Versatile electronic states in epitaxial thin films of (Sn-Pb-In)Te: from topological crystalline insulator and polar semimetal to superconductor

TL;DR

This study demonstrates the growth and characterization of (Sn-Pb-In)Te epitaxial thin films, revealing a range of electronic phases including topological insulators, semimetals, and superconductors, controlled by composition tuning.

Contribution

It provides a comprehensive phase diagram of (Sn-Pb-In)Te thin films, showing how composition influences topological, semimetallic, and superconducting states, and highlights the potential for exploring novel quantum phenomena.

Findings

Superconductivity appears at high Sn and In compositions.

Charge carrier type switches from p-type to n-type with In doping.

High mobility semimetal states observed near specific compositions.

Abstract

Epitaxial thin films of (Sn$_{x}$Pb$_{1-x}$)$_{1-y}$In$_{y}$Te were successfully grown by molecular-beam-epitaxy (MBE) in a broad range of compositions (0 $\leq$ x $\leq$ 1, 0 $\leq$ y $\leq$ 0.23). We investigated electronic phases of the films by the measurements of electrical transport and optical second harmonic generation. In this system, one can control the inversion of band gap, the electric polarization that breaks the inversion symmetry, and the Fermi level position by tuning the Pb/Sn ratio and In composition. A plethora of topological electronic phases are expected to emerge, such as topological crystalline insulator, topological semimetal, and superconductivity. For the samples with large Sn compositions (x > 0.5), hole density increases with In composition (y), which results in the appearance of superconductivity. On the other hand, for those with small Sn compositions (x < 0.5), increase in In composition reduces the hole density and changes the carrier type from p-type to n-type. In a narrow region centered at (x, y) = (0.16, 0.07) where the n-type carriers are slightly doped, charge transport with high mobility exceeding 5,000 cm$^{2}$V$^{-1}$s$^{-1}$ shows up, representing the possible semimetal states. In those samples, the optical second harmonic generation measurement shows the breaking of inversion symmetry along the out-of-plane [111] direction, which ensures the presence of polar semimetal state. The thin films of (Sn$_{x}$Pb$_{1-x}$)$_{1-y}$In$_{y}$Te materials systems with a variety of electronic states would become a promising materials platform for the exploration of novel quantum phenomena.