Magnetic-field dependence of the oxygen isotope effect on the magnetic penetration depth of hole-doped cuprate superconductors

The magnetic-field dependence of the oxygen isotope (O-16/O-18) effect (OIE) on the in- plane magnetic- field penetration depth gimel(ab) was studied in the hole- doped high- temperature cuprate superconductors YBa2Cu4O8, Y0.8Pr0.2Ba2Cu3O7-infinity, and Y0.7Pr0.3Ba2Cu3O7-delta. It was found that the OIE on gimel(ab) decreases by more than a factor of 2 with increasing magnetic field from mu H-0= 0.2 T to mu H-0= 0.6 T. This effect can be explained by the isotope dependence of the in- plane charge carrier mass m*(ab).