X-ray probes of orbital configurations in f-electron systems*

The interplay of structural, orbital, charge and spin degrees of freedom is at the heart of many emergent phenomena, including superconductivity, hidden order, and unique quantum phenomena like topological insulating states. We will show that modern x-ray spectroscopy [1,2] which aim specifically at the ground state symmetry are invaluable tools for determining the occupied ground state orbital.
Cerium-based heavy fermion compounds are an ideal playground for investigating orbital and ground state interdependencies. We will present evidence for a correlation between orbital anisotropy and the ground states properties in a representative family of materials: The strongly correlated compounds CeMIn₅, with M = Co, Rh and Ir, exhibit superconducting and magnetic ground states as well as Fermi surface changes upon substituting one M element for another. Our soft x-ray absorption (XAS) study at the cerium M-edge of CeRh_1−xIr_xIn₅ reveals that the anisotropy of the Ce 4f-wave function is a significant parameter for the ground state formation and should be taken into account when modeling these systems [3]. We extended this study to quantum critical CeCoIn₅ where In is substituted with Sn and Cd, thus suppressing superconductivity and driving the system into a magnetically ordered (Cd) or paramagnetic ground state (Sn). Also here we see the impact of hybridization on the wave function [4].

References:
[1] P. Hansmann, A. Severing et al., Phys. Rev. Lett. 100, 066405 (2008)
[2] T. Willers, A. Severing et al., Phys. Rev. Lett. 109, 046401 (2012)
[3] T. Willers, A. Severing et al, Proc. Nat. Acad. Sci. 112, no. 8, 2384 (2015)
[4] K. Chen, A. Severing et al., submitted