Infrared phonon spectrum of the tetragonal helimagnet Ba2CuGe2O7 (art.: PRB 90, 014304)

A. NUCARA; W. S. MOHAMED; L. BALDASSARRE; S. KOVAL; J. LORENZANA; R. FITTIPALDI; G. BALAKRISHNAN; A. VECCHIONE; P. CALVANI

PHYSICAL REVIEW B - CONDENSED MATTER AND MATERIALS PHYSICS

American Physical Society (APS)

Año: 2014 vol. 90 p. 143041 - 143047

0163-1829

The lattice dynamics of Ba2CuGe2O7 , a compound which develops Dzyaloshinsky-Moriya (DM)helical magnetism below T_N = 3.2 K, has been studied by measuring the infrared reflectivity ofa single crystal with the radiation polarized both in the the ab plane and along the c axis of itstetragonal cell, from 7 K to 300 K. In this compound, where the unit cell has no inversion symmetry,fourteen E phonon modes of the ab plane, out of the eighteen predicted, and nine B2 modes ofthe c axis, out of the ten predicted, have been observed. They have been assigned to the atomicmotions by a comparison with shell-model calculations, which provided vibrational frequencies ingood agreement with the experiment, while most calculated intensities turned to be much lowerthan the experimental values. This discrepancy has been tentatively explained by assuming strongelectron-phonon interactions, a hypothesis supported by the failure of the f - sum rule if restrictedto the phonon region. Indeed, we observe a remarkable increase in the oscillator strengths between300 and 200 K fro the phonons of the ab plane (between 300 and 100 K for the c axis), whichimplies an increase in the dielectric constant of Ba2CuGe2O7 below room temperature.