Quantum and classical vortex ratchets in a trigonal 2D superconductor

One of the unique features of recently emerging 2D superconductors is the quantum metallic state, which is a temperature-independent finite resistive state that appears once finite magnetic field is switched on [1, 2]. This quantum metallic state exhibits a sharp contrast with the conventional superconductor-insulator transition in conventional 2D systems, where the metallic state appears only at a single critical point.
To investigate the vortex dynamics which governs the resistance in the quantum metallic state, we have investigated the nonreciprocal transport in gated MoS₂, an archetypal noncentrosymmetric 2D superconductor with trigonal symmetry. We found that the second harmonic resistance R_xx^2ω appears when the vortex motion is controlled by the classical vortex flow, while R_xx^2ω is substantially suppressed when the vortex motion is in quantum creep region. The present result indicates that the trigonal 2D superconductor is a new model system for investigation of quantum and classical ratchets.
[1] Y. Saito et al., Science. 350, 409–413 (2015).
[2] A. W. Tsen et al., Nat. Phys. 12, 208–212 (2016).