Giant Vortices Below the Surface of NbSe$_2$ Detected Using Low Energy $\beta$-NMR

TL;DR

This study uses low energy $eta$-NMR to detect and analyze giant vortices near the surface of NbSe$_2$, revealing field-dependent vortex core sizes and providing insights into vortex behavior in superconductors.

Contribution

First application of depth-resolved $eta$-NMR to observe giant vortices and their properties near the surface of NbSe$_2$.

Findings

Giant vortices with large core radii observed at low magnetic fields.

Vortex core size varies significantly with magnetic field.

Characteristic vortex lattice lineshape consistent with theoretical models.

Abstract

A low energy radioactive beam of polarized $^8$Li has been used to observe the vortex lattice near the surface of superconducting NbSe$_2$. The inhomogeneous magnetic field distribution associated with the vortex lattice was measured using depth-resolved $\beta$-detected NMR. Below $T_c$ one observes the characteristic lineshape for a triangular vortex lattice which depends on the magnetic penetration depth and vortex core radius. The size of the vortex core varies strongly with magnetic field. In particular in a low field of 10.8 mT the core radius is much larger than the coherence length. The possible origin of these giant vortices is discussed.