Non-Ohmic behaviour and phase transitions in YBa₂Cu ₃O_7-δ with high normal resistivity

The electrical resistivity in polycrystalline YBa₂Cu ₃O_{7- delta} with high normal resistivity has been investigated extensively in the temperature range 78<or=T<or=100 K by the four-probe method. The electrical resistivity exhibits four pronounced features: (i) the resistivity rho is independent of the current density above T _c0=92.24 K; (ii) for 84 K<T<T_c0 the I-V relation deviates slightly from Ohm's law, but if it is approximated by V approximately= alpha ₁I, the parameter alpha ₁ can be described by alpha ₁ approximately=exp(-2 mod b(T_c0-T_c)/(T-T _c) mod ¹2/), with T_c=78.9 K and b=3.2; (iii) in the temperature range T_c<T<84 K the I-V relation deviates strongly from Ohm's law and has the form V approximately=I^a(T), where the exponent a(T) increases with decreasing temperature and reaches a approximately=3 at T_c; and (iv) below T_c, the resistivity rho is reduced to zero. The non-Ohmic behaviour of this compound with high normal resistivity may be explained qualitatively in terms of current-induced dissociation of bound vortex pairs occurring in islands inside the 2D CuO layers. The two phase transitions of this compound at T_c0 and T _c may be explained in terms of a model used to describe the phase transitions of the stage-2 CoCl₂-graphite intercalation compounds at T_cl=8.0 K and T_cu=9.2 K, where islands play important roles.