NbSe2

Nd<sub>2</sub>Fe<sub>14</sub>B domain structure imaged with MOKE
Figure 1. Crystal structure of 2H-NbSe2.

2H-NbSe2 is a layered transition metal dichalcogenide. It undergoes a phase transition to an incommensurate (IC) (~ 3a0), triangular charge density wave phase at a temperature TCDW~ 33 K. It also becomes a prototypical anisotropic s-wave superconductor below a temperature TSC~7.2 K. The coexistence of the density wave and superconducting phases and the nature of their interplay has been a subject of much recent interest, especially in the context of competing order in strongly correlated systems. Though NbSe2 has been extensively studied by thermodynamic and spectroscopic probes, several key questions pertaining to the CDW viz. what determines the CDW wavevector, the magnitude of the CDW gap and the role of FS nesting, remain unresolved.

2H-NbSe2 has a layered hexagonal structure (Fig. 1). The unit cell consists of two sandwiches of Se-Nb-Se. The crystal typically cleaves between the weakly coupled neighboring Se layers, and the top Se layer is imaged by STM.