Investigations of two-magnon Raman scattering (RS) were carried out on ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6+\mathit{x}}$ single crystals with oxygen content near the metal-insulator transition. It was found that the chain oxygen reordering processes strongly influence the two-magnon RS spectrum.

The site-selective substitution of ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$ for ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ in ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{Ca}}_{0.8}$${\mathrm{Y}}_{0.2}$${\mathrm{Cu}}_2$${\mathrm{O}}_{8+\mathrm{\delta }}$ has been investigated by Raman spectroscopy. Three prominent Raman bands are observed at 297, 464, and 623 ${\mathrm{cm}}^{\mathrm{-}1}$. They all shift to lower wave numbers as the ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$ content increases. The bands at 297 and 464 ${\mathrm{cm}}^{\mathrm{-}1}$ shift at similar rates but the band at 623 ${\mathrm{cm}}^{\mathrm{-}1}$ shifts at a slower rate. These results show that all three bands are due to the vibrations involving oxygen atoms, and that the rate of oxygen substitution is site dependent. Based on these experimental results, tentative mode assignments have been made assuming a tetragonal symmetry: 623 ${\mathrm{cm}}^{\mathrm{-}1}$→${\mathrm{O}}_{\mathrm{pl}}$(${\mathit{A}}_{\mathit{g}}$), 464 ${\mathrm{cm}}^{\mathrm{-}1}$→${\mathrm{O}}_{\mathrm{ap}}$, and 297 ${\mathrm{cm}}^{\mathrm{-}1}$→${\mathrm{O}}_{\mathrm{Bi}}$.

We report a measurement of an axially symmetric g tensor for the substitutional nitrogen center in type-IIa synthetic isopure ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ diamond. Because the nitrogen concentration of the diamond studied is exceptionally low the electron-paramagnetic-resonance linewidth is sufficiently narrow to reveal the g-tensor anisotropy. Studies of the angular dependence reveal [111] symmetry and an axial g tensor in which ${\mathit{g}}_{\mathit{p}\mathit{e}\mathit{r}\mathit{p}}$-${\mathit{g}}_{\mathrm{\parallel }}$=0.0001±0.000 03. From the narrow linewidth, we observe that the deviation from the [111] symmetry exhibited by the nitrogen center is less than 0.5°.

Through the coupling between the electrical capacitance of a spherical piezoceramic shell and the mechanical stiffness of a liquid contained therein, the frequency-dependent adiabatic bulk modulus ${\mathit{K}}_{\mathit{s}}$(ω) of the liquid can be derived. Using this method, ${\mathit{K}}_{\mathit{s}}$(ω) of glycerol in the range 15 Hz–15 kHz has been measured at the glass transition. The loss peak frequencies of the compressibility ${\mathrm{\kappa }}_{\mathit{s}}$(ω)=${\mathit{K}}_{\mathit{s}}^{\mathrm{-}1}$(ω) and the specific heat ${\mathit{c}}_{\mathit{p}}$(ω) have the same temperature dependence, but the frequency dependences are different.

The ground-state wave function and energy of the Hubbard model with unlimited hopping can be obtained for arbitrary fillings using the transformation recently introduced by Brandt and Giesekus. A simple linear behavior is obtained for the ground-state energy dependence on the number of particles: ${\mathit{E}}_{\mathrm{GS}}$=-2t(L-N), where -t is the hopping energy (t>0), L is the number of sites, and N is the number of electrons. The ground state shows a very large total spin degeneracy. The system is shown to behave as a Fermi liquid with a spectral weight Z=1-N/L for all fillings less than 1 (N≤L).

Systematic impurity doping in the Cu-O plane of the hole-doped cuprate superconductors may allow one to decide between unconventional (‘‘d-wave’’) and anisotropic conventional (‘‘s-wave’’) states as possible candidates for the order parameter in these materials. We show that potential scattering of any strength always increases the gap minima of such s-wave states, leading to activated behavior in temperature with characteristic impurity concentration dependence in observable quantitites such as the penetration depth. A magnetic component to the scattering may destroy the energy gap and give rise to conventional gapless behavior, or lead to a nonmonotonic dependence of the gap on impurity concentration. We discuss how experiments constrain this analysis.

A cylindrical Josephson junction with a spatially dependent Josephson coupling averaging to zero over the length of the junction is studied as a model for the physics of a disk of a d-wave superconductor embedded in a superconducting film of a different symmetry. It is found that the system always introduces Josephson vortices in order to gain energy at the junction. The critical current is calculated. It is argued that a recent experiment that was claimed to provide evidence for s-wave superconductivity in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ may also be consistent with d-wave superconductivity.

The puzzling appearance of a large resistance peak in the superconducting state near ${\mathit{T}}_{\mathit{c}}$ of mesoscopic superconducting wires is analyzed. It is shown that this resistance anomaly can be explained in terms of thermal fluctuations producing phase slips of the superconducting order parameter in quasi-one-dimensional wires. Good quantitative agreement with the experimental observations is obtained within the framework of the modified Langer-Ambegaokar and McCumber-Halperin models.

Tunneling spectra of a polycrystalline bulk ${\mathrm{HgBa}}_2$${\mathrm{Ca}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{8+\mathrm{\delta }}$ with ${\mathit{T}}_{\mathit{c}}$=132 K have been measured by point-contact spectroscopy using a low-temperature scanning tunneling microscope. Unlike other high-${\mathit{T}}_{\mathit{c}}$ superconductors, the conductance of this material shows a BCS-like gap structure with a relatively small smearing parameter Γ. Conductances with a linear density of states for ‖eV‖<Δ and a small cusp at the gap are also observed. From a simple calculation based on the assumption that the surface is covered with a normal layer, we explained the linear dI/dV for ‖eV‖<Δ and obtained two distinctively different energy gaps Δ=48 meV and 22 meV with the resultant 2Δ/${\mathit{k}}_{\mathit{B}}$${\mathit{T}}_{\mathit{c}}$=8.5 and 3.9, respectively.

The peak effect in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ single crystals is observed by transport-current measurements. Comparison with the shielding current from magnetization shows good agreement between the results, which exclude granularity as being necessarily involved in the origin of the peak effect. The behavior of the current-voltage characteristics in a wide voltage range (${10}^{\mathrm{-}9}$<E<${10}^{\mathrm{-}4}$ V/cm) is analyzed. Different types of scaling are established in magnetic fields below and above the position of the current peak. Possible mechanisms responsible for the peak effect are discussed.

Systematic effects of driving current and magnetic field on the observed reentrant resistive transition in ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_8$/${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{Ca}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{10}$ intergrowth single crystals have been investigated. The results are explained in terms of our previously proposed model where the Josephson coupling between noncontinuous ${\mathrm{CuO}}_2$ trilayers is weakened by thermally activated resistance between these layers. The present results reveal that the Josephson coupling between these trilayers is further weakened by applying a magnetic field or a large driving current, leading to a systematic change in the minimum resistivity value and the associated temperature for the reentrant region. The good agreement between the experimental data and theoretical calculation further shows the suitability of our model to layered superconductors like ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_8$.

The hydrogen ordering processes in the β phase of the rare-earth hydride system ${\mathrm{CeH}}_{2+\mathit{x}}$ are analyzed. It is assumed that, within the discussed concentration range (x<0.75), the stable states of the metal-hydrogen system are homogeneous monophase solutions with fixed concentrations. The description of ordering is based upon the mean-field approximation of the static-concentration-waves theory. The relevant energy constants describing the H-H interactions are determined using available experimental data corresponding to the order-disorder transformation temperatures. It is shown that in the system under consideration there exists (in the concentration range x≥0.35), besides the usual order-disorder transformation, an additional order-order transition at low temperatures which, in some cases (for x>0.44), has a two-step character.

NMR signal intensities of ${}_{}{}^{181}{}_{}{}^{}\mathrm{Ta}$ in two crystals of ${\mathrm{KTa}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Nb}}_{\mathit{x}}$${\mathrm{O}}_3$ (KTN) (x=0.021, 0.033; 10 K<T<200 K and p≤4 kbar) are compared with predictions of a continuum percolation model, using parameters extrapolated from pressure-dependent dielectric data in the literature (x=0.012, 0.023). The essential observation is that at low temperature, a p- and x-dependent fraction of ${\mathrm{KTaO}}_3$ unit cells remains unpolarized. The (p,x) variation of this fraction can be explained by the percolation model. Extrapolation to x=0 predicts a phase transition at zero K for negative pressures, in quantitative agreement with an analysis of available data (at positive p) for pure ${\mathrm{KTaO}}_3$.

Owing to macroscopic electric fields arising from thermal fluctuations of a ferroelectric domain wall, the attraction energy of two parallel walls decays at large distances between the walls (h) as ${\mathit{h}}^{\mathrm{-}2}$ with a prefactor that is proportional to T and to a ratio of dielectric constants that is equal to unity for the isotropic medium. For proper ferroelectrics, and improper ones with a quadratic dependence of the spontaneous polarization on the order parameter, this law is valid for all h greater than the domain wall thickness (which is the minimum length of the problem). For improper ferroelectrics with cubic (or higher-order) dependence of the polarization on the order parameter, there exists a region with a logarithmic dependence of the interaction energy on h. This attraction, which proves to prevail over the Van der Waals one, leads to a discontinuity of the ferroelectric-incommensurate transition. Estimation of the period of the incommensurate phase at the transition provides a reasonable order of magnitude and shows that this period does not depend much on the crystal parameters other than the width of the domain wall. The interaction of the ferromagnetic and ferroelastic domain walls is qualitatively analogous to that of the ferroelectric ones. The role of quantum fluctuations becomes important at not very low temperatures. For nonferroelectric, nonferroelastic, nonferromagnetic domain walls the power-law fluctuation-induced attraction is shown to arise as well.

Extrapolating the results of hybrid quantum Monte Carlo simulations to the zero temperature and infinite-chain-length limits, we calculate the ground-state phase diagram of a system of quantum particles on a chain of harmonically coupled, symmetric, quartic double-well potentials. We show that the ground state of this quantum chain depends on two parameters, formed from the ratios of the three natural energy scales in the problem. As a function of these two parameters, the quantum ground state can exhibit either broken symmetry, in which the expectation values of the particle’s coordinate are all nonzero (as would be the case for a classical chain), or restored symmetry, in which the expectation values of the particle’s coordinate are all zero (as would be the case for a single quantum particle). In addition to the phase diagram as a function of these two parameters, we calculate the ground-state energy, an order parameter related to the average position of the particle, and the susceptibility associated with this order parameter. Further, we present an approximate analytic estimate of the phase diagram and discuss possible physical applications of our results, emphasizing the behavior of hydrogen halides under pressure.

In this paper we report the different hyperfine structures of the 594-nm optical transition in several ${\mathrm{CaF}}_2$:${\mathrm{Pr}}^{3+}$:${\mathrm{Sr}}^{2+}$ crystals as well as the associated optical-microwave double-resonance behavior. Using the observed signals with different ${\mathrm{Sr}}^{2+}$ dopant concentrations, the consequences of strain on the optical line shape and the optical-microwave double-resonance transitions is discussed.

A critical analysis of the meaning of universality at shock pressures is presented, and a careful comparison is made of previous work by the present authors and other approaches to universality. To illustrate some relevant points in the discussion we shall consider in particular an interesting approach recently proposed to obtain universal relationships describing the high-pressure behavior of matter. The approximations made, the basic equations, the different kinds of scaled variables, and the various types of plots currently used are some of the points thoroughly discussed and compared in different approaches to universality.