Large and Recoverable Electrostrain in Strained Ferroelectric Superlattices due to Domain Switching

The ferroelectric superlattices have been widely studied due to their distinguished electromechanical coupling properties.Under different biaxial mismatch strains,ferroelectric superlattices exhibit different domain structures and electromechanical coupling properties.A three-dimensional phase field model is employed to investigate the detailed domain evolution and electromechanical properties of the PbTiO_(3)/SrTiO_(3)(PTO/STO)superlattices with different biaxial mismatch strains.The phase field simulations show that the ferroelectric superlattice exhibits large electrostrain in the stacking direction when an external field is applied.Under a large compressive mismatch strain,vortex domains appear in ferroelectric layers with the thickness of 4 nm.The vortex domains become stable cdomain under a large external electric field,which remains when the electric field is removed.When the initial compressive mismatch strain decreases gradually,the waved or a1/a2 domains replaces the initial vortex domains in the absence of electric field.The fully polarized c-domain by a large electric field switches to diagonal direction domain or a/c domain when the electric field is small.Furthermore,when a biaxial tensile strain is applied to the superlattice,ferroelectric domains switch back to the initial a1/a2 twin-like domain structure,resulting in the recoverable and large electrostrain.This provides an effective way to obtain the large and recoverable electrostrain for the engineering application.

DIELECTRIC,PIEZOELECTRIC,AND ELASTIC PROPERTIES OF BaTiO_(3)/SrTiO_(3)FERROELECTRIC SUPERLATTICES FROM FIRST PRINCIPLES

The effect of epitaxial strain on the phonon spectra,crystal structure,spontaneous polarization,dielectric,piezoelectric,and elastic properties of(001)-oriented ferroelectric(BaTiO_(3))m=(SrTiO_(3))n superlattices(m=n=1-4)was studied using thefirst-principles density functional theory.The ground state of free-standing superlattices is the monoclinic Cm polar phase.Under the in-plane biaxial compressive strain,it transforms to tetragonal P4mm polar phase,and under the in-plane biaxial tensile strain,it transforms to orthorhombic Amm2 polar phase.When changing the in-plane lattice parameter,a softening of several optical and acoustic modes appears at the boundaries between the polar phases,and corresponding components of dielectric,piezoelectric,and elastic tensors diverge critically.The comparison of the mixing enthalpy of disordered Ba_(0.5)Sr_(0.5)TiO_(3)solid solution modeled using two special quasirandom structures SQS-4 with the mixing enthalpy of the superlattices reveals a tendency of the BaTiO_(3)-SrTiO_(3)system to shortrange ordering and shows that these superlattices are thermodynamically quite stable.

Spin-Polarization of Ferroelectric Supperlattice with Spin-1/2 Transverse Ising Model

By using the simple decoupling approximation to Fermi-type Green＇s function for the transverse Ising model under pseudospin theory, we systemically study the influence of different exchange interactions （transverse fields） of the two distinct materials on the polarization and Curie temperature of finite alternating superlattice. Meanwhile, we analyze the effect of the whole parameters of the top surface, present their influence on the polarization of each layer （including the mean polarization of the whole ferroeleetric superlattiee） and on the Curie temperature. The results show the ratio of the exchange interactions （the transverse fields）, which are of the two alternating materials have deeply impact on the polarization and Curie temperature of the supperlattice. Moreover, the top surface also has great influence on the whole ferroelectric superlattice.