Organic crystal reveals how Joule heating stabilizes resistive switching
Metal-insulator transitions (MITs), in which a material changes from a metallic state with low resistivity to an insulating state because of a change in an external parameter, such as temperature, pressure or an electric field, are a central topic in fundamental physics research.
This study delves into the intricate phenomenon of resistive switching in a bulk organic conductor, specifically the organic conductor (d7-DMe-DCNQI)2Cu. This material undergoes a sharp metal-insulator transition (MIT) around 79 K, transitioning from a metallic phase above this temperature to an insulating phase below it. The research team, led by Professor Tetsuaki Itou from Tokyo University of Science, aimed to elucidate the underlying mechanisms of resistive switching in such systems.
The study employed a unique experimental setup, suspending the needle-like crystal inside a Teflon tube in a helium gas atmosphere, with electrical current supplied by gold wires. This configuration minimized heat dissipation, allowing the researchers to observe the resistive-switched state with minimal interference.
The researchers measured the sample's resistance at zero and three different applied currents (0.3, 0.5, and 2.0 mA), finding a sharp resistance jump at zero current, as expected. However, below the transition temperature, resistance values gradually increased for applied currents, ultimately stabilizing at the lowest measured ambient temperatures when using 2.0 mA current.
Proton nuclear magnetic resonance (1H-NMR) measurements revealed that both metallic and insulating phases coexist in the intermediate resistance state. Furthermore, the team analyzed the effect of Joule heating using 1H-NMR signal intensity. They discovered that under equilibrium conditions, the sample temperature matched the ambient temperature.
However, when an applied current of 2.0 mA was applied above the transition temperature, the sample temperature rose well above the ambient temperature and locked close to the MIT temperature, leading to a temperature-locking phenomenon.
This temperature-locking effect resulted in an inverse Ohm's law, where voltage and current have an inverse proportionality. The researchers attribute this unusual behavior to spatial self-organization within the bulk crystal, where a metallic current filament forms and adjusts its thickness based on the applied current.
The findings provide a microscopic understanding of the resistive switching phenomenon in bulk organic MIT systems under extreme conditions. The temperature-locking phenomenon discovered could potentially lead to the development of durable and efficient resistive switching devices, as it challenges the notion of resistive switching being solely reliant on uniform Joule heating.
Instead, the study highlights the complex interplay between phase transition, heat flow, and electrical transport through thermal self-organization.
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