Anisotropy, Phonon Modes, and Lattice Anharmonicity from Dielectric Function Tensor Analysis of Monoclinic Cadmium Tungstate

Authors

Alyssa Mock, University of Nebraska-LincolnFollow
Rafal Korlacki, University of Nebraska at LincolnFollow
Sean Knight, University of Nebraska-LincolnFollow
Mathias Schubert, University of Nebraska-LincolnFollow

Document Type

Article

Date of this Version

2017

Citation

Physical Review B 95, 165202 (2017)

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.95.165202

DOI: 10.1103/PhysRevB.95.165202

Comments

Copyright 2016 by the American Physical Society

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.95.165202

DOI: 10.1103/PhysRevB.95.165202

Abstract

We determine the frequency dependence of four independent Cartesian tensor elements of the dielectric function for CdWO₄ using generalized spectroscopic ellipsometry within mid-infrared and far-infrared spectral regions. Different single crystal cuts, (010) and (001), are investigated. From the spectral dependencies of the dielectric function tensor and its inverse we determine all long-wavelength active transverse and longitudinal optic phonon modes with A_u and B_u symmetry as well as their eigenvectors within the monoclinic lattice. We thereby demonstrate that such information can be obtained completely without physical model line-shape analysis in materials with monoclinic symmetry. We then augment the effect of lattice anharmonicity onto our recently described dielectric function tensor model approach for materials with monoclinic and triclinic crystal symmetries [M. Schubert et al., Phys. Rev. B 93, 125209 (2016)], and we obtain an excellent match between all measured and modeled dielectric function tensor elements. All phonon mode frequency and broadening parameters are determined in our model approach. We also perform density functional theory phonon mode calculations, and we compare our results obtained from theory, from direct dielectric function tensor analysis, and from model line-shape analysis, and we find excellent agreement between all approaches. We also discuss and present static and above reststrahlen spectral range dielectric constants. Our data for CdWO₄ are in excellent agreement with a recently proposed generalization of the Lyddane-Sachs-Teller relation for materials with low crystal symmetry [M. Schubert, Phys. Rev. Lett. 117, 215502 (2016)].