Investigation of the optical reflectivity of misfit layer compounds: (MS)nTS2(T=Ta, Nb; M=Sn, Pb, Sm, Tb, La; 1.08

Abstract

The room temperature reflectivity of the misfit layer compounds (MS)nTS2 (T=Nb, Ta) is investigated in the spectral range 1000-20000 cm -1. The reflectivity for the E-vector parallel to the (001) planes and the incident light along (001) is dominated by the charge carrier response. This part of the spectra can be described using the Drude formula with the plasma frequency given by omega p=(Ne2/ epsilon 0 epsilon infinity m*)0.5 (N=charge carrier concentration, m*=effective mass, epsilon infinity =high-frequency dielectric constant). For compounds with M=Sn, Pb the number of charge carriers is equal to that of the parent compound TS2. The observed anisotropy in the (001) plane is rather small and can be related to small differences in the effective masses (m(010)*/m(100)* approximately=0.98). This effect is attributed to the orthorhombic distortion of the (TS2) sublattice. For compounds with M=La, Sm, Tb, electron donation is indicated and the plasma frequency is determined by about 0.5, 0.2, 0.25 holes per T ion, respectively, in a single (TS2)-related conduction band.