Persistent and time-resolved transient holes were burnt into the ${}_{}{}^1{}_{}{}^{}\mathrm{D}_2^{}{}_{}{}^{}$${\leftarrow }^3$${\mathrm{H}}_4$ transition of ${\mathrm{Pr}}^{3+}$ and the ${}_{}{}^5{}_{}{}^{}\mathrm{D}_0^{}{}_{}{}^{}$${\leftarrow }^7$${\mathrm{F}}_0$ transition of ${\mathrm{Eu}}^{3+}$ in silicate glasses. The holewidths were studied as a function of burning-fluence density, wavelength, and temperature between 0.4 and 12 K. Transient holes arising from optical pumping of ground-state nuclear hyperfine levels were found for both ions at T<3 K. Their lifetime decreases with increasing temperature. They are accompanied by symmetrically placed antiholes. For ${\mathrm{Eu}}^{3+}$, two pairs of antiholes were found which are attributed to the two europium isotopes, ${}_{}{}^{151}{}_{}{}^{}\mathrm{Eu}$ and ${}_{}{}^{153}{}_{}{}^{}\mathrm{Eu}$. For ${\mathrm{Pr}}^{3+}$, an additional mechanism leading to persistent hole burning dominates above 3 K. The holewidth follows a ${\mathit{T}}^{(1.3\mathrm{}\pm 0.1)}$ dependence for ${\mathrm{Pr}}^{3+}$ and a ${\mathit{T}}^{(1.0\mathrm{}\pm 0.1)}$ dependence for ${\mathrm{Eu}}^{3+}$. The temperature dependence is the same as that of the contribution of the two-level systems to the specific heat measured on the same samples and in the same temperature range. The quadrupole splittings of the ${}_{}{}^7{}_{}{}^{}\mathrm{F}_0^{}{}_{}{}^{}$ ground state and the crystal-field splittings of the ${}_{}{}^7{}_{}{}^{}\mathrm{F}_1^{}{}_{}{}^{}$ and ${}_{}{}^7{}_{}{}^{}\mathrm{F}_2^{}{}_{}{}^{}$ levels of ${\mathrm{Eu}}^{3+}$ as a function of excitation wavelength indicate that the strength of the noncubic component of the crystal field varies monotonically for excitation across the ${}_{}{}^5{}_{}{}^{}\mathrm{D}_0^{}{}_{}{}^{}$${\leftarrow }^7$${\mathrm{F}}_0$ absorption band, falling to zero at the long-wavelength edge. The hole width shows a similar variation with excitation frequency. No influence of the experimental time scale on the hole width was detected for either sample between ${10}^{\mathrm{-}2}$ and ${10}^2$ s.

We provide the electrostatic solution in mean field theory to the electrical response of heterogeneous systems in which the inclusions have arbitrary structure and permanent multipole moments of all orders. The system is placed between two parallel electrode plates, and all the resulting multipole moments of the inclusions and their images are taken into account. By performing a statistical average over the orientations of the inclusions, we obtain the effective dielectric function, depending on temperature and applied potential at the electrode plates.

We study the linear and nonlinear electric response of heterogeneous systems containing inclusions of arbitrary structure and permanent multipole moments of all orders with L=0 and L=1 pair distributions, within mean field theory. We find that for these distributions only the dipole moments contribute to the effective dielectric function. In particular, we provide the results for inclusions without permanent dipole moments or polarizabilities, and spherical inclusions. We show that the Debye’s effective dielectric function represents the linear part of the general result, in the special case of spherical inclusions with isotropic (L=0) pair distributions.

Isotope-dilution neutron spectroscopy was used to investigate the dynamics of the H and D optic vibrations in ${\mathrm{YH}}_2$ and ${\mathrm{YD}}_2$, respectively. The broad, complex, densities of states of the tetrahedrally coordinated H and D atoms in the isotopically pure materials collapse to single, sharp, triply degenerate features when the H and D atoms are sufficiently diluted with their companion isotopes. This demonstrates that the vibrational dispersion for the pure materials is due to the presence of significant H-H and D-D dynamic-coupling interactions. Moreover, a spectral doublet (with 5-meV splitting) superimposed on the sharp D-defect-mode feature for Y(${\mathrm{H}}_{0.9}$${\mathrm{D}}_{0.1}$${)}_2$ is suggestive of localized acoustic and optic branches of dynamically coupled, near-neighbor D-D pairs isolated in the predominantly hydrided material. The magnitude of the splitting implies a D-D/Y-D force-constant ratio of ∼6%. The dynamics of both light (H) and heavy (D) mass defects are in general agreement with simple mass-defect theory.

This paper is concerned with the design of optimal surface-heating patterns that result in focusing acoustic energy inside a subsurface target volume at a specified target time. The surface of the solid is heated by an incident laser beam which gives rise to shear and compressional waves propagating into the solid. The optimal heating design process aims to achieve the desired energy focusing at the target with minimal laser power densities and minimal system disturbance away from the target. Due to the slow motion of the thermal conduction process relative to the propagation of acoustic waves, a general formulation is derived which is the limiting case for a heat-absorbing nonconducting solid. This is consistent with the laser heating of the surface where, for the time duration of interest, thermal effects are confined to a thin layer at the surface. This simplification allows for the solution of the problem through the use of the system Green’s function for the purely elastic medium where thermal expansion enters as an external force. The problem is then posed as an optimal control problem, the solution of which is the required heating pattern at the surface. The optimality conditions are secured via the conjugate gradient method and the mechanics of the elastic medium is treated by the finite element method along with using the half-space Green’s function matrix. Good quality energy focusing is achieved, with the optimal designs reflecting the high directivity of the photothermally generated shear-wave patterns.

The domain dynamics of a quenched system with many nonconserved order parameters was investigated by using the time-dependent Ginzburg-Landau kinetic equations. Our computer simulation of a model two-dimensional system produced microstructures remarkably similar to experimental observations of normal grain growth. After a short transient, the average domain or grain radius was found to increase with time as ${\mathit{t}}^{1/2}$, in agreement with most of previous mean-field predictions and more recent Q-state Potts model Monte Carlo simulations.

The main purpose of the present paper is to study the effect of third- and fourth-order anharmonicity on the heat conductivity in the high-temperature (i.e., classical or saturation) regime of disordered systems. We show that the fourth- and third-order contributions, respectively, enhance and lower the conductivity, which can be understood by a strengthening or weakening of the stiffness of the bonds in the system. On comparing the heat conductivity in percolating and compositionally disordered systems, we find that disorder loosening the structure (percolating systems) favors the effect of anharmonicity.

Nuclear magnetic relaxation rates and linewidths are reported for the ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$ and ${}_{}{}^{93}{}_{}{}^{}\mathrm{Nb}$ resonances in the layered semiconducting compound ${\mathrm{LiNbO}}_2$. $ sup 7—Li line narrowing and ${\mathit{T}}_{1\mathrm{\rho }}$ measurements provide unambiguous signatures of Li diffusive motion and the Li long-range hopping rate is determined as a function of temperature. Rotating-sample experiments show that the frequency dependence of the Li spin-lattice relaxation rate at low resonance frequencies results from cross relaxation to the quadrupole-broadened ${}_{}{}^{93}{}_{}{}^{}\mathrm{Nb}$ spins. A crucial aspect of the cross relaxation is the large frequency variations of the ${}_{}{}^{93}{}_{}{}^{}\mathrm{Nb}$ transitions resulting from the diffusion of Li vacancies. ${}_{}{}^{93}{}_{}{}^{}\mathrm{Nb}$ relaxation rates are reported and shown to support both the cross relaxation and the Li hopping interpretations. In high magnetic fields, cross relaxation is suppressed and the ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$ and ${}_{}{}^{93}{}_{}{}^{}\mathrm{Nb}$ spin-lattice relaxation rates differ by three orders of magnitude. The frequency and temperature dependences of these high field relaxation rates indicate that the relaxation mechanisms are not associated with diffusive Li motion. For both Li and Nb, nearly linear temperature dependences of ${\mathit{T}}_1^{\mathrm{-}1}$ result from interaction with conduction electrons. The electrons are believed to be due to the acceptorlike behavior of Li vacancies. The much slower ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$ relaxation rate ${\mathit{T}}_1^{\mathrm{-}1}$ reflects the much smaller electron density (at the Fermi surface) at the Li sites than at the Nb sites. Sample-to-sample variations in the Li ion mobility and in the Nb ${\mathit{T}}_1^{\mathrm{-}1}$ are believed to reflect variations of the Li deficiency of the material.

The spin-glass domain is according to the droplet model predicted to be unstable to any perturbation (bond ΔJ, temperature ΔT, or field ΔH) on length scales larger than the overlap length ${\mathit{L}}^{\mathrm{*}}$. Specifically designed magnetic relaxation experiments on an amorphous metallic spin glass (${\mathrm{Fe}}_{0.15}$${\mathrm{Ni}}_{0.85}$${)}_{75}$${\mathrm{P}}_{16}$${\mathrm{B}}_6$${\mathrm{Al}}_3$ confirm that the overlap length concept holds for magnetic-field perturbations. The results apply both to a spin glass in zero field and in an applied field below the freezing temperature ${\mathit{T}}_{\mathit{f}}$(H,t) contour in the (H,T) plane.

Both dc and ac magnetic susceptibility have been carefully measured and analyzed in order to determine the features of the low-temperature magnetic state of the highly frustrated ${\mathrm{SrCr}}_8$${\mathrm{Ga}}_4$${\mathrm{O}}_{19}$ compound. The scaling analysis of the nonlinear part of the dc magnetic susceptibility gives clues that indicate that the nature of the magnetic state below ${\mathit{T}}_{\mathit{f}}$ does not correspond to a conventional spin glass. This idea is further confirmed by the study of the dynamic scaling that also differs from what is expected for a conventional spin glass. A progressive change towards a more conventional magnetic ordering below ${\mathit{T}}_{\mathit{f}}$ is observed by substituting Ga atoms by Fe atoms in the compound ${\mathrm{SrCr}}_8$${\mathrm{Ga}}_{4\mathrm{-}2\mathit{x}}$${\mathrm{Fe}}_{\mathit{x}}$${\mathrm{O}}_{19}$. For x=1.5, both the scaling analysis of the nonlinear part of the dc magnetic susceptibility and the dynamic scaling seem to indicate that a true spin-glass phase transition does exist at ${\mathit{T}}_{\mathit{f}}$. This change in the magnetic behavior is attributed to the fact that the presence of a magnetic atom in the 4${\mathit{f}}_{\mathrm{VI}}$ sites as a first neighbor of the Cr atoms, located in the 12k sites, will impose a ‘‘local preferential direction’’ for the magnetic moments removing somehow the strong intrinsic magnetic frustration of the kagomé lattice.

Spin-polarized low-energy-electron-diffraction (SPLEED) rotation curves have been measured for 2-ML ultrathin iron films grown on a tungsten (001) substrate. This LEED technique emphasizes intraplane multiple scattering and is found to be very sensitive to the magnetism and geometry of the film. The contributions of the exchange and spin-orbit polarization, as well as the intensity asymmetry upon magnetization reversal, are shown theoretically and experimentally to be separable using symmetry-based arguments. These symmetries confer particular advantages to SPLEED rotation curves for the study of ultrathin magnetic films: they provide many independent checks of the internal consistency of the data, allow a direct evaluation of the effects of instrumental asymmetries, allow a means (symmetry averaging) for correcting for small misalignments, and permit qualitative conclusions to be drawn directly from the data. For example, the remanent axis of magnetization for Fe/W(001) is shown to be in-plane along [10]. Large magnetic asymmetries, ${\mathit{A}}_{\mathit{u}}$, of 2% are observed for this system, showing that it is experimentally feasible to perform SPLEED with neither a spin-polarized electron source nor a polarization detector.

In mesoscopic samples of disordered ${\mathrm{Fe}}_{100\mathrm{-}\mathit{x}}$${\mathrm{Zr}}_{\mathit{x}}$ evaporated films with x≊8, complex discrete resistance switching due to collective rearrangements of dissolved hydrogen appeared at temperatures from 4 to 300 K. In larger samples, 1% irreproducible hysteretic changes in the resistance as a function of temperature were observed near the nominal ferromagnetic transition, ${\mathit{T}}_{\mathit{C}}$≊250 K, also apparently due to clustering of dissolved H, coupled to the ferromagnetism. The unidirectional anisotropy of the low-temperature reentrant spin-glass magnetic regime was detected via magnetoresistance in mesoscopic samples with hydrogen removed. However, mesoscopic samples exhibiting such anisotropy still showed no detectable spin-glass resistance noise, unlike larger samples.

A Monte Carlo method for finite-temperature studies of the two-dimensional quantum Heisenberg antiferromagnet with random ferromagnetic bonds is presented. The scheme is based on an approximation which allows for an analytic summation over the realizations of the randomness, thereby significantly alleviating the ‘‘sign problem’’ for this frustrated spin system. The approximation is shown to be very accurate for ferromagnetic bond concentrations of up to 10%. The effects of a low concentration of ferromagnetic bonds on the antiferromagnetism are discussed.

A realistic theoretical treatment of the influence of lattice vibrations on electronic structure and photoemission has been extended to ferromagnetic 3d metals by simultaneously including correlated fluctuations of local moments. Numerical applications to photoemission from Ni(110) yield, for temperatures from 121 K up to the Curie temperature, spectra in good agreement with experimental data. The phonon-induced lifetime broadening, which is inherent in these calculations, enhances the merging behavior between opposite-spin peaks with increasing temperature. Comparison with experiment implies a sizeable short-range magnetic order near ${\mathit{T}}_{\mathit{c}}$.

An exact analysis of a three-state Potts model with infinite-range interactions in the presence of an asymmetric random field is carried out. Complete phase diagrams at zero temperature are obtained. By finding lines of critical points and lines of tricritical points, the asymmetrical random field is shown to have strong effects on phase transitions. Under certain circumstances, a sufficient asymmetry in the distribution of the random field can either terminate a first-order phase transition at a critical point or bring it to a second-order phase transition through a tricritical point.

An experimental study of the spectral distribution of magnetic correlations in the quasi-one-dimensional antiferromagnet ${\mathrm{CsNiCl}}_3$ above the Néel temperature has been performed. Using the cold-source three-axis spectrometer with the energy resolution 0.02 THz, we made a number of q scans around the antiferromagnetic Bragg point (1/3, 1) / 3 ,1) at different energies. This allowed us to distinguish between the quasielastic and dynamic contributions to S(q,ω) in ${\mathrm{CsNiCl}}_3$ at different temperatures. Attention was also paid to the separate examination of the temperature evolution of the in-chain and interchain correlations. It is shown that the interchain and the low-energy correlations (including three-dimensional critical scattering) gradually disappear above ${\mathit{T}}_{\mathit{N}}$=4.84 K, giving rise to well-defined gapped excitations, as predicted by Haldane. These excitations, notably broadening, persist up to 21 K, in agreement with the previous studies.

The electronic structure of the antiferromagnetic heavy-fermion superconductor ${\mathrm{UPd}}_2$${\mathrm{Al}}_3$ is determined by means of self-consistent density-functional calculations in the local approximation treating the U 5f states as band states. The magnetic moment is obtained in agreement with experiment as well as the observed magnetocrystalline anisotropy. State densities and the effects of hybridization and of different magnetic configurations are discussed in detail. The Fermi surface is determined and illustrated; it is compared with recent de Haas–van Alphen measurements and is found to explain the data satisfactorily.

We have used Monte Carlo simulations, combined with microscopic Heisenberg equations of motion, to calculate accurately the dynamics of a classical spin assembly. The system consists of two kinds of nuclear spins which are coupled by exchange interactions, either ferromagnetic or antiferromagnetic. The fluctuation-dissipation theorem is employed to calculate the NMR lineshapes from the autocorrelation matrix for magnetization. We investigate the polarization and field dependence of the line shapes in the paramagnetic phase. Our simulations reproduce the observed exchange-induced merging of isotopic NMR lines of thallium at low polarizations, as well as the suppression-enhancement effect found experimentally for highly polarized copper and silver isotopes. The results are employed to evaluate the accuracy of a previously used mean-field model in the description of the NMR response. The observed deviations from the model highlight the usefulness of accurate numerical simulations. The line shapes are analyzed by separating the total NMR absorption into contributions from like and unlike spins. The decomposition shows how the nature of exchange narrowing changes with polarization. The effect of isotopic site randomness on the NMR line shapes is also investigated and found to be important at high polarizations.

A Green-function approach is used to determine the microwave-stimulated steady-state enhancement to the energy gap of low-${\mathit{T}}_{\mathit{c}}$ superconducting thin films. Consequently, the following results are being reported. The first is that a closed form expression for the energy gap is obtained for weak-coupling superconductors (the general solution is given in terms of integral equations). The second is that the transparency of this approach results in an explanation of the physical mechanism underlying the enhancement of the energy gap. This explanation differs in some respects from previous interpretations. We interpret energy-gap enhancement as resulting from virtual transitions to energies within the energy gap due to the stimulated emission and subsequent reabsorption of photons by correlated electron pairs. The fact that these pairs can now be virtually closer to the bottom of the energy gap enhances their probability of momentarily being a part of the superfluid, thereby enhancing the energy gap.

Magnetic susceptibility, Mössbauer spectroscopy, nuclear quadrupole resonance of Cu (NQR), nuclear magnetic resonance of Cu in the hyperfine field of the antiferromagnetic state (AFNMR), and x-ray diffraction measurements were performed to study the superconducting and magnetic properties of hydrogen charged bulk ${\mathrm{YBa}}_2$${\mathrm{Cu}}_4$${\mathrm{O}}_8$${\mathrm{H}}_{\mathit{y}}$. For y<1 the c lattice parameter decreases with y. In contrast to Fe-doped ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$${\mathrm{H}}_{\mathit{y}}$, where ${\mathit{T}}_{\mathit{c}}$ decreases continuously with increasing y, we find here that for 1% Fe doped, ${\mathit{T}}_{\mathit{c}}$=59 K is unaffected by hydrogen charging, but the shielding fraction is reduced sharply from 38% for y=0 to 17% and 3% for y=1 and 1.85, respectively. The Mössbauer spectra for y=0 at T>90 K contain one doublet. The spectra for hydrogen charged samples, in addition to this doublet, contain a magnetic pattern composed of two sextets, between 4.1 K and ${\mathit{T}}_{\mathit{N}}$=360 K. The relative amount of the magnetic pattern depends on hydrogen concentration. The Mössbauer spectrum obtained for y=1 is a superposition of spectra obtained for y=0 and those for y=2, showing a phase separation into magnetic and superconducting regions in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_4$${\mathrm{O}}_8$${\mathrm{H}}_{\mathit{y}}$. NQR-AFNMR studies of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_4$${\mathrm{O}}_8$${\mathrm{H}}_{\mathit{y}}$ and ${\mathrm{GdBa}}_2$${\mathrm{Cu}}_4$${\mathrm{O}}_8$${\mathrm{H}}_{\mathit{y}}$ samples indicate that magnetic order is induced only in the Cu(2) planes.