S. De Gennaro

Electron-ion pseudopotentials in the noble metals

Abstract Pseudopotential form-factors for Cu, Ag and Au are evaluated by means of a simple model potential.

Elastic moduli of polycrystalline Si and Ge

Elastic moduli of polycrystalline, silicon and germanium are derived, using different approximations, for both an isotropic distribution of crystallites and a small number of textures. A simple application to the piezoresistance in polysilicon is outlined.

Knight shifts in solid noble metals

Abstract The direct contribution to the Knight shift in solid noble metals is evaluated. The entire calculation is accomplished in the framework of the pseudopotential formalism and the effective shape of the Fermi surface is taken into account by using the simple “eight-cone” model by Ziman. The theoretical results are discussed and compared with the available experimental data.

On the Kondo effect in rare-earth alloys: Role of the crystal field

Using the s-f exchange model we examine the influence of the crystal-field on the magnetic resistivity of a Kondo system. Taking into account crystal-field splitting of the localized levels we can explain the Kondo effect in rare-earth alloys with a value of the coupling constant Jsf < 0, as it is usually required for transition-metal alloys.

Susceptibility and evidence of charge carriers interaction above {Tc} in Nd{sub 1.85}Ce{sub 0.15}CuO{sub 4+{delta}}

The charge carrier density plays a crucial role in the transition between normal and superconducting state in the copper-oxide superconductors. In Nd1.85Ce0.15CuO4+δ sample, for charge carrier density n = nc≃ 0.07 the superconductivity sets in. In order to investigate the magnetic properties around nc, a set of magnetic susceptibility measurements in the temperature range 3-260K for samples with different charge carriers concentrations were realized. Magnetic susceptibility curves of Nd1.85Ce0.15CuO4+δ revealed novel behavior well above the critical temperature Tc: at fixed T≥45K, the susceptibility χ, reported as a function of the charge carrier density n, shows a peak around n=0.06 and drops to significantly lower values for n greater than 0.07, the value at which the samples become superconducting. The effective magnetic moment μeff shows the same behavior. The comparison of the present experimental results with recent transport ones gives evidence of a possible interaction between charge carriers above Tc. The possibility that this effect could be due to a bipolarons-like mechanism of charge carriers interaction is discussed.

Occurence and correlation of two non-equivalent sites for europium in EuPd3B

Mössbauer spectroscopy has been used to investigate the mechanism which affects the valence state of europium in EuPd3B. The obtained results are interpreted in terms of a correlation between two non equivalent sites each in a fluctuating valence state.

Magnetic characterization of Ag-sheathed Bi(Pb)2223 tapes

Ag/BPSCCO (2223) tapes were characterized by SQUID magnetometry over a wide range of temperatures (4.2-95 K) and under a magnetic field H (up to 5 T) applied either parallel or perpendicular to the c axis of the tape. For the two different field orientations, taking into account (when required) the anisotropy of the currents and demagnetization effects, the values of the critical current density were evaluated as a function of the applied magnetic field H and temperature. The magnetization decay for H perpendicular to the c axis exhibited a non-linear dependence of the flux creep activation energy on the current J. From behaviour and from the relaxation data M(t) some conclusions about dissipation mechanisms were drawn.

Magnetic Measurements of Critical Currents in Ag‐BPSCCO Tapes: A Comparison

Critical currents in Ag-BPSCCO (2223) tapes obtained both by means of dc magnetization hysteresis loops and by ac magnetic susceptibility are reported. In this context a new method to distinguish the intergranular and intragranular contributions is suggested and the relative importance of the two contributions is discussed.

On the thermoelectric power of YBa2Cu3O7−γ

Abstract Electrical resistivity and thermoelectric power (TEP) measurements on YBa 2 Cu 3 O 7− γ as a function of oxygen content are reported here for very low values of γ. A broad minimum in the TEP versus T curves is observed at temperatures between T c and approximately 150–180 K. The temperature of the minimum is seen to shift towards higher temperatures as γ increases, and at the same time the minimum itself becomes broader. Our experimental data on the TEP are discussed taking into account also the results previously reported by several authors, and a tentative explanation for the general behaviour of the TEP when varying the oxygen content and the temperature is proposed.

Elastic constants and moduli of polycrystalline YBa2Cu3O7 and La2CuO4

Elastic constants and moduli of polycrystalline YBa2Cu3O7 and La2CuO4 are derived, using different approximations, for both an isotropic distribution of crystallites and a uniaxial texture. The results obtained are compared with the available experimental data.