Raman spectroscopy and phonon dynamics in strained ${\mathrm{V}}_2{\mathrm{O}}_3$

Transition metal oxides are known to have a strong interplay of many degrees of freedom giving rise to their rich phase diagrams with competing ground states. The Mott material ${\mathrm{V}}_2{\mathrm{O}}_3$ hosting a room- and low-temperature metal-insulator transition is a great example where electronic, structural and magnetic ordering are the directors at play. By combining first-principle calculations and Raman spectroscopy, we study the phonon dynamics of ${\mathrm{V}}_2{\mathrm{O}}_3$ to gain further understanding in the interplay of these ordering mechanisms driving the transitions. First-principle calculations show that the Raman active vibrations correspond to the structural distortions occurring in the phase diagram. Additionally, Raman spectroscopy is performed on a unique series of epitaxial strained $1.5\%$ Cr-doped ${\mathrm{V}}_2{\mathrm{O}}_3$ thin films, where both paramagnetic insulating, metallic, as well as intermediate electronic states are stabilized. This has led to identifying the importance of the local V-V dimer elongation that drives both the room- and low-temperature MIT in ${\mathrm{V}}_2{\mathrm{O}}_3$ compounds.

Figure 1

The $c/a$ ratio change in the RT Mott MIT induced by $\mathrm{Cr}$ doping in bulk ${\mathrm{V}}_2{\mathrm{O}}_3$ (gray circles) and by the use of ${\left({\mathrm{Cr}}_x{\mathrm{Fe}}_{1-x}\right)}_2{\mathrm{O}}_3$ buffer layers to induce epitaxial strain in $1.5\%$ Cr-doped ${\mathrm{V}}_2{\mathrm{O}}_3$ thin films (red circles). The bulk data was taken from [1], while the thin film data was adopted from [5].

Figure 2

Crystal structure and atomic-resolved phonon dynamics of room-temperature ${\mathrm{V}}_2{\mathrm{O}}_3$. (a) The conventional corundum unit cell with the marked short V-V bond. (b) The calculated phonon spectrum projected on a rhombohedral Brillouin zone path and the corresponding phonon density of states. (c) An upper and side view of the $E_g\left(1\right)$ mode. (d) A side view of the $A_{1g}\left(1\right)$ mode. Note that the eigenvectors are not scaled, the amplitude of the oxygen eigenvectors has been increased to make their motion visible. Cyan and red atoms correspond to vanadium and oxygen, respectively.

Figure 3

DFPT calculated phonon spectrum and corresponding phonon DOS of low-temperature monoclinic AFI ${\mathrm{V}}_2{\mathrm{O}}_3$.

Figure 4

Room-temperature Raman spectra comparison of $1.5\%$ Cr-doped ${\mathrm{V}}_2{\mathrm{O}}_3$ thin films upon applying epitaxial strain through different buffer layers. The $0.6\%$ Cr-doped ${\mathrm{V}}_2{\mathrm{O}}_3/{\mathrm{Cr}}_2{\mathrm{O}}_3/\mathrm{sapphire}$ is included as a reference of a pure and relaxed case of ${\mathrm{V}}_2{\mathrm{O}}_3$ thin film. The Raman spectra of ${\mathrm{Cr}}_2{\mathrm{O}}_3$ thin film on sapphire (with rescaled intensity) is included to show the Raman peaks originating from ${\mathrm{Cr}}_2{\mathrm{O}}_3$ (square) and sapphire (star). The dotted lines indicate the PM and PI $A_{1g}\left(1\right)$ vibration mode of bulk ${\mathrm{V}}_2{\mathrm{O}}_3$. (b) A zoom in on the $A_{1g}\left(1\right)$ mode indicating an abrupt change between PM and PI with the thin film on the $34\%$ Fe buffer layer showing a broadened peak with a mixed nature. (c) The frequency of $A_{1g}\left(1\right)$ mode under the different strain levels. The room-temperature resistivity (RTR) is plotted in the same range to compare to the trend of the $A_{1g}\left(1\right)$ frequency with strain. Bulk PI lattice parameter is defined as $0.0\%$ as a reference for the different in-plane strain. Note that for the $A_{1g}\left(1\right)$ frequency of the sample with $34\%$ Fe buffer layer, two empty data points are used to indicate the two deconvoluted peaks inferred from (b), corresponding to PM and PI.

Figure 5

Temperature-dependent Raman spectrum of $0.6\%$ Cr-doped ${\mathrm{V}}_2{\mathrm{O}}_3/{\mathrm{Cr}}_2{\mathrm{O}}_3/\mathrm{sapphire}$. All vibrational modes are labeled according to the DFT calculation except the Magnon mode (marked as M). The frequency labeled by star and square represent the peaks from the sapphire substrate and ${\mathrm{Cr}}_2{\mathrm{O}}_3$ buffer layer, respectively. Finally, the intensity of the spectrum is plotted on a logarithmic scale, and the software interpolates the data between the data points.

Figure 6

Experimental temperature dependence and Cochran law fit of (a) $A_g\left(7\right)$, (b) Magnon, and ($\mathrm{c})A_g\left(2\right)$.

Figure 7

Low-temperature Raman spectra of $1.5\%$ Cr-doped ${\mathrm{V}}_2{\mathrm{O}}_3$ thin films upon epitaxial strain comparison at 80 K. The vibration mode contributions from sapphire and the ${\mathrm{Cr}}_2{\mathrm{O}}_3$ buffer layer are labeled as star and square, respectively.

Figure 8

Low-temperature phase transition of $1.5\%$ Cr-doped ${\mathrm{V}}_2{\mathrm{O}}_3$ under different strain levels. The phase transition change from PM-to-AFI and PI-to-AFI transition by increasing Fe percentage in the buffer layer.