Line-shape analyses of magnon Raman spectra of a PrBa2Cu2.7Al0.3O7 single crystal.

Abstract

We present line-shape analyses of magnon Raman spectra in ${B}_{1g}$ geometry of a Pr${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{2.7}$${\mathrm{Al}}_{0.3}$${\mathrm{O}}_{7}$ crystal for various excitation energies. The description is based on the Heisenberg model of antiferromagnetism. Out of resonance, we describe the experimental data by taking into account phonons and a two-magnon excitation using the experimental superexchange energy $J=720$ ${\mathrm{cm}}^{\ensuremath{-}1}$ and a damping parameter $\ensuremath{\Gamma}=200$ ${\mathrm{cm}}^{\ensuremath{-}1}$ of the one-magnon states. After subtraction of the fitted line shapes, we identify three additional spectral features at 730 ${\mathrm{cm}}^{\ensuremath{-}1}$, 1170 ${\mathrm{cm}}^{\ensuremath{-}1}$, and 3500 ${\mathrm{cm}}^{\ensuremath{-}1}$. All three show enhanced intensities close to the two-magnon resonance at 2.79 eV excitation energy. This strongly suggests that these excitations have a magnetic rather than a phononic origin.