Development of Organic Superconductors by Reduction of Intermolecular Interaction

Tetrachalcogenafulvalene(TCF)donors are no longer essential to the production of organic superconductors，excluding fullerene superconductors。Indeed，the non-TCF-containingπ-electron donor BDA-TTP[which contains the bis-fused1，3-dithiol-2-ylidene(BDY)unit instead of the TCF unit as aπ-electron system]and the 1，3-dithiol-2-ylidene(DT)-basedπ-donor DODHT do give superconducting salts。Our molecular design strategy for theseπ-donors is based on the following requirements：(I)the extension of theσ-bond framework and(Ii)the reduction of theπ-system。These requirements are thought to lead to a decrease of intermolecular interaction，which would be crucial in achieving superconductivity。In addition to their superconductivity，we have found the outstanding physical properties of the SbF_6and FeCl_4salt of BDA-TTP。β-(BDA-TTP)_2SbF_6was proved to have the largest effective cyclotron mass(12.4±1.1m_e)yet found in organic superconductors.This result indicates that the electron-electron interaction is probably enhanced in the BDA-TTP superconductor。Angular dependent and variable-temperature magnetoresistance(MR)measurement forβ-(BDA-TTP)_2FeCl_4under pressure showed an abrupt drop of MR.A similar anomaly in MR has been reported inκ-(BETS)_2FeBr_4andλ-(BETS)_2FeCl_4，which exhibit magnetic-field-induced superconductivity.Similar to the superconductorsβ“-(DODHT)_2X(X=AsF_6and PF_6)at 16.5kbar，theβ”-(DODHT)_2BF_4·H_2O hydrate was found to undergo an superconducting transition under an applied pressure of 15.5kbar.The common noticeable feature of the three DODHT superconductors is that they all show a variety of pressure-induced conducting behaviors，including the insulating(Or Semiconducting)，metallic，and superconducting phases。In this way，we succeeded in finding a series of non-TCF-based organic superconductors with notable features。

Tetrachalcogenafulvalene(TCF) donors are no longer essential to the production of organic superconductors, excluding fullerene superconductors. Indeed, the non-TCF-containing π-electron donor BDA-TTP [which contains the bis-fused 1,3-dithiol-2-ylidene(BDY) unit instead of the TCF unit as a π-electron system] and the 1,3-dithiol-2-ylidene (DT)-based π-donor DODHT do give superconducting salts. Our molecular design strategy for these π-donors is based on the following requirements : (i)the extension of the σ-bond framework and (ii)the reduction of the π-system. These requirements are thought to lead to a decrease of intermolecular interaction, which would be crucial in achieving superconductivity. In addition to their superconductivity, we have found the outstanding physical properties of the SbF_6 and FeCl_4 salt of BDA-TTP. β-(BDA-TTP)_2SbF_6 was proved to have the largest effective cyclotron mass (12.4±1.1 m_e) yet found in organic superconductors. This result indicates that the electron-electron interaction is probably enhanced in the BDA-TTP superconductor. Angular dependent and variable-temperature magnetoresistance(MR) measurement for β-(BDA-TTP)_2FeCl_4 under pressure showed an abrupt drop of MR. A similar anomaly in MR has been reported in κ-(BETS)_2FeBr_4 and λ-(BETS)_2FeCl_4, which exhibit magnetic-field-induced superconductivity. Similar to the superconductors β"-(DODHT)_2X (X=AsF_6 and PF_6) at 16.5 kbar, the β"-(DODHT)_2BF_4・H_2O hydrate was found to undergo an superconducting transition under an applied pressure of 15.5 kbar. The common noticeable feature of the three DODHT superconductors is that they all show a variety of pressure-induced conducting behaviors, including the insulating (or semiconducting), metallic, and superconducting phases. In this way, we succeeded in finding a series of non-TCF-based organic superconductors with notable features.