Observation of metallic mosaic phase in 1T-TaS2 at equilibrium

The transition-metal dichalcogenide tantalum disulphide (1T-TaS2) hosts a commensurate charge density wave (CCDW) at temperatures below 165 K where it also becomes insulating. The low temperature CCDW phase can be driven into a metastable "mosaic" phase by means of either laser or voltage pulses which shows a large density of CDW domain walls as well as a closing of the electronic band gap. The exact origins of this pulse-induced metallic mosaic are not yet fully understood. Here, we observe the occurrence of such a metallic mosaic phase on the surface of TaS2 without prior pulse excitation using scanning tunnelling microscopy and spectroscopy (STM/STS) over continuous areas larger than 100x100 nm2 and macroscopic areas on the millimetre scale. We attribute the appearance of the mosaic phase to the presence of surface defects which arrange into the characteristic dense domain wall network. Based on our STM measurements, we further argue how the appearance of the metallic behaviour in the mosaic phase can be explained by local stacking differences of the top two layers induced by the large number of domain walls. Thus, we provide a potential avenue to explain the origin of the pulse induced mosaic.