Critical behaviour of thermal relaxation in composites

C.D. Mukherjee and K.K. Bardhan

ECMP Division
Saha Institute of Nuclear Physics
1/AF Bidhannagar
Kolkata 700 064

At a composition far above the percolation threshold, the resistance of a composite sample increases with time as a constant current is passed through the sample due to Joule heating. For a current less than the breakdown current, the resistance eventually reaches a steady value. The increase is found to be well described by a simple first-order exponential term with a characteristic relaxation time tau_h. Similarly, when the sample is allowed to cool down from the steady state by reducing the constant current to a small value the resistance relaxation is again described by a first-order exponential with a relaxation time tau_c which is however different from tau_h. Thus, relaxations during heating and cooling appear to possess different characteristic times. Both tau_h and tau_c exhibit critical behaviour as a function of the current I. Interestingly, it is found that the product (tau_h)(tau_c) is a constant independent of I. The relaxation time tau_h diverges with I as (1-I^2/{{I_b}^2})^{-alpha} where I_b is the breakdown current and alpha is an exponent equal to 0.14. Consequently, tau_c goes to zero as I approaches I_b. Attempts to understand this unusual phenomena will be discussed.