Evolution of the pseudogap temperature dependence in YBa${}_2$Cu${}_3$O${}_{7-\delta }$ films under the influence of a magnetic field

The evolution of the temperature dependence of pseudogap $\mathrm{\Delta }$(T) in optimally doped (OD) YBa${}_2$Cu${}_3$O${}_{7-\delta }$ (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 $\mathrm{\Delta }$(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 $\mathrm{\Delta }$($T_{pair}$) 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 $\mathrm{\Delta }$(T) at $T_{min}$, which becomes very pronounced with a further increase in the field. As a result, the overall value of $\mathrm{\Delta }$(T) decreases noticeably most likely due to pair-breaking affect of a magnetic field. A comparison of $\mathrm{\Delta }$(T) near $T_c$ with the Peters-Bauer theory shows that the density of fluctuating Cooper pairs actually decreases from <<n${}_{\uparrow }$n${}_{\downarrow }$>> $\approx$ 0.31 at $B$ = 0 to <<n${}_{\uparrow }$n${}_{\downarrow }$>> $\approx$ 0.28 in the field 8 T. The observed behavior of $\mathrm{\Delta }$*(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 $T_c$.