Distinct surface and bulk charge density waves in ultrathin 1T-TaS2

TL;DR

This study uses low-frequency Raman spectroscopy to reveal distinct surface and bulk charge density wave behaviors in ultrathin 1T-TaS2, showing surface-specific phase transitions and new phonon modes.

Contribution

It demonstrates the existence of separate surface and bulk CDW transitions in ultrathin 1T-TaS2 and identifies surface-specific phonon modes, advancing understanding of surface effects in layered materials.

Findings

Surface undergoes a separate NC-C phase transition

New surface-specific CDW phonon modes identified

Surface transition exhibits low hysteresis

Abstract

We employ low-frequency Raman spectroscopy to study the nearly commensurate (NC) to commensurate (C) charge density wave (CDW) transition in 1T-TaS2 ultrathin flakes protected from oxidation. We identify new modes originating from C phase CDW phonons that are distinct from those seen in bulk 1T-TaS2. We attribute these to CDW modes from the surface layers. By monitoring individual modes with temperature, we find that surfaces undergo a separate, low-hysteresis NC-C phase transition that is decoupled from the transition in the bulk layers. This indicates the activation of a secondary phase nucleation process in the limit of weak interlayer interaction, which can be understood from energy considerations.