=∞ Hubbard model on a 4×4 cluster using a Lanczos technique. The quantum numbers of the ground-state, its momentum distribution, the magnetic structure factor, some 〈n${\mathrm{⁁}}_{\mathbf{k}}$n${\mathrm{⁁}}_{\mathbf{k}\prime }$〉 dynamic correlation functions, and the overlap of the ground state with the noninteracting one (both before and after a Gutzwiller projection) are given for all fillings. As the number of electrons approaches half-filling the system undergoes a transition from a conventional ground state that overlaps appreciably with the noninteracting one to a new ground state that shows local ferromagnetic correlations and negligible overlap with the corresponding U=0 ground state. A detailed analysis of the wave function corresponding to the ground state of 14 electrons (two holes) proves the existence of a finite shift of the Fermi surface. Our results support the existence of a stable magnetic phase at small but finite hole doping. This new magnetic phase is locally ferromagnetic although the total spin is not maximum.

The free energy and magnetization for the general SU(N) one-impurity Kondo model in the magnetic field h are calculated by extending the previous 1/N expansion technique: the saddle point is determined self-consistently to the 1/N order. The obtained universal field-dependent magnetization M(h/${\mathit{T}}_{\mathit{K}}$) by this simple method is shown analytically to be asymptotically exact at both h≪${\mathit{T}}_{\mathit{K}}$ and h≫${\mathit{T}}_{\mathit{K}}$ limits. For general ‘‘f-electron’’ fillings, except half filling, the M(h/${\mathit{T}}_{\mathit{K}}$) curves cross continuously from the weak to strong coupling limit, but overestimate the curavature in the crossover region for moderate N. The magnetic Wilson crossover numbers are calculated approximately. Our results explicitly verify that the 1/N parameter is nonsingular under the adiabatic continuation.

The Eliashberg equations are solved in two dimensions for the transition temperature of a superconductor. The density of states has a logarithmic singularity. We show that the singularity in the density of states has only a small influence on the numerical solutions to the Eliashberg equations, apart from its contributions to the electron-phonon interaction λ. We also consider the effects of the angular dependence of the gap around the Fermi line which occur for s-wave pairing.

We investigate the influence of a periodic potential and electron-phonon interactions on the de Haas-van Alphen (dHvA) effect in the presence of very strong Coulomb correlations. The inclusion of these correlations is essential for creating the insulating state at half-filling. We use the almost localized Fermi-liquid approach to include the Coulomb correlation induced metal-insulator transition at half-filling and to calculate the correlation effects within the context of renormalized band structures. The effects of a periodic potential on the electronic motion in a magnetic field are calculated by using a series of magnetic subbands called the Hofstadter spectrum. In contrast to Landau levels, these magnetic subbands have nonlinear field dependence and have nonuniform energy gaps between two adjacent subbands. The results of these differences appear as the field and the hole doping concentration x, dependence on the effective mass, chemical potential, and magnetization in two dimensions. The inclusion of strong Coulomb correlations leads to a substantial reduction in the electron-phonon coupling constant near the metal-insulator transition. Electron-phonon interactions are screened in the presence of strong Coulomb correlations because charge fluctuations are progressively suppressed by the correlation effects as half-filling is approached. Hence, the coupling constant decreases as x decreases. The screened coupling constant is estimated self-consistently by including the contributions from both the renormalized band and the dynamical screening effects. We show that both a periodic potential and electron-phonon interactions lead to mass enhancement as a function of x, thereby, changing both the amplitude and frequency of the dHvA oscillations. We discuss the feasibility of observing these effects in copper oxides.

A theoretical analysis is presented for the temperature dependence of the diffusion of ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ impurities in solid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ in the regime where the coherent impurity motion at low temperatures is interrupted by scattering from vacancies or phonons. It is shown that the observed temperature dependences can be accurately described by a model involving vacancy scattering.

Density functionals that reproduce the helium liquid-gas interface as a function of temperature have been used, within an improved homogeneous nucleation approach, to investigate thermal nucleation and cavitation in both helium isotopes. The results are compared with available experimental data on cavitation in ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ and ${}_{}{}^4{}_{}{}^{}\mathrm{He}$. Predictions are made for cavitation in ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ at negative pressures and for nucleation in both isotopes.

A full account of positive-muon spin-rotation (${\mathrm{\mu }}^{+}$SR) studies on the systematics of the magnetic penetration depth λ in the isotropic Chevrel-phase superconductors ${\mathrm{SnMo}}_6$${\mathrm{S}}_{8\mathrm{-}\mathit{x}}$${\mathrm{Se}}_{\mathit{x}}$ (x=0,1,4,7) and ${\mathrm{PbMo}}_6$${\mathrm{S}}_{8\mathrm{-}\mathit{x}}$${\mathrm{Se}}_{\mathit{x}}$ (x=0,4) is presented. The absolute determination of λ from the μSR data necessitates a careful analysis of the μSR line shape and a comparison with simulated data, taking into account distortions of the flux-line lattice and a distribution of demagnetization factors in the sintered granular polycrystalline probes investigated. A correlation between ${\mathit{T}}_{\mathit{c}}$ and 1/${\lambda }^2$ is found which supports a recently developed quantum percolation model.