Dynamical gap generation in a two-dimensional Dirac semimetal with a deformed Dirac cone

Abstract

According to the extensive theoretical and experimental investigations, it is widely accepted that the long-range Coulomb interaction is too weak to generate a dynamical excitonic gap in graphene with a perfect Dirac cone. We study the impact of the deformation of Dirac cone on dynamical gap generation. When a uniaxial strain is applied to graphene, the Dirac cone is made elliptical in the equal-energy plane and the fermion velocity becomes anisotropic. The applied uniaxial strain has two effects: it decreases the fermion velocity; it increases the velocity anisotropy. After solving the Dyson-Schwinger gap equation, we show that dynamical gap generation is promoted by the former effect, but is suppressed by the latter one. For suspended graphene, we find that the systems undergoes an excitonic insulating transition when the strain is roughly 7.34$\%$. We also solve the gap equation in case the Dirac cone is tiled, which might be realized in the organic material $\alpha$-(BEDT-TTF)$_{2}$I$_{3}$, and find that the tilt of Dirac cone can suppress dynamical gap generation. It turns out that the geometry of the Dirac cone plays an important role in the formation of excitonic pairing.