Optical study of crystal-field excitation in (R)Ba2Cu3O7-delta single crystals

A large number of high-temperature superconductors (HTS) contain rare-earth (R) ions with 4f-electron multiplets which are split by the crystal field (CF) of the neighboring ligands and, in many cases, their energy difference s are in the range of optical phonons. When these two excitations (phonons and CF) have the same symmetry and are close in energy they can couple to e ach other, leading to a doublet structure at low temperature. Since the R i ons are located between the CuO2 superconducting planes, this crystal-field -phonon coupling can be used as a probe to investigate superconductivity re lated phenomena. In this paper, we discuss the coupling of phonons to CF ex citations in (R)Ba2Cu3O7-delta and demonstrate that Raman and infrared spec troscopy can be successfully used as a novel tool for CF studies in HTS, si nce they are not limited to low energy transfer and can be applied to study small single crystals.