We have found strong experimental evidence of the dependence of the shape of the magnetic hysteresis loops on the thickness d of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$ single crystals when the width over the thickness is much larger than 1. For crystals with similar widths, the full penetration field ${\mathrm{\mu }}_0$${\mathit{H}}_{\mathit{p}}$, defined experimentally as the merging point between the virgin magnetization curve and the external envelope, increases as d increases. Similarly, the magnetic-field variation needed to reverse completely the magnetization increases with d. Our data are found to support the theoretical work performed by Clem and Sanchez using the Bean model for thin disks. Reverse legs measured at temperatures up to 70 K and for different reversal fields ${\mathrm{\mu }}_0$${\mathit{H}}_{\mathit{m}}$ up to 10 T are described well by this theory.

The Raman spectra of polycrystalline ${\mathrm{YBa}}_2$${\mathrm{Cu}}_4$${\mathrm{O}}_8$ have been measured under various pseudohydrostatic pressures up to 10.6 GPa using a diamond anvil cell at room temperature. Most of the observed phonon scattering peaks shift linearly with increasing pressure. Only the phonon frequency for the in-phase O(2)-O(3) ${\mathit{A}}_{\mathit{g}}$ mode (∼431 ${\mathrm{cm}}^{\mathrm{-}1}$) exhibits nonlinear pressure dependence, which is in contrast to the behavior of the out-of-phase O(2)-O(3) ${\mathit{A}}_{\mathit{g}}$ mode (∼338 ${\mathrm{cm}}^{\mathrm{-}1}$). The frequency shift saturates above ∼6 GPa, which is similar to the pressure dependence of ${\mathit{T}}_{\mathit{c}}$. This suggests that the degree of hybridization between the atomic vibrations of apical and ${\mathrm{CuO}}_2$ plane oxygens may change under a certain pressure.

The double-time Green’s-function decoupling scheme used by Li, Tian, and Jiang [Phys. Rev. B 47, 11 905 (1993)] is non-self-consistent and non-moment-preserving. When replaced by a self-consistent, moment-preserving decoupling scheme, important differences arise. In particular, the results presented by Li et al. for a system of spin S and spin coupling constant ${\mathit{J}}_{\mathit{i}\mathit{j}}$ are, instead, appropriate for a system with spin S/2 and spin coupling constant 2${\mathit{J}}_{\mathit{i}\mathit{j}}$.

We report detection of the low-field–high-field transition in ${\mathrm{KZnF}}_3$:${\mathrm{Cr}}^{3+}$, induced by external pressure, through photoluminescence measurements at both room temperature and T=90 K. Direct measurement of the lowest electronic level separation Δ was accomplished through lower-temperature experiments. Pressure coefficients of Δ and values of the crystal-field and covalence quenched free-ion parameters are given.

We derive an equation for a single-plaquette array of Josephson junctions for an arbitrary number of flux quanta per unit cell f. We show that for f=1/2 this equation is equivalent to one derived for a 2×2 Josephson-junction array in the f=1/2 ground state. We show that in the presence of an rf drive with frequency ν, the system exhibits, for all nonzero values of f, integer and half-integer Shapiro steps at 〈V〉=nhν/4e, where n=1,2,3,. . . . In addition, very small subharmonic Shapiro steps at 〈V〉=nhν/2em, where m=1,2,3,. . . and not equal to n, are observed for all f not equal to zero. These particular steps, however, are found to be consistently smaller than the integer and half-integer steps concurrently present in the array I-V characteristics. Contrary to recent suggestions, we show that single-plaquette array behavior is not entirely consistent with that of large arrays of Josephson junctions and hence not responsible for the display of fractional giant Shapiro steps in these large arrays. Such behavior may, however, be responsible for the recent observation of half-integer steps in high-${\mathit{T}}_{\mathit{c}}$ grain-boundary junctions.

Monte Carlo simulations are performed to study the properties of type-II superconducting films in a magnetic field in which the vortices move in the two-dimensional geometry represented by the surface of a sphere. No numerical evidence is found for any melting transition, although the correlation length over which vortex crystalline order exists grows as the temperature is reduced. At low temperatures, the relaxation time scales are of an Arrhenius form which can be associated with the creation of dislocations of a size comparable to the correlation length.

We report asymmetric current-voltage characteristics in superconducting films in a parallel magnetic field. The critical current, ${\mathit{I}}_{\mathit{c}}$, is found to differ by as much as 30% for negative and positive currents, and beyond ${\mathit{I}}_{\mathit{c}}$, the magnitude of the voltage is also different for positive and negative currents, ‖V(+I)‖≠‖V(-I)‖. The critical current difference ‖${\mathit{I}}_{\mathit{c}+}$‖-‖${\mathit{I}}_{\mathit{c}\mathrm{-}}$‖ has complicated behavior, changing sign as temperature and magnetic field are varied. We propose a model based on the Bean-Livingston surface barrier and inhomogeneous bulk pinning that accounts for all observed behavior.

A study of the structural and electronic phase diagram of Pr-doped ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_4$ is presented in order to investigate whether there is an influence of the high-temperature tetragonal-to-orthorhombic phase transition on the disappearance of superconductivity in the overdoped region. We find that Pr doping shifts the structural phase boundary to higher Sr concentrations, but that superconductivity occurs in the same Sr-concentration range as in ‘‘pure’’ ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_4$. Thus, the disappearance of superconductivity is not related to the high-temperature structural transition, but is determined predominantly by the hole concentration in the CuO planes.

Detailed magnetic hysteresis measurements have been performed with the applied field parallel to the ab plane in single crystals of ${\mathrm{Tl}}_2$${\mathrm{Ba}}_2$${\mathrm{CuO}}_6$. Magnetization in the descending branch of the hysteresis loop is almost zero, indicating the presence of surface barriers which determine the penetration field of vortices. Magnetization with the field tilted slightly to the ab plane shows that the two components of magnetization due to ${\mathit{H}}_{\mathrm{\parallel }}$ and ${\mathit{H}}_{\mathrm{\perp }}$ act independently. The data present clear evidence of a lock-in transition in the compounds studied.

Specific-heat measurements on ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ in magnetic fields to 7 T have shown the presence of Schottky anomalies with the field and temperature dependence expected for a fixed concentration of ${\mathrm{Cu}}^{2+}$ magnetic moments. The field-dependent part of the specific heat also includes a temperature-proportional contribution that is approximately proportional to field. These results are qualitatively different from those reported recently from another laboratory.

We have measured electrical conductances for single-crystal ${\mathrm{Nd}}_2$${\mathrm{CuO}}_{4\mathrm{-}\mathit{x}\mathrm{-}\mathrm{\delta }}$${\mathrm{F}}_{\mathit{x}}$ samples by changing the parameters x(=0.02–0.30) and δ. We confirm that observed conductances can be unifiedly described by a single β function from the weakly to the strongly localized regime. We claim, from our analysis of the data using the β function, that the nonsuperconducting state of ${\mathrm{Nd}}_2$${\mathrm{CuO}}_{4\mathrm{-}\mathit{x}\mathrm{-}\mathrm{\delta }}$${\mathrm{F}}_{\mathit{x}}$ systems is the two-dimensional Fermi-liquid system described by orthogonal symmetry.

Time-differential measurements of nuclear-resonance scattering using synchrotron radiation sources have been performed since 1984. In the present contribution the evaluation of this type of spectra is described. Starting from the theoretical point of view numerical methods are applied to gain hyperfine interaction parameters from time spectra. The conuss program package that was developed can be used to interpret either nuclear Bragg-Laue scattering or nuclear forward scattering. It is also possible to investigate combinations of several resonant samples which is important for comparing measurements.

Combining a diamond anvil cell and a laser heating system, pure ${\mathrm{ZrO}}_2$ powders were heated above 1000 °C under high pressures above 20 GPa. Thermally quenched samples were investigated by using x-ray diffraction under high pressure. The results indicated that a new high-pressure phase was formed. This new phase was retained in the course of releasing pressure down to 1 GPa but reverted to the monoclinic phase on the complete release of pressure. Thirteen diffraction lines were confirmed to be from the new phase and indexed on the basis of hexagonal symmetry. The molar volume of this phase is larger than that of orthoII, which appears in the low-temperature region of the same pressure range. Its structure is derived from that of orthoII which is composed of pseudohexagonal packing of oxygen.

The phason dispersion and the linear electro-optic response in the ferroelectric Sm ${\mathit{C}}^{\mathrm{*}}$ liquid-crystalline phase have been measured in an external magnetic field up to 14 T. In the presence of an external field the originally continuous phason spectrum splits into an opticlike and an acousticlike phason branch, separated by a field-dependent gap. The band structure of the phason spectrum in the presence of the field is a consequence of the broken helical symmetry of the Sm ${\mathit{C}}^{\mathrm{*}}$ phase and is analogous to the energy spectrum of an electron in a crystal lattice. The observed magnetic-field dependence of the phason spectrum is in good agreement with theory.

Hyper-Raman and Raman spectra of ferroelectric ${\mathrm{LiTaO}}_3$ are observed in the temperature range from 14 to 1200 K, which includes the phase-transition temperature at about 890 K. In the ferroelectric phase, no evidence of the softening of the ${\mathit{A}}_1$ phonon is found in the spectra. However, the hyper-Raman spectra show a strong Debye-type relaxational mode in both phases. The hyper-Raman spectra clearly show that the phase transition of ${\mathrm{LiTaO}}_3$ is of the order-disorder type. The anomalous line shape of an ${\mathit{A}}_1$ phonon band at 203 ${\mathrm{cm}}^{\mathrm{-}1}$ is interpreted in terms of the coupling between this phonon and the relaxational modes. The coupling model also explains the difference between the line shapes of the Raman and hyper-Raman spectra.

A method for calculating ‘‘constant volume’’ pair potentials in Al-rich transition-metal compounds is presented. The method is based on a combination of a Green’s-function analysis of the interaction between transition-metal d shells, and a perturbative treatment of the Al and transition-metal pseudopotentials. In addition to the d-shell interactions, the pair potentials include terms describing interactions between pseudopotentials on different atoms, as well as interactions between the pseudopotential on one atom and the charge density induced by the d shell on another atom. The potentials are parametrized by use of both ab initio total-energy results and experimental inputs. The methodology is tested by calculating the energies of many different structures at the Al-rich end of the Al-Co phase diagram. The potentials correctly predict the sequence of complex phases that occurs with increasing Co content, with just one exception. The complex phases are favored relative to simpler fcc-based phases by a large energy due mainly to short-ranged spacing constraints. The Co-Co potential has a secondary minimum at 4.4 Å which coincides with the Co-Co spacings in the complex structures, and thus further enhances their stability.

${}_{}{}^{27}{}_{}{}^{}\mathrm{Al}$ NMR results are presented for three Ni-Al alloys with composition 62%, 62.5%, and 63% Ni undergoing a thermoelastic martensitic phase transformation below room temperature. By a method of analysis of the NMR data recently developed by us [Phys. Rev. B 46, 10 563 (1992)], we measure the growth of the volume fraction of martensite on cooling for samples having different particle sizes (1 mm–50 μm). We observe an incomplete transformation in the fine-particle sample (less than 150 μm). The effect can be explained as due to the presence of austenite trapped in intramartensitic regions near the center of the particles.

Extensive molecular-dynamics simulations have been performed within a classical force-field model for the pressure-induced amorphization of quartz. In agreement with earlier molecular-dynamics studies, we find that a phase transition occurs within the experimental pressure range of the amorphization transformation. However, at variance with previous interpretations, we find that the resulting phase is not amorphous. The correlation functions of the equilibrated structure can be shown to be consistent with those of a crystalline phase. Two transformations to ordered structures occur sequentially during the simulations. The first transformation is likely to be related to the recently discovered transition of quartz to an intermediate crystalline phase before its amorphization. The second transformation, instead, yields a compact octahedrally coordinated Si sublattice. The latter structure may be an artifact of the classical force field.

The lattice constants of diamond crystals consisting of an isotopic mixture of ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}_{\mathit{x}}^{12}{}_{}{}^{}$${\mathrm{C}}_{\left(1\mathrm{}\mathrm{-}\mathit{x}\right)}$ were measured as a function of the isotopic composition by single-crystal diffractometry using the synchrotron x-ray radiation. The isotope ratio of the synthesized diamond samples was determined by secondary-ion-mass spectroscopy. The lattice constant can be expressed in quadratic form as a(X) =3.567 12-9.0×${10}^{\mathrm{-}4}$X+3.7×${10}^{\mathrm{-}4}$${\mathit{X}}^2$, where X=${[}^{13}$C]/(${[}^{12}$C]+${[}^{13}$C]), in contrast to the linear relation reported by Holloway, Hass, and Tamor [Phys. Rev. B 45, 6353 (1992)], namely a(X)=3.567 14-5.4×${10}^{\mathrm{-}4}$X.