Physics and Chemistry of Chalcogenide Quantum Materials with Lacunar Spinel Structure

This paper aims to review the physical properties and crystal chemistry of the family of correlated quantum materials AM4Q8 (A = Ga, Ge; M = V, Nb, Ta, Mo; Q = S, Se). These compounds exhibit a lacunar spinel structure with tetrahedral transition metal clusters which favor a correlated state. But compared to most other inorganic Mott insulators, the AM4Q8 compounds show very small Mott–Hubbard gaps (0.1–0.3 eV). These small values originate from the weak electronic repulsion occurring on the scale of the tetrahedral clusters and from the effect of spin–orbit coupling. As a consequence, AM4Q8 quantum materials show a great variety of ground states and astonishing electronic properties depending on cluster filling, compression, or distortion. For example, they exhibit a multiferroic behavior related to an orbital ordering on the clusters and a variety of magnetic ordering like skyrmions. Under external pressure they undergo a bandwidth-controlled insulator to metal transition (IMT) that leads to a superconducting state at low temperature. When chemically doped, the AM4Q8 compounds undergo a filling-controlled insulator to metal transition with appearance of a half ferromagnetic metallic behavior or colossal negative magnetoresistance. The AM4Q8 compounds are also sensitive to the electric field and exhibit a striking resistive switching above a threshold electric field of a few kV/cm which is related to the breakdown of the Mott insulating state at the nanoscale. This phenomenon unlocks new functionalities that may be used to build up a new type of resistive random-access memory (RRAM) or an artificial neuron. All these examples show the potential of this family of quantum materials whose exploration has only just begun.