Bulk photovoltaic effect in hexagonal LuMnO3 single crystals

Abstract

Hexagonal manganites, such as $h\text{\ensuremath{-}}{\mathrm{LuMnO}}_{3}$, are ferroelectric and have a narrow electronic band gap of $\ensuremath{\approx}1.5\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$. Here we report on the photoresponse of $h\text{\ensuremath{-}}{\mathrm{LuMnO}}_{3}$ single crystals. It is found that the short circuit photocurrent density (${J}_{\mathrm{sc}}$) and the open circuit voltage (${V}_{\mathrm{oc}}$) 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 ${J}_{\mathrm{sc}}$. 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 ${\mathrm{BiFeO}}_{3}$. We argue that the smaller band gap of $h\text{\ensuremath{-}}{\mathrm{LuMnO}}_{3}$, its distinctive bipyramidal crystal field, and electronic configuration ($3{d}^{4}$ vs $3{d}^{5}$), account for the difference and suggest a path towards ferroelectrics of higher photoconversion efficiency.