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

TL;DR

This study investigates the infrared phonon spectrum of Ba$_2$CuGe$_2$O$_7$, revealing detailed phonon modes, their atomic assignments, and temperature-dependent oscillator strength changes, indicating possible electron-phonon interactions.

Contribution

First comprehensive infrared phonon analysis of Ba$_2$CuGe$_2$O$_7$ including mode assignments and temperature effects, supported by shell-model calculations.

Findings

Observed 14 $E$ phonon modes in the $ab$ plane and all 10 $B_2$ modes along $c$ axis.

Discrepancy between experimental intensities and shell-model predictions suggests strong electron-phonon coupling.

Oscillator strengths increase at temperatures above $T_N$, implying changes in dielectric properties.

Abstract

The lattice dynamics of Ba$_2$CuGe$_2$O$_7$, a compound which develops Dzyaloshinsky-Moriya (DM) helical magnetism below $T_N$ = 3.2 K, has been studied by measuring the infrared reflectivity of a single crystal with the radiation polarized both in the $ab$ plane and along the $c$ axis of its tetragonal cell, from 7 K to 300 K. In this compound, where the unit cell has no inversion symmetry, fourteen $E$ phonon modes of the $ab$ plane, out of the eighteen predicted, and all the ten $B_2$ modes of the $c$ axis, have been observed. They have been assigned to the atomic motions by a comparison with shell-model calculations, which provided vibrational frequencies in good agreement with the experiment, while most calculated intensities turned to be much lower than the experimental values. This discrepancy has been tentatively explained by assuming strong electron-phonon interactions, a hypothesis supported by the failure of the $f$- sum rule if restricted to the phonon region. 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 of Ba$_2$CuGe$_2$O$_7$ may increase at those temperatures.