Electronic structure and anomalous band-edge absorption feature in multiferroicMnWO4: An optical spectroscopic study

Abstract

We investigated the electronic structure and lattice dynamics of multiferroic MnWO 4 by optical spectroscopy. With variation of polarization, temperature, and magnetic field, we obtained optical responses over a wide range of photon energies. The electronic structure of MnWO 4 near to the Fermi level was examined, with inter-band transitions identified in optical conductivity spectra above a band-gap of 2.5 eV. As for the lattice dynamics, we identified all the infrared transverse optical phonon modes available according to the group-theory analysis. Although we did not observe much change in global electronic structure across the phase transition temperatures, an optical absorption at around 2.2 eV showed an evident change depending upon the spin configuration and magnetic field. The behavior of this band-edge absorption indicates that spin-orbit coupling plays an important role in multiferroic MnWO 4 .

Infrared-active phonon modes in monoclinic multiferroicMnWO4

“…[40,41] and our data agree very well with each other. With the exception of the lowest A u mode, the values for ω 0,i agree within about 1-2 %.…”

“…With the exception of the lowest A u mode, the values for ω 0,i agree within about 1-2 %. In MnWO 4 , we and Choi et al 40 found the lowest mode at 168 cm −1 at room temperature, whereas it was reported at 180 and 182 cm Note that the values of the damping γ i and the oscillator strength ∆ε i are not reported for the single-crystal data in Refs. [40,41].…”

“…Using a generalized Drude-Lorentz model, we determine the temperature dependence of all phonon parameters, including the orientation of the B u modes within the ac plane. A comparison to previous studies [40][41][42] shows that in particular the strongest modes have been overlooked thus far. This surprising result can be explained easily.…”

“…To the best of our knowledge, an analysis of the full dielectric tensor in the frequency range of the phonons has not been reported for any monoclinic tungstate AWO 4 with divalent A metal ions thus far. Choi et al 40 found no anomalies of the phonons as a function of temperature and reported only representative data on the temperature dependence for two high-energy features above 80 meV.…”

“…As far as the infrared-active phonon modes are concerned, a consistent description is still lacking. Choi et al 40 measured polarized reflectivity spectra for three different polarization directions of the electric field E (E a, b, and c), which is not sufficient for a proper identification of the eigenmodes in this monoclinic compound where a and c are not mutually perpendicular. To the best of our knowledge, an analysis of the full dielectric tensor in the frequency range of the phonons has not been reported for any monoclinic tungstate AWO 4 with divalent A metal ions thus far.…”