Polarized reflectivity of $\beta$-Sr$_{0.17}$V$_2$O$_{5}$
V. Ta Phuoc, C. Sellier, E. Janod, and C. Marin

In order to investigate phonon dynamics, infrared polarized reflectivity was measured along the $b$-axis and on the low symmetry $ac$ face of a monoclinic $\beta$-Sr$_{0.17}$V$_2$O$_{5}$ single crystal. Whereas 11 $A_u$ modes are expected along the $b$-axis, only four broad and asymmetric modes are observed. Such a discrepancy with Factor Group Analysis presumably results from the coupling of the phonons with an electronic background. Thus, both reflectivity and optical conductivity, obtained from Kramers-Kronig analysis, were fitted with Fano lineshapes. In the $ac$ face, almost all the 22 $B_u$ modes predicted by Factor Group Analysis are observed. Reflectivity spectra are analyzed by using tensor dispersion formula. To extract phonon parameters, including dipole moments, not predicted by group theory, the model requires to measure the reflectivity for at least three polarizations. The assignement of high frequency phonon modes is made by comparing bond length and mode frequencies with $\alpha$-'NaV$_2$O$_{5}$ and V$_2$O$_{5}$, and by using the dipole moment directions. Phonon modes assigned to V-O-V rungs stretching show unusually large dampings. We interpret these anomalous dampings as due to charge delocalization within V-O-V rungs. The mean Born effective charge tensors obtained from the model show a clear anisotropy in $ac$-plane.

© 2008 The American Physical Society.