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

Nickelate superconductors, R(1−x)A(x)NiO(2) (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 T(c) (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: see text] , which can be nicely fitted to two-dimensional (2D) models; however, the deduced film thickness, [Formula: see text] , exceeds the physical film thickness, [Formula: see text] , by a manifold. To address the latter, it should be noted that 2D models assume that [Formula: see text] is less than the in-plane and out-of-plane ground-state coherence lengths, [Formula: see text] and [Formula: see text] , respectively, and, in addition, that the inequality [Formula: see text] satisfies. Analysis of the reported experimental [Formula: see text] data showed that at least one of these conditions does not satisfy for R(1-x)A(x)NiO(2) 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: see text] in nickelates. The model is based on a heuristic expression for temperature-dependent coherence length anisotropy: [Formula: see text] , where [Formula: see text] is a unitless free-fitting parameter. The proposed expression for [Formula: see text] , perhaps, has a much broader application because it has been successfully applied to bulk pnictide and chalcogenide superconductors.