Field-induced superconductivity

k-(BEDT-TTF)Cu[N(CN)2]Br (k-Br) is an organic superconductor whose electronic state is Mott-insulating at room-temperature but turns into metallic at low temperature through a crossover around 50-100 K, possibly because of an increase of bandwidth upon thermal contraction. In our previous works, a tensile strain altered its ground state into a Mott-insulating state, too, when its thin (100-300 nm) crystal is laminated on top of SiO2/Si++ substrate and cooled down to low temperature. Although the electronic state at low temperature became completely insulating in this experiment because of the very small thermal expansion coefficient of Si substrate (2 ppm/K), one can anticipate from the T-P (temperature vs. pressure) phase diagram that somehow mixed electronic state between superconducting and Mott-insulating states can be realized when the tensile strain is much weaker. To achieve such a mixed state in the device, where phase-separation occurs between superconducting and Mott-insulating states, we have chosen Nb-doped SrTiO3 as a back-gate substrate because of its larger thermal expansion coefficient (ca. 10 ppm/K) than Si. An aluminum oxide layer was grown by atomic layer deposition technique to form a gate dielectric on the substrate. After lamination process of k-Br on the substrate, the Mott-FET device which showed a weakly insulating behavior at low temperature was fabricated (Figure 2). Upon applying a positive gate voltage, however, the resistivity goes down and weakly metallic behavior was observed at VG > 2 V. By further increasing the gate voltage up to 8 V, the device showed a sudden drop of resistivity around 5 K, which can be attributed to superconductivity. Taking account of bistable IV characteristics observed in the low resistance region, the above transition can be understood as a percolation transition of superconducting islands that is induced by the electrostatic doping of electrons. The transition temperature increases as the gate voltage rises and saturates around VG = 11 V. The above result is the first example of field-induced superconductivity in organic materials, and can be utilized for uncovering a phase diagram of organic Mott system in the band filling controlled regime.

1. H. M. Yamamoto, M. Nakano, M. Suda, Y. Iwasa, M. Kawasaki and R. Kato 
   "A strained organic field-effect transistor with a gate-tunable superconducting channel" Nature Commun. 4, 2379/1–2379/7 (2013)