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The transport coefficients of the Anderson model require knowledge of both the temperature and frequency dependence of the single-particle spectral densities and consequently have proven difficult quantities to calculate. Here they show how these quantities can be calculated via an extension of Wilson's numerical renormalization group method. Accurate results are obtained in all parameter regimes and for the full range of temperatures of interest ranging from the high-temperature perturbative regime T>>T_K, through the cross-over region T approximately=T_K, and into the low-temperature strong coupling regime T<<T_K. The Fermi liquid relations for the T² coefficient of the resistivity and the linear coefficient of the thermopower are satisfied to a high degree of accuracy. The techniques used here provide a new highly accurate approach to strongly correlated electrons in high dimensions. Sacks, J. Chang, Y.F.

Measurements of the temperature dependence of the ultrasonic attenuation at 5, 10, and 28 MHz in molybdenum single crystals of 99.995% purity, oriented along (111), (110) and (100), have provided a detailed study of the alpha complex of dislocation relaxation peaks following compressive stressing at room temperature. At 5 MHz the peaks were at 148, 167 and 206 K, independent of the orientation of the crystal. As expected, each of the peaks moved to higher temperatures as the frequency of measurement was increased and from these data the activation enthalpies and attempt frequencies were 0.18 eV and 6.6*10¹¹ Hz, respectively, for alpha ₁, 0.22 eV and 1.84*10¹³ Hz, respectively for alpha ₂, and 0.20 eV and 4.95*10¹¹ Hz, respectively, for alpha ₃. Similar measurements at 10 MHz on polycrystals of the same purity showed the effects of increasing the applied stress from 0.2 to 29 MPa and of irradiation with doses D_y of gamma -rays up to 8.6*10⁸ gamma vt and doses D_n of neutrons up to 10.8*10⁹ nvt. It was found that the peaks' strength Q_max^-1 varied as D^-m, where m is a parameter for each irradiated pack.

The solid state amorphization reaction in Ni₅₀Mo₅₀ alloys has been investigated by neutron diffraction and neutron small-angle scattering. A series of specimens has been mechanically alloyed for 2,4,8,16 and 32 hours in a high-energy ball mill. The initial stages of the reaction are characterized by the deformation of the parent crystalline phases. After 8 hours of treatment a change in the samples is observed and those treated for 16 and 32 hours consist of an amorphous NiMo phase with some free molybdenum. The intensity of the small-angle scattering is much greater for the 2 hour specimen than for the parent, but decreases regularly for longer milling times. The Guinier plots show considerable curvature indicative of particles with a range of sizes. Graphs of log I(Q) versus log Q show a sequence of changes which correspond to the sequence of changes seen in the diffraction data. The graphs for the 2 and 4 hour samples are linear over an extended range of scattering vector Q. The slope of these graphs is about -3.4, which is indicative of a rough surface having the fractal dimension of 2.6. The authors tentatively associate this with the convoluted surfaces between the nickel and molybdenum particles which are drawn into an interpenetrating filamentary structure in the early stages of the mechanical alloying process.

Hydrogen and deuterium atoms are stabilized in a xenon matrix from the gas phase and investigated by electron spin resonance (ESR). It is shown that H and D atoms are trapped in substitutional positions of the matrix crystal lattice. The ESR linewidths of H and D show an isotope effect and are found to be greatly increased (up to 80%) by the zero-point vibrations of the atoms. The charge-transfer coefficient v(r) identical to gamma exp(- eta r) between matrix atom and hydrogen atom is estimated. A comparison is carried out between experimental and theoretical values for the relative matrix shift of the H atom hyperfine constant.

The electronic structure of small icosahedral (I_h) silicon clusters has been studied using the discrete variational method within the local density functional theory. The I_h Si₁₃ cluster is not a closed-shell structure, in contrast to the predictions of empirical calculations. The Si₁₃²⁺ cluster is found to be stable, with an exceptionally large LUMO-HOMO gap.

Self-consistent band structure calculations have been performed for the antirutile structure of epsilon-Ti₂N and for the metastable long-range ordered defect structure of delta'-Ti₂N. In accordance with recent neutron diffraction data a relaxation of the Ti atoms in direction of the crystallographic c-axis away from the vacancies was assumed. Band structures, densities of states, partial local densities of states and charge density contour plots are discussed with regard to the chemical bonding in these compounds. On the basis of the band structure data, a qualitative explanation for the occurrence of the metastable long-range ordered delta'-phase in the Ti-N_x system near a composition x=0.5 and for the high stability of the in -phase can be given. Contrary to hypothetically ordered TiN_0.75, no so-called vacancy states with sigma bonding of Ti e_g-like states across the vacancies are found for delta'-Ti₂N.

An elastic anomaly, observed in the heavy Fermi liquid state of Ce alloys (for example, CeCu₆ and CeTe), is analysed by using the infinite-U Anderson lattice model. Four atomic energy levels are assumed for f-electrons. Two of them are mutually degenerate. A small crystalline splitting 2 Delta is assumed between two energy levels. The fourfold degenerate conduction bands are also considered in the model. The authors solve the model using the mean-field approximation to slave bosons, changing the Fermi energy in order to keep the total electron number constant. The non-zero value of the mean field of the slave bosons persists over temperatures much higher than the Kondo temperature. This is an effect of the constant electron number. Next, the linear susceptibility with respect to Delta is calculated in order to obtain the renormalized elastic constant. The resulting temperature dependence of the constant shows a downward dip. The authors discuss the relation of their findings to experimental data.

Monte Carlo simulations of a system of point charges occupying a fraction of a random array of sites in two and three dimensions are reported. The authors focus on the Coulomb-interaction-driven ordering phenomena in the system. The model corresponds directly to the details of physics of charges of partially filled resonant impurities in a semiconductor, but qualitative results are of wider significance. Spatial correlations of charges, due to long-range repulsive interactions between them, are studied as a function of temperature by means of pair correlation functions and one-particle density of states. The presence of inherent disorder of the allowed positions of the impurity charges is found to influence the ordering process, leading to a saturation of correlation range at low temperatures. Such behaviour is not observed in a pseudo-liquid model also studied for the sake of comparison. Within the latter model the charges are assumed to have the same densities, and interactions to have the same strength, but instead of being restricted to a random array of sites the charges are allowed to take any position. Comparison of the two models allows the authors to introduce a phenomenological effective temperature, which includes the effects of the built-in disorder on the same footing as the thermal disorder. The formation of the Coulomb gap in the density of states is discussed and its relation to the Madelung gap in crystalline materials pointed out.

This work is a study on the incorporation phenomena of cobalt magnetic ions in ZnSe. Zn_1-xCo_xSe (x<0.02) polycrystals were prepared by the melt growth technique. The influences of Co impurities on the ZnSe crystal were investigated using X-ray diffractometry and Raman spectroscopy. The solid solubility of melt grown crystals is about x=0.02(+or-0.0015). It was found that the lattice constant of the crystal increases linearly up to x=0.0086 and then saturates as the amount of Co impurities increases. It is also observed that the TO₂ and LO₁ Raman modes in Zn_1-xCo_xSe soften for x<0.0086 by the addition of Co atoms. The oscillator strength, which is calculated using the energies of the TO₂ and LO₁ phonon modes, decreases as the cobalt content increases. There is a linear relationship between the ionicity and the oscillator strength with a slope of -7.02*10²⁵ s^-2. From these observations, it is clear that the instability of the Zn_1-xCo_xSe crystal is mainly due to the ionicity difference between Zn and Co atoms.

The polarized absorption spectra of Tm³⁺ ions in LiNbO₃ and LiNbO₃(MgO) crystals, at 20 K, are presented. The Stark levels of the different multiplets up to 28000 cm^-1 are identified. The results are consistent with C₃ symmetry for the rare-earth ions and a singlet (A) character for the lower Stark level of the ground state. Several transitions show additional structure, which indicates multi-site occupancy for the Tm³⁺ ions. The effect of MgO codoping which includes the appearance of new absorption bands, is also discussed.

Quasi-critical percolation systems are prepared from conducting metal-insulator granular composite samples by subjecting them to high-density current pulses. This treatment drastically reduces the room-temperature conductivity of the mixture, due to the explosion of a fraction of the metallic grains in percolation channels. Provided that the pulse current density is high enough, the samples show the hopping conductivity characteristic of an insulator. Abnormal dielectric properties of quasi-critical percolation systems have been previously investigated, and the objective of the present work is to study the transport and magnetotransport phenomena. Two distinctly different nearest-neighbour hopping (NNH) conduction regimes, which depend on the sample preparation conditions, are observed in the quasi-critical samples. The first type of conduction is dominated by hopping between adjacent metallic grains embedded in a dielectric host medium. The grain diameter, evaluated from the resistance versus temperature data, is found to be several nanometres. Negative orbital magnetoresistance (NMR) with a quasi-linear field dependence, is observed at both 77 and 300 K. In the sample of the second type (those which have been subjected to higher-current pulses or to several sequential pulses of the same amplitude), conduction is also found to be dominated by NNH between conducting sites in the host medium, but an evaluation of the 'grain' size shows them to be of atomic dimensions.

The resistance of self-heated mesoscopic structures has been used for some time as a way to prove the characteristics of electrons and phonons in these systems. In this paper, the authors consider the differences between low-frequency dynamic and static resistance measurements of mesoscopic systems in the steady state. They present a general description of these non-equilibrium experiments, and discuss the connection to an integration procedure used to determine the differences between the two methods of resistance measurements. The authors have applied the analysis to electron-heating experiments and have shown that the difference between the static and dynamic resistance depends on the electron-phonon coupling and the acoustic coupling appropriate for the mesoscopic structure.

The magnetic properties of a ferromagnetic or ferrimagnetic mixed spin-1/2 and spin-3/2 Ising system are studied by the use of the effective-field theory. The general expressions for evaluating these properties are given. In particular, the internal energy, specific heat and susceptibility of the system with the honeycomb lattice are numerically examined. Some characteristic phenomena are found in these properties.

Self-consistent field electronic structure calculations were performed for embedded clusters representing FCC disordered Fe-Pd alloys. The discrete variational method was employed in the framework of local-spin-density theory. The hyperfine magnetic fields and the magnetic moments on several distinct Fe sites and Pd sites in the Fe-Pd alloys were derived from the calculations. The results for magnetic moments show that the Fe-Pd alloys are strong ferromagnets and that the Fe magnetic moment is almost independent of the number of Pd nearest neighbours, slightly increasing with increasing number of Pd nearest neighbours, while Pd atoms are strongly polarized. When a Pd atom is surrounded by six Fe and six Pd nearest neighbours, it has a maximum induced magnetic moment. The changes in hyperfine fields due to the variation in the number of Pd nearest neighbours are in good agreement with experiment.

Measurements are reported on the temperature dependence of the elastic constants of dilute Cr-Ru and Cr-Re alloy single crystals, containing respectively 0.3 at.% Ru, 0.5 at.% Ru, 0.3 at.% Re and 0.5 at.% Re. Neutron diffraction experiments were also performed on the two Cr-Ru crystals. Well defined anomalies were observed in all the elastic constants as well as in the integrated neutron intensities at the magnetic phase transitions of the alloys. The commensurate (C) spin density wave (SDW) to paramagnetic (P) and incommensurate (I). SDW to P transitions are second-order phase transitions, while the ISDW-CSDW and CSDW-ISDW transitions are of first order. A thermodynamic model describes the temperature dependence of the magnetic contributions ( Delta B) to the bulk modulus (B) reasonably well, but fails to give the observed relationship between Delta B and the magnetization.

The isotropic Compton profiles of tantalum were measured by a 59.54 keV gamma -ray Compton profile spectrometer. Comparisons with the renormalized free-atom model and band structure calculations have been made. The valence electron configuration is found close to 5d⁴6s¹, and the band structure calculations give a sharper Compton profile. The Compton profiles of V, Nb and Ta normalized to their respective Fermi momentum have also been compared and discussed. The d electrons are seen to play a more significant role than the orthogonality effect, as observed in the IVA group elements.