Abstract
Transient electric polarization in single crystalline SrTiO3 (STO) substrates was studied by measuring the transmission of surface acoustic waves (SAWs). We applied a large dc electric field (8 × 106 Vm−1) to interdigital transducers (IDTs) on STO substrates in order to induce local piezoelectricity, which is required to generate and transmit SAWs. The resulting electric polarization and the retention thereof were analyzed as a function of time after the applied electric field was removed, by measuring transient SAW signals. The retained polarization turned out to provide strong electromechanical coupling, comparable to that resulting from the dc-field-induced piezoelectricity. SAW signals were observed for more than 30 h (in absence of an external dc electric field), which is evidence for a long-lasting retention of electric polarization. Remarkably, this polarization was found to be rapidly suppressed as the sample was exposed to visible light. By measuring the transient SAW transmission under illumination with light at different wavelengths, we identified photoconductivity and/or acceleration of oxygen vacancy migration by photon-induced splitting of bound vacancy pairs as the main mechanisms behind this photoresponsive memory effect.
Highlights
Among the innumerous intriguing features of SrTiO3 (STO), one of the most debated is ferroelectricity [1]
After that we analyzed the time-evolution of surface acoustic waves (SAWs) signals when the dc-bias was switched off and the sample was left for relaxation
When a 20 V dc bias was applied to both interdigital transducers (IDTs) at the same time, SAW transmission was measured at 657 MHz corresponding to the 3rd harmonic resonance frequency of the device
Summary
Among the innumerous intriguing features of SrTiO3 (STO), one of the most debated is ferroelectricity [1]. SAW generation requires piezoelectricity, which can be induced in STO by applying an external dc electric field [19, 20]: Due to the ionic nature of STO, comprising Sr2+, Ti4+ cations and O2- anions, an electric field displaces these charges in opposite directions and the lattice inversion symmetry is broken and a polarization is induced [17]. This approach has been used to excite surface acoustic waves (SAWs) on STO substrates [21,22,23]. We conclude that the transient electric polarization in STO is mainly due to oxygen vacancy migration, and is not ferroelectric in nature
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