Finite-thickness Ginzburg–Landau constraints on the 2D–3D upper-critical-field crossover in infinite-layer La0.8Sr0.2NiO 2

The angle-dependent upper critical field of superconducting La[Formula: see text]Sr[Formula: see text]NiO 2 reveals a finite-size crossover controlled by the relation between the film thickness and the out-of-plane orbital coherence length. Solving the full finite-thickness anisotropic Ginzburg–Landau nucleation problem shows that the 10[Formula: see text]K cusp occurs because the film is thinner than the effective out-of-plane coherence length, whereas upon cooling the coherence length falls below the film thickness and the orbital response becomes three-dimensional. The central result is not the existence of this geometric crossover, but its rate: A constant-parameter extrapolation of the one-component GL description does not provide a common effective representation of the observed angular evolution from 10[Formula: see text]K to 2[Formula: see text]K. Paramagnetic depairing is required to describe the large in-plane-field scale and the reduction of the apparent field anisotropy, yet it does not remove the need for an additional temperature dependence in the out-of-plane orbital sector. The data therefore constrain the effective ratio controlling [Formula: see text] to evolve beyond the standard constant-coefficient GL law. This provides a quantitative mesoscopic constraint on multiband and interlayer-hybridization descriptions of infinite-layer nickelate superconductivity.

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