Hard mode spectroscopy for the investigation of structural and superconducting phase transitions

Abstract The temperature evolution of high frequency phonons and bound charge excitations is used to quantify the order parameter of phase transitions. The characteristic length of the vibrational excitations is in the order of the lattice repetition length, so that structural states are analysed on a local length scale. The effects of chemical mixing and short range order are demonstrated using ferroelastic Pb 3 (PO 4 ) 2 antiferroelectric CaTiSiO 5 and superconducting YBa 2 Cu 3 O 7 as examples. The advantages and disadvantages of absorption measurements compared with reflection spectra are discussed.