Temperature depending studies of multiferroic TbMnO$_3$ by spectral ellipsometry and Raman scattering

TbMnO$_3$ has been studied in order to understand the orbital, structural, and magnetic structure as a function of temperature by Raman scattering and spectral ellipsometry. We present optical spectra, in which anomalies can be observed above and below the N\'eel temperature of 41 K. Below T$_N$, TbMnO$_3$ develops a complex magnetic structural phase. Another characteristic temperature of this material is T$_F$ = 28 K. Below this temperature, TbMnO$_3$ develops a multiferroic state. From the ellipsometry data, we find a rearrangement of the spectral weight at 41 K. Raman scattering with an excitation energy of 1.91 eV shows Jahn-Teller mode changes below T$_N$. Below T$_F$ we also detected a new ferroelectric mode at 128 cm$^{-1}$. In the ellipsometry data as well as in the Raman spectra we identify another critical temperature at T$^*$ $\sim$ 150 K. From our observations we can conclude that there is a strong coupling between the electronic and lattice degrees of freedom, which influence both the formation of the multiferroic state and the Jahn-Teller-distortions.