Quasiparticle Excitations outside the Vortex Cores in MgB$_2$ Probed by Muon Spin Rotation

TL;DR

This study uses muon spin rotation to investigate the magnetic penetration depth in MgB$_2$, revealing delocalized quasiparticles and anisotropic superconducting gaps through field and temperature dependence analysis.

Contribution

It provides microscopic evidence of delocalized quasiparticles and anisotropic energy gaps in MgB$_2$ using muon spin rotation measurements.

Findings

Detection of field-dependent magnetic penetration depth

Observation of Doppler shift effects indicating anisotropic gaps

Evidence of delocalized quasiparticles outside vortex cores

Abstract

The magnetic penetration depth $\lambda$ in the mixed state of MgB$_2$ has been determined microscopically from the field distribution around magnetic vortices probed by muon spin rotation. Both the temperature and magnetic field dependence of $\lambda$ strongly suggest the presence of delocalized quasiparticles around the vortex cores. In particular, the effect of Doppler shift has been clearly observed as a finite gradient of the field dependence of $\lambda$, strongly suggesting an anisotropic order parameter with the region of a vanishingly small energy gap.