EVIDENCE OF ANOMALOUS HOPPING AND TUNNELING EFFECTS ON THE CONDUCTIVITY OF A FRACTAL PT-FILM SYSTEM

The temperature dependence of the conductance and the nonlinear electrical response of a Pt-film percolation system, deposited on fracture surfaces of -Al2O3 ceramics, have been measured over three decades of sheet resistance. We find that in the temperature interval of T=77300 K, the resistance temperature coefficient =(1/R)dR/dT is not a constant, which is different from that for flat films. A dc I-V characteristic which strongly depends on the thickness of the film is found and it can be interpreted as a competition among the local Joule heating, hopping, and tunneling effects. The third-harmonic measurement suggests that the critical exponent comes from 1/f noise, which obeys the power-law dependences SR(p-pc)-, R(p-pc)-t, and then SRRw with w=/t, where SR is the mean square of resistance fluctuations, p the surface coverage fraction, and pc its percolation critical value. We find that w=0.450.06, which is lower than the flat-film exponent. This result indicates that the tunneling and hopping effects in the fractal samples are much stronger than that of flat films. © 1994 The American Physical Society.