Abstract:

In cluster Mott insulators, electrons are delocalized over a cluster (dimer, trimer, tetramer, …) while intercluster delocalization is suppressed by Coulomb repulsion. This leads to unconventional localized quasimolecular magnetic moments. Compared to standard Mott insulators, the localized magnetic moments have additional internal degrees of freedom that may lead to exotic magnetic interactions.

A particularly interesting example is the large family of lacunar spinels AM4X8 (M = V, Ti, Mo, Nb, Ta; A = Ga, Ge, Al; X = S, Se, Te) in which the transition metals form a tetrahedral cluster. These materials show a large variety of physical phenomena, which include multiple multiferroic and skyrmion-lattice phases, (anti-)ferroelectric states and non-magnetic singlet states. 
In the talk we focus on GaTa4Se8 which shows a pressure-induced insulator-to-metal transition followed by superconductivity. We show how we can use Resonant Inelastic X-ray Scattering (RIXS) at the L3 edge of Ta to prove the quasimolecular character and determine the cluster wavefunction. We achieve this by studying the modulation of the RIXS intensity as a function of the transferred momentum q, which, in the case of quasimolecular wavefunctions, is a measure of the structure factor S(q, ω).
The RIXS modulation reveals that the cluster wavefunction is very sensitive to the relative strength of competing intracluster hopping terms that mix states with different character and remonalize the effective spin-orbit coupling.

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