Superconductivity for lanthanide platinum boron carbide quaternary intermetallic phases is reported. For R${\mathrm{Pt}}_2$${\mathrm{B}}_2$C, superconducting ${\mathit{T}}_{\mathit{c}}$’s are approximately 10 K for R=La and Y and 6 K for R=Pr, reflecting the influence of magnetic pair breaking. The crystal structure for one of the analogs, ${\mathrm{LaPt}}_2$${\mathrm{B}}_2$C, was determined by single-crystal x-ray diffraction. These materials are isostructural with the recently discovered R${\mathrm{Ni}}_2$${\mathrm{B}}_2$C superconductors, but apparently can accommodate some nonstoichiometry. The density of states at the Fermi level for ${\mathrm{LaPt}}_2$${\mathrm{B}}_2$C is estimated to be approximately 5 mJ/mole ${\mathrm{K}}^2$ through measurements of the critical fields and the normal-state susceptibility.

Detailed measurements of Josephson tunneling between ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{y}}$ (YBCO) and Pb using YBCO/[MgO (or native barrier)]/Pb junctions with YBCO films either c-axis or a-axis oriented are presented. In the case of the c-axis-oriented films, the dependence of the Josephson critical current on the applied magnetic field exhibits diffraction patterns close to that characteristic of a d-wave superconductor. The temperature dependence of the Josephson critical current also shows an anomalous behavior. Incidentally, the diffraction pattern exhibits less-sensitive behavior to magnetic fields at low temperatures. These results suggest the possibility of d-wave symmetry for a YBCO superconductor.

Sound velocity and powder x-ray diffraction in ${\mathrm{La}}_{1.9\mathrm{-}\mathit{x}}$${\mathit{R}}_{0.1}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_4$ (R=Nd,Sm,Ge,Tb) reveal that the transition temperature to the tetragonal low-temperature (TLT) phase increases with decreasing the ionic radius of R up to 150 K (R=Tb around x∼0.125). Prominent change of the resistivity ρ and the thermoelectric power S is observed in the narrow region of x around 0.125 and at almost the same temperature near 80 K in the Tb-, Gd-, and Sm-substituted samples whose crystal structure is the TLT phase. In the ND-substituted sample, less prominent but significant change of S is also observed below ∼50 K where the crystal structure is the orthorhombic midtemperature phase. These results suggest that the change of the transport properties is responsible for the change of the electronic state itself rather than for the structural phase transition and this modification of the electronic state is more pronounced in the TLT phase.

A maximum-entropy method has been applied to transverse-field muon-spin-rotation vortex data of R${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ (R 1:2:3:7; R=Er, Gd, and Eu); this produces spectra representing estimates for the magnetic-field distributions. The information on the field distribution is of better quality than that resulting from Fourier transformation and curve fitting of the same data. Significant deviations from Abrikosov predictions have been observed for the estimated R 1:2:3:7 vortex-field distributions. Below 10 K, for Er 1:2:3:7 and Gd 1:2:3:7, the non-Abrikosov features appear to be influenced by the magnetism of the rare-earth layers.

We derive an approximate integral line-shape expression for conversion-electron Mössbauer spectroscopy and for the single-foil microfoil internal conversion-electron detector, which includes asymmetry (interference) effects, finite thickness of foils, and the presence of resonance absorption in the source material. We then give an analytical expansion that is equivalent to this integral expression and which is usable for computer least-squares fitting. We also give the exact integral line-shape expression for Mössbauer transmission experiments using finite thickness absorbers, which includes asymmetry effects and source resonance self-absorption, along with its convergent analytic expansion. The inclusion of hyperfine interactions and the practical approach to using the line-shape analysis to minimize the correlation of parameters is discussed.

Using multi-ion interatomic potentials derived from first-principles generalized pseudopotential theory (GPT) together with molecular-dynamics (MD) simulation, a detailed study of melting and related high-temperature solid and liquid properties in molybdenum has been performed. The energetics in such bcc transition metals are dominated by d-state interactions that give rise to both many-body angular forces and enhanced electron-thermal contributions. The angular forces are accounted for in the GPT through explicit three- and four-ion potentials, ${\mathit{v}}_3$ and ${\mathit{v}}_4$, which in the present work are applied in analytic model-GPT (MGPT) form. With the MGPT potentials, ion-thermal melting in Mo has been investigated both mechanically, by cycling up and down through the observed MD melting point at constant volume, and thermodynamically, by calculating solid and liquid free energies. In the former approach, parallel MD simulations have also been done with a corresponding effective-pair potential ${\mathit{v}}_2^{\mathrm{eff}}$ in which the angular dependence of ${\mathit{v}}_3$ and ${\mathit{v}}_4$ has been averaged. The multi-ion angular forces, which are essential to an accurate description of the bcc solid, are found to lower the dynamically observed melting point by about 1000 K. Above the melting transition, however, ${\mathit{v}}_2^{\mathrm{eff}}$ gives a reasonbly good account of the structure and thermal energy of the liquid and the accuracy of this description improves with increasing temperature.

Both the multi-ion and effective pair potentials also permit a large amount of supercooling of the liquid before the onset of freezing. With ${\mathit{v}}_2^{\mathrm{eff}}$ a bcc structure is nucleated at freezing, while with the multi-ion potentials an amorphous glasslike structure is obtained, which appears to be related to the energetically competitive A15 structure. In our second approach to melting, the multi-ion potentials have been used to obtain accurate solid and liquid free energies from quasiharmonic lattice dynamics and MD calculations of thermal energies and pressures. The resulting ion-thermal melting curve exactly overlaps the dynamically observed melting point, indicating that no superheating of the solid occurred in our MD simulations. To obtain a full melting curve, electron-thermal contributions to the solid and liquid free energies are added in terms of the density of electronic states at the Fermi level, ρ(${\mathit{E}}_{\mathit{F}}$). Here the density of states for the solid has been calculated with the linear-muffin-tin-orbital method, while for the liquid tight-binding calculations have been used to justify a simple model. In the liquid ρ(${\mathit{E}}_{\mathit{F}}$) is increased dramatically over the bcc solid, and the net effect of the electron-thermal contributions is to lower the calculated melting temperatures by about a factor of 2. A full melting curve to 2 Mbar has thereby been obtained and the calculated melting properties near zero pressure are in generally good agreement with experiment.

The decrease in the proton hyperfine interaction of ${\mathrm{HC}}_{60}$ with increasing temperature is analyzed on the basis of quantum-chemical calculations of the vibrational force field of the radical. The changes of the hyperfine interaction with respect to normal-mode displacements are evaluated and used to simulate the temperature dependence. It is shown that the modulation by low-frequency A’’ (${\mathit{C}}_{\mathit{s}}$ symmetry) vibrational modes of the radical that correspond to the lowest ${\mathit{h}}_{\mathit{g}}$ and ${\mathit{h}}_{\mathit{u}}$ vibrations of ${\mathrm{C}}_{60}$ dominates the observed temperature dependence. The experimentally observed negative hyperfine anomaly, vis-à-vis the muon coupling, is also attributed to vibrational motions, specifically to the three modes that describe hydrogen motions.

The temperature dependence of the local distortions in ${\mathrm{SrTiO}}_3$ has been studied by EXAFS spectroscopy at the titanium K edge (4982 eV). The oxygen-ion Debye-Waller factor ${\mathrm{\sigma }}_0^2$ has been determined from 4.5 to 240 K. The antiferrodistortive transition at 105 K is evidenced by a step in this Debye-Waller factor. At about 31 K, a maximum of ${\mathrm{\sigma }}_0^2$ is detected and the EXAFS oscillations due to the first oxygen shell increase. This is the signature of a maximum disorder in the lattice vibrations in this temperature range. A quasiharmonic model with a sinusoidal modulation of the Ti-O distance cannot account for these observations. The lattice and electronic excitations which could support our data are qualitatively described.

α-quartz has been irradiated with heavy ions: ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$, ${}_{}{}^{32}{}_{}{}^{}\mathrm{S}$, and ${}_{}{}^{63}{}_{}{}^{}\mathrm{Cu}$ at an energy of about 1 MeV/amu in order to cover a range of electronic stopping powers dE/dx between 2.4 and 9 keV/nm and ${}_{}{}^{58}{}_{}{}^{}\mathrm{Ni}$, ${}_{}{}^{86}{}_{}{}^{}\mathrm{Kr}$, ${}_{}{}^{128}{}_{}{}^{}\mathrm{Te}$, ${}_{}{}^{129}{}_{}{}^{}\mathrm{Xe}$, ${}_{}{}^{181}{}_{}{}^{}\mathrm{Ta}$, and ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$ between 1 and 5.8 MeV/amu for dE/dx>7 keV/nm. The extent of the induced damage is determined using Rutherford backscattering ion channeling with a 2-MeV ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ beam. The damage cross section A is obtained using a Poisson law ${\mathit{F}}_{\mathit{d}}$=1-exp(-Aφt), where φ is the flux and t the irradiation time. This damage cross section is linked to the effective radius ${\mathit{R}}_{\mathit{e}}$ through the relation A=π${\mathit{R}}_{\mathit{e}}^2$, where ${\mathit{R}}_{\mathit{e}}$ is the radius of an equivalent cylinder of damage. Using high-resolution electron microscopy, cylinders of amorphous matter have been observed, whose radius corresponds to ${\mathit{R}}_{\mathit{e}}$ when the track is continuous (i.e., for A≥1.3×${10}^{\mathrm{-}13}$ ${\mathrm{cm}}^2$; ${\mathit{R}}_{\mathit{e}}$≥2 nm). A thermal-spike model is applied to calculate the radii of the observed tracks assuming that the observed amorphous cylinders correspond to a rapid quench of a molten liquid phase along the ion path. The model is applied only when the latent track is continuous and cylindrical. A good agreement is obtained taking into account that the initial spatial energy deposition on the electrons depends on the ion velocity.

We introduce a Monte Carlo model for the evolution of equilibrium interface morphologies from initially fully faceted (straight) forms. The two-dimensional model integrates the stochastic occupation of surface sites and the diffusive exchange of particles with the surrounding vapor phase, other surface sites and the underlying bulk solid. Neither lattice gas nor solid-on-solid (SOS) restrictions are imposed. The exchange processes are associated with energy barriers, the absence of which is shown to result, in particular at low temperatures, in physically unrealistic transition probabilities. For medium-sized model crystals, the averaged shapes obtained through time averaging over a large number of instantaneous configurations agree well with equilibrium shapes resulting from analytical models. The interface widths are considerably larger than those resulting from SOS models, without significant increase in the surface roughness. Crystal-size-dependent collective motions of solid particles in the interfacial region are observed. On larger crystals the surface shape fluctuations have the character of capillary waves. These results provide a new link between the macroscopic, thermodynamic shape of the interface and the microscopic origin of interface roughness.

The dimer compounds ${\mathrm{Cs}}_3$${\mathrm{Er}}_2$${\mathit{X}}_9$ (X=Cl,Br,I) were synthesized and grown as crystals. Their f-f absorption spectra were measured and their upconversion luminescence behavior was studied as a function of temperature. Both ${}_{}{}^4{}_{}{}^{}$${\mathit{I}}_{15/2}$${\to }^4$${\mathit{I}}_{11/2}$ and ${}_{}{}^4{}_{}{}^{}$${\mathit{I}}_{15/2}$${\to }^4$${\mathit{I}}_{9/2}$ excitations were used. The highest-energy phonons were found by Raman spectroscopy at 285, 190, and 160 ${\mathrm{cm}}^{\mathrm{-}1}$ in ${\mathrm{Cs}}_3$${\mathrm{Er}}_2$${\mathit{X}}_9$ (X=Cl,Br,I), respectively. The upconversion and cross-relaxation behavior is essentially determined by two factors: (i) As a result of the lower phonon energies multiphonon-relaxation processes are orders of magnitude less competitive than in fluorides; (ii) as a result of their dimeric character with very short intradimer ${\mathrm{Er}}^{3+}$-${\mathrm{Er}}^{3+}$ distances and the relatively short interdimer separations all types of nonradiative energy-transfer processes are very competitive. The following trends were observed: a redshift of the f-f multiplet baricenters of up to 1% between chloride and iodide, a reduction of crystal-field splittings up to 50% between chloride and iodide, enhanced near-infrared to visible upconversion efficiency between chloride and bromide or iodide. All these trends are rationalized with simple models. For some selected crystal-field transitions energy splittings resulting from exchange interactions within the dimers were observed.

The parameters A and m in the power-law expression for the kinetic-energy dependence of the secondary-electron cascade of solid elements has been measured as a function of the atomic number of the sample. A pattern has been observed in the values of both A and m which is related to the variations in the Periodic Table. m is found to fall between 0.5 and 1.5 as predicted by theory. Also, it has been established that the coefficient A is correlated with the exponent m. The measured values of m are expected to be related to the exponent of another power law of the kinetic energy which is found to describe the energy dependence of the electron attenuation length for energies above about 400 eV. This expectation is checked against several existing models or empirical expressions for the attenuation length. Moderate agreement is found between m and (2-q) where q is the exponent of the energy dependence of the attenuation length described by Tokutaka et al. and the agreement is poorer with values of q derived from various theoretical treatments of the energy dependence of the inelastic mean free path. m is found to be correlated to the free-electron energy ${\mathit{E}}_{\mathit{p}}$ and A to the density of valence electrons in the solid.

Quadrature photopyroelectric spectra of the optical-to-thermal (nonradiative) energy conversion efficiency of ${\mathrm{Ti}}^{3+}$:${\mathrm{Al}}_2$${\mathrm{O}}_3$ crystals were found to yield good agreement between the experimental quantum-yield spectrum of this optical material and detailed quantum-mechanical calculations of the nonradiative and radiative deexcitation kinetics of the ${\mathrm{Ti}}^{3+}$ ion in the ${\mathrm{Al}}_2$${\mathrm{O}}_3$ host matrix. Both inter- and intraconfigurational decay mechanisms of the excited-state manifold were considered including fast nonradiative relaxations. The experimental quantum-yield spectra were found to be dominated by intraconfigurational nonradiative relaxation below the excited- and ground-electronic-state manifold crossover energy, ${\mathit{E}}_{\mathrm{nr}}$; and by internal conversion processes in the highly vibronically excited ${\mathrm{Ti}}^{3+}$ ion above ${\mathit{E}}_{\mathrm{nr}}$. A one-parameter fit of the quantum-yield model to the data gave the intraconfigurational relaxation time constant ${\mathrm{\tau }}_{\mathrm{intra}}$ for the ${\mathrm{Ti}}^{3+}$:${\mathrm{Al}}_2$${\mathrm{O}}_3$ system. The present combined experimental and theoretical approach can be more generally applicable to other optoelectronic systems exhibiting strong electron-lattice coupling.

The structural development of the ordered intermetallic compounds ${\mathit{T}}_3$${\mathit{X}}_2$ (T=Ni,Fe,Mn; X=Sn,Ge) upon mechanical milling was investigated by x-ray diffraction, magnetic measurements, and subsequently by differential scanning calorimetry (DSC). It is found that the magnetization at 4.2 K increases continuously with increasing milling time in ferromagnetic ${\mathrm{Ni}}_3$${\mathrm{Sn}}_2$ and ${\mathrm{Fe}}_3$${\mathrm{Ge}}_2$. In contrast, in ferrimagnetic ${\mathrm{Mn}}_3$${\mathrm{Sn}}_2$ it decreases. The unit-cell volume of both ${\mathrm{Mn}}_3$${\mathrm{Sn}}_2$ and ${\mathrm{Fe}}_3$${\mathrm{Ge}}_2$ continuously increases. These results are explained well in terms of a special type of atomic disorder: redistribution of transition-metal atoms over two different types of transition-metal sites, induced by ball milling. Exothermic heat effects corresponding to atomic reordering are observed in the DSC scans of ${\mathrm{Mn}}_3$${\mathrm{Sn}}_2$ and ${\mathrm{Fe}}_3$${\mathrm{Ge}}_2$ as well as ${\mathrm{Ni}}_3$${\mathrm{Sn}}_2$ after various periods of milling. The heat evolved in the atomic reordering process increases gradually with milling time. After long-time milling all physical parameters tend to become constant. After prolonged periods of milling, a phase transformation in ${\mathrm{Ni}}_3$${\mathrm{Sn}}_2$ from the orthorhombic-structure low-temperature phase (LTP) to the hexagonal-structure high-temperature phase (HTP) is observed accompanied by a sharp increase in magnetization. ${\mathrm{Mn}}_3$${\mathrm{Sn}}_2$ and ${\mathrm{Fe}}_3$${\mathrm{Ge}}_2$ remain in the hexagonal structure.

The exothermic heat effect corresponding to the phase restoration of the ball-milled metastable HTP to the original equilibrium LTP is evident from DSC scans of ${\mathrm{Ni}}_3$${\mathrm{Sn}}_2$ after long milling periods. The occurrence of the ball-milling-induced phase transformation in ${\mathrm{Ni}}_3$${\mathrm{Sn}}_2$ is also confirmed by a comparison of the ball-milled phase to the high-temperature phase obtained by rapid quenching. The excellent agreement of all experimental results obtained by different techniques proves that by mechanical milling well-defined metastable states are generated in these B8-like compounds and that atomic disorder is the main source of energy storage during ball milling of intermetallic compounds. The particular type of atomic disorder in these B8-like compounds cannot be obtained by rapid quenching from high temperatures.

In the Ni-Ta system, several amorphous alloys were synthesized by room-temperature 200-keV xenon-ion irradiation of multilayered films with various compositions. The metallic-glass-forming range was determined experimentally to extend from 25 to 75 at. % Ni. In addition, a hcp metastable crystalline (MC) phase was observed in the Ni-rich ${\mathrm{Ni}}_{80}$${\mathrm{Ta}}_{20}$ and ${\mathrm{Ni}}_{75}$${\mathrm{Ta}}_{25}$ multilayers at different irradiation stages, and a fcc MC phase was formed in the Ta-rich ${\mathrm{Ta}}_{65}$${\mathrm{Ni}}_{25}$ and ${\mathrm{Ta}}_{75}$${\mathrm{Ni}}_{35}$ multilayers before and after the films were amorphized, respectively. Interesting evolution behavior of the above nonequilibrium solid phases upon thermal annealing was also observed. A free-energy diagram of the system was constructed based on the model of Miedema, de Boer, and de Boer and the method of Alonso and Simozar. The calculation included the free-energy curves of all the competing phases, especially the MC phases, which were considered as compoundlike, and the enthalpies were calculated based on well-identified structural parameters. The calculated energy sequence of the phases was verified at certain compositions by the phase appearance order in the respective as-deposited multilayers upon steady-state thermal annealing, thus confirming the relevance of the diagram in its general outline. The free-energy diagram can explain the formation and the thermal stability of the above solid phases. The growth kinetics of the hcp and fcc MC phases were also discussed in terms of the structural compatibility between the newly formed phase and the matrix in the ion-irradiation process.

We have investigated the structural properties of five different crystalline forms of ${\mathrm{SiO}}_2$ using a first-principles approach. An ultrasoft Vanderbilt pseudopotential is generated for oxygen which enables us to use a small plane-wave cutoff of 25 Ry. The relative stability, the equation of state, and pressure-dependent structural parameters of all five polymorphs have been calculated and found to be in very good agreement with available experimental results.

The charge-unbinding transitions in the two-dimensional classical Coulomb-plasma model are analyzed by use of the equivalence between the two-dimensional Coulomb plasma and (1+1)-dimensional quantum sine-Gordon models. The grand canonical potential and the screening length are obtained from the ground states of the (1+1)-dimensional quantum sine-Gordon model as derived by use of a variational Gaussian wave functional. We have shown that the charge-unbinding transition at small fugacity is a Kosterlitz-Thouless transition, while at large fugacity it is a first-order transition. The dependence of the potential on the mass is similar to that of the phenomenological Landau theory.

The pressure dependence of the structural parameters of deuterated ice VIII has been studied by time-of-flight neutron powder diffraction up to 10 GPa. Variations of the a and c parameters of tetragonal ice VIII, as well as those of the internal atomic coordinates of oxygen and deuterium are reported and discussed. Isothermal equations of state for ice VIII between 0 and 300 K are established. A periodic Hartree-Fock calculation performed at the all-electron level is shown to reproduce the equation of state at 0 K and the pressure dependence of the structural parameters, including the unexpectedly small variation of the O-D bond length up to 10 GPa. It also exactly accounts for the decrease under pressure of the ${\mathit{A}}_{1\mathit{g}}$ vibron frequency in ice VIII, which is shown not to be caused by an increase of the covalently bonded O-D distance.

In several recent studies orthogonal correlation-crystal-field (CCF) operators for the ${\mathit{f}}^{\mathit{N}}$ configuration have been used to improve fits to the energy levels of several lanthanide compounds. Here we report results for ${\mathrm{Sm}}^{3+}$:${\mathrm{Na}}_3$[Sm(oxydiacetate${)}_3$]⋅${2\mathrm{N}\mathrm{a}\mathrm{C}\mathrm{l}\mathrm{O}}_4$⋅${6\mathrm{H}}_2$O. We show that the addition of fourth rank orthogonal CCF operators ${\mathit{g}}_2^{\mathrm{}\left(4\right)\mathrm{}}$, ${\mathit{g}}_4^{\mathrm{}\left(4\right)\mathrm{}}$, ${\mathit{g}}_{10\mathit{A}}^{\mathrm{}\left(4\right)\mathrm{}}$, and ${\mathit{g}}_{10\mathit{B}}^{\mathrm{}\left(4\right)\mathrm{}}$ to the Hamiltonian for the 4${\mathit{f}}^5$ configuration improves the fit. Our fits are in qualitative agreement with ab initio calculations of CCF effects for lanthanide ions.

It is shown, by investigating the ESR parameters, that the impurity ${\mathrm{Fe}}^{3+}$ ion substitutes the ${\mathrm{Nb}}^{5+}$ ion rather than the ${\mathrm{Li}}^{+}$, with use of the generalized crystal-field theory and a complete diagonalization procedure.