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 T-K=2 K 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.