Seminars

  • Manipulation of physical properties in oxide thin films by a local inversion symmetry breaking induced by flexoelectricity

  • Speaker : 박성민 박사/Sung Min Park
    Affiliation : Center for Correlated Electron Systems, Institute for Basic Science & Seoul National University
    Date : July 5, 2019 2:00 PM
    Place : Buliding 108 Room 401
    Contact : ysoh@unist.ac.kr
    Host : Prof.Yoon Seok Oh
  • Abstract

  • Manipulation of physical properties in oxide thin films by a local inversion symmetry breaking induced by flexoelectricity

     

    While conventionally tuned by the electrical bias of a scanning probe, the physical properties in oxide thin films can also be controlled by mechanical force by virtue of a flexoelectricity. The flexoelectricity refers to the electromechanical coupling between electrical polarization and strain-gradient, which appears in every dielectric materials [1]. This universal effect is inversely proportional to the length scale, thereby promising novel, enhanced functionalities at reduced dimensions.
    In 2011, our group discovered that the flexoelectricity can play very important roles in oxide thin films [2]. We proved that the strain gradients, which are generated during strain relaxation process in film growth, can 6 or 7 orders of magnitude larger than typical values reported for bulk oxides. We demonstrated that such large strain gradient in nanoscale can be used to tune domain configuration and hysteric behaviors of ferroelectric HoMnO3 epitaxial films. Since then, lots of works have followed to tune many physical properties of epitaxial thin films, including polarization rotation, anisotropic photocurrents in morphotropic phase boundaries and defect formations. However, reversible change of physical properties by this method is inevitably challenging since the built-in strain gradients by strain relaxation remains unchanged.
    Recently, we used flexoelectricity created by the mechanically loaded scanning probe microscopy (SPM) tip [3–7]. This method is called SPM tip pressing that is applying pressure using sharp SPM tip to induce local stain gradients, thus, break the local inversion symmetry. In this way, we can not only generate huge strain gradients locally but also control the location and amount of strain gradients in systematic manner. In this talk, I will show that it is possible to tune the physical properties of oxide thin films by breaking the space inversion symmetry induced by flexoelectricity, and three works related to this will be addressed. Our studies on oxide thin films suggest that many interesting control of local physical properties can be possible by means of pure mechanical force.

    [1] S. M. Kogan, Sov. Phys. Solid State 5, 2069 (1964).
    [2] D. Lee et al., Phys. Rev. Lett. 107, 057602 (2011).
    [3] H. Lu et al., Science 336, 59 (2012)
    [4] S. Das et al., Nat. Commun. 8, 615 (2017).
    [5] S. M. Park et al., Nat. Nanotech. 13, 366 (2018).
    [6] S. Das and S. M. Park et al., Nat. Commun. 10, 537 (2019).
    [7] S. M. Park et al., Submitted