Resonating holes vs molecular spin-orbit coupled states in group-5 lacunar spinels

Abstract

The valence electronic structure of magnetic centers is one of the factors that determines the characteristics of a magnet. This may refer to orbital degeneracy, as for jeff = 1/2 Kitaev magnets, or near-degeneracy, e.g., involving the third and fourth shells in cuprate superconductors. Here we explore the inner structure of magnetic moments in group-5 lacunar spinels, fascinating materials featuring multisite magnetic units in the form of tetrahedral tetramers. Our quantum chemical analysis reveals a very colorful landscape, much richer than the single-electron, single-configuration description applied so far to all group-5 GaM4X8 chalcogenides, and clarifies the basic multiorbital correlations on M4 tetrahedral clusters: while for V strong correlations yield a wave-function that can be well described in terms of four V4+V3+V3+V3+ resonant valence structures, for Nb and Ta a picture of dressed molecular-orbital jeff = 3/2 entities is more appropriate. These internal degrees of freedom likely shape vibronic couplings, phase transitions, and the magneto-electric properties in each of these systems.

Cluster Rearrangement by Chiral Charge Order in Lacunar Spinel GaNb4Se8

“…Lacunar spinel compounds GaM 4 X 8 (M = V, Nb, Mo, Ta, and X = S, Se) are Mott insulator systems that exhibit various electronic and magnetic properties, such as pressure-induced superconductivity, − negative colossal magnetoresistance, spin-singlet state, , spin–orbit entangled molecular J eff state, − and magnetic skyrmions. − Since the formal valence of M is 3.25+, the average number of d electrons are 7/4 and 11/4 for group V (M = V, Nb, Ta) and group VI (M = Mo) transition metals, respectively. Since only half of the tetrahedral sites are occupied by Ga atoms, the pyrochlore network of M atoms takes on a breathing-type structure, resulting in M 4 tetrahedral clusters.…”