Low energy excitation and time-resolved dynamics in heavy-fermion systems
by Dr. Shovon Pal (Department of Materials, ETH Zurich, Zurich, Switzerland)
Monday, October 14, 2019 from to (Asia/Kolkata)
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 . 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 . 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 . 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 . 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 . 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.  H. v. Lohneysen et al., Rev. Mod. Phys. 79, 1015 (2007).  C. Wetli, S. Pal et al., Nat. Phys. 14, 1103 (2018).  K. Kummer et al., Phys. Rev. X 5, 011028 (2015).  S. Pal et al., Phys. Rev. Lett. 122, 096401 (2019).