Modern Physics Letters B

Polycrystalline samples of Y_1−xPr_xBa₂Cu_3−yLi_yO_7−δ (y=0, 0.05) have been prepared and characterized by X-ray powder diffraction and temperature dependence of resistivity. The T_c of Y_1−xPr_xBa₂Cu_2.95Li_0.05O_7−δ decreases monotonically with increasing Pr content, but the normalized T_c (T_c/T_c(max)) of Y_1−xPr_xBa₂Cu_2.95Li_0.05O_7−δ is higher than that of Y_1−xPr_xBa₂Cu₃O_7−δ for a fixed Pr content. This result indicates that the effect of T_c suppression by Pr doping becomes weaker in the Li-doped system. Considering the Li substitution effect on the structural details, we argue that the Pr–O covalent bonding effect is responsible for the superconductivity suppression.

Multi-band superconductors are analyzed and the relevance of overlapping energy bands to the high-T_c of these materials is studied. Within the BCS framework, a two band model with generalized Fermi surface topologies is developed. Values of the overlapped occupancy parameters for typical cuprate superconductors are obtained as a function of the ratio R and the effective coupling constant, λ, in the weak-coupling limit. The overlap scale is of the order or lower than the cutoff (Debye) energy. The typical behavior of the isotope effect is obtained. As these superconductors have transition temperatures above the phonon barrier, the results of this approach are important to the generic understanding of the high-T_c superconducting mechanism.

The second-order Doppler shift (SODS) of the Mössbauer spectra in a superconductor, δ_SC, is calculated for the first time. The shift depends on the fourth power of the ratio of the gap energy of the superconductor to that of the matrix element of the electron–phonon interaction. At T=T_c where the gap vanishes, the shift reduces to that in the normal state. The relative values of the SODS and the isomer shifts are found in several superconductors. The isomer shift for ¹¹⁹Sn in EuBa₂Cu_2.98Sn_0.02O_7−δ has been found. The recoilless fraction of the ¹¹⁹Sn in EuBa₂Cu_2.98Sn_0.02O_7−δ is compared with a previous theory. It is found that the ordinary BCS type gap agrees with the experimental measurements of the recoilless fraction only when the gap is modified to include a lattice distortion in the superconducting state.

The influence of different treatments of quartz substrate on morphology and adhesion of carbon coatings was studied. The surfaces of quartz substrates were abraded by using fine and coarse abrasive papers respectively to develop rough surface features. Adherent carbon coatings with different surface roughness were obtained. In order to grow adherent carbon coatings with very smooth surfaces, a silicon intermediate layer was grown by means of electron beam evaporation. Carbon coatings deposited on the intermediate layers were adherent to the quartz substrates.

We have applied the Singwi, Tosi, Land and Sjölander approximation for the two-particle distribution function in the BBGKY hierarchy equations to investigate the properties of the hard-sphere Yukawa systems. The static structure factor and the radial distribution function are evaluated and compared with other approximations of the theory of liquids and computer simulations.

Ultrafine/nanosize powders of iron oxide have been synthesized from a sintered α-Fe₂O₃ target by pulsed excimer laser ablation and cold condensation. The influence of target porosity and temperature of condensation on the formation of nanosize particles has been studied. The nanosize powders have been characterized by X-ray diffraction, Mössbauer spectroscopy, vibrating sample magnetometry and scanning electron microscopy. It is shown that the formation of ultrafine/nanosize particles is strongly governed by the target porosity, whereas, the temperature of condensation, over a range from 27°C to −130°C, does not affect the magnetic properties of the nanoparticles.