Orientation-dependent strain effects on the metal-insulator transitions in (V0.99Cr0.01)2O3/Al2O3 thin films

Abstract

${\mathrm{V}}_{2}{\mathrm{O}}_{3}$ is an archetypical Mott material which exhibits Cr doping and/or temperature-induced paramagnetic metal to paramagnetic insulator/antiferromagnetic insulator (PM-PI/AFI) transitions. However, the strain effects on the metal-insulator transition (MIT) in thin films and the mechanisms behind them are not yet fully understood. In this study, we investigate the strain effects on the MIT in $({\mathrm{V}}_{0.99}{\mathrm{Cr}}_{0.01}{)}_{2}{\mathrm{O}}_{3}$ (VC1) thin films growing on $c$-plane (001), $r$-plane (012), and $a$-plane (110) ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ substrates. The strains can be tuned in a wide range over 1% in $\mathrm{VC}1\text{/}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ thin films by changing the substrate temperature. Remarkably, the influence of strain on the MIT differs dramatically from (001)-oriented and non-(001)-oriented $\mathrm{VC}1\text{/}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ thin films. The PM-PI transition can be effectively modulated by strain tuning in the (001) $\mathrm{VC}1\text{/}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ thin films accompanied by gradual transformation from the PM/PI phase into the AFI phase upon cooling. On the contrary, both (012) and (110) $\mathrm{VC}1\text{/}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ thin films exhibit sharp PM-AFI transitions irrespective of the strain states. The orientation-dependent strain effects on the MIT in the $\mathrm{VC}1\text{/}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ thin films could be explained by the trigonal distortion within electronic correlation scenario. The trigonal distortion of $\mathrm{V}(\mathrm{Cr}){\mathrm{O}}_{6}$ octahedra along (within) the $c$ axis ($ab$ plane) in the (001) $\mathrm{VC}1\text{/}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ thin films can be directly modulated by the strains. However, the trigonal distortion in the (012) and (110) $\mathrm{VC}1\text{/}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ thin films would hardly be affected due to the deviation of the film orientation from the $c$ axis. This explanation is experimentally confirmed by Raman spectra as a sensitive probe of the trigonal distortion in VC1 thin films. These results highlight the importance of trigonal distortion to understand the MIT in the ${\mathrm{V}}_{2}{\mathrm{O}}_{3}$ compound.