Feynman's variational path-integral model for the Fröhlich polaron. Calculates T-dep ab-initio polaron mobilities, and other polaron observables.
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README.md

PolaronMobility.jl

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DOI

PolaronMobility.jl is a Julia package which calculates the temperature-dependent polaron mobility for a material.

This is based on the Feynman variational solution to the Polaron problem. The electron-phonon coupling is treated as an effective α (alpha) Frohlich Hamiltonian parameter. The band structure is treated with an effective mass theory. The variational problem is solved numerically for finite-temperature free energies, rather than by the asymptotic solutions to the athermal energies, as often found in the textbooks.
The mobility is calculated numerically by integrating the polaron self-energy along the imaginary axis, as well as with the Kadanoff Boltzmann equation approximation, and the FHIP low-temperature asymptotic solution.

We provided parameters for various metal-halide Perovskites, and other interesting systems.

The motivation for developing these codes was to enable polaron mobility calculations on arbitrary materials. They also provide the only extant implementation of Feynman's variational method.
They offer a convenient basis for writing codes that build on these variational solutions.

More extensive documentation, is perhaps easiest to read and understand alongside the first paper: ArXiv:1704.05404 / Frost2017PRB.

Local installation

These codes require Julia >0.6 . They are structured as a full Julia package, but and are registered with the central METADATA package repository.

To install, type the following at the Julia REPL:

julia> import Pkg; Pkg.add("PolaronMobility")

Common errors

Often errors discoverd when trying to run these codes are related to changes in the interface to Optim.jl

This general appears as a MethodError with the internal Optim type, perhaps similar to: MethodError: Cannot convert an object of type Optim.BFGS{...

You may want to check the Optim package version and status: Pkg.status("Optim")

The present versions of the code require Optim 0.15-.

JuliaBox

There is an example notebook which can be run interactively on the (free) JuliaBox notebook server. This is the fastest way to calculate a few polaron parameters, if you do not have Julia.

  1. Login to Juliabox.
  2. Click on the 'git' button
  3. Enter https://github.com/jarvist/PolaronMobility.jl.git (this repository) as the git URL
  4. Auto-filled defaults are fine; click '+' and wait for it to clone
  5. You should now have a PolaronMobility directory in your JuliaBox, with the above notebook inside. 5b) As of 2nd Aug 2018, you a more modern version of the Optim package than comes as default on JuliaBox. Click on 'Packages' from the root page. Within the new pop-up window type Optim into the 'registered package box', click '+' to add it to your list, then click on the 'build' button. Hopefully a new Julia image with an updated Optim will be built.
  6. Once setup, you can save the Notebook URL as a bookmark: [https://juliabox.com/notebook/notebooks/PolaronMobility/JuliaBox-Example.ipynb]

Using

As an example:

using PolaronMobility
MAPIe=polaronmobility(300, 4.5, 24.1, 2.25E12, 0.12)

Will calculate the polaron mobility for methyl-ammonium lead halide perovskite (f=2.25 THz; ϵoptical=4.5; ϵstatic=24.1; effective-mass=0.12 electron-masses) at 300 K.

An abbreviated output should look like:

T: 300.000000 β: 2.41e+20 βred: 0.36 ħω  = 9.31 meV     Converged? : true
 VariationalParams v= 19.86  w= 16.96   ||   M=0.371407 k=106.835753    
 POLARON SIZE (rf), following Schultz1959. (s.d. of Gaussian polaron ψ )
     Schultz1959(2.4): rf= 0.528075 (int units) = 2.68001e-09 m [SI]
 Polaron Free Energy: A= -6.448815 B= 7.355626 C= 2.911977 F= -3.818788  = -35.534786 meV
Polaron Mobility theories:
    μ(FHIP)= 0.082049 m^2/Vs    = 820.49 cm^2/Vs
     Eqm. Phonon. pop. Nbar: 2.308150 
    μ(Kadanoff1963 [Eqn. 25]) = 0.019689 m^2/Vs      = 196.89 cm^2/Vs
    Tau=1/Gamma0 = 1.15751e-13 = 0.115751 ps
    μ(Hellwarth1999)= 0.013642 m^2/Vs   = 136.42 cm^2/Vs

Further details in the documentation.

Research outputs

The central output of this model are temperature-dependent polaron mobilities:

MAPI Polaron mobility, plotted vs experimental data

From the variational solution, you have characterised the polarons in your system. This gives access to the effective mass renormalisations (phonon drag), polaron binding energies, effective electron-phonon coupling parameters, etc.

Community guidelines

Contributions to the code (extending that which is calculated), or additional physical systems / examples, are very welcome.

If you have questions about the software, scientific questions, or find errors, please create a GitHub issue.

Reference

If you find this package (or snippets, such as the entered and tested free-energy expressions) useful for your work, please cite the paper Frost2017PRB.

@article{Frost2017,
  doi = {10.1103/physrevb.96.195202},
  url = {https://doi.org/10.1103/physrevb.96.195202},
  year  = {2017},
  month = {nov},
  publisher = {American Physical Society ({APS})},
  volume = {96},
  number = {19},
  author = {Jarvist Moore Frost},
  title = {Calculating polaron mobility in halide perovskites},
  journal = {Physical Review B}
}

These codes use the Optim.jl optimisation library to do the essential calculation in the Feynman variational theory. DOI