software to fit optical spectra - Quantum Materials Group

The model file stores information about the parameters in the internal RefFIT format,
which cannot be viewed or accessed by other programs. However, every time a model is saved,
an extra file is created automatically, which lists all parameter values and error bars in the text
format, that can be opened, e.g, by the Notepad program. This file has the same name as the
model file, but the extension “ASC”. For instance, after saving a model under file name
“MOD12.RFM”, a text file “MOD12.ASC” is created. Note, that error bars are not calculated,
unless the corresponding fitting option is specified (section 4.10.2).
There is another ‘side’ action, which accompanies model saving. Namely, a new line is
added to a file called “PAR.DAT”, which lies in the current directory. This line has the
following format “ModelFileName Chi-Square Einf Wo[1] Wp[1] G[1] Wo[2] Wp[2] G[2]
Wo[3] Wp[3] G[3] …”, where ModelFileName is the filename of the model saved, Chi-Square
is the current value of the fitting chi-square, Einf is the value of ε ∞ , Wo[1], Wp[1] and G[1]
are the values parameter ω 0 , ω p and γ from the first row in the Lorentzian table, Wo[2],
Wp[2] and G[2] are the parameters from the second row and so on. File “PAR.DAT” is very
useful, when, for example, the spectra for different temperatures are fitted in a cycle and for
each temperature the model is saved under a different name. Then one can easily plot the
temperature dependence of model parameters, such as plasma frequency or conductivity gap,
using any standard scientific graphics package (see an example in section 3.3.2).
A saved model can be loaded into an existing model window. One should create a new
model window or activate the existing one and press the F3 button. After that the model file to
be loaded has to be selected. It is a good idea to save models after each modification (e.g.,
fitting run) to avoid the potential loss of data.
4.5.2. Editing models
The rows in the ‘Lorenzian’ section can be added and deleted by the buttons and
, or, alternatively, by the Insert and Delete keys. A row can be moved up and down by
clicking and . They can also be sorted in different ways by mouse clicking on the
column headers. For instance, in order to sort the Lorenzians by increasing transversal
frequencies one should click on the first column header “Wo”.
4.5.3. Model types
The meanings of parameters and output optical spectra depend on the model type. At the
moment there exist several types of models. The most common one is the “Dielectric Function”
type (see section 4.6). Each newly created model is assumed to be of this type. In this case the
meaning of the parameters exactly corresponds to the text labels in the Model window.
Initially the “Dielectric Function” was the only model type in RefFIT, but recently other
types were introduced (see section 4.7). Due to technical reasons it was easier to keep the same
appearance of the Model window for other model types and use special ‘tricks’ in order to
explain RefFIT that the model type is not “Dielectric Function”.
According to the convention, the way to change the model type is to put a negative
integer number (code) into the parameter “Einf” and make this parameter fixed (look red on the
screen). The code -1 corresponds to the extra (special) model type 1, code -2 corresponds to the
Guide to RefFIT Page 55