Hexagonal manganites, such as h-LuMnO3, are ferroelectric with its polar axis along the hexagonal axis and have a narrow electronic bandgap (≈ 1.5 eV). Using Pt electrodes, h-LuMnO3 single crystals display a strong rectification, characteristic of a Schottky diode, and a large photoresponse. It is found that the short circuit photocurrent Jsc along the polar axis is modulated (up to 25 %) by the direction of the ferroelectric polarization P, leading to a short circuit photocurrent loop that mimics the ferroelectric polarization, and by polarization backswitching. However, a non-switchable Jsc persists. Diffusion photocurrent is shown to dominate current-in-plane measurements and contributes to the non-switchable Jsc. This observation illustrates the dramatic role of the large optical absorption in hexagonal manganites. The accompanying optical dichroism shall challenge disentangling a genuine bulk photovoltaic response in h-LuMnO3 single crystal contributing to the non-switchable Jsc. Epitaxial thin films may offer a suitable alternative.
Hexagonal manganites, such as h-LuMnO 3 , are ferroelectric and have a narrow electronic band gap of ≈ 1.5 eV. Here we report on the photoresponse of h-LuMnO 3 single crystals. It is found that the short circuit photocurrent density (J sc ) and the open circuit voltage (V oc ) are dependent on the direction of the polarization plane of a linearly polarized impinging light. Its angular dependence indicates the contribution of bulk photovoltaic effect to the short circuit photocurrent. It is also observed that a switchable drift photocurrent, originating from the depoling field of the ferroelectric and thus tunable (<10%) by its polarization direction, also contributes to J sc . Although its presence precludes accurate determination of the bulk photovoltaic tensor elements and Glass coefficients, some bounds can be established. The Glass coefficients are found to be significantly larger than those obtained in BiFeO 3 . We argue that the smaller band gap of h-LuMnO 3 , its distinctive bipyramidal crystal field, and electronic configuration (3d 4 vs 3d 5 ), account for the difference and suggest a path towards ferroelectrics of higher photoconversion efficiency.
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