Optical property and pseudogap study of FeSe thin films on different substrates

Among Fe-based superconductors, beta-FeSe has received a lot of attention from researchers because it has unique and complex physical properties under the simplest crystal structure. In this paper, FeSe films were prepared on CaF2, LiF, and SrTiO3 substrates by pulsed laser deposition (PLD) technique with the same growth parameters. The superconducting transition temperatures of the samples were tested at 11.44 K, 10.91 K, and 5.4 K, respectively, by applying electrodes on the surface of the films. We used X-ray diffraction (XRD) and atomic force microscopy (AFM) to obtain the crystal structure and roughness of the samples, respectively. Two phonons (A(1g), B-1g) of different samples were found in the Raman spectra, indicating that different strain effects were introduced by the substrates. In order to accurately reflect the properties of FeSe films, a dispersion model with Lorentz + Drude oscillations was established. The physical properties such as complex refractive index and scattering rate were analyzed by spectroscopic ellipsometry (SE) inversion. Meanwhile, we adopted the standard critical point (SCP) model to fit the second derivatives of the imaginary part of the dielectric function and calculate the transition energy. In combination with the first-principles calculation based on density functional theory (DFT), the physical origin of the transition peaks was obtained. On this basis, we propose a new method to acquire the FeSe pseudogap by making the difference between the two transition energies observed by SE. The experimental result indicates that pseudogap of FeSe film is negatively correlated with the superconducting transition temperature. This work can offer a new perspective for the prediction of the superconductivity of FeSe and a reference for the application of FeSe on different substrates.