R. Hilfer and R. Orbach

Department of Physics and Solid State Science Center,
University of California, Los Angeles, CA 90024, USA

###### Abstract.

We discuss correlated hopping motion
in a dynamically disordered environment.
Particles of type A with hopping rate 1/τA
diffuse in a background of B-particles with hopping rate 1/τB.
Double occupancy of sites is forbidden.
Without correlations the limit τB/τA→∞
corresponds to diffusion on a percolating network,
while the case τB=τA is that of self-diffusion in a lattice gas.
We consider also the effect of correlations.
In general these will change the transition rate of the A-particle
to the previously occupied site as compared to the rate
for transitions to all other neighbouring sites.
We calculate the frequency dependent conductivity for this model with arbitrary
ratio of hopping rates and correlation strength.
Results are reported for the two dimensional hexagonal lattice
and the three dimensional face centered cubic lattice.
We obtain our results from a generalization of the effective medium approximation
for frozen percolating networks.
We predict the appearance of new features in real and imaginary part
of the conductivity as a result of correlations.
Crossover behaviour resulting from the combined effect
of disorder and correlations leads to apparent
power laws Reσω∼ωy with 0<y<1
over roughly one to two decades in frequency.
In addition we find a crossover between a low frequency regime
where the response is governed by the rearrangements in the geometry
and a high frequency regime where the geometry appears frozen.
We calculate the correlation factor for the d. c. limit
and check our results against Monte Carlo simulations
on the hexagonal and face centered cubic lattices
for the case τ=1 and τ=∞.
In all cases we find good agreement.