Heavy carrier effective masses in van der Waals semiconductor Sn(SeS) revealed by high magnetic fields up to 150 T

The ${\mathrm{SnSe}}_{2(1\ensuremath{-}x)}{\mathrm{S}}_{2x}$ alloy is a van der Waals semiconductor with versatile, tunable electronic properties and prospects for future applications ranging from electronics to thermoelectrics and superconductivity. Its band structure and carrier effective masses underlie the quantum behavior of charge carriers and hold great promise in future technologies. However, experimental measurement of these properties remains a challenging task. Here magnetotransmission spectroscopy of ${\mathrm{SnSe}}_{2(1\ensuremath{-}x)}{\mathrm{S}}_{2x}$ thin films at pulsed magnetic fields $B$ of up to 150 T reveals a large electron-hole reduced cyclotron mass ${\ensuremath{\mu}}^{*}>$ 0.454 ${m}_{e}$ (${m}_{e}$ is the free electron mass). This finding is supported by first-principle calculations of the band structure and by semiclassical Boltzmann transport theory, which predict a pronounced anisotropy of the carrier effective masses and electrical conductivity over two orthogonal directions (namely in the layer plane and out-of-plane) with a different anisotropy for electrons and holes. These properties are unique and important features of this class of compounds and are critical for understanding and using the tunable band structure of ${\mathrm{SnSe}}_{2(1\ensuremath{-}x)}{\mathrm{S}}_{2x}$ in fundamental and applied research.