Charge Density Wave Phase Transition in TaSe2 Low-Dimensional Material
Abstract: Although periodic atomic displacements in transition metal dichalcogenides formed due to the commensurate charge density wave (CCDW) phase transition have been modeled by diffractometric methods, they have not been confirmed by direct observation in real space. Against this background, we have attempted direct observation of the periodic atomic displacements in real space by high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) of the CCDW phase of 1T-TaSe2. In consequence, periodic image contrast with six-fold symmetry superimposed on the image contrasts due to the atomic columns is observed in HAADF-STEM image of the CCDW phase of 1T-TaSe2. The periodicity of the image contrast corresponds to that of the long range ordered structure formed in the CCDW phase of 1T-TaSe2. Also it is confirmed that the periodic image contrast is disappeared at 673 K, which is the phase transition temperature of the 1T-TaSe2, or higher temperature by HAADF-STEM observation of the 1T-TaSe2 at elevated temperatures. These experimental results and comparison of the experimental HAADF-STEM image with simulated image calculated by the multi slice method suggests that the periodic image contrast reflects the static displacement of Ta atom positions. These results show that the periodic atomic displacement, which corresponds with the model predicted by the diffractometric methods, is observed experimentally in the CCDW phase of 1T-TaSe2 shown by HAADF-STEM observation.
Key words: Transition metal dichalcogenides, Charge density wave, 1T-TaSe2, HAADF-STEM