Optical sensing of weak magnetic fields in superconductors using NV centers in diamond*

Visible light fluorescent nitrogen-vacancy centers in diamond have emerged as high sensitivity sensors for nanoscale magnetometry. Here, we study spatial distributions of the magnetic induction upon penetration and expulsion of weak magnetic fields in several representative superconductors. In particular, vector magnetic field was measured with diffraction-limited spatial resolution on the surface of elemental (Pb and Nb), intermetallic (LuNi₂B₂C), and iron-based (Ba_0.6K_0.4Fe₂As₂, Ba(Fe_0.93Co_0.07)₂As₂, CaKFe₄As₄) superconductors in zero-field-cooling (ZFC) and field-cooling (FC) experiments. While ZFC shows robust screening, the structure of the Meissner state after FC ranges from conventional flux expulsion in Pb and LuNi₂B₂C, to paramagnetic Meissner effect in Nb, to virtually no expulsion in iron-based superconductors. We further apply this technique for accurate measurements of the first critical field and use it to estimate London penetration depth in several superconductors (arXiv:1709.02769).