Thermodynamic and Dynamical Signatures of a Quantum Spin-Hall Insulator to Superconductor Transition.

Using quantum Monte Carlo simulations, we study the thermodynamic and dynamical properties of a model of Dirac fermions in 2+1 dimensions with a deconfined quantum critical point (DQCP) separating an interaction-generated quantum spin-Hall insulator from an s-wave superconductor [Nature Comm. 10, 2658 (2019)]. Our focus is the distinct physics inside and outside the deconfined quantum critical region, located below the single-particle gap, in which spinons with spin-1/2 and no charge, as well as skyrmions carrying charge 2e and no spin, emerge. Since total spin and charge are conserved, and our model has a single length scale, these excitations lead to a distinct linear temperature dependence of the uniform spin and charge susceptibilities. At the DQCP, the order parameter spectral functions show remarkable similarities that hint at an emergent SO(5) symmetry. Superconductivity disappears at a Berezinskii-Kosterlitz-Thouless phase transition due to the temperature-driven proliferation of spin-1/2 vortices.