Condensed Molecular Materials Laboratory

Outline

Synthesis, characterization, and design of molecular materials, especially molecular conductors (including superconductors), have been undertaken. Molecular conductors exhibit a variety of physical properties which can be systematically understood on the basis of simple and clear electronic structures. From a chemical point of view, the most fascinating character of a molecular conductor is its designability, that is, we can finely control solid state properties with chemical modifications of the molecule. The newly synthesized materials are characterized by X-ray diffraction and physical measurements (electrical conductivity...etc.). The electronic structure is investigated by simple band structure calculation. All these results are devoted to the design of new molecular materials.

Recent Research Topic

First observation of the filling-controlled Mott transition in an organic FET

Fig. 1 Optical image of a κ-(BEDT-TTF)2Cu[N(CN)2]Br single crystal on a SiO2 / Si substrate (left) and Schematic diagram of the FET structure (right)

A Mott-insulator is an insulator whose conduction carriers are localized due to electron-electron Coulomb repulsion. The correlated electrons inside Mott-insulators show many interesting phenomena such as high-Tc cuprate superconductivity and metal-insulator transition, the so-called Mott-transition. Since correlation strength among the carriers can be tuned by the filling of the conduction band of the Mott-insulator, FET (Field Effect Transistor) device structure has been long believed to enable “band-filling controlled Mott-transition” by means of electrostatic doping. Despite tremendous efforts by many material scientists however, Mott-transition in a FET device has never been observed. This year, we realized an organic FET device with organic Mott-insulators, and demonstrated sudden increase of mobile carrier density in the device, which indicates that this device is the first Mott-transition FET.

Molecular conductors (organic charge transfer salts) provide various Mott-type semiconductors whose insulating phase is directly connected to a superconducting phase in their phase diagrams. We have fabricated FET structure with a thin-layer single crystal of κ-(BEDT-TTF)2Cu[N(CN)2]Br laminated on a SiO2/Si substrate in order to study whether the Mott-transition FET is possible with this material (Fig. 1). At low temperatures, the device showed a clear n-type FET behavior as shown in Fig. 2. The device mobility of the best sample measured by the four-probe method reached 94 cm2/Vs. To understand the mechanism of this device, the Hall coefficient was measured and the carrier concentration determined. Despite the n-type behavior of this device, the carrier under positive gate voltage was a hole and its number was almost 100 % of the 1st Brillouin zone (Fig. 3). This abnormal Hall effect can be interpreted on the basis of the Mott-transition in this organic FET.

Fig. 2 Arrhenius plot of the resistivity of the FET device based on κ-(BEDT-TTF)2Cu[N(CN)2]Br measured at various gate voltages

The inset shows the gate-voltage dependency of the activation energy.

Fig. 3 Gate-voltage dependency of the hole concentration determined by Hall effect measurement

Fig. 1

Reproduced, with permission, from Y. Kawasugi, et al. Strain-induced superconductor/insulator transition and field effect in a thin single crystal of molecular conductor, Appl. Phys. Lett. 2008, 92, 243508. © (2012) American Institute of Physics

Fig. 2

Reproduced, with permission, from Y. Kawasugi, et al. Field-Induced Carrier Delocalization in the strain-Induced Mott Insulating state of an Organic Superconductor, Phys. Rev. Lett. 2009, 103, 116801. © (2012) American Physical Society