Investigation of electronic structure of transition metal silicides MnSi1.75 and CoSi for enhanced thermoelectric properties

Transition metal silicides that have low band gap or semimetallic states have gained huge interest due to enhanced thermoelectric efficiency. We present high resolution photoemission studies on two such systems like MnSi1.75 a low band gap semiconductor and CoSi a semimetal to understand the inherent electronic properties. Valence band studies on these systems reveal that there are both localized and delocalized 3d states present where as in the pure transition metals more localized states present near the Fermi level. In both MnSi1.75 and CoSi, the localized 3d states are found to be shifted to higher binding energy due to the strong hybridization with the Si 3s - 3p states that gives rise to the hybridization gap in these systems. The delocalized 3d states in both the systems has much lower electronic density near the Fermi level than the pure transition metals that contributes to the conduction. Supporting evidences of delocalized screening in MnSi1.75 and weak correlated satellite in CoSi have been observed in the 2p core-level spectra. We find that the presence of both hybridization gap and the finite carrier concentration at the Fermi level in these transition metal silicides are necessary to have the enhanced thermoelectric properties.