Extreme strange metal behavior and unconventional superconductivity in YbRh₂Si₂

The heavy fermion compound YbRh₂Si₂ continues to surprise. It is a key example for beyond-order parameter quantum criticality [1], as evidenced most notably by a jump in the Fermi surface volume at its quantum critical point [2] and dynamical energy-over-temperature scaling in the THz conductivity [3]. We have recently succeeded to expand the temperature range of electrical resistivity measurements down to ultralow temperatures – into the micro-Kelvin regime – and discovered that the strange metal state in YbRh₂Si₂ is even more extreme than previously known: Linear resistivity extends over 3.5 orders of magnitude in temperature [4]. At even lower temperatures, superconductivity condenses directly out of this strange metal state. We propose that the Cooper pairing is mediated by the same fluctuations that are responsible for the unconventional quantum criticality and discuss possible implications for other strange metal superconductors [4].