Intrinsic Coherence Length Anisotropy in Nickelates and Some Iron-Based Superconductors

Area of Science:

• Superconductivity
• Materials Science
• Condensed Matter Physics

Background:

• Nickelate superconductors (R_1-xA_xNiO₂) were discovered in 2019, exhibiting superconductivity (T_c up to 18 K) only in thin films, a phenomenon not observed in bulk materials.
• The temperature-dependent upper critical field (Bc2(T)) of nickelates is typically analyzed using 2D models, but deduced parameters often contradict physical film dimensions.
• Existing 2D models for superconductivity assume specific relationships between film thickness and coherence lengths (ξ_ab(0), ξ_c(0)), which are found to be violated in nickelate films.

Purpose of the Study:

• To address the discrepancies in analyzing nickelate superconductor critical field data.
• To propose and validate a new analytical three-dimensional (3D) model for fitting both in-plane and out-of-plane Bc2(T) in nickelates.
• To investigate the underlying reasons for the observed superconducting behavior in thin-film nickelates.

Main Methods:

• Analysis of reported experimental data for the temperature-dependent upper critical field (Bc2(T)) in R_1-xA_xNiO₂ films.
• Development of an analytical 3D model incorporating a heuristic expression for temperature-dependent coherence length anisotropy: γξ(T) = γξ(0) [1 - (T/T_c)^a], where 'a' is a fitting parameter.
• Application of the 3D model for a global data fit of Bc2(T) across different dimensions.

Main Results:

• Experimental data analysis revealed that conditions for 2D superconductivity models are not met for R_1-xA_xNiO₂ films.
• The proposed 3D model successfully provides a global fit for in-plane and out-of-plane Bc2(T) data in nickelates.
• The heuristic expression for coherence length anisotropy demonstrated broader applicability, also fitting data for bulk pnictide and chalcogenide superconductors.

Conclusions:

• Nickelate thin films exhibiting superconductivity do not behave as purely 2D superconductors, contrary to initial assumptions based on fitting models.
• The developed 3D analytical model offers a more accurate description of the superconducting properties of nickelates.
• The proposed model and its underlying expression for coherence length anisotropy show potential for wider application in understanding various superconducting materials.