Raman phonon spectrum of the Dzyaloshinskii-Moriya helimagnet Ba2CuGe2 O7

The Raman spectrum of Ba2CuGe2O7, a tetragonal insulator which develops Dzyaloshinskii-Moriya helical magnetism below T-N = 3.2 K, has been detected at temperatures varying from 300 to 80 K in a single crystal, with the radiation polarized either in the ab plane or along the c axis of its tetragonal cell. Twenty-nine phonon lines out of the 35 allowed by the Raman selection rules for the present geometry were observed, and their vibrational frequencies were found in overall good agreement with those provided by shell-model calculations. Together with the previous report on the infrared-active phonons [A. Nucara et al., Phys. Rev. B 90, 014304 (2014)], the present study provides an exhaustive description, both experimental and theoretical, of the lattice dynamics in Ba2CuGe2O7.