2022/03/03 by Schuberth, Erwin, Wirth, S., Steglich, F.
#FOS: Physical sciences #Strongly Correlated Electrons (cond-mat.str-el) #Superconductivity (cond-mat.supr-con)
paper · doi:10.48550/arxiv.2203.01617
The tetragonal heavy-fermion metal YbRh2Si2 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 4f-electronic antiferromagnetic (AF) order is strongly suppressed by a nuclear-dominated hybrid order (`A-phase') at T\rm A ≤ 2.3 mK, such that quantum critical fluctuations develop at B ≈ 0 (Schuberth et al., Science 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 ≈ 6.5 mK. Recent electrical-resistivity results by Nguyen et al. (Nat. Commun. 12, 4341 (2021)) confirm the existence of superconductivity in YbRh2Si2 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.