Low Energy Excitation And Time-Resolved Dynamics In Heavy-Fermion Systems

2019-10-11 Dr. Shovon Pal, ETH Zurich

ABSTRACT :

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 fluctuations [1]. Rare-earth heavy-fermion systems such as CeCu6−xAux show a QPT between a fully Kondo-screened, paramagnetic Fermi-liquid phase and an antiferromagnetically 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 first 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 quasiparticles in heavy-fermion compounds. The spectroscopic method mentioned above is capable of distinguishing contributions from the heavy Kondo band and from the crystalelectric-field (CEF) split satellite bands by different terahertz response delay times [4]. We find 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.

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11:00

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12:00

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