Speaker: Heike Pfau, Max Planck Institute for Chemical Physics of Solids, Dresden
Time: November 13, 2014, 14:00 - 15:00
The Kondo effect couples a local magnetic moment with conduction electrons to a composite heavy quasiparticle leading to a non-magnetic ground state. At the same time, a coupling between the moments favors a magnetically ordered state. An applied magnetic field influences the ground state and the nature of the heavy quasiparticles in two ways: On one side, it can tune the competition between the Kondo interaction and the magnetic exchange leading to a quantum critical point (QCP). On the other side, a magnetic field competes with the Kondo effect itself leading to a breakup of the heavy quasiparticles. We studied these two effects in YbRh2Si2 – the first in low, the second in high magnetic fields – using thermal and electrical transport experiments. At a low field of 60mT, YbRh2Si2 exhibits an unconventional QCP, the nature of which is highly debated. Our experiments suggest a violation of the fundamental Wiedemann-Franz law at the QCP and hence a breakdown of the quasiparticle picture at this point. At a high magnetic field of 10T, a transition was reported, the origin of which is debated. We performed thermopower and resistivity measurements and detected in total three transitions between 3T ans 12T. An excellent agreement between our experiments, previous ones, and renormalized band structure calculations enables us to describe the observed high field behavior as a combination of a smooth suppression of the Kondo effect and three successive Lifshitz transitions.