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

Recently, we reported on the discovery of superconductivity in YbRh₂Si₂ 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 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 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 B₁ ≈ 50 mT and B₂ ≈ 60 mT, which imply a more complex T - B phase diagram for YbRh₂Si₂ with B ⊥ c. These features possibly indicate two second-order phase transitions or domain reorientation at B₁ and a phase transition from the antiferromagnetic to the paramagnetic state at B₂.