A paradigmatic case of multi-band Mott physics including spin-orbit and Hund's coupling is realized in Ca2RuO4. Progress in understanding the nature of this Mott insulating phase has been impeded by the lack of knowledge about the low-energy electronic structure. Here we provide—using angle-resolved photoemission electron spectroscopy—the band structure of the paramagnetic insulating phase of Ca2RuO4 and show how it features several distinct energy scales. Comparison to a simple analysis of atomic multiplets provides a quantitative estimate of the Hund's coupling J=0.4 eV. Furthermore, the experimental spectra are in good agreement with electronic structure calculations performed with Dynamical Mean-Field Theory. The crystal field stabilization of the dxy orbital due to c-axis contraction is shown to be essential to explain the insulating phase. These results underscore the importance of multi-band physics, Coulomb interaction and Hund's coupling that together generate the Mott insulating state of Ca2RuO4.

Lead-halide perovskite (LHP) semiconductors are emergent optoelectronic materials with outstanding transport properties which are not yet fully understood. We find signatures of large polaron formation in the electronic structure of the inorganic LHP CsPbBr 3 by means of angle-resolved photoelectron spectroscopy. The experimental valence band dispersion shows a hole effective mass of 0.26 AE 0.02 m e , 50% heavier than the bare mass m 0 ¼ 0.17 m e predicted by density functional theory. Calculations of the electron-phonon coupling indicate that phonon dressing of the carriers mainly occurs via distortions of the Pb-Br bond with a Fröhlich coupling parameter α ¼ 1.81. A good agreement with our experimental data is obtained within the Feynman polaron model, validating a viable theoretical method to predict the carrier effective mass of LHPs ab initio.

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In nodal-line semimetals, linearly dispersing states form Dirac loops in the reciprocal space with a high degree of electron-hole symmetry and a reduced density of states near the Fermi level. The result is reduced electronic screening and enhanced correlations between Dirac quasiparticles. Here we investigate the electronic structure of ZrSiSe, by combining time-and angle-resolved photoelectron spectroscopy with ab initio density functional theory (DFT) complemented by an extended Hubbard model (DFT þ U þ V) and by time-dependent DFT þ U þ V. We show that electronic correlations are reduced on an ultrashort timescale by optical excitation of high-energy electrons-hole pairs, which transiently screen the Coulomb interaction. Our findings demonstrate an all-optical method for engineering the band structure of a quantum material.

Black phosphorus (BP), the most stable allotrope of elemental phosphorus, is a layered semiconductor. It recently experienced a resurgence of interest after exfoliation down to few layers was accomplished [1-4]. Among two-dimensional (2D) materials, BP combines the optoelectronic properties of gapless graphene [5] and of wide-gap transition metal dichalcogenides (TMDs) [6]. The charge mobility of bulk BP is comparable to that of graphene (10 5 cm 2 V −1 s −1 ) [7]. Even if it decreases by one-to-two orders of magnitude in an isolated monolayer [3,8,9], it still surpasses that of TMDs [10-13], which proved to be sufficient to realize field-effect transistors [2]. In contrast to TMDs, in the few-layers limit ambipolar charge transport can be achieved by gating, enabling the realization of p-n junctions [14]. Also, BP-based saturable absorbers have already been successfully implemented in lasers technology [15][16][17], and polarization-sensitive photodetectors have been designed to exploit its strong in-plane absorption dichroism [4,18].BP has an orthorhombic crystal structure, with space group Cmca (Nr. 64) and point group D 18 2h [10].