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Severing Group: X-ray and neutron spectroscopies on narrow band materials
(The Severing group was in 2009-2014 part of the Freimuth group, since 2015 it is linked to the van Loosdrecht group.)
Solid state materials containing transition metal, rare-earth or actinide elements exhibit a wealth of fascinating phenomena, including unconventional superconductivity, quantum criticality, and strongly correlated non-trivial topology. The origin of these phenomena can be traced back to the intricate interplay between the local atomic character of the electrons and the tendency to form bands in the crystalline solid. Important hereby is that the Coulomb interactions in the 3d-4d-5d or 4f-5f shells make the bands to be effectively very narrow, so that the system becomes extremely sensitive to which of the atomic quantum states (orbital, spin and charge) contribute to the formation of the ground state and the excited states as well. The objective of our research is to unravel the electronic structure of narrow band materials with special emphasis on identifying the relevant atomic quantum states so that we can obtain a better understanding of the intriguing properties of these phenomena.
We use advanced synchrotron based spectroscopic tools for determining the orbital-spin-charge occupations, symmetries and energy scales in these narrow band materials. The methods are versatile and applicable to many different material classes and have shown to be particularly useful for transition metal (3d, 4d, 5d) and rare earth (4f) or uranium (5f) systems. One of the strengths of core level spectroscopy is also that the information is highly element specific.
We have recently focused our research on the material class of so-called Kondo lattice or Heavy Fermion compounds. They are conducting materials where the charge carriers move as if they were 1000 times heavier than a free electron. Strong interactions of the electrons with one another are responsible for this mass enhancement. The heavy fermion phenomenon can be found in many solids containing cerium (Ce) or ytterbium (Yb) where the 4f electrons hybridize with the conduction electrons, forming ultra-narrow bands. Many heavy fermion compounds also exhibit unconventional superconductivity, so that we may also gain new inside into the mechanism of high-temperature superconductivity in the cuprates when we understand the heavy fermion phase diagram. It seems generally accepted that magnetic fluctuations are a necessity to form Copper pairs in these materials, but a specific description of how these superconducting states with their non-isotropic order parameters form, does not yet exist. Hence, there is a clear need for investigating properties such as symmetry, occupation, or energy scales of the electronic states involved. We have recently shown that the 4f orbital anisotropy correlates with the CeRh1-xIrxIn5 phase diagram (PNAS 2015).
Strongly correlated topological insulators: Kondo insulators are related materials where the physics is also driven by the hybridization of 4f and conduction electrons. Here a hybridization driven gap opens when the temperature becomes lower than the energy scale for forming a Kondo singlet state and the material’s behaviour changes from metallic to insulating. These materials are presently experiencing a great revival because they have all the ingredients for topological non-trivial behavior. These ingredients include strong spin orbit coupling, orbitals with opposite parity, and an insulating ground state. Very exciting is that we are dealing here with a strongly correlated system, i.e. very different from the topological insulators that are based on semiconductors. This is important since very recently theoretical predictions have been put forward that strongly correlated topological insulators offer phenomena that are not available in non-interacting topological insulators. Yet, the field of correlated topological insulators is so far mostly theory driven and it is crucial to identify compounds for experimental research. Here we set out to investigate the electronic structure of a variety of 4f Kondo insulators with attention to the question whether the 4f orbital-spin symmetry of the ground state is favorable for the formation of the highly correlated topological insulator.
Currently we are extending our investigations to uranium (5f) and also to 4d and 5d transition metal compounds. Uranium compounds exhibit a manifold of properties but the access to the electronic states in question is hard due to the strong delocalization of the 5f electrons. The research on 4d and 5d compound is stimulated by the recent studies in iridate materials where the strong atomic spin-orbit interaction leads to unexpected ground states. Obviously not only the strength of the spin-orbit interaction is important, but also its relative magnitude to the crystal field as well as the symmetry of the crystal field and the occupation of the d-shell determine whether the spin-orbit interaction will become effective. In particular the work on d materials is in close collaboration with the Max-Planck Institute for Chemical Physics of Solids (https://www.cpfs.mpg.de/de)
Single crystalline samples come from our collaborations with institutions worldwide (for example: Los Alamos National Laboratory – New Mexico – US, University of California – Irvine – US, CNRS – Grenoble – France, CEA – Grenoble – France, Hiroshima University – Japan, TU Vienna – Austria, MPI – CPfS - Dresden and others).
Spectroscopic tools like x-ray absorption (XAS) with soft or hard x-rays, inelastic scattering with hard x-rays (IXS) with different set-ups [resonant inelastic x-ray scattering (RIXS), resonant x-rays emission spectroscopy (RXES), non-resonant inelastic x-ray scattering (NIXS)], photo emission with hard x-rays (HAXPES) or inelastic neutron scattering (INS) give access to orbital symmetries (e.g. crystal-field wave functions) and occupation (e.g. valences), and also energy scales (e.g. hybridization strength, Kondo temperatures). The state-of-the art methods with hard x-rays have the great advantage of being bulk sensitive. Experiments are preformed at various synchrotron or neutron facilities and the extensive data analysis is performed in Cologne. The synchrotron efforts of the group are in close collaboration with the group “Physics of Correlated Matter” of Prof. Liu Hao Tjeng at the Max-Planck Institute for Chemical Physics of Solids
We carry out experiments at several synchrotrons and neutron sources in Europe and Asia, (e.g. ESRF and ILL – Grenoble – France, SOLEIL – Saclay –France, ALBA – Barcelona – Spain, DIAMOND and ISIS – Oxfordshire – UK, BESSY – HZ-Berlin, SPring-8 – Japan and NSRRC-Taiwan, and in the future also at PETRA-III in Hamburg). The group has about 24 to 30 days of beamtime per year via peer review proposal processes and another 10 ten days through collaborations.
DFG project (2012-ongoing): Crystal-field investigations in rare earth compounds using linear polarized soft x-ray absorption spectroscopy.
DFG project (2013-2017): Valence and orbital states of rare earth Heavy Fermion compounds close to the quantum critical point: Resonant and non-resonant inelastic X-ray scattering investigations.
DFG project (2016-ongoing): Correlated topological insulators: spectroscopic investigations combined with band structure calculations.
DFG project (starting 2018): From hidden to large-moment-antiferromagnetic order in URu2Si2: an x-ray study of 5f occupation and wave function symmetry.
M. Sundermann, G. van der Laan, A. Severing, L. Simonelli, G.H. Lander, M.W. Haverkort, R. Caciuffo
U 5f crystal-field ground state of UO2 determined by nonresonant inelastic x-ray scattering
H. Yasav, M. Sundermann, K. Chen, A. Severing, M.W. Haverkort, L.H. Tjeng
Direct observation of local wave functions in strongly correlated materials
Andrea Amorese, Kurt Kummer, Nicholas B. Brookes, Oliver Stockert, Devashibhai T. Adroja, Andr´e M. Strydom, Andrey Sidorenko, Hannes Winkler, Diego A. Zocco, Andrey Prokofiev, Silke Paschen, Maurits W. Haverkort, Liu Hao Tjeng, and Andrea Severing
Determining the local low-energy excitations in the Kondo semimetal CeRu4Sn6 using resonant inelastic x-ray scattering
W.J. Gannon, K. Chen, M. Sundermann, F. Strigari, Y. Utsumi, K.-D. Tsuiei, J.-P. Rueff, P. Bencok, A. Tanaka, A. Severing, and M.C. Aronson
Intermediate valence in single crystalline Yb2Si2Al
accepted at PRB, arXiv:1802.07907
S. Agrestini, C.-Y Kuo, K. Chen, Y. Utsumi,D. Mikhailova, A. Rogalev, F. Wilhelm, T. Förster, A. Matsumoto, T. Takayama, H. Takagi, M.W. Haverkort, Z. Hu, L.H. Tjeng
Probing the Jeff=0 ground state and the VanVleck paramagnetism of the Ir5+ ions in layered Sr2Co0.5O4
Phys. Rev. B 97, 214436 (2018)
K. Chen, M. Sundermann, F. Strigari, J. Kawabata, T. Takabatake, A. Tanaka, P. Bencok, F. Choueikani, and A. Severing
Tuning the hybridization and magnetic ground state of electron and hole doped CeOs2Al10: An x-ray spectroscopy study
Phys. Rev. B 97, 155106 (2918)
K. Chen, F. Strigari, M. Sundermann, Z. Hu, Z. Fisk, E. D. Bauer, P. F. S. Rosa, J. L. Sarrao, J. D. Thompson, J. Herrero-Martin, E. Pellegrin, D. Betto, K. Kummer, A. Tanaka, S. Wirth, and A. Severing
Evolution of gournd state wave function in CeCoIn5 upon Cd or Sn doping
Phys. Rev. B 97, 045134 (2918)