Hierarchy among the crystal lattice, charge density wave and superconducting orders in transition metal dichalcogenides

Layered TMDs exhibit a variety of lattice structures and collective electronic states, including charge density waves (CDW)s and superconductivity (SC). Studies have indicated that, generally, the CDW and SC orders in TMDs compete unless mutually exclude each other but lack details. We studied exemplary Ta(Te/Se)₂ solid solutions extending between the stable polymorphs of their end members, hexagonal 2H-TaSe₂ and monoclinic 1T’-TaTe₂, using high energy x-ray diffraction and large scale structure modeling. We will show that the emergence of CDWs and SC in TMDs does not indeed depend critically on the type of the underlying crystal lattice but depends strongly on its perfection. In particular, TMDs exhibiting strong lattice distortions, such as buckling of TM planes, do not exhibit CDWs and SC. The presence of a perfect lattice order in 2D is a prerequisite to the emergence of CDWs but insufficient to achieve a SC ordered state. For the SC order to emerge, the TM-sublattice should also appear periodic in 3D. We will also present evidence that distortions of TM-chalcogenide coordination polyhedra degrade the SC order to a certain extent, and this may well explain the observed irregular evolution of the SC critical temperature, Tc, with the degree of chalcogenide substitution.