C.D. Mukherjee and K.K. Bardhan

ECMP Division

Saha Institute of Nuclear Physics

1/AF Bidhannagar

Kolkata 700 064

India

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.