Dresden 2017 – scientific programme
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TT: Fachverband Tiefe Temperaturen
TT 69: Focus Session: Superconductivity in the Vicinity of a Quantum Critical Point
TT 69.1: Invited Talk
Thursday, March 23, 2017, 15:00–15:30, HSZ 03
The Antiferromagnet YbRh2Si2 - a New Heavy-Fermion Superconductor — •Frank Steglich — MPI CPfS Dresden, Germany — CCM, ZJU Hangzhou, China — IOP, CAS, Beijing, China
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 TA slightly above 2 mK and the subsequent development of SC at Tc = 2 mK. The initial slope of the upper critical field curve, Bc2(T), at Tc 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 mel, 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.