Dimensionality of vortex matter in superconducting infinite-layer nickelates

Characterizing the dimensionality of the superconducting state in infinite-layer (IL) nickelates is essential for understanding its nature. Most studies have addressed this by examining the anisotropy of the upper critical fields. However, the dominance of Pauli paramagnetic effects over orbital effects complicates the interpretation of these experiments in terms of dimensionality. Here, we approach the question from a different perspective by mapping the vortex phase diagram. We show that superconducting Pr0.8Sr0.2NiO2 thin films with low disorder exhibit a vortex liquid-to-glass transition of a quasi-two-dimensional (2D) nature. In contrast, increasing disorder drives a crossover into a pure 2D state. This demonstrates that pure bidimensionality is an extrinsic property, resulting from the decoupling of NiO2 planes due to enhanced disorder. Our findings establish disorder as a key control parameter of superconductivity in IL nickelates and suggest that it resides within the NiO2 planes, providing two fundamental insights for understanding these materials.

💡 Research Summary

The authors investigate the dimensionality of the superconducting state in infinite‑layer (IL) nickelate Pr₀.₈Sr₀.₂NiO₂ (PSNO) thin films by mapping their vortex phase diagram rather than relying on upper‑critical‑field anisotropy, which is obscured by dominant Pauli‑paramagnetic limiting. Four samples with varying normal‑state resistivity (ρₙ) and critical temperature (T_c) are prepared by different reduction methods, providing a controlled disorder series. Structural analysis confirms the IL phase and film thickness of ~7–8 nm for all samples; disorder is attributed mainly to oxygen‑related defects (residual apical O, interstitial O).

Transport measurements under magnetic fields reveal a strong anisotropy of the transition width, larger for fields perpendicular to the film plane, consistent with previous reports on IL nickelates. However, the extracted H_c2⊥ shows a linear temperature dependence while H_c2∥ follows the √H behavior expected for thin‑film orbital limiting. Both fields are significantly lower than Ginzburg‑Landau predictions, indicating that Pauli limiting dominates and that H_c2 anisotropy cannot be straightforwardly linked to dimensionality.

The core of the study is the analysis of vortex dynamics. Arrhenius plots of resistance versus 1/T display a broad thermally activated flux‑flow (TAFF) regime for all samples. The extracted activation energy U₀ scales with magnetic field as U₀∝H^‑α with α≈0.5–0.7, a hallmark of low‑dimensional vortex matter, contrasting with α≈1 typical of three‑dimensional superconductors. At lower temperatures a deviation from the TAFF line appears in the low‑disorder samples, suggesting a crossover to quantum vortex motion, a behavior characteristic of two‑dimensional systems.

Current‑voltage (I‑V) characteristics are then examined. In the lowest‑disorder film (sample #4, lowest ρₙ, highest T_c) the I‑V curves evolve from an ohmic response at high temperature to a non‑linear, zero‑resistance state below a glass transition temperature T_g≈7.9 K. Scaling according to vortex‑glass (VG) theory yields a dimensionality d=2 (quasi‑2D), static exponent ν≈4.9 and dynamic exponent z≈2–3, indicating a finite vortex correlation length both within and perpendicular to the NiO₂ planes. This implies that inter‑plane coupling, though weak, remains finite.

In contrast, the higher‑disorder films (samples #1, #2, #3) never develop a true zero‑resistance state down to the base temperature (≈1.8 K). Their I‑V curves retain a linear low‑current regime, and scaling with the pure 2D vortex‑liquid (or vortex‑glass) form provides an excellent collapse, with no discernible T_g. This behavior corresponds to a vanishing vortex correlation length perpendicular to the planes; the vortices become independent “pancake” vortices confined to each NiO₂ layer.

The authors estimate the vortex line length ℓ_v from the critical current I_c using the balance between Lorentz force and thermal energy. For the clean sample ℓ_v is comparable to the film thickness, consistent with quasi‑2D scaling. As disorder increases, ℓ_v shrinks dramatically, eventually becoming comparable to the inter‑plane spacing (~0.4 nm). This demonstrates that disorder shortens the vortex line, suppresses inter‑layer vortex correlations, and drives a crossover from quasi‑2D to strictly 2D vortex matter.

Overall, the work shows that pure two‑dimensional superconductivity in IL nickelates is not intrinsic but emerges when disorder decouples the NiO₂ planes. The superconducting coherence length ξ_c is already shorter than the inter‑plane distance, consistent with Pauli‑limited behavior, and the remaining coupling is governed by Josephson and magnetostatic interactions. Enhanced disorder (likely oxygen‑related) overwhelms these weak links, producing pancake vortices and eliminating a finite vortex‑glass transition.

The findings have broader implications: they identify disorder as a key control parameter for dimensional crossover, a concept already familiar in cuprates where oxygen stoichiometry tunes the effective dimensionality. For nickelates, controlling oxygen defects and achieving high‑quality epitaxy will be essential to preserve inter‑plane coherence and to explore intrinsic pairing mechanisms. The study therefore provides a crucial piece of the puzzle in understanding the nature of superconductivity in infinite‑layer nickelates and highlights vortex‑matter analysis as a powerful tool for probing dimensionality when conventional H_c2 anisotropy is ambiguous.