${\mathrm{YbRh}}_2{\mathrm{Si}}_2$: 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 clean Yb compound at ambient pressure and zero magnetic field. The electrical resistivity and the specific-heat coefficient of high-quality single crystals of ${\mathrm{YbRh}}_2{\mathrm{Si}}_2$ present a linear and a logarithmic temperature dependence, respectively, in more than a decade in temperature. We ascribe this NFL behavior to the presence of (presumably) quasi-2D antiferromagnetic spin fluctuations related to a very weak magnetic phase transition at $T_N\simeq 65\mathrm{mK}$. Application of hydrostatic pressure induces anomalies in the electrical resistivity, indicating the stabilization of magnetic order.