Mott-Hubbard excitations in hole doped RTiO3 (R = rare earth ion or Y) studied via spectroscopic ellipsometry

Hole doping of the 3d1 Mott-Hubbard insulators RTiO3 (R=rare earth ion or Y) leads to an interesting phase diagram including spin, charge and orbital order. Temperature dependent (20 K<T<500 K) spectroscopic ellipsometry (0.75-5.5 eV) offers a very sensitive method to study these phenomena as the Mott-Hubbard excitations from the lower to the upper Hubbard-Band vertical stroke dn right angle vertical stroke dn right angle → vertical stroke dn+1 right angle vertical stroke dn-1 right angle are sensitively dependent on nearest-neighbor spin-spin and orbital-orbital correlations. The optical spectra of the undoped 3d1 compounds RTiO3 show a complex multipeak structure which has been attributed to the Mott-Hubbard excitations into the different 3d2 multiplets, while the temperature dependence has been analyzed on the basis of nearest-neighbor spin-spin correlations. We now study the impact of hole doping on the optical spectra. In a perfect charge ordered scenario of a half doped compound, the hopping of an electron to the neighboring lattice site should not result in a double occupancy. Thus, instead of the Hubbard energy U the excitation should only cost the nearest-neighbor repulsion V.