Extended van Hove Singularity and Superconducting Instability in Doped Graphene

Abstract

We have investigated the effects of doping on a single layer of graphene using angle-resolved photoemission spectroscopy. We show that many-body interactions severely warp the Fermi surface, leading to an extended van Hove singularity (EVHS) at the graphene M point. The ground state properties of graphene with such an EVHS are calculated, analyzing the competition between a magnetic instability and the tendency towards superconductivity. We find that the latter plays the dominant role as it is enhanced by the strong modulation of the interaction along the Fermi line, leading to an energy scale for the onset of the pairing instability as large as 1 meV when the Fermi energy is sufficiently close to the EVHS. DOI: 10.1103/PhysRevLett.104.136803 PACS numbers: 73.22.Pr, 73.20.At, 74.70.Wz, 79.60.Ài The fundamental interactions between charge, spin, and lattice depend very much on the topology of electronic bands, particularly in two dimensions when the Fermi level E F is near a saddle point. Such a saddle point occurs where the curvature of the bands has opposite sign in two orthogonal directions, leading to a van Hove singularity (VHS), i.e., a divergence in the density of states (DOS).Much attention has been given to a ''VHS scenario'' in the cuprates, whereby the superconductivity is strongly influenced or even induced by a saddle point VHS [1,2]. Among the important effects of the VHS are (1) the presence of both electronlike and holelike carriers, leading to an attractive potential favoring pairing, (2) a high DOS, held to favor not only superconductivity but structural and magnetic instabilities, and (3) a perfect screening at wave vectors connecting VHSs, which acts to reduce repulsion. The topology of the VHS is important, especially when the curvature of one band vanishes; i.e., the electron or hole mass diverges. This increases the strength of the divergence in the DOS at such a so-called extended VHS (EVHS), which can increase the critical temperature for superconductivity [3].Graphene's band structure has saddle points at the M point of the Brillouin zone (BZ) occurring around AE2 eV from the charge neutrality point at the Dirac energy E D [4]. Since superconductivity occurs in graphene-related systems such as graphite-intercalation compounds and nanotubes, it is interesting to explore whether superconductivity or other instability can appear in graphene itself due to the influence of this VHS.In this Letter, we present the band structure of highly doped graphene determined by angle-resolved photoemission spectroscopy (ARPES). We show that by chemically doping graphene on both sides, a much higher level of doping can be achieved than previously obtained. By this method, we can induce an electronic topological transition in graphene for the first time, whereby the Fermi energy E F is brought to the position of the saddle point VHS in graphene. We find that the electronic structure is strongly renormalized by the resulting DOS divergence such that the VHS has an extended, not pointlike, character [5,6]...

Superconductivity from repulsive interaction

“…Such a high doping of a single layer graphene has been achieved in Ref. [115] by placing Ca and K dopants above and below a graphene layer. At x = x c the system passes through a Van-Hove singularity which results in an enhanced density of states at the six saddle points where nearest pockets merge.…”

“…The KL mechanism has been also applied to somewhat smaller dopings, when the FS still contains six disconnected pieces. In this doping range, KL-based analysis yields a novel spin-triplet f-wave superconductivity [115,116,46,122].…”

Chirald-wave superconductivity in doped graphene

“…Some promising experimental work on achieving high doping levels in graphene exist, see e.g. [142,143], but further theoretical and experimental work is clearly needed to understand these issues more systematically.…”

“…In the existing proposals for doping graphene close or to the van Hove point [142,143], the deposition of charge-donating adatoms can lead to a significant amount of disorder. Hence the studies that show that the chiral d-wave pairing state is only perturbed locally provide important indications that the d-wave state can be experimentally realized.…”

Role of the van Hove singularity in the quantum criticality of the Hubbard model

“…10 The presence of saddle points in the energy dispersion is also argued to lead to a superconducting instability in other correlated systems, e.g. graphene 11 . If the Fermi level is doped to coincide with the van Hove singularity, then the superconducting transition temperature can be greatly enhanced.…”