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 . 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 . 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 . 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 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 . 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.
 H. v. L¨ohneysen 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).
About the speaker: Shovon Pal obtained his Master’s degree in Physics at IIT Madras, doing a masters’s project with Prof. Kasiviswanathan. He then went abroad as an international Max Planck research fellow at the Ruhr University Bochum (Germany), where he obtained his Ph.D. under the supervision of Prof. Andreas Wieck in 2015. His doctoral research was on investigating and electrically tuning intersubband and intersublevel spacings in semiconductor heterostructures, like quantum dots, 2DEGs and quantum cascade lasers and probing by THz and infrared spectroscopy. He continued his postdoc for a year in NEST, Pisa (Italy) with Prof. Miriam Vitiello, working on passive mode locking of THz cascade lasers. Since 2017, he is in ETH Zurich (Switzerland) with Prof. Manfred Fiebig, changing his research direction from semiconductor materials towards complex material systems with a strong focus on revealing ultrafast and low energy carrier dynamics in strongly-correlated electronic systems.