phonon-assisted optical absorption

[Jinlong.ma]

Dear all,

I wish your help about the code in EPW.

When I use EPW in QE 6.7 to calculate the phonon-assisted optical absorption, I found the code in "indabs.f90" is a little different from the formula publised in the original paper [PRL 108, 167402 (2012), Eq. (1)]. The "c*nr"(light speed times refractive index) is replaced by "omegap*wq" (photon energy times phonon frequency). I guess that a transformation is used. Anyone can tell me what is the relationship used for this transformation or approximation?

Bests,

[xiaozha]

Dear Jinlong,

It is not a transformation or approximation.

Please note that in the original paper, the equation is for the absorption coefficient and what's implemented in the code is the imaginary part of the dielectric function. This is why c and n_r are not there but an additional omegap is present. In principle, if you want to calculate absorption coefficient, you can either assume a constant n_r (In some cases this could be a reasonable approximation for the indirect spectra), or calculate explicitly calculate the real part using Kramers-Kronig relationship.

For phonon frequency, please see Eq(5) of the following paper: https://journals.aps.org/prb/abstract/1 ... .76.165108. It is g^{SE} that we need to calculate indirect optical properties and because the S parameters enters squared into epsilon_2, we have the 1/wq term.

Thanks!
Best!
Xiao Zhang