Dresden 2020 – wissenschaftliches Programm
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TT 56.1: Vortrag
Donnerstag, 19. März 2020, 09:30–09:45, HSZ 304
Quasiparticle critical slowing down in a heavy-fermion system — •S. Pal1, C.-J. Yang1, F. Zamani2, K. Kliemt3, C. Krellner3, H. v. Löhneysen4, J. Kroha2, and M. Fiebig1 — 1ETH Zürich, Switzerland. — 2Universität Bonn, Germany. — 3Goethe-Universität Frankfurt, Germany. — 4KIT Karlsruhe, Germany.
Rare-earth heavy-fermion systems such as YbRh2Si2 show a quantum phase transition between a fully Kondo-screened paramagnetic Fermi-liquid phase and an antiferromagnetically ordered phase. When excited by THz pulses, the Kondo state disintegrates and coherently recovers on timescales in the order of 2 ps, corresponding to the Kondo lattice temperature of TK* = 25 K. We use THz time-domain spectroscopy[1,2] to probe the coherent Kondo spectral weight of YbRh2Si2 in presence of an in-plane magnetic field. By tuning the magnetic field, we are able to map the logarithmic build-up of quasiparticle spectral weight around 25 K across the quantum phase transition. We find that near the critical magnetic field of Bc = 67 mT, first the Kondo weight rises logarithmically with decreasing temperature, reaching a maximum near the Kondo lattice temperature. It is then followed by a reduction until 10 K indicating Kondo breakdown. Upon decreasing the temperature further, we find that the THz response signal starts building up logarithmically until the lowest temperatures measured. A critical, two-band Fermi-liquid theory indicates that this signals quasiparticle critical slowing down for a fermionic field - an observation of which in our experiments is indeed intriguing.
 Nat. Phys. 14, 1103 (2018)
 Phys. Rev. Lett. 122, 096401 (2019).