Sie sind hier: ICP » R. Hilfer » Publikationen

# 5 Results

[190.1.1] The results for will be expressed via the so called correlation factor . [190.1.2] It is a measure which is often used to characterize the amount by which the self diffusion coefficient of a tracer particle in a lattice gas differs from its mean field value given by the vacancy concentration. [190.1.3] All results will be presented and discussed with the conventions , , and assuming a unit lattice constant. [190.1.4] In these dimensionless units is defined by the equation or, more generally,

 (5.1)

because for our hopping models . [190.1.5] In the limit exact results for the correlation factor are known for the case (and )[11, 13, 14]. [page 191, §0]    [191.0.1] For the hexagonal lattice one has , while for the fcc lattice. [191.0.2] These values will be used below to determine in eq. (4.2).

[191.1.1] We now have to solve eq. (3.10) in conjunction with eqs. (3.13) and (4.4) for the cases of interest, the hexagonal and face centered cubic lattice. [191.1.2] The lattice Greens functions for these situations are well known and can be expressed in terms of the complete elliptic integral of the first kind

 (5.2)

[191.1.3] For the hexagonal lattice we have

 (5.3)

For the fcc lattice the Greens function is given by

 (5.4)

where

 (5.5)

[191.1.4] We now solve eq. (3.10) iteratively on the computer. [191.1.5] We stop the iteration when the maximal relative error between two consecutive solutions falls below . [191.1.6] The resulting is then used to calculate according to eqs. (3.13) and (4.4). [191.1.7] The results are displayed in Figures 2 through 10.

[191.2.1] First we determine the proportionality constant in eq. (4.2). [191.2.2] This is achieved by requiring that the calculated correlation factor reproduces the known exact results for the limit . [191.2.3] We find for the hexagonal lattice, and for the fcc lattice. [191.2.4] These numbers are difficult to determine numerically, and we estimate the error to be roughly . [191.2.5] In all subsequent calculations we then use these values for .

[page 193, §1]    [193.1.1] In Figure 2 we have extracted the correlation factor from and plotted it versus blocker concentration . [193.1.2] All curves are for the uncorrelated case, i. e.  , on the fcc lattice. [193.1.3] We give results for and . [193.1.4] The crosses are the results of the Monte Carlo simulation for the case taken from Ref. [7]. [193.1.5] The circles are MC-results for and were taken from Ref. [10]. [193.1.6] Clearly there will be a discrepancy for this case because our results are approximate and for bond percolation while the simulation is exact and for site percolation. [193.1.7] An immediate problem is the value of for which the effective medium theory gives while the exact value is [8]. [193.1.8] If we simply use the exact value for in our calculation we obtain the dashed line displayed in Fig. 2 which is found to be in good agreement. [193.1.9] In Fig. 3 we plot vs.  for the hexagonal lattice. [193.1.10] Here the simulations have been taken from Ref. 10. [193.1.11] Keeping in mind that there are no free parameters (remember ) we find very good agreement for both lattices. [193.1.12] However, additional simulation data especially for in the range , and a more accurate determination of are required to fully evaluate the quality of the theoretical results. [page 194, §0]    [194.0.1] We now turn to the results for our primary objective, the frequency dependent diffusion coefficient.

[194.1.1] We consider first the uncorrelated case on the fcc-lattice. [194.1.2] In Figure 4 and Figure 5 we plot over ten decades in frequency on a log-log plot. [194.1.3] Figure 4 corresponds to a blocker concentration which is below the percolation threshold for vacancies, and shows the results for and . [194.1.4] Figure 5 has and . [194.1.5] From Figure 4 we see immediately that below the percolation threshold vanishes quadratically with frequency for . [194.1.6] This behaviour is well known from the analysis of the EM theory for the frozen case. [194.1.7] For we find a crossover to a constant proportional to . [194.1.8] This could have been expected because the blocker motion now allows the Aâparticle to get through the network although the vacancy concentration at each instant is below . [194.1.9] The mobility of the A-particles will be completely determined by the mobility of the blockers. [page 197, §0]    [197.0.1] The crossover frequency is seen to vary as . [197.0.2] This will be discussed further in the next section. [197.0.3] On the other hand above the vacancy threshold Figure 5 shows that the effect of the blocker rearrangement is only noticeable for values smaller than roughly . [197.0.4] Indeed one expects that the effect of blocker motion will become negligible if is much smaler than the d. c. conductivity in the frozen case which is proportional to .

[197.1.1] In Figures 6 and 7 we now turn to the correlated case, i. e.  . [197.1.2] Again we consider the fcc-lattice and plot the real (Fig. 6) and imaginary (Fig. 7) part of for the two concentrations and with fixed but variable . [197.1.3] We have chosen for the correlation factor. [197.1.4] The case is included as a reference and has been distinguished graphically by a dashed line. [197.1.5] As before the real part approaches a constant as irrespective of because is finite. [197.1.6] A new phenomenon however is the appearance of nonmonotonous behaviour for . [197.1.7] In this case is found to increase at low frequencies, and to decrease at high frequencies thereby exhibiting a maximum at a finite frequency. [197.1.8] In general is found to decrease as at high frequencies, and to increase at low frequencies. [197.1.9] The reverse is seen for . [197.1.10] This will also be discussed in the next section in more detail. [197.1.11] For the imaginary part of we find a change of sign for sufficiently small . [197.1.12] See for example the case , . [197.1.13] On the other hand for , there is no change of sign in the imaginary part while the real part still shows a maximum.

[197.2.1] The same calculations have been performed for the hexagonal lattice. [197.2.2] The results are displayed in Figures 8 and 9. [197.2.3] The only difference lies in the parameter values. [197.2.4] We have chosen different concentrations, , and fixed at . [197.2.5] The results show qualitatively the same behaviour as for the fcc-lattice.

[197.3.1] In Figure 10 we have plotted some results for the correlated case () in a log-log plot. [197.3.2] We show for , and on the hexagonal lattice. [197.3.3] We note that as a consequence of the correlations the crossover into the constant high frequency limit is smeared out and resembles a power law over more than a decade in frequency. [197.3.4] This is particularly apparent for the case .

[page 198, §0]    [198.1.1] For reference we have included a straight line into the graph whose slope is found to be roughly . [198.1.2] We remark that such a power law behaviour for the frequency dependent conductivity is often found experimentally in disordered systems. [198.1.3] As a particular example we mention --alumina where the ionic transport is also known to be highly correlated[34].