Uniaxial strain investigation on the metal-insulator transition of (EDO-TTF)2PF6

The quasi-one dimensional organic conductor (EDO-TTF)2PF6 shows the novel metal-insulator transition, which complies with the cooperative features of the tetramerization of EDO-TTF, the charge ordering of 0110 pattern, and the ordering of anion at ca. 280 K. In this paper, the effect of the uniaxial strain on the metal-insulator transition of (EDO-TTF)2PF6 was investigated by means of the transport measurement. In low pressure region, the transport behavior was differently modified depending on the direction of the compression. In the cases that the strain was applied along the b and a′ axes within the donor layer (intralayer), the transition temperature (TMI) was slightly decreased or not changed and the jump of resistivity was suppressed with increasing the strain. Contrary, the TMI was enhanced remarkably to above 340 K by the strain along the interlayer direction. The present results indicate that the strain along the interlayer direction is more effective for stabilization of insulating phase than those along the interlayer directions. It has been reported that the intralayer direction is expanded while the interlayer distance is shortened below TMI because the anion slightly inserts into the donor layer by means of the low temperature X-ray measurement at ambient pressure. The enhancement of the metal-insulator transition by the uniaxial strain along the interlayer direction is consistent with this anisotropic distortion of the unit cell through the transition. With applying the large strain along each direction, the insulating phase was suppressed and the metallic phase appeared around the room temperature. The metal-insulator transition occurred below about 200 K with the feature of the second-order.