Superconductivity normally becomes less stable as a magnetic field increases. In field-reentrant superconductivity, however, a superconducting state that has been suppressed at low field reappears at a higher field before disappearing again. Earlier examples were largely confined to low-transition-temperature materials. Infinite-layer nickelates, a newer family of unconventional superconductors with transition temperatures approaching 40 K, therefore provide an unusual setting in which to study how magnetism can both compete with and support superconductivity.
Europium-doped infinite-layer nickelates are especially intriguing because Eu 2+ plays two roles at once: it supplies holes to the electronic system and contributes a localized, half-filled 4 f 7 magnetic moment. That combination creates several possible interactions among charge carriers, magnetic fluctuations and superconducting pairs. Systematic tests have nevertheless been difficult because perovskite nickelate precursors containing later rare-earth elements become increasingly metastable and are hard to grow with conventional thin-film methods.
The research team addressed this materials bottleneck with a solution-based chemical route assisted by high oxygen pressure. Rare-earth nitrate and nickel acetate precursors were dissolved, spin-coated on single-crystal substrates and crystallized at 700-900 °C under 1-20 MPa oxygen. A subsequent low-temperature topotactic reduction with CaH2 converted the perovskite precursors into infinite-layer films about 20 nm thick. By changing the rare-earth composition in solution, the researchers prepared (RE 1-y RE' y ) 1-x Eu x NiO 2 films containing Pr, Nd, Sm, Gd or Dy. X-ray diffraction, synchrotron spectroscopy and atomic-resolution microscopy verified the structural and valence changes produced by the process.
This broader composition space exposed a consistent phase-diagram pattern. In Nd 1-x Eu x NiO 2 , the optimally doped range, x approximately 0.35-0.45, showed robust superconductivity with an onset temperature up to 37.6 K and no reentrance in fields as high as 35 T. At both the underdoped and overdoped edges of the superconducting dome, however, magnetic-field sweeps produced a reentrant zero-resistance state. The same boundary-selective behavior appeared in Pr1-xEuxNiO2 and on a second substrate, indicating that it is not confined to one film composition or growth interface.
Rare-earth substitution provided a second control knob. Starting from an optimally doped Nd-Eu composition that did not show reentrance, partial replacement of Nd by Pr, Sm or Dy preserved conventional superconductivity. Gd substitution, by contrast, restored the reentrant response. In (Nd 0.95 Gd 0.05 ) 0.65 Eu 0.35 NiO 2 , zero resistance reappeared in an intermediate magnetic-field window and was quenched again at still higher fields. Measurements to 35 T captured this second loss of superconductivity, while angle-dependent transport showed that the reentrant window is strongest when the field lies close to the crystal c axis and contracts rapidly as the temperature rises.
The Gd result is consistent with a Jaccarino-Peter-type exchange-field compensation picture. Gd 3+ and Eu 2+ both carry half-filled 4 f 7 moments, and an internal exchange field associated with these moments may partially oppose the applied field. Within a limited field range, the reduced effective field can allow superconductivity to recover; once the effective field again exceeds the upper critical field, superconductivity disappears. The authors emphasize that Eu also changes the carrier concentration, so the full behavior cannot be reduced to exchange compensation alone. Competing charge, spin and orbital states near the edges of the dome may also help determine where reentrance can emerge.
The same rare-earth freedom could also strengthen the robust superconducting state. Pr, Sm and mixed Pr/Sm substitution raised the onset transition temperature beyond the 37.6 K maximum of the Nd-Eu parent series. The highest value, 40.1 K, was observed in (Nd 0.8 Pr 0.2 ) 0.65 Eu 0.35 NiO 2 . Optimized Eu-doped films also reached a critical current density of 266 kA cm -2 at 2 K and retained substantial current-carrying capability in a 9 T field. Compared with Sr- or Ca-doped nickelates having similar infinite-layer spacing, the Eu-containing films showed higher transition temperatures, suggesting that rare-earth magnetism may provide an additional pairing-enhancement channel.
Low-energy muon-spin spectroscopy supplied an independent view of the magnetic side of the problem. In optimally doped Nd 0.6 Eu 0.4 NiO 2 , which shows no reentrant response, the zero-field muon relaxation rate rose sharply across the superconducting transition range, while the stretch exponent approached 0.5-0.6 at low temperature. These observations point to stronger spin correlations and a possible spin-glass-like state within the superconducting regime. They do not by themselves establish that the magnetic response causes the higher transition temperature, but they identify a close link that can now be tested with further spectroscopic and theoretical work.
Together, the results place robust high-temperature superconductivity, field-reentrant superconductivity and rare-earth 4 f magnetism on one chemically tunable platform. The work offers a route to study possible quantum-critical competition at the boundaries of the nickelate superconducting dome and suggests that non-vacuum chemical processing may eventually be relevant to coated-conductor approaches. The study was led by corresponding authors Jikun Chen of the University of Science and Technology Beijing, Jia-Cai Nie of Beijing Normal University and Binghui Ge of Anhui University. Haowen Han, Yusong Zhao and Yi Bian contributed equally as first authors.
National Science Review
Experimental study