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-TaS2, 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 modes running 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.
[1] A. Ribak et al., Sci. Adv. 6, eaax9480 (2020).
[2] E. Persky et al., Nature 607, pages 692–696 (2022).
[3] A. K.Nayak et al., Nature Phys. 17,1413 (2021).