The sign of interband interaction and stability of the superconducting ground state in the multiband model

Abstract

(ricevuto il 25 Febbraio 1994; approvato il 12 Marzo 1994) Summary. --It is stressed that the stability of the superconducting ground state in the two-band model is guaranteed for both signs of the leading interband interaction W. Thereby the requirement for the energy minimum fixes the phase differences of two order parameters as I~1 -~21 = 0, 2= .... if W < 0 and t~1 -~21 = =, 3= .... if W > 0, and this difference is reflected in the ground-state wave function.PACS 63.90 -Other topics in lattice dynamics and crystal statistics.In the multiband models of superconductivity [1][2][3] it, is well known that the pairing may be achieved for both signs of the intei, band interactions [3,4]. For high-Te superconductors there is a number of new multiband approaches, see, e.g., review [5]. In the scheme developed in papers [6-8] the total leading interband interaction W, consisting of the Coulomb and the electron-phonon part, is assumed to be positive. In the multigap approach adopted in papers [9,10] the negative value of W leading to stable superconductive state has been emphasized [10] as a consequence of the assumption of a model exclusively relying upon electron-phonon interactions near the Fermi surface.The aim of the present note is to show that, regardless of the signature of W, the stability of the superconducting state can be guaranteed if the phase differences of the order parameters are properly fixed. For simplicity a two-band model is used.The starting Hamiltonian is of the same form as in [6] (1) a~-k~ a:,_k,~ a:,,k,~ 9 ~,k,s ~,~' k,k' Here the electron band energies ~(k) are measured with respect to the chemical potential, the arrows indicate the spin (s) orientation, and the second term--the (*) Permanent address:

MgB 2 two-gap superconductivity with intra- and interband couplings

“…The gaps are taken to be real. Note that for the repulsive interband coupling the gaps have different signs [30]. This circumstance can be of significance for the description of the anisotropy of superconductivity in MgB 2 characterized by two totally symmetric order parameters.…”

Superconductivity in non-adiabatic systems with variable charge-carrier density

“…Hence it is of undoubted interest to study superconducting properties taking into account some of the striking features characteristic of high-T C materials. Among them are: a layered structure (multiband character of the system), the existence of van Hove-Lifshitz singularities in the electron-state density and a variable density of charge carriers [2][3][4][5][6][7][8][9]. Also, oxide ceramics (organic superconductors and fullerenes) have low values of the Fermi energy (E F ) that can be of the same order as the Debye energy (ω D ).…”