Excitons in molybdenum disulphide

Optical spectra of Mo${\mathrm{S}}_2$ in the excitonic region were obtained by photoconductivity, photovoltaic effect and wavelength-modulated reflectivity at temperatures of 300 to 4.2 K and in magnetic fields of up to 70 kG. Cleaved natural crystals and synthetic crystals studied in both the Faraday and Voigt configurations showed nearly the same characteristics. The results indicate the possibility of up to four excitonic series in Mo${\mathrm{S}}_2$ as in other Mo dichalcogenides, and support band-structure calculations predicting flat conduction and valence bands originating from Mo orbitals. A ground-state anomaly of the $A$ exciton is explained by a central cell correction of the type used by Harbeke and Tosatti to explain a similiar anomaly in Pb${\mathrm{I}}_2$. The reduced mass of the $A$ exciton is found to be ${\mu }^{*}\simeq 0.4m_0$, both from the study of the series itself, and from its magneto-optical properties, thus removing a large discrepancy found in previous work.