Impact of small polarons on the properties of transition-metal oxides

Chris Van de Walle (UC, Santa Barbara)

Dec 12. 2019, 09:40 — 10:20

The impact of polarons on transport properties is well known. First-principles calculations based on density functional theory with hybrid functionals now allow us to accurately describe the role of polarons in conductivity [1-4]. Polarons also significantly affect the optical properties of materials [5,6]. In the case of complex oxides, this can even obscure the true band gap of the material. Optical conductivity measurements are frequently applied to determine gaps. In the rare-earth titanates (RTiO3), which behave as Mott insulators, the onset of the measured spectra (in the range of 0.2-0.7 eV) is commonly interpreted as the Mott-Hubbard gap. First-principles calculations, however, produce band gaps close to 2 eV for YTiO3 and GTiO3 [7,8]. This raises the question of the origin of the absorption below 1 eV observed in optical experiments. We have attributed this signal to excitation of small hole polarons. The rare-earth titanates commonly exhibit unintentional p-type conductivity, and holes localize in the form of small polarons. Configuration-coordinate diagrams enable us to model the lineshape of optical absorption. Good agreement with experiment indicates that the infrared absorption is indeed polaron-related in GdTiO3. . . . . . . . . . Work performed in collaboration with A. Janotti, J. Varley, J. Lyons, L. Bjaalie, B. Himmetoglu, L. Weston, D. Wickramaratne, M. Swift, C. Franchini, S. Stemmer, and U. Diebold, and supported by NSF, ARO and ONR. . . . . . . . . . [1] A. Janotti, M. Choi, J. B. Varley, and C. G. Van de Walle, Phys Rev. B 90, 085202 (2014). [2] J. L. Lyons, A. Janotti, and C. G. Van de Walle, J. Appl. Phys. 115, 012014 (2014). [3] M. Setvin et al., Phys. Rev. Lett. 113, 086402 (2014). [4] L. Weston, D. Wickramaratne, and C. G. Van de Walle, Phys. Rev. B 96, 100102(R) (2017). [5] J. Varley, A. Janotti, C. Franchini, and C. G. Van de Walle, Phys. Rev. B 85, 081109 (2012). [6] M. Swift, A. Janotti, and C. G. Van de Walle, Phys. Rev. B 92, 214114 (2015). [7] B. Himmetoglu, A. Janotti, L. Bjaalie, and C. G. Van de Walle, Phys. Rev. B 90, 161102(R) (2014). [8] L. Bjaalie et al., Appl. Phys. Lett. 106, 232103 (2015).

Further Information
ESI Boltzmann Lecture Hall
Associated Event:
Polarons in the 21st Century (Workshop)
Jozef Devreese (U Antwerpen)
Cesare Franchini (U of Vienna)
Georg Kresse (U of Vienna)
Jacques Tempere (U Antwerpen)