Spatially - resolved study of the Meissner effect in superconductors using NV-centers-in-diamond optical magnetometry

TL;DR

This study uses NV-center-based optical magnetometry to spatially resolve the Meissner effect in various superconductors, revealing diverse behaviors in magnetic field expulsion and providing microscopic insights.

Contribution

It introduces a non-invasive, high-resolution method to study the spatial distribution of magnetic fields in superconductors, uncovering new details about the Meissner effect across different materials.

Findings

Conventional superconductors show strong Meissner expulsion.

Iron-based superconductors exhibit minimal expulsion.

Magnetic induction profiles correlate with macroscopic magnetic measurements.

Abstract

Non-invasive magnetic field sensing using optically - detected magnetic resonance of nitrogen-vacancy (NV) centers in diamond was used to study spatial distribution of the magnetic induction upon penetration and expulsion of weak magnetic fields in several representative superconductors. Vector magnetic fields were measured on the surface of conventional, Pb and Nb, and unconventional, LuNi$_2$B$_2$C, Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$, Ba(Fe$_{0.93}$Co$_{0.07}$)$_2$As$_2$, and CaKFe$_4$As$_4$, superconductors, with diffraction - limited spatial resolution using variable - temperature confocal system. Magnetic induction profiles across the crystal edges were measured in zero-field-cooled (ZFC) and field-cooled (FC) conditions. While all superconductors show nearly perfect screening of magnetic fields applied after cooling to temperatures well below the superconducting transition, $T_c$, a range of very different behaviors was observed for Meissner expulsion upon cooling in static magnetic field from above $T_c$. Substantial conventional Meissner expulsion is found in LuNi$_2$B$_2$C, paramagnetic Meissner effect (PME) is found in Nb, and virtually no expulsion is observed in iron-based superconductors. In all cases, good correlation with macroscopic measurements of total magnetic moment is found. Our measurements of the spatial distribution of magnetic induction provide insight into microscopic physics of the Meissner effect.