The metal-insulator transition of nanoscaled ${\mathrm{VO}}_2$ devices is drastically different from the smooth transport curves generally reported. The temperature driven transition occurs through a series of resistance jumps ranging over 2 decades in magnitude, indicating that the transition is caused by avalanches. We find a power law distribution of the jump sizes, demonstrating an inherent property of the ${\mathrm{VO}}_2$ films. We report a surprising relation between jump magnitude and device size. A percolation model captures the general transport behavior, but cannot account for the statistical behavior.

Watching a metal filament grow: Resistive switching is a process in which the electrical resistance of a sample changes abruptly in response to a voltage pulse, often by orders of magnitude. This process is at the heart of many neuromorphic computing approaches but visualizing it in both space and time is tricky. del Valle et al . monitored the resistive switching in three different vanadium oxide compounds by measuring time- and space-resolved optical reflectivity (see the Perspective by Hilgenkamp and Gao). A characteristic conducting filament was quickly nucleated on the inhomogeneities in the sample and then propagated due to Joule heating. —JS

Periodic pinning arrays can give rise to commensurability effects for the vortex pinning in superconductors. We studied the influence of size and separation of the individual pinning centers made out of magnetic and nonmagnetic dots. While there is some dependence on the dot size, the periodic pinning changes its character drastically, as soon as the separation between dots becomes comparable to the coherence length of the superconductor. This can be understood by a crossover from a weak pinning regime to a superconducting wire network regime with decreasing dot separation. In addition, the comparison between the periodic pinning of magnetic dot arrays and nonmagnetic dot arrays shows that the former generally show more pronounced periodic pinning. This indicates conclusively a magnetic contribution to the pinning mechanism.

We have performed first-order reversal-curve measurements of the temperature-driven metal-insulator transition in ${\text{VO}}_2$ thin films, which enable quantitative analysis of the hysteresis behavior. An unexpected tail-like feature in the contour plot of the reversal-curve distribution indicates the existence of metallic domains, even at temperatures below the closing of the hysteresis. These domains interact with the surrounding medium and change the reversal path relative to a path from a fully insulating state. With this in mind, and assuming that such interaction persist through the entire phase transition, we develop a model where the driving force (or energy barrier) in charge of opening a hysteresis in ${\text{VO}}_2$ are interdomain interactions. This model is intrinsically different from the Preisach model usually used to describe hysteresis; given that it looks for the microscopic origin of the hysteresis, and provides physical parameters to characterize it.

The interdependences of different phase transitions in Mott materials are fundamental to the understanding of the mechanisms behind them. One of the most important relations is between the ubiquitous structural and electronic transitions. Using IR spectroscopy, optical reflectivity, and x-ray diffraction, we show that the metal-insulator transition is coupled to the structural phase transition in ${\mathrm{V}}_2{\mathrm{O}}_3$ films. This coupling persists even in films with widely varying transition temperatures and strains. Our findings are in contrast to recent experimental findings and theoretical predictions. Using ${\mathrm{V}}_2{\mathrm{O}}_3$ as a model system, we discuss the pitfalls in measurements of the electronic and structural states of Mott materials in general, calling for a critical examination of previous work in this field. Our findings also have important implications for the performance of Mott materials in next-generation neuromorphic computing technology.

In this paper we investigate the effect of disorder on highly correlated electron systems, which exhibit metal-insulator transition (MIT) and structural-phase transition (SPT). We show that the effect of ion irradiation is strikingly different between ${\mathrm{V}}_2{\mathrm{O}}_3$ and $\mathrm{V}{\mathrm{O}}_2$, two otherwise similar materials. Upon irradiation, the MIT and SPT temperatures in ${\mathrm{V}}_2{\mathrm{O}}_3$ decrease drastically at low absolute dosages, much lower than for $\mathrm{V}{\mathrm{O}}_2$. At a low threshold dose, the insulating state of ${\mathrm{V}}_2{\mathrm{O}}_3$ drastically collapses into a metallic state. Contrary to this, irradiation of $\mathrm{V}{\mathrm{O}}_2$ leads to a much milder reduction of the MIT and SPT temperatures and to a weak, gradual decrease of the insulating state resistivity—not suppressed even at one order of magnitude higher doses than the ${\mathrm{V}}_2{\mathrm{O}}_3$ threshold. These major differences imply that the phase transition in ${\mathrm{V}}_2{\mathrm{O}}_3$ arises from global (rather than local as in $\mathrm{V}{\mathrm{O}}_2$) physical mechanisms that are extremely sensitive to disorder. This shows that the MIT and SPT may have substantially different physical origins in different systems, with the consequent major implications for theoretical descriptions of the MIT in highly correlated electron systems.