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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, B c 2 (T) , which can be nicely fitted to two-dimensional (2D) models; however, the deduced film thickness, d s c , G L , exceeds the physical film thickness,   d s c , by a manifold. To address the latter, it should be noted that 2D models assume that d s c is less than the in-plane and out-of-plane ground-state coherence lengths, d s c < ξ a b (0) and d s c < ξ c (0) , respectively, and, in addition, that the inequality ξ c (0) < ξ a b (0) satisfies. Analysis of the reported experimental B c 2 (T) 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 B c 2 (T) in nickelates. The model is based on a heuristic expression for temperature-dependent coherence length anisotropy: γ ξ (T) = γ ξ (0) 1 − 1 a × T T c , where a > 1 is a unitless free-fitting parameter. The proposed expression for γ ξ (T) , perhaps, has a much broader application because it has been successfully applied to bulk pnictide and chalcogenide superconductors. [ABSTRACT FROM AUTHOR]

Tin-based nanoparticles, proposed to be barium stannate, have been formed in YBCO films fabricated by metal-organic deposition, through modification of the precursor solution. These randomly-oriented 30-50 nm particles are dispersed throughout the film thickness, providing an enhancement in the isotropic flux pinning relative to undoped YBCO. For a low level, of nanopartide addition of 2 vol%, a flux-pinning force enhancement of up to 32% is achieved. We report field and field-angle dependence of the transport critical current, transmission electron microscopy, and x-ray diffraction of our YBCO films on RABiTS substrates, and compare with similar barium zirconate additions. [ABSTRACT FROM AUTHOR]