TY - JOUR
T1 - Phase Coexistence and Electric-Field Control of Toroidal Order in Oxide Superlattices
JF - Nature Materials
Y1 - 2017/08//
SP - 1003
EP - 1009
A1 - Anoop R. Damodaran
A1 - James D. Clarkson
A1 - Zijian Hong
A1 - Haihua Liu
A1 - Ajay K. Yadav
A1 - Christopher T. Nelson
A1 - Shang-Lin Hsu
A1 - Margaret R. McCarter
A1 - Kyoung-Duck Park
A1 - Vasily Kravtsov
A1 - Alan Farhan
A1 - Y. Dong
A1 - Zhonghou Cai
A1 - Hua Zhou
A1 - Pablo Aguado-Puente
A1 - Pablo García-Fernández
A1 - Jorge Íñiguez
A1 - Javier Junquera
A1 - Andreas Scholl
A1 - Markus B. Raschke
A1 - Long-Qing Chen
A1 - Dillon D. Fong
A1 - Ramamoorthy Ramesh
A1 - Lane W. Martin
KW - Ferroelectrics and multiferroics
KW - interfaces and thin films
KW - Phase transitions and critical phenomena
KW - surfaces
AB - 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.
VL - 16
ER -
TY - JOUR
T1 - Stability of Polar Vortex Lattice in Ferroelectric Superlattices
JF - Nano Letters
Y1 - 2017/04//
SP - 2246
EP - 2252
A1 - Zijian Hong
A1 - Anoop R. Damodaran
A1 - Fei Xue
A1 - Shang-Lin Hsu
A1 - Jason Britson
A1 - Ajay K. Yadav
A1 - Christopher T. Nelson
A1 - Jian-Jun Wang
A1 - James F. Scott
A1 - Lane W. Martin
A1 - Ramamoorthy Ramesh
A1 - Long-Qing Chen
KW - Ferroelectric superlattices
KW - geometric length scale
KW - phase-field simulations
KW - topological structures by design
KW - ultrafine polar vortex
AB - A novel mesoscale state comprising of an ordered polar vortex lattice has been demonstrated in ferroelectric superlattices of PbTiO3/SrTiO3. Here, we employ phase-field simulations, analytical theory, and experimental observations to evaluate thermodynamic conditions and geometric length scales that are critical for the formation of such exotic vortex states. We show that the stability of these vortex lattices involves an intimate competition between long-range electrostatic, long-range elastic, and short-range polarization gradient-related interactions leading to both an upper and a lower bound to the length scale at which these states can be observed. We found that the critical length is related to the intrinsic domain wall width, which could serve as a simple intuitive design rule for the discovery of novel ultrafine topological structures in ferroic systems.
VL - 17
ER -
TY - JOUR
T1 - Observation of Polar Vortices in Oxide Superlattices
JF - Nature
Y1 - 2018/03//
SP - 198
EP - 201
A1 - Ajay K. Yadav
A1 - Christopher T. Nelson
A1 - Shang-Lin Hsu
A1 - Zijian Hong
A1 - James D. Clarkson
A1 - Christian M. Schlepüetz
A1 - Anoop R. Damodaran
A1 - Padraic Shafer
A1 - Elke Arenholz
A1 - Liv R. Dedon
A1 - Deyang Chen
A1 - Ashvin Vishwanath
A1 - Andrew M. Minor
A1 - Long-Qing Chen
A1 - James F. Scott
A1 - Lane W. Martin
A1 - Ramamoorthy Ramesh
KW - Electronic properties and materials
KW - Ferroelectrics and multiferroics
KW - interfaces and thin films
KW - surfaces
AB - The complex interplay of spin, charge, orbital and lattice degrees of freedom provides a plethora of exotic phases and physical phenomena. In recent years, complex spin topologies have emerged as a consequence of the electronic band structure and the interplay between spin and spin–orbit coupling in materials. Here we produce complex topologies of electrical polarization—namely, nanometre-scale vortex–antivortex (that is, clockwise–anticlockwise) arrays that are reminiscent of rotational spin topologies—by making use of the competition between charge, orbital and lattice degrees of freedom in superlattices of alternating lead titanate and strontium titanate layers. Atomic-scale mapping of the polar atomic displacements by scanning transmission electron microscopy reveals the presence of long-range ordered vortex–antivortex arrays that exhibit nearly continuous polarization rotation. Phase-field modelling confirms that the vortex array is the low-energy state for a range of superlattice periods. Within this range, the large gradient energy from the vortex structure is counterbalanced by the corresponding large reduction in overall electrostatic energy (which would otherwise arise from polar discontinuities at the lead titanate/strontium titanate interfaces) and the elastic energy associated with epitaxial constraints and domain formation. These observations have implications for the creation of new states of matter (such as dipolar skyrmions, hedgehog states) and associated phenomena in ferroic materials, such as electrically controllable chirality.
VL - 530
ER -