We report our studies on the charging effect in metal-insulator-superconductor, thin-film structures with thick (∼100 nm), partially oxygen-depleted ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{y}}$ (YBCO) channels. The devices were fabricated in YBCO-on-${\mathrm{SrTiO}}_3$ films with a laser-writing patterning technique. The ${\mathrm{SrTiO}}_3$ substrates were used as gate insulators. A number of test structures, consisting of the oxygen-deficient (superconducting at 10–60 K) channel and fully oxygenated (superconducting above 85 K) source and drain electrodes, were measured. We found that the electric field applied to our as-fabricated structures complexly affected their superconducting properties, causing both reversible and irreversible modifications of the channel’s critical temperature and critical current. The channel sensitivity to the electric field disappeared in our samples after several days of room-temperature aging; however, it could be subsequently recovered, again only temporarily, by hydrogen doping. The charging effect was particularly pronounced in structures with the lowest critical temperatures of the channel. We associate the behavior observed in our samples with charge-transfer effects related to electric-field-enhanced changes of the crystalline order in oxygen-deficient YBCO.

We have calculated the widths of the integer and half-integer voltage steps in a square Josephson-junction single plaquette as a function of ac level for various filling factors f. The characteristic features of the step widths, corresponding to n=0, 1/2, and 1, show clear differences between small and large values of f, and are in reasonable agreement with the results observed experimentally in high-${\mathit{T}}_{\mathit{c}}$ single grain-boundary junctions. When the inhomogeneity in the critical current of the junctions parallel and perpendicular to the external current is introduced to the model the equations of motion for a single plaquette become equivalent to those of a dc superconducting quantum interference device in the limit of small loop inductance.

A series of atomistic calculations is performed in order to explore dislocation core-core interactions and the Peierls stress in a model hexagonal lattice. The method of calculation is the lattice Green’s function method, using several pair potentials with variable parameters. We confirm that dislocation cores broaden as a pair of dislocations with opposite sign move closer to each other. Continuum theories are surprisingly accurate in describing the dislocation-dislocation interaction force even in the range of strong core-core overlap. However, our atomistic calculations show that while the relation between the Peierls stress and dislocation width is exponential as the Peierls-Nabarro model predicts, that model underestimates the Peierls stress by nearly a factor of ${10}^4$.

The formation of thermal vacancies, which is a key issue with respect to the high-temperature properties of ordered intermetallic compounds, was studied in γ-TiAl by means of positron lifetime spectroscopy between ambient temperature and 1400 K. An estimate of the atomic concentration of thermal vacancies yields a value similar as in pure fcc metals and an effective vacancy formation enthalpy of 1.41±0.06 eV in good agreement with nearest-neighbor-bond model calculations. A comparison of these data with the results of recent self-diffusion studies suggests a low mobility of vacancies as typical for ordered intermetallic compounds. No evidence for structural vacancies at ambient temperature could be observed.

Effects of a noncubic crystal field on the ${\mathit{s}}^2$ ion are investigated. Optical-absorption spectra of thallous ion centers produced in single crystals of ${\mathrm{KH}}_2$${\mathrm{PO}}_4$ (KDP) and ${\mathrm{RbH}}_2$${\mathrm{PO}}_4$ (RDP) are presented. They consist of polarized absorption bands. Since it is confirmed that the ${\mathrm{Tl}}^{+}$ centers in KDP and RDP exhibit quite similar absorption characteristics, a model is proposed to interpret the interaction mechanisms between the ${\mathit{s}}^2$ ions and the KDP-like crystal lattices. Among five typical absorption bands named ${\mathit{A}}_{\mathit{z}}$, ${\mathit{A}}_{\mathit{x}\mathit{y}}$, ${\mathit{B}}_{\mathit{x}\mathit{y}}$, ${\mathit{C}}_{\mathit{x}\mathit{y}}$, and ${\mathit{C}}_{\mathit{z}}$, the ${\mathit{A}}_{\mathit{x}\mathit{y}}$ band has a noticeable doublet structure and the ${\mathit{B}}_{\mathit{x}\mathit{y}}$ band is as intense as the ${\mathit{A}}_{\mathit{x}\mathit{y}}$ band even at liquid-nitrogen temperature and is strongly temperature dependent. The observed results are qualitatively explained by taking into account the spin-orbit, crystal-field, and electronic-vibrational interactions.

We present an experimental and theoretical study of the phase effect in the energy loss of fast hydrogen beams colliding with gas and solid zinc targets. The experiments show a maximum phase effect of 50% around 50 keV/u, the energy loss per atom in the solid target being smaller than in the gas target. An extensive theoretical study of all the processes contributing to the energy loss in the two phases shows that the experimental findings can be explained primarily by the screening of the projectile field by the valence electrons in the solid.

The phase stability is studied for the β-phase ${\mathrm{YH}}_{2+\mathit{x}}$ system based on first-principles total energy calculations. Our study predicts that the ${\mathrm{D}0\mathrm{}}_{22}$, ‘‘40’’, and ${\mathrm{D}1\mathrm{}}_{\mathit{a}}$ structures are stable near x=0.25, 0.5, and 0.8, respectively. Using the effective cluster interactions obtained from the first-principles total-energy data, the phase diagram for the ${\mathrm{D}0\mathrm{}}_{22}$ and ‘‘40’’ ordered phases is calculated by the cluster variational method. The calculated order-disorder transition temperature at x=0.1 for the ${\mathrm{D}0\mathrm{}}_{22}$ structure is around 280 K, which is consistent with the recent observation of the metal-semiconductor transition near 230–280 K and resistivity anomalies near 200–250 K for the system with x near 0.1 [Daou and Vajda, Phys. Rev. B 45, 10 907 (1992)].

The photon energy dependence of the photoionization cross section in a solid has been measured in the UV region (10<hν<25 eV) using an experimental method based on inverse photoemission. Measurements have been performed on low-d-occupancy transition metals (Ti, Zr, Hf, V, Nb, and Ta). The observed trend is in good qualitative agreement with the available atomic calculations, at variance with measurements around the Cooper minimum region (50<hν<200 eV), where strong modifications with respect to the isolated atom have been observed in previous experiments based on direct photoemission.

Molecular-dynamics simulations of stress relaxation have been performed for models of metals and polymers. A method that employs coupling between the simulation cell and an applied stress as well as an external thermal bath has been used. Two-dimensional models of the materials are defined with interactions described by the Lennard-Jones (Mie 6-12) and harmonic potentials. A special method is employed to generate chains in dense polymeric systems. In agreement with experiments, simulated stress-relaxation curves are similar for metals and polymers. At the same time, there exists an essential difference in the stress-strain behavior of the two kinds of simulated materials. During the relaxation, trajectories of the particles in different materials display a common feature: There exist domains in which movement of the particles is highly correlated. Thus, the simulation results support the cooperative theory of stress relaxation.

Fluorescence of ${\mathrm{Cr}}^{3+}$ ions in the two different octahedral sites of the spinel block of β- and β’’-alumina have been measured by time-resolved fluorescence line-narrowing spectroscopy. For the excitation either a pulsed or a chopped cw-dye laser was used. Lifetimes and splittings of the ground state ${(}^4$${\mathit{A}}_2$) and of the ${}_{}{}^2{}_{}{}^{}$E state have been measured by tuning the excitation energy within the inhomogeneous profile of the ${\mathit{R}}_1$ and ${\mathit{R}}_2$ lines of the split ${}_{}{}^2{}_{}{}^{}$E${\mathrm{-}}^4$${\mathit{A}}_2$ transition. In sodium β’’-alumina, we have isolated several inequivalent emitting centers, which are related to different configurations of the stabilizing ${\mathrm{Mg}}^{2+}$ ions.

The complete set of elastic constants of the single crystal [N(${\mathrm{CH}}_3$${)}_4$${]}_2$${\mathrm{MnCl}}_4$ has been measured at 15 MHz in the temperature range 250–310 K, including the normal-incommensurate transition at ${\mathit{T}}_{\mathit{I}}$=292.3 K, the incommensurate-improper ferroelastic lock-in transition at ${\mathit{T}}_{\mathit{L}}$=291.7 K, and the improper ferroelastic–improper ferroelastic transition at ${\mathit{T}}_1$=266.7 K. The Landau theory has been used to explain the experimental data. In addition, the attenuation of the longitudinal wave associated with the ${\mathit{C}}_{11}$ mode was measured as a function of temperature and frequency (12.7–71 MHz) around ${\mathit{T}}_{\mathit{I}}$ and analyzed in terms of a dynamic-scaling model. Above ${\mathit{T}}_{\mathit{I}}$ the results agree with both three-dimensional-Ising and mean-field theory. In the commensurate phase the critical exponents have values consistent with mean-field theory. The relaxation time of the amplitudon below ${\mathit{T}}_{\mathit{I}}$ has been determined to be ${\mathrm{\tau }}_{\mathit{A}}$=${\mathrm{\tau }}_{\mathit{A}0}$/(${\mathit{T}}_{\mathit{I}}$-T) with ${\mathrm{\tau }}_{\mathit{A}0}$≊4×${10}^{\mathrm{-}10}$ s K.

The Landau theory of phase transitions is used to study the cubic-to-tetragonal phase transformation which occurs in group-IV semiconductors at high pressure. Following various authors we assume that a spontaneous tetragonal strain, η∼(2${\mathit{e}}_3$-${\mathit{e}}_1$-${\mathit{e}}_2$), superimposed on the hydrostatic pressure, is the order parameter of ${\mathit{E}}_{\mathit{g}}$(${\mathrm{\Gamma }}_{12}$) symmetry which drives the ${\mathit{O}}_{\mathit{h}}^7$→${\mathit{D}}_{4\mathit{h}}^{19}$ structural phase transition. The theoretical approach is equivalent to a higher-order nonlinear elastic theory. The static aspect of the phase transformation and the anomalies of the elastic properties are studied. Failures of the criteria of Vaidya and Demarest et al., which predict the occurrence of pressure-induced phase transitions involving shear strains, are shown. The Landau criterion for diamond-structure semiconductors involves the ratio (${\mathit{C}}_{11}$-${\mathit{C}}_{12}$)/(${\mathit{C}}_{111}$-3${\mathit{C}}_{112}$+2${\mathit{C}}_{123}$) of the second- to third-order elastic constants associated with the tetragonal strain, or equivalently in terms of valence force-field model, the ratio β/δ of the second- to third-order bond-bending force constants, i.e., the harmonic and the anharmonic interatomic interactions which tend to stabilize the equilibrium angle between covalent bonds. The contribution of the electronic bands to the elastic constants under pressure is also reviewed.

An atomistic simulation model for evaluating the stability of crystal under uniaxial tensile force is presented for diamond. The method is based upon the tight-binding approximation, and an interatomic potential which explicitly has an additive cutoff term having a directional dependence is proposed. In the static relaxation procedure for this model, a rigid tetrahedron placed at each atomic position is moved toward the equilibrium configuration by six driving forces, where three are translational and three rotational. As a result, there exists a critical strain of the crystal stability under the uniaxial tensile strain of the [111] direction in diamond. In crystals including vacancies, the strength of the crystal decreased as predicted by Griffith’s theoretical treatment. The critical strength of the perfect diamond crystal is found to be about 80 GPa. The nonlinear relationship between stress and strain of diamond under larger external forces is obtained from the results for equilibrium states. The functional form in a cubic polynomial of strain reproduces very well the calculated plots.

The high-accuracy universal polarimeter (HAUP) opened a way for revealing optical activity in incommensurate phases of several ${\mathit{A}}_2$${\mathit{BX}}_4$-type crystals. Ortega, Etxebarria, Breczewski, and Tello found recently that a gyration tensor component ${\mathit{g}}_{23}$ of ${\mathrm{Rb}}_2$${\mathrm{ZnBr}}_4$, which had been reported by Meekes and Janner to appear in the incommensurate phase, vanished. They drew the conclusion that the incommensurate phase of this material lacked optical activity. In order to review these controversial results, we made a critical study of all the components ${\mathit{g}}_{\mathit{i}\mathit{j}}$ of ${\mathrm{Rb}}_2$${\mathrm{ZnBr}}_4$ by using the refined HAUP. It was found that the component ${\mathit{g}}_{23}$ of ${\mathrm{Rb}}_2$${\mathrm{ZnBr}}_4$ clearly appeared at the incommensurate transition point and increased conspicuously with decreasing temperature. Therefore the incommensurate phase of ${\mathrm{Rb}}_2$${\mathrm{ZnBr}}_4$ was found to be optically active.

The electronic structure of Fe impurity in MgO was calculated by the linear muffin-tin-orbital–full-potential method within the conventional local-density approximation (LDA) and making use of the LDA+U formalism. The importance of introducing different potentials, depending on the screened Coulomb integral U, is emphasized for obtaining a physically reasonable ground state of the ${\mathrm{Fe}}^{2+}$ ion configuration. The symmetry lowering of the ion electrostatic field leads to the observed Jahn-Teller effect; related ligand relaxation confined to tetragonal symmetry has been optimized based on the full-potential total-energy results. The electronic structure of the ${\mathrm{Fe}}^{3+}$ ion is also calculated and compared with that of ${\mathrm{Fe}}^{2+}$.

The phase transitions in ${\mathrm{Cs}}_2$${\mathrm{CdBr}}_4$ have been studied by means of Brillouin scattering and ultrasonic measurements. The sequence Pnma ${\mathrm{\leftrightarrows }}^{252\mathrm{}\mathrm{K}}$ incommensurate ${\mathrm{\leftrightarrows }}^{235{\mathrm{}}^{\mathrm{K}}}$ P$2_1$/n11 gives rise to significant anomalies of both real and imaginary parts of the elastic constants. In the case of ${\mathit{C}}_{44}$, a strong asymmetric broadening of the Brillouin lines was observed in the IC phase and several degrees above the Pnma ⇆ IC transition. The differences between Brillouin and ultrasonic results concerning ${\mathit{C}}_{22}$ and ${\mathit{C}}_{44}$ suggest a strong frequency dependence of the process governing the above-mentioned sequence of phase transitions. A low-temperature transition at 156 K has been studied; it presents a considerable softening of shear stiffness constants. Selection rules for Brillouin scattering are greatly affected at the transitions to monoclinic and triclinic phases, allowing to measure the shear stiffness constants with a good accuracy. A detailed discussion of the phenomena linked to the monoclinicity is provided. In addition, the ultrasonic experiments have confirmed the existence of a previously reported phase transition at 208 K.

The Brillouin and ultrasonic results are interpreted using a phenomenological free-energy development. The ferroelastic phase transition at 156 K is of a displacive type and is well described by a static model: the variation of the elastic constants bilinearly coupled to the order parameter is in quantitative agreement with recent Raman data. It is shown that the order parameter ${\mathit{Q}}_1$ of the sequence Pnma (I) ${\mathrm{\leftrightarrows }}^{252\mathrm{}\mathrm{K}}$ incommensurate (IC) ${\mathrm{\leftrightarrows }}^{235\mathrm{}\mathrm{K}}$ P$2_1$/n11 (C) is a relaxator which significantly contributes to the scattering in the Brillouin frequency range. The differences between the ultrasonic and the Brillouin results are well understood. (i) A qualitative interpretation of the behavior of ${\mathit{C}}_{22}$ is given: it is strongly influenced by the fluctuations of the order parameter and, concerning the Brillouin scattering, by a significant Landau-Khalatnikov contribution below ${\mathit{T}}_{\mathit{i}}$. (ii) In the vicinity of the Pnma⇆ IC transition, the ultrasonic data concerning ${\mathit{C}}_{44}$ are well interpreted in terms of a static model with a bilinear ${\mathit{Q}}_1$${\mathrm{\varepsilon }}_4$ coupling, while the Brillouin scattering, in the geometry which usually gives access to ${\mathit{C}}_{44}$ through the dynamics of the strain ${\mathrm{\varepsilon }}_4$, is interpreted as resulting from the cumulative contribution of three terms involving three pair correlation functions of the order parameter ${\mathit{Q}}_1$ and the strain ${\mathrm{\varepsilon }}_4$ which allow to fit the characteristic strongly asymmetric shape of the spectra and which explain the considerable shift of the Brillouin maxima below the ‘‘static’’ ultrasonic curve. Finally, we show that an improved interpretation of the Brillouin and ultrasonic data is found assuming that the virtual Pnma⇆P$2_1$/n11 transition is of the first order.