T. Örd

MULTIBAND MODEL OF CUPRATE SUPERCONDUCTIVITY

A minimal model for the description of cuprate superconductor characteristics on doping scale (hole and electron) is developed. The leading interband pairing channel couples an itinerant band and defect states created by doping. Bare gaps between them are supposed and become closed by extended doping. Band overlap conditions determine special points in the phase diagram. Nodal and antinodal momentum regions are distinguished. Illustrative calculations have been made using a mean-field pair-transfer multiband Hamiltonian and corresponding free-energy expansions. The results are self-consistent and demonstrate that the elaborated approach is able to reproduce characteristic features of cuprate superconductors as, e.g., the doping dependence of Tc, superconducting gaps and pseudogaps, supercarrier density and effective mass, coherence length and penetration depth, critical magnetic fields and some other properties. Interband pairing scheme is suggested to be an essential aspect of cuprate superconductivity.

Multiband superconductivity of doped cuprates

Numerous indications point to the multiband nature of the superconductivity in doped cuprates. The electronic structure of these compounds evolves with doping. Based on experimental findings, a simple model for the description of cuprate superconductor characteristics on the doping is developed. The leading interband pairing channel between an itinerant band and defect states created by doping is postulated. Bare gaps between them are supposed and to be closed by extended doping. Band overlap conditions determine the phase diagram critical points. The supercarrier density varies with the disposition of the chemical potential and bands. Nodal and antinodal momentum regions are distinguished. In the case of hole doping the relevant regime lies near the top of the oxygen band. Illustrative calculations have been made using a mean-field multiband Hamlitonian. Manifestations of various gaps and pairing strength characteristics on the doping level are discussed. A nonmonotonic dependence of the critical coherence length is calculated. The coherence of noncritical mode is also investigated. The supercarrier effective mass diminishes moderately out of underdoping. The penetration depth curve is of the observed type. The isotope effect on supercarrier density is predicted. In the case of electron doping the events concentrate near the bottom of the upper Hubbard band. The gross features of the phase diagram with two pseudogaps remind the hole doping case. The pairing strength and the phase coherence develop simultaneously. The agreement of the results of our model calculations with experimental findings suggests that multiband pairing is an essential aspect of cuprate superconductivity.

On critical and non-critical fluctuation modes in two-band superconductors

We show that in a two-band superconductor with interband pairing the fluctuations of the order parameters are characterized by two coherence scales exhibiting critical and non-critical behavior near the phase transition temperature. The critical coherence length determines the exponential damping of fluctuations in space. However, the non-critical length as an imaginary quantity is associated with spatially periodic fluctuations. It is established that, for the appearance of non-critical fluctuations, the excitation of the Leggett mode is needed. The dependences of the coherence lengths on doping is studied for copper oxide superconductors.

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.

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.