Unveiling the mechanisms of metal-insulator transitions in V 2 O 3 : The role of trigonal distortion

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 ${\mathrm{V}}_{2}{\mathrm{O}}_{3}$.