R. Hlubina

Degenerate Ground State in a Mesoscopic YBa2Cu3O7x Grain Boundary Josephson Junction

We have measured the current-phase relationship I(varphi) of symmetric 45 degrees YBa2Cu3O7-x grain boundary Josephson junctions. Substantial deviations of the Josephson current from conventional tunnel-junction behavior have been observed: (i) The critical current exhibits, as a function of temperature T, a local minimum at a temperature T*. (ii) At T approximately T*, the first harmonic of I(phi) changes sign. (iii) For T<T*, the second harmonic of I(phi) is comparable to the first harmonic, and (iv) the ground state of the junction becomes degenerate. The results are in good agreement with a microscopic model of Josephson junctions between d-wave superconductors.

Anomalous Periodicity of the Current-Phase Relationship of Grain-Boundary Josephson Junctions in High-Tc Superconductors

The current-phase relation (CPR) for asymmetric 45\ifmmode^\circ\else\textdegree\fi{} Josephson junctions between two d-wave superconductors has been predicted to exhibit an anomalous periodicity. We have used the single-junction interferometer to investigate the CPR for these kinds of junctions in

Observation of the second harmonic in superconducting current-phase relation of Nb/Au/(001)YBa2Cu3Ox heterojunctions

{\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}x}

First-order transition between a small gap semiconductor and a ferromagnetic metal in the isoelectronic alloy FeSi1-xGex.

thin films. A remarkable amplitude of the

Microwave study of the critical state in granular YBa2Cu3Ox thin films

\ensuremath{\pi}

Luttinger liquid in a solvable two-dimensional model

-periodical component of the CPR has been experimentally found, providing an additional source of evidence for the d-wave symmetry of the pairing state of the cuprates.

Superconducting parameters of YBCO and BSCCO from ‘tunneling’ spectroscopy

The superconducting current-phase relation (CPR) of Nb/Au/(001)YBa2Cu3Ox heterojunctions prepared on epitaxial c-axis–oriented YBa2Cu3Ox thin films has been measured in a single-junction interferometer. For the first time, the second harmonic of the CPR of such junctions has been observed. The appearance of the second harmonic and the relative sign of the first and second harmonics of the CPR can be explained assuming that the macroscopic pairing symmetry of our YBa2Cu3Ox thin films is of the d + s type.

Microscopic interpretation of the Dynes formula for the tunneling density of states

The contrasting ground states of isoelectronic, isostructural FeSi and FeGe are explained within an extended local density approximation scheme (LDA+U) by an appropriate choice of the Coulomb repulsion U on the Fe sites. A minimal two-band model with interband interactions leads to a phase diagram for the alloys FeSi1-xGex. A mean field approximation gives a first-order transition between a small gap semiconductor and a ferromagnetic metal as a function of magnetic field, temperature, and concentration x. Unusually the transition from metal to insulator is driven by broadening, not narrowing, the bands and it is the metallic state that shows magnetic order.

Consistent two-lifetime model for spectral functions of superconductors

Abstract Modulated microwave absorption measurements at low magnetic fields in YBa 2 Cu 3 O x thin films in a wide temperature range are reported. The microwave losses are proposed to arise from the weak link network which we treat as a medium in the critical state. A modification of the standard critical state model is introduced considering the mutual interaction of intergranular vortices and vortices trapped within the grains: the interaction of both these media described in terms of the modified critical state model can account for the irreversible features of microwave as well as magnetization or transport measurements.

Superconducting transport properties of YBCO grain boundary Josephson junctions

We consider spinless electrons in two dimensions with the bare spectrum