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, B-c2(T), which can be nicely fitted to two-dimensional (2D) models; however, the deduced film thickness, d(sc,GL), exceeds the physical film thickness, d(sc), by a manifold. To address the latter, it should be noted that 2D models assume that d(sc) is less than the in-plane and out-of-plane ground-state coherence lengths, d(sc)1 is a unitless free-fitting parameter. The proposed expression for ?(?)(T), perhaps, has a much broader application because it has been successfully applied to bulk pnictide and chalcogenide superconductors.

Automated summary

Nickelate superconductors discovered in 2019 exhibit unexplained mysteries such as the presence of a superconducting state in thin films but not in bulk materials, and the temperature-dependent upper critical field that can be fitted to two-dimensional models. The deduced film thickness exceeds the physical film thickness significantly.