Robust Gapless Superconductivity in 4Hb-TaS$_2$

Abstract

The superconducting TMD 4Hb-TaS$_2$ consists of alternating layers of H and T structures, which in their bulk form are metallic and Mott-insulating, respectively. Recently, this compound has been proposed as a candidate chiral superconductor, due to an observed enhancement of the muon spin relaxation at $T_c$. 4Hb-TaS$_2$ also exhibits a puzzling $T$-linear specific heat at low temperatures, which is unlikely to be caused by disorder. Elucidating the origin of this behavior is an essential step in discerning the true nature of the superconducting ground state. Here, we propose a simple model that attributes the $T$-linear specific heat to the emergence of a robust multi-band gapless superconducting state. We show that an extended regime of gapless superconductivity naturally appears when the pair-breaking scattering rate on distinct Fermi-surface pockets differs significantly, and the pairing interaction is predominantly intra-pocket. Using a tight-binding model derived from first-principle calculations, we show that the pair-breaking scattering rate promoted by slow magnetic fluctuations on the T layers, which arise from proximity to a Mott transition, can be significantly different in the various H-layer dominated Fermi pockets depending on their hybridization with T-layer states. Thus, our results suggest that the ground state of 4Hb-TaS$_2$ consists of Fermi pockets displaying gapless superconductivity, which are shunted by superconducting Fermi pockets that are nearly decoupled from the T-layers.