Researchers in Japan have identified how Joule heating can stabilise an unusual resistive state in a bulk organic conductor, offering new insight into a phenomenon that could influence the development of next-generation memory, optoelectronic and neuromorphic computing technologies.
The study, led by Professor Tetsuaki Itou of the Department of Applied Physics at Tokyo University of Science (TUS), shows that resistive switching in the material is not simply the result of uniform heating. Instead, the researchers found that electrical transport, heat flow and a metal-insulator transition work together to create a self-organised state inside the crystal.
The findings were published in Volume 26, Issue 2 of Physical Review Applied on August 17, 2026, and were selected as an Editors’ Suggestion.
Study reveals temperature-locking effect
Metal-insulator transitions occur when a material changes between a low-resistivity metallic state and a high-resistivity insulating state in response to an external condition such as temperature, pressure or an electric field.
In some materials, applying an electric field or current near the transition point can cause the resistance to suddenly fall while the material is still in its insulating phase. This behaviour, known as volatile resistive switching, has attracted interest for applications including resistive memory, optoelectronics and neuromorphic computing.
Despite that interest, the physical processes behind the switching remain difficult to pin down. Joule heating is known to be involved, but understanding exactly how heat generation and heat dissipation interact with the metal-insulator transition has remained a challenge.
The latest study addresses this question by examining a bulk organic conductor with an unusually sharp transition and weak heat dissipation.
“While resistive switching is actively studied from a device application perspective, what actually happens inside the material during this process is not always fully understood,” said Prof. Itou. “In this study, we present an experimental elucidation of the volatile resistive-switched state, aiming to establish a microscopic basis for understanding the thermal self-organization that links phase coexistence, heat flow, and electrical transport.”
Organic crystal provides a different test environment
The researchers studied a needle-shaped crystal of the organic conductor (d7-DMe-DCNQI)2Cu, a deuterated derivative of N,N?-dicyanoquinonediimine, or DCNQI.
The material undergoes a sharp metal-insulator transition around 79 K during heating and around 78 K during cooling. Above approximately 79 K, it behaves as a metal, while below approximately 78 K it enters the insulating phase.
For the experiment, the crystal was suspended inside a Teflon tube so that it did not touch the walls. Gold wires attached to both ends supplied the electrical current. The setup was placed in a helium gas atmosphere, limiting the main routes through which heat could escape to the surrounding gas and the connecting wires.
This arrangement gave the researchers a system with considerably weaker heat dissipation than the substrate-supported thin films commonly used in resistive-switching studies.
The research team included Riku Ishii and Assistant Professor Takayoshi Kouchi from TUS, Dr Hiroshi Oike from the National Institute for Materials Science, Professor Fumitaka Kagawa from the Institute of Science Tokyo and Dr Reizo Kato from RIKEN’s Cluster for Pioneering Research.
Current creates an intermediate resistance state
The team first measured the crystal’s resistance as the ambient temperature changed, both without an applied current and under currents of 0.3, 0.5 and 2.0 mA.
With no current applied, the material showed the expected sharp change in resistance around the transition. Under an applied current, however, the resistance changed more gradually as the temperature fell below the transition point.
The most notable behaviour appeared at 2.0 mA. At this current, an intermediate resistance state remained stable even at the lowest temperatures measured, indicating the formation of a resistive-switched state.
The researchers then turned to proton nuclear magnetic resonance, or 1H-NMR, to determine what was happening inside the crystal.
The measurements showed that the intermediate resistance state was not a completely separate phase. Instead, metallic and insulating regions existed together within the material.
Joule heating keeps the material near its transition point
Further 1H-NMR measurements provided evidence for the role of Joule heating.
Under equilibrium conditions, the temperature of the sample followed the surrounding ambient temperature, as expected. A similar relationship was observed when a 2.0 mA current was applied above the metal-insulator transition.
The behaviour changed below the transition temperature. In the intermediate resistance state, the NMR signal intensity remained almost unchanged even as the ambient temperature varied.
This indicated that the sample itself was being heated significantly above the surrounding temperature by the applied current. More importantly, its temperature remained close to the metal-insulator transition temperature.
The researchers describe this as a temperature-locking phenomenon.
In effect, Joule heating prevents the sample from simply cooling into a fully insulating state. Instead, the electrical heating and heat loss reach a balance that keeps the material close to its transition point.
Metallic filament explains unusual electrical behaviour
The team also observed an unusual inverse Ohm’s law in the resistive-switched state, meaning that voltage decreased as current increased rather than following the conventional relationship between voltage and current.
The researchers attribute this behaviour to the temperature-locking effect.
As the current changes, the crystal adjusts its internal structure to maintain the balance between Joule heating and heat dissipation. A metallic current filament forms within the bulk crystal, with its thickness changing as the applied current increases or decreases.
When the current rises, the filament becomes thicker. When the current falls, it becomes thinner.
This spatial self-organisation allows the crystal to maintain a relatively constant level of Joule heating while keeping its temperature close to the metal-insulator transition.
Findings could guide future resistive switching technologies
The results suggest that volatile resistive switching in this material cannot be explained simply by assuming that the entire crystal heats uniformly.
Instead, the researchers found a nonequilibrium steady state in which metallic and insulating phases coexist while thermal and electrical processes continuously influence one another.
“ These findings form the foundation for understanding the intermediate resistance state induced by Joule heating in bulk organic MIT systems under extreme conditions,” said Prof. Itou. “Moreover, the temperature-locking phenomenon uncovered in our tests provides a strategy for developing durable and efficient resistive switching devices.”
The findings could help researchers better understand how heat and phase transitions interact in resistive-switching materials. Controlling this thermal self-organisation may eventually offer a route towards switching technologies that operate more efficiently while limiting energy dissipation.
For emerging applications such as resistive memory and neuromorphic computing, where controlled changes in resistance are central to device operation, a clearer understanding of these mechanisms could prove important in designing more reliable next-generation electronic systems.
Reference
Title of original paper: Volatile resistive-switched state in a bulk organic conductor with a sharp metal-insulator transition
Journal: Physical Review Applied
DOI: 10.1103/3yjz-8f9d
