Pressure effect and Superconductivity in $\beta$-Bi$_4$I$_4$ Topological Insulator

TL;DR

This study investigates how pressure influences the electronic properties and induces superconductivity in the topological insulator $eta$-Bi$_4$I$_4$, revealing a pressure-induced transition from charge density wave order to superconductivity.

Contribution

It provides the first detailed analysis of pressure effects on $eta$-Bi$_4$I$_4$, demonstrating superconductivity emergence above 10 GPa and characterizing its transport properties under varying pressures.

Findings

Charge density wave order shifts to higher temperatures under pressure.

Superconductivity appears above 10 GPa with a transition temperature of 4.0 K.

Pressure causes amorphization of $eta$-Bi$_4$I$_4$ without decomposition.

Abstract

We report a detailed study of the transport coefficients of $\beta$-Bi$_4$I$_4$ 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 $\beta$-Bi$_4$I$_4$ 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 $\beta$-Bi$_4$I$_4$ above 10 GPa with of 4.0 K which is slightly decreasing upon increasing the pressure up to 20 GPa. Chemical characterisation of the pressure-treated samples shows amorphization of $\beta$-Bi$_4$I$_4$ under pressure and rules out decomposition into Bi and BiI$_3$ at room-temperature conditions.