Cooperative evolution of polar distortion and nonpolar rotation of oxygen octahedra in oxide heterostructures

Taewon Min; Wooseon Choi; Jinsol Seo; Gyeongtak Han; Kyung Song; Sangwoo Ryu; Hyungwoo Lee; Jungwoo Lee; Kitae Eom; Chang-Beom Eom; Hu Young Jeong; Young-Min Kim; Jaekwang Lee; Sang Ho Oh

doi:10.1126/sciadv.abe9053

Cooperative evolution of polar distortion and nonpolar rotation of oxygen octahedra in oxide heterostructures

Sci Adv. 2021 Apr 21;7(17):eabe9053. doi: 10.1126/sciadv.abe9053. Print 2021 Apr.

Authors

Taewon Min¹, Wooseon Choi², Jinsol Seo², Gyeongtak Han², Kyung Song³, Sangwoo Ryu⁴, Hyungwoo Lee⁴, Jungwoo Lee⁴, Kitae Eom⁴, Chang-Beom Eom⁴, Hu Young Jeong⁵, Young-Min Kim⁶, Jaekwang Lee⁷, Sang Ho Oh⁶

Affiliations

¹ Department of Physics, Pusan National University, Busan 46241, Republic of Korea.
² Department of Energy Science, Sungkyunkwan University, Suwon 16419, Republic of Korea.
³ Materials Testing and Reliability Division, Korea Institute of Materials Science (KIMS), Changwon 51508, Republic of Korea.
⁴ Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, WI 53706, USA.
⁵ UNIST Central Research Facilities (UCRF), Ulsan National Institute of Science and Technology, Ulsan 44919, Republic of Korea.
⁶ Department of Energy Science, Sungkyunkwan University, Suwon 16419, Republic of Korea. sanghooh@skku.edu jaekwangl@pusan.ac.kr youngmk@skku.edu.
⁷ Department of Physics, Pusan National University, Busan 46241, Republic of Korea. sanghooh@skku.edu jaekwangl@pusan.ac.kr youngmk@skku.edu.

Abstract

Polarity discontinuity across LaAlO₃/SrTiO₃ (LAO/STO) heterostructures induces electronic reconstruction involving the formation of two-dimensional electron gas (2DEG) and structural distortions characterized by antiferrodistortive (AFD) rotation and ferroelectric (FE) distortion. We show that AFD and FE modes are cooperatively coupled in LAO/STO (111) heterostructures; they coexist below the critical thickness (t _c) and disappear simultaneously above t _c with the formation of 2DEG. Electron energy-loss spectroscopy and density functional theory (DFT) calculations provide direct evidence of oxygen vacancy (V _O) formation at the LAO (111) surface, which acts as the source of 2DEG. Tracing the AFD rotation and FE distortion of LAO reveals that their evolution is strongly correlated with V _O distribution. The present study demonstrates that AFD and FE modes in oxide heterostructures emerge as a consequence of interplay between misfit strain and polar field, and further that their combination can be tuned to competitive or cooperative coupling by changing the interface orientation.

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