Anisotropic Pressure and Career Doping Effect for
Molecular Conductor EtMe3P[Pd(dmit)2]2
Takaaki Minamidate1, Yoshitaka Kawasugi1, Hiroshi Yamamoto2, and R. Kato1
1

MolecularCondensed Molecular Materials Laboratory, RIKEN, 2Institute for Molecular Science

conductor

EtMe3P[Pd(dmit)2]2(dmit=1,3-dithiole-2-thione-4,5-dithiolate) consists of alternate stackof cation layer of EtMe3P+ and anion dimer layer of[Pd(dmit)2]2-. A spin correlation between dimers in the anion
dimer layer has a geometry close to a regular triangular lattice.
In addition, the system has a half-filled band structure atambient pressure, and electrons are localized on each dimersite one by one due to the strong on-site Coulomb repulsion,
resulting in the Mott insulator. From previous study for thissalt, it is known that, at ambient pressure, the transition to thevalence-bond order (VBO) phase with lattice modulation
occurs at 25 K and leads to the nonmagnetic state with a spin
gap [1]. Furthermore, the VBO transition is suppressed by theweak hydrostatic pressure and superconducting phase appears
above 0.4 GPa [2, 3].
In order to investigate the electronic state of theFig. 1: Schematic figure of the setup forsuperconducting phase adjacent to the Mott insulating phaseone-dimensional pressure measurement andusing device fabrication, we have mainly investigated κ-(ET)2X

the(ET = Bis(ethylenedithio)tetrathiafulvalene, X : monovalentlaminated on the PET substrate.

thin

crystalofEtMe3P[Pd(dmit)2]2anion). Using a thin crystal laminated on a flexible substrate,
we have attempted control of the bandwidth by the bending of
the substrate and doping of carriers by the field effect [4,5]. In
the present research, we newly investigated EtMe3P[Pd(dmit)2]2,and studied universal physics of the Mott insulating phase andsuperconducting phase, and compared with other molecular
conductors having a triangular lattice. We aim to elucidate therelationship between the VBO phase, which appears only in
EtMe3P[Pd(dmit)2]2, and the magnetic frustration and the bandstructure.
Figure 1 shows the setup used in this study to apply theuniaxial pressure. A single crystal of EtMe3P[Pd(dmit)2]2 with athickness of the several tens of nanometers is laminated on thesoft PET (polyethylene terephthalate) substrate and the center

Fig.2:

Temperaturedependenceoftheelectrical resistance of EtMe3P[Pd(dmit)2]2 forvarious uniaxial pressure.

of the substrate is pushed by the piezo actuator. The sample isbent according to the bending of the substrate and compressed in the direction of the long axis of thesubstrate. The PET substrate has larger coefficient of thermal
expansion than that of EtMe3P[Pd(dmit)2]2 and when it cooled,a weak two-dimensional positive pressure is applied to thesample.
Figure 2 shows temperature dependence of the electricalresistance for EtMe3P[Pd(dmit)2]2 thin crystal with uniaxialpressure. The units “µm” and “S” in the legend represent theFig. 3: Schematic picture of organic FETwith ionic liquid.

elongation of the piezo actuator and the magnitude of thedistortion, relatively. When the substrate is not bent, the systemremained insulating in the whole temperature range. When theuniaxial pressure was applied to the sample, the insulationbehavior was suppressed, and in the high-pressure region, asuperconducting phase appeared with a transition temperatureof 4 K. This behavior is roughly same as that of the bulk
sample under the hydrostatic pressure [2, 3]. In the intermediatepressure region, reentrant behavior was observed. In the bulk
sample under the hydrostatic pressure, the VBO transition
accompanied by the hysteresis is observed [3]. However, theFig. 4: Electrical resistance of FET

sharp resistance jump or clear hysteresis was not observed atprocessed EtMe3P[Pd(dmit)2]2 under thethe present case. Magnetic frustration due to the triangularuniaxial pressure for various gate voltage.

lattice structure is considered to play an important role for theVBO transition and it is possible that the differences in thetransport behavior are caused by the difference between hydrostatic and uniaxial pressure.
Finally, in order to investigate the effect of the carrier doping near the boundary between thesuperconducting and VBO phases, we fabricated a field effect transistor structure using an electric double
layer structure and attempted the hole-doping. Figure 3 shows a schematic picture of an electric double layertransistor. When the gate voltage is applied, carriers are induced on the sample surface. Figure 4 shows
temperature dependent electric resistance of hole-doped EtMe3P[Pd(dmit)2]2 for various gate voltage whenthe expansion of piezo actuator fixed at 700 µm. Although the measurement is insufficient, we can say thatthe conductivity rose with increasing hole density and the small drop of the resistance accompanying thesuperconducting transition in the high-doped region was observed. In the research of the next year, we willverify the gate voltage dependence in more detail, and conduct the Hall resistance measurement to discuss
the relation between the carrier concentration and the electronic state.
Reference[1]M. Tamura et al., J. Phys. Soc. Jpn., 75 093701 (2006).
[2]R. Kato et al., J. Am. Chem. Soc., 128, 10016 (2006).
[3]Y. Shimizu et al., Phys. Rev. Lett., 99, 256403(2007)[4]Y. Kawasugi et al., Nat. commun., 7, 12356 (2016)[5]M. Suda et al., Adv. Mater., 26, 3490 (2014)