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An antiferromagnetic quantum critical point (QCP) is conventionally described by the quantum gen- eralization of finite temperature phase transitions leading to a spin-density-wave QCP in heavy- fermion metals. In recent years, a new class of quantum criticality emerged based on the breakdown of the Kondo effect. YbRh2Si2 is a prototypical system which is best described within the latter class with the Kondo breakdown and the AF QCP coinciding. Here we report on experiments on YbRh2Si2 under positive and negative chemical pressure as realized by Co and Ir substitution on the Rh side. Our results indicate a detaching of the AF QCP from the Kondo breakdown. For positive pressure the AF QCP seems to be transformed to a SDW QCP. We believe therefore, that YbRh2Si2 provides the test ground to study the link between the different classes of quantum criticality. On the negative pressure side an intermediate spin-liquid type ground state is found. |
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