Javier Junquera Seminar

Thursday, February 1, 2018

Lecture Details

12:30 pm - 1:15 pm, plus Q & A
SERC Building, room 703
Title: Complex topological phases in polar oxide superlattices

Speaker: Javier Junquera, Department of Earth Science and Condensed Matter Physics, Universidad de Cantabria, Spain

Remote Viewers:
For individuals who will not be on Temple's campus, Dr. Junquera's lecture can be accessed through WebEx.  The meeting number is: TBD

Abstract

The study of systems exhibiting phase competition, order parameter coexistence, and emergent order parameter topologies constitutes a major part of modern condensed matter physics. Here, we observe that in PbTiO3/SrTiO3 superlattices all three of these effects can be found. By exploring superlattice period-, temperature-, and field-dependent evolution of these structures using an array of experimental and computational approaches, we observe several new features: 1) It is possible to engineer phase coexistence between an emergent vortex phase with electric toroidal order and a ferroelectric a1/a2 phase. 2) The vortex phase forms upon cooling from the high-temperature ferroelectric phase via a first-order transition. 3) At room temperature, the coexisting vortex and ferroelectric phases spontaneously assemble in a mesoscale, fiber-textured, hierarchical superstructure. 4) The vortex phase is found to possess 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 electrotoridal compounds that are potentially chiral and naturally gyrotropic. 5) Application of electric fields to this mixed-phase system, results in deterministic interconversion between the vortex and the ferroelectric phases which corresponds to an order of magnitude change in piezoelectric and nonlinear optical responses and is promising for new cross-coupled functionalities. The study of systems exhibiting phase competition, order parameter coexistence, and emergent order parameter topologies constitutes a major part of modern condensed matter physics. Here, we observe that in PbTiO3/SrTiO3 superlattices all three of these effects can be found. By exploring superlattice period-, temperature-, and field-dependent evolution of these structures using an array of experimental and computational approaches, we observe several new features: 1) It is possible to engineer phase coexistence between an emergent vortex phase with electric toroidal order and a ferroelectric a1/a2 phase. 2) The vortex phase forms upon cooling from the high-temperature ferroelectric phase via a first-order transition. 3) At room temperature, the coexisting vortex and ferroelectric phases spontaneously assemble in a mesoscale, fiber-textured, hierarchical superstructure. 4) The vortex phase is found to possess 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 electrotoridal compounds that are potentially chiral and naturally gyrotropic. 5) Application of electric fields to this mixed-phase system, results in deterministic interconversion between the vortex and the ferroelectric phases which corresponds to an order of magnitude change in piezoelectric and nonlinear optical responses and is promising for new cross-coupled functionalities.

Biography

Javier Junquera is a tenure professor at the Department of Earth Science and Condensed Matter Physics at the Universidad de Cantabria. He is a recognized expert on first-principles and second-principles simulations of ferroelectric oxide heterostructures writing two monographies on this topic. From the methodological point of view he is a member of the siesta (first-principles) and of the scale-up (second-principles) development team. He has published more than 60 papers in peer-review journals (including Nature, Nature Materials, Nature Communications, Physical Review Letters, Nano Letters, etc) and invited to the most important conferences in the field (including three invited talks in the March Meeting of the American Physical Society). He has been cited over 13000 times, with an h-index of 31. In 2014 the American Physical Society recognized him as an outstanding referee. http://personales.unican.es/junqueraj/