Low energy excitations and dynamic Dzyaloshinskii-Moriya interaction in $\alpha'$-NaV$_2$O$_5$ studied by far infrared spectroscopy

TL;DR

This study investigates low energy excitations in alpha'-NaV2O5 using far infrared spectroscopy, revealing magnetic field effects, zone-folded phonons, and two dynamic Dzyaloshinskii-Moriya mechanisms that enable forbidden transitions.

Contribution

The paper identifies two distinct dynamic Dzyaloshinskii-Moriya mechanisms contributing to singlet-triplet transitions in alpha'-NaV2O5, expanding understanding of spin-phonon interactions.

Findings

Magnetic field splits the triplet excitation at 65.4cm-1.

The spin gap remains field-independent up to 33T.

Dynamic DM interactions enable otherwise forbidden singlet-triplet transitions.

Abstract

We have studied far infrared transmission spectra of alpha'-NaV2O5 between 3 and 200cm-1 in polarizations of incident light parallel to a, b, and c crystallographic axes in magnetic fields up to 33T. The triplet origin of an excitation at 65.4cm-1 is revealed by splitting in the magnetic field. The magnitude of the spin gap at low temperatures is found to be magnetic field independent at least up to 33T. All other infrared-active transitions appearing below Tc 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. 1. 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, and is allowed by the dynamic DM interaction created by a high frequency optical a-axis phonon. 2. In the incident light polarization perpendicular to the ladder planes 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 68cm-1 c-axis phonon energy. The origin of this mechanism is the dynamic DM interaction created by the 68cm-1 c-axis optical phonon. The strength of the dynamic DM is calculated for both mechanisms using the presented theory.