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The lattice dynamics of Ba$_2$CuGe$_2$O$_7$, a compound which developsDzyaloshinsky-Moriya (DM) helical magnetism below $T_N$ = 3.2 K, has beenstudied by measuring the infrared reflectivity of a single crystal with theradiation polarized both in the $ab$ plane and along the $c$ axis of itstetragonal cell, from 7 K to 300 K. In this compound, where the unit cell hasno inversion symmetry, fourteen $E$ phonon modes of the $ab$ plane, out of theeighteen predicted, and all the ten $B_2$ modes of the $c$ axis, have beenobserved. They have been assigned to the atomic motions by a comparison withshell-model calculations, which provided vibrational frequencies in goodagreement with the experiment, while most calculated intensities turned to bemuch lower than the experimental values. This discrepancy has been tentativelyexplained by assuming strong electron-phonon interactions, a hypothesissupported by the failure of the $f$- sum rule if restricted to the phononregion. Indeed, we observe a remarkable increase in the oscillator strengths at$T$'s low but higher than $T_N$, which suggests that the dielectric constant ofBa$_2$CuGe$_2$O$_7$ may increase at those temperatures.

Infrared phonon spectrum of the tetragonal helimagnet Ba$_2$CuGe$_2$O$_7$