Nuclear-order-induced quantum criticality and heavy-fermion superconductivity at ultra-low temperatures in YbRh$_2$Si$_2$
The tetragonal heavy-fermion metal YbRh$_2$Si$_2$ orders antiferromagnetically at $T_{\rm N} = 70$ mK and exhibits an unconventional quantum critical point (QCP) of Kondo-destroying type at $B_{\rm N} = 60$ mT, for the magnetic field applied within the basal ($a,b$) plane. Ultra-low-temperature magnetization and heat-capacity measurements at very low fields indicate that the 4$f$-electronic antiferromagnetic (AF) order is strongly suppressed by a nuclear-dominated hybrid order (`A-phase') at $T_{\rm A} \le 2.3$ mK, such that quantum critical fluctuations develop at $B \approx 0$ (Schuberth et al., Science \textbf{351}, 485 (2016)). This enables the onset of heavy-fermion superconductivity ($T_{\rm c} = 2$ mK) which appears to be suppressed by the primary AF order at elevated temperatures. Measurements of the Meissner effect reveal bulk superconductivity, with $T_{\rm c}$ decreasing under applied field to $T_{\rm c} < 1$ mK at $B > 20$ mT. The observation of a weak but distinct superconducting shielding signal at a temperature as high as 10 mK suggests the formation of insulated random islands with emergent A-phase order and superconductivity. Upon cooling, the shielding signal increases almost linearly in temperature, indicating a growth of the islands which eventually percolate at $T \approx 6.5$ mK. Recent electrical-resistivity results by Nguyen et al. (Nat. Commun. \textbf{12}, 4341 (2021)) confirm the existence of superconductivity in YbRh$_2$Si$_2$ at ultra-low temperatures. The combination of the results of Schuberth et al. and Nguyen et al. at ultra-low temperatures below $B_{\rm N}$, along with those previously established at higher temperatures in the paramagnetic state, provide compelling evidence that the Kondo-destruction quantum criticality robustly drives unconventional superconductivity.