Field effect on molcular conductors

Hiroshi M. Yamamoto, Yoshitaka Kawasugi, Naoya Tajima, Takeo Fukunaga, and Reizo Kato
RIKEN, Saitama Univ. / Japan
Field effect transistor is not only an electronic device for application use but also a powerfultool for solid state physics. For example, gate-modulated behavior of zero-gap conductors
such as graphene is one of the most interesting subjects in recent material research. Anotherexample is the gate-induced enhancement of Tc of superconductivity of highly correlated
electron systems such as Nb-doped SrTiO3, which gives rise to significant insight about theorigin of high-Tc superconductivity. Molecular conductor shares many low-dimensional
electronic properties such as zero-gap state, charge ordering, Pierls instability, and non-BCS
superconductivity with inorganic correlated systems, but precise control of the physicalproperties of molecular conductors has been long relying on physical and/or chemicalpressure that tunes transfer integrals between molecules. For example, enhancement of thetransfer integral effectively reduces the electron-electron correlation (U/W) in kappa-(BEDTTTF)2Cu[N(CN)2]Cl and in the vicinity of antiferromagnetic insulator to metal transition
unconventional superconductivity appears. The band filling control by the field effect isbelieved to possess similar correlation-reducing effect, but lack of appropriate procedure tofabricate device configuration with molecular conductors has kept researchers away fromconsidering such experiments. Several groups have reported field effect on molecularconductors recently, but the ranges of material and temperature in their measurements arequite limited. In this presentation, we report versatile method for the fabrication of field effectdevice of molecular conductor and the results of the measurement. (BEDT-TTF =bis(ethylenedithio)tetrathiafulvalene)
Our method employs electrochemical growth of thin crystals and transfer of them onto asubstrate with source, drain, and gate electrodes. Before examining the field effect, it isnecessary to survey the thermal influence of the substrate on the single crystal of molecularconductors. When the substrate is made of SiO2/Si, the thermal expansion coefficient of thesubstrate is far smaller than those of the organic samples. Therefore, the sample isexpanded in directions parallel to the substrate surface at low temperature. For example,
kappa-(BEDT-TTF)2Cu[N(CN)2]Br is normally a superconductor in its ground state, but when
it is fixed on a hard silicon substrate it falls into the insulating ground state due to the pseudo
negative pressure from the substrate. We have applied gate voltage to this insulating sample
on SiO2/Si substrate and observed its transistor behavior. The ON/OFF ratio reaches morethan 1000 and the activation energy has been considerably reduced by applying positivegate voltage (Fig. 1, Fig. 2). It is interesting that there is no apparent threshold behavior forthis device, which seems to be a unique character of Mott-insulator based transistor. alpha(BEDT-TTF)2I3 is in a zero-gap state under hydrostatic pressure, and at ambient pressure it
exhibit metal-insulator transition at 135 K. The insulating phase is a charge ordering state.
Before we apply gate voltage and pressure simultaneously, we have measured transistor
behavior of the charge ordered insulating phase at ambient pressure to check the device
quality. With this insulating sample again, we have observed n-type transistor behavior withmoderate ON/OFF ratio. Results using plastic substrates will also be presented.