Extended Data Fig. 1: SQUID-based setup for ultra-sensitive impedance measurements (a), and signatures of superconductivity (c,d) and antiferromagnetism (e) in YbRh2Si2 at zero (Earth’s) field.
a, Schematic diagram of the measurement circuit. Details in Supplementary Note 1. b, Step in kinetic inductance at TA aligned with a sharp heat capacity peak23. We attribute negative \({\rm{Im}}\,Z\) to a small offset due to unidentified parasitic coupling in the circuit. c, Gradual drop of sample and contact resistance and development of imaginary parts of the impedances on cooling below 10 mK. Small increases in \({\rm{Re}}\,Z(T)\) on cooling coincide with abrupt drops in \({\rm{Re}}\,{Z}_{{\rm{c}}}(T)\), indicative of current path redistribution in heterogeneous samples. Electronic heat capacity Ce23 shows no clear signs of a BCS transition. The data represent mean ± SD of the measured total heat capacity Ce+n, with the fitted nuclear contribution Cn subtracted. Large errorbars below 8 mK are a result of Cn dominating Ce+n at low temperatures23. d, The Néel transition between AFM Fermi liquid (R = R0 + AT2) and PM non-Fermi liquid (\(R={R}_{0}+{A}^{{\prime} }T\)) in purely resistive impedance of samples and contacts.
