Penetration depth isotope effect and anisotropy in the low temperature mixed state of MgB_2

The magnetic penetration depth λ(0) in polycrystalline MgB₂ for different boron isotopes (^10B/^11B) was investigated by transverse field muon spin rotation (μSR). No boron isotope effect on the penetration depth λ(0) was found within experimental error (Δλ(0)/λ(0) = -0.7(8)%) in any field H, in contrast to the substantial oxygen isotope effect on λ(0) observed in cuprate high-temperature superconductors. This suggests non-adiabatic effects in MgB₂ to be negligibly small. μSR indicates a strong field dependence of λ at low temperatures, not attributable to pinning. Magnetization and torque measurements on single crystals yield H dependences of λ at 2 K in agreement with μSR and furthermore provide evidence for a strong H dependence of the λ anisotropy below ≈ 1 T, less above.