1T-TaS2 as a 45 year old spin liquid.

IT-TaS2 has been studied for a long time as a charge density wave system. The new unit cell consists of 13 site "star of David" clusters forming a triangular lattice. It was proposed 40 years ago that these cluster give rise to a Mott insulator state with S=1/2 per cluster. However, the expected local moments have never been observed and no magnetic ordering has been found. We proposed [1] that this material may be an example of the quantum spin liquid that has somehow gone unnoticed for 45 years. Recent experiments have provided strong evidence that this is indeed the case, and that the low energy excitations are characterized by a spinon Fermi surface. We propose that the mechanism has to do with the proximity to the Mott transition, which can be modelled by a spin model with NN exchange J and a 4 site ring exchange term K. We performed DMRG calculations on this model [2], extending earlier work to 6 and 8 wide cylinders and found that a modest K/J ratio is sufficient to stabilize a spin liquid ground state with Fermi surface. A strong signature is that the peaks in the spin-spin correlator lie on a circle with radius 2k_F. We have also performed doping studies on the spin liquid and found that the leading pair correlation is oscillatory in space, ie, a pair density wave. [3] It will be very interesting to dope this and related systems very gently (less than 1% carrier per Ta) by gating to minimize localization of the doped carriers.
[1] K. T. Law and P. A. Lee, PNAS 114, 6996 (2017).
[2] Wen-Yu He, Xiao Yan Xu, Gang Chen, K. T. law and P. A. Lee, Phys. Rev. Lett. 121, 046401 (2018).
[3] Xiao Yan Xu, K. T. Law and P. A. Lee, to be published.