Experimental realization of a ferroelectric topological crystalline insulator based on IV–VI compounds

Abstract

Ferroelectric topological materials combining ferroelectricity and non-trivial band topology open promising frontiers in topological matter by introducing ferroelectric-driven inversion symmetry breaking in the system [1-3]. In particular, introducing ferroelectricity in topological crystalline insulator has been predicted to support the emergence of inversion breaking Weyl semimetal phase, which so far has been experimentally realized only in time-reversal breaking systems, such as TaAs material family [4-6]. Furthermore, controlling novel topological phases via ferroelectric polarization switching induced by electric fields is a core idea in topological spintronics [7].
Here, we report the experimental realization of a new Pb1-x-ySnxGeySe ferroelectric topological crystalline insulator (FETCI) that hosts non-trivial topological surface states in co-existence with emergent ferroelectric phase. Using molecular beam epitaxy, high-quality quantum well heterostructures (QWs) as well as thin film geometries were grown on (111)-oriented BaF2 substrates using PbSe, SnSe, and GeSe sources. We demonstrate the dual functionality in FETCI QWs by means of temperature-dependent angle resolved photoemission spectroscopy (ARPES), spanning over the whole temperature range of the ferroelectric and topological phase transitions from room temperature down to 20 K. In particular, ARPES measurements at the high symmetry M-point reveal simultaneously ferroelectric Rashba-split states and topological surface states emerging as the system undergoes a topological phase transition below the ferroelectric Curie temperature. The character of the observed topological and ferroelectric phase transitions is further proven by temperature-dependent x-ray diffraction, transport and magneto-optical measurements.
Our results evidence that ferroelectric phase and topological crystalline insulator phase co-exist simultaneously in a single material based on the group of IV-VI compounds. This opens a new platform for electrically tunable topological phases, promising for demonstrating and manipulating novel band topologies.

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