High Polarization Stability Induced by Light-activated Defect Engineering in Ferroelectric Single Crystals
Xinyu JinYu WangXiangda MengMingxuan LiuBohan XingX. Y. WenXiaolin HuangXiaoou WangChengpeng HuPeng TanHao Tian
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Non-monotonous thickness-dependent ferroelectric and ferroelectric-ferroelastic domain size scaling behaviors were revealed in ferroelectric films, including three distinct regions: (I) a classical ½ power law relationship for thick films, (II) a deviation from the ½ scaling relationship for an intermediate thickness range, and (III) an exponential increase in ultrathin films when decreasing the film thickness. The calculations indicate a much narrower region (II) in ferroelectric films with ferroelectric domains than that with ferroelectric-ferroelastic ones. As the film thickness decreases, the stable domain pattern also changes from a ferroelectric-ferroelastic domain to a ferroelectric one, which leads to the divergence of domain size scaling.
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A new type of ferroelectric liquid crystal, the 4′-alkoxybiphenyl-4-yl (2S,3S)-3-methyl-2-halogenopentanoates, has been synthesized; these compounds possess novel ferroelectricity, with spontaneous polarization and dielectric constant larger than 10–7 C/cm2 and 6000, respectively.
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In improper ferroelectric materials, the order parameter of the phase transition is not the polarization but another physical quantity, where transformation properties are different from those of the polarization. The spontaneous polarization arises in the phase transition as a secondary effect [ 1 ]. For example, geometric ferroelectric such as the hexagonal manganites are improper ferroelectrics in which geometric structural constrains induce ferroelectric polarization [ 2 ]. An another example is a hybrid improper ferroelectricity such as (Ca,Sr) 3 Ti 2 O 7 , whichresults from the combination of two or more non-ferroelectric structural order parameters [ 3 ]. The coupling between the spontaneous ferroelectric polarization and other physical quantities should result in unique domain structures in the improper ferroelectric materials. Figure 1 is a domain structure in the improper ferroelectric compound BaAl 2 O 4 with the hexagonal structure [ 4 ]. Characteristic structural antiphase domains are observed, in which structurally modulated superstructure with 2 a × 2 a × c . In the presentation, unique charged domain walls found in some improper ferroelectric compounds will be reported.
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Geometric ferroelectrics are called as improper ferroelectrics where geometric structural constraints, rather than typical cation-anion paring, induce proper ferroelectric polarization. Hybrid improper ferroelectricity, one kind of geometric ferroelectricity, results from the combination of two or more of non-ferroelectric structural order parameters. In recent, hybrid improper ferroelectricity has been theoretically predicted in ordered perovskites and the Ruddlesden-Popper compounds. However, the ferroelectricity of these compounds has never been experimentally confirmed and even their polar nature has been under debate. In this talk, we report our experimental results of exploring switchable electric polarization and domain structures in the single crystals of the n = 2 layered Ruddlesden-Popper compounds.
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It is shown that the constant current method, in which a dielectric is charged with a constant current while the voltage is monitored, allows one to determine the dependence of the stable ferroelectric polarization with the electric field. The determination is based on two successive experiments separated in time by a short-circuit period: a charging process in which polarization switching occurs followed by a recharging with the same current polarity. Analysis of the recharging experiments for poly(vinylidene fluoride), PVDF, shows that the polarization appearing in it is a metastable ferroelectric polarization, due to the reorientation of ferroelectric polarization lost during the short-circuit period. The method was applied to measure the ferroelectric polarization in PVDF samples with different β-phase contents and in an exploratory way for a few other ferroelectric polymers.
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Valley polarization and ferroelectricity are the two basic concepts in electronic device applications. However, the coexistence of these two scenarios in one material has not been reported. Here, using first-principles calculations, we demonstrated that the two-dimensional GaAsC6 monolayer which is a hybrid structure of GaAs and graphene has a pair of inequivalent valleys with opposite Berry curvatures and an intrinsic out-of-plane spontaneous electric polarization. It also has a direct band gap of about 1.937 eV and a high carrier mobility of about 1.80 × 105 cm2 V-1 s-1, which are promising for electronic device applications. The integration of valley polarization and ferroelectricity in a single material offers a promising platform for the design of electronic devices.
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Abstract Interest in the synthesis of optically active smectic liquid crystals has increased considerably since the advent of a fast switching, bistable, electrooptic device configuration based on their ferroelectric properties. A number of structurally separate ferroelectric liquid crystal phases have been defined which possess differing properties. These types of phase can be utilized in different forms of application. The structures of the various ferroelectric smectic phases and the types of material which exhibit these modifications are discussed in detail. The design and engineering of materials to suit certain device criteria is related to the properties of the smectic ferroelectric phases. A relationship between the absolute spacial configuration of the optically active materials which exhibit ferroelectric smectic phases, the twist direction of the phase and the direction of the spontaneous polarization is developed.
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Ferroelectric materials are a special type of polar substances, including solids or liquid crystals. However, obtaining a material to be ferroelectric in both its solid crystal (SC) and liquid crystal (LC) phases is a great challenge. Moreover, although cholesteric LCs inherently possess the advantage of high fluidity, their ferroelectricity remains unknown. Here, through the reasonable H/F substitution on the fourth position of the phenyl group of the parent nonferroelectric dihydrocholesteryl benzoate, we designed ferroelectric dihydrocholesteryl 4-fluorobenzoate (4-F-BDC), which shows ferroelectricity in both SC and cholesteric LC phases. The fluorination induces a lower symmetric polar P1 space group and a new solid-to-solid phase transition in 4-F-BDC. Beneficial from fluorination, the SC and cholesteric LC phases of 4-F-BDC show clear ferroelectricity, as confirmed by well-shaped polarization-voltage hysteresis loops. The dual ferroelectricity in both SC and cholesteric LC phases of a single material was rarely found. This work offers a viable case for the exploration of the interplay between ferroelectric SC and LC phases and provides an efficient approach for designing ferroelectrics with dual ferroelectricity and cholesteric ferroelectric liquid crystals.
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