Chiral superconductivity in the alternate stacking compound 4Hb-TaS2.

Van der Waals materials offer unprecedented control of electronic properties via stacking of different types of two-dimensional materials. A fascinating frontier, largely unexplored, is the stacking of strongly correlated phases of matter. 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. We find a superconductor with a T c of 2.7 Kelvin and anomalous properties, of which the most notable one is a signature of time-reversal symmetry breaking, abruptly appearing at the superconducting transition. This observation is consistent with a chiral superconducting state.

A universal critical density underlying the physics of electrons at the LaAlO3/SrTiO3 interface.

The two-dimensional electron system at the interface between the insulating oxides LaAlO(3) and SrTiO(3) exhibits ferromagnetism, superconductivity and a range of unique magnetotransport properties. An open experimental challenge is to identify, out of the multitudinous energy bands predicted to exist at the interface, the key ingredients underlying its emergent transport phenomena. Here we show, using magnetotransport measurements, that a universal Lifshitz transition between d orbitals of different symmetries lies at the core of the observed phenomena. We find that LaAlO(3)/SrTiO(3) systems generically switch from one- to two-carrier transport at a universal carrier density, which is independent of the LaAlO(3) thickness and electron mobility. Interestingly, the maximum superconducting critical temperature occurs also at the Lifshitz density, indicating a possible connection between the two phenomena. A simple band model, allowing for spin-orbit coupling at the atomic level, connects the observed transition to a variety of previously reported properties. Our results demonstrate that the fascinating behaviour observed so far in these oxides follows from a small but fundamental set of bands.