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Abstract

The magnetic vortex lattice (VL) of polycrystalline MgB2 has
been investigated by transverse-field muon spin relaxation (TF-
muSR). The evolution of the TF-muSR depolarization rate sigma,
which is proportional to the second moment of the field
distribution of the VL, has been studied as a function of
temperature and applied magnetic field. The low-temperature
value sigma exhibits a pronounced peak near H-ext = 75 mT. This
behavior is characteristic of strong-pinning-induced
distortions of the VL which put into question the
interpretation of the low-field TF-muSR data in terms of the
magnetic penetration depth lambda(T). An approximately constant
value of sigma, such as expected for an ideal VL in the London
limit, is observed at higher fields of H-ext>0.4 T. The TF-muSR
data at H-ext=0.6 T are analyzed in terms of a two-gap model.
We obtain values for the gap size of Delta(1)=6.0(3) meV
[2Delta(1)/k(B)T(c)=3.6(2)], Delta(2)=2.6(2) meV
[2Delta(2)/k(B)T(c) = 1.6(1)], acomparable spectral weight of
the two bands, and a zero-temperature value for the magnetic
penetration depth of lambda(ab) approximate to 100 nm. In
addition, we performed muSR measurements in zero external
field. We obtain evidence that the muon site (at low
temperature) is located on a ring surrounding the center of the
boron hexagon. Muon diffusion sets in already at rather low
temperature of T> 10 K. The nuclear magnetic moments can
account for the observed relaxation rate and no evidence for
electronic magnetic moments has been obtained.