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

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.

arXiv comment4 pages, 3 figures included. to be published in J. Phys. Soc. Jpn

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