Orbitally Defined Field-induced Electronic State In A Kondo Lattice

CeRhIn5 is a Kondo-lattice prototype in which a magnetic field B* similar or equal to 31 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* 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 pronounced decrease in the In-115 formal Knight shift, without changes in crystal or magnetic structures, of CeRhIn5 at fields (parallel to c) spanning B*. We discuss the emergent state above B* in terms of 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 hybridization between the 4f and the conduction electrons that leads ultimately to an itinerant quantum-critical point at B-c0 similar or equal to 50 T.