Signature of quantum criticality in photoemission spectroscopy at elevated temperature

TL;DR

This study uses ultraviolet photoemission spectroscopy to observe the breakdown of heavy quasiparticles in a heavy-fermion compound near a quantum critical point, revealing a sudden change in the Kondo energy scale at elevated temperatures.

Contribution

It provides the first direct high-temperature evidence of heavy quasiparticle breakdown at a quantum critical point in CeCu$_{6-x}$Au$_x$, linking it to a non-linear renormalization of local spin coupling.

Findings

Abrupt step in Kondo energy scale near critical concentration

High-temperature indicator for quasiparticle breakdown

Theoretical explanation via non-linear spin coupling renormalization

Abstract

A quantum phase transition (QPT) in a heavy-fermion (HF) compound may destroy the Fermi liquid groundstate. However, the conditions for this breakdown have remained obscure. We report the first direct investigation of heavy quasiparticle formation and breakdown in the canonical system CeCu$_{6-x}$Au$_x$ by ultraviolet photoemission spectroscopy at elevated temperatures without the complications of lattice coherence. Surprisingly, the single-ion Kondo energy scale $T_K$ exhibits an abrupt step near the quantum critical Au concentration of $x_c=0.1$. We show theoretically that this step is expected from a highly non-linear renormalization of the local spin coupling at each Ce site, induced by spin fluctuations on neighboring sites. It provides a general high-temperature indicator for HF quasiparticle breakdown at a QPT.