Abstract Body

Understanding electronic properties of materials similar to the cuprates can provide insight into the mechanism of superconductivity in the cuprates and other high-T_c superconductors. The infinite-layer nickel-based 112 thin films consist of square plane-coordinated NiO₂ planes and Ln/Sr/Ca atomic layers, which is the isostructure of cuprates [1]. For LaNiO₂ and NdNiO₂, x-ray absorption spectroscopy and resonant inelastic x-ray scattering confirmed the nominal 3d⁹ electronic configuration, equivalent to Cu²⁺ in the cuprates [2]. Such isostructural and isoelectronic characteristics show great promise in deconstructing the various cooperative mechanisms responsible for high-temperature superconductivity. However, the hybridization effect of Ni 3d and O 2p orbitals is decreased compared to the hybridization of Cu 3d and O 2p orbitals in cuprates (i.e., La₂CuO₄ ), while the coupling strength of Ni 3d and La/Nd 5d states is increased compared to Cu 3d and La 5d states [2]. In this configuration, the Ni state can have a charge transfer to the rare-earth cation, thus leaving holes in Ni orbitals and electrons in R 5d (R = La, Pr, Nd) orbitals, which is known as the self-doping effect [3,4]. The change in sign of the low-temperature Hall coefficient as a function of Sr doping indicates the presence of multiple bands at the Fermi level [5,6]. From the theoretical point of view, electronic structure calculations for the nickelates indicate the presence of a large hole pocket with 3dx²−y² character, and electron pockets arising from Nd 5d and Ni 3d hybridization [1,7,8]. As is well known, the parent phase of cuprates is depicted as a charge transfer insulator with an antiferromagnetic (AFM) order [4,9]. In sharp contrast, a definitive resolution of long-range AFM in infinite-layer nickelates is still lacking, even though hints of short-range magnetic order or spin glass behavior have been picked up by various measurements [10–13]. Recent STM experiments uncovered a mixed s- and d-wave superconductivity (SC) gap feature on the rough surface of Nd_1−xSr_xNiO₂ thin films [14]. At present, the research on nickelate superconductors is still in the exploratory stage. The similarities and differences between the SC mechanisms of nickelates and cuprates are still unclear.

The upper critical field (H_c2) is a fundamental parameter for getting information such as the pair-breaking mechanism, coherence length ξ, and pairing symmetry, which plays an important role in the understanding of unconventional superconducting mechanisms. We studied the upper critical field (H_c2 ) of a high-quality La_0.8Sr_0.2NiO₂ thin film with superconducting transition temperature, T^onset_c = 18.8 K, using high magnetic field up to 56 T. A very large H_c2,∼40 T for H // c and ∼52 T for H ⊥ c, was confirmed, which suggests that infinite-layer nickelates also have great application potential. The anisotropy of H_c2 monotonically decreases from ∼10 (γ = H^ab_c2 /H^c _c2 ) near T_c to ∼1.5 at 2 K. Angle dependence of H_c2 confirms the crossover of superconductivity from two-dimensional to three-dimensional as the temperature decreases. We discussed that the interstitial orbital effect causes the weakening of anisotropy. The observed abnormal upturning of H_c2 at low temperatures is found to be a universal behavior independent of film quality and rare-earth elements. Therefore, it should not be the Fulde-Ferrell-Larkin-Ovchinnikov state due to the fact that it is in the dirty limit and insensitive to disorder.

References

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Acknowledgment

This work was partly supported by the National Key R&D Program of China (Grant No. 2018YFA0704300), the Strategic Priority Research Program (B) of the Chinese Academy of Sciences (Grant No. XDB25000000), and the National Natural Science Foundation of China (Grant No. U1932217).