Low-Temperature Magnetic and Transport Properties of the Clean NFL System YbRh2(Si1-xGex)2

High-quality single crystals of YbRh2(Si1-xGex)2 ( x = 0, 0,05 ) have been studied by low-temperature (T) and high-magnetic-field (B) measurements of the electrical resistivity ρ(T,B), dc-magnetization M(T,B) and magnetic ac-susceptibility χac(T,B). For the undoped compound (x=0) a weak Anti Ferromagnetic (AF) transition at TN≅70 mK is observed which can be suppressed for small fields Bc≅0.06 T and 0.7 T applied along the crystallographic a - and c -direction, respectively. Below TN a ρ(T)=ρ0 + aT2 behavior is found with very low residual resistivity ρ0≅μΩcm. Above TN pronounced Non-Fermi-Liquid (NFL) effects are observed with a linear T-dependence of the electrical resistivity. A small volume expansion of ΔV∼+0.3% is sufficient to tune the system YbRh2(Si1-xGex)2 to its Quantum Critical Point (QCP) at xc=(0.06±0.01). The x=0.05 compound shows a ρ(T)=ρ0+bT dependence from below 10 mK up to above 10 K. The low ρ0≅5μΩcm proves that this NFL behavior is intrinsic due to the proximity to the QCP and not related to the disorder induced by Ge-alloying. No evidence for a metamagnetic phase transition in fields up to 56 T applied along the magnetic hard direction (BIIc) has been observed. (author)