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Angular Dispersion of Polar Phonons in Biaxial Crystals
Author(s) -
Graf L.,
Schaacx G.,
Unger B.
Publication year - 1972
Publication title -
physica status solidi (b)
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.51
H-Index - 109
eISSN - 1521-3951
pISSN - 0370-1972
DOI - 10.1002/pssb.2220540126
Subject(s) - degeneracy (biology) , polar , phonon , dispersion (optics) , condensed matter physics , curvature , dielectric dispersion , physics , transverse plane , dispersion relation , dielectric , function (biology) , raman scattering , raman spectroscopy , optics , quantum mechanics , geometry , permittivity , mathematics , bioinformatics , structural engineering , engineering , evolutionary biology , biology
The angular dispersion ( k ph ↫ ω/ c ) of polar phonons in the region of the ν 3 ,‐states of the SO 4 2− ‐ions in biaxial MgSO 4 , · 7 H 2 O has been observed with Raman‐scattering experiments at T = 300 K and T = 1.9 K. The agreement of the experimental data with the existing theories of angular dispersion is very satisfying. Some accidental degeneracies of transverse with longitudinal states of different symmetry could be observed. This degeneracy, where the two branches cross with equal slopes of opposite signs, can be lifted due to the temperature dependence of the frequencies. This behaviour is also interpreted theoretically and some simple relations are derived for the slopes and the curvature of dispersion branches, which relate these data to the effective charges and the dielectric function ε(ω), and which characterize the critical points occuring on the dispersion branches.