Scaling of thin-film Nd1.85Ce0.15CuO4-y resistivity-current isotherms at low fields: Implications for vortex phase transitions and universality

We have investigated a second-order phase transition of the vortex lattice in thin-film Nd1.85Ce0.15CuO4-y from 1 mT to 1 T applied magnetic fields. The dc resistivity-current density data for each field exhibit critical scaling consistent with a second-order vortex-liquid to vortex-solid phase transition. The high-field data support the vortex-glass model and have the same critical exponents as thin-film YBa2Cu3O7-δ. At low fields we observe a change in the critical exponents and universal functions. The critical regime diminishes as the field is decreased but remains nonzero even at 1 mT. These results suggest that the universality class of the transition changes at low fields