作者
Anoop R. Damodaran,James D. Clarkson,Zijian Hong,Huajun Liu,Ajay K. Yadav,Christopher T. Nelson,S.-L. Hsu,Margaret McCarter,K. D. Park,Vasily Kravtsov,Alan Farhan,Yongqi Dong,Zhonghou Cai,Hua Zhou,Pablo Aguado‐Puente,Pablo García‐Fernández,Jorgé Íñiguez,Javier Junquera,A. Schöll,Markus B. Raschke,Lei Chen,Dillon D. Fong,R. Ramesh,Lane W. Martin
摘要
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. Metal-oxide superlattices were found to possess coexisting phases; a ferroelectric phase and a vortex phase with electric toroidal order. Electric fields interconverted from one phase to another, potentially enabling new functionality.