Raman Scattering from Charge Density Waves and Superconducting Gap Excitations in 2H-TaSe2 and 2H-NbSe2

Many transition-metal dichalcogenides undergo phase transitions of the charge-density-wave (CDW) type.1 The metal atoms are found at the center of hexagonal layers where they bind covalently and metallically to chalcogen atoms in adjacent planes. Weak interlayer bonding is of the Van der Waals type. In the 2H polytype of 2H-TaSe2 and 2H-NbSe2 there is one electron per metal atom in a narrow half-filled “dz2” conduction band above a filled s-p valence band and below a broad, empty d-like conduction band. This normal high temperature structure is unstable against the formation of a periodic structural distortion (PSD) of the lattice associated with an electronic CDW.