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Abstract

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.