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

TL;DR

This paper investigates the complex electronic and magnetic structures of group-5 lacunar spinels, revealing multiorbital correlations and differing behaviors in V, Nb, and Ta compounds that influence their physical properties.

Contribution

It provides a detailed quantum chemical analysis showing multiorbital correlations and clarifies the nature of magnetic moments in group-5 lacunar spinels, extending beyond single-electron models.

Findings

V exhibits strong correlations with resonant valence structures.

Nb and Ta are better described by dressed molecular-orbital-like $j_{eff}=3/2$ states.

Internal degrees of freedom influence vibronic couplings and phase transitions.

Abstract

The valence electronic structure of magnetic centers is one of the factors that determines the characteristics of a magnet. It may refer to orbital degeneracy, as for $j_\text{eff}=1/2$ 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 Ga$M_4X_8$ chalcogenides, and clarifies the basic multiorbital correlations on $M_4$ tetrahedral clusters: while for V strong correlations yield a wave-function that can be well described in terms of four V$^{4+}$V$^{3+}$V$^{3+}$V$^{3+}$ resonant valence structures, for Nb and Ta a picture of dressed molecular-orbital-like $j_\text{eff}=3/2$ entities is more appropriate. These internal degrees of freedom likely shape vibronic couplings, phase transitions, and magneto-electric properties in each of these systems.