The strain in an organic field-effect channel was controlled using a pressure-sensitive organic superconductor -͑BEDT-TTF͒ 2 Cu͓N͑CN͒ 2 ͔Br ͓BEDT-TTF= bis͑ethylenedithio͒tetrathia-fulvalene͔. The difference in thermal properties between the channel and the substrate induced strain with cooling, resulting in a phase transition. An unshrinkable SiO 2 / Si substrate was used to strain the subject material to the insulating state at low temperature, whereas a shrinkable polymer-based substrate allowed it to be a superconductor. The strain-induced insulating phase on SiO 2 / Si is at the vicinity of the phase boundary with the superconducting phase, actualizing field-effect transistor behavior.

We report the first observation of Shubnikov-de Haas (SdH) oscillations and quantized Hall resistance in the multilayered massless Dirac fermion system α-(BEDT-TTF) 2 I 3 with tilted cones.Holes were injected into the thin crystal fixed on a polyethylene naphthalate (PEN) substrate by contact electrification. The detection of SdH oscillations whose phase was modified by Berry's phase π strongly suggested that the carrier doping was successful in this system. We succeeded in detecting the quantum Hall effect (QHE) with the steps which is the essence of two dimensional Dirac fermion systems. The number of effectively doped layers was examined to be two in this device. We reveal that the correlation between effective layers plays an important role in QHE.

It is widely recognized that the effect of doping into a Mott insulator is complicated and unpredictable, as can be seen by examining the Hall coefficient in high Tc cuprates. The doping effect, including the electron–hole doping asymmetry, may be more straightforward in doped organic Mott insulators owing to their simple electronic structures. Here we investigate the doping asymmetry of an organic Mott insulator by carrying out electric-double-layer transistor measurements and using cluster perturbation theory. The calculations predict that strongly anisotropic suppression of the spectral weight results in the Fermi arc state under hole doping, while a relatively uniform spectral weight results in the emergence of a non-interacting-like Fermi surface (FS) in the electron-doped state. In accordance with the calculations, the experimentally observed Hall coefficients and resistivity anisotropy correspond to the pocket formed by the Fermi arcs under hole doping and to the non-interacting FS under electron doping.

We report the influence of the field effect on the dc resistance and Hall coefficient in the strain-induced Mott insulating state of an organic superconductor kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]Br. Conductivity obeys the formula for an activated transport sigma(square)=sigma(0)exp(-W/k(B)T), where sigma(0) is a constant and W depends on the gate voltage. The gate-voltage dependence of the Hall coefficient shows that, unlike in conventional field-effect transistors, the effective mobility of dense hole carriers ( approximately 1.6x10(14) cm(-2)) is enhanced by a positive gate voltage. This implies that carrier doping involves delocalization of intrinsic carriers that were initially localized due to electron correlation.

A Mott insulator sometimes induces unconventional superconductivity in its neighbors when doped and/or pressurized. Because the phase diagram should be strongly related to the microscopic mechanism of the superconductivity, it is important to obtain the global phase diagram surrounding the Mott insulating state. However, the parameter available for controlling the ground state of most Mott insulating materials is one-dimensional owing to technical limitations. Here, we present a two-dimensional ground-state mapping for a Mott insulator using an organic field-effect device by simultaneously tuning the bandwidth and bandfilling. The observed phase diagram showed many unexpected features such as an abrupt first-order superconducting transition under electron doping, a recurrent insulating phase in the heavily electron-doped region, and a nearly constant superconducting transition temperature in a wide parameter range. These results are expected to contribute toward elucidating one of the standard solutions for the Mott-Hubbard model.

A novel type of flexible organic field-effect transistor in which strain effects can be finely tuned continuously has been fabricated. In this novel device structure, electronic phases can be controlled both by "band-filling" and by "band-width" continuously. Finally, co-regulation of "band-filling" and "band-width" in the strongly-correlated organic material realize field-induced emergence of superconducting fractions at low temperature.

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We present the carrier transport properties in the vicinity of a doping-driven Mott transition observed at a field-effect transistor (FET) channel using a single crystal of the typical two-dimensional organic Mott insulator κ-(BEDT-TTF)CuN(CN)Cl (κ-Cl). The FET shows a continuous metal-insulator transition (MIT) as electrostatic doping proceeds. The phase transition appears to involve two-step crossovers, one in Hall measurement and the other in conductivity measurement. The crossover in conductivity occurs around the conductance quantum e/h, and hence is not associated with "bad metal" behavior, which is in stark contrast to the MIT in half-filled organic Mott insulators or that in doped inorganic Mott insulators. Through in-depth scaling analysis of the conductivity, it is found that the above carrier transport properties in the vicinity of the MIT can be described by a high-temperature Mott quantum critical crossover, which is theoretically argued to be a ubiquitous feature of various types of Mott transitions.