Study of fluctuation conductivity in YBa$_2$Cu$_3$O$_7$--$\delta$ films in strong magnetic fields

Abstract

We report the effect of the \emph{ab}-plane magnetic field $B$ up to $8\,\mathrm{T}$ on the resistivity $\rho(T)$ and fluctuation conductivity $\sigma'(T)$ in YBa$_2$Cu$_3$O$_{7-\delta}$ thin films. As expected, up to $\sim 2.5\,\mathrm{T}$ the magnetic field monotonously increases $\rho$, the width of the resistive transition $\Delta T_c$, and the coherence length along the $c$ axis, $\xi_c(0)$, but decreases both $T_c$ and the range of superconducting (SC) fluctuations $\Delta _{\mathrm{fl}}$. The fluctuation conductivity exhibits a crossover at characteristic temperature $T_0$ from the 3D Aslamazov--Larkin (AL) theory near $T_c$ to the 2D fluctuation theory of Maki--Thompson (MT). However, at $B = 3\,\mathrm{T}$, the MT term is completely suppressed, and above $T_0$ $\sigma'(T)$ is unexpectedly described by the fluctuation contribution of 2D AL, suggesting the formation of a two-dimensional vortex lattice in the film under the action of a magnetic field. At the same time, $\Delta T_{\mathrm{fl}}$ sharply increases by a factor of about 7, and $\xi_c(0)$ demonstrates a very unusual dependence on $T_c$ when $B$ increases above $3\,\mathrm{T}$. Our results demonstrate the possibility of the formation of a vortex state in YBCO and its evolution with increasing $B$.