Observation of the metallic mosaic phase in 1$T$-TaS$_2$ at equilibrium

Abstract

The transition-metal dichalcogenide tantalum disulphide (1$T$-TaS$_2$) 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, using scanning tunneling microscopy and spectroscopy (STM/STS), we observe the occurrence of such a metallic mosaic phase on the surface of TaS$_2$ without prior pulse excitation over continuous areas larger than $100 \times 100$~nm$^2$ and macroscopic areas on the millimetre scale. We attribute the appearance of the mosaic phase to the presence of surface defects which cause the formation of 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 could be explained by local stacking differences of the top layer. Thus, we provide a potential avenue to explain the origin of the pulse induced mosaic phase.