s- and d-wave superconductivity in a two-band model

Abstract

Superconductivity in strongly correlated systems is a remarkable phenomenon that attracts a huge interest. The study of this problem is relevant for materials as the high T c oxides, pnictides and heavy fermions. In this work we study a realistic model that includes the relevant physics of superconductivity in the presence of strong Coulomb correlations. We consider a two-band model, since most of these correlated systems have electrons from at least two different atomic orbitals coexisting at their Fermi surface. The Coulomb repulsion is taken into account through a local repulsive interaction. Pairing is considered among quasiparticles in neighbouring sites and we allow for different symmetries of the order parameter. In order to deal with the strong local correlations, we use the well known slave boson approach that has proved very successful for this problem. Here we are interested in obtaining the zero temperature properties of the model, specifically its phase diagram and the existence and nature of superconducting quantum critical points. We show that these can arise by increasing the mixing between the two bands. Since this can be controlled by external pressure or doping, our results have a direct relation with experiments. We show that the superconductor-to-normal transition can be either to a metal, a correlated metal or to an insulator. Also we compare the relative stability of s and d-wave paired states for different regions of parameter space and investigate the BCS-BEC crossover in the two-band lattice model as function of the strength of the pairing interaction.

Influence of the symmetry of hybridization on the critical temperature of multiband superconductors

“…For instance, for n ≈ 0.5, as well as n ≈ 3.5 and even symmetry of hybridization, two superconducting regions are obtained: one with the usual initial decrease of the superconducting critical temperature with V and a second with a dome for higher values of hybridization. The Coulomb repulsion is also an important ingredient affecting superconductivity: for certain values of band-filling it can lead to a suppression of the superconducting phase, instead of a continuous asymptotic decrease as reported in previous works 8,20 .…”

“…In this work, we approach this complicated many-body problem using the mean-field slave boson formalism [27][28][29] , which has been shown suitable for studying coexistence between superconductivity and magnetism 25 , crossover from BCS-type to local pairing 30 , magnetic instabilities [31][32][33] , and the effect of infinite 11,34,35 and finite 8,20,36 Coulomb repulsion in narrow bands. It has also been shown to be in remarkable agreement with more elaborated numerical Monte Carlo results over a wide range of interactions and particle densities 37 .…”

“…Notice that the ratio between hybridization and bandwidth, which depends on the overlap of different wave functions is sensitive to these external parameters. Consequently, theoretical phase diagrams [8][9][10][11][12][13][14] obtained as a function of this ratio have a direct resemblance to those obtained experimentally when pressure or doping is varied [5][6][7][15][16][17] . It turns out that in realistic cases, the parity of the hybridization is very important.…”

“…An interesting experimental fact observed in multiband superconductors is the existence of a quantum phase transition associated with a superconducting quan-tum critical point (SQCP) [5][6][7] . Varying an external control parameter, such as doping or pressure, these systems can be driven to a non-superconducting metallic state or even to an insulator 8 . Notice that the ratio between hybridization and bandwidth, which depends on the overlap of different wave functions is sensitive to these external parameters.…”

Excitonic insulator states in molecular functionalized atomically-thin semiconductors

“…If not stated otherwise we use an hBN substrate ( = 4.5) entering the Coulomb potential and a surrounding of air ( = 1) for the Figure 2(a) displays the numerical solution of ∆ k and Σ k at room temperature, and the resulting Bogoliubov dispersion E k with the original band gap of E G = 0.12 eV. The ordering parameter ∆ k is symmetric to k = 0 and displays a monotonous decrease with the wave number, comparable to the gap function of s-wave superconductors [59][60][61] . Together with Σ k it yields a sombrero-like Bogoliubov dispersion.…”