Re: [SI-LIST] : Atomic animation

Christian Schuster (schuster@iis.ee.ethz.ch)
Thu, 18 Feb 1999 09:45:46 +0100

Hi Doug,

> Introducing this concept for the first time, I'd recommend
> the mean free path model (I think I know where you're doing
> your presentation). I don't remember the exact mean free
> path for an electron in Cu but it's something like
> 400 angstroms (don't bet on this number). In contrast,
> super conductors can have mean free paths above a kilometer.
> That's why super conductors are so good at conducting.
>
> I'm sure someone here would have exact numbers,
> but if not, I'll try to dig them up.

you were pretty close to a value I found in a text book. There
it says for the mean free path (in brackets: mean time between
collisions):

Au: 410 Angstroms (tau = 2.9e-14 s)
Cu: 430 Angstroms (tau = 2.7e-14 s)
Ag: 560 Angstroms (tau = 4.1e-14 s)

The electric conductivity sigma is then given by:

sigma = (n * e^2 * tau) /m

where n is the electron density, e the electron charge, and
m the electron mass (a formula derived by Drude in 1900).

By the way, from the value of tau you can estimate at which
frequency this simple conductivity model fails (certainly above
10e14 Hz).

Hope this is of interest for you, kind regrads,

CH. Schuster

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Christian Schuster, Dipl.-Phys. IIS, ETH Zurich

Integrated Systems Laboratory
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