Electron density of states and superconducting Tc in A15-compounds

We compare the superconductivity parameters of V₃Ga and V₃Ge by the method of McMillan with the help of new neutron diffraction data obtained by P. Schweiss. We consider the strongly varying density of states in V₃Ga at the Fermi energy, a complication suggested by Labbé, Barisic, and Friedel. We find that increased electronic density of states contributes about equally with lattice softening to the enhancement of T_c in V₃Ga as compared to V₃Ge, although the values of N(O)J² = Mλ〈ω²〉 are roughly consistent with those obtained for some other metals by McMillan. The reason N(O) is not more effective in raising T_c is the partial compensation obtained from averaging over the narrow density-of-states peak which gives N ? 0.6N(O).     

Self-consistent treatment of superconducting hole systems and anharmonic lattices in oxides

An extention of Hirsch's Hamiltonian describing the hole system is proposed by including a coupling to the anharmonic lattice and a modulation of the hopping interaction due to lattice displacements. The model is discussed within the variational Bogoliubov's approach, assuming a BCS-like trial Hamiltonian for the hole-like subsystem and a self-consistent phonon approximation for the lattice. The model allows in general to discuss an interplay between superconductivity and ferroelectricity. The results for the critical temperature and the isotope effect coefficient in the superconducting paraelectric phase in one dimension are presented. In particular, the strong influence of the phonon softening on the superconducting transition is widely discussed.     

Coexistence of the superconducting condensate and the quasi-stationary states of hole pairs

The formation of bound states in the case of a direct Coulomb repulsion between two holes whose reciprocal effective mass tensor has principal values of opposite signs is considered as a possible mechanism of high-T _C superconductivity. The study of the specific features of the scattering amplitude shows that, under certain conditions, in addition to the quasi-stationary states, states with a negative attenuation are possible, which corresponds to the tendency toward the formation of a hole pair condensate. The coexistence of the quasi-stationary states and the condensate qualitatively agrees with the phase diagram of p-type doped high-T _C superconducting cuprates.     

High-spin states in97Tc

The level scheme of⁹⁷Tc was studied by gamma ray spectroscopy using the⁹⁴Zr(⁶Li, 3nγ)⁹⁷Tc reaction. New levels and spins and parities were established.⁹⁷Tc is compared with the neighbouring even-even nucleus and with the results of an IBFM calculation.     

High-spin states in99Tc

The level scheme of⁹⁹Tc was studied byγ-ray spectroscopy with the⁹⁶Zr(⁶Li, 3n γ)⁹⁹Tc reaction. New levels were established and spins and parities assigned.⁹⁹Tc is compared with^{93, 95, 97}Tc, with the neighbouring even-even Mo-isotopes and with the results of an IBFM calculation.     

Interstitial vortex in superconducting film with periodic hole arrays

The response of superconducting Nb films with a diluted triangular and square array of holes to a perpendicular magnetic field are investigated.Due to small edge-to-edge separation of the holes,the patterned films are similar to multi-connected superconducting islands.Two regions in the magnetoresistance R(H) curves can be identified according to the field intervals of the resistance minima.Moreover,in between these two regions,variation of the minima spacing was observed.Our results provide strong evidence of the coexistence of interstitial vortices in the islands and fluxoids in the holes.     

Favoured superdeformed states in 89Tc

A superdeformed band consisting of a cascade of ten γ-ray transitions has been identified and assigned to the nucleus ⁸⁹Tc, close to the proton dripline. The quadrupole moment of the band (Q _t= 6.7^+3.0 _−2.3 eb, as measured by the Residual Doppler Shift Method) as well as a large dynamic moment of inertia point to a highly elongated shape. With a relative population of approximately 15% of the γ-ray flux in the ⁸⁹Tc exit channel, the band is among the most intense superdeformed bands observed to date. Received: 6 July 1999     

High spin states in91Tc

Zeitschrift für Physik A Hadrons and nuclei - A shell model calculation for the states of91Tc has been carried out using the (1g 9/2, 2p 1/2) model space with residual interactions determined...     

Calculation of the superconducting transition temperature Tc for metals with phonon-anomalies

It is shown that the electron-phonon coupling constant λ and T_c of the refractory transition-metals VC, VN, NbC, etc. can be obtained from λ and T_c of the corresponding transition-metals V, Nb, etc. The results obtained suggest that superconductivity in the refractory metals involves mainly d-electron transitions between the transition-metal atoms.     

Quasiparticle states in superconducting superlattices

The energy bands and the global density of states are computed for superconductor / normal-metal superlattices in the clean limit. Dispersion relations are derived for the general case of insulating interfaces, including the mismatch of Fermi velocities and effective band masses. We focus on the influence of finite interface transparency and compare our results with those for transparent superlattices and trilayers. Analogously to the rapid variation on the atomic scale of the energy dispersion with layer thicknesses in transparent superlattices, we find strong oscillations of the almost flat energy bands (transmission resonances) in the case of finite transparency. In small-period transparent superlattices the BCS coherence peak disappears and a similar subgap peak is formed due to the Andreev process. With decreasing interface transparency the characteristic double peak structure in the global density of states develops towards a gapless BCS-like result in the tunnel limit. This effect can be used as a reliable STM probe for interface transparency.     

Electronic structure of superconducting Cu oxides

On the basis of photoemission results on La_2-xSr_xCuO_4-y and YBa₂Cu₃O_y it is suggested that carriers doped in the otherwise insulating samples are oxygen p holes rather than Cu d holes. The p holes interact with the localized Cu d states through hybridization and may give rise to an anomaly near the Fermi surface as described by a periodic Anderson model with low carrier density.     

EXAFS spectroscopy of perovskite-type superconducting oxides

On the basis of the investigation of the temperature dependences of EXAFS spectra of the Ba_1?x K _x BiO₃ superconducting oxide, a model of relationship between the local crystalline and electronic structures is proposed. It is found that oxygen ions vibrate in a double-well potential and their vibrations are correlated with the local electron pair transfer. Analogous specific features are also observed in the EXAFS spectra of La_2?x Sr _x CuO₄, which gives grounds to extend the proposed model to Cu-based superconductors. The model based on the unified approach makes it possible to explain the ground antiferromagnetic state, the insulator-metal transition, and the occurrence of superconductivity upon doping of La₂CuO₄ with strontium.     

Symmetry of electronic states in garnets

The paper presents the classification of the electronic empty lattice eigenvalues and the classification of the Bloch sums for the garnet structureIa3d (O _h ¹⁰ ) at the symmetry points, H, P, andN of the Brillouin zone. This provides a starting point for the energy band studies of these technologically important materials.The author thanks Dr. V. Frei for useful comments.     

Structural instability in high-T c superconducting oxides

Two broad internal friction (attenuation as well) peaks in YBCO measured at around 120 K and 250 K are found to exhibit the characteristics of a first-order phase transition. X-ray diffraction indicates no symmetry change on cooling from room temperature but only a downward jump of the lattice parameters was observed. This is referred to as a phase-like transition (PLT) and further confirmed by stress-strain, specific heat and Debye-Waller factor measurements. There always occurs a phase-like transition at 10-30 K above T _c in YBCO, BSCCO and TBCCO which disappears in the non-superconducting phases. Ultrasonic studies on single-crystal BSCCO reveal a pronounced elastic anisotropy in the c-plane and a velocity minimum associated with PLT. The overall trend of elastic stiffening below room temperature and some discrepancies and questions in elasticity measurements so far observed for high-T _c oxides have been explained or clarified in terms of an abnormal fast change of lattice parameters.     

Optical conductivity in superconducting layered cuprate oxides

Various properties of the cuprate oxides are explained by a layer model in which normal and superconducting layers are alternately stacked and the bulk superconductivity is maintained by the proximity effect. By using this model, the optical conductivity of the cuprate oxides is calculated. The result for YBa₂Cu₃O₇ is that the optical conductivity for the light polarized parallel to the CuO chain has finite values even for frequencies below the gap, although the conductivity for the light perpendicular to the chain vanishes below the gap, as in the conventional superconductors. The character of the optical conductivities is seen in the results of the optical conductivities measured by Schlesinger et al. using untwinned YBa₂Cu₃O₇ single crystals.