Khian-Hooi Chew

Hysteresis loops of ferroelectric bilayers and superlattices

A ferroelectric superlattice with an antiferroelectric interfacial coupling is considered; the same model describes a bilayer with antiferroelectric coupling. By mapping minimum points in the Landau free energy expression and plotting them against the applied electric field, a triple hysteresis loop pattern is obtained. The loop patterns vary between typically ferroelectric and typically antiferroelectric depending on the layer thicknesses and the magnitude of the interfacial-coupling constant. This work suggests the possibility of designing multilayer elements for computer memories with four or more different storage states.

Dielectric tunability of (Ba0.90Ca0.10)(Ti0.75Zr0.25)O3 ceramics

We have performed a study on the dielectric properties of (Ba0.90Ca0.10)(Ti0.75Zr0.25)O3 bulk ceramics prepared by a sol–gel technique in the cooling temperature range from 400to213K. The ceramics are found to exhibit an interesting dielectric response under the presence of a dc bias field. The field dependence of dielectric constant is stronger at T≈Tm (Tm is the temperature of dielectric maximum) and the field-induced variation of dielectric constant Δe is nonzero for T−Tm<150K. One striking feature of these ceramics is that they possess a broad optimum value of figure of merit (as high as 200 at 30kV∕cm) between the temperature range from 295 and 330K. At room temperature, the high tunability (K=60%), low-loss tangent (≈0.003), and large figure of merit (200), clearly indicate that these ceramics are suitable candidates for microwave applications.

Dynamic Control of Collapse in a Vortex Airy Beam

Here we study systematically the self-focusing dynamics and collapse of vortex Airy optical beams in a Kerr medium. The collapse is suppressed compared to a non-vortex Airy beam in a Kerr medium due to the existence of vortex fields. The locations of collapse depend sensitively on the initial power, vortex order, and modulation parameters. The collapse may occur in a position where the initial field is nearly zero, while no collapse appears in the region where the initial field is mainly distributed. Compared with a non-vortex Airy beam, the collapse of a vortex Airy beam can occur at a position away from the area of the initial field distribution. Our study shows the possibility of controlling and manipulating the collapse, especially the precise position of collapse, by purposely choosing appropriate initial power, vortex order or modulation parameters of a vortex Airy beam.

Primary and secondary pyroelectric effects of ferroelectric 0-3 composites

Simple and tractable analytical expressions for determining the pyroelectricity in ferroelectric 0-3 composites have been developed. For the dilute suspension limit, expressions for the effective pyroelectric and other thermal electromechanical properties are derived within the framework of the Maxwell–Wagner approach. Then, an effective-medium theory is employed to examine the first and second pyroelectric coefficients in the concentrated suspension regime. The effective-medium approach as compared to the Maxwell–Wagner approach results in a better agreement with known experimental data up to higher volume fraction of inclusions. The pyroelectricity of 0-3 composites of ceramic inclusions embedded in the P(VDF–TrFE) copolymer matrix and of P(VDF–TrFE) inclusions embedded in a ceramic matrix are analyzed numerically under different polarization configurations. The theoretical predictions show that the secondary pyroelectric effects in composite systems with ceramic as the matrix are stronger than those wi...

Theory of interface structures in double-layer ferroelectrics

Interface structures in double-layer ferroelectrics comprising two distinct layers of different polarization properties, with bilinear interface coupling across the interface, are examined analytically within the framework of Landau–Ginzburg phenomenological theory. The structures are investigated for coupled layers with: (i) one layer in paraelectric phase and the other in ferroelectric phase (ferroelectric–paraelectric), and (ii) both layers in ferroelectric phases (ferroelectric–ferroelectric). The effects of interfacial coupling on the total interface structure energies, interfacial coupling energies and mismatch in polarization across the interface are discussed.

A Lattice Model for Ferroelectric Superlattices

A one-dimensional lattice model based on the Landau–Ginzburg theory to study the intrinsic properties of a periodic superlattice with interfacial coupling is developed. The effects of thickness and interfacial coupling on the properties of a superlattice consisting of alternating ferroelectric and paraelectric constituent layers are investigated. Competition between the thickness effects of each constituent layer is expected and indeed can play an important role in governing the properties of an interface-coupled superlattice, depending on the strength of the interfacial coupling. This work may provide useful information on the possibility of manipulating structures to obtain the desired properties for specific applications.

Improvement of ferroelectric fatigue endurance in multiferroic (Ba0.5Sr0.5)TiO3/(Bi1.05La0.05)FeO3/(Ba0.5Sr0.5)TiO3 sandwich structures

We report the improved ferroelectric properties in dielectric/multiferroic/dielectric sandwich structures composed of Ba0.5Sr0.5TiO3 (BST) and Bi1.05La0.05FeO3 (BLF). Compared with the single BLF film, the trilayer structures exhibit a lower dielectric loss and weaker frequency dependence of dielectric properties. At room temperature, the remnant polarization and saturation polarization of the trilayer structures are 34.3 and 46.9μC∕cm2, respectively. More interestingly, the polarization of BST/BLF/BST trilayer exhibits a fatigue-free characteristic of up to 109 switching cycles. Applying Dawber’s model, the concentration of oxygen vacancies and barrier energy of oxygen vacancies migration in BST/BLF/BST trilayer are calculated as 6.1×1017cm−3 and 1.33eV, respectively.

Bi-relaxation behaviors in epitaxial multiferroic double-perovskite BiFe0.5Mn0.5O3/CaRuO3 heterostructures

Multiferroic double-perovskite BiFe0.5Mn0.5O3 thin film heterostructures were epitaxially grown on CaRuO3-buffered (001) SrTiO3 by pulse laser deposition. Typical Vogel-Fulcher relaxorlike dielectric and magnetic susceptibilities were observed, implying the film exhibits the properties of an electric relaxor and a magnetic relaxor. Polarization and size of polar nanoregions (PNRs) were determined by fitting the dielectric constant to a multi-polarization mechanism model. It was found that PNRs of 7-11 nm decrease from 0.67 μC/cm2 to 0.11 μC/cm2, as the temperature increases from 380 K to 460 K. A weak ferromagnetism was observed via magnetic hysteresis loops up to 300 K.

Structure of Interfaces in Layered Ferroelectrics of First and/or Second Order Transition

We studied the structure of interfaces in layered ferroelectrics comprising two different ferroelectric materials with first and/or second order transitions. The layered structure is described using the Landau–Ginzburg theory by including a bilinear coupling at the interface between the two neighboring layers. The interfacial coupling leads to the variation of polarization across the interface from one layer to another. An abrupt or continuous change of polarization across the interface is found to depend on the strength of coupling. For a layered structure having a layer in paraelectric phase, an interface-ordered state is predicted (in the paraelectric layer) as a manifestation of interfacial coupling. The Tilley–Zeks model is compared with the present approach to discuss the relationship between the bilinear coupling parameter and the extrapolation lengths.

Structured caustic vector vortex optical field: manipulating optical angular momentum flux and polarization rotation

A caustic vector vortex optical field is experimentally generated and demonstrated by a caustic-based approach. The desired caustic with arbitrary acceleration trajectories, as well as the structured states of polarization (SoP) and vortex orders located in different positions in the field cross-section, is generated by imposing the corresponding spatial phase function in a vector vortex optical field. Our study reveals that different spin and orbital angular momentum flux distributions (including opposite directions) in different positions in the cross-section of a caustic vector vortex optical field can be dynamically managed during propagation by intentionally choosing the initial polarization and vortex topological charges, as a result of the modulation of the caustic phase. We find that the SoP in the field cross-section rotates during propagation due to the existence of the vortex. The unique structured feature of the caustic vector vortex optical field opens the possibility of multi-manipulation of optical angular momentum fluxes and SoP, leading to more complex manipulation of the optical field scenarios. Thus this approach further expands the functionality of an optical system.