Nonequilibrium-induced metal-superconductor quantum phase transition in graphene

Abstract

We study the effects of dissipation and time-independent nonequilibrium drive on an open superconducting graphene. In particular, we investigate how dissipation and nonequilibrium effects modify the semi-metal-BCS quantum phase transition that occurs at half filling in equilibrium graphene with attractive interactions. Our system consists of a graphene sheet sandwiched by two semi-infinite three-dimensional Fermi-liquid reservoirs, which act both as a particle pump/sink and a source of decoherence. A steady-state charge current is established in the system by equilibrating the two reservoirs at different but constant chemical potentials. The graphene sheet is described using the attractive Hubbard model in which the interaction is decoupled in the s-wave channel. The nonequilibrium BCS superconductivity in graphene is formulated using the Keldysh path-integral formalism, and we obtain generalized gap and number density equations valid for both zero and finite voltages. The behavior of the gap is discussed as a function of both attractive interaction strength and electron densities for various graphene-reservoir couplings and voltages. We discuss how tracing out the dissipative environment (with or without voltage) leads to decoherence of Cooper pairs in the graphene sheet, hence, to a general suppression of the gap order parameter at all densities. For weak enough attractive interactions we show that the gap vanishes even for electron densities away from half filling and illustrate the possibility of a dissipation-induced metal-superconductor quantum phase transition. We find that the application of small voltages does not alter the essential features of the gap as compared to the case when the system is subject to dissipation alone (i.e., zero voltage). The possibility of tuning the system through the metal-superconductor quantum critical point using voltage is presented.

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