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# Isotropic Pauli-limited superconductivity in the infinite-layer nickelate Nd0.775Sr0.225NiO2

## Abstract

The recent observation of superconductivity in thin-film infinite-layer nickelates1,2,3 offers a different angle from which to investigate superconductivity in layered oxides4. A wide range of candidate models have been proposed5,6,7,8,9,10, which emphasize single- or multi-orbital electronic structure, Kondo or Hund’s coupling and analogies to cuprates. Further experimental characterization of the superconducting state is needed to develop a full understanding of the nickelates. Here we use magnetotransport measurements to probe the superconducting anisotropy in Nd0.775Sr0.225NiO2. We find that the upper critical field is surprisingly isotropic at low temperatures despite the layered crystal structure. In a magnetic field, the superconductivity is strongly Pauli-limited, such that the paramagnetic effect dominates over orbital de-pairing. Underlying this isotropic response is a substantial anisotropy in the superconducting coherence length, which is at least four times longer in-plane than out-of-plane. A prominent low-temperature upturn in the upper critical field indicates the presence of an unconventional ground state.

## Relevant articles

• ### Observation of perfect diamagnetism and interfacial effect on the electronic structures in infinite layer Nd0.8Sr0.2NiO2 superconductors

Nature Communications Open Access 08 February 2022

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## Data availability

Source data are provided with this paper. All other data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.

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## Acknowledgements

We thank R. L. Greene, A. Kapitulnik, S. A. Kivelson, P. B. Littlewood, B. Maiorov, G. A. Sawatzky, H. Takagi, R. Thomale, A. Viswanathan and Y.-H. Zhang for discussions. The work at SLAC and Stanford was supported by the US Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering (contract no. DE-AC02-76SF00515) and the Gordon and Betty Moore Foundation’s Emergent Phenomena in Quantum Systems Initiative (grant no. GBMF9072, synthesis equipment). B.H.G. and L.F.K. acknowledge support by the US Department of Defense Air Force Office of Scientific Research (no. FA 9550-16-1-0305). This work made use of the Cornell Center for Materials Research Shared Facilities, which are supported through the US National Science Foundation (NSF) MRSEC Program (no. DMR-1719875). The FEI Titan Themis 300 was acquired through NSF grant no. MRI-1429155, with additional support from Cornell University, the Weill Institute and the Kavli Institute at Cornell. The Thermo Fisher Helios G4 X focused ion beam was acquired with support from the NSF Platform for Accelerated Realization, Analysis and Discovery of Interface Materials (PARADIM) under Cooperative Agreement no. DMR-1539918.

## Author information

Authors

### Contributions

D.L., K.L. and M.O. grew the nickelate films and conducted the reduction experiments and structural characterization. B.H.G. and L.F.K. conducted electron microscopy. B.Y.W. performed the transport measurements and analysis with M.R.B. B.Y.W., S.P.H. and H.Y.H. wrote the manuscript with input from all authors.

### Corresponding authors

Correspondence to Bai Yang Wang or Harold Y. Hwang.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

Peer review information Nature Physics thanks Neil Harrison and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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## Supplementary information

### Supplementary Information

Supplementary Figs. 1–9, Table 1 and Sections 1–5.

## Source data

### Source Data Fig. 1

Resistivity superconducting transition under magnetic fields.

### Source Data Fig. 2

Upper critical field as function of temperature.

### Source Data Fig. 3

Arrhenius plot of resistivity transition and activation energy of dissipative vortex motion.

### Source Data Fig. 4

Magnetic field versus temperature phase diagram and extraction of electron susceptibility.

## Rights and permissions

Reprints and Permissions

Wang, B.Y., Li, D., Goodge, B.H. et al. Isotropic Pauli-limited superconductivity in the infinite-layer nickelate Nd0.775Sr0.225NiO2. Nat. Phys. 17, 473–477 (2021). https://doi.org/10.1038/s41567-020-01128-5

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• DOI: https://doi.org/10.1038/s41567-020-01128-5

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