Direct observation of mono-layer, bi-layer, and tri-layer charge density waves in 1T-TaS2 by transmission electron microscopy without a substrate

Condensed Matter Physics: Charge density wave occurs in the two dimensional limit Charge density wave (CDW), a wave-like charge modulation, occurs in tri-, bi- and mono-layer crystals. A team led by Satoshi Tanda at Hokkaido University and collaborators at AIST in Japan observed CDWs in mono-, bi-, and tri-layer 1T-TaS2 crystal without a substrate using a low voltage scanning transmission electron microscope. The CDW in a mono-layer sample does not exhibit a topological phase with vortex type point defects, which had been previously anticipated from a two-dimensional model. Instead, new topological CDW phases with intra-layer line defects are observed in mono-layer and bi-layer samples, suggesting a significant role of strong electron-phonon interaction in thinned samples. (In tri-layer, commensurate CDW phase without defects was observed.) These results shed a light to understand the stripe type structures in high temperature superconductors and call for new understandings in a pure material to the two dimensional limit. (141 words)