Dynamics of electronic states in the insulating intermediate surface phase of 1 T-TaS2

This article reports a comparative study of bulk and surface properties in the transition metal dichalcogenide 1T-TaS2. When heating the sample, the surface displays an intermediate insulating phase that persists for ∼10 K on top of a metallic bulk. The weaker screening of Coulomb repulsion and a stiffer charge density wave (CDW) explain such resilience of a correlated insulator in the topmost layers. Both time-resolved angle-resolved photoelectron spectroscopy and transient reflectivity are employed to investigate the dynamics of electrons and CDW collective motion. It follows that the amplitude mode is always stiffer at the surface and displays variable coupling to the Mott-Peierls band, stronger in the low-Temperature phase and weaker in the intermediate one.