Quasiparticle excitations outside the vortex cores in MgB2 probed by Muon spin rotation

The magnetic penetration depth lambda in the mixed state of MgB2 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.