Anisotropy and penetration depth of MgB2 from 11B NMR

The 11B NMR spectra in polycrystalline MgB2 were measured for several magnetic fields (1.97 and 3.15 T) as a function of temperature from 5 to 40 K. The composite spectra in the superconducting state can be understood in terms of anisotropy of the upper critical field, γH, which is determined to be 5.4 at low temperature. Using Brandt's algorithm (Brandt 1997 Phys. Rev. Lett. 78 2208) the full spectrum, including satellites, was simulated for the temperature 8 K and a magnetic field of 1.97 T. The penetration depth λ was determined to be 1152 ± 50 Å, and the anisotropy of the penetration depth, γλ, was estimated to be close to one at low temperature. Therefore, our findings establish that there are two different anisotropies for upper critical field and penetration depth at low temperatures.

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