Artjom Vargunin

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.

Correlation functions and coherence lengths in a two-gap superconductor

We derive analytically the spatial correlation functions for gap fluctuations in two-band scenario with intraand interband pair-transfer interactions. These functions demonstrate the changes in spatial functionality due to the presence of two channels of coherency described by the divergent and finite correlation lengths. Even at the phase transition point both channels are essential for two-band superconductivity. Generally their relative contributions depend on the temperature and system parameters.

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.

Dynamic heat capacity in spatially restricted systems with phase transition

The frequency-dependent complex heat capacity has been derived for spatially restricted systems with a bulk phase transition. It is demonstrated that internal energy relaxation is determined by the eigenvalues of the Fokker-Planck operator, providing an insight into the symmetry of the reduced free energy. The size driven properties of the energy relaxation rates are discussed and the universal ratio between them has been found.

On phase transition and the critical size in spatially restricted systems

We study the finite-size effects on the critical temperature in spatially restricted systems with bulk second-order phase transition using the Fokker–Planck equation approach. It is established that the dependence of the transition temperature on system size is characterized by the competition of two length scales. The first scale is similar to the correlation length, determining the critical behavior in sufficiently large samples. The second scale appears as a consequence of the stochastic nature of the order parameter and controls the transitional features in small samples, particularly in the vicinity of the critical size. It is also found that the rate of the critical slowdown of relaxation of the order parameter fluctuations increases as the volume of the system decreases.

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.

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 ...

Vortex motion and flux-flow resistivity in dirty multiband superconductors

The conductivity of vortex lattices in multiband superconductors with high concentration of impurities is calculated based on microscopic kinetic theory at temperatures significantly smaller than t ...