Abstract

The heavy fermion compound YbRh2Si2 exhibits antiferromagnetic ordering at 70 mK. Suppressing this transition to zero temperature by a tiny magnetic field leads to the quantum critical point. Conventionally, a quantum critical point is described by the quantum generalization of finite-temperature phase transitions. In this talk I present high-precision Hall effect measurements which are at variance with the predictions of the standard theory for both the static and dynamic behaviour.

On the one hand, the data suggest a reconstruction of the Fermi surface and on the other hand, they allow to exclude the dynamical scaling predicted by the conventional theory. The results rather suggest a breakdown of the Kondo effect as the dominant mechanism. In addition, I show results revealing the global phase diagram of YbRh2Si2 under positive and negative chemical pressure as realized by Co and Ir substitution on the Rh side. Surprisingly, this leads to a detachment of the antiferromagnetic quantum critical point from the Fermi surface reconstruction. In particular, negative pressure induces a separation of the two with an intermediate non-Fermi-liquid ground state emerging in an extended field range. These results indicate a new quantum phase arising from the interaction of the Kondo breakdown and the AF QCP .