X-ray Irradiation-induced Carrier Doping in Organic Mott Insulators

X-ray irradiation usually induces disorders in a crystal, for example, displacement of the atomic position. In the case of molecular materials, the irradiation produces the molecular defects, which are radiolysed by ionizing radiation. This kind of molecular defect permanently remains, while the irradiation damage in inorganic materials in general is only due to atomic displacements which can be restored by a proper heat treatment. These defects and disorders make the electrical conductivity worse because of increasing the electron scattering [1]. Increase of the conductivity by X-ray irradiation, however, has been found in organic Mott insulators [2]. The irradiation-induced defects expected at the donor molecule sites might cause a local imbalance of the charge-transfer in the crystal. Such local modulation of the charge-transfer comes into being the effective doping of carriers into the halffilled Mott insulators. Figure 1 shows a schematic crystal structure of κ-type BEDT-TTF molecule based organic conductor. The anion and donor molecule layers are stacked alternatively. Charge transfer between donor and anion molecules introduces a hole carrier for a BEDT-TTF dimer. In the case of the Mott insulating state that the band width W is smaller than the effective on-site Coulomb energy U, the hole is localized at the dimer site. When the sample is irradiated by X-ray, some molecular defects could be introduced into the donor layer. The expected defects at donor molecule sites might cause a local imbalance of the charge transfer between BEDT-TTF donor and anion molecules. Such molecular defects result in the effective carrier doping into the half-filled dimer-Mott insulators. As shown in Figure 2, the in-plane resistivity of the organic Mott insulator κ-(BEDT-TTF)2Cu[N(CN)2]Cl decreases drastically by X-ray irradiation with the tungsten tube (40kV, 20mA). It is noted, moreover, that a metal-like temperature dependence appears with rather small irradiation dose. These large changes of the transport properties may indicate that the carrier-doping by X-ray irradiation breaks down of the Mott insulating state due to small shift from half-filling of the conduction band. By controlling the molecular defects by X-ray irradiation, we could tune the carrier doping continuously for the Mott insulator, which has not been possible for the organic system by chemical substitution and other methods. This new way of carrier-doping technique will be useful not only for studying the Mott physics but also for molecular device fabrications.