Enhanced Hybridization Sets the Stage for Electronic Nematicity in CeRhIn5

High magnetic fields induce a pronounced in-plane electronic anisotropy in the tetragonal antiferromagnetic metal CeRhIn5 at H* ≳ 30 T for fields ≃20° off the $c$ axis. Here in this paper, we investigate the response of the underlying crystal lattice in magnetic fields to 45 T via high-resolution dilatometry. At low fields, a finite magnetic field component in the tetragonal $ab$ plane explicitly breaks the tetragonal (C4) symmetry of the lattice revealing a finite nematic susceptibility. A modest a-axis expansion at H * hence marks the crossover to a fluctuating nematic phase with large nematic susceptibility. Magnetostriction quantum oscillations confirm a Fermi surface change at H * with the emergence of new orbits. By analyzing the field-induced change in the crystal-field ground state, we conclude that the in-plane Ce $f$ hybridization is enhanced at H * , in agreement with the in-plane lattice expansion. We argue that the nematic behavior observed in this prototypical heavy-fermion material is of electronic origin, and is driven by the hybridization between 4$f$ and conduction electrons which carries the $f$-electron anisotropy to the Fermi surface.