Study of fluctuation conductivity in YBa₂Cu₃O_7–δ films in strong magnetic fields

We report the effect of the ab-plane magnetic field B up to 8 T on the resistivity ρ(T) and fluctuation conductivity σ'(T) in YBa₂Cu₃O_7–δ thin films. As expected, up to ~ 2.5 T the magnetic field monotonously increases ρ, the width of the resistive transition, ΔTc, and coherence length along the c axis, ξ_c(0), but decreases both T_c and the range of superconducting (SC) fluctuations ΔT_fl. The fluctuation conductivity exhibits a crossover at characteristic temperature T₀ from the 3D Aslamasov–Larkin (AL) theory near T_c to the 2D fluctuation theory of Maki–Thompson (MT). However, at B = 3 T, the MT term is completely suppressed, and above T₀ σ'(T) is unexpectedly described by the fluctuation contribution of 2D AL, suggesting the formation of a 2D vortex lattice in the film under the action of a magnetic field. At the same time, ΔT_fl sharply increases by a factor of about 7, and ξ_c(0) demonstrates a very unusual dependence on T_c when B increases above 3 T. Our results demonstrate the possibility of the formation of a vortex state in YBCO and its evolution with increasing B. (author)