Abstract
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 YBa2Cu3O7-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, ξC(0), but decreases both Tc and the range of superconducting (SC) fluctuations ΔTfl. The fluctuation conductivity exhibits a crossover at characteristic temperature T0 from the 3D Aslamasov-Larkin (AL) theory near Tc 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, ΔTf sharply increases by a factor of about 7, and ξC(0) demonstrates a very unusual dependence on Tc 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.