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Photoelectron spectra of calaverite, AuTe₂, show a Au 5d band extending from -3 to -8 eV below the Fermi energy. Photoemission peaks from Au 4f core levels are sharp, indicating that all Au atoms have the same valency. The binding energies of Au 4f electrons in AuTe₂ and metallic Au are approximately the same. According to these observations all Au atoms in AuTe₂ are monovalent Au(I). Photoelectron spectra of Te 4d and 3d core levels show single sharp peaks, characteristic of a single valency for Te. These investigations clearly establish the absence of appreciable valency fluctuations or mixed valency Au(I)-Au(III) in AuTe₂.

The authors present a study of Si 2p and Ge 3d core levels and of the valence Si 3p and conduction Si p and Ge s,d states by, respectively, X-ray-induced photoelectron spectroscopy and soft X-ray emission and absorption spectroscopy, for hydrogenated amorphous silicon-germanium a-Si_1-xGe_x:H alloys with 0.16<x<0.60 prepared by the glow-discharge technique. As the Ge concentration increases, the core level distributions are gradually broadened; they attribute this effect to an increase in the local disorder by alloying. The low-binding-energy edge of the valence p distribution is shifted towards E_F while the bottom of the conduction band remains at the same energy position. Thus they clearly show that the decrease in the optical gap as x_Ge increases is only due to a shift of the valence band edge. At the bottom of the conduction band, Si p and Ge s,d empty states are totally mixed; a spreading of the edge is observed as the Ge concentration increases. Eventually, the X-ray emission of a core excitonic state is observed as a faint structure in the Si K beta spectra, for x_Ge=0.45 and 0.60.

The electrical resistivities of the low-dimensional conductors (Na_1-xLi_x)_0.9Mo₆O₁₇ (x=0, 0.2) have been measured between 4.2 and 300 K under hydrostatic pressure. The magnitude of the resistivity anomaly associated with the charge-density wave (CDW) transition became smaller with increasing pressure, while the transition temperature itself was found to be enhanced by the pressure in contrast to the simple cases of the inorganic low-dimensional conductors.

Polycrystalline samples of Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O_Y with the 108 K superconducting phase show a magnetic field dependent broadening of the resistive transition for temperatures below 118 K. The temperature of the onset of dissipation, T_d(H), decreases as a function of increasing field strength, H. The temperature dependence of dr/dT, where r is the normalized resistance of the samples, indicates the presence of two scattering mechanisms that are responsible for the broadening of the transition when H not=0. The first mechanism causes a steep increase of dissipation for temperatures that are close to T_d(0). The second one is turned on at T_d(H). The authors fit the shape of the resistive transition for the temperatures between T_d(H) and 118 K by using the Halperin-Nelson form for the excess of conductivity: Delta sigma varies as sigma _N xi _T²(T) and assuming that xi _T(T) is related to xi _E(T) and xi ₁(T) via: xi _T^-2(T)= xi _E^-2(T)+ xi _I^-2(T), where xi _E(T) and xi ₁(T) are the correlation lengths attributed to each scattering mechanism.

Substitution of Fe for Cu in Bi₂Ca₁Sr₂Cu₂O_y was found to be more effective in reducing T_c than in YBa₂Cu₃O_y. The authors discovered that the superconductivity which has been lost in Fe-doped Bi₂Ca₁Sr₂Cu₂O_y can be greatly recovered in vacuum at about 400 degrees C. This suggests that the exchange interaction between the magnetic moments and superconducting pairs is not predominantly in pair-breaking mechanisms, leading to the loss of superconductivity in high-T_c ceramics. As the non-doped and Fe-doped Bi₂Ca₁Sr₂Cu₂O_y samples were subjected to vacuum annealing, the lattice parameter c was found to increase and Mossbauer data revealed that both the local structure around Fe (or Cu) sites and the relative number of Fe (or Cu) sites associated with different values of quadrupole splittings Delta Q₁ and Delta Q₂ varied. This implies that the increase in T_c for the annealed samples is related to the local distortion of Cu-O layers. The optimum value of T_c ( rho =50%) for Bi₂Ca₁Sr₂Cu₂O_y as high as 94 K can be obtained by annealing the sample in a vacuum at 500 degrees C for 30 min.

The phase diagram of the spin-one Ising model with a random crystal field is investigated by the use of the effective-field theory with correlations. The important differences from the results obtained by the standard mean-field theory are indicated. The differences are very similar to those found in the dilution problem of magnetic systems.

The pressure dependence of the Curie point of weakly ferromagnetic Y(Co_1-xAl_x)₂ alloys for x approximately 0.15 has been measured at pressures up to 8 kbar. The pressure dependence of the spontaneous magnetization per unit mass at 0 K ( sigma ₀) has been deduced for the same alloys from measurements of the forced volume magnetostriction in fields up to 12 T. It was found that ( delta ln T_c/ delta ln V)=( delta ln sigma ₀/ delta P)=120+or-17, one of the largest values known for a ferromagnetic material. The critical pressure for the collapse of ferromagnetism is approximately 9+or-1 kbar for 0.14<x<0.18, while the chemical pressure exerted by the Al in Y(Co_0.85Al_0.15)₂ is approximately -40 kbar, so weak ferromagnetism would not be possible in this Y-Co-Al system if the material was clamped to the volume of YCo₂.

Explicit relations are given which allow the calculation of the infrared reflectivity spectra dependence on the angle of incidence for both polarizations in anisotropic absorbing crystals of symmetry as high or higher than orthorhombic. As an example, these relations are applied to the case of orthorhombic sodium nitrite. The calculated and experimental results are compared.

Many electron excitations in photoemission are observed to occur at selected photon energies leading to resonant photoemission enhancements of the 4f multiplet levels of terbium and dysprosium. The authors find that final state symmetry effects can alter the photon energy of the photoemission resonances of the 4f levels creating a slightly different photon energy dependence for the various 4f multiplets. Additional complications are observed because of unresolved multiplets resulting from inter-atomic correlation effects. It is final state 5d-4f interactions that give rise to a variety of valence band 4f multiplets not predicted by one-electron theory.

By means of classical density-functional theory the authors study the density distribution of mobile Ag⁺ ions in alpha -AgI-type superionic conductors taking into account the thermal fluctuations in the anion host lattice. Their model is based on an expansion of the 'external' potential v(r) acting on the cation-component up to second order in the displacements of anions, the latter being regarded as Einstein oscillators. Calculated density profiles are compared with molecular-dynamics simulations. For alpha -AgI the agreement is almost quantitative. In the case of beta -Ag₂S the inclusion of anion vibrations leads to significant corrections in the temperature-dependent profiles.

For pt.II see ibid., vol.2, p.2687 (1990). A model calculation is attempted for the electrical resistivity and the electronic density of states of liquid and amorphous metals using the muffin-tin EMA formalism developed by the present authors. A single s-phase shift model is adopted here with different scattering strengths. It is found that the short- and the medium-range order in the atomic structure causes a deep minimum in the electronic density of states and, furthermore, a very sharp rise of the resistivity when the Fermi energy approaches the minimum. It causes in turn a strong temperature dependence of resistivity for strong scattering cases through the temperature dependence of the Fermi-Dirac distribution function. The temperature dependence also comes from the structure factor, and both effects support the Mooij correlation between the resistivity and the temperature coefficient of resistivity.

Adhesive energies are computed for flat and atomically sharp tips as a function of the normal distance to the substrate. The dependence of binding energies on tip shape is investigated. The magnitudes of the binding energies for the atomic force microscope are found to depend sensitively on tip material, tip shape and the sample site being probed. The form of the energy-distance curve, however, is universal and independent of these variables, including tip shape.

The authors report the results of a variational calculation of the binding energy of shallow donors in quantum wells along with the effects of the image potential.