Effect of a magnetic field up to 9 T on the temperature dependence of the pseudogap in YBa2Cu3O7–δ films

The work analyzes the effect of a magnetic field B directed along the c axis (B || c) up to 9 T on the resistivity ρ(T), fluctuation conductivity (FLC) σ′(T) and pseudogap Δ*(T) in thin films of YBa2Cu3O7–δ with a critical temperature of the superconducting transition Tc = 88.8 K. In contrast to previous work (Low Temp. Phys. 51, 1061 (2025) [Fiz. Nyzk. Temp. 51, 1180 (2025)]), where the magnetic field was directed along the ab plane (B || ab), the influence of the field on the sample is stronger due to the contribution of both spin-orbit and Zeeman effects. As expected, the magnetic field does not affect ρ(T) in the normal state. However, at the superconducting (SC) transition, it sharply increases ρ(T), the width of the SC transition ΔTc, and the coherence length along the c axis, ξc(0), but at the same time reduces both Tc and the range of the SC fluctuations ΔTfl. The FLC reveals a transition at a characteristic temperature T0 from the three-dimensional Aslamazov–Larkin (3D–AL) theory near Tc to the two-dimensional Maki–Thompson (2D–MT) fluctuation theory. However, at B = 1 T, the 2D–MT contribution is completely suppressed, and above T0, σ′(T) is unexpectedly described by the 2D–AL fluctuation contribution, indicating the formation of a two-dimensional vortex lattice in the film under the action of a magnetic field. It was found that the BEC–BCS transition temperature, Tpair, which corresponds to the maximum of the Δ*(T) dependence, shifts to the region of lower temperatures with increasing B, and the maximum value of Δ*(Tpair) decreases in fields B > 5 T. It was found that with increasing field, the low-temperature maximum near T0 is smeared and disappears at B > 1 T. In addition, above the Ginzburg temperature TG, for B > 1 T, a minimum appears on Δ*(T) at Tmin, which becomes very pronounced with a subsequent increase in B. As a result, the overall value of Δ*(TG) decreases noticeably, most likely due to the pair-breaking effect. At the same time, ΔТfl and ξс(0) increase sharply by approximately 3 times with increasing B above 1 T. Our results confirm the possibility of the formation of a vortex state in YBa2Cu3O7–δ by a magnetic field and its evolution with increasing B.