Emergence of charge density wave in ultrathin orthorhombic-CoSe₂ film

Among transition metal chalcogenides (TMC), CoSex system has several kinds of stable phases, such as orthorhombic-CoSe2(o-CoSe2), hexagonal-CoSe2, 1T-CoSe2, tetragonal-CoSe. These stable phases of CoSex system have consecutive structural transitions across thermal annealing procedure.[1] We successfully fabricated ultrathin o-CoSe2 on epitaxial graphene substrates using MBE. We propose that o-CoSe2 possesses the Peierls-type charge density wave (CDW) in ultrathin thickness limit. In our DFT calculation results, Fermi surface of o-CoSe2 satisfies nesting condition with two parallel vectors unlike that of corresponding bulk. We experimentally confirmed the corresponding lattice distortion from scanning tunneling microscopy (STM) measurements and also analyzed related band structure maps from angle-resolved photoemission spectroscopy (ARPES) measurements. Our results suggest that the ultrathin CoSe2 film can serve as a interesting material platform for investigating two-dimensional electronic correlations.