2D Superconductivity: Classification of Universality Classes by Infinite Symmetry

Abstract

I consider superconducting condensates which become incompressible in the infinite gap limit. Classical 2D incompressible fluids possess the dynamical symmetry of area-preserving diffeomorphisms. I show that the corresponding infinite dynamical symmetry of 2D superconducting fluids is the coset ${{W_{1+\infty} \otimes \bar W_{1+\infty}} \over U(1)_{\rm diagonal}}$, with $W_{1+\infty}$ the chiral algebra of quantum area-preserving diffeomorphisms and I derive its minimal models. These define a discrete set of 2D superconductivity universality classes which fall into two main categories: conventional superconductors with their vortex excitations and unconventional superconductors. These are characterized by a broken $U(1)_{\rm vector} \otimes U(1)_{\rm axial}$ symmetry and are labeled by an integer level $m$. They possess neutral spinon excitations of fractional spin and statistics $S = {\theta \over 2\pi} = {{m-1} \over 2m}$ which carry also an $SU(m)$ isospin quantum number; this hidden $SU(m)$ symmetry implies that these anyon excitations are non-Abelian. The simplest unconventional superconductor is realized for $m=2$: in this case the spinon excitations are semions (half-fermions). My results show that spin-charge separation in 2D superconductivity is a universal consequence of the infinite symmetry of the ground state. This infinite symmetry and its superselection rules realize a quantum protectorate in which the neutral spinons can survive even as soft modes on a rigid, spinless charge condensate.