Infrared-active phonon modes in monoclinic multiferroicMnWO4

Abstract

We report on polarized infrared reflectivity measurements of multiferroic, monoclinic ${\mathrm{MnWO}}_{4}$ between 10 and 295 K. All five nonvanishing components of the dielectric tensor have been determined in the frequency range of the phonons. All infrared-active phonon modes (7 ${A}_{u}$ modes and 8 ${B}_{u}$ modes) are unambiguously identified. In particular, the strongest ${B}_{u}$ modes have been overlooked in previous studies, in which the monoclinic symmetry was neglected in the analysis. The combined analysis of reflectance data measured in different experimental geometries (${R}_{ac}$ and ${R}_{p}$) is particularly helpful for a proper identification of the ${B}_{u}$ modes. Using a generalized Drude-Lorentz model, we determine the temperature dependence of the phonon parameters, including the orientation of the ${B}_{u}$ modes within the $ac$ plane. The phonon parameters and their temperature dependence were discussed controversially in previous studies, which did not include a full polarization analysis. Our data do not confirm any of the anomalies reported above 20 K. However, in the paramagnetic phase we find a drastic reduction of the spectral weights of the weakest ${A}_{u}$ mode and of the weakest ${B}_{u}$ mode with increasing temperature. Below 20 K, the parameters of the ${A}_{u}$ phonon modes for $E\phantom{\rule{0.16em}{0ex}}\ensuremath{\parallel}\phantom{\rule{0.16em}{0ex}}b$ show only subtle changes, which demonstrate a finite but weak coupling between lattice dynamics and magnetism in ${\mathrm{MnWO}}_{4}$. A quantitative comparison of our infrared data with the quasistatic dielectric constant ${\ensuremath{\varepsilon}}_{b}$ yields a rough estimate for the oscillator strength $\ensuremath{\Delta}{\ensuremath{\varepsilon}}_{\mathrm{em}}⪅0.02$ of a possible electromagnon for $E\phantom{\rule{0.16em}{0ex}}\ensuremath{\parallel}\phantom{\rule{0.16em}{0ex}}b$. Furthermore, we report on a Kramers-Kronig-consistent model which is able to describe non-Lorentzian line shapes in compounds with monoclinic symmetry.