Finite-energy spin fluctuations as a pairing glue in systems with coexisting electron and hole bands

Abstract

We study, within the fluctuation exchange approximation, the spin-fluctuation-mediated superconductivity in Hubbard-type models possessing electron and hole bands, and compare them with a model on a square lattice with a large Fermi surface. In the square lattice model, superconductivity is more enhanced for better nesting for a fixed band filling. By contrast, in the models with electron and hole bands, superconductivity is optimized when the Fermi surface nesting is degraded to some extent, where finite energy spin fluctuation around the nesting vector develops. The difference lies in the robustness of the nesting vector, namely, in models with electron and hole bands, the wave vector at which the spin susceptibility is maximized is fixed even when the nesting is degraded, whereas when the Fermi surface is large, the nesting vector varies with the deformation of the Fermi surface. We also discuss the possibility of realizing in actual materials the bilayer Hubbard model, which is a simple model with electron and hole bands, and is expected to have a very high Tc.

Pressure dependence of superconductivity in low- and high- Tc phases of (NH3)yNaxTerao1, Yang2, Xing3 et al. 2018Phys. Rev. B104180View full textAdd to dashboardCite

“…Such a suppression of T c with greater FeSe-plane separation is observed in metaldoped HfNCl [27]. Recently, Kuroki et al suggested that the superconductivity is optimized when Fermi surface nesting is degraded to some extent [36], which allows finite energy spin fluctuations around the nesting vector to develop. This may explain the existence of an optimum FeSe plane spacing for superconductivity.…”

Superconducting mechanism for the cuprate Ba2CuO3+δ based on a multiorbital Lieb lattice model

“…We have found that s±-wave superconductivity is strongly enhanced in the Lieb-lattice model when the d 3z 2 −r 2 band is raised so that it becomes incipient. Such a strong enhancement of s±-wave superconductivity reminds us of the bilayer Hubbard model on a square lattice [39,42,43,[76][77][78][79][80][84][85][86][87][88][89][90][91][92], where enhanced superconductivity is found when one of the bands is incipient [ Hubbard model is a single-orbital (one orbital per site) system with two sites per unit cell. The enhancement of superconductivity in such a system is mediated by spin fluctuations originating from the on-site U , which gives rise to the pairing interaction between the bonding and antibonding bands, both of which have equal weight of the two orbitals in a unit cell.…”

“…Recently, such a band lying just above or below the Fermi level is referred to as an "incipient band" and has received attention, especially in the context of the iron-based superconductors [27][28][29][30][31][32][33][34][35]38], where hole bands lying just below the Fermi level are observed in some materials [30,31,[68][69][70][71][72][73][74][75]. In a wider context, the possibility of the occurrence or strong enhancement of superconductivity due to an incipient band has long been proposed for multiband Hubbard models on various types of lattices [23,26,36,37,[39][40][41][42][43][44][76][77][78][79][80][81][82]. A salient feature in these cases is that the gap function typically exhibits nodeless "s±-wave" symmetry.…”

“…In the bilayer Hubbard model, whose noninteracting band structure is depicted in Fig. 18(b), superconductivity is found to be strongly enhanced when one of the bands becomes (nearly) incipient upon increasing t ⊥ [39,42,43,[76][77][78][79].…”

Possible high- Tc superconductivity due to incipient narrow bands originating from hidden ladders in Ruddlesden-Popper compounds

“…Superconductivity involving flat bands has also been discussed recently in the context of the flat-band superconductivity 15 , where the virtual pair scattering between the flat and dispersive bands is also at work. The incipient-band-induced superconductivity exploits finiteenergy spin fluctuations, whose importance has recently been noted in other models as well 16 .…”