Optical Properties and Electronic Transitions of YbFe$_{\mathrm{2}}$O$_{\mathrm{4\thinspace }}$Thin Films*

We present growth, structural, optical and electronic properties of Ytterbium-Iron-oxide, YbFe$_{\mathrm{2}}$O$_{\mathrm{4}}$, thin films. YbFe$_{\mathrm{2}}$O$_{\mathrm{4\thinspace }}$exhibits the unique physical properties due to the presence of Fe$^{\mathrm{2+}}$ and Fe$^{\mathrm{3+}}$ valance states within the triangular lattice structure. We prepared the compound by a solid state reaction starting with stoichiometric proportion of Yb$_{\mathrm{2}}$O$_{\mathrm{3}}$, Fe$_{\mathrm{2}}$O$_{\mathrm{3}}$, and FeO. The material was then deposited on c-axis sapphire substrates using a reactive electron beam deposition technique to produce \textasciitilde 100 nm thick films. Absorption, reflectance, and transmittance of the YbFe$_{\mathrm{2}}$O$_{\mathrm{4}}$ films were measured in the temperature range of 10 -- 450 K. The optical spectra contain Fe d to d on-site transitions as well as O 2p to Fe 3d, Yb 6$s$, and Yb 5$d$ charge-transfer transitions. In addition, the optical spectra exhibit strong temperature dependence, indicating evidence of a structural distortion of the crystal structure at \textasciitilde 180 \textpm 10 K as well as a magnetic transition at $\sim $250 K. The detail analysis of the optical data in comparison with theoretical studies will be presented.