An applied current has revealed an intermediate-resistance state in a bulk organic crystal that changes sharply between metallic and insulating phases. Nuclear magnetic resonance measurements support the idea that the state contains metallic and insulating regions at the same time.
The material, (d7-DMe-DCNQI)2Cu, was metallic above 79 K, with a two-wire resistance of about 10 Ω. Below 78 K it was insulating, with resistance above the 1 MΩ measurement limit, putting the insulating-to-metallic resistance ratio above 10^5. Heating and cooling transitions occurred within ±0.5 K, with an approximately ±1 K hysteresis loop.
A current-dependent switch
The switching temperature changed sharply with the applied current. During cooling at constant current, the resistance switched to the insulating state at 67.8 K with 0.3 mA and at 12.0 K with 0.5 mA. At 2.0 mA, the intermediate-resistance state persisted down to zero ambient temperature.
The reported transport and NMR measurements were made on a needlelike bulk single crystal measuring 0.13 × 0.13 × 6.3 mm³. The researchers paired resistance readings with NMR measurements of signal relaxation and total intensity to examine the phases present and the sample’s average temperature under current.
Two phases in one crystal
NMR relaxation data from the intermediate state were described by a double-exponential fit. The fast component matched the insulating phase, while the slow component matched the metallic phase. That correspondence supports spatial coexistence of both phases.
The NMR spectrum reinforced that reading, combining characteristics of the metallic and insulating phases, with its fast component matching the insulating phase and its slow component coinciding with the metallic phase. As ambient temperature decreased under 2.0 mA, the estimated metallic-phase volume fraction fell, a pattern consistent with a thinning current filament.
Heat holds the state near the transition
Below the 79 K transition, NMR intensity remained nearly constant as ambient temperature decreased under 2.0 mA. That indicated the sample’s average temperature stayed nearly locked near 79 K even as the surroundings became colder.
The authors interpret this temperature locking as a balance between Joule heating, the heat generated by current, and heat escaping from a crystal that loses heat only weakly. They propose that the material’s very sharp transition helps keep the average temperature near 79 K.
The transport pattern was consistent with that interpretation. In the intermediate state, voltage varied inversely with current, so power—the current multiplied by voltage—stayed constant as current changed at a fixed ambient temperature. As ambient temperature fell, the power associated with the state increased and was reported to follow P ∝ (79 K − T_ambient).
A mechanism still tied to one material
The result remains a laboratory observation in a specific bulk organic crystal and measurement setup. The study does not report how many crystals or independent experimental replicates were examined, and it provides no uncertainty estimates for the NMR fits or reported trends. NMR intensity gives an average temperature, not a direct map of local temperature gradients or current-filament geometry.
The findings therefore support an interpretation of switching in this bulk organic system, rather than a demonstration of a practical device. Whether the behavior repeats across independent crystals, how the filament’s local geometry changes, and whether the effect can become durable or scalable remain open questions.
Paper data and sources
Original title: Volatile resistive-switched state in a bulk organic conductor with a sharp metal-insulator transition
Authors: Riku Ishii, Ryo Motohashi, Keitaro Tada et al.
Journal/Repository: Physical Review Applied 26, 024041 (2026)
Status: Peer-reviewed
First online: 2026-08-20
DOI: 10.1103/3yjz-8f9d
Original paper · Full text