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- ItemNesting-driven multipolar order in CeB6 from photoemission tomography(London : Nature Publishing Group, 2016) Koitzsch, A.; Heming, N.; Knupfer, M.; Büchner, B.; Portnichenko, P.Y.; Dukhnenko, A.V.; Shitsevalova, N.Y.; Filipov, V.B.; Lev, L.L.Some heavy fermion materials show so-called hidden-order phases which are invisible to many characterization techniques and whose microscopic origin remained controversial for decades. Among such hidden-order compounds, CeB6 is of model character due to its simple electronic configuration and crystal structure. Apart from more conventional antiferromagnetism, it shows an elusive phase at low temperatures, which is commonly associated with multipolar order. Here we show that this phase roots in a Fermi surface instability. This conclusion is based on a full 3D tomographic sampling of the electronic structure by angle-resolved photoemission and comparison with inelastic neutron scattering data. The hidden order is mediated by itinerant electrons. Our measurements will serve as a paradigm for the investigation of hidden-order phases in f-electron systems, but also generally for situations where the itinerant electrons drive orbital or spin order.
- ItemAbsorption and photoemission spectroscopy of rare-earth oxypnictides(Milton Park : Taylor & Francis, 2009) Kroll, T.; Roth, F.; Koitzsch, A.; Kraus, R.; Batchelor, D.R.; Werner, J.; Behr, G.; Büchner, B.; Knupfer, M.The electronic structure of various rare-earth oxypnictides has been investigated by performing Fe L2, 3 x-ray absorption spectroscopy, and Fe 2p and valence band x-ray photoemission spectroscopy. As representative samples the non-superconducting parent compounds LnFeAsO (Ln=La, Ce, Sm and Gd) have been chosen and measured at 25 and 300 K, i.e. below and above the structural and magnetic phase transition at ~150 K. We find no significant change of the electronic structure of the FeAs layers when switching between the different rare-earth ions or when varying the temperature below and above the transition temperatures. Using a simple two-configuration model, we find qualitative agreement with the Fe 2p3/2 core-level spectrum, which allows for a qualitative explanation of the experimental spectral shapes.
- ItemNonlocal dielectric function and nested dark excitons in MoS2(London : Nature Publishing Group, 2019) Koitzsch, A.; Pawlik, A.-S.; Habenicht, C.; Klaproth, T.; Schuster, R.; Büchner, B.; Knupfer, M.Their exceptional optical properties are a driving force for the persistent interest in atomically thin transition metal dichalcogenides such as MoS2. The optical response is dominated by excitons. Apart from the bright excitons, which directly couple to light, it has been realized that dark excitons, where photon absorption or emission is inhibited by the spin state or momentum mismatch, are decisive for many optical properties. However, in particular the momentum dependence is difficult to assess experimentally and often remains elusive or is investigated by indirect means. Here we study the momentum dependent electronic structure experimentally and theoretically. We use angle-resolved photoemission as a one-particle probe of the occupied valence band structure and electron energy loss spectroscopy as a two-particle probe of electronic transitions across the gap to benchmark a single-particle model of the dielectric function ϵ(q, ω) against momentum dependent experimental measurements. This ansatz captures key aspects of the data surprisingly well. In particular, the energy region where substantial nesting occurs, which is at the origin of the strong light–matter interaction of thin transition metal dichalcogenides and crucial for the prominent C-exciton, is described well and spans a more complex exciton landscape than previously anticipated. Its local maxima in (q≠0,ω) space can be considered as dark excitons and might be relevant for higher order optical processes. Our study may lead to a more complete understanding of the optical properties of atomically thin transition metal dichalcogenides.