Missing spectral weight in a heavy-fermion system far above N\'eel temperature

TL;DR

This study reveals a suppression of Kondo quasiparticles in heavy-fermion CeCu$_{6-x}$Au$_x$ systems well above the Néel temperature, highlighting the role of RKKY interactions and quantum frustration in the quantum critical behavior.

Contribution

It provides systematic high-temperature spectroscopic evidence of RKKY-induced quasiparticle suppression on the magnetically ordered side of the quantum phase transition.

Findings

Suppression of Kondo quasiparticle weight above $T_N$

Quantum frustration induced by RKKY interaction

RKKY interaction influences quantum-critical behavior

Abstract

The competition between the Kondo spin-screening effect and the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction in heavy-fermion systems drives the quantum phase transition between the magnetically ordered and the heavy-Fermi-liquid ground states. Despite intensive investigations of heavy quasiparticles on the Kondo-screened side of the quantum phase transition and of their breakdown at the quantum critical point, the magnetically ordering side has not systematically been studied. Using terahertz time-domain spectroscopy, we report a suppression of the Kondo quasiparticle weight in CeCu$_{6-x}$Au$_x$ samples on the antiferromagnetic side of the quantum phase transition at temperatures as much as two orders of magnitude above the N\'{e}el temperature $T_\text{N}$. With our systematic investigations into the high-temperature, paramagnetic region on the antiferromagnetic side of the phase diagram of CeCu$_{6-x}$Au$_x$, i.e., with $x =$ 0.2, 0.3, and 0.5, we show that the suppression results from a quantum frustration effect induced by the temperature-independent RKKY interaction. Hence, our results emphasize that besides critical fluctuations, the RKKY interaction may play an important role in the quantum-critical scenario.