Local measurement of temperature dependent London Penetration Depth in Superconductors using Nitrogen Vacancy Centers in Diamond*

Local optical magneto-sensing based on nitrogen-vacancy (NV) centers in diamond [1] was used to measure temperature-dependent London penetration depth, λ (T), in two different unconventional superconductors, Ba(Fe_1-xCo_x)₂As₂ (BaCo122) and YBa₂Cu₃O_7-δ (YBCO). The results are compared with the sensitive bulk measurements of the same samples using tunnel-diode resonator (TDR). Each sample was measured in two orientation to separate the effects of platelet geometry and surface morphology, imaged using SEM. Not only this study compares the absolute numbers from two different techniques, it also allows us to discuss a more realistic picture of the magnetic field penetration into superconducting samples.