Symmetry and three-dimensional anisotropy of polar domain boundaries observed in ferroelastic LaAlO_3 in the complete absence of ferroelectric instability

摘要

The domain boundaries of ferroelastic LaAlO_3 (LAO) are examined using a nonlinear optical second harmonic generation (SHG) microscope. Although bulk crystalline LAO possesses centro-symmetry and is inherently non-SH active, our three-dimensional observations reveal that the domain boundaries of LAO exhibit SH activity with almost the same magnitude within a boundary. The polarization dependence of the SH intensities shows strong anisotropy, which is explained by the polar trigonal point group, 3m. The polar direction is found to be along the [111]_(pc) axis and inclined from the domain boundary plane. It implies that the biquadratic coupling scheme of two order parameters could be the main origin of the emergence of polar nature at the domain boundary. Our experimental results together with those of CaTiO_3 suggest that the polar nature of the domain boundary in ferroelastic materials may be a ubiquitous phenomenon.