Microstructure in the incommensurate and the commensurate charge-density-wave states of2H-TaSe2: A direct observation by electron microscopy

We have obtained images of the microstructure in the incommensurate and the commensurate charge-density-wave states of $2H$-Ta${\mathrm{Se}}_2$ with the use of superlattice dark-field electron microscopy. Large orthorhombic domains are observed in the commensurate phase. With warming to higher temperatures the incommensurate stripe phase appears. The incommensurate phase on a cooling cycle, however, is found to be more complicated. Instead of the hexagonal symmetry indicated by early x-ray diffraction results, we find that the charge-density waves actually adopt orthorhombic symmetry on a microscopic scale. The thermal evolution and detailed configuration of domains and discommensurations in these phases are studied in detail. It is found that charge-density-wave dislocations play the most prominent role in various charge-density states of $2H$-Ta${\mathrm{Se}}_2$.