Orbitally defined field-induced electronic state in a Kondo lattice

CeRhIn${}_5$ is a Kondo-lattice prototype in which a magnetic field B${}^{\ast }\simeq$ 30 T induces an abrupt Fermi-surface (FS) reconstruction and pronounced in-plane electrical transport anisotropy all within its antiferromagnetic state. Though the antiferromagnetic order at zero field is well-understood, the origin of an emergent state at B${}^{\ast }$ remains unknown due to challenges inherent to probing states microscopically at high fields. Here, we report low-temperature Nuclear Magnetic Resonance (NMR) measurements revealing a discontinuous decrease in the ${}^{115}$In formal Knight shift, without changes in crystal or magnetic structures, of CeRhIn${}_5$ at fields spanning B${}^{\ast }$. We show that the emergent state above B${}^{\ast }$ results from a change in Ce’s 4f orbitals that arises from field-induced evolution of crystal-electric field (CEF) energy levels. This change in orbital character enhances hybridisation between the 4f and the conduction electrons (c.e.) that leads ultimately to an itinerant quantum-critical point at B${}_{\mathbf{c}\mathbf{0}}\simeq$ 50 T.