Kavli Affiliate: David A. Muller
| First 5 Authors: Yu-Tsun Shao, Sujit Das, Zijian Hong, Ruijuan Xu, Swathi Chandrika
| Summary:
Polar skyrmions are predicted to emerge from the interplay of elastic,
electrostatic and gradient energies, in contrast to the key role of the
anti-symmetric Dzyalozhinskii-Moriya interaction in magnetic skyrmions. With
the discovery of topologically-stable polar skyrmions, it is of both
fundamental and practical interest to understand the microscopic nature and the
possibility of temperature- and strain-driven phase transitions in ensembles of
such polar skyrmions. Here, we explore the emergence of a two-dimensional,
tetratic lattice of merons (with topological charge of -1/2) from a skyrmion
state (topological charge of -1) upon varying the temperature and elastic
boundary conditions in [(PbTiO3)16/(SrTiO3)16]8 lifted-off membranes. This
topological phase transition is accompanied by a change in chirality, from
net-handedness (in skyrmionic phase) to zero-net chirality (in meronic phase).
To map these changes microscopically required developing new imaging methods.
We show how scanning convergent beam electron diffraction provides a robust
measure of the local polarization simultaneously with the strain state at
sub-nm resolution, while also directly mapping the chirality of each skyrmion.
Using this, we demonstrate strain as a crucial order parameter to drive
isotropic-to-anisotropic structural transitions of chiral polar skyrmions to
non-chiral merons, validated with X-ray reciprocal space mapping and
theoretical phase-field simulations. These results revealed by our new
measurement methods provide the first illustration of systematic control of
rich variety of topological dipole textures by altering the mechanical boundary
conditions, which may offer a promising way to control their functionalities in
ferroelectric nanodevices using the local and spatial distribution of chirality
and order.
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