Küllike Rägo

Critical and Non-Critical Coherence Lengths in a Two-Band Superconductor

We study the peculiarities of coherency in a two-gap superconductor. Both intraband couplings, inducing superconductivity in the independent bands, and interband pair-transfer interaction have been taken into account. On the basis of the Ginzburg–Landau equations derived from the Bogoliubov–de Gennes equations and the relevant self-consistency conditions for a two-gap system, we find critical and non-critical coherence lengths in the spatial behaviour of the fluctuations of order parameters. The character of the temperature dependencies of these length scales is determined by the relative contributions from intra- and interband interaction channels.

Two-gap superconductivity: interband interaction in the role of an external field

We show that weak interband pair-transfer interaction affects the weaker-superconductivity component of a two-gap system as an external field associated with the order parameter, similarly to an external magnetic field for ferromagnetics or an external electric field for ferroelectrics.

Band filling effects on coherence lengths and penetration depth in the two-orbital negative-U Hubbard model of superconductivity

Abstract The two-orbital superconducting state is modeled by on-site intra-orbital negative- U Hubbard correlations together with inter-orbital pair-transfer interactions. The critical temperature is mainly governed by intra-orbital attractive interactions and it can pass through an additional maximum as a function of band filling. For the certain number of electrons the clear interband proximity effect is observable in the superconducting state of the band with a smaller gap. The influence of band fillings and orbital site energies on the temperature dependencies of two-component superconductivity coherence lengths and magnetic field penetration depth is analyzed. The presence of proximity effect is probably reflected in the relative temperature behaviour of characteristic lengths.

On the interband pairing in doped graphane

Abstract An estimation shows that the interband pairing channel between the valence band components of doped graphane can support a superconducting transition temperature (or a contribution into this expected event) of the order of 100 K at the coupling strength near 1 eV.

Critical and Non-Critical Channel in the Damping of Superconducting Fluctuations in Two-Band Systems

We investigate the damping of fluctuations of order parameters in a two-band model of superconductivity in dependence on intra- and interband interactions. The relevant free energy and kinetic equations have been derived. On the basis of these equations one can distinguish two different time scales in the relaxation of superconducting fluctuations. The considerable non-monotonic temperature dependence of the relaxation times has been established below the phase transition point. The behavior is caused by the certain competition between intra- and interband couplings and it allows one to estimate the ratio of these interactions.

Unusual boundary effect on coherency in two-band superconductors

Abstract We demonstrate that restoration of two-band superconductivity near the surface of the system is governed by length scales which are drastically different from correlation lengths. Similar to the one-band case, one of the characteristic lengths diverges at critical temperature T c , while another one shows unusual behaviour having singularity at T c + T c . By moving away from the boundary, these scales approach correlation lengths in the bulk state, where the divergence at T c + is removed by arbitrary weak interband coupling. The peculiarity can be manifested in proximity sandwich where T c + becomes an inflection point for critical temperature being a function of superconducting layer thickness.

Superconductivity-induced shift of phonon frequency in the system with itinerant and strongly correlated electrons

We investigate a system consisting of strongly correlated localized and itinerant electron states mixed by optical vibration. The linear vibronic interaction causes the softening of active optical mode and it may induce a structural instability. The modification of renormalized phonon frequency by the gap in the energy of itinerant electrons in the superconducting phase has been established. The disposition of the electron spectrum and chemical potential influences substantially the corresponding effect. It is demonstrated that the presence of van Hove singularity in the center of itinerant electron band introduces qualitative changes into the superconductivity-induced shifts of phonon frequency.

Cooper pairs in a two-orbital superconductor: bands filling effect on pair sizes

The two-orbital superconducting state is modeled by on-site intra-orbital negative-U Hubbard correlations together with inter-orbital pair-transfer interactions. The influence of bands filling on t ...

A Description of MgB2 Superconductivity Including σ–π Bands Coupling

A two-band scheme of the MgB2 superconductivity has been developed. The pairing channels incorporate σ-intraband electron–phonon attraction besides the Coulombic repulsion, together with pair transfer between effective σ- and π-bands. Various MgB2 superconducting characteristics calculated using a plausible parameter set (which is narrower than the amount of considered properties) agree on a quantitative level with the observations.