Universität zu Köln

Moeller, T., Becker, P., Bohaty, L., Hemberger, J. and Grueninger, M. (2014). Infrared-active phonon modes in monoclinic multiferroic MnWO4. Phys. Rev. B, 90 (15). COLLEGE PK: AMER PHYSICAL SOC. ISSN 2469-9969

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Abstract

We report on polarized infrared reflectivity measurements of multiferroic, monoclinic MnWO4 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 vertical bar vertical bar 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 quasistatic dielectric constant epsilon(b) yields a rough estimate for the oscillator strength Delta epsilon(em) less than or similar to 0.02 of a possible electromagnon for E vertical bar vertical bar b. Furthermore, we report on a Kramers-Kronig-consistent model which is able to describe non-Lorentzian line shapes in compounds with monoclinic symmetry.

Item Type:	Journal Article
Creators:	Creators Email ORCID ORCID Put Code Moeller, T. UNSPECIFIED UNSPECIFIED UNSPECIFIED Becker, P. UNSPECIFIED UNSPECIFIED UNSPECIFIED Bohaty, L. UNSPECIFIED UNSPECIFIED UNSPECIFIED Hemberger, J. UNSPECIFIED UNSPECIFIED UNSPECIFIED Grueninger, M. UNSPECIFIED UNSPECIFIED UNSPECIFIED
URN:	urn:nbn:de:hbz:38-425912
DOI:	10.1103/PhysRevB.90.155105
Journal or Publication Title:	Phys. Rev. B
Volume:	90
Number:	15
Date:	2014
Publisher:	AMER PHYSICAL SOC
Place of Publication:	COLLEGE PK
ISSN:	2469-9969
Language:	English
Faculty:	Unspecified
Divisions:	Unspecified
Subjects:	no entry
Uncontrolled Keywords:	Keywords Language TEMPERATURE-DEPENDENT RAMAN; CRYSTAL-STRUCTURE; OPTIC MODES; DISPERSION; SPECTRA; IR Multiple languages Materials Science, Multidisciplinary; Physics, Applied; Physics, Condensed Matter Multiple languages
URI:	http://kups.ub.uni-koeln.de/id/eprint/42591