Large and Tunable Polar-Toroidal Coupling in Ferroelectric Composite Nanowires toward Superior Electromechanical Responses

Abstract

The collective dipole behaviors in (BaTiO3)m/(SrTiO3)n composite nanowires are investigated based on the first-principles-derived simulations. It demonstrates that such nanowire systems exhibit intriguing dipole orders, due to the combining effect of the anisotropic electrostatic interaction of the nanowire, the SrTiO3-layer-modified electrostatic interaction and the multiphase ground state of BaTiO3 layer. Particularly, a strong polar-toroidal coupling that is tunable by the SrTiO3-layer thickness, temperature, external strains and electric fields is found to exist in the nanowires, with the appearance of fruitful dipole states (including those being purely polar, purely toroidal, both polar and toroidal, or distorted toroidal) and phase boundaries. As a consequence, an efficient cross control of the toroidal (polar) order by static (curled) electric field, and superior piezoelectric and piezotoroidal responses, can be achieved in the nanowires. The result provides new insights into the collective dipole behaviors in nanowire systems.

Controlling polar-toroidal multi-order states in twisted ferroelectric nanowires

“…Recently, it has been revealed that topological dipole states like vortices with a toroidal order [11][12][13] can be stabilized in ferroelectric nanostructures under certain conditions. Interestingly, the toroidal order can coexist with polar order forming polar-toroidal multiorder (PTMO) states 14,15 and even skyrmion-like states 16 in specific ferroelectric nanostructures.…”

“…25 The second kind of systems are particular nanosystems made of rhombohedral phase ferroelectrics and under anisotropic screening condition, such as BaTiO 3 and BaTiO 3 /SrTiO 3 composite nanowires. [14][15][16]23 The most important feature of a PTMO state is the coexistence and possible coupling between the polar order and toroidal order, which gives rise to the distinguished behaviors of a PTMO state compared with purely toroidal or polar state. For example, as a result of the polar-toroidal coupling, superior piezoelectric and piezotoroidal responses 15 and interesting dynamical characteristics 26 are found in PTMO states.…”

“…[14][15][16]23 The most important feature of a PTMO state is the coexistence and possible coupling between the polar order and toroidal order, which gives rise to the distinguished behaviors of a PTMO state compared with purely toroidal or polar state. For example, as a result of the polar-toroidal coupling, superior piezoelectric and piezotoroidal responses 15 and interesting dynamical characteristics 26 are found in PTMO states. More importantly, we note that the coupling between polar order and toroidal order plays a significant role in dipole order transformation.…”

Phase coexistence and electric-field control of toroidal order in oxide superlattices

“…In the case of ferroelectrics, several theoretical studies [20][21][22][23][24] have proposed complex polarization topologies (reminiscent of rotational spin topologies) in low-dimensional structures. The models have predicted the formation of polarization waves, vortices and so on that can be characterized by an emergent order parameter, a so-called electric toroidal moment (G = (1/2V ) r × P(r) d 3 r; refs 20,22,25).…”

“…The presence of this axial polarization component is interesting because it suggests the formation of a multi-order-parameter state. In turn, electric-field control of toroidal order is of great interest because it offers the potential for new cross-coupled functions [20][21][22][23][24] . To explore this concept, the time-dependent evolution of both the vortex and a 1 /a 2 regions of a mixed-phase structure was explored under an applied out-of-plane voltage (15 V) using phase-field models.…”