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Quantum phase transitions (QPTs) occur at zero temperature when parameters such as magnetic field or pressure are varied. In heavy fermion compounds, superconductivity often accompanies QPTs, a seeming exception being the material YbRh2Si2, which undergoes a magnetic QPT. Schuberth et al. performed magnetic and calorimetric measurements at extremely low temperatures and magnetic fields and found that it does become superconducting after all. Almost simultaneously with superconductivity, another order appeared that showed signatures of nuclear spin origin.Science, this issue p. 485The smooth disappearance of antiferromagnetic order in strongly correlated metals commonly furnishes the development of unconventional superconductivity. The canonical heavy-electron compound YbRh2Si2 seems to represent an apparent exception from this quantum critical paradigm in that it is not a superconductor at temperature T ≥ 10 millikelvin (mK). Here we report magnetic and calorimetric measurements on YbRh2Si2, down to temperatures as low as T ≈ 1 mK. The data reveal the development of nuclear antiferromagnetic order slightly above 2 mK and of heavy-electron superconductivity almost concomitantly with this order. Our results demonstrate that superconductivity in the vicinity of quantum criticality is a general phenomenon.
