Recently attention to the dichalcogenide superconductors has been revived thanks to the observation of charge order in the cuprate high temperature superconductors. Indeed, since 2008, more and more experiments report the observation of a charge density wave order close to the superconducting state in the cuprates. Those observations revive the debate of the role of electronic fluctuations versus the previous dominant paradigm of antiferromagnetic fluctuations as keys ingredients for high Tc. It turns out that the dichalcogenide superconductor familly is a perfect playground to study the interaction between superconductivity and charge order without any magnetic degree of freedom.
Dichalcogenides are lamellar metallic or semi-metallic materials known to exhibit charge-density wave instabilities related to their low-dimensional character : the basic building block consists of MX2 layers, with M a transition metal and X a chalcogen atom, separated by a van der Waals gap providing a 2D-character to their electronic properties. The van der Waals gap allows the tuning of the charge order instability by inserting ions in-between the layers.
When going in the chalcogen column from S to Se to Te, the electronic structure evolves from semiconducting to metallic, giving rise to a variety of charge ordering instabilities. Depending on the system, the mechanism responsible for the onset of the CDW instability can involve electron-phonon interaction, charge-charge interaction (Mott or excitonic type) or a combination of them. The competition between local physics due to the strong electronegativity of the chalcogen and delocalized, band-structure effects can result in a complex behavior of the CDW.
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