Anisotropic excitonic magnetism from discrete C4 symmetry in CeRhIn5

Anisotropy in strongly correlated materials is a central parameter in determining the electronic ground state and is tuned through the local crystalline electric field. This is notably the case in the CeCoxRh1−xIn5 system where the ground-state wave function can provide the basis for antiferromagnetism and/or unconventional superconductivity. We develop a methodology to understand the local magnetic anisotropy and experimentally investigate with neutron spectroscopy applied to antiferromagnetic (TN=3.8 K) CeRhIn5 which is isostructural to d-wave superconducting (Tc=2.3 K) CeCoIn5. Through diagonalizing the local crystal field Hamiltonian with discrete tetragonal C4 point group symmetry and coupling these states with the Random Phase Approximation (RPA), we find two distinct modes polarized along the crystallographic c and a−b planes, agreeing with experiment. The anisotropy and bandwidth, underlying the energy scale of these modes, are tuneable with a magnetic field which we use experimentally to separate in energy single and multiparticle excitations thereby demonstrating the instability of excitations polarized within the crystallographic a−b plane in CeRhIn5. We compare this approach to a Seff=12 parameterizations and argue for the need to extend conventional SU(2) theories of magnetic excitations to utilize the multi-level nature of the underlying crystal-field basis states constrained by the local point-group C4 symmetry.