Bandwidth controlled quantum phase transition between an easy-plane quantum spin Hall state and an s-wave superconductor

TL;DR

This paper demonstrates a continuous quantum phase transition from a quantum spin Hall state to an s-wave superconductor driven by bandwidth control, involving topological excitations called merons, in an easy-plane anisotropic system.

Contribution

It reveals a new topological route to superconductivity via meron condensation in an easy-plane quantum spin Hall system, supported by large-scale quantum Monte Carlo simulations.

Findings

Continuous transition between quantum spin Hall and s-wave superconductor.

Merons carry a unit charge and condense to induce superconductivity.

Results applicable in both strong and weak anisotropy regimes.

Abstract

The quantum spin Hall state can be understood in terms of spontaneous O(3) symmetry breaking. Topological skyrmion configurations of the O(3) order parameter vector carry a charge 2e, and as shown previously, when they condense, a superconducting state is generated. We show that this topological route to superconductivity survives easy-plane anisotropy. Upon reducing the O(3) symmetry to O(2)$\times$ Z$_2$, skyrmions give way to merons that carry a unit charge. On the basis of large-scale auxiliary field quantum Monte Carlo simulations, we show that at the particle-hole symmetric point, we can trigger a continuous and direct transition between the quantum spin Hall state and s-wave superconductor by condensing pairs of merons. This statement is valid in both strong and weak anisotropy limits. Our results can be interpreted in terms of an easy-plane deconfined quantum critical point. However, in contrast to the previous studies in quantum spin models, our realization of this quantum critical point conserves $U(1)$ charge, such that skyrmions are conserved.