Effect of intercalation on structural instability and superconductivity of layered 2H-type Nbse₂ and NbS₂

Effects of intercalation on the lattice instability and the superconductivity of NbSe₂ and NbS₂ are studied in the framework of the rigid band approximation. In the case of doping acceptor-type intercalants into NbSe₂, the point of lattice instability in q-space is moved to q = ΓM from q = 2/3 ΓM Further, the frequency renormalization is enhanced and the CDW transition temperature is expected to increase. In the case of doping donor-type intercalants, on the other hand, the frequency renormalization is weakened and the lattice instability is suppressed. The same tendency is found also in NbS₂. The effect of intercalation on the superconducting transition temperature T_c of NbS₂ is investigated by calculating the functional derivative of T_c with respect to the spectral function. The donor-type intercalant lowers T_c as observed in the case of alkali-metal doping. On the other hand, the acceptor-type intercalant raises T_c as long as the lattice instability does not take place.