Pressure effect and superconductivity in the β-Bi4I4 topological insulator

We report a detailed study of the transport properties of a beta-Bi4I4 quasi-one-dimensional topological insulator. Electrical resistivity, thermoelectric power, thermal conductivity, and Hall coefficient measurements are consistent with the possible appearance of a charge-density-wave order at low temperatures. Both electrons and holes contribute to the conduction in beta-Bi4I4 and the dominant type of charge carrier changes with temperature as a consequence of temperature-dependent carrier densities and mobilities. Measurements of resistivity and Seebeck coefficient under hydrostatic pressure up to 2 GPa show a shift of the charge-density-wave order to higher temperatures suggesting a strongly one-dimensional character at ambient pressure. Surprisingly, superconductivity is induced in beta-Bi4I4 above 10 GPa with T-c of 4.0 K which is slightly decreasing upon increasing the pressure up to 20 GPa. Chemical characterization of the pressure-treated samples shows amorphization of beta-Bi4I4 under pressure and rules out decomposition into Bi and BiI3 at room-temperature conditions.