Evaluating Exciton-Phonon Interactions and Assisted Absorption

Running EPW
1

Dear all

I am working on optical properties of indirect band gap materials and would like to ask a specific question regarding EPW.

Is it possible within EPW (or in combination with standard workflows) to evaluate phonon-assisted exciton absorption, exciton self-energy, and exciton lifetimes arising from electron–phonon interactions?

Any brief clarification or relevant insight would be highly appreciated.

Best Regards,
Agathyan

2

Hi,

It is possible to calculate exciton-phonon coupling using EPW in combination with BerkeleyGW. See the lecture by Dr. Dai here https://drive.google.com/file/d/1mXatJhFLgKZEUSGKhX0TGDU4N5JZveCC/view.

Within the EPW repo you can find an example for calculating exciton-phonon coupling and exciton-polaron formation energies. The link is here: https://gitlab.com/epw/q-e/-/tree/develop/EPW/examples/lif_hdf5?ref_type=heads.

However, the phonon assisted exciton absorption is still not implemented in EPW and may become available in a future version of the code.

Best regards,
Sabya.

3

Dear Sabya,

Thank you for pointing me to those resources.

Following up, for a material of GW Indirect Gap = 3.22 eV, I tried running the lindabs (absorption module) alongside the exciton-polaron flags after the finite-q grid calculations specified in the tutorial (input below), and plotted the optical absorption against my BSE results (see attached linear- and log-scale images).

The EPW spectrum (black) captures only phonon-assisted effects.
The BSE spectrum (blue) captures the strongly bound excitons.
The BSE+EPW spectrum (red) shows a phonon-assisted sub-gap tail.

Do these plots hold any physical significance?

Dataset | Peak 1 (eV) |

Phonon (EPW) - 4.8100
Exciton (BSE) - 2.6400
Exciton-Phonon (BSE+EPW) - 2.0100

Images:
epw_bse_comp.jpg
epw_bse_comp_log.jpg


Input section:

! Exciton-Polaron Flags
  lpolar       = .true.
  exciton      = .true.
  explrn       = .true.

  negnv_explrn = 20
  nbndv_explrn = 13   ! Match the valence band in BSE calcs.
  nbndc_explrn = 13   ! Match the conduction band in BSE calcs.

! Optical Absorption Flags and Energy Parameters
  lindabs      = .true.
  omegamin   = 0.05
  omegamax   = 10.00
  omegastep  = 0.01
  fsthick    = 11.0 ! eV
  temps      = 0 100 300 ! K
  degaussw     = 0.05 ! eV
  efermi_read  = .true.
  fermi_energy = -3.179447

  elecselfen   = .false.
  phonselfen   = .false.
  a2f          = .false.