Charge and Spin electrodynamics of SrTiO3 and EuTiO3 studied by optical spectroscopy

This work is a threefold optical study of two perovskites: the very well known but still surprisingly intriguing SrTiO3 and its almost unexplored magnetic relative EuTiO3. A major extension to previous work is the very broad energy scale from the microwave to the ultra-violet region on which phenomena have been studied. Central in the thesis is the novel technique of time-domain spectroscopy in the THz range, a relatively unexplored range which clarifies and illustrates textbook phenomena of low energy physics. The thesis built up as follows. Chapter 1 introduces the theory of the interaction of light with matter and existing optical concepts needed in the following chapters. One part needs to be highlighted, it is a magnetic sum rule specially derived for the use in chapter 4, which relates the spectral weight of magnetic dipole transitions to the total magnetization of the system. Chapter 2 describes how SrTiO3 is tuned towards the formation of electron-phonon coupling by using chemical substitution of niobium as a control parameter. In chapter 3 it is shown that pristine SrTiO3 tuned by an external static electric field approaches a quantum ferroelectric state and shows an intriguing universal behavior below 20 K. Chapter 4 is devoted to influence of a magnetic field on the low energy optical properties of EuTiO3, which leads to a strong absorption due to the resonance of the electron spins, a so-called ferromagnetic spin resonance