Unveiling the mechanisms of metal-insulator transitions in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi mathvariant="normal">V</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mn>3</mml:mn></mml:msub></mml:mrow></mml:math>: The role of trigonal distortion
Ling Hu, Chuang Xie, S. J. Zhu, Min Zhu, Renhuai Wei, Xianwu Tang, W. J. Lu, Wenhai Song, J. M. Dai, Rihong Zhang, C. J. Zhang, X. B. Zhu, Yuping Sun
Abstract
The mechanisms underlying the paramagnetic metal to paramagnetic insulator (PM-PI) and antiferromagnetic insulator (PM-AFI) transitions in the archetypical correlated oxide of ${\mathrm{V}}_{2}{\mathrm{O}}_{3}$ are long-standing yet not completely resolved topics in condensed matter physics. Herein, utilizing large differences of thermal expansion coefficient between ${\mathrm{V}}_{2}{\mathrm{O}}_{3}$ and ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$, a large variation of trigonal distortion in a continuous way is realized in pure ${\mathrm{V}}_{2}{\mathrm{O}}_{3}$ thin films grown on $c$-plane ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ substrates by changing the substrate temperature during deposition. The PM-PI transition is successfully reproduced in pure ${\mathrm{V}}_{2}{\mathrm{O}}_{3}$ thin films through enhancing trigonal distortion. Furthermore, the PM phase cannot be taken for granted to exhibit identical orbital occupations and consequently, play a negligible role in triggering the PM-AFI transitions. Instead, the ${a}_{1g}$ orbital occupation gauged by the ${A}_{1g}$ phonon mode in the PM phase strongly varies with the trigonal distortion and directly determines the PM-AFI transition characteristics. Our findings unambiguously demonstrate the essential role of trigonal distortion for understanding the multiple metal-insulator transitions and open up an opportunity for manipulating them by trigonal distortion in ${\mathrm{V}}_{2}{\mathrm{O}}_{3}$.