Loi d'échelle dans YBa$\\mathsf{_2}$Cu$\\mathsf{_3}$O${_{\\mathsf{7}-x}}$ céramique

Abstract

\nOur measurements $V$-$I$ in YBa$_2$Cu$_3$O$_{7-x}$ ceramic, in the limit $I=0$, confirm a second-order phase transition at a critical temperature $T_{\\rm c}(0)<T_{\\rm cs}$ where $T_{\\rm cs}$ is the critical temperature of the superconducting intragranular phase transition. As $I\\neq 0$, we show the existence of a crossover temperature $T^{*}(I)$ depending on the value of the current $I$. For the temperatures $T\\gg T^{*}(I)>T_{\\rm c}(0),\\ \\sigma\\sim\\bar{t}^{-\\gamma}$ with $\\gamma\\simeq 0.9$ and $\\bar{t}=(T-T_{\\rm c}(I))/T_{\\rm c}(0)$ where $T_{\\rm c}(I)$ corresponds to the critical temperature shift due to $I$. This conductivity critical behaviour is identical to the one observed as $T\\rightarrow T_{\\rm c}(0)^+$ and $I=0$. For the temperature range $T_{\\rm c}(I)<T\\ll T^{*}(I)<T_{\\rm c}(0)$ \n $ \\sigma\\sim A(I)\\bar{t}^{-\\tilde{\\gamma}}$ with $\\tilde{\\gamma}=1.3$ and $A(I)=I^{(\\tilde{\\gamma}-\\gamma)/\\phi}$.\n