The insulator-insulator and the insulator-mixed-metal-insulator transitions in self-heated Al${}_x$V${}_{1-x}$O${}_2$ single crystals and their visualization

In addition to the well-known insulator-metal transition above room temperature in pure VO${}_2$, Al${}_x$V${}_{1-x}$O${}_2$ exhibits an insulator-insulator transition at temperatures above room temperature and below the insulator-metal transition temperature, consisting of an increase in resistance by a factor ≤ 2. The wealth of static and dynamic domain structures discovered in the mixed-metal-insulator state of free-standing, high-quality, pure VO${}_2$ single crystals via optical microscopy synchronous with dc I-V tracing has inspired the use of this technique in these more complex materials. Reported herein are I(V) measurements conducted on crystals of Al${}_x$V${}_{1-x}$O${}_2$ (0.007≤ × ≤0.02) at various ambient temperatures (T${}_o$). Prominent features (switching) on the I(V) loops are easily identified with the onset of the insulator-insulator and of the mixed-metal-insulator states, both induced by self-heating (Joule heating). The switching results from instabilities of two different types of negative differential resistance regimes. The applied power at the onset of each switching scales with the excess temperature above ambient. For both transitions, optical microscopy synchronous with dc I-V tracing visualizes the expansion of each phase within the background upon the current increasing, and its contraction upon the current decreasing. (© 2021 Wiley‐VCH GmbH)