Electronic ferroelectrics are materials in which the electric polarization originates from the ordering of electrons, rather than from the displacement of ions. This distinction is important because electrons respond much faster than ions, enabling the development of ferroelectric devices with ultrafast switching and novel functionalities. YbFe₂O₄ belongs to the mixed-valence iron oxide family RFe₂O₄ (R: rare-earth ion) and exhibits charge ordering of Fe²⁺ and Fe³⁺ ions, which leads to electronic polarization.

A key challenge in studying YbFe₂O₄ is the direct detection of its electronic polarization. In bulk YbFe₂O₄ crystals, the correlation length along the c-axis polarization is short (only tens of nanometers), making it difficult to observe the second-order nonlinear optical effects and to determine all tensor components. To overcome this limitation, we prepared a c-axis-oriented epitaxial thin film of YbFe₂O₄, approximately 50 nm thick—comparable to the correlation length—and measured its second-harmonic generation (SHG). SHG, a second-order nonlinear optical effect, occurs only when inversion symmetry is broken and provides information about the direction and magnitude of ferroelectric polarization. By systematically varying both the incident and azimuthal angles of the incident light while recording the emitted 400 nm SHG from an 800 nm excitation, we obtained a comprehensive dataset that could be fit under the symmetry of the space group C_m. These analyses quantitatively reproduced the observed angle dependence and allowed us to obtain a complete set of relative ratios for all d_ij tensor components with respect to d₁₁. Notably, the obtained components associated with the c-axis, including d₃₁, d₃₂, d₃₃, and d₃₅, are more than one order of magnitude larger than the other tensor elements. This pronounced anisotropy provides direct optical evidence that the electronic polarization in the thin film is well-aligned along the c-axis, indicating the potential of YbFe₂O₄ thin films for future electronic ferroelectric devices.

(Written by Xiaopu Wang and Yoichi Okimoto on behalf of all authors)

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