Clarifying the fate of collective metallic quantum states

Many complex metals exhibit collective states in which electrons appear to collaborate to generate novel and frequently functional behavior. These states develop when metals are cooled down to remove the effects of thermal fluctuations, enabling collective states in which electrons move coherently through the material. These collective electronic states are of tremendous importance because they are the foundation for many quantum states of interest such as unconventional superconductivity, frustrated magnetism, hidden order, as well as topologically non-trivial and electronic-nematic states.

Although, the observation of coherent electronic behavior has long been already established, the fate of high-energy degrees of freedom, such as crystal electric field excitations and spin-orbit interactions, in the coherent state of Kondo lattice materials remains unclear. This is a key scientific question as recent state-of-the-art electronic structure calculations suggest that high-energy degrees of freedom are crucial in understanding the nature of these collective quantum states. Here, we use resonant inelastic x-ray scattering (RIXS) in CePd₃ to show how Kondo-quasiparticle excitations are renormalized and coherently become incorporated in the electronic state.