Phonon order and reststrahlen bands of polar vibrations in crystals with monoclinic symmetry

Abstract

We derive from an eigendielectric displacement vector summation approach the frequency order of the polar phonon modes and the polarization-dependent structure of the reststrahlen bands within the monoclinic plane of materials with monoclinic crystal symmetry. We show that phonon modes occur in associated pairs of transverse and longitudinal optical modes, and that pairs either belong to inner or outer phonon modes. Inner mode pairs are nested within outer mode pairs. Outer mode pairs cause polarization-dependent reststrahlen bands. Inner mode pairs cause polarization-independent reststrahlen bands. The directional limiting frequencies within the Born-Huang approach are bound to within outer mode regions not occupied by inner mode pairs. Hence, an unusual phonon mode order can occur where both lower-frequency as well as upper-frequency limits for the directional modes can be both transverse and/or longitudinal modes. We exemplify our findings using experimental data for the recently unraveled case of monoclinic symmetry $\beta$-Ga${}_2$O${}_3$ [Phys. Rev. B~\textbf{93}, 125209 (2016)] and demonstrate excellent agreement with results from density functional theory calculations.

arXiv comment5 pages, 2 figures. Submitted to Physical Review Letters

References in corpus (9)

• Quantum ESPRESSO: a modular and open-source software project for quantum simulations of materials
• Phonons and related properties of extended systems from density-functional perturbation theory
• Ab initio pseudopotentials for electronic structure calculations of poly-atomic systems using density-functional theory
• Anisotropy, phonon modes, and free charge carrier parameters in monoclinic $\beta$-gallium oxide single crystals
• Band-to-band transitions, selection rules, effective mass and exciton binding energy parameters in monoclinic β-Ga2O3
• Laser frequency stabilization based on steady-state spectral-hole burning in Eu${}^{3+}$:Y${}_2$SiO${}_5$
• The Lyddane-Sachs-Teller relationship for polar vibrations in materials with monoclinic and triclinic crystal systems
• Anisotropy, phonon modes, and lattice anharmonicity from dielectric function tensor analysis of monoclinic cadmium tungstate
• Anisotropy and phonon modes from analysis of the dielectric function tensor and inverse dielectric function tensor of monoclinic yttrium orthosilicate

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