Evidence for time reversal symmetry breaking in a van der Waals Superconductor

Low dimensional materials have been a major subject of interest in recent years. In particular, the transitions metal dichalcogenides (TMDs), quasi-2D layered materials with weak van der Waals coupling between layers, received a lot of attention. TMDs exhibit strong electron-electron and electron-phonon interactions that

lead to complicated phase diagrams showing a variety of ground states. A fascinating frontier, largely unexplored, is the stacking of strongly correlated phases of TMDs. We study 4Hb-TaS 2, which naturally realizes an alternating stacking of 1T-TaS₂ and 1H-TaS₂ structures. The former is a well known Mott insulator, which has recently been proposed to host a gapless spin-liquid ground state. The latter is a superconductor known to also host a competing charge density wave state. This raises the question of how these two components affect each other when stacked together. Using Muon Spin Relaxation, we show that 4Hb-TaS₂ is a superconductor that breaks time-reversal symmetry, abruptly at the superconducting transition [1]. Using scanning superconducting quantum interference device (SQUID) microscopy we found a spontaneous vortex phase whose vortex density depends on the magnetic history of the sample above Tc [2]. In addition, using scanning tunneling spectroscopy we find spectroscopic evidence for the existence of topological surface superconductivity. These include edge modesrunning along the 1H-layer terminations as well as under the 1T-layer terminations, where they separate between superconducting regions of distinct topological nature [3]. While specific heat measurements find a fully gapped superconductor, we show using the Little-Parks experiment that 4Hb-TaS₂ is not a s-wave superconductor. Together, all the accumulated data strongly suggests that 4Hb-TaS₂ is a chiral superconductor.