Strong interactions with phonons cause nonlinearity to appear in the evolution of quasiparticles such as electrons, or electronic and vibrational excitations. Our study of the evolution of such nonlinear quantum systems, when they are placed in contact with a thermal reservoir, has been found to exhibit rich behavior including a Hopf bifurcation. The nature of this bifurcation is studied and experimental manifestations are indicated.

It has been recently demonstrated that dynamical models of nonlinear lattices admit approximate solutions in the form of self-supported intrinsic modes (IM’s). In this work, the intensity of the emission of radiation (‘‘phonons’’) from the one-dimensional IM is calculated in an analytical approximation for the case of a moderately strong anharmonicity. Contrary to the emission in nonintegrable continuum models, which may be summarized as fusion of several vibrons into a phonon, the emission in the lattice may be described in terms of fission of a vibron into several phonons: as the IM’s internal frequency lies above the phonon band of the lattice, the radiative decay of the IM in the discrete system can be only subharmonic. It is demonstrated that the corresponding lifetime of the IM may be very large. Then, the threshold (minimum) value of the amplitude of an external ac field, necessary to support the IM in a lattice with dissipative losses, is found for the limiting cases of the weak and strong anharmonicity.

The effects of the external magnetic field on the spin-density-wave (SDW) order and on accompanying fluctuations are calculated within the random-phase approximation for the extended Hubbard model with imperfectly nested quasi-one-dimenisonal Fermi surfaces. Both Pauli and orbital mechanisms are treated in parallel. It is shown that the Pauli coupling leads to a finite hybridization of the SDW component in the direction of the external field and the charge-density wave. The mean value of this SDW component remains zero below the critical temperature in the isotropic system, but may be activated in systems with an internal spin anisotropy. The mean-field expression for the corresponding spin-flop field is derived. Furthermore, the Pauli coupling renormalizes two of six fluctuative SDW modes. In order to establish ways of qualitative and quantitative comparison between effects belonging to the Pauli and orbital couplings, we analyze the characteristic parameters for these two modes as well as for the other four modes affected only by the orbital coupling. In particular we evaluate the field dependence of long-wavelength gaps, correlation lengths, and attenuation coefficients.

We define a prescription to generate the low-temperature expansion of the free energy of the Ising spin glass. We test our ideas by computing the first few coefficients in D=2. Our results indicate that the series coefficients are not numbers but averages over a nontrivial distribution.

The spin-1/2 antiferromagnetic Heisenberg model on a square lattice is studied by a Monte Carlo method. The ground-state energy and the lowest energies of the sectors ${\mathit{S}}_{\mathit{z}}$=1 and 2 are determined for lattice sizes ranging from 4×4 to 16×16. The extrapolation to infinite lattice gives an energy per site equal to -0.6690±0.0002. The staggered magnetization was also measured for lattice sizes up to 24×24. The extrapolation to infinite lattice gives a value 0.307±0.006 in units where the saturated value is 1/2.

Electron-spin-resonance studies of films of the conducting form of polyaniline (PANI), doped with camphor-sulfonic acid (CSA) and cast from metacresol solutions, show a temperature-independent Pauli susceptibility between 300 and 50 K; in PANI-CSA, there is no Curie contribution to the paramagnetic susceptibility at high temperatures. Below 50 K, a Curie-like contribution to the electronic susceptibility (χ∝1/T) is observed, indicative of singly occupied states at the Fermi energy. Thus, at approximately 50 K, the electron-electron interactions are comparable to the thermal energy in this disordered metal near the boundary of the metal-insulator transition. We conclude from this crossover that the intrasite electron-electron interaction parameter U is ∼50–60 K (∼4–5 meV). In many samples, evidence of a spin-glass transition is observed around 10 K due to spin-spin coupling between the singly occupied states. The average spin-spin coupling (random in sign) is thereby estimated to be 0.5–1 meV. The indirect exchange interaction between localized moments via π-electrons is proposed to dominate at these low temperatures.

The spin-dependent absorption of circularly polarized x rays at the K edge of Fe and the ${\mathit{L}}_2$ and ${\mathit{L}}_3$ edges of Gd in amorphous and crystalline ${\mathrm{GdFe}}_2$ has been studied. Large differences in the magnitude of the dichroic signal are observed between the two samples. The application of recently derived sum rules indicates substantial quenching of the orbital moment in the amorphous sample. The results are compared to a theoretical spectrum for crystalline ${\mathrm{GdFe}}_2$.

Measurements of the longitudinal and transverse magnetization components (${\mathit{M}}_{\mathit{L}}$,${\mathit{M}}_{\mathit{T}}$) were performed on a field-cooled thin-disk-like sample of disordered ${\mathrm{Ni}}_{76}$${\mathrm{Mn}}_{24}$ as it was rotated in various fixed fields. Our experimental data were presented at low temperatures which are well below the spin-glass freezing temperature (≊116 K). The observed rotational behavior of the anisotropy is consistent with neither a purely directional character nor a unidirectional and uniaxial character in the assumption of a rigid rotation. We also take into account the possible incoherent domain rotations. Such a possibility also does not seem solely to explain this rotational behavior. However, the data are well described by the fact that the unidirectional anisotropy is not rigidly linked to the lattice along its initial direction, but it can be elastically rotated by the applied field. These experimental results are also in good agreement with a ‘‘domain anisotropy model’’ suggesting a multidomain configuration even in a field-cooled state, which affects the rotation of the anisotropy.

Self-consistent calculations with the discrete variational method in the framework of local spin-density theory were performed for 43-atom embedded clusters representing a Mn impurity in Al and Cu hosts. Effects of local lattice relaxation were explored by varying the distance between Mn and the nearest-neighbor host atoms. It was found that the magnetic moment of Mn in Al is much more sensitive to local lattice distortion than Mn in Cu. At the equilibrium Mn nearest-neighbors distance, as determined by x-ray absorption fine-structure measurements, a reduction of 34% of μ in Al is obtained, relative to the host lattice distance. Mechanisms related to the effect of the environment on μ are discussed. The Mn 3s exchange splitting observed in x-ray photoelectron spectra is also investigated, and it is found that it may not be taken as evidence of a 3d moment in the ground state.

Just as Invar describes a system with a smaller than ‘‘normal’’ thermal expansion in a magnetically ordered state, a system with a larger than ‘‘normal’’ thermal expansion in a magnetically disordered state can be described as anti-Invar. Anti-Invar had been already observed in a number of fcc binary and ternary alloys of the 3d series well above magnetic ordering temperatures. While Invar is characterized by low-temperature moment-volume instabilities, anti-Invar signifies the importance of high-temperature moment-volume instabilities. Both are closely related to the ground-state properties. We have reinvestigated the ${\mathrm{Fe}}_{50}$${\mathrm{Ni}}_{\mathit{x}}$${\mathrm{Mn}}_{50\mathrm{-}\mathit{x}}$ alloy system and confirmed the existence of enhanced thermal expansion at high temperatures, i.e., the anti-Invar effect. With the aid of the thermal properties of these alloys we deduce the temperature dependence of the thermal expansion of γ-Fe and show its anti-Invar nature. The enhanced volume expansion in γ-Fe amounts to 2.8%, which is larger than the Invar effect (2%) in ${\mathrm{Fe}}_{65}$${\mathrm{Ni}}_{35}$.

Small Ising lattices with both ferromagnetic (F) and antiferromagnetic (AF) exchange interactions (or bonds) and increasing numbers of spins are studied by means of two independent methods: computational solutions to the Hamiltonian problem and topological counting of frustration paths. Equal magnitudes and concentrations are assumed for both types of bonds. Two different geometries are considered: square lattices (SL’s) with coordination number 4 and triangular lattices (TL’s) with coordination number 6. Two-dimensional samples with a total number of spins N between 4 and 64 are considered for SL’s, while N is varied between 4 and 44 for TL’s. They are distributed in two-dimensional arrays where periodic boundary conditions are imposed. After an array is selected, bond distributions (samples) are independently and randomly generated in fixed positions. The physical parameters are then calculated exactly for each sample. The emphasis here is on the ground-state properties and their dependence with size and shape for the two kinds of lattices. All magnitudes correspond to a basic statistics over a large number of samples for each array. The following magnitudes are reported: ground-state energy per bond, frustration segment, abundance of first excited states, remnant entropy, low-temperature specific heat, and site order parameters q, p, and h. Parameters p and h are introduced here, showing advantages over other similar magnitudes. The results are in good correspondence with analytic studies for the thermodynamic limit. This means that the spin site correlation (p) tends to vanish as N grows. However, we have found that the shape dependence modulates the behavior of these systems toward the thermodynamic limit. There is no tendency to vanish for the bond correlation parameter (h). For both kinds of lattices h might be a constant independent of size and shape.

The longitudinal relaxation time ${\mathit{T}}_1$ and the transverse relaxation time ${\mathit{T}}_2$ of ${}_{}{}^{133}{}_{}{}^{}\mathrm{Cs}$ NMR were measured in the three-dimensional ordered phase of a quasi-one-dimensional S=1/2 Ising-like antiferromagnet ${\mathrm{CsCoCl}}_3$. The experimental results were well explained by a pulselike fluctuation due to spin inversion in the ${\mathrm{Co}}^{2+}$ chains. This fact strongly suggests the presence of domain-wall pairing in the linear chain. The spin-inversion rate, the spin-inversion duration, and the correlation time were evaluated from an analysis of fluctuating internal field at the Cs nuclei. The mechanism of the phase transition at ${\mathit{T}}_{\mathit{N}2}$ (9 K) was made clear from the dynamical point of view.

In this paper the two-dimensional Heisenberg model is considered, with a possible anisotropy in the Heisenberg interaction in the 1 direction. Helical boundary conditions are introduced and an exact mapping of the two-dimensional lattice to a one-dimensional lattice is given. This mapping allows for a determination of the form of the wave function for 1 and 2 reversed spins. For this latter nontrivial case, the quantization condition is also derived.

The magnetic flux loss induced in sintered Nd-Fe-B permanent magnets by proton and α irradiation has been measured. The irradiations have been made with 14–20 MeV protons and with 56 MeV α particles. The dependence of the magnetic flux loss on the temperature, external magnetic field, and on the particle energy has been studied. An earlier theoretical model of the magnetic damage is refined to take into account the effects of the external magnetic field as well as the shape of the magnet sample. The good agreement between the experimental and theoretical results supports our theoretical model according to which the local heating induced by particle irradiation causes the magnetic flux loss.

An approximate partition functional is derived for the infinite-dimensional Hubbard model. This functional naturally includes the exact solution of the Falicov-Kimball model as a special case, and is exact in the uncorrelated and atomic limits. It explicitly keeps spin symmetry. For the case of the Lorentzian density of states, we find that the Luttinger theorem is satisfied at zero temperature. The susceptibility crosses over smoothly from that expected for an uncorrelated state with antiferromagnetic fluctuations at high temperature to a correlated state at low temperature via a Kondo-type anomaly at a characteristic temperature ${\mathit{T}}^{\mathrm{*}}$. We attribute this anomaly to the appearance of the Hubbard pseudogap. The specific heat also shows a peak near ${\mathit{T}}^{\mathrm{*}}$. The resistivity goes to zero at zero temperature, in contrast to other approximations, rises sharply around ${\mathit{T}}^{\mathrm{*}}$, and has a rough linear temperature dependence above ${\mathit{T}}^{\mathrm{*}}$.

We provide an explanation of the spin-freezing transition recently observed by Chou et al. [Phys. Rev. Lett. 71, 2323 (1993)] in ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_4$ for x≲0.02. We propose that topological excitations of the two-dimensional Heisenberg quantum antiferromagnet having noncoplanar transverse components have a pair-interaction energy that qualitatively and quantitatively agrees with the observed values of spin-freezing temperature as a function of doping.

We study the effect of long-range Coulomb interaction on the phason in spin-density waves (SDW’s) within mean-field theory. In the longitudinal limit and in the absence of SDW pinning, the phason is completely absorbed by the plasmon due to the Anderson-Higgs mechanism. In the presence of SDW pinning or when the wave vector q is tilted from the chain direction, though the plasmon still almost exhausts the optical sum rule, another optical mode appears at ω<2Δ(T), with small optical weight. This low-frequency mode below the SDW gap may be accessible to electron-energy-loss spectroscopy.

A spin-rotationally-symmetric path-integral formulation with a vector order parameter is presented for a generalized scrN-orbital Hubbard model which yields the correct Hartree-Fock result in the saddle-point approximation, and also the spin-wave mode representing transverse Gaussian fluctuations. A systematic expansion of the action in powers of inverse scrN, wherein spin-rotational symmetry and hence the Goldstone mode are preserved order by order, is developed, and the first order O(1/scrN) quantum corrections to the ground-state energy and sublattice magnetization obtained. Setting scrN=1 yields a quantitatively correct description of the antiferromagnetic state of the one-orbital Hubbard model at half-filling.

The linear upturn of the electrical resistivity, Δρ=-${\mathit{C}}_1$T, first reported below the freezing temperature ${\mathit{T}}_{\mathit{f}}$ of ${\mathrm{Fe}}_{2.4}$${\mathrm{Ni}}_{77.6}$${\mathrm{Si}}_8$${\mathrm{B}}_{12}$, is shown to be a rather universal feature of metallic glasses containing localized 3d and 4f moments in a wide range of concentrations. The normalized amplitudes, ${\mathit{C}}_1$${\mathit{T}}_{\mathit{f}}$, are almost independent of composition and the magnetoresistance also indicates that impurity scattering of the conduction electrons is not effective below ${\mathit{T}}_{\mathit{f}}$. We discuss these results by assuming that the spin-glass order parameter q∼(1-T/${\mathit{T}}_{\mathit{f}}$) increases the resistivity structure factor of the amorphous matrix.

A detailed theoretical analysis of the weak ferromagnetic (WF) transition in the low-temperature orthorhombic and Pccn phases of the ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Nd}}_{\mathit{x}}$${\mathrm{CuO}}_4$ system is presented. This transition arises from a subtle interplay between intraplanar anisotropic Dzyaloshinski-Moriya (DM) interactions and the interplanar coupling between adjacent layers. We model the DM interactions using a modified form of a spin Hamiltonian recently derived by Shekhtman et al. [Phys. Rev. Lett. 69, 836 (1992)], and use symmetry arguments to show that there are only two possible interplanar coupling mechanisms: (1) a distortion-induced isotropic interaction; and (2) a pseudodipolar interaction. For (1) to account for the antiferromagnetic arrangement of WF moments in low field the interplanar coupling must be ferromagnetic, and an explicit calculation for the effective interaction including antiferromagnetic superexchange, ferromagnetic Hund’s-rule interaction, and direct ferromagnetic exchange, shows that this is plausible. In the Pccn phase, we show that the critical magnetic field required to induce the WF transition depends strongly on whether (1) or (2) is the relevant interplanar coupling mechanism. These results are discussed in the context of recent experiments on the magnetic properties of ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Nd}}_{\mathit{x}}$${\mathrm{CuO}}_4$ system.