Yambopy tutorial: electron-phonon coupling

In this tutorial we will see some examples on how to read and analyse the data contained in the Yambo netCDF databases, which are not readily available as human readable outputs.

• Electron-phonon matrix elements (Yambo databases: ndb.elph_gkkp*, Yambopy class: YamboElectronPhononDB).

Command line: importing electron-phonon matrix elements

You may have seen how to calculate and import electron-phonon matrix elements in yambo in the electron-phonon tutorial.

With Yambopy, we can generate a yambo SAVE folder and import the matrix elements with a single command. Typing

$yambopy gkkp

will print the necessary documentation:

Produce a SAVE folder including elph_gkkp databases

Arguments are:
  -nscf, --nscf_dir  -> <Optional> Path to nscf save folder
  -elph, --elph_dir  -> Path to elph_dir folder
  -y, --yambo_dir    -> <Optional> Path to yambo executables
  -e, --expand       -> <Optional> Expand gkkp databases

The necessary quantum espresso databases are stored in ELPH_SAVES/QE_SAVES/hBN.save (nscf calculation) and ELPH_SAVES/QE_SAVES/elph_dir (matrix elements from the dfpt calculation). For this tutorial we also need to expand the electron-phonon matrix elements to the full Brillouin zone. Since this is a different calculation with respect to the previous section, please generate this SAVE in a different directory than the one you used for the previous SAVE, which should be the current directory. For example:

$mkdir yambo-with-elph
$cd yambo-with-elph

Now we can run yambopy as in the instructions:

$yambopy gkkp -nscf ../ELPH_saves/QE_saves/hBN.save -elph ../ELPH_saves/QE_saves/elph_dir --expand

This should generate a yambo SAVE folder which contains the ndb.elph_gkkp_expanded* databases.

Electron-phonon intro: plots of el-ph matrix elements on k-BZ and q-BZ

In this section we will use the script elph_plot.py and read the electron-phonon databases that you generated in the previous section.

In order to read the ndb.elph_gkkp_expanded* databases in python we use the Yambopy class YamboElectronPhononDB, which can be instanced like this:

yelph = YamboElectronPhononDB(ylat,folder_gkkp='path/to/elph/folder',save='path/to/SAVE')

(notice that it requires a previous instance of YamboLatticeDB).

Now, the yelph object contains phonon frequencies, phonon eigenvectors, qpoint information and, of course, the electron-phonon matrix elements g_{nm\nu}(k,q) where n, m are electron band states, \nu is a phonon branch, and k and q are the electronic and transfer momenta.

We can print the docstring of the YamboElectronPhononDB class with

print(yelph.__doc__)

to get an idea of the information stored and of its capabilities.

Now check the elph_plot.py script. You will see that it performs two plots:

• Plot of |g(k)| in the k-BZ for selected n,m,\nu and q [Kspace_Plot=True].
• Plot of |g(q)| in the q-BZ for selected n,m,\nu and k [Qspace_Plot=True].

You can change the electronic, phononic and momentum indices to see what happens.