Low energy excitation and time-resolved dynamics in heavy-fermion systems

Quantum phase transitions (QPT) describe a change between two ground states of a many-body system, controlled by a nonthermal control parameter and resulting from quantum uctuations [1]. Rare-earth heavy-fermion systems such as CeCu6-xAux show a QPT between a fully Kondo-screened, paramagnetic Fermi-liquid phase and an antiferro-magnetically ordered phase. When excited by a terahertz pulse, the heavy quasiparticles disintegrate and coherently recover on a picosecond timescale, characteristic of the Kondo coherence time or inverse Kondo temperature [2]. We use terahertz time-domain spectroscopy to probe the Kondo quasiparticle spectral weight at such ultrafast timescales. Temperature-dependent examination of samples with different Au concentrations reveals that in the heavy-fermion (CeCu6) and the quantum-critical (CeCu5:9Au0:1) samples, the Kondo weight _rst increases upon lowering the temperature down to 30 K, followed by a decrease as we enter the quantum critical regime [2]. While in CeCu6 the Kondo weight drops by about 40%, in CeCu5:9Au0:1 it is completely destroyed below 5 K. The CeCu5Au sample, being deep in the antiferromagnetic phase, does not exhibit a visible Kondo weight at any temperature, despite the fact that low-temperature specific heat measurements reveal a sizeable Fermi liquid-like contribution.

Recent observations of large Fermi volume at temperatures much higher than the Kondo lattice temperature raised controversies on the validity of this long-known scale [3]. This is because an enlarged Fermi volume is a hallmark of the existence of Kondo quasi-particles in heavy-fermion compounds. The spectroscopic method mentioned above is capable of distinguishing contributions from the heavy Kondo band and from the crystal-electric-field (CEF) split satellite bands by di_erent terahertz response delay times [4]. We _nd that an exponentially-enhanced, high-energy Kondo scale controls the formation of heavy bands, once the CEF states become thermally occupied. We corroborate these observations by temperature-dependent, high-resolution dynamical mean-field calculations for the multi-orbital Anderson lattice model and discuss its relevance for quantum critical scenarios.
[1] H. v. Lohneysen et al., Rev. Mod. Phys. 79, 1015 (2007).
[2] C. Wetli, S. Pal et al., Nat. Phys. 14, 1103 (2018).
[3] K. Kummer et al., Phys. Rev. X 5, 011028 (2015).
[4] S. Pal et al., Phys. Rev. Lett. 122, 096401 (2019).