Physical Review Research (Oct 2020)

Free and defect-bound (bi)polarons in LiNbO_{3}: Atomic structure and spectroscopic signatures from ab initio calculations

  • Falko Schmidt,
  • Agnieszka L. Kozub,
  • Timur Biktagirov,
  • Christof Eigner,
  • Christine Silberhorn,
  • Arno Schindlmayr,
  • Wolf Gero Schmidt,
  • Uwe Gerstmann

DOI
https://doi.org/10.1103/PhysRevResearch.2.043002
Journal volume & issue
Vol. 2, no. 4
p. 043002

Abstract

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Polarons in dielectric crystals play a crucial role for applications in integrated electronics and optoelectronics. In this work, we use density-functional theory and Green's function methods to explore the microscopic structure and spectroscopic signatures of electron polarons in lithium niobate (LiNbO_{3}). Total-energy calculations and the comparison of calculated electron paramagnetic resonance data with available measurements reveal the formation of bound polarons at Nb_{Li} antisite defects with a quasi-Jahn-Teller distorted, tilted configuration. The defect-formation energies further indicate that (bi)polarons may form not only at Nb_{Li} antisites but also at structures where the antisite Nb atom moves into a neighboring empty oxygen octahedron. Based on these structure models, and on the calculated charge-transition levels and potential-energy barriers, we propose two mechanisms for the optical and thermal splitting of bipolarons, which provide a natural explanation for the reported two-path recombination of bipolarons. Optical-response calculations based on the Bethe-Salpeter equation, in combination with available experimental data and new measurements of the optical absorption spectrum, further corroborate the geometries proposed here for free and defect-bound (bi)polarons.