The ABINIT Users Mailing List ( CLOSED )

[Matthieu Verstraete <mjv500@york.ac.uk>]

> the matrix element. If I understand correctly, the matrix element is
> calculated through the ground state wavefunction at least for getting
> the lamda (e-p constant).
yes: these are matrix elements of the GS wfct from the phonon run. Any 
temperature or energy window you select are post-hoc and concern 0K GS 
states.

> So if the system is excited by a laser pulse, can the matrix element be 
> the same? I mean, since the screen potential is from the fact that the 
> electrons react to contribute to the effective potential. Wouldn't this 
> effective potential change when the electrons are excited in terms of 
> non-zero electron temperatures, temperature dependent?
Yes, but it depends which time scales you are looking at. In a first 
approximation, immediately after the pulse, the electrons are excited in a 
really unequilibrated way. Then (normally) electronic processes allow 
thermalization of the electron population and the distribution of the 
states should be fermi-like. Later, the e-p coupling kicks in and the heat 
is progressively (sometimes quite quickly!) transferred or shared with the 
lattice.


> This sounds that one has to do calculation of
> the wavefunctions at conditions of non-zero electron temperatures.
Yes, but it's always more complex
1) your distribution of hot electrons will not be spatially homogeneous
2) the distribution of energies will not be a simple fermi one. All of the 
standard distributions correspond to thermodynamic potentials with a given 
equilibrium and ensemble. You should be completely out of equilibrium, and 
the time scale analysis above presumes you will go rapidly to a sequence 
of pseudo equilibrium states with thermalized electrons then phonons 
etc...


> And here comes another questions while I was reading your previous email 
> about the physical electron temperature tphysel and the laser paper by 
> CEA group PRL 2006.  I assume that they specify the value of tphysel to 
> be 6eV in abinit?
tphysel is probably not necessary. To my knowledge they used just a fermi 
distribution. tphysel is for intermediate physical temperatures, when the 
physical smearing is not enough to improve your kpoint convergence. Then 
you can add a second (rapidly decaying) smearing function in order to 
improve your k convergence without destroying your electronic temperature 
dependence.

> BTW, the umklapp processes is included in the calculations of matrix element?
hmmmm. I'm not sure I know the answer to that one. You mean for q vectors 
out of the first BZ? I'll ponder that some more. Anyone have an idea?


Matthieu