Power laws in the resistive state in high-Tcsuperconductors

Temperature dependence of power laws in the mixed state of an oxide superconductor film, ${\mathrm{ErBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{x}}$, is investigated under the influences of both a transport current and an external magnetic field. A critical jump of the exponent n(T,H)=logV/logI is found at the universal value of n=3 in the presence of the magnetic field. The exponent m(T,I)=logV/logH shows a sharp bend universally at m=2. Clear evidence of the Kosterlitz-Thouless transition is given in the exponents of two kinds of power laws. The coexistence of these power laws and the relationship of the exponents are an important key in the total understanding of the resistive state in an oxide superconductor.