Possible Spin-Tripletf-Wave Pairing Due to Disconnected Fermi Surfaces In NaxCoO2·yH2O

Abstract

We propose that the spin-triplet pairing mechanism due to disconnected Fermi surfaces proposed in our previous study [Phys. Rev. B 63, 174507 (2001)]] may be at work in a recently discovered superconductor NaxCoO2.yH2O. We introduce a single band effective model that takes into account the pocketlike Fermi surfaces along with the van Hove singularity near the K point found in the band calculation results. Applying the fluctuation exchange method and solving the linearized Eliashberg equation, the most dominant pairing is found to have spin-triplet f-wave symmetry, where the nodes of the gap function do not intersect the pocket Fermi surfaces. The presence of finite Tc is suggested in sharp contrast to cases when the gap nodes intersect the Fermi surface.

Chirald-wave superconductivity in doped graphene

“…Early proposals for the symmetry of the order parameter included, among other orders, both spin-triplet f -wave and chiral d-wave symmetries. For example, disconnected Fermi surfaces have been suggested to favor spin-triplet fwave superconductivity [183]. Considering instead RVB physics and antiferromagnetic exchange interactions the spin-singlet chiral d-wave state has been proposed as the leading pairing instability [167,168,169,170,184].…”

Elliptical hole pockets in the Fermi surfaces of unhydrated and hydrated sodium cobalt oxides

“…The manner in which electrons form these pairs can be strongly influenced by the shape of the Fermi surface. Questions regarding the origin of the pairing interaction and the nature of the superconductivity in the cobaltates has stimulated significant theoretical speculation, most of which has focused heavily on the properties of some small elliptically shaped pockets predicted to be the electronically dominant Fermi surface sheet, [5][6][7][8] but the outcome of direct attempts to look for them has instead cast serious doubt over their existence. [10][11][12][13][14] Here we present evidence that these pockets do indeed exist, based on bulk measurements of the electron momentum distribution in unhydrated and hydrated sodium cobalt oxides using the technique of x-ray Compton scattering.…”

“…7 With 70% of the density-ofstates at the Fermi level, these pockets are central to several models of spin-fluctuation-mediated superconductivity, developing either via imperfect nesting between the Fermi surface pockets, 5 or as a consequence of these disconnected pieces of Fermi surface. 6,8 A series of angle-resolved photoemission spectroscopy ͑ARPES͒ results, the consensus of which report observation of the a 1g sheet at ⌫ but crucially not the e g Ј pockets, has fuelled the controversy surrounding the elliptical pockets. Indeed, they observe the e g Ј band as lying completely below the Fermi level.…”

Possible spin triplet superconductivity in NaxCoO2·yH2O —59Co NMR studies

“…Some groups studied a single band t-J model as a prototype of strong coupling electrons theories to understand the low energy electronic phenomena based on the resonating-valence-bond (RVB) state [3,4,5,6,7]. Kuroki et al introduced a single band effective model with taking into account pocket-like Fermi surfaces along with van Hove singularity near the K point in the first Brillouin zone [8,9,10,11]. They showed that the large density of states near the Fermi level gives rise to ferromagnetic spin fluctuations, leading to the f -wave superconductivity due to disconnected Fermi surfaces near the Γ point.…”