Evolution of the pseudogap temperature dependence in YBa_2Cu_3O_7-δ films under the influence of a magnetic field

The evolution of the temperature dependence of pseudogap Δ*(T) in optimally doped (OD) YBa_2Cu_3O_7-δ (YBCO) films with T_c = 88.7 K under the influence of a magnetic field B up to 8 T has been studied in detail. It has been established that the shape of Δ*(T) for various B over the entire range from the pseudogap opening temperature T* to T_01, below which superconducting fluctuations occur, has a wide maximum at the BEC-BCS crossover temperature T_pair, which is typical for OD films and untwinned YBCO single crystals. T* was shown to be independent on B, whereas T_pair shifts to the low temperature region along with increase of B, while the maximum value of Δ*(T_pair) remains practically constant regardless of B. It was revealed that as the field increases, the low-temperature maximum near the 3D-2D transition temperature T_0 is blurred and disappears at B > 5 T. Moreover, above the Ginzburg temperature T_G, which limits superconducting fluctuations from below, at B > 0.5 T, a minimum appears on Δ*(T) at T_min, which becomes very pronounced with a further increase in the field. As a result, the overall value of Δ*(T) decreases noticeably most likely due to pair-breaking affect of a magnetic field. A comparison of Δ*(T) near T_c with the Peters-Bauer theory shows that the density of fluctuating Cooper pairs actually decreases from <> ≈ 0.31 at B = 0 to <> ≈ 0.28 in the field 8 T. The observed behavior of Δ*(T) around T_min is assumed to be due to the influence of a two-dimensional vortex lattice created by the magnetic field, which prevents the formation of fluctuating Cooper pairs near T_c.