Low-energy excitations and dynamic Dzyaloshinskii-Moriya interaction in α′-NaV2O5 studied by far-infrared spectroscopy -: art. no. 1444410

We have studied far-infrared transmission spectra of alpha(')-NaV2O5 between 3 and 200 cm(-1) in polarizations of incident light parallel to a, b, and c crystallographic axes in magnetic fields up to 33 T. The temperature dependence of the transmission spectra was studied close to and below the phase-transition temperature T-c=34 K. The triplet origin of an excitation at 65.4 cm(-1) (8.13 meV) is revealed by splitting in the magnetic field. The g factors for the triplet state are g(a)=1.96+/-0.02, g(b)=1.975+/-0.004, and g(c)=1.90+/-0.03. The magnitude of the spin gap at low temperatures is found to be magnetic-field independent at least up to 33 T. All other infrared-active transitions appearing below T-c are ascribed to zone-folded phonons. Two different dynamic Dzyaloshinskii-Moriya (DM) mechanisms have been discovered that contribute to the oscillator strength of the otherwise forbidden singlet to triplet transition. First, the strongest singlet to triplet transition is an electric dipole transition where the polarization of the incident light's electric field is parallel to the ladder rungs (E(1)parallel toa). This electric dipole active transition is allowed by the dynamic DM interaction created by a high-frequency optical a-axis phonon. Second, in the incident light polarization perpendicular to the ladder planes (E(1)parallel toc) an enhancement of the singlet to triplet transition is observed when the applied magnetic field shifts the singlet to triplet resonance frequency to match the 68 cm(-1) c-axis phonon energy. The origin of the second mechanism is the dynamic DM interaction created by the 68 cm(-1) c-axis optical phonon. The strength of the dynamic DM is calculated for both mechanisms using the presented theory.