Lattice dynamics and electronic excitations in a large family of lacunar spinels with a breathing pyrochlore lattice structure

Abstract

Reproducing the electronic structure of AM4X8 lacunar spinels with a breathing pyrochlore lattice is a great theoretical challenge due to the interplay of various factors. The character of the M4X4 cluster orbitals is critically influenced by the Jahn-Teller instability, the spin-orbit interaction, and also by the magnetic state of the clusters. Consequently, to reproduce the narrow-gap semiconducting nature of these moderately correlated materials requires advanced approaches, since the strength of the inter-cluster hopping is strongly affected by the character of the cluster orbitals. In order to provide a solid experimental basis for theoretical studies, we performed broadband optical spectroscopy on a large set of lacunar spinels, with systematically changing ions at the A and M sites as well as the ligand (A=Ga, Ge, Al; M=V, Mo, Nb, Ta; X=S, Se). Our study covers the range of phonon excitations and also electronic transitions near the gap edge. In the phonon excitation spectrum a limited subset of the symmetry allowed modes is observed in the cubic state, with a few additional modes emerging upon the symmetry-lowering structural transition. All the infrared active modes are assigned to vibrations of the ligands and ions at the A sites, with no obvious contribution from the M-site ions. Concerning the electronic states, we found that all compounds are narrow-gap semiconductors (Eg = 130 − 350 meV) already in their room-temperature cubic state and their structural transitions induce weak, if any, changes in the band gap. The gap value is decreased when substituting S with Se and also when replacing 3d ions by 4d or 5d ions at the M sites. :1912.11079v2 [cond-mat.str-el] arXiv

WITHDRAWN: Discovery of New Metallic and Superconducting Phase in GaTa4Se8 Under High Pressure

“…In contrast, most of the other lacunar spinels' insulating states are stable in a slightly distorted cubic lattice (i.e. rhombohedral lattice) and magnetically ordered phase 13,15,16 .…”

Macroscopic manifestation of domain-wall magnetism and magnetoelectric effect in a Néel-type skyrmion host

“…Lacunar spinels with the chemical formula AB 4 X 8 (A = Al, Ga, Ge; B = V, Mo, Nb, Ta; X = S, Se, Te) are a family of narrow-gap Mott insulators 31,32 , exhibiting a plethora of correlation and spin-orbit effects, including pressure-induced superconductivity 33 , bandwidth-controlled metal-to-insulator transition 34 , electric-field-driven avalanche of the Mott gap 35 , large negative magnetoresistance 36 , twodimensional topological insulator state 37 and orbitally driven ferroelectricity [38][39][40][41] . In addition to these charge related phenomena and particularly important for the present study, lacunar spinels, such as GaV 4 S 8 , GaV 4 Se 8 and GaMo 4 S 8 , were the first material family found to host the Néel-type skyrmion lattice (SkL) state [42][43][44] .…”

Spin dynamics in the skyrmion-host lacunar spinel GaV4S8

“…GaV 4 S 8 belongs to the lacunar spinel family of compounds with a chemical formula AM 4 X 8 (A = Ga, Ge; M = V, Mo, Nb, and Ta; X = S and Se) [20][21][22][23][24][25][26][27][28][29][30][31], where the magnetic M ions form a breathing pyrochlore lattice as shown in Fig. 1(a).…”