Local and Global measurements of London Penetration Depth in Inhomogeneous Superconductors*

An important parameter, the London penetration depth (LPD), in superconductors can be estimated from the global (whole sample) response [1] or measuring the first critical field locally near the surface [2]. We compare LPD from the global tunnel diode resonator (TDR) technique [1] with the local values obtained by using NV-centers in diamond optical magnetometry [2]. We deliberately chose a crystal of Ba(Fe_1-xCo_x)₂As₂ pnictide superconductor that looks like an old book under an electron microscope, revealing a peculiar planar stack-like structure. We also examined a standard single crystal of YBa₂Cu₃O_7-δ. While the temperature dependence of LPD is the same between TDR and NV-centers magnetometry, the absolute values differ by a factor of two in FeCo122, but are practically the same in YBCO. We model this situation numerically by finite-element method solving the London equations for our particular sample geometry and find unobvious behavior apparently due to the non-local long-range electromagnetic behavior of the finite-sized superconductor in an external field.
[1] R. Prozorov and R. W. Giannetta, SUST 19, R41 (2006)
[2] K. R. Joshi et al. PRApp. 11, 014035 (2019).