Colloquium 114: Superconductivity and magnetism in doped graphene (A.V. Chubukov, Apr.24, 2013)

Title: Superconductivity and magnetism in doped graphene

Speaker: A.V. Chubukov,Department of Physics, University of Wisconsin-Madison, USA

Location: Room 111, Physics Building

Time: 15:00-16:00p, April 24th, 2013

Abstract:

I discuss possible realization, in doped graphene monolayer, of a chiral superconductivity which breaks time-reversal symmetry, and exotic spin-density-wave (SDW) order which preserves full Fermi surface. A unique situation arises in graphene at the critical doping where the Fermi surface is nested and the density of states is singular. In this regime, d-wave superconductivity and SDW order emerge from repulsive electron-electron interactions. Using a renormalization group method, I show that superconductivity wins at the critical doping, but SDW wins at slightly smaller or slightly larger doping. Superconductivity develops simultaneously in two degenerate d-wave pairing channels and is of chiral type, with the phase of the superconducting order parameter winding by 4 around the Fermi surface. Such a state breaks the time reversal symmetry and exhibit many other fascinating properties. SDW state is uniaxial, with order parameter breaking O(3) * Z4 symmetry. I show that this SDW state is a half-metal – excitations in one spin branch are gapped, but excitations in the other spin branch remain gapless and preserve full original Fermi surface. Such a state is highly desirable for nano-science as it allows for electrical control of spin currents.