Magnetic Order Within the Heavy Lattice in SmCoIn5

by Dr David Tam (PSI)

Tuesday 24 Jan 2023, 11:00 → 12:00 Europe/Zurich

WHGA/121

The microscopic mechanism of heavy band formation, relevant for superconductivity in CeCoIn5 and other Ce-based materials, depends strongly on the efficiency with which f electrons are delocalized and can participate in a coherent Kondo lattice. Replacing Ce3+ (4f1, S = 1/2, L = 3, J = 5/2) with Sm3+ (nominally 4f5, S = 5/2, L = 5, J = 5/2), which share the same J, allows the nearly-half-filled Sm3+ ion to act as a hole-like analog of Ce3+. We find that SmCoIn5 exhibits a Kondo-like mixed-valence scenario below a temperature scale set by the crystal field, Tv ≈ 60 K, in which we directly observe a charge fluctuation degree of freedom due to two Sm configurations with different nonzero f filling. Our result adds to the evidence that the crystal field controls the efficiency of f electron hybridization near a heavy fermion quantum critical point, and shows that charge fluctuations driven by Coulomb repulsion may play a role in the hybridized Kondo lattice state of “115” materials. To further understand the effect of the delocalization in SmCoIn5, we study the electronic structure and magnetically ordered ground state. We find evidence for a direct magnetic exchange mechanism between Sm and Co that couples the electronic states at the Fermi energy to a flat band of Co d electrons, one which also appears in other “115” family members. Finally, in the magnetically ordered state below TN = 11 K, we show that SmCoIn5 has an incommensurate-to-commensurate magnetic phase transition with a topological multi-q magnetic structure, and contains an internal degree of freedom that is sensitive to the conduction electrons. Together, our results give a detailed picture of how a multi-f-electron system can exhibit both delocalization and magnetic order near a heavy fermion quantum critical point, and some of the complexities resulting from the coexistence of these two paradigms.