1T-TaS 2 is unique among transition metal dichalcogenides in that it is understood to be a correlation-driven insulator, where the unpaired electron in a 13-site cluster experiences enough correlation to form a Mott insulator. We argue, based on existing data, that this well-known material should be considered as a quantum spin liquid, either a fully gapped Z2 spin liquid or a Dirac spin liquid. We discuss the exotic states that emerge upon doping and propose further experimental probes.spin liquid | Mott insulator | transition metal dichalcogenide T he transition metal dichalcogenide (TMD) is an old subject that has enjoyed a revival recently due to the interests in its topological properties and unusual superconductivity. The layer structure is easy to cleave or intercalate and can exist in singlelayer form (1, 2). This material was studied intensively in the 1970s and 1980s and was considered the prototypical example of a charge density wave (CDW) system (3). Due to imperfect nesting in two dimensions, in most of these materials, the onset of CDW gaps out only part of the Fermi surface, leaving behind a metallic state that often becomes superconducting. Conventional band theory and electron-phonon coupling appear to account for the qualitative behavior (3). There is, however, one notable exception, namely 1T-TaS2. The Ta forms a triangular lattice, sandwiched between two triangular layers of S, forming an ABCtype stacking. As a result, the Ta is surrounded by S, forming an approximate octahedron. In contrast, the 2H-TaS2 forms an ABA-type stacking, and the Ta is surrounded by S, forming a trigonal prism. In a single layer, inversion symmetry is broken in 2H structure. The system is a good metal below the CDW onset around 90 K, and, eventually, the spin-orbit coupling gives rise to a special kind of superconductivity called Ising superconductivity (4-6). In 1T-TaS2, inversion symmetry is preserved. The system undergoes a CDW transition at about 350 K with a jump in the resistivity. It is known that this transition is driven by an incommensurate triple-Q CDW (ICDW). A similar transition is seen in 1T-TaSe2 at 470 K. However, whereas TaSe2 stays metallic below this transition, 1T-TaS2 exhibits a further resistivity jump around 200 K that is hysteretic, indicative of the firstorder nature of this transition. These transitions are also visible in the spin susceptibility data shown in Fig. 2. In early samples, the resistivity rises only by about a factor of 10 as the temperature is lowered from 200 K to 2 K and, below that, obeys Mott hopping law (log resistivity goes as T −1/3 ) (7). More recent samples show better insulating behavior, and it is generally agreed that the ground state is insulating. The 200 K transition is accompanied by a lock-in to a commensurate CDW (CCDW), forming a √ 13 × √ 13 structure. As shown in Fig. 1, this structure is described as clusters of stars of David where the sites of the stars move inward toward the site in the middle. The stars of David are packed in such a way that they form ...
