We report the two-dimensional angular-correlation-of-annihilation-radiation measurement of the ${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{-}}$ (pair) momentum distribution in the organic superconductor κ-(BEDT-TTF${)}_2$Cu(NCS${)}_2$. Our data, when compared with tight-binding (Hückel) electronic-band calculations, show features consistent with predicted Fermi surfaces. The structure around Z in our spectrum supports the results from earlier de Haas–van Alphen and Shubnikov–de Haas measurements. In addition, our data show features consistent with the predicted open Fermi surface that runs nearly parallel to YM.

We have measured the superconducting and normal tunneling characteristics of quality controlled planar junctions based on ${\mathrm{YBa}}_2$${\mathrm{Cu}}_4$${\mathrm{O}}_8$ (Y124) single crystals. Our data show that the in-plane properties are similar to the ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ (Y123) system. A higher value of the zero-bias conductance is measured in the Y124, indicating an increased number of normal electrons due to the presence of the double chain structure. Linearity of the background conductance is observed in both compounds. Less steep slopes are found in Y124 that might indicate a reduced interlayer coupling in this system.

We study the effect of superconducting fluctuations on the spin susceptibility ${\mathrm{\chi }}_{\mathit{s}}$ and NMR relaxation rate 1/${\mathit{T}}_1$ just above ${\mathit{T}}_{\mathit{c}}$ for arbitrary impurity concentrations. Fluctuations are found to reduce ${\mathrm{\chi }}_{\mathit{s}}$ below the Pauli susceptibility. 1/${\mathit{T}}_1$T is enhanced above the Korringa value for weak pair breaking, but suppressed when the pair breaking is large.

From the theory of Yip, Li, and Kumar, we derive convenient expressions relating the slopes of the phase boundaries near the tetracritical point (TP) in the superconducting heavy-fermion compound ${\mathrm{UPt}}_3$ to the jumps in the specific heat across the transitions. We apply these relations to our recent measurements on the pressure dependence of the phase boundaries near the TP for magnetic field, H, parallel to both the a and c axes. Comparison with published data on the specific heat jumps at zero pressure or at zero field implies that the low-field inner transition line is first order for the field orientation H∥a for H>0, and is likely to be second order for H∥c. For H=0 the corresponding transition line in the pressure-temperature plane is likely to be second order as well.

Changes in the elastic properties of Ge induced by room-temperature irradiation with 3.5-MeV Kr ions have been determined and correlated with changes in the microstructure determined by transmission electron microscopy. Elastic-shear-moduli changes were measured by Brillouin scattering, and changes in local atomic arrangement were determined by Raman scattering. Amorphization decreased the elastic shear modulus of Ge by 17%. The fractional decrease was correlated with the amorphous volume fraction with a cross section of 4.5±0.5 ${\mathrm{nm}}^2$/ion. No change was observed in the shear modulus during void formation and growth. The elastic properties of the voided material are described by the Voigt averaging. However, as the voids evolved into a fibrous spongelike microstructure, a second dramatic elastic softening occurs which we attribute to the inability of the fibrous structure to support shear stresses. Raman scattering showed that, once formed, there was no change in the structure of the amorphous material at the atomic scale during void formation and subsequent void coalescence.

Femtosecond time-domain observations of soft-mode dynamics in crystals near structural phase transitions have been conducted. Impulsive stimulated Raman scattering (ISRS) experiments are reported for both the orthorhombic phase of ${\mathrm{KNbO}}_3$ and the tetragonal phase of ${\mathrm{BaTiO}}_3$. The data from potassium niobate clearly show a heavily damped soft mode of ${\mathit{B}}_2$ symmetry and relaxational modes of ${\mathit{A}}_1$ symmetry but not ${\mathit{B}}_2$ symmetry. Similarly, the data from barium titanate clearly show a heavily damped E-symmetry soft mode and no relaxational modes of this symmetry. The absence of relaxational modes of the same symmetries as the soft modes is consistent with an eight-site order-disorder model of the phase transitions in this crystal class. The present results demonstrate significant advantages of ISRS over conventional Raman spectroscopy of low-frequency, heavily damped soft modes.

The kinetics of the martensitic transition in In-Tl alloys has been studied by x-ray- and neutron-diffraction methods. A characteristic waiting time appears at fixed temperature above ${\mathit{M}}_{\mathit{s}}$, the normal martensitic phase transition start temperature. A similar waiting time is also seen for inverse phase transitions. A peculiar temperature ${\mathit{T}}_{\mathit{p}}$, which is above ${\mathit{M}}_{\mathit{s}}$, but below ${\mathit{T}}_0$, is found to exist. The time development of x-ray-diffraction patterns shows a very different behavior whether the temperature is above ${\mathit{T}}_{\mathit{p}}$ or below ${\mathit{T}}_{\mathit{p}}$. It is found that ideal nucleation and growth take place between ${\mathit{T}}_{\mathit{p}}$ and ${\mathit{T}}_0$. Between ${\mathit{T}}_{\mathit{p}}$ and ${\mathit{M}}_{\mathit{s}}$, the kinetics of the phase transition is more complicated. In order to understand the kinetics of the phase transition, the temperature-dependent waiting time curve in In-Tl alloys is calculated using the Roitburd method [A. L. Roitburd, Mater. Sci. Eng. A127, 229 (1990)] and is found to be in good agreement with experimental results.

Distortions appear in x-ray-absorption spectra obtained by monitoring the fluorescence from thick samples with concentrated absorbing species. The glancing-emergence-angle technique for obtaining spectra from this type of sample eliminates distortions from the measured spectra by monitoring the fluorescence leaving the sample at a small angle relative to the sample surface. This technique is limited by the small signal available from the inherently limited detector solid angle. In addition, no precise estimate of the required restriction on maximum emergent angle ${\mathrm{\theta }}_{\max }$ has been available. We have calculated residual extended x-ray-absorption fine structure distortions as a function of ${\mathrm{\theta }}_{\max }$, and performed experimental tests of the calculations. These calculations provide a means to estimate the required detector geometry for negligible distortions, or alternatively, allow the use of a larger ${\mathrm{\theta }}_{\max }$, increasing the available signal, with the remaining residual distortions removed by application of the calculations. The calculations are also applicable to other detector geometries, and account for detectors subtending a large solid angle by an integration over the subtended angle. This represents an improvement over previous calculations. The application to more general detector configurations is also discussed.

The temperature-dependent homogeneous width of the ${}_{}{}^5{}_{}{}^{}$${\mathit{D}}_{0\mathrm{-}}^7$${\mathit{F}}_0$ transition of ${\mathrm{Eu}}^{3+}$ in a pure silicate glass fiber has been measured by accumulated photon echoes between 1.6 and 44 K. The temperature dependence of the homogeneous width shows a crossover from T-linear (T<7 K) to ${\mathit{T}}^2$ (T>7 K). The whole temperature dependence of the homogeneous width between 1.6 and 44 K can be explained by the configurational changes of the glass which is modeled by two-level systems (TLS’s) and a Raman process involving low-frequency vibrational modes of the silicate glass (the rotations of ${\mathrm{SiO}}_4$ tetrahedra). The T-linear temperature dependence is attributed to the TLS process. The anomalous ${\mathit{T}}^2$ dependence down to 10 K cannot be explained by only one process, but can be explained by the contributions of both the TLS process and the Raman process. Above 40 K, the Raman process becomes the dominant dephasing mechanism.

Threshold-excited soft-x-ray-fluorescence spectroscopy has been used to study several selectively excited electronic configurations in ${\mathrm{CaF}}_2$, enabling the determination of specific interaction energies. In the decay of the Ca 2p core excitons via core-to-core fluorescence, final states of the 3${\mathit{s}}^{\mathrm{-}1}$3d configuration are populated. Here the exchange interaction between the remaining 3s electron and the excited 3d electron was measured, as well as the Coulomb interaction between the core hole and the excited electron. Furthermore, the influence of crystal-field splitting on the 3${\mathit{s}}^{\mathrm{-}1}$3d states has been studied, and it was found that all of these three interaction energies are interrelated. Using the Ca core-to-core fluorescence data as fingerprints for the 2p excited states, we determine upper limits for the 2${\mathit{p}}_{1/2}^{\mathrm{-}1}$3d→2${\mathit{p}}_{3/2}^{\mathrm{-}1}$ Coster-Kronig yields and for the 2${\mathit{p}}_{3/2}$ ionization threshold which in the absorption spectrum is masked by the 2${\mathit{p}}_{1/2}^{\mathrm{-}1}$3d resonances. Our data are in conflict with optical and photoemission data, a conflict which is resolved by assuming the existence of an excitonic 2${\mathit{p}}^{\mathrm{-}1}$ns state unidentifiable in the Ca L absorption spectrum. In the threshold-excited F K fluorescence spectra a satellite is assigned to the decay of core hole states with an additional valence vacancy, making it possible to discuss the p-p correlation energy in the valence band. Finally, the influence on the fluorine valence band of an excitonic screening electron localized in various states primarily localized on the neighboring Ca ions is determined.

A study of the entropy change ΔS between the β phase and the martensite in Cu-based shape-memory alloys is presented. The critical resolved shear stress as a function of the temperature [τ(T)] was determined by compression experiments. This allowed the calculation of ΔS by means of a Clausius-Clapeyron type relationship. The results obtained were compared with those resulting from tension tests and calorimetric determinations taken from the literature. A good agreement was found between the three methods for the wide range of compositions available. Moreover, a correlation seems to exist between the experimentally determined ΔS and the corresponding electron concentration e/a. The entropy change can be expressed in terms of the specific heats, using the models of Debye and Einstein. However, by relating the characteristic temperatures (Debye or Einstein) to the elastic constants (C’ and ${\mathit{C}}_{44}$) of the β phase at the transformation temperature, it is found that the dependence of ΔS on e/a cannot be explained. It is shown that a relation between the phonon frequency at the Brillouin zone and C’ with composition must be considered.

Results of atomistic modeling studies are reported which concern the axial relaxation of a niobium-substituting ${\mathrm{Fe}}^{3+}$ cation relative to an associated oxygen vacancy. The employed methods are shell-model simulations and embedded-cluster calculations. These simulation studies consistently show that the iron impurity is displaced in the opposite direction to the oxygen vacancy. The present results are in contrast to known interpretations based on the superposition model (SPM). It is shown finally that slight modifications of iron-specific superposition-model parameters are sufficient in order to achieve agreement between SPM-based interpretations and shell-model as well as embedded-cluster-type calculations. The relaxation pattern found in this special case is expected to be valid for most axial defect aggregates of this particular type.

We have studied the structural and bonding properties of the equilibrium and high-pressure phases of tellurium by means of first-principles total-energy calculations, performed within the local-density approximation. The calculated characteristics of the various polymorphs under pressure show good general agreement with existing experiments. However, some systematic discrepancies occur between computed and measured structural parameters for the open linear-chain (trigonal) and layer-type (orthorhombic) phases. The interchain and interlayer distances are underestimated within the local-density approach, and the relative stability of the trigonal and of the orthorhombic phase with respect to compact metallic structures is lower in the calculations than indicated by the experimental phase diagram. The complex structural changes of Te under pressure are discussed in terms of the trends under pressure of the different types of bonds involved in the various crystal structures.

Pressure-induced amorphization has been observed in isostructural oxides such as α-quartz (${\mathrm{SiO}}_2$), ${\mathrm{GeO}}_2$, and α-berlinite (${\mathrm{AlPO}}_4$). Here we focus on the mechanical behavior of α-berlinite under pressure. We have computed the second-order elastic constants of α-berlinite and applied the Born stability criteria. Our analysis reveals an elastic instability associated with soft shear modes. We have also computed the phonon spectrum of α-berlinite as a function of pressure. A soft phonon mode corresponding to the shear instability is observed at the Brillouin-zone edge. We have analyzed the displacements of cations associated with the soft phonon modes in α-quartz and α-berlinite. They exhibit the same qualitative behavior. This strongly suggests that the mechanism of amorphization is similar in these materials.

The synchrotron-radiation-induced electron emission spectra from evaporated KF films have been measured by varying the photon energy across the K 2p ionization threshold. The excitation of the K 2p electrons to the pre-edge resonances creates resonant Auger spectra which are characterized by essentially sharper line shapes and by energy shifts of 7 eV to higher kinetic energies as compared with the normal Auger spectra.

A tetragonal three-dimensional model of particles with displacive degree of freedom and interacting via a potential energy with harmonic and anharmonic fourth-order terms has been studied by the molecular-dynamics technique. The temperature–potential-energy-parameter phase diagram of the model exhibits normal, commensurate k=1/2, one-dimensional 1q and two-dimensional 2q incommensurate phases. The 2q phase can be seen as a sequence of columns, oriented along the unique fourth-fold axis. The 2q incommensurate phase is more stable at higher temperature than the 1q one. The simulation has shown that (1/2,1q)→(1/2,2q) and (I,1q)→(I,2q) phase transitions between commensurate and incommensurate phases, respectively, are driven by anisotropic gaps nucleated equidistantly on the modulation stripes of the 1q phases.

We present experimental data and the underlying physics of spatial-phase-modulation effects associated with the thermal nonlinearity of semiconductor-doped glasses. Transient and steady-state self-focusing, laser-induced birefringence, and spatial ringing of the thermal origin were studied in this class of optical materials.

The Faber-Ziman total structure factors of a series of rf-sputtered and melt-spun ${\mathrm{Ni}}_{\mathit{x}}$${\mathrm{Zr}}_{1\mathrm{-}\mathit{x}}$ glassy metal alloys have been measured for 0.25≤x≤0.86. The radial distribution functions and nearest-neighbor coordination numbers have been calculated. The measurements indicate that there is no discernible difference between the as-made structure of the sputtered and melt-spun materials. These data are reinforced by crystallization enthalpy measurements which also show no difference between the two preparation methods.

In this work the short-time dynamics of quenched Frenkel excitons in binary, coiled chains is investigated by numerical calculations. Quenching is modeled by means of a non-Hermitian damping Hamiltonian accounting for interstitional traps. The dependence of donor and acceptor response functions to δ-pulse excitation is examined as a function of diagonal disorder, off diagonal disorder induced by random non-nearest-neighbor contacts (degree of coiling), coupling strength, and two types of initial conditions (localized and nonlocalized). The results clearly demonstrate that the relaxation behavior shows significant oscillations even after averaging over a representative ensemble of chain configurations and over all possible initial localized conditions over a wide range of model parameters.

The presence of oscillatory instabilities during directional solidification of thin lamellar eutectics is explored. A linear stability analysis is performed in which the triple points are perturbed tangential to the interface, i.e., no Mullins-Sekerka type of morphological instability is allowed. A locally planar interface is assumed except in the thermal boundary condition where a curvature term is included. Neither the quasistationary approximation nor the equal-average-undercooling assumption is used; instead, the average concentration over a phase is forced to be consistent with the thermal boundary condition. This analysis reveals two oscillatory modes, an ‘‘acoustic’’ mode of wavelength twice that of the lamellar spacing and an ‘‘optical’’ mode of wavelength equal to the lamellar spacing.