Optical Spectroscopy of Orbital and Magnetic Excitations in Vanadates and Cuprates

Within the scope of this thesis the low-energy excitations of undoped Mott insulators RVO(3) with R = Y, Ho, and Ce, (Sr,Ca)CuO(2) and La(8)Cu(7)O(19) have been investigated by means of optical spectroscopy. The compounds RVO(3) with R = rare-earth ion recently have attracted a lot of interest because of their unusual structural, orbital, and magnetic properties. The compounds undergo a series of temperature-induced phase transitions accompanied by a change of orbital and magnetic order. Furthermore, it has been proposed that YVO(3) represents the first realization of a one-dimensional orbital liquid and an orbital Peierls phase, with a transition to an orbitally ordered phase at lower temperatures. In this thesis, we present the optical conductivity sig(w) of RVO(3) with R = Y, Ho, and Ce for energies from 0.1 to 1.6 eV as a function of temperature (10-300 K) and polarization of the incident light parallel to the crystallographic axes (sig_a, sig_b, sig_c). Our main experimental result is the observation of two absorption features at 0.55 eV in sig_a and 0.4 eV in sig_c which are assigned to collective orbital excitations, in contrast to conventional local crystal-field transitions. Altogether our results strongly suggest that in RVO(3) with R = Y, Ho, and Ce the orbital exchange interactions play a decisive role. In a second study we have investigated the magnetic excitations of low-dimensional quantum magnets, namely the spin chain (Sr,Ca)CuO(2) and the five-leg ladder La(8)Cu(7)O(19). For (Sr,Ca)CuO(2), two absorption features around 0.4 eV in sig_c (chain direction) and sig_b (inter-chain direction) are identified as magnetic contributions to the optical conductivity. The analysis of sig_c enables the very precise determination of the nearest-neighbor exchange coupling Jc as a function of temperature and Ca substitution. We have found Jc = (227 +- 4) meV for SrCuO(2) at low temperatures and no effect on Jc upon Ca-substitution of 10%. Furthermore, we ascertained that the theoretically predicted two-spinon-plus-phonon contribution describes roughly 74-97% of the observed spectral weight. We suggest that the remaining weight has to be ascribed to multi-spinon excitations. To explain the small magnetic absorption features in sig_b we sketch a new mechanism which gives rise to a dipole moment perpendicular to the chains when holes are virtually exchanged along the chain. In particular, we point out the possible relevance of this contribution to the optical conductivity of many quasi-1D quantum spin systems. Our data of La(8)Cu(7)O(19) are the first experimental result on the magnetic excitations of n-leg ladders (n > 2) at high energies. Amongst others, we compared our data with the well-investigated spectrum of the two-leg ladder and found a surprising resemblance. Although a detailed understanding of the observed line shape of the 5-leg ladder is missing up to now, our data strongly suggest the existence of bound states of magnetic excitations also in this system. The comparison of different low-dimensional S = 1/2 systems suggests that the magnetic excitations in the high-energy continua are similar and develop smoothly from 1D to 2D.