Theory of polarization enhancement in epitaxial BaTiO3_3/SrTiO3_3 superlattices

The spontaneous polarization of epitaxial BaTiO$_3$/SrTiO$_3$ superlattices is studied as a function of composition using first-principles density functional theory within the local density approximation. With the in-plane lattice parameter fixed to that of bulk SrTiO$_3$, the computed superlattice polarization is enhanced above that of bulk BaTiO$_3$ for superlattices with BaTiO$_3$ fraction larger than 40%. In contrast to their bulk paraelectric character, the SrTiO$_3$ layers are found to be {\it tetragonal and polar}, possessing nearly the same polarization as the BaTiO$_3$ layers. General electrostatic arguments elucidate the origin of the polarization in the SrTiO$_3$ layers, with important implications for other ferroelectric nanostructures.Comment: 4 pages, 2 Figures, 1 Tabl

Superlattice-induced ferroelectricity in charge-ordered La1/3_{1/3}Sr2/3_{2/3}FeO3_{3}

Charge-order-driven ferroelectrics are an emerging class of functional materials, distinct from conventional ferroelectrics, where electron-dominated switching can occur at high frequency. Despite their promise, only a few systems exhibiting this behavior have been experimentally realized thus far, motivating the need for new materials. Here, we use density functional theory to study the effect of artificial structuring on mixed-valence solid-solution La$_{1/3}$Sr$_{2/3}$FeO$_{3}$ (LSFO), a system well-studied experimentally. Our calculations show that A-site cation (111)-layered LSFO exhibits a ferroelectric charge-ordered phase in which inversion symmetry is broken by changing the registry of the charge order with respect to the superlattice layering. The phase is energetically degenerate with a ground-state centrosymmetric phase, and the computed switching polarization is 39 $\mu$C/cm$^{2}$, a significant value arising from electron transfer between Fe ions. Our calculations reveal that artificial structuring of LSFO and other mixed valence oxides with robust charge ordering in the solid solution phase can lead to charge-order-induced ferroelectricity

First-principles study of symmetry lowering in relaxed BaTiO3/SrTiO3 superlattices

The crystal structure and local spontaneous polarization of (BaTiO3)m/(SrTiO3)n superlattices is calculated using a first-principles density functional theory method. The in-plane lattice constant is 1% larger than the SrTiO3 substrate to imitate the relaxed superlattice structure and the symmetry is lowered to monoclinic space group Cm which allows polarization to develop along the [110] and [001] directions. The polarization component in the [110] direction is found to develop only in the SrTiO3 layers and falls to zero in the BaTiO3 layers, whereas the polarization in the [001] direction is approximately uniform throughout the superlattice. These findings are consistent with recent experimental data and first-principles results for epitaxially strained BT and ST.Comment: 4 pages, 2 figure

Ideal barriers to polarization reversal and domain-wall motion in strained ferroelectric thin films

The ideal intrinsic barriers to domain switching in c-phase PbTiO_3 (PTO), PbZrO_3 (PZO), and PbZr_{1-x}Ti_xO_3 (PZT) are investigated via first-principles computational methods. The effects of epitaxial strain on the atomic structure, ferroelectric response, barrier to coherent domain reversal, domain-wall energy, and barrier to domain-wall translation are studied. It is found that PTO has a larger polarization, but smaller energy barrier to domain reversal, than PZO. Consequentially the idealized coercive field is over two times smaller in PTO than PZO. The Ti--O bond length is more sensitive to strain than the other bonds in the crystals. This results in the polarization and domain-wall energy in PTO having greater sensitivity to strain than in PZO. Two ordered phases of PZT are considered, the rock-salt structure and a (100) PTO/PZO superlattice. In these simple structures we find that the ferroelectric properties do not obey Vergard's law, but instead can be approximated as an average over individual 5-atom unit cells.Comment: 9 pages, 13 figure

Learning from Failure: An Action Research Case Study on Developing Growth Mindset Through Academic Risk-Taking in an Athletic Training Program

The employment of academic risk-taking and growth mindset instructional practices in educational fields, though supported in literature, is limited and not commonly documented in the classroom. The purpose of this action research case study was to determine the impact of facilitation of growth mindset through modification of instructional practices in athletic training clinical courses at an Institution of Higher Learning. Constructive failure and growth mindset theories supported the mixed-methods research approach and the modification of instructional practices made during the study. Data were collected through growth mindset and academic risk-taking instruments and through semi-structured survey questions. The data analysis was performed through thematic coding and descriptive statistics. The findings of this case study revealed that even small adjustments to instructional practices generated improvements in participant views of their abilities and responses to challenging situations. When participants were provided with immediate feedback during applied decision-making activities, they reported the feedback helpful and supportive in critical thinking. Allowing small choice in challenging situations provided ownership for the participants, which in turn resulted in the selection of challenging activities. Small classroom adjustments to allow for formative participation opportunities where accuracy was not the focus were successful at providing participants with supportive environments where learning was not feared, and knowledge was the primary objective. The researcher suggests applying the instructional methods utilized in the present research study, such as academic risk-taking activities through formative activities where the focus of the outcome is to increase learning rather than accuracy

Effects of Vacancies on Properties of Relaxor Ferroelectrics: a First-Principles Study

A first-principles-based model is developed to investigate the influence of lead vacancies on the properties of relaxor ferroelectric Pb(Sc1/2Nb1/2)O3 (PSN). Lead vacancies generate large, inhomogeneous, electric fields that reduce barriers between energy minima for different polarization directions. This naturally explains why relaxors with significant lead vacancy concentrations have broadened dielectric peaks at lower temperatures, and why lead vacancies smear properties in the neighborhood of the ferroelectric transition in PSN. We also reconsider the conventional wisdom that lead vacancies reduce the magnitude of dielectric response.Comment: 11 pages, 1 figur