Infrared-active phonon modes in monoclinic multiferroicMnWO4

Abstract

We report on polarized infrared reflectivity measurements of multiferroic, monoclinic MnWO4 between 10 K and 295 K. All five non-vanishing components of the dielectric tensor have been determined in the frequency range of the phonons. All infrared-active phonon modes (7 Au modes and 8 Bu modes) are unambiguously identified. In particular the strongest Bu 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 (Rac and Rp) is particularly helpful for a proper identification of the Bu modes. Using a generalized Drude-Lorentz model, we determine the temperature dependence of the phonon parameters, including the orientation of the Bu 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 Au mode and of the weakest Bu mode with increasing temperature. Below 20 K, the parameters of the Au phonon modes for E b show only subtle changes, which demonstrate a finite but weak coupling between lattice dynamics and magnetism in MnWO4. A quantitative comparison of our infrared data with the quasi-static dielectric constant ε b yields a rough estimate for the oscillator strength ∆εem 0.02 of a possible electromagnon for E b. Furthermore, we report on a Kramers-Kronig-consistent model which is able to describe non-Lorentzian line shapes in compounds with monoclinic symmetry.

A new polar symmetry of huebnerite (MnWO4) with ferrodistortive domains

“…We have detected two additional Raman signals below 890 cm À 1 in cross polarized spectra (Fig. 11), but their positions do not match well the reported IR-peak positions of MnWO 4 [35]. The additional Raman peaks near 642 and 784 cm À 1 most probably arise from combination modes of type A g þ B g or A u þB u .…”

Anisotropy, phonon modes, and free charge carrier parameters in monoclinicβ-gallium oxide single crystals

“…In this context, a 4th spectroscopic response function is described, whose physical meaning, however, remained unexplained. Kuz [40,41]. Spectra were obtained as a function of incident light polarization relative to the crystallographic axes.…”

“…These were obtained and reported in Refs. [40,41,43]. However, numerical integrations were required to guarantee Kramers-Kronig consistency for the 4 tensor element spectra, since neither of these elements can be obtained independently and as complex-valued functions from reflectance data analysis.…”

Electromagnon in theZ-type hexaferrite(BaxSr1−x)3Co2Kadlec1, Kadlec2, Vít3 et al. 2016Phys. Rev. B28080View full textAdd to dashboardCite

“…We note that, due to this fact, no new phonons below T C were reported also in other spin-induced ferroelectrics, only small shifts of phonon frequencies were observed due to the spin-phonon coupling. 26,27 Anyway, the factor-group analysis is useful for the discussion of electromagnon activity in both IR and Raman spectra below. Fig.…”