Tutorials

JDOS and OPTICS

JDOS

See examples/Si2_JDOS. This is a simple example of using optados for calculating joint electronic density of states. We choose to recalculate the Fermi level using the calculated DOS, rather than use the Fermi level suggested by castep, and so EFERMI: OPTADOS is included in the Si2.odi file.

  1. Execute castep and optados using the example files. The JDOS is written to Si2.jadaptive.dat. A file suitable for plotting using xmgrace is written to Si2.jadaptive.agr.
  2. Check the effect of changing the sampling by increasing and decreasing the value of JDOS_SPACING in the Si2.odi file.

OPTICS

Two sets of example files are provided for calculations of optical properties. For each example, the castep files containing all the cell and simulation parameters are included, along with an optados input file. We assume that the reader is familiar with the previous sections on DOS and JDOS.

Silicon

examples/Si2_OPTICS/ This is a simple example of using optados to calculate the optical properties of crystalline silicon, which is an insulator.

INSTRUCTIONS:

  • Execute optados to calculate the optical properties. Several *.dat files are produced:
    • Si2_OPTICS_absorption.dat : This file contains the absorption coefficient (second column) as function of energy (first column).
    • Si2_OPTICS_conductivity.dat : This file contains the conductivity outputted in SI units (Siemens per metre). The columns are the energy, real part and imaginary part of the conductivity respectively.
    • Si2_OPTICS_epsilon.dat : This file contains the dielectric function. The columns are the energy and real and imaginary parts of the dielectric function respectively. The file header also includes the result of the sum rule

      Neff is the effective number of electrons contributing to the absorption process, and is a function of energy.
    • Si2_OPTICS_loss_fn.dat : This file contains the loss function (second column) as a function of energy (first column). The header of the file shows the results of the two sum rules associated with the loss function

      and
    • Si2_OPTICS_reflection.dat : This file contains the reflection coefficient (second column) as a function of energy (first column).
    • Si2_OPTICS_refractive_index.dat : This file contains the refractive index. The columns are the energy and real and imaginary parts of the refractive index respectively.

Corresponding *.agr files are also generated which can be plotted easily using xmgrace.

  • Change parameters JDOS_SPACING and JDOS_MAX and check the effect on the optical properties. Note: all of the other optical properties are derived from the dielectric function.
  • The optados input file has been set up to calculate the optical properties in the polycrystalline geometry (optics_geom = polycrystalline). It is possible to calculate either polarised or unpolarised geometries, or to calculate the full dielectric tensor. To calculate the full dielectric tensor set optics_geom = tensor. This time only the file Si2_OPTICS_epsilon.dat is generated. The format of this file is the same as before (the columns are the energy and the real and imaginary parts of the dielectric function respectively), but this time the six different components of the tensor are listed sequentially in the order εxx, εyy, εzz, εxy, εxz and εyz.
  • Additional broadening can be included in the calculation of the loss function. This is done by including the keyword optics_lossfn_broadening in the optados input file. If you include this keyword and re-run optados, you will find that the file Si2_OPTICS_loss_fn.dat now has three columns. These are the energy, unbroadened spectrum and broadened spectrum respectively.

Aluminium

  • Aluminium is a metal so we need to include both the interband and intraband contributions to the dielectric function. To include the intraband contribution optics_intraband = true must be included in the optados input file. When you run optados, the same files are generated as when only the interband term is included.
  • The Al_OPTICS_epsilon.dat file has the same format as before, but it now contains sequentially the interband contribution, the intraband contribution and the total dielectric function. The file Al_OPTICS_epsilon.agr only contains the interband term. In the same way, Al_OPTICS_loss_fn.dat contains the interband contribution, intraband contribution and total loss function. All other optical properties are calculated from the total dielectric function and the format of the output files remains the same.
  • In the case where the dielectric tensor is calculated and the intraband term is included, only the Al_OPTICS_epsilon.dat file is generated. As before it contains each component, but this time it lists sequentially the interband contribution, intraband contribution and total dielectric function for each component.
  • This time, if additional broadening for the loss function is included by using the key word optics_lossfn_broadening, AL_OPTICS_loss_fn.dat will contains four sequential data sets. These are the interband contribution, the intraband contribution, the total loss function without the additional broadening and the broadened total loss function.