Vortex core excitations in superconductors with frustrated antiferromagnetism

Abstract

Motivated by recent discovery of cobalt oxide and organic superconductors, we apply an effective model with strong antiferromagnetic and superconducting pairing interaction to a related lattice structure. It is found that the antiferromagnetism is highly frustrated and a broken-time-reversalsymmetry chiral d + id ′ -wave pairing state prevails. In the mixed state, we have solved the local electronic structure near the vortex core and found no local induction of antiferromagnetism. This result is in striking contrast to the case of copper oxide superconductors. The calculated local density of states indicates the existence of low-lying quasiparticle bound states inside the vortex core, due to a fully gapped chiral pairing state. The prediction can be directly tested by scanning tunneling microscopy.PACS numbers: 74.20.Rp, 74.25.Jb Recently, superconductivity in the oxyhydrate Na 0.35 CoO 2 · 1.3H 2 O below ∼ 5K was discovered by Takata et al.[1] and confirmed immediately by several groups [2,3,4,5]. Interesting features of this material include: (i) It is believed that the superconductivity comes from CoO 2 layers, similar to that in copper-oxide cuprates.(ii) Co 4+ atoms have spin-1 2 but form a triangular lattice, which frustrates the antiferromagnetism (AF) and thus is a promising candidate for the occurrence of spin-liquid phases [6]. There are also organic superconductors like the κ-(BEDT-TTF) 2 X materials which have a lattice structure very similar to the triangular lattice [7,8]. (iii) Theoretically, the analysis based on the resonant valence bond theory in the framework of the t-J model [9,10,11,12] or on the renormalization group theory within the framework of t-U -J model [13] indicates a wide window of broken-time-reversal-symmetry (BTRS) d+id ′ -wave pairing state in the phase diagram. Other theoretical groups [14,15,16] proposed a p x + ip y -wave pairing state mediated by ferromagnetic fluctuations. Since the ferromagnetism is insensitive to the detailed lattice structure, no frustration effect is expected on a triangular lattice. At this stage, we are not at a position to resolve the pairing state issue. Motivated by recent observation of a superconducting phase diagram of Na x CoO 2 · 1.3H 2 O similar to that of the cuprate superconductors [5], we consider in this Letter a spin-singlet pairing and study the nature of low-lying excitations around a vortex in these new superconductors with frustrated antiferromagnetism. The results can be directly tested by further experiments such as scanning tunneling microscopy (STM), which likely will be carried out soon.In conventional s-wave superconductors such as NbSe 2 , the observed quasiparticle tunneling spectrum at the vortex core by Hess et al. [17] can be explained successfully in terms of the low-lying quasiparticle bound states as shown by Caroli, de Gennes, and Matricon [18]. In copper-oxide cuprates, the condensate has a d-wave t',V' FIG. 1: Lattice structure of a superconductor with frustrated antiferromagnetism. It has a nearest-neighbor h...

Vortex Core Ordering in Abrikosov Lattices

“…We have used large scale real-space Bogoliubov de-Gennes mean field simulations, a method that has been widely used to study competing phases [26][27][28][29][30][31][32][33][34][35][36] . This is the best available microscopic method to study vortex lattices, as the orbital field introduces a sign problem in quantum Monte Carlo simulations.…”