Linear-in-temperature resistivity for optimally superconducting (Nd,Sr)NiO₂

The occurrence of superconductivity in proximity to various strongly correlated phases of matter has drawn extensive focus on their normal state properties, to develop an understanding of the state from which superconductivity emerges^1-4. The recent finding of superconductivity in layered nickelates raises similar interests^5-8. However, transport measurements of doped infinite-layer nickelate thin films have been hampered by materials limitations of these metastable compounds: in particular, a high density of extended defects^9-11. Here, by moving to a substrate (LaAlO₃)_0.3(Sr₂TaAlO₆)_0.7 that better stabilizes the growth and reduction conditions, we can synthesize the doping series of Nd_1-xSr_xNiO₂ essentially free from extended defects. In their absence, the normal state resistivity shows a low-temperature upturn in the underdoped regime, linear behaviour near optimal doping and quadratic temperature dependence for overdoping. This is phenomenologically similar to the copper oxides^2,12 despite key distinctions-namely, the absence of an insulating parent compound^5,6,9,10, multiband electronic structure^13,14 and a Mott-Hubbard orbital alignment rather than the charge-transfer insulator of the copper oxides^15,16. We further observe an enhancement of superconductivity, both in terms of transition temperature and range of doping. These results indicate a convergence in the electronic properties of both superconducting families as the scale of disorder in the nickelates is reduced.