Magnetic field dependence of the oxygen isotope effect on the magnetic   penetration depth in hole-doped cuprate superconductors
  The magnetic field dependence of the oxygen-isotope (^{16}O/^{18}O) effect (OIE) on the in-plane magnetic field penetration depth \lambda_{ab} was studied in the hole-doped high-temperature cuprate superconductors YBa_2Cu_4O_8, Y_0.8Pr_0.2Ba_2Cu_3O_7-\delta, and Y_0.7Pr_0.3Ba_2Cu_3O_7-\delta. It was found that \lambda_ab for the ^{16}O substituted samples increases stronger with increasing magnetic field than for the ^{18}O ones. The OIE on \lambda_ab decreases by more than a factor of two with increasing magnetic field from \mu_0H=0.2 T to \mu_0H=0.6 T. This effect can be explained by the isotope dependence of the in-plane charge carrier mass m^\ast_{ab}. 
R. Khasanov, A. Shengelaya, D Di Castro, D.G. Eshchenko, I.M. Savich,   K. Conder, E. Pomjakushina, J. Karpinski, S. Kazakov...