Non-Ohmic Behaviour and Phase Transitions in YBa sub 2 Cu sub 3 O sub 7-- delta With High Normal Resistivity

The electrical resistivity in polycrystalline YBa sub 2 Cu sub 3 O sub 7-- delta with high normal resistivity has been investigated extensively in the temperature range 78 < = T < = 100K by the four-probe method. The electrical resistivity exhibits four pronounced features: (i) the resistivity rho is independent of the current density above T sub c0 = 92.24K; (ii) for 84K < T < T sub c0 the I--V relation deviates slightly from Ohm's law, but if it is approximated by V approx = alpha sub 1 I, the parameter alpha sub 1 can be described by alpha sub 1 approx = exp(|2--b(T sub c0 --T sub c )/(T--T sub c )| exp 1Maps2 ), with T sub c = 78.9K and b = 3.2; (iii) in the temperature range T sub c < T < 84K the I--V relation deviates strongly from Ohm's law and has the form V approx = 1 exp a(T) , where the exponent a(T) increases with decreasing temperature and reaches a approx = 3 at T sub c ; and (iv) below T sub c , the resistivity rho is reduced to zero. The non-Ohmic behaviour of this compound with high 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 sub c0 and T sub c may be explained in terms of a model used to describe the phase transitions of the stage-2 CoCl sub 2 --graphite intercalation compounds at T sub c1 = 8.0K and T sub cu = 9.2K, where islands play important roles. 10 ref.--AA