Cluster dynamical mean-field theory study on deficient spinel chalcogenide GaV$_{\mathrm{4}}$S$_{\mathrm{8}}$

Here I will present my current research progress about a deficient~spinel compound GaV$_{\mathrm{4}}$S$_{\mathrm{8}}$ (GVS) using a cluster dynamical mean-field (DMFT) approach. GVS has been of growing interest recently because of its multiferroicity below T$_{\mathrm{C}}$\textasciitilde 13K. It has an additional structural transition around T$_{\mathrm{f}}$ \textasciitilde 70K, lowering the symmetry of the high-temperature cubic phase to a rhombohedral structure as it is cooled. Across the two structural transitions GVS remains insulating with the gap size estimated to be \textasciitilde 0.3 eV, and the insulating nature of its high-temperature cubic paramagnetic phase is suspected to be a Mott-type. However, there has not been a systematic study about the Mott-insulating phases of GVS, which should be important to understand the low- temperature ferroelectric and multiferroic phases. Hence we apply a cluster DMFT method to GVS with V$_{\mathrm{4}}$ as our cluster problem, employing a molecular orbital basis set instead of an atomic one. ~Comparing with single-V-site DMFT calculations, we show that intra-cluster correlations are essential in reproducing the Mott-insulating phase of the high-T cubic paramagnetic phase. Correlation effects on the crystal structure of GVS will be also discussed, and especially the role of Hund's coupling in tuning V$_{\mathrm{4}}$-clustering by switching low to high spin states will be shown.