software to fit optical spectra - Quantum Materials Group

Figure 3-15 The model and experimental reflectivity curves after the automated fitting. The model contains only
two oscillators.
Honestly, I am still not quite happy with the fitting quality. It looks like that our sample
has a small metallic conductivity, because reflectivity is bending upwards at very low
frequencies. Let us put an extra Drude peak, corresponding to the response of charge carriers. It
is a special case of Lorenzian with zero transverse frequency ω 0 . What we should do is to
simply add an extra Lorenzian to the model (by clicking the button) and set ω 0
explicitly to zero. You may try to adjust ω p and γ in order to get a better match. If you find it
difficult, I suggest setting both of them to 500 cm -1 .
Now it makes sense to start the automated fitting again. But wait! We want ω 0 of the
Drude term to be always zero, while at the moment it is an adjustable parameter. There are
several ways to tell RefFIT that we do not want a parameter to be varied. Perhaps, the fastest is
to click on it with the right mouse button. Another way is to move the marker (light-green) to a
specific parameter and press the SPACEBAR. The third option is to uncheck “Active in fit” in
the Parameter window (Figure 3-4). The fixed parameter always looks red.
OK, we can start fitting. Wow, we’ve got an improvement (Figure 3-16)! The model now
is shown at Figure 3-17. If you prefer to have oscillators to be sorted by their transverse
frequency ( ω 0 ), then click on the column header “Wo”. It is time to save the model again.
Guide to RefFIT Page 33