Yb-based heavy-fermion metal situated close to a quantum critical point

We report low-temperature thermodynamic, magnetic, and transport measurements on high-quality single crystals of the tetragonal heavy-fermion system YbIr₂Si₂. Depending on synthesis conditions it crystallizes either in the I-type body-centered ThCr₂Si₂ or P-type CaBe₂Ge₂ structure. The P-type system has a low Kondo scale TK=20.3em0exK and orders magnetically below 0.7 K whereas the I-type system remains paramagnetic and shows, above 0.5 K, a temperature dependence of the resistivity and the specific heat similar to that of YbRh₂Si₂. In contrast to YbRh₂Si₂ which exhibits non-Fermi-liquid behavior, YbIr₂Si₂ reaches a Landau Fermi-liquid state below 0.2 K and is thus ideally suited for pressure studies across the quantum critical point.