Fig. 4: Temperature–magnetic field phase diagrams of YbRh₂Si₂ and ¹⁷⁴YbRh₂Si₂.

a Results for YbRh₂Si₂. b Results for ¹⁷⁴YbRh₂Si₂. Open (T_c) and full (B_c2) circles of all colors represent midpoints (50%) of the resistive transitions of iso-B ρ(T) curves (Fig. 1c, d) T iso-T ρ(B) curves (Fig. 2a, b), respectively. Full diamonds represent the B_c2 values extracted from a multi-transition model (Supplementary Note 1: Analysis of resistivity vs magnetic field isotherms and Supplementary Fig. 1) for the three involved transitions (green for leaving phase I, violet for entering and leaving phase II). The good agreement between diamonds and circles in the regions of minimal multi-phase interference justifies this procedure. The upper boundaries of the pale shaded areas away from data points represent 90% resistance lines. The black dashed line in both panels indicates the boundary of the antiferromagnetic (AFM) phase. It represents an extrapolation of data from refs. ^28,29, to T = 0, the quantum critical point (QCP, black dot). The red lines are linear fits to the low-field (50%) data, and determine both the upper critical field slopes \(-d{B}_{{{{\rm{c}}}}2}/dT{| }_{{T}_{{{{\rm{c}}}}}}\) and the zero-field superconducting transition temperatures T_c [=T_c(H = 0)]. The dashed lines represent (inverted) \({B}^{\prime}_{{{{\rm{c}}}}2}(T)={B}^{\prime}_{{{{\rm{c}}}}2}(T=0)\cdot [1-{(T/{T}_{{{{\rm{c}}}}})}^{2}]\) mean-field curves determined from \(-d{B}_{{{{\rm{c}}}}2}/dT{| }_{{T}_{{{{\rm{c}}}}}}\) and T_c, as estimates of the (low-field) phase boundaries. Below 0.5 mK, all other phase/shading boundaries correspond to linear-in-B extrapolations of fits to the lowest T data points. Note that the perfect overlap of the full and open symbols demonstrates the high reproducibility of the results.