Ultrahigh-resolution (ΔE up to 10 meV) angle-resolved photoemission spectroscopy on single crystals of ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{8+\mathrm{\delta }}$ (Bi-2212) show a highly anisotropic superconducting energy gap: the gap is minimized, with a value close to zero, along the Γ-X and Γ-Y symmetry directions, while a large gap (Δ=22 meV) is observed along the Γ-M¯ (Cu-O bond) direction. However, the observation of gap anisotropy may depend on sample conditions; in a few cases the gap becomes more isotropic. A dip in the spectral weight at 65 meV below the Fermi energy is clearly observed only in samples with an anisotropic gap.

A theoretical technique is described, employing Monte Carlo simulation, which allows the investigation of strain effects associated with transformations in solids. The method applies a grand canonical stress ensemble in the study of elasticity during phase evolution in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{z}}$ where both oxygen occupancy and atomic positions in the basal plane, as well as total sample volume, can vary. Quenching experiments yield both metastable √2${\mathit{a}}_0$×√2${\mathit{a}}_0$ superstructures from short-wavelength modulations and a tweed texture from long-period 〈110〉 modulations of the ordered z=7.0 compound.

Two cluster algorithms, based on constructing and flipping loops, are presented for world-line quantum Monte Carlo simulations of fermions and are tested on the one-dimensional repulsive Hubbard model. We call these algorithms the loop-flip and loop-exchange algorithms. For these two algorithms and the standard world-line algorithm, we calculated the autocorrelation times for various physical quantities and found that the ordinary world-line algorithm, which uses only local moves, suffers from very long correlation times that makes not only the estimate of the error difficult but also the estimate of the average values themselves difficult. These difficulties are especially severe in the low-temperature, large-U regime. In contrast, we find that new algorithms, when used alone or in combinations with themselves and the standard algorithm, can have significantly smaller autocorrelation times, in some cases being smaller by three orders of magnitude. The new algorithms, which use nonlocal moves, are discussed from the point of view of a general prescription for developing cluster algorithms. The loop-flip algorithm is also shown to be ergodic and to belong to the grand canonical ensemble. Extensions to other models and higher dimensions are briefly discussed.

Electronic-Raman (ER) transitions from the ground level of the ${}_{}{}^2{}_{}{}^{}$${\mathit{F}}_{5/2}$ manifold to other 4f levels of trivalent cerium (${\mathrm{Ce}}^{3+}$) in yttria (${\mathrm{Y}}_2$${\mathrm{O}}_3$) have been studied as a function of excitation wavelength and temperature. ER transitions were observed for ${\mathrm{Ce}}^{3+}$ occupying both the ${\mathit{C}}_{3\mathit{i}}$ and ${\mathit{C}}_2$ sites in ${\mathrm{Y}}_2$${\mathrm{O}}_3$. Energy-level positions of the Kramer doublets of ${\mathrm{Ce}}^{3+}$ in the ${\mathit{C}}_{3\mathit{i}}$ site were determined from this measurement. The levels of the ${}_{}{}^2{}_{}{}^{}$${\mathit{F}}_{5/2}$ manifold were at 978 and 1503 ${\mathrm{cm}}^{\mathrm{-}1}$, while three of the four levels of the ${}_{}{}^2{}_{}{}^{}$${\mathit{F}}_{7/2}$ manifold were at 2270, 2996, and 3402 ${\mathrm{cm}}^{\mathrm{-}1}$ above the ground level. Previous literature values of three of the four energy levels of the ${}_{}{}^2{}_{}{}^{}$${\mathit{F}}_{7/2}$ manifold of the ${\mathit{C}}_2$ site were confirmed. A level of the ${}_{}{}^2{}_{}{}^{}$${\mathit{F}}_{5/2}$ manifold of ${\mathrm{Ce}}^{3+}$ in the ${\mathit{C}}_2$ site at 666 ${\mathrm{cm}}^{\mathrm{-}1}$ was also identified. For resonant excitation involving a 5d intermediate state, two-orders-of-magnitude enhancement in the ER transition strength was observed. This resonant enhancement effect makes the electronic-Raman-scattering technique site selective, since for resonant excitation of the intermediate state of a particular site the ER lines of that site become more intense than those belonging to the other site. The differential scattering cross section for the transition from the ground state to the lowest ${}_{}{}^2{}_{}{}^{}$${\mathit{F}}_{7/2}$ level was measured to be 2.2×${10}^{\mathrm{-}30}$ ${\mathrm{cm}}^2$ ${\mathrm{sr}}^{\mathrm{-}1}$ ${\mathrm{ion}}^{\mathrm{-}1}$ for the ${\mathit{C}}_2$ site, and 8.8×${10}^{\mathrm{-}30}$ ${\mathrm{cm}}^2$ ${\mathrm{sr}}^{\mathrm{-}1}$ ${\mathrm{ion}}^{\mathrm{-}1}$ for the ${\mathit{C}}_{3\mathit{i}}$ site. These experimental values of cross sections are compared to the theoretical estimates.

A method of determination of the local diffusion coefficients in the grain bounday (GB) core as well as coefficients of atomic-probe (AP) ‘‘pumping’’ from GB’s into adjacent crystalline regions is developed. This method is based on (i) measurements of the absolute areas of the components of the nuclear γ-resonance spectra after the introduction by diffusion of atomic probes into the polycrystal, which is capable of giving spectral information about the populated states in the GB diffusion zone; (ii) measurements of residual activities, characterizing absolute AP quantities in the GB diffusion zone. It is shown that, only at high temperatures (T≥0.5${\mathit{T}}_{\mathrm{melt}}$), the measured ‘‘pumping’’ coefficients coincide with those of the volume diffusion coefficients. At low temperatures (T≃0.2${\mathit{T}}_{\mathrm{melt}}$) the ‘‘pumping’’ coefficients are much higher than the volume diffusion coefficients, which provides the known peculiarities of temperature dependencies of the two-dimensional flux parameter in polycrystals at low temperatures. Diffusion coefficients in the GB core are 10 orders of magnitude higher than the ‘‘pumping’’ coefficients in the investigated temperature interval.

A variational calculation is presented for large bipolarons in the strong-coupling limit. By means of the phonon coherent state, two coupled self-consistent integrodifferential equations for the center-of-mass and relative motions have been derived. Compared with previous works, lower binding energies for large bipolarons in three dimensions (3D) and two dimensions (2D) have been calculated. As a result, the values of lower bounds to critical ratios between dielectric constants below which bipolarons can exist are obtained as ${\mathrm{\eta }}_{\mathit{c}}$=1.23 in 3D and ${\mathrm{\eta }}_{\mathit{c}}$=1.64 in 2D.

In this paper we describe an electron heating experiment performed on thin, free-standing metallic films in a vacuum. In the steady state, the resistance change with electric field is found to be different to that observed on similar films supported by a substrate. By considering the power balance between the electrons and phonons in the film, we show that the phonon mean free path is close to the film thickness and that the electron-phonon coupling strength is consistent with other measurements.

The Brillouin light scattering technique was used to determine the complex mechanical modulus, which describes the dynamic response of molecular structures, for ${\mathrm{B}}_2$${\mathrm{O}}_3$ and binary alkali borate. The effects of temperature, alkali concentration, oxygen and water vapor partial pressures on the structural developments and on the thermal activation of dissipative processes were examined. The glass transition in these systems is characterized by a discontinuity in the temperature dependence of the elastic component of this modulus. Above ${\mathit{T}}_{\mathit{g}}$, the elastic modulus decreases with a faster rate, the higher the alkali concentration. The complete structural evolution from a room temperature glass to a liquid near the boiling point was found to involve several distinct mechanisms, which become gradually activated with increasing temperature. By using a mechanical relaxation formalism, the activation energies and preexponential time constants describing the mechanical degradation, as well as the molecular rearrangements associated with each mechanism were determined. For a given system, the initial network degradation is characterized by the smallest activation energy. The motion involved in this process is that of boron atoms oscillating between triangular and tetrahedral coordination, upon exchanging one of their oxygen neighbors. During this phase boroxol rings open, without the formation of nonbridging oxygens. At intermediate temperatures the motion of alkali cations between network segments is activated, and at very high temperatures complete network disintegration takes place, leaving ionic species whose motion occurs by complete dissociation from their immediate neighbors.

We report the measurement of infrared transmission of large ${\mathrm{C}}_{60}$ single crystals. The spectra exhibit a very rich structure with over 180 vibratonal absorptions visible in the 100–4000 ${\mathrm{cm}}^{\mathrm{-}1}$ range. Many silent modes are observed to have become weakly IR active. We also observe a large number of higher-order combination modes. The temperature (77–300 K) and pressure (0–25 kbar) depedences of these modes were measured and are presented. Careful analysis of the IR spectra, in conjuction with Raman scattering data showing second-order modes and neutron scattering data, allows the selection of the 46 vibrational modes of ${\mathrm{C}}_{60}$. We are able to fit all of the first- and second-order data seen in the present IR spectra and the previously published Raman data (∼300 lines total), using these 46 modes and their group-theory-allowed second-order combinations.

Ferromagnetic resonance (FMR) can reveal important information on the size and shape of the ferromagnetic particles which are dispersed in granular giant magnetoresistive (GMR) materials. We have investigated the FMR spectra of three different types of granular GMR material, each with different properties: (1) melt-spun ribbons of ${\mathrm{Fe}}_5$${\mathrm{Co}}_{15}$${\mathrm{Cu}}_{80}$ and ${\mathrm{Co}}_{20}$${\mathrm{Cu}}_{80}$, (2) thin films of ${\mathrm{Co}}_{20}$${\mathrm{Cu}}_{80}$ produced by pulsed laser deposition, and (3) a granular multilayer film of [Cu(50 Å)/Fe(10 Å)]×50. We interpret the linewidth of these materials in as simple a manner as possible, as a ‘‘powder pattern’’ of noninteracting ferromagnetic particles. The linewidth of the melt-spun ribbons is caused by a completely random distribution of crystalline anisotropy axes. The linewidth of these samples is strongly dependent upon the annealing temperature: the linewidth of the as-spun sample is 2.5 kOe (appropriate for single-domain particles) while the linewidth of a melt-spun sample annealed at 900 °C for 15 min is 4.5 kOe (appropriate for larger, multidomain particles). The linewidth of the granular multilayer is attributed to a restricted distribution of shape anisotropies, as expected from a discontinuous multilayer, and is only 0.98 kOe when the applied magnetic field is in the plane of the film.

We have measured magnetic excitations in dilute crystals of the three-dimensional anisotropic antiferromagnet ${\mathrm{Fe}}_{\mathit{x}}$${\mathrm{Zn}}_{1\mathrm{-}\mathit{x}}$${\mathrm{F}}_2$ with x=0.69, 0.59, and 0.31 using inelastic-neutron-scattering techniques. Well-defined spin excitations at the higher concentrations are not observed above the transition temperature. A magnetic excitation spectrum with significant structure is observed at low temperature. In the case of x=0.59, we observe three nearly dispersionless intensity peaks in the energy scans. The lowest-energy peak dominates near the zone center. The three energy peaks become comparable in intensity as the zone boundary is approached. The structure in the peaks, if measured under conditions of low instrumental resolution, could appear as damping away from the zone center, consistent with the behavior reported previously for the similar but less anisotropic system ${\mathrm{Mn}}_{0.5}$${\mathrm{Zn}}_{0.5}$${\mathrm{F}}_2$, where the apparent damping has been identified with ‘‘fracton’’ effects. We argue that such an interpretation is, for the most part, inappropriate for ${\mathrm{Fe}}_{\mathit{x}}$${\mathrm{Zn}}_{1\mathrm{-}\mathit{x}}$${\mathrm{F}}_2$ for x≥0.31, where the percolation threshold is ${\mathit{x}}_{\mathit{p}}$=0.24, and highly questionable for the ${\mathrm{Mn}}_{0.5}$${\mathrm{Zn}}_{0.5}$${\mathrm{F}}_2$ system.

Nuclear magnetic resonance (NMR) and low-temperature magnetization measurements are used to demonstrate antiferromagnetic (AF) coupling in molecular beam epitaxy grown (111)-oriented Co/Pt superlattices. Drastic changes in the NMR spin-echo spectra as a function of the in-plane dc magnetic-field strength provide conclusive evidence of AF interlayer coupling in these structures, consisting of 35- and 52-Å Co and 18-Å Pt bilayers. The structure of the Co films is mainly hcp (70%) and the coupling energy per unit area, as determined from in-plane magnetization loops is found to range up to 1 erg/${\mathrm{cm}}^2$.

Stage-2 ${\mathrm{Cu}}_{\mathit{c}}$${\mathrm{Co}}_{1\mathrm{-}\mathit{c}}$${\mathrm{Cl}}_2$-graphite intercalation compounds (GIC’s) (0≤c≤1) provide ideal two-dimensional random spin systems with a spin frustration effect arising from competing ferromagnetic and antiferromagnetic interactions. The magnetic properties of these compounds have been studied by dc and ac magnetic susceptibility, and superconducting quantum interference device magnetization. The sign of the Curie-Weiss temperature changes from positive to negative with increasing concentration around c=0.80 to 0.85. The intraplanar exchange interaction J(Cu-Co) between ${\mathrm{Cu}}^{2+}$ and ${\mathrm{Co}}^{2+}$ spins is ferromagnetic and depends on the Cu concentration. These systems with c<0.9 undergo a ferromagnetic phase transition at the critical temperature ${\mathit{T}}_{\mathit{c}}$. The irreversible effect of magnetization is observed below ${\mathit{T}}_{\mathit{c}}$. The low-temperature phase below ${\mathit{T}}_{\mathit{c}}$ may correspond to a cluster glass phase where the spin direction of ferromagnetic clusters is frozen because of frustrated interisland interactions which include a dipole-dipole interaction and an interplanar antiferromagnetic interaction. The critical temperature ${\mathit{T}}_{\mathit{c}}$ increases as c increases and exhibits a broad maximum around c=0.5. This enhancement of ${\mathit{T}}_{\mathit{c}}$ is partly due to the ferromagnetic interaction J(Cu-Co). No magnetic phase transition is observed for 0.9<c<1 partly because of the spin frustration effects arising from (i) the competition between ferromagnetic J(Cu-Co) and antiferromagnetic J(Cu-Cu) interactions, and (ii) the fully frustrated nature of the antiferromagnet on the triangular lattice.

The structural and magnetic properties of vermiculite intercalation compounds (VIC’s) with two-water-layer hydration states having ${\mathrm{Ce}}^{3+}$, ${\mathrm{Pr}}^{3+}$, ${\mathrm{Nd}}^{3+}$, ${\mathrm{Sm}}^{3+}$, ${\mathrm{Eu}}^{3+}$, ${\mathrm{Gd}}^{3+}$, ${\mathrm{Tb}}^{3+}$, ${\mathrm{Dy}}^{3+}$, ${\mathrm{Ho}}^{3+}$, and ${\mathrm{Er}}^{3+}$ ions in the interlamellar space between the host silicate layers have been studied by means of (00L) x-ray scattering, dc magnetic susceptibility, and thermogravimetric measurements. The intercalate layer sandwiched between two water layers consists of u cations and w water molecules per unit rectangular cell of (a×b): u=0.5–0.69, w=0–4.0. The c-axis repeat distance is found to be roughly proportional to the ionic radius, giving indirect evidence for the strong rigidity of the host silicate layers. The magnetic susceptibility of these compounds shows a Curie-Weiss behavior. The Curie-Weiss temperature FTHETA for ${\mathrm{Ce}}^{3+}$, ${\mathrm{Pr}}^{3+}$, ${\mathrm{Nd}}^{3+}$, and ${\mathrm{Sm}}^{3+}$ is described by FTHETA=-1.58J(J+1) [K], while FTHETA for ${\mathrm{Gd}}^{3+}$, ${\mathrm{Tb}}^{3+}$, ${\mathrm{Dy}}^{3+}$, ${\mathrm{Ho}}^{3+}$, and ${\mathrm{Er}}^{3+}$ is described by FTHETA=0.93(${\mathit{g}}_{\mathit{J}}$-1${)}^2$J(J+1) [K]. The ${\mathrm{Eu}}^{3+}$ VIC shows a Van Vleck susceptibility in the temperature range between 50 and 300 K.

First-principles calculations of the itinerant-electron magnetic form factor for ferromagnets which include both the spin and orbital contributions have been completed for ferromagnetic nickel and iron. To our knowledge, these are the first calculations of this type for 3d ferromagnets. These calculations also include, by necessity, the spin-orbit interaction. Numerical results based on spin polarized bands used previously by Cooke et al. [Phys. Rev. B 21, 4118 (1980)] have been found to be in good agreement with experiment while results based on local density bands were found to be in good agreement for iron but not for nickel. In addition, it has been shown that if the energy dependence of the radial part of the wave functions can be ignored then the dominant terms in the itinerant-electron form factor can be written in a form almost identical to the atomic form factor. This provides an explanation for why fits to the atomic form factor have been so successful.

The observed crystal-field magnetostriction and its complex temperature dependence have been calculated for the itinerant iron-rich uniaxial ferromagnet ${\mathrm{Y}}_2$${\mathrm{Fe}}_{17}$, using a simple, although highly efficient, type of rigid-band Stoner model. We have evaluated the irreducible magnetostriction modes ${\mathrm{\epsilon }}^{\mathrm{\alpha },1\mathrm{}}$ and ${\mathrm{\epsilon }}^{\mathrm{\alpha },2\mathrm{}}$, which represent the volume and c-axis pure tetragonal strains, respectively, as well as their temperature dependencies. Good agreement with the available experimental results has been attained. As a result, values of the corresponding microscopic magnetoelastic coupling coefficients, ${\mathit{M}}_{12}^{\mathrm{\alpha }}$ and ${\mathit{M}}_{22}^{\mathrm{\alpha }}$, have been obtained. Those values are very large (around -${10}^3$ K/Fe atom), as expected for an intermetallic compound with a 3d shell transition element.

In this work we study the Heisenberg XXZ antiferromagnetic model for spin S and dimension d in the presence of an external longitudinal magnetic field. First, we essay a variational approach which uses as trial function a version of the analytic expression for the ground state given by the paired nonmagnetic excitation (PNME) theory [M. Lagos and G. G. Cabrera, Solid State Commun. 67, 221 (1988); Phys. Rev. B 38, 659 (1988)], generalized in order to incorporate the external field. The ground-state energy, sublattice magnetization, and magnetic susceptibility are obtained. Subsequently, we solve the problem numerically for a chain of 12 spins (S=1 and S=3/2) using the Lanczös method [E. Dagotto and A. Moreo, Phys. Rev. D 31, 865 (1985)]. The two approaches give excellent concordance over a wide range of the parameters of the model. We show that our analytic trial function represents accurately the ground state of the system for anisotropies ranging from the Ising limit to the almost isotropic Heisenberg model for all values of the field. Moreover, it accounts for the several antiferromagnetic and ferromagnetic phases occurring for different values of the magnetic field with the same precision. The critical fields of the transitions are predicted correctly. Also, results for various spin values S in two and three dimensions are presented.

The equiatomic ternary compound YbPtGa has been investigated by neutron diffraction, magnetic susceptibility, electrical resistivity, and low-temperature heat capacity measurements. A profile refinement of neutron diffraction data reveals that YbPtGa crystallizes in the orthorhombic TiNiSi-type structure. A λ-type anomaly is observed in the heat capacity at 3.8 K, and a broad maximum in the susceptibility is seen at 4 K, suggesting magnetic ordering of the Yb moment. The temperature dependence of the magnetic scattering resistivity exhibits a behavior consistent with Kondo-type interactions in the presence of crystalline electric field effects.

Applying the bosonization procedure to weakly coupled Hubbard chains, we discuss the fixed points of the renormalization-group procedure where all spin excitations are gapful and a singlet pairing becomes the dominant instability.

Consider a d-dimensional antiferromagnet with a quantum disordered ground state and a gap to bosonic excitations with nonzero spin. In a finite external magnetic field, this antiferromagnet will undergo a phase transition to a ground state with nonzero magnetization, describable as the condensation of a dilute gas of bosons. The finite-temperature properties of the Bose gas in the vicinity of this transition are argued to obey a hypothesis of zero scale-factor universality for d<2, with logarithmic violations in d=2. Scaling properties of various experimental observables are computed in an expansion in ε=2-d, and exactly in d=1.