Quantum critical point scenarios in heavy-fermion systems

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Different types of heavy-fermion (HF) quantum critical points (QCPs) are investigated by systematic low-temperature experiments on high-quality single crystals. Two kinds of QCPs are known to exist in HF metals: The standard “itinerant” one and the “local” Kondobreakdown QCP where a selective Mott transition of the 4f-electrons is considered at a lowenergy scale T*(H). While CeNi2Ge2 is located very close to a QCP of the former type, YbRh2Si2 displays many features which are described by the latter one. Upon tuning the unitcell volume of YbRh2Si2, by partially substituting Rh with isoelectronic but smaller Co or larger Ir, the energy scale T*(H) remains unchanged whereas the antiferromagnetic (AF) phase is enlarged (Co-doping) or suppressed (Ir-doping). In Ir-doped single crystals a metallic spin-liquid type of ground state emerges, which will be characterized. Metallic spinliquid type behavior will also be investigated in the geometrically frustrated Kondo lattice Pr2Ir2O7. Moreover, we will fine-tune the ground state properties of single crystals of CeNi2Ge2 across the QCP by slight Ni:Ge variation and Pd-doping in Ce(Ni1−xPdx)2Ge2. Finally, ferromagnetic, metamagnetic and antiferromagnetic instabilities in various new 4felectron Ce- and Yb-based HF and also in few 3d-electron metals will be investigated.