Nanomaterials 2021, 11, 2595. https://doi.org/10.3390/nano11102595 www.mdpi.com/journal/nanomaterials Article Topological Anderson Insulator in Cation‐Disordered Cu2ZnSnS4 Binayak Mukherjee 1, *, Eleonora Isotta 1 , Carlo Fanciulli 2 , Narges Ataollahi 1 and Paolo Scardi 1, * 1 Department of Civil, Environmental and Mechanical Engineering, University of Trento, via Mesiano 77, 38123 Trento, Italy; eleonora.isotta@unitn.it (E.I.); narges.ataollahi@unitn.it (N.A.) 2 Lecco Unit, National Research Council of Italy, Institute of Condensed Matter Chemistry and Technologies for Energy (CNR‐ICMATE), via Previati 1/E, 23900 Lecco, Italy; carlo.fanciulli@cnr.it * Correspondence: binayak.mukherjee@unitn.it (B.M.); paolo.scardi@unitn.it (P.S.) Abstract: We present the first candidate for the realization of a disorder‐induced Topological An‐ derson Insulator in a real material system. High‐energy reactive mechanical alloying produces a polymorph of Cu2ZnSnS4 with high cation disorder. Density functional theory calculations show an inverted ordering of bands at the Brillouin zone center for this polymorph, which is in contrast to its ordered phase. Adiabatic continuity arguments establish that this disordered Cu2ZnSnS4 can be connected to the closely related Cu2ZnSnSe4, which was previously predicted to be a 3D topological insulator, while band structure calculations with a slab geometry reveal the presence of robust sur‐ face states. This evidence makes a strong case in favor of a novel topological phase. As such, the study opens up a window to understanding and potentially exploiting topological behavior in a rich class of easily‐synthesized multinary, disordered compounds. Keywords: quaternary chalcogenides; topological insulators; disordered systems; kesterite; thermoelectrics 1. Introduction Topologically non‐trivial materials present a novel and exciting field of research in condensed matter [1]. They are valued both for their importance to fundamental science as exotic states of quantum matter as well as their inherent potential for application in new and future technologies including thermoelectrics [2–4], spintronics [5–7], and quan‐ tum computation [5,6,8]. Starting with the discovery of the Quantum Hall Effect (QHE) by von Klitzing et al. [9], this class of materials has grown to include many candidates in 2‐,3‐, and higher dimensional systems, a growing (albeit still small) fraction of which have been experimentally realized. Three‐dimensional (3D) topological insulators (TIs) present a sub‐class of these exotic materials. They may generally be described as hosting insulat‐ ing bands in the bulk with band inversion at high‐symmetry points, coupled with sym‐ metry‐protected gapless surface states [10]. In the absence of symmetry breaking, these surface states support high‐mobility electron transport along specific directions on the surface, without backscattering. Large spin‐orbit coupling (SOC) was originally under‐ stood to be driving the topologically non‐trivial behavior [11–15]. Subsequently, Fu [16] demonstrated that topological surface states can also be protected by crystalline symme‐ tries in the absence of SOC (topological crystalline insulators). This allows for topologi‐ cally non‐trivial materials with a weak SOC [17]. The possibility of TIs in the quaternary chalcogenide class has been investigated by Chen et al. [18], using density functional theory (DFT) band structures. They showed that HgTe, a 3D semimetal with the zinc‐blende structure, may be transformed into a TI by introducing a strong crystal field splitting (ΔCF). This can be achieved either by epitaxial straining or by substituting two group‐II Hg ions with one group‐I ion and one group‐III Citation: Mukherjee, B.; Isotta, E.; Fanciulli, C.; Ataollahi, N.; Scardi, P. Topological Anderson Insulator in Cation‐Disordered Cu2ZnSnS4. Nanomaterials 2021, 11, 2595. https://doi.org/10.3390/nano11102595 Academic Editor: Jory Yarmoff Received: 30 July 2021 Accepted: 29 September 2021 Published: 1 October 2021 Publisher’s Note: MDPI stays neu‐ tral with regard to jurisdictional claims in published maps and insti‐ tutional affiliations. Copyright: © 2021 by the authors. Li‐ censee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and con‐ ditions of the Creative Commons At‐ tribution (CC BY) license (http://crea‐ tivecommons.org/licenses/by/4.0/).