Hayashida Takeshi, Kimura Kenta, Kimura Tsuyoshi
Department of Advanced Materials Science, Graduate School of Frontier Sciences, University of Tokyo, Kashiwa, Chiba 277-8561, Japan.
Proc Natl Acad Sci U S A. 2023 Aug 22;120(34):e2303251120. doi: 10.1073/pnas.2303251120. Epub 2023 Aug 17.
In a chiral medium, any mirror symmetries are broken, which induces unique physical properties represented by natural optical rotation. When electromagnetic waves propagate through a chiral medium placed in a magnetic field, the refractive index, or equivalently, the absorption encountered by the electromagnetic waves differs depending on whether it travels parallel or antiparallel to the magnetic field. Such a phenomenon is known as magnetochiral dichroism (MChD), which is the characteristic interplay between chirality and magnetism. Similar to chirality, the so-called ferroaxial order, an emergent ferroic state of crystalline materials, is also characterized by mirror symmetry breaking. In contrast to chiral materials, however, the mirror symmetry perpendicular to the crystalline principal axis is allowed in ferroaxial materials. In other words, chirality and thus phenomena unique to chirality can be induced by breaking the remaining mirror symmetry by applying an electric field. Here, we show electric control of chirality and resulting electric field-induced MChD (-MChD) of the short-wavelength infrared region in a ferroaxial crystal, NiTiO. We performed spectroscopy measurements of -MChD by taking a difference of absorption coefficients obtained with and without electric and magnetic fields. As a result, -MChD was observed around the excitation energy corresponding to Ni magnetic-dipole transitions. The result is nicely explained by adopting the theory of MChD concerning the pseudo-Stark splitting of the energy state. Ferroaxial materials therefore provide platforms to achieve electric control of chirality-related phenomena.
在手性介质中，任何镜面对称性都会被打破，这会引发以自然旋光性为代表的独特物理性质。当电磁波在置于磁场中的手性介质中传播时，电磁波所遇到的折射率（或者等效地说，吸收率）会因传播方向与磁场平行或反平行而有所不同。这种现象被称为磁手性二向色性（MChD），它是手性与磁性之间的独特相互作用。与手性类似，所谓的铁轴序，即晶体材料的一种新兴铁电态，同样以镜面对称性的破缺为特征。然而，与手性材料不同的是，铁轴材料中允许存在垂直于晶体主轴的镜面对称性。换句话说，通过施加电场打破剩余的镜面对称性，可以诱导出手性以及因此产生的手性特有的现象。在此，我们展示了在铁轴晶体NiTiO中对手性的电控制以及由此产生的短波长红外区域的电场诱导磁手性二向色性（-MChD）。我们通过测量有电场和磁场以及无电场和磁场时的吸收系数之差，进行了-MChD的光谱测量。结果，在对应于Ni磁偶极跃迁的激发能量附近观察到了-MChD。采用关于能态的伪斯塔克分裂的磁手性二向色性理论可以很好地解释这一结果。因此，铁轴材料为实现与手性相关现象的电控制提供了平台。
