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

Abstract

Nickelate superconductors, R1−xAxNiO2 (where R is a rare earth metal and A = Sr, Ca), experimentally discovered in 2019, exhibit many unexplained mysteries, such as the existence of a superconducting state with Tc (up to 18 K) in thin films and yet absent in bulk materials. Another unexplained mystery of nickelates is their temperature-dependent upper critical field, (Formula presented.), which can be nicely fitted to two-dimensional (2D) models; however, the deduced film thickness, (Formula presented.), exceeds the physical film thickness, (Formula presented.), by a manifold. To address the latter, it should be noted that 2D models assume that (Formula presented.) is less than the in-plane and out-of-plane ground-state coherence lengths, (Formula presented.) and (Formula presented.), respectively, and, in addition, that the inequality (Formula presented.) satisfies. Analysis of the reported experimental (Formula presented.) data showed that at least one of these conditions does not satisfy for R1-xAxNiO2 films. This implies that nickelate films are not 2D superconductors, despite the superconducting state being observed only in thin films. Based on this, here we propose an analytical three-dimensional (3D) model for a global data fit of in-plane and out-of-plane (Formula presented.) in nickelates. The model is based on a heuristic expression for temperature-dependent coherence length anisotropy: (Formula presented.), where (Formula presented.) is a unitless free-fitting parameter. The proposed expression for (Formula presented.), perhaps, has a much broader application because it has been successfully applied to bulk pnictide and chalcogenide superconductors. © 2023 by the author.