Orbitally defined field-induced electronic state in a Kondo lattice

Abstract

${\mathrm{CeRhIn}}_{5}$ is a Kondo-lattice prototype in which a magnetic field ${B}^{*}\ensuremath{\simeq}31\phantom{\rule{0.28em}{0ex}}\mathrm{T}$ 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 $^{115}\mathrm{In}$ formal Knight shift, without changes in crystal or magnetic structures, of ${\mathrm{CeRhIn}}_{5}$ at fields ($\ensuremath{\parallel}c$) 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 ${B}_{c0}\ensuremath{\simeq}50\phantom{\rule{0.28em}{0ex}}\mathrm{T}$.