Assessing the Ubiquity of Bloch Domain Walls in Ferroelectric Lead Titanate Superlattices

The observation of unexpected polarization textures such as vortices, skyrmions, and merons in various oxide heterostructures has challenged the widely accepted picture of ferroelectric domain walls as being Ising-like. Bloch components in the 180° domain walls of <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mrow><a:msub><a:mrow><a:mi>PbTiO</a:mi></a:mrow><a:mn>3</a:mn></a:msub></a:mrow></a:math> have recently been reported in <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mrow><c:msub><c:mrow><c:mi>PbTiO</c:mi></c:mrow><c:mn>3</c:mn></c:msub><c:mo stretchy="false">/</c:mo><c:msub><c:mrow><c:mi>SrTiO</c:mi></c:mrow><c:mn>3</c:mn></c:msub></c:mrow></c:math> superlattices and linked to domain wall chirality. While this opens exciting perspectives, the ubiquity of this Bloch component remains to be further explored. In this work, we present a comprehensive investigation of domain walls in <f:math xmlns:f="http://www.w3.org/1998/Math/MathML" display="inline"><f:mrow><f:msub><f:mrow><f:mi>PbTiO</f:mi></f:mrow><f:mn>3</f:mn></f:msub><f:mo stretchy="false">/</f:mo><f:msub><f:mrow><f:mi>SrTiO</f:mi></f:mrow><f:mn>3</f:mn></f:msub></f:mrow></f:math> superlattices, involving a combination of first- and second-principles calculations, phase-field simulations, diffuse scattering calculations, and synchrotron-based diffuse x-ray scattering. Our theoretical calculations highlight that the previously predicted Bloch polarization in the 180° domain walls in <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"><i:mrow><i:msub><i:mrow><i:mi>PbTiO</i:mi></i:mrow><i:mn>3</i:mn></i:msub><i:mo stretchy="false">/</i:mo><i:msub><i:mrow><i:mi>SrTiO</i:mi></i:mrow><i:mn>3</i:mn></i:msub></i:mrow></i:math> superlattices might be more sensitive to the boundary conditions than initially thought and is not always expected to appear. Employing diffuse scattering calculations for larger systems, we develop a method to probe the complex structure of domain walls in these superlattices via diffuse x-ray scattering measurements. Through this approach, we investigate depolarization-driven ferroelectric polarization rotation at the domain walls. Our experimental findings, consistent with our theoretical predictions for realistic domain periods, do not reveal any signatures of a Bloch component in the centers of the 180° domain walls of <l:math xmlns:l="http://www.w3.org/1998/Math/MathML" display="inline"><l:mrow><l:msub><l:mrow><l:mi>PbTiO</l:mi></l:mrow><l:mn>3</l:mn></l:msub><l:mo stretchy="false">/</l:mo><l:msub><l:mrow><l:mi>SrTiO</l:mi></l:mrow><l:mn>3</l:mn></l:msub></l:mrow></l:math> superlattices, suggesting that the precise nature of domain walls in the ultrathin <o:math xmlns:o="http://www.w3.org/1998/Math/MathML" display="inline"><o:mrow><o:msub><o:mrow><o:mi>PbTiO</o:mi></o:mrow><o:mn>3</o:mn></o:msub></o:mrow></o:math> layers is more intricate than previously thought and deserves further attention. Published by the American Physical Society 2024