Mott breakdown effect in spinel MgTi1.2V0.8O4

Materials in which a Mott insulator to metal transition (Mott breakdown) can be induced using a moderate dc current (with an electric field that is much smaller than the corresponding Zener breakdown voltage) is extremely promising for electronic device applications and offer a unique platform for investigation of the underlying emergent mechanism. Here we report a Mott breakdown phenomenon in a spinel compound, namely, ${\mathrm{MgTi}}_1.2{\mathrm{V}}_0.8{\mathrm{O}}₄$, which undergoes a sharp switching of resistivity by several orders of magnitude by application of a dc current. The observed Mott breakdown effect, which is associated with a negative differential resistance as seen in the corresponding electric-voltage ($\mathit{V}$) vs electric-current ($\mathit{I}$) curve (opposite to Ohm's law dependence), occurs beyond a temperature-dependent threshold electric current (${\mathit{I}}_{\mathrm{th}}$), which corresponds to an extremely small threshold electric-field (${\mathit{E}}_{\mathrm{th}}$) (for example, ${\mathit{E}}_{\mathrm{th}}$ is found to be $\ensuremath{\sim}40\phantom{\rule{0.28em}{0ex}}\mathrm{V}/\mathrm{cm}$ at 50 K and further decreases with an increase in temperature). The system remains in the metastable metallic state only in the presence of high-current flow upto the lowest measured temperature and switches back to the highly insulating state when the current flow is switched off. Associated experimental observations suggest against a Joule-heating driven change in state and hint towards an electronic mechanism. Using first-principles calculation and several experimental probes, we further investigate the origin of the driving mechanism. The presence of two Jahn-Teller active ions, i.e., ${\mathrm{Ti}}³⁺$ ($3{d}¹$---one electron among three ${t}_{2\mathrm{g}}$ levels) and ${\mathrm{V}}³⁺$ ($3{d}²$---two electrons among three ${t}_{2\mathrm{g}}$ levels) in ${\mathrm{MgTi}}_1.2{\mathrm{V}}_0.8{\mathrm{O}}₄$ along with the obtained density of states offers a unique opportunity which seems critical for the Mott breakdown phenomenon as it supports high-current induced excitation of electrons through an activated avalanche process from ${\mathrm{Ti}}³⁺\phantom{\rule{0.28em}{0ex}}{t}_2g$ levels (located at the edge of the valence band) into ${\mathrm{V}}³⁺\phantom{\rule{4pt}{0ex}}{t}_2g$ levels (located at the bottom of the conduction band) thereby removing the orbital degrees of freedom of the V and Ti electrons and leading to melting of the Jahn-Teller driven structural transition into the insulating tetragonal phase.