Detaching the antiferromagnetic quantum critical point from the Fermi-surface reconstruction inYbRh 2 Si 2

Abstract

A continuous phase transition driven to zero temperature by a non-thermal parameter, such as pressure, terminates in a quantum critical point (QCP). At present, two main theoretical approaches are available for antiferromagnetic QCPs in heavy-fermion systems. The conventional one is the quantum generalization of finite-temperature phase transitions, which reproduces the physical properties in many cases. More recent unconventional models incorporate a breakdown of the Kondo effect, giving rise to a Fermi-surface reconstruction YbRh 2 Si 2 is a prototype of this category. In YbRh 2 Si 2, the antiferromagnetic transition temperature merges with the Kondo breakdown at the QCP. Here, we study the evolution of the quantum criticality in YbRh 2 Si 2 under chemical pressure. Surprisingly, for positive pressure we find the signature of the Kondo breakdown within the magnetically ordered phase, whereas negative pressure induces their separation, leaving an intermediate spin-liquid-type ground state over an extended range. This behaviour suggests a new quantum phase arising from the interplay of the Kondo breakdown and the antiferromagnetic QCP. © 2009 Macmillan Publishers Limited.