Exotic properties and emergent functionalities of chalcogenide Mott insulators AM4Q8

The AM4Q8 (A = Ga, Ge; M = V, Nb, Ta, Mo; Q = S, Se) compounds represent a family of narrow gap Mott insulators with very interesting electronic properties. These compounds exhibit a lacunar spinel structure with tetrahedral transition metal clusters M4 [1]. Compare to most other inorganic Mott insulators, the AM4Q8 compounds show very small Mott-Hubbard gap (0.1-0.3 eV) as the electronic repulsion occurs on the scale of these tetrahedral clusters and not on the scale of single atoms [2]. As a consequence these compounds show a great variety of ground states and astonishing electronic properties depending on clusters filling, compression or distortion. GeV4S8 for example exhibit a multiferroic behaviour related to an orbital ordering on the clusters [3]. GaTa4Se8 and GaNb4Se8 undergo a bandwidth-control Insulator to Metal Transition (IMT) and superconductivity when placed under external pressure [2, 4]. The substitution of V per Ti in the GaV4S8 leads to an IMT driven by disorder on the clusters and to the emergence of a half ferromagnetic metal [5]. While chemical doping of the ferromagnetic Mott insulator Ga1-xGexV4S8 leads to a bulk, colossal and negative magnetoresistance that may be understood on the basis of the cluster distortion at low temperature [6]. Finally, the AM4Q8 compounds reveal 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 [7]. This phenomenon opens new functionalities that may be employed to build up a new type of Resistive Random Access Memory (RRAM) [8] or an artificial neurons [9]. 


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