We present a spectroscopic overview of ${\mathrm{Cr}}^{4+}$-doped ${\mathrm{Y}}_3$${\mathrm{Al}}_5$${\mathrm{O}}_{12}$ (YAG), a new laser material for the near-infrared region (NIR). It is found that only 2% of the Cr ions are tetrahedrally coordinated ${\mathrm{Cr}}^{4+}$. However, the optical spectra are dominated by this ion. The missing inversion symmetry for the tetrahedral site causes higher cross sections than are common for octahedrally coordinated ions. The experimental data include a survey of the absorption and emission features. Lifetime measurements, as well as piezospectroscopic experiments yield additional spectroscopic information. Our data suggest an energy-level assignment that is different from the one previously used to describe the spectrum. Instead of ascribing the strong NIR absorption band, centered at around 1000 nm, to the ${}_{}{}^3{}_{}{}^{}$${\mathit{B}}_1$${(}^3$${\mathit{A}}_2$)${\to }^3$E${(}^3$${\mathit{T}}_2$) transition, we assign it to the ${}_{}{}^3{}_{}{}^{}$${\mathit{B}}_1$${(}^3$${\mathit{A}}_2$)${\to }^3$${\mathit{A}}_2$${(}^3$${\mathit{T}}_1$) transition. This assignment is able to describe the polarization-dependent spectra, the piezospectroscopic results, and the results from earlier polarization-dependent saturation experiments. An analysis of the luminescence decay rate yields quantum efficiencies of around 49% at 10 K and 15% at room temperature, resulting in a peak emission cross section of 3.5×${10}^{\mathrm{-}19}$ ${\mathrm{cm}}^2$.

Recent studies, simulating the behavior of insulating materials in a fusion device environment, show that under concurrent applications of radiation, applied electric field, and elevated temperature, they suffer degradation of their electrical properties. The goal of the present study is to address the mechanism of this radiation-induced electrical degradation and the defects involved. Our results show that when an ${\mathrm{Al}}_2$${\mathrm{O}}_3$ crystal under a moderate electric field is irradiated with 1.8 MeV electrons at 773 K, the dc conductivity during and after irradiation increases rapidly above a critical dose and saturates after the conductivity increases by a factor of ${10}^3$. There are two main conclusions. First, the electrical degradation is due to the charge of the electrons and holes created during radiation, rather than due to displacements of indigenous ions by elastic collisions with the energetic electrons. Second, the defects attending the observed electrical degradation are dislocations. Transmission electron microscopy studies revealed regions of large dislocation density distributed nonuniformly throughout the degraded area, with an overall average density of ∼${10}^9$ ${\mathrm{cm}}^{\mathrm{-}2}$, as opposed to ∼${10}^4$ ${\mathrm{cm}}^{\mathrm{-}2}$ in regions which were not electron irradiated nor subjected to an electric field. The concentration of point defects, as characterized by optical absorption and electron paramagnetic resonance, was below detectable limit. In addition, no second phase was observed.

The local structure and electronic properties of an interstitial ${\mathrm{Ni}}^{+}$ impurity in diamond are studied using the discrete-variational local-density-functional method with cluster models. The existence of a trigonal distortion about the impurity site is verified by the binding-energy calculation. The electronic-structure calculation shows that two optical absorption bands with zero-phonon lines at 1.40 and 3.1 eV and a broad absorption band centered near 1.4 eV, observed in synthetic diamond, are associated with the single interstitial ${\mathrm{Ni}}^{+}$ impurity. The calculated results of the spin-orbit splitting of the ground state and the energy positions as well as polarization properties of the optical transitions are in good agreement with the experimental ones. An optical transition of about 2.5 eV is predicted, which reveals a different polarization property compared with the observed 2.51-eV absorption band and 2.56-eV luminescence band.

The effect of multiple scattering (MS) on the x-ray absorption fine structure (XAFS) spectrum is investigated by means of calculations using the full curved wave theory with exact polarization dependence and including all orders of MS up to five. We performed model calculations for the hypothetical half-monolayer c(2×2) overlayer of atomic O on an unreconstructed Cu(100) or (110) surface and assuming adsorption in hollow, bridge or atop sites at an O-Cu distance of 1.85 Å. From these calculations it is found that MS is important for some adsorption geometries and polarization directions. Failure to include at least the dominant double scattering terms in an analysis may result in errors in the parameters for some distances beyond the nearest neighbors. We derive general rules for when MS is likely to influence surface EXAFS (extended x-ray absorption fine structure) spectra. Most of the significant three-atom paths involve either one small (<45°) or large (>150°) scattering angle, with the most favorable geometry for MS being when forward scattering occurs at a light atom and backscattering at a heavy atom. The dominant MS paths involve the shortest bonds in the material. Contributions tend to be largest where the ɛ vector is roughly parallel to one of these bonds, and there is often a substantial polarization dependence to the MS terms. Multiple-scattering effects are likely to be strongest when adsorbate atoms occupy sites close to coplanar with the surface. The presence of MS peaks at high-R in an experimental spectrum is a useful indication of high symmetry and relatively long-range order in surface structures. We also show that much of the near-edge region can be fitted using an EXAFS algorithm which includes a sufficiently small number of multiple-scattering paths to make routine calculation feasible, provided that the exact curved wave polarization dependence is included.

We use a simple intermolecular potential to describe a large number of properties of solid ${\mathrm{C}}_{60}$. Minimizing their free energies we establish an equation of state for both phases, and calculate the phase diagram, discontinuities at the phase transition, and, in general, statical, dynamical and thermodynamical properties as a function of temperature and pressure, in good overall agreement with experiment. We find that in the lower temperature phase several secondary orientational energy minima appear, and that therefore the simple two-level picture for the residual orientational disorder must be revised.

A model is proposed to describe the production of light induced by energetic ions in scintillator materials, based on the distribution of energy deposited by the secondary electrons produced along the ion’s track. The initial energy of the electrons is determined using an impulse approximation in which their motion is constrained to the radial direction, perpendicular to the ion’s track. The residual energy of the electrons along the radial coordinate is obtained from an expression for the specific energy loss obtained from Lindhard’s potential theory. Contributions from backscattered electrons to the energy deposition are included in the calculation. Local production of energy carriers is assumed to be proportional to the local density of deposited energy, in the absence of quenching effects. The latter are introduced by assuming the existence of a maximum energy density greater than which prompt quenching predominates and the energy carrier density reaches a maximum constant value. Light production is related to the process of energy transport through thermal diffusion of energy carriers to luminescence centers. Simple algebraic expressions are given for the energy deposition profile and for the specific luminescence. Model predictions are compared with published experimental data from various organic and inorganic scintillators.

We report inelastic scattering measurements of the neutron Compton profile, J(y), in liquid Ne, over a broad range of neutron wave-vector transfers for liquid at a density of 36.1 atoms ${\mathrm{nm}}^{\mathrm{-}3}$ at 27 K. The measurements used a chopper spectrometer at a pulsed-spallation neutron source. The wave vectors span the range 105 to 276 ${\mathrm{nm}}^{\mathrm{-}1}$, a range in which deviations from impulse-approximation scattering are expected to be dominated by final-state effects. Spectrometer resolution and sample-dependent multiple scattering were taken into account using a sophisticated Monte Carlo simulation. The data were analyzed, using the series expansion of Sears, to determine the magnitude of the first antisymmetric term in the expansion as a function of wave-vector transfer. Special care was taken to assess possible contributions arising from systematic errors. We find that the measurements agree very well with a calculation using a pair-distribution function, g(r), from path-integral Monte Carlo simulation of liquid Ne near this density and temperature.

An extensive structural analysis of a decagonal ${\mathrm{Al}}_{63}$${\mathrm{Cu}}_{17.5}$${\mathrm{Co}}_{17.5}$${\mathrm{Si}}_2$ alloy is reported. The structure is analyzed within the context of both microcrystalline and quasicrystalline models through the examination of single-crystal x-ray diffraction patterns. We conclude that our sample is in a microcrystalline state formed of coherent twins of an approximant state of the quasicrystal. We underline that diffraction patterns exhibiting sharp peaks and perfect tenfold symmetry do not necessarily correspond to long-range quasiperiodic order, which could have implications for the physical properties of quasicrystal-like alloys. The present work also gives us the opportunity to point out and discuss some general features: (i) the intensity cutoff problem for quasicrystal structure determinations, (ii) the consequence in structure determination if a sample supposed to be a quasicrystal is in fact a microcrystal, and (iii) the proper determination of basis vectors in decagonal quasicrystals.

We report on low-frequency Raman-scattering results obtained on ${\mathrm{K}}_{1\mathrm{-}\mathit{x}}$(${\mathrm{NH}}_4$${)}_{\mathit{x}}$I mixed crystals and on its deuterated isomorph (x=0.46 and 0.62) as a function of temperature. With decreasing temperature, the Raman susceptibility is dominated by the growth and the narrowing of a strong component around 30 ${\mathrm{cm}}^{\mathrm{-}1}$ and a much weaker one at 44 ${\mathrm{cm}}^{\mathrm{-}1}$. A comparison of the present results with recent inelastic neutron scattering data on the same mixed crystals enabled us to assign the components at 30 and 44 ${\mathrm{cm}}^{\mathrm{-}1}$ to the density of states of transverse and longitudinal acoustic phonons at the Brillouin-zone boundary. For the most prominent band, it is also shown that the drastic increase of the intensity coincides with the onset of freezing of antiferroelectric correlations evidenced by diffuse scattering measurements (${\mathit{T}}_{\mathit{F}}$=45 and 58 K for x=46% and 62%, respectively). The strong activity of the zone-boundary acoustic branches below ${\mathit{T}}_{\mathit{F}}$ is interpreted in terms of breaking of the local symmetry of the mixed crystal. Finally, the temperature dependences of the main Raman active components identified below 250 ${\mathrm{cm}}^{\mathrm{-}1}$ are also discussed, with particular attention to aspects relevant to the freezing transition and to the orientational disorder.

Monte Carlo results are obtained for random walks of excitation on a one-dimensional lattice with a Gaussian energy distribution of site energies. The distribution Ψ(t) of waiting times is studied for different degrees of energetic disorder. It is shown that at T=0, Ψ(t) is described by a biexponential dependence and at T≠0 the distribution Ψ(t) broadens due to the power-law ‘‘tail’’ ${\mathit{t}}^{\mathrm{-}1\mathrm{-}\gamma }$ that corresponds to the description of Ψ(t) in the framework of the continuous-time random walk model. The parameter γ depends linearly on T for strong (T→0) and moderate disorder. For the case of T=0 the number of new sites S(t) visited by a walker is calculated at t→∞. The results are in accordance with Monte Carlo data. The survival probability Φ(t) for strong disorder in the long-time limit is characterized by the power-law dependence Φ(t)∼${\mathit{t}}^{\mathrm{-}\mathrm{\beta }}$ with β=cγ, where c is the trap concentration and for moderate disorder the decay Φ(t) is faster than ${\mathit{t}}^{\mathrm{-}\gamma }$.

We examine the possibility of quasicrystalline metastable states for one- and two-component hard-sphere solids, systems whose excess free energies are entirely entropic. The investigations are carried out using the self-consistent modified weighted density approximation of density functional theory as applied to inhomogeneous systems. By a method entirely formulated in reciprocal space, we find that the best quasicrystalline phases have significantly higher free energies than the crystalline or liquid states. These results indicate that quasicrystals probably cannot be stabilized entirely by dynamic entropy.

We found that the decay rate, λ, of the correlation function, G, in glass-forming liquids can be expressed in terms of G itself. A three-parameter function of the form λ=${\lambda }_0$+${\lambda }_1$${\mathit{G}}^{\mathit{k}}$ leads to an analytic solution for the function G in the time domain. This assumption also allows one to express the function G as an infinite series of Lorentzians in the frequency domain. This model has been applied to Mössbauer absorption spectra of aqueous ${\mathrm{FeCl}}_2$ solutions in glycerol (14.4-keV line of ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$) and higher temperature quasielastic (Rayleigh) scattering data obtained by the scattering of the ${}_{}{}^{183}{}_{}{}^{}\mathrm{W}$ 46.5-keV Mössbauer line from pure glycerol. The model describes the Mössbauer data as well as the empirical Kohlrausch or Cole-Davidson (CD) laws. Also, the function λ(G) can be expressed in a two-parameter form as ${\lambda }_0$exp(αG). It has been shown, however, that a closed-form expression for the function G does not exist in the time or frequency domain for this latter form. On the other hand, the exponential form gives a two-parameter fit to existing data, suggesting a physical basis to this form and implies that small changes in G are proportional to fractional changes in λ. It has been found that the parameter α has some universal meaning as it remains constant over a significant low-temperature range accessible experimentally, decreasing to the zero value with increasing temperature. Our analysis suggests that the parameter α may change in steps as sample temperature is increased. Such a behavior suggests that some processes (degrees of freedom) are ‘‘freezing out’’ at well-defined temperatures.

We investigate the quantum states of a light particle (positronium, or Ps) in a disordered medium (fluid xenon). The Ps atom is modeled as a hard sphere which has thermalized in a Lennard-Jones fluid. The purpose of this paper is threefold: first, to test the efficacy of a recent analytic theory by comparing its predictions with available results of path-integral Monte Carlo (PIMC) simulations; second, to explore the Ps-xenon system over a much wider range of circumstances than is possible with PIMC; third, to report predictions for the lifetime of ortho-Ps and the momentum distribution of para-Ps at high density which should be of interest to condensed-matter experimentalists. In sharp contrast with the case of an electron or positron, the reference-interaction-site-model (RISM)-polaron theory produces Ps-fluid pair-distribution functions in good agreement with the PIMC results. As a result, the pick-off decay rate of ortho-Ps in the transition region between localized and extended states is reproduced successfully. We also find that the variance of the momentum distribution of para-Ps agrees qualitatively with experimental measurements of the angular correlation of the annihilation photons. Compared with the behavior at low density, above the critical density up to about 3${\mathrm{\rho }}_{\mathit{c}}$ the RISM-polaron theory predicts strong confinement of the localized Ps atom and different distortion of the local fluid density, resulting in the monotonic increase of the decay rate and the momentum variance with mean fluid density. As a consequence, the slope of the decay rate is much greater than the extrapolated low density limit predicted by the older density functional theories. These predictions of RISM-polaron theory for the behavior of positronium in a dense fluid suggest that the traditional picture of self-trapping in a fluid is incomplete and call for more careful experimental investigations to resolve this issue.

In a ternary alloy, there are three independent pair-correlation functions for each orbit. Diffraction experiments cannot extract this information unless variations in the scattering factors of each atomic species are exploited. Such an experiment, using x-ray anomalous scattering with a synchrotron radiation, is described in this paper on a (${\mathrm{Ni}}_3$Fe${)}_{0.93}$${\mathrm{Cr}}_{0.07}$ single crystal. Due to the small amount of Cr, only two pair-correlation functions have been obtained directly from the diffraction data. The missing information has been supplied using an original method based on the inverse cluster variation method. As a by-product, the method gives plausible effective pair potentials which have been used in Monte Carlo simulations. Atom distributions of the alloy and intensity maps of the diffuse scattering are calculated as a function of the temperature, through the order-disorder transition. The method used opens a way towards a better knowledge of ternary alloys, for which very few direct determinations of the degree and nature of the ordering are possible.

A prototype exciton-phonon Hamiltonian is subjected to the Fulton-Gouterman (FG) transformation which diagonalizes the excitonic subspace. The evolving FG equations elucidate most clearly the two counteracting tendencies inherent in the model: self-trapping and anti-self-trapping. Two types of test functions for the FG ground state are devised by further unitary transformations which aim at simulating the competing influences in the FG equations. It is found that the displaced antisqueezing type is a suitable description for the ground state in the low coupling regime but it is inappropriate for the large coupling case. The displaced reflective type yields acceptable results for all coupling-transfer combinations. A nonunitary generated as well as the adiabatic trial wave function are also discussed.

Explicit formulas for one-electron propagators x and y between same and distal corner sites are given for several families of hierarchical lattices. These give x and y as a function of lattice size for a set of energies dense in the Cantor portion of the spectrum of the large-lattice limit. With linear chains as quantum lead wires, the Kubo-Greenwood conductance is expressed succinctly in terms of x and y. Lattice families treated are (1) the Sierpiński simplex family scrS of Dhar in Euclidean dimension k, (2) the scrR family introduced previously, also k dimensions, (3) a star-delta dual to the scrR family, (4) a two-parameter family of k dimensional, q sheeted lattices related to those of de Menezes and de Magãlhaes, (5) a dual to the two-parameter family, and (6) the first several members of a family related to the lattices of Hilfer and Blumen. The exact results derive from a formalism for solving the dynamics of the renormalization map explicitly. Special properties that make the renormalization maps solvable are enumerated, and the general solution procedure is reviewed. Power-law scaling of conductance with lattice size is found at special energies for each lattice. In several cases there are uncountably many energies at which conductance becomes independent of lattice size.

We analyze the Mössbauer quadrupole data of several biferrocenium salts, using a recent two-orbital extension of a previous cooperative vibronic Piepho-Krausz-Schatz (PKS) model. A discontinuous (continuous) localized-delocalized transition is described in terms of an order-disorder transition of first (second) order. The first-order results from a crossover of vibronic states based on the two orbitals. The thermal variations of the specific heat are calculated and compared to the available experimental data.

For a complete description of out-of-plane magnetic surface anisotropy and its angular dependence, higher-order terms are needed beyond the quadratic Néel-type approximation. The spin-reorientation transition between perpendicular and in-plane magnetization is influenced by these higher-order terms in an elementary manner. Second- and fourth-order terms of the magnetic surface anisotropy were determined by torsion-oscillation magnetometry for uncovered Fe(110) films prepared on Cr(110), and for Pd-covered Co films prepared on Pd(111). These samples show different types of the spin-reorientation transition.

Time-dependent spin-autocorrelation functions at T=∞ and (in particular) their spectral densities for the bulk spin and the boundary spin of the semi-infinite spin-1/2 XXZ model (with exchange parameters ${\mathit{J}}_{\mathit{x}}$=${\mathit{J}}_{\mathit{y}}$≡J, ${\mathit{J}}_{\mathit{z}}$) are investigated on the basis of (i) rigorous bounds in the time domain and (ii) a continued-fraction analysis in the frequency domain. We have found strong numerical evidence for spin diffusion in quantum spin models. For ${\mathit{J}}_{\mathit{z}}$/J increasing from zero, the results of the short-time expansion indicate a change of the bulk-spin xx-autocorrelation function from Gaussian decay to exponential decay. The continued-fraction analysis of the same dynamic quantity signals a change from exponential decay to power-law decay as ${\mathit{J}}_{\mathit{z}}$/J approaches unity and back to a more rapid decay upon further increase of that parameter. By contrast, the change in symmetry at ${\mathit{J}}_{\mathit{z}}$/J=1 has virtually no impact on the bulk-spin zz-autocorrelation function (as expected). Similar contrasting properties are observable in the boundary-spin autocorrelation functions.

We have measured partially angle-integrated photoemission with linearly polarized undulator radiation on the valence band and on the 3p and 3s core levels of iron in an Fe(100) single crystal. A large asymmetry is measured for 3p core levels when reversing the magnetization direction perpendicular to the photoemission plane. A smaller but clear asymmetry is also measured in the valence (3d) band spectrum, and no asymmetry is measured for the 3s core levels. The results are explained by the transverse spin polarization of the near-normal-emission photoelectrons which is induced by the light p polarization and by spin-orbit interaction in the initial state. From the line-shape change of the 3p core level as a function of magnetization reversal the fine structure of the 3p multiplet is derived. This effect provides a very efficient diagnostic of the surface magnetization direction allowing to measure magnetic ordering at surfaces similarly to the Kerr effect, but in a chemically specific and intrinsically surface sensitive way as shown by examples on ferromagnetically coupled and antiferromagnetically coupled Fe/Cr interfaces.