Ultra-low temperature ac susceptibility of the heavy-fermion superconductor YbRh₂Si₂

Recently, we reported on the discovery of superconductivity in YbRh 2 Si 2 at the very low temperature T c = 2 mK. We present here complementary measurements of the temperature and field dependence of the ac susceptibility χ ac ( T ) of this heavy-fermion system for temperatures down to 1 mK and magnetic fields in the basal plane up to 75 mT. The in-phase response IMG [http://ej.iop.org/images/1742-6596/807/5/052007/JPCS_807_5_052007_eqn001.gif] {{χ }^{^{\prime} }}_{ac}(T)} shows a steep drop at T c , but it does not become negative, possibly because of the presence of the earth field that was not compensated. The out-of-phase response IMG [http://ej.iop.org/images/1742-6596/807/5/052007/JPCS_807_5_052007_eqn002.gif] {{{χ }^{^{\prime\prime} }}_{ac}(T) shows an increase right below T c indicating that the transition is first order. We also observe two kinks in the field dependence of χ ac ( B ) at B 1 ≈ 50 mT and B 2 ≈ 60 mT, which imply a more complex T – B phase diagram for YbRh 2 Si 2 with B ⊥ c . These features possibly indicate two second-order phase transitions or domain reorientation at B 1 and a phase transition from the antiferromagnetic to the paramagnetic state at B 2