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

TL;DR

This study investigates how strong magnetic fields affect resistivity and fluctuation conductivity in YBa₂Cu₃O₇−δ films, revealing a transition from 3D to 2D fluctuation behavior and the possible formation of a vortex lattice.

Contribution

It provides new insights into the fluctuation conductivity crossover and vortex state formation in YBCO films under high magnetic fields.

Findings

Magnetic field increases resistivity and transition width up to 2.5 T.

Suppression of Maki–Thompson fluctuation contribution at 3 T.

Evidence of 2D vortex lattice formation at high 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 $B$.