Evolution of the pseudogap temperature dependence in YBa₂Cu₃O_7-δ films under the influence of a magnetic field

The evolution of the temperature dependence of the pseudogap Delta*(T) in optimally doped (OD) YBa2Cu3O7-delta (YBCO) films with the superconducting critical temperature T-c = 88.7 K under the influence of a magnetic field B has been studied in detail. It has been established that the shape of Delta*(T) for various B over the entire range from the pseudogap opening temperature T* to T01, below which superconducting fluctuations occur, has a wide maximum at the BEC-BCS crossover temperature Tpair, which is typical for OD films and untwinned YBCO single crystals. T* was shown to be independent on B, whereas Tpair shifts to the low-temperature region along with the increase in B, while the maximum value of Delta*(Tpair) 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, for B > 0.5 T, a minimum appears on Delta*(T) at Tmin, which becomes very pronounced with a further increase in B. As a result, the overall value of Delta*(T) decreases noticeably most likely due to the pair-breaking effect. A comparison of Delta*(T) near T-c with the Peters-Bauer theory shows that the density of fluctuating Cooper pairs actually decreases from < n up arrow n down arrow > approximate to 0.31 at B = 0 to < n up arrow n down arrow > approximate to 0.28 in the field of 8 T. The observed behavior of Delta*(T) around Tmin 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 Tc.