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

Reviews and Highlights	Quantum Science	Molecular and Soft-matter	Ultrafast Nano-optics and Nanophotonics	Mineralogy and Geochemistry

Anoop R. Damodaran, James D. Clarkson, Zijian Hong, Huiyun Liu, Ajay K. Yadav, C. T. Nelson, Shang-Lin Hsu, Margaret R. McCarter, Kyoung-Duck Park, Vasily Kravtsov, Alan Farhan, Yonki Dong, Zonghou Cai, Pablo Aguado-Puente, Francisco P. García-Fernández, Jorge Íñiguez, Javier Junquera, Andreas Scholl, Markus B. Raschke, Long-Qin Chen, Dillon D. Fong, Ramamoorthy Ramesh, and Lane W. Martin
Nature Materials 16, 1003 (2017).
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Systems that exhibit phase competition, order parameter coexistence, and emergent order parameter topologies constitute a major part of modern condensed-matter physics. Here, by applying a range of characterization techniques, and simulations, we observe that in PbTiO3/SrTiO3 superlattices all of these effects can be found. By exploring superlattice period-, temperature- and field-dependent evolution of these structures, we observe several new features. First, it is possible to engineer phase coexistence mediated by a first-order phase transition between an emergent, low-temperature vortex phase with electric toroidal order and a high-temperature ferroelectric a1/a2 phase. At room temperature, the coexisting vortex and ferroelectric phases form a mesoscale, fibre-textured hierarchical superstructure. The vortex phase possesses an axial polarization, set by the net polarization of the surrounding ferroelectric domains, such that it possesses a multi-order-parameter state and belongs to a class of gyrotropic electrotoroidal compounds. Finally, application of electric fields to this mixed-phase system permits interconversion between the vortex and the ferroelectric phases concomitant with order-of-magnitude changes in piezoelectric and nonlinear optical responses. Our findings suggest new cross-coupled functionalities.