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

We studied the light-induced effects in BiFeO 3 single crystals as a function of temperature by means of optical spectroscopy. Here we report the observation of several light-induced absorption features, which are discussed in terms of the photostriction effect and are interpreted in terms of excitons. The temperature dependence of their energy position suggests a possible coupling between the excitons and the lattice vibrations. Moreover, there are hints for anomalies in the temperature evolution of the excitonic features, which might be related to the temperature-induced magnetic phase transitions in BiFeO 3 . Our findings suggest a coupling between light-induced excitons and the lattice and spin degrees of freedom, which might be relevant for the observed ultrafast photostriction effect in multiferroic BiFeO 3 .

A single ferromagnetic kagome layer is predicted to realize a Chern insulator with quantized Hall conductance, which upon stacking can become a Weyl semimetal with a large anomalous Hall effect (AHE) and magnetooptical activity. Indeed, in the kagome bilayer material Fe 3 Sn 2 , a large AHE was detected. In order to directly probe the responsible band structure features, we measure the optical Hall conductivity spectra in addition to the diagonal optical conductivity over a broad frequency range. Since the former is the energy selective measure of the intrinsic contributions to the AHE, we identify their common origin with the help of momentum-and banddecomposed optical conductivity spectra obtained from first principles calculations. We find that low-energy transitions, tracing "helical volumes" in momentum space reminiscent of the formerly predicted helical nodal lines, substantially contribute to the AHE, which is further increased by contributions from multiple higherenergy interband transitions. Our study also reveals that in this kagome magnet, local Coulomb interactions lead to remarkable band reconstructions near the Fermi level.

The pressure dependence of light-induced effects in single-crystalline BiFeO 3 is studied by optical spectroscopy. At low pressures, we observe three light-induced absorption features with energies just below the two crystal field excitations and the absorption onset, respectively. These absorption features were previously ascribed to excitons, possibly connected with the ultra-fast photostriction effect in BiFeO 3 . The pressure-induced redshift of the absorption features follows the pressure dependence of the corresponding crystal field excitations and absorption onset, suggesting the link between them. Above the structural phase transition at P c1 ≈ 3.5 GPa the three absorption features disappear, suggesting their connection to the polar phase in BiFeO 3 . The pressure-induced disappearance of the photo-induced features is irreversible upon pressure release.