Organic crystal reveals Joule heating

A team led by Professor Tetsuaki Itou from Tokyo University of Science has experimentally elucidated how Joule heating stabilizes a volatile resistive-switched state in an organic crystal. Their findings, published in Physical Review Applied on August 31, 2026, reveal a temperature-locking phenomenon and phase coexistence driven by thermal self-organization.

While resistive switching is promising for memory and neuromorphic computing, its fundamental mechanism has remained unclear. The phenomenon has been difficult to study in conventional inorganic thin films, where heat flow into substrates can obscure the thermal response. The research team addressed this by using a bulk organic conductor, (d7-DMe-DCNQI)2Cu, which exhibits an extremely sharp metal-insulator transition (MIT) at 79 K and allows for weak heat dissipation.

Investigating a sharp transition

The crystal was suspended in a helium gas atmosphere with gold wire contacts to minimize heat loss. The researchers measured resistance under varying ambient temperatures and applied currents. At zero current, the resistance showed a sharp jump at the transition temperature. Under applied currents, however, an intermediate resistance state emerged. This state was fully stabilized down to the lowest temperatures only at a current of 2.0 mA.

Proton nuclear magnetic resonance (1H-NMR) measurements were then used to probe the material's interior. Analysis of relaxation curves indicated that metallic and insulating phases coexist within the intermediate resistance state.

The temperature-locking effect

The NMR signal intensity provided a direct measure of sample temperature. Under equilibrium conditions, sample temperature matched ambient temperature. With a 2.0 mA current applied above the MIT, the behavior was similar. Below the transition temperature, however, the NMR signal intensity remained nearly constant despite changes in the ambient temperature. This showed that Joule heating raised the sample temperature well above the ambient level and locked it close to the MIT temperature of 79 K.

In this locked state, the material exhibited an inverse Ohm's law, where voltage was inversely proportional to current. The researchers explain that this unusual behavior arises because the sample temperature stays fixed near the transition point. Joule heating continuously balances heat dissipation to maintain this constant temperature.

Self-organization and current filaments

The constant level of Joule heating is sustained by spatial self-organization within the bulk crystal. A metallic current filament forms, thickening or thinning as the applied current increases or decreases. The filament's adjustment ensures the heat generated matches the heat lost to the surroundings, preserving the temperature-locked state.

Professor Itou stated that these findings "form the foundation for understanding the intermediate resistance state induced by Joule heating in bulk organic MIT systems under extreme conditions." He also noted that the uncovered temperature-locking phenomenon "provides a strategy for developing durable and efficient resistive switching devices."

The study demonstrates that resistive switching in this system is not simple uniform heating. It is a nonequilibrium steady state where the phase transition, heat flow, and electrical transport are coupled through thermal self-organization. The team suggests that understanding this coupling may lead to resistive switching with lower energy dissipation. The research was conducted with collaborators from the National Institute for Materials Science, the Institute of Science Tokyo, and RIKEN. For more on the latest research stats and scientific fixtures, follow our coverage.