Current-voltage characteristics and vortex dynamics in highly underdoped La_(2-x)Sr_(x)CuO₄

The temperature dependence of the nonlinear current-voltage ($I$-$V$) characteristics in highly underdoped La$_{2-x}$Sr$_{x}$CuO$_{4}$ ($x=0.07$ and 0.08) thick films has been studied in both zero and perpendicular magnetic fields $H$. Power-law behavior of $V(I)$ is found for both $H=0$ and $H \neq 0$. The critical current $I_{c}$ was extracted, and its temperature and magnetic field dependences were studied in detail. The Berezinskii-Kosterlitz-Thouless physics dominates the nonlinear $I$-$V$ near the superconducting transition at $H=0$, and it continues to contribute up to a characteristic temperature $T_x(H)$. Nonlinear $I$-$V$ persists up to an even higher temperature $T_{h}(H)$ due to the depinning of vortices.