Variable-range hopping

Variable-range hopping, or Mott variable-range hopping, is a model describing low-temperature conduction in strongly disordered systems with localized charge-carrier states.^[1]

It has a characteristic temperature dependence of

\sigma =\sigma _{0}e^{-(T_{0}/T)^{1/4}}

for three-dimensional conductance, and in general for d-dimensions

\sigma =\sigma _{0}e^{-(T_{0}/T)^{1/(d+1)}}

Hopping conduction at low temperatures is of great interest because of the savings the semiconductor industry could achieve if they were able to replace single-crystal devices with glass layers.^[2]

Derivation

The original Mott paper introduced a simplifying assumption that the hopping energy depends inversely on the cube of the hopping distance (in the three-dimensional case). Later it was shown that this assumption was unnecessary, and this proof is followed here.^[3] In the original paper, the hopping probability at a given temperature was seen to depend on two parameters, R the spatial separation of the sites, and W, their energy separation. Apsley and Hughes noted that in a truly amorphous system, these variables are random and independent and so can be combined into a single parameter, the range $\textstyle {\mathcal {R}}$ between two sites, which determines the probability of hopping between them.

Mott showed that the probability of hopping between two states of spatial separation $\textstyle R$ and energy separation W has the form:

P\sim \exp \left[-2\alpha R-{\frac {W}{kT}}\right]

where α⁻¹ is the attenuation length for a hydrogen-like localised wave-function. This assumes that hopping to a state with a higher energy is the rate limiting process.

We now define $\textstyle {\mathcal {R}}=2\alpha R+W/kT$ , the range between two states, so $\textstyle P\sim \exp(-{\mathcal {R}})$ . The states may be regarded as points in a four-dimensional random array (three spatial coordinates and one energy coordinate), with the "distance" between them given by the range $\textstyle {\mathcal {R}}$ .

Conduction is the result of many series of hops through this four-dimensional array and as short-range hops are favoured, it is the average nearest-neighbour "distance" between states which determines the overall conductivity. Thus the conductivity has the form

\sigma \sim \exp(-{\overline {\mathcal {R}}}_{nn})

where $\textstyle {\overline {\mathcal {R}}}_{nn}$ is the average nearest-neighbour range. The problem is therefore to calculate this quantity.

The first step is to obtain $\textstyle {\mathcal {N}}({\mathcal {R}})$ , the total number of states within a range $\textstyle {\mathcal {R}}$ of some initial state at the Fermi level. For d-dimensions, and under particular assumptions this turns out to be

{\mathcal {N}}({\mathcal {R}})=K{\mathcal {R}}^{d+1}

where $\textstyle K={\frac {N\pi kT}{3\times 2^{d}\alpha ^{d}}}$ . The particular assumptions are simply that $\textstyle {\overline {\mathcal {R}}}_{nn}$ is well less than the band-width and comfortably bigger than the interatomic spacing.

Then the probability that a state with range $\textstyle {\mathcal {R}}$ is the nearest neighbour in the four-dimensional space (or in general the (d+1)-dimensional space) is

P_{nn}({\mathcal {R}})={\frac {\partial {\mathcal {N}}({\mathcal {R}})}{\partial {\mathcal {R}}}}\exp[-{\mathcal {N}}({\mathcal {R}})]

the nearest-neighbour distribution.

For the d-dimensional case then

{\overline {\mathcal {R}}}_{nn}=\int _{0}^{\infty }(d+1)K{\mathcal {R}}^{d+1}\exp(-K{\mathcal {R}}^{d+1})d{\mathcal {R}}

This can be evaluated by making a simple substitution of $\textstyle t=K{\mathcal {R}}^{d+1}$ into the gamma function, $\textstyle \Gamma (z)=\int _{0}^{\infty }t^{z-1}e^{-t}\,\mathrm {d} t$

After some algebra this gives

{\overline {\mathcal {R}}}_{nn}={\frac {\Gamma ({\frac {d+2}{d+1}})}{K^{\frac {1}{d+1}}}}

and hence that

\sigma \propto \exp \left(-T^{-{\frac {1}{d+1}}}\right)

Non-constant density of states

When the density of states is not constant (odd power law N(E)), the Mott conductivity is also recovered, as shown in this article.

Notes

↑ Mott, N.F. (1969). "Conduction in non-crystalline materials". Phil. Mag. 19: 835.
↑ P.V.E. McClintock, D.J. Meredith, J.K. Wigmore. Matter at Low Temperatures. Blackie. 1984 ISBN 0-216-91594-5.
↑ Apsley, N. & Hughes, H.P. (1974). "Temperature-and field-dependence of hopping conduction in disordered systems". Phil. Mag. 30: 963.

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Variable-range hopping

Derivation

Non-constant density of states

See also

Notes