Interband Pair-transfer Interaction Research Articles

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.

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

We study the peculiarities of coherency in a two-gap superconductor. The 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.

Relaxation of Superconducting Fluctuations in a Two-Component Scenario with Intra- and Interband Pairings

We study the relaxation of superconducting fluctuations in a two-component (two-band) model where the interband pair-transfer interaction and intraband effective attractions of electron-phonon nature are taken into account. The corresponding free energy and the system of Landau–Khalatnikov equations are derived. One can distinguish two different time scales in the damping of the fluctuations of superconducting order parameters in a two-band scenario. The formation of these relaxation channels is caused by the interband interaction leading to the redistribution of damping processes of the initially independent bands. The relevant relaxation times characterize the kinetics of critical and noncritical fluctuations which appear as the certain linear combinations of the deviations from the equilibrium band superconducting orders. The peculiarities of the temperature behavior of the relaxation times in dependence on intra- and interband couplings are established.

Cuprate superconductivity characteristics in a multiband model controlled by doping

A multiband model of cuprate superconductivity with the electron spectrum created and evolving by doping is suggested. Bare gaps between the itinerant and two components of the defect subsystem disappear with extended doping. The interband pair-transfer interaction between itinerant and defect states drives the pairing. The behaviour of T c, supercarrier density, superconducting, normal state and pseudogaps, as also the condensation energy, the second critical field, in-plane coherence length, penetration depth, paired carrier effective mass and the isotope effect exponents have been calculated on the whole doping scale. Self-consistent qualitative agreement with experimental data is obtained. Cuprates can be expected to be multi(two)-component superconductors.

Vibronic Effects in Cuprate Superconductors with a Two-Component Spectrum Created by Doping

A model [1] of the electron spectrum of a cuprate superconductor involves a new band borning with hole doping above the itinerant band. Vibronic coupling of these components leads to the renormalization of the gaps created by the interband pair transfer interaction. The phonon softening is maximal at the under/overdoped separation line where the bands begin to overlap. The vibronic hybridization determines the normal state joint pseudogap at optimal doping and one of the two pseudogaps of the underdoped material. The larger pseudogap changes its nature with doping.

Superconducting Phase Diagram of the Two-Band Model Investigated by the Exact-Diagonalization Method

By means of the exact-diagonalization method we have studied the finite-size two-band model with the interband pair-transfer interaction by calculating the superconducting (SC) correlation function P s etc. in the ground state. Occurrence of SC was clearly indicated by the increase of P s . The diagram of the observed SC region was constructed in the plane of the relative position of the second band versus on-site Coulomb energy U . In the low-density limit it proved close to the region where a negative- U situation is realized by the pair-transfer interaction. On the high-density side, the SC region was found to be more extended than the latter in accord with the previous interband-ladder-diagram results. Inclusion of the exchange interaction was found to reduce the SC region in a restricted region where two bands substantially share electrons. The two-band situations in cuprates are argued to be relevant to the high T c through the present mechanism.

Negative-U superconductivity in the two-band model with the pair-transfer interaction

The superconducting transition is studied in a two-band model which has an interband pair-transfer interaction between the conducting band and a hidden band not crossing the Fermi level. The interband ladder-diagram process enhances the effect of the pair-transfer interaction, remarkably reducing the effective on-site Coulomb energy and leading to high- T c superconductivity with a negative U. The isotope effect is shown to weaken with increasing T c if in addition we take account of the BCS interaction. The tendency of increase of T c with decrease of the Madelung potential difference Δ V M between Cu and O sites in cuprates is understandable in the present model.

Superconducting transition of 2D two-band systems with interband pair-transfer interaction

An interband BCS-type pair-transfer process arises from an off-diagonal matrix element of the Coulomb interaction. This is shown to be divergently intensified through a ladder-diagram process in a two-dimensional (2D) two-band systems with concentric circular electron and hole Fermi surfaces. This effect comes from a peak of the interband polarization function for the wave number equal to the difference of the wave numbers of the two bands; the peak can lead to the divergence of the above-mentioned ladder-diagram process, resulting in an s-wave superconductivity with Tc possibly reaching 200 K despite large onsite Coulomb energies. This mechanism is argued to be substantiated in high-Tc oxides.