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 the 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 the X-ray diffraction method and physical measurements (electrical conductivity...etc.). The electronic structure is investigated by the simple band structure calculation. All these results are devoted to the design of new molecular materials.
- Development of molecular conductors based on metal-dithiolene complexes
(1) Control of the electronic state in a series of Pd(dmit)2 salts, a strongly correlated electron system with a quasi-triangular lattice structure (dmit=1,3-dithiol-2-thione-4,5-dithiolate)
(2) Novel phase transition to a spin-gapped ground state in EtMe3P[Pd(dmit)2]2 (P21/m phase)
(3) Interlayer magnetoresistance peak effect in a superconducting Pd(dmit)2 salts under high pressure
(4) Magnetic susceptibility measurements of a Pd(dmit)2 salt under high pressure
(5) Physical properties of molecular conductors based on [(ppy)AuIII(S-S)] (ppy- = C-deprotonated-2-penylpyridine; S-S = dithiolene ligand) type unsymmtrical square planar metal dithiolene complexes - Single crystal formation of molecular conductors on silicon substrate
- Organic zero-gap-conductor
- Photo-induced insulator-metal transition
- Development of the instrument for the electrical resistivity measurement under the anisotropic extension for fragile molecular crystals
- Study on charge ordering state of molecular conductors
(1) Charge ordering state of ET5Te2I6 and BETS5Te2I6
(2) Analysis of the vibrational spectra in the CO state of [Pd(dmit)2] salts - Supramolecular Ni(dmit)2 salt with π-character electron based metallic and antiferromagnetic states
- Application of lasers for the development of new technology
- Anisotropy of nonlinear optical responses in organic molecules and semiconductor nanocrystals
- Development of molecular conductors based on metal-dithiolene complexes
- Single crystal formation of molecular conductors on silicon substrate
- Organic zero-gap-conductor
- Photo-induced insulator-metal transition
- Development of the instrument for the electrical resistivity measurement under the anisotropic extension for fragile molecular crystals
- Study on charge ordering state of molecular conductors
- Supramolecular Ni(dmit)2 salt with π-character electron based metallic and antiferromagnetic states
- Application of lasers for the development of new technology
- Anisotropy of nonlinear optical responses in organic molecules and semiconductor nanocrystals
(1) Control of the electronic state in a series of Pd(dmit)2 salts, a strongly correlated electron system with a quasi-triangular lattice structure (dmit=1,3-dithiol-2-thione-4,5-dithiolate)
Anion radical salts of metal dithiolene complex Pd(dmit)2 with tetrahedral counter cations (Me4Z+ and Et2Me2Z+; Z=N, P, As, Sb) are known to form a strongly correlated two-dimensional system. At ambient pressure, they are Mott-insulators where spin-1/2 dimer ([Pd(dmit)2]2-) units form a two-dimensional quasi-triangular lattice. The electronic state of this system is governed by three parameters, the effective on-site Coulomb energy on the dimer (Ueff.), the band width (W), and the degree of frustration. These parameters can be controlled by the choice of the counter cation and/or the application of (hydrostatic or uni-axial) pressure. In this fiscal year, we have studied new series of Pd(dmit)2 salts with EtMe3Z+ (Z=N, P, As, Sb) cations.
Five conducting EtMe3Z (Z=N, P, As, Sb) salts of Pd(dmit)2 have been prepared by the air-oxidation of the corresponding 2:1 salts. There are three structural types with the space groups, P21/m, P1, and C2/c, all of which are based on a strongly dimerized Pd(dmit)2 unit. The [Pd(dmit)2]2- dimers form a two-dimensional quasi triangular lattice.
At ambient pressure, all of them are Mott insulators where one electron is located on each dimer. The localized electrons exhibit various magnetic behaviors. The EtMe3Sb salt, where the deviation from the regular triangular lattice is small, shows frustrated paramagnetism down to the lowest temperature. The EtMe3As salt with larger deviation from the regular triangular lattice undergoes an antiferromagnetic transition at 23 K. The EtMe3P salt (P21/m) shows a spin-Peierls-like phase transition to a non-magnetic state at 25 K, which is the first case in a two-dimensional spin system. The EtMe3N salt shows a second-order phase transition to a non-magnetic state at 85 K and the EtMe3P (P1) salt also shows a second-order phase transition at 73 K. Mechanisms for these second-order transitions consistent with low-temperature crystal structures have not been found out.
The application of hydrostatic pressure can release the Mott-insulating state and induce a metallic or superconducting state. The EtMe3P (P21/m) and EtMe3As salts exhibit the superconductivity at 5 K (3.3 kbar) and at 4 K (7 kbar), respectively. It is quite interesting that these two superconducting states would be associated with different types of Mott-insulating states: the spin Peierls-like non-magnetic state and the antiferromagnetic state.
(2) Novel phase transition to a spin-gapped ground state in EtMe3P[Pd(dmit)2]2 (P21/m phase)
A novel Pd(dmit)2 salt, EtMe3P[Pd(dmit)2]2 (P21/m phase), has been found to undergo a second-order phase transition at 25 K to a non-magnetic spin-gapped ground state. Low temperature X-ray diffraction study has revealed a doubled periodicity in the low temperature phase, similarly to those of the spin-Peierls transition in a quasi-one-dimensional case. It is the first case that this kind of phase transition is observed in a two-dimensional spin system. The frustration of quantum spins induces this transition to the spin-gapped phase, by suppressing the magnetic long-range order.
(3) Interlayer magnetoresistance peak effect in a superconducting Pd(dmit)2 salts under high pressure
Longitudinal interlayer magnetoresistance measured for the superconducting state of Et2Me2P[Pd(dmit)2]2 under pressure exhibits a peak around 1 T. This effect is explained by a resistively shunted Josephson junction model. The peak is suppressed with increasing pressure above 0.6 GPa.
(4) Magnetic susceptibility measurements of a Pd(dmit)2 salt under high pressure
Experimental techniques have been developed for the measurements of susceptibility under pressure, to study the pressure-induced phenomena in molecular conductors and related materials. Suppression of the transition to the spin-gapped state followed by bulk superconductivity below 5 K has been found in EtMe3P[Pd(dmit)2]2 (P21/m phase) at 0.2 GPa, for the first time in the Pd(dmit)2 salts.
(5) Physical properties of molecular conductors based on [(ppy)AuIII(S-S)] (ppy- = C-deprotonated-2-penylpyridine; S-S = dithiolene ligand) type unsymmtrical square planar metal dithiolene complexes
Magnetic behavior of [(ppy)AuIII(C8H4S8)]2[PF6] indicates that this material is a quasi-one-dimensional strongly correlated system. Antiferromagnetic transition has been observed near 50 K.
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Nano-/micro-sized crystals of the molecular conductors were directly formed on SiO2/Si substrates, and their electrical properties have been examined. (DMe-DCNQI)2Ag microcrystal showed metal-insulator transition and rectifying behavior after light-irradiation. In addition, it exhibited bistable resistance behavior that can be applied to resistive random access memory. The phase transition temperature of α-(BEDT-TTF)2I3 microcrystal has been raised compared to the corresponding bulk material, and the salt worked as n-type field effect transistor. The nanocrystal of supramolecular nanowire (EDT-TTF)4BrI2(TIE)5 retained its conductivity down to 4 K, while the bulk crystal became an insulator at low temperatures.
(BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene; EDT-TTF = ethylenedithiotetrathiafulvalene; DMe-DCNQI = 2,5-Dimethyl-N,N'- Dicyanobenzoquinonediimine)
Massless Dirac Fermions have been realized in an organic conductor α-(BEDT-TTF)2I3 under high hydrostatic pressures (p). Electrical conductivity and Hall coefficient of this material under p=18 kbar were investigated down to 100 mK. The conductivity is nearly constant from 300 K to 2 K. Interestingly, the sheet resistance per BEDT-TTF molecules layer corresponds to quantum resistance h/e2=25.8 kΩ. In the same temperature region, on the other hand, both carrier density and mobility change by about 6 orders of magnitude, in a manner so that the effects just cancel out giving rise to a quantum resistance h/e2 per layer. Another significant point is that the temperature (T) dependence of carrier density (n) obeys not the exponential law but the power law. It obeys n∝T2 from 10 K to 50 K. This result strongly suggests that the system is a zero-gap conductor with 2-dimensional (2D) Dirac cone type energy dispersion near the Fermi level. Below 10 K, on the other hand, the carrier density is proportional to T3, which indicates that the ground state of this system is 3D zero-gap conductor.
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Photo-switching between a charge-ordered insulating state and a metallic state has been successfully realized in an organic conductor α-(BEDT-TTF)2I3 at low temperatures. The large photocurrent induced by the pulsed laser excitation with photon energy of 2.6 eV had two components. The fast-decaying component had a life time of 120 ns. The subsequent one rose slowly and remained as long as high electric field was applied. This transition is accompanied by a drastic resistivity drop more than 7 orders of magnitude.
We have found the photo-induced insulator to metal transition in (BEDT-TTF)5Te2I6 and (BEDT-TTF)3(ClO4)2 at low temperatures.
There have been few efforts to give the negative pressure for the molecular conductors since most of the molecular conductors are fragile. We can expect that the negative pressure induces changes in the structural, electronic and magnetic properties, which would not occur under pressure. As a first step, the instrument for the resistivity measurement under the uni-axial extension has been developed. We have applied newly developed instrument to TTF-TCNQ, θ-(BEDT-TTF)2RbZn(SCN)4 and (DMe-DCNQI-d8)2Cu, which are the one-, two- and three-dimensional conductors, respectively. The insulating behavior is enhanced for TTF-TCNQ. The temperature of the resistivity jump is increased for θ-(BEDT-TTF)2RbZn(SCN)4. The insulator-metal transition is observed for (DMe-DCNQI-d8)2Cu. These phenomena can be explained from the increase in the inter-molecular distance along the direction of the anisotropic extension.
(TTF = tetrathiafulvalene; TCNQ = tetracyanoquinodimethane)
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(1) Charge ordering state of ET5Te2I6 and BETS5Te2I6
Charge ordering (CO) state of the ET-salts has attracted attention since the inhomogeneous site-charge is often observed for the molecular conductors including highly conducting materials and the superconductors. In order to investigate what dictates the CO state, we have examined the CO state of ET5Te2I6 and BETS5Te2I6 using the vibrational spectroscopy. We have evidenced the inhomogeneous site-charge not only in the insulator phase but also in the highly conducting state. The site-charges are localized in the low temperature phase. The distribution of the site charge takes the "inner-distribution" in the pentamer for both compounds. The crystal structures of the present compounds are almost identical to each other, but the magnitude of the overlap integrals in the BETS-salt remarkably differs from that in the ET-salt. The observation of the "inner-distribution" confirms that the inter-site Coulomb interaction mostly contributes to the pattern in the CO state. The high-temperature phase is characterized as the nearly localized state wherein the most stable distribution (inner-distribution) and the secondary stable distribution can coexist owing to thermally activated carrier.
(ET = BEDT-TTF =bis(ethylenedithio)tetrathiafulvalene; BETS = bis(ethylenedithio)tetraselenafulvalene)
(2) Analysis of the vibrational spectra in the CO state of [Pd(dmit)2] salts
[Pd(dmit)2]2X [X=Cs and Et2Me2Sb] salts undergoes the CO phase transition. This phase transition was evidenced by the X-ray crystal structural analysis. On the other hand, the CO state has not been analyzed by using the vibrational spectroscopy since the assignment of the vibrational mode has not been settled for the Pd(dmit)2 system. Based on the accumulation of the vibtational spectra of some [Pd(dmit)2] salts, we have given the assignments of the two kinds of the C=C stretching modes; the asymmetric and symmetric mode. The former is the charge sensitive mode. The latter exhibits the factor group splitting, so that the distribution of the site-charge is deduced from the observation of the symmetric mode. Therefore, we have confirmed the methodology for analyzing the vibrational spectra in the CO state of the dmit complexes. Interestingly, the symmetric mode is observed in both IR and Raman spectra of [Pd(dmit)2]2Cs salt above the transition temperature. This phenomenon leads to the inversion symmetry breaking even in the high temperature phase. This observation reflects that the vibrational spectroscopy is a powerful method to investigate the CO state and related phenomenon.
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Ni(dmit)2 molecules can form supramolecular interactions with halogenated pyridinium cations. We have found the supramolecular Ni(dmit)2 anion radical salt with diiodopyridinium cation. This compound has two crystallographically independent anion layers (Layer I and II), and the one contains localized spins and the other metallic electrons. Measurements of electrical resistivity and magnetic susceptibility revealed metallic conduction down to 4.2 K and the antiferromagnetic transition at 10 K. Consequently, two contrastive features of the electron, localization and itinerary, coexist in the same crystal and both features are derived from only π-character electrons.
We are applying laser ablation for the production and the detection of atoms and ions in the solid sample. Femtosecond laser ablation (fsLA) is unique as compared with conventional nanosecond laser ablation (nsLA) because fsLA produces atoms and ions with high kinetic energies (K.E.). In the case of Samarium ion, for example, we found K.E. amounted as large as 250 eV corresponding to that of 80 photons of ablation laser. High K.E. can be attributed to the Coulomb explosion mechanism in the case of fsLA, although thermal evaporation works in the case of nsLA. We found that the once ablated surface produces atoms and ions with only low K.E. at the second or later ablation because the surface is changed into amorphous phase.
Microscopic molecular symmetry and macroscopic molecular orientation affect nonlinear optical responses and their anisotropy. Thus polarization-dependent nonlinear optical spectroscopy is a powerful tool to investigate dimensionality of molecules and symmetry of molecular orientation. In this fiscal/academic year, second-order nonlinear optical spectroscopy based on electroabsorption technique, which has been established in the previous year, was applied to investigate molecular orientation of vacuum-deposited tris(8-hydroxyquinolinato) aluminum(III) (Alq3) thin films which show spontaneous buildup of giant surface potential (GSP) as high as 28 V at 560 nm thickness. Irradiation of visible light reduces GSP, but the mechanism is not understood yet. By electroabsorption technique, we revealed that non-centrosymmetric molecular orientation exists before and after GSP reduction, and that the order parameter is similar to that obtained from magnitude of GSP. These results indicate that GSP reduction is due to electrostatic screening by photo-generated carrier, rather than photo-induced randomization of molecular orientation.
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