|標題：||新物質セミナー：Dynamical Magnetoelectric ﬀects of Spin Waves in the Multiferroic Ba2CoGe2O7|
|講師：||Dr. Bordacs Sandor|
|要旨：||Lack of inversion and time reversal symmetry leads to cross-coupling between oscillating magnetic and electric polarization, namely dynamical magnetoelectric response. Perhaps, the most exotic manifestation of this phenomena is the non-reciprocal directional dichroism, that is the diﬀerence of the light absorption for counter propagating light beams irrespective of their polarization. Despite the early prediction of this phenomenon  it has been found minimal in conventional materials and hence of little technological potential.
We proposed that multiferroic compounds endowed with electrically-active magnetic excitations (electromagnons) may provided a key to produce large optical magnetoelectric eﬀect for long wavelengths of light . By exploiting the control of ferroelectric polarization and magnetization in a multiferroic oxide Ba2CoGe2O7 , we demonstrated the enhancement of the directional dichroism – as large as 50% – at terahertz frequencies in resonance with the electromagnon absorption [2, 3].
Due to the non-centrosymmetric crystal structure of Ba2CoGe2O7 chirality can be induced via a magnetic ﬁeld . As a direct proof of such a chiral phase, we found large optical activity for propagation along each crystallographic direction . This is the ﬁrst demonstration that optical rotation from spin-waves at terahertz frequencies can be a sensitive indicator of chirality. In this chiral phase, another type of directional dichroism, the magnetochiral eﬀect, was observed. Our results imply that this hidden potential is present in a broad variety of multiferroic materials.
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|世話人：||大串 研也 (内線：63250)
e-mail: ohgushi :at: issp.u-tokyo.ac.jp