Abstract
Ferroelectric domain walls are interfaces between areas of a material that exhibits different directions of spontaneous polarization. The properties of domain walls can be very different from those of the undisturbed material. Metallic-like conductivity of charged domain walls (CDWs) in nominally insulating ferroelectrics was predicted in 1973 and detected recently. This important effect is still in its infancy: The electric currents are still smaller than expected, the access to the conductivity at CDWs is hampered by contact barriers, and stability is low because of sophisticated domain structures or proximity of the Curie point. Here, we report on large, accessible, and stable conductivity at CDWs in lithium niobate (LN) crystals – a vital material for photonics. Our results mark a breakthrough: Increase of conductivity at CDWs by more than 13 orders of magnitude compared to that of the bulk, access to the effect via ohmic and diode-like contacts, and high stability for temperatures T ≤ 70 °C are demonstrated. A promising and now realistic prospect is to combine CDW functionalities with linear and nonlinear optical phenomena. Our findings allow new generations of adaptive-optical elements, of electrically controlled integrated-optical chips for quantum photonics, and of advanced LN-semiconductor hybrid optoelectronic devices.
Highlights
The ferroelectric state can be characterized by the spatial distribution of the vector of spontaneous polarization
During the last years, pronounced DC conductivity at charged domain walls (CDWs) was detected in many materials including PbZr0.2Ti0.8O3 films[15], BaTiO312, 16, BiFeO317, ErMnO318, hexagonal h-HoMnO319, and (Ca,Sr)3Ti2O7 crystals[20]
The most advanced data for CDWs has been presented for BaTiO3 single crystals: An enhancement factor of ∼109 as compared to the bulk conductivity was obtained for 45° head-to-head walls, and signs of metallic-like behaviour were indicated[12]
Summary
The ferroelectric state can be characterized by the spatial distribution of the vector of spontaneous polarization. Owing to the local breaking of spatial symmetry, they can exhibit properties entirely different from those of the undisturbed material[1,2,3,4] It is found, in particular, that neutral ferroelectric domain walls exhibit pronounced conductive properties[5,6,7,8]. For these head-to-head walls, the bound surface charge is + 2PS and + 2PS sinθ, respectively, where θ is the inclination angle, and the compensating negative surface charge is almost opposite in value. Correspondence and requests for materials should be addressed to www.nature.com/scientificreports/
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