Broken hexagonal symmetry in the locked-in state of 2Ha-TaSe₂and the discommensurate microstructure of its incommensurate CDW states

Abstract

Discommensuration arrays have been seen directly in the electron microscope for the first time in a charge-density-wave-bearing material. The material, 2H-TaSe2, is also discovered to be orthorhombic in its commensurate phase (projected space group Cmcm) and not hexagonal. This leads to a natural description of the occurrence of the incommensurate stripe phase, observed only on warming. It likewise is orthorhombic. A new double-honeycomb discommensurate model is supported for the symmetric incommensurate phase. The remarkable dynamics and inhomogeneity in this wealth of microstructure is of much relevance to the new generation of fluctuation and transition theory papers on these and related incommensurate systems.