Interplay of charge density modulations and superconductivity

Although charge density waves (CDW) in transition metal dichalcogenides (TMDs) have been studied for over three decades, there is yet no consensus on the mechanism for CDW in two-dimensional TMDs. Moreover, the layered 2H-type TMDs which exhibit coexistence of CDW order and superconductivity (SC) present an intriguing opportunity for studying the interplay of SC and charge density modulations. In particular, 2H-NbSe$_2$ is most suitable for such investigation as its transition temperatures for CDW and SC are well separated, and as it is easy to cleave is ideal for surface-sensitive measurements. The relation between CDW and SC in this material is still under debate, with one experiment indicating an enhancement of SC by CDW [1] and another implying a competition of the two kinds of orders [2]. Motivated by these experiments, we study the effects of Fermi surface structure on CDW and its interplay with SC in terms of the Bogoliubov-de Gennes formalism. \\[4pt] [1] Kiss {\it et al.}, Nat. Phys. {\bf 3}, 720 (2007). \\[0pt] [2] Borisenko {\it et al.}, Phys. Rev. Lett. {\bf 102}, 166402 (2009).