YbRh2Si2: Pronounced non-Fermi-liquid effects above a low-lying magnetic phase transition

We report the first observation of non-Fermi-liquid (NFL) effects in a clea n Yb compound at ambient pressure and zero magnetic field. The electrical r esistivity and the specific-heat coefficient of high-quality single crystal s of YbRh2Si2 present a linear and a logarithmic temperature dependence, re spectively, in more than a decade in temperature. We ascribe this NFL behav ior to the presence of (presumably) quasi-2D antiferromagnetic spin fluctua tions related to a very weak magnetic phase transition at T-N similar or eq ual to 65 mK. Application of hydrostatic pressure induces anomalies in the electrical resistivity, indicating the stabilization of magnetic order.