The antiferromagnet YbRh₂Si₂. A new heavy-fermion superconductor

While unconventional superconductivity (SC) often occurs in the vicinity of quantum critical points (QCPs) in antiferromagnetic (AF) heavy-fermion metals, no SC has so far been observed near the QCP induced by a small magnetic field in YbRh₂Si₂. Here, we explore results of magnetic and calorimetric measurements on YbRh₂Si₂ down to T = 1 mK (E. Schuberth et al., Science 351, 485 (2016)). They reveal the onset of a hybrid nuclear-electronic type of AF order dominated by the Yb-derived nuclear spins at T_A slightly above 2 mK and the subsequent development of SC at T_c = 2 mK. The initial slope of the upper critical field curve, B_c2(T), at T_c is found to be as large as -B^'_c2 ≅ 25 T/K. This indicates that the effective charge-carrier mass must be of the order of several 100 m_el, implying that the superconducting state is associated with the Yb-derived 4f-electronic rather than nuclear spins. Therefore, the theoretical possibility of superheavy-fermion SC based upon an underlying nuclear Kondo effect can be ruled out. In conclusion, we ascribe the formation of Cooper pairs in YbRh₂Si₂ to the critical fluctuations associated with the unconventional, Mott-type, QCP of this antiferromagnet, which are revealed when the primary electronic order is diminished by the competing nuclear-dominated hybrid order. Our results demonstrate a new means to reach a field-induced QCP and provide further evidence that SC in the vicinity of AF QCPs is a general phenomenon.