Fig. 2. Resistivity upturn across optimal doping in Nd_1−xSr_xNiO₂.

a Magnetoresistivity isotherms ρ_ab(H) for 0.3 K ≤T≤ 30 K with H ∣∣ c, for the specified x values. b Resistivity in the field-induced normal state (μ₀H = 34 T), normalised by the minimal values $\rho /{\rho }_{\min }$. Data for each x is vertically shifted by 0.1 for clarity. Above x = 0.17, the magnitude of each resistivity upturn is small ($\rho /{\rho }_{\min }\lesssim$ 1%; illustrated by the coloured shadings) and neither the magnitude of the upturn nor the corresponding T₀ shows any strong dependence on x. c Upper critical field μ₀H_c2 extracted using the 50% ρ_n criterion (${\rho }_{\mathrm{n}}\equiv {\rho }_{ab}\left(T_{\mathrm{c}}^{\prime }\right)$). A small but pronounced upturn in H_c2 below 2 K is observed for all x. d Evolution of the SC dome under varying magnetic field strengths. T_c values in finite fields are inferred from isothermal field sweeps shown in a using the 50% ρ_n criterion (Supplementary Fig. 1). Error bars in T_c reflect the difference between the two neighbouring temperatures with ρ_ab(μ₀H) closest to 50% of ρ_n. At μ₀H = 12.5 and 20 T, the maximal T_c remains centred around x_opt ≈ 0.17.