Unveiling the mechanisms of metal-insulator transitions in ${\mathrm{V}}_2{\mathrm{O}}_3$: 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 ${\mathrm{V}}_2{\mathrm{O}}_3$.

Figure 1

(a) The face-sharing octahedra along the $c$ axis of the hexagonal unit cell (left). Schematic energy splitting of the $t_{2g}$ orbitals in ${\mathrm{V}}_2{\mathrm{O}}_3$ determined by the trigonal distortion (quantified by the $c/a$ ratio) (right). The vertical and horizontal inward (outward) arrows represent the contraction (expansion) of $a$- and $c$-axis lattice parameters, respectively. (b) The variation of room-temperature $a$ and $c$ values of ${\mathrm{V}}_2{\mathrm{O}}_3$ thin films grown on $c$-plane ${\mathrm{Al}}_2{\mathrm{O}}_3$ with $T_S$. The inset shows the change of $c/a$ ratio with $T_S$. The dashed line represents the $c/a$ ratio of ${\mathrm{V}}_2{\mathrm{O}}_3$ bulk at room temperature. (c) The influence of $a$-axis thermal expansion coefficient differences between ${\mathrm{Al}}_2{\mathrm{O}}_3$ and ${\mathrm{V}}_2{\mathrm{O}}_3$ on the ${\mathrm{V}}_2{\mathrm{O}}_3$ thin films during deposition and the subsequent cooling process.

Figure 2

Temperature-dependent resistivity of the ${\mathrm{V}}_2{\mathrm{O}}_3$ thin films with different $c/a$ ratios deposited under various $T_S$ in the range of 673–973 K in steps of 50 K. The solid (dotted) lines represent the cooling (warming) curves. The up arrows denote the transition temperatures of the PM-AFI or PI-AFI transitions in the cooling runs extracted from the resistivity derivatives with respect to temperatures indicating the destruction of percolation conduction (see Supplemental Material, Fig. S13). The inset shows the reproduced phase diagram of ${\mathrm{V}}_2{\mathrm{O}}_3$ by tuning the trigonal distortion.

Figure 3

(a) Temperature-dependent Raman spectra of the ${\mathrm{V}}_2{\mathrm{O}}_3$ thin film with $c/a=2.786$ ($T_S=923$ K). (b) Raman shift as a function of temperature.

Figure 4

(a) Room-temperature Raman spectra for the PM phases of the ${\mathrm{V}}_2{\mathrm{O}}_3$ thin films with various $c/a$ ratios. The short dotted lines are present to guide the eyes of the Raman modes. (b) Room-temperature $A_{1g}^L$ phonon mode (left), resistivity ratio (${\rho }_{50\text{K}}/{\rho }_{300\text{K}}$), and transition temperature ($T_{\mathrm{MIT}}$) of the PM-AFI transition (right) as a function of $c/a$ ratio.