A chemical avenue to manipulate field-reentrant superconducting competition in infinite-layer nickelates.

Recently, preliminary magnetic-field-reentrant superconductivity manifested in high-temperature (T c) Eu-doped infinite-layer (IL) nickelates, beyond analogous discoveries exclusively in low-T c systems. This evokes intriguing fundamental issues about potential quantum-phase boundary and criticality between unconventional superconductivity and field-reentrant one, which are inexplicable owing to formidable challenges in growing IL nickelates toward later-series rare-earths. Herein, we open up chemical avenues to enable effective growth of (RE 1 - yRE'y )1 - x Eu x NiO2 (RE/RE': Pr, Nd, Sm, Gd, Dy), giving rise to discoveries of RE-4f-related quantum competition between high-T c and reentrant superconductivity. Robust magnetic-field-reentrant superconductivity with uniaxial anisotropy is observed at superconducting dome boundaries, stemming from Eu2+-4f 7-associated competition between magnetic-fluctuation-promoted pairing and exchange-field interactions. Their quantum criticality is further modulable via RE(RE') magnetism, which either reinforces reentrancy or elevates T c (∼40.1 K) with more robust critical current density (∼266 kA/cm2 at 2 K) beyond Sr-/Ca-doped counterparts. Our synthetic route enables the establishment of an ideal platform via IL nickelates for studying 4f-related unconventional superconductivity and quantum criticality.