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 novel metal dithiolene complexes
- Field effect measurement of molecular conductors on silicon substrate and organic Mott-FET
- Massless Dirac fermions in organic conductors
- Photo-induced insulator-to-metal transition in BEDT-TTF salts
- Control of the electronic states in molecular conductors by use of dynamical external fields
- Strain-induced Mott transition in strongly-correlated molecular conductors
- Dielectric anomaly in dimer Mott insulators
(2) High pressure electrical properties of molecular conductor β'-Et2Me4P[Pd(dmit)2]2 by using a diamond anvil cell
(3) The low temperature structure and high pressure electrical property of α-Me4N[Pd(dmit)2]2
(4) Electrical conductivity of metal-dmit complexes with fluorinated ammonium
(5) Development of novel Ni(dmit)2 anion radical salts with 2,5-dihalopyridinium cations
(2) Low temperature and high magnetic field
- Development of molecular conductors based on novel metal dithiolene complexes
- Field effect measurement of molecular conductors on silicon substrate and organic Mott-FET
- Massless Dirac fermions in organic conductors
- Photo-induced insulator-to-metal transition in BEDT-TTF salts
- Control of the electronic states in molecular conductors by use of dynamical external fields
- Strain-induced Mott transition in strongly-correlated molecular conductors
- Dielectric anomaly in dimer Mott insulators
Metal dithiolene complexes have provided a variety of molecular conductors. Among them, most of Pd(dmit)2 salts belong to a strongly correlated two-dimensional system with a quasi triangular lattice of [Pd(dmit)2]2- dimers. The conduction band originates from HOMO, which is associated with the srong dimerization and a small HOMO-LUMO energy splitting. Their electronic state is associated with various degrees of freedom (including charge, spin, orbital, and lattice) and can be tuned by pressure and counter cations.
(1) Quantum spin liquid state in a molecular conductor β'-EtMe3Sb[Pd(dmit)2]2
Assign: Kato, A. Tajima, N. Tajima, H. M. Yamamoto, Cui, Yamashita; Ishii, Fukunaga, Kubo
At ambient pressure, most of all Pd(dmit)2 salts are in the Mott insulating state and are the spin-1/2 Heisenberg antiferromagnets where the spin frustration operates and exhibit variour ground states. Among them, EtMe3Sb salt with a nearly regular-triangular lattice is a candidate of quantum spin liquid. We are continuing 13C-NMR, μSR, thermal conductivity, and specific heat measurements. We also tried preparation of mixed crystals of EtMe3Sb salt with Me4Sb (Antiferromagnetic order) or Et2Me2Sb (Charge order: 2[Pd(dmit)2]2- → [Pd(dmit)2]20 + [Pd(dmit)2]22-) salt, in order to tune the anisotropy of the triangular lattice and thus magnetic properties.
The 13C nuclear spin-lattice relaxation rate clearly indicates a kink around 1 K. This suggests that the low temperature phase has a nodal spin gap. On the other hand, low temperature heat capacity shows a T-linear term, as if this is a typical metal. There is no magnetic field dependence in heat capacity. These results suggest the realization of gapless spin liquid state. The low temperature heat capacities of EtMe3Sb salt and mixed salts of EtMe3Sb and Et2Me2Sb show the Wilson ratio close to 1. This fact suggests that the spin liquid state in the Pd(dmit)2 system is a Fermi-liquid like state.
All these results indicate possibility of a new kind of quantum liquid state, but we do not have a unified picture for this system yet.
( dmit= 1,3-dithiole-2-thione-4,5-dithiolate )
Figure 1: Pd(dmit)2
↑Top
(2) High pressure electrical properties of molecular conductor &beta'-Et2Me4P[Pd(dmit)2]2 by using a diamond anvil cell
Assign: Cui, Kato
Recently, attractive physical properties such as quantum spin liquid and valence bond solid states were discovered in the quantum spin system Pd(dmit)2 salts. Most Pd(dmit)2 salts are Mott insulators at ambient pressure, and easily exhibit metallic or superconducting behavior under high pressure. Compared with Me4P salt with the highest antiferromagnetic transition temperature (TN), Et2Me2P salt has a narrower conduction band and lower TN. Under high pressure, Et2Me2P salt turns superconducting around 1GPa and shows a non-metallic state again in the higher pressure region. In the previous high pressure experiments for Et2Me2P salt by using a cubic anvil cell, the non-metallic state could not be suppressed up to 8GPa. Here we measured resistivity up to 15.8GPa by using our new four-probe diamond anvil cell technique. We successfully found that Et2Me2P salt turned to a complete metal under 13.6GPa. We suggest that this new metallic phase is associated with HOMO-LUMO band overlapping.
( dmit= 1,3-dithiole-2-thione-4,5-dithiolate )
Figure 2: The Tc -P phase diagram of β'-Et2Me2P[Pd(dmit)2]2
↑Top
(3) The low temperature structure and high pressure electrical property of α-Me4N[Pd(dmit)2]2
Assign: Cui, A. Tajima, Oshima, Kato
Recently, interesting physical properties were discovered in a Mott-insulator system Pd(dmit)2 salts, such as quantum spin liquid and valence bond solid states. Under high pressure, most Pd salts show metallic and/or superconducting transitions. Among them, α-Me4N[Pd(dmit)2]2 exhibits a semiconductor-to-semiconductor transition at 110K, and metal like behavior around 30K. Surprisingly, it shows insulating behavior again below 10K. We have studied low temperature structure that is shown in Figure 3. Below 110K, the unit cell changes from (a0, b0) to (aa0+b0, b-a0+b0). The unit cell contains two crystallographically independent conducting layers A and B, and each layer is formed by two independent Pd(dmit)2 molecules. In Layer B, two different interdimer distances were observed at low temperature. This phenomenon is also observed in Et2Me2Sb salt where the charge order (2[Pd(dmit)2]2- → [Pd(dmit)2]20 + [Pd(dmit)2]22-) occurs. The charge order disappeared around 30K, and appeared again below 10K. On the other hand, there was no significant change of interdimer distance in Layer A. We also measured resistivity of α-Me4N salt up to 17.1GPa by using a diamond anvil cell. We found that the charge order was suppressed under 9.3GPa and the system turned complete metal.
( dmit= 1,3-dithiole-2-thione-4,5-dithiolate)
Figure 3: The low temperature structure of α-Me4N[Pd(dmit)2]2
↑Top
(4) Electrical conductivity of metal-dmit complexes with fluorinated ammonium
Assign: Nomura, A. Tajima, Cui, Oshima, H. M. Yamamoto, Kato
We attempted molecular modifications for (R4E)[M(dmit)2]2 (E = N, P, As or Sb; M = Ni, Pd, Pt, Au) salts, which had been previously reported. There are many possibilities to modify these metal complex salts: (1) Modification of the counter cation R4E+, (2) Substitution of the central transition metal and (3) Replacement of some sulfur atoms in the dmit ligand. Namely, one may develop numerous molecular (super)conductors on the basis of these molecular modifications. The larger counter cations do not tend to provide β- or Tbeta;'-type crystals that frequently show the superconductivity. Accordingly, we carried out 'slight modification' of the counter cation by fluorination of the R group, and then the fluorinated ammonium was used to develop new conducting salts (Figure 4). The crystal structure of β-[Me3(CH2F)N][Pd(dmit)2]2 is isostructural with that of the non-fluorinated β-(Me4N)[Pd(dmit)2]2 (Figure 5). This partially originates from similar molecular size in both [Me3(CH2F)N]+ and (Me4N)+ cations, because the 'fluorine mimic effect' can be considered. In contrast to the structural similarity, we observed different physical properties. This β-Me3(CH2F)N salt showed higher conductivity and higher superconducting transition temperature (TSC) under pressure, than those of -Me4N salt (Figure 6). In addition, temperature dependence of the resistivity for -Me3(CH2F)N salt exhibited metallic behavior at ambient pressure (Figure 6), while most β- or Tbeta;'-(R4E)[Pd(dmit)2]2 salts are Mott insulators at ambient pressure. Similarly, we also obtained β-type crystals of [Pt(dmit)2]2 and selenated [Pd(dsit)2]2 salts with Me3(CH2F)N cation (Figure 4 ), and their resistivity indicated metallic behavior as well at ambient pressure. In conclusion, the introduction of fluorine atom to the counter cation would be promising for the development of excellent molecular conductors based on the 'slight size modification'.
( dmit= 1,3-dithiole-2-thione-4,5-dithiolate)
Figure 4: β-[Me3(CH2F)][M(dmit)2]2and its selenium analogue
Figure 5: Crystal structure of β-[Me3(CH2F)][Pd(dmit)2]2
Figure 6: Temperature- and pressure-dependences of electrical resistivity ofβ-[Me3(CH2F)][Pd(dmit)2]2
↑Top
(5) Development of novel Ni(dmit)2 anion radical salts with 2,5-dihalopyridinium cations
Assign: Kusamoto, H. M. Yamamoto, N. Tajima, Ohshima, Kato
We recently discovered that some Ni(dmit)2 anion radical salts have two different kinds of anion layers in the unit cell, which we named ''Bi-layer system''. For instance, (methyl-3,5-diiodopyridinium) [Ni(dmit)2]2 have two-dimensional metallic layer and Mott insulating layer, realizing 'Dual-functional π-electron system'. I…S halogen bonds between the cations and the anions play a crucial role to construct the bi-layer system in this salt. In this study, we prepared new [Ni(dmit)2]2- anion radical salt with asymmetric cation, ethyl-2,5-dibromopyridinium (Et-2,5-DBP), to expand the bi-layer system (Figure 7). Single crystal X-ray diffraction measurement for (Et-2,5-DBP)[Ni(dmit)2]2 revealed that two crystallographically independent Ni(dmit)2 anions (anions A and B) form crystallographically independent layers in the unit cell, indicating a bi-layer nature of this salt (Figure 8). Effective halogen bonding between Br (at 2-position on the cation)…S (on Ni(dmit)2 anion) atoms is observed. Band calculations, as well as conducting and magnetic measurements suggested that both Ni(dmit)2 anion layers are in the Mott insulating state, which is a novel electronic structure in the bi-layer system. We next prepared new Ni(dmit)2 anion radical salts with 2,5-dihalopyridinium cations, in which Br atom at 2- or 5-position was replaced by other halogen atoms, to investigate how the halogen bond affects the structural and physical properties. It was found that (Et-2I-5BrP)[Ni(dmit)2]2, in which the Br atom at 2-position is replaced by an I atom, shows identical crystal structure with that of (Et-2,5-DBP)[Ni(dmit)2]2. Their magnetic properties, however, were quite different. On the other hand, (Et-2Br-5IP)[Ni(dmit)2]2, in which the Br atom at 5-position is replaced by an I atom, has different crystal structure from that of (Et-2,5-DBP)[Ni(dmit)2]2, which is so-called mono-layer system, as a result of halogen bonding between both Br…S and I…S atoms.
Figure 7: (Et-2,5-DBP)[Ni(dmit)2]2
Figure 8: Crystal structure of(Et-2,5-DBP)[Ni(dmit)2]2
↑Top
Assign: H. M Yamamoto, Ueno, Suda, Kimura, N. Tajima, Kato; Kawasugi
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@(Figure 9). We have fabricated FET structure with a thin-layer single crystal of κ-(BEDT-TTF)Cu[N(CN)2]X (X = Br or Cl) laminated on a SiO2/Si substrate in order to realize a Mott-transition FET as well as a superconducting FET with organic materials. We have measured Seebeck coefficient for X = Br device and found that the polarity of the conduction carrier at high gate voltage depends on the crystal axes, reflecting the anisotropy of the original Fermi surface (without correlation). This result suggests the absence of Hubbard gap at high doping region. Although X = Br FETs showed only n-type behavior, X = Cl FETs always exhibited ambipolar field effect (Figure 10). The device mobility of the best sample measured by four-probe method reached 87cm2/Vs. The ambipolar behavior allowed us a numerical analysis of the Mott transition. According to our analysis, a Coulomb gap seems to exist after Mott-transition takes place at the interface.
(BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene, FET = Field Effect Transistor)
Figure 9: Filling-correlation phase diagram of κ-BEDT-TTF salts.
At the OFF state, the surface of the FET stays on the green line (= Mott-insulator),
while at the ON state it can be shifted in horizontal directions due to an electro-static doping.
Figure10: Characteristics of Mott-FET based on κ-(BEDT-TTF)2Cu[N(CN)2]Cl at various temperatures.
The black lines indicate simulation results with Coulomb-gap scenario.
↑Top
Assign: N. Tajima, Kato
In this work, we investigated the magnetotransport of massless Dirac Fermions system α-(BEDT-TTF)2I3. The purpose is to clarify the nature of Dirac particles in this system. This system exhibits various types of electronic states when the magnetic field is applied along in-plane or out-of-plane.
(1) Effects of zero-mode
One of the characteristic feature of the Dirac fermion system is the n=0 Landau level called zero-mode that always appears at the Dirac point in the magnetic field. When the energy gap between n=1 and 0 is sufficiently large compared with the thermal energy at low temperature and/or high magnetic field, carriers on the zero-mode Landau level dominate the conduction. This situation is called quantum limit. In this work, in-plane resistivity ρxx and Hall resistivity ρxy in the quantum limit state were investigated. We succeeded in finding the characteristic transport of the zero-mode Landau carriers;
1. ρxx ~ ρxy.
2. Hall angle is always 45 degree (θH~45).
3. The mobility is proportional to the inverse of magnetic field (με1/B).
4. The mean free path is equal to the magnetic length (l~lc).
Those relationships can be derived from the Boltzmann equation.
(BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene)
Figure 11(left): Magnetic field dependences of ρxx, ρxy, θH, μ (inset)
Figure 12(right): Temperature dependence of θH and resistance (inset)
↑Top
(2) Low temperature and high magnetic field
We succeeded in observing the spin splitting of the zero-mode at low temperature and high magnetic field. Furthermore, we demonstrated that the electronic structure of the edge state is different from the bulk. The effects of the broken two-hold valley degeneracy of the edge state were detected in the high magnetic field. This result leads us to the investigation of the quantum Hall state.
↑Top
Assign: Takubo, N. Tajima, Kato
We have studied photo-induced phase transition by focusing on photo-induced charge-ordered melting in BEDT-TTF salts. Previously, photo-induced insulator-to-metal transitions have been observed in α-(BEDT-TTF)2I3, (BEDT-TTF)3(ClO4)2, (BEDT-TTF)5Te2I6 and α-(BEDT-TTF)2RbZn(CNS)4 (fast cooling) by transport measurements. In this year, we simultaneously measured conductivity and transmittance in order to clarify the mechanism of the photo-induced phase transition in charge-ordered BEDT-TTF salts. Figure 13 shows a scheme of the simultaneous measurement. We observed drastic conductivity and transmittance change caused by a pulsed laser irradiation at charge- ordered state in α-(BEDT-TTF)2I3 thin film (Figure 14). This result suggests that macroscopic photo-induced metallic phase is generated. The detailed mechanism is now under investigation.
(BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene)
Figure 13: Simultaneous measurement system of conductivity and transmittance change
caused by a pulsed laser irradiation
Figure 14: Conductivity and transmittance change caused by a pulsed laser irradiation
in α-(BEDT-TTF)2I3 thin film at 4K.
Pump light: YAG-OPO pulsed laser (450 nm, 12 mJ/cm2, E//b)
↑Top
Assign: Oshima, Cui, Kato
The π-d molecular conductors show interesting physical phenomena, such as field-induced superconductivity or giant magneto-resistance, due to its non-negligible π-d interaction. The aim of this study is to control such interesting phenomena by flipping the local d spins or disturbing itinerant -electrons by the use of oscillatory external fields. Here, we have focused on the λ-(BETS)2FexGa1-xCl4 system which shows field-induced superconductivity (FISC) at high magnetic fields, and have studied whether the FISC state can be controlled by ESR transitions, which correspond to the spin-flips of d-electrons. Simultaneous ESR and transport measurements were performed on λ-(BETS)2FexGa1-xCl4 (x=0.6), and we have observed a change in the resistance due to the ESR transitions since the resistance anomaly occurs at the same field position of the resonance field. As shown in the figures below, the ESR is observed in the paramagnetic state and even in the FISC state. This suggests that the FISC state is inhomogeneous. Moreover, the resistance does not show any anomalies in the low and high temperature range, but only shows anomalies at the boundary of paramagnetic and FISC states. These results indicate that the FISC phase is only partially destroyed due to the weak power of the millimeter-wave irradiation. Upgrade of millimeter-wave oscillators is planned for the future.
(BETS= bis(ethylenedithio)tetraselenafulvalene)
Figure 15: Temperature dependence of simultaneous ESR and transport measurements of λ-(BETS)2FexGa1-xCl4(x=0.6)
The resistance changes when ESR transition occurs at the phase boundary
↑Top
Assign: Suda, H. M. Yamamoto, Kato
κ-(BEDT-TTF)2Cu[N(CN)2]Cl is a strongly-correlated molecular conductor and the competition between the electron-electron Coulomb repulsion and the kinetic energy is highlighted, resulting in the pressure-induced bandwidth-controlled Mott transition. In this study, we have successfully controlled the ground state of κ-(BEDT-TTF)2Cu[N(CN)2]Cl thin single crystal laminated on a plastic substrate by strain effects. A variable strain was applied by a nano-positioner from the back-side of the substrate. κ-(BEDT-TTF)2Cu[N(CN)2]Cl on the substrate was found to be a percolated superconductor (TC = 11K) due to the positive pressure exerted through the compressible substrate. The observation of the strain-induced huge resistance jump under 37.5K provides an unambiguous evidence for the first-order nature of the band-width-controlled Mott transition between the metal (superconductor) and the insulator while the jump vanishes at over 40.0K (Figure 16). This result indicates a possibility of realizing novel phase transition-type devices co-regulation of bandwidth and band filling in molecular conductors.
(BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene)
Figure 16: Strain-induced Mott transition (left) and strain-temperature phase diagram (right)
for κ-(BEDT-TTF)2Cu[N(CN)2]Cl
↑Top
Assign: Abdel Jawad, N. Tajima, Kato
We have found an anomalous dielectric response in molecular conductors with dimer packing where identical molecules are arranged in pairs and share one charge per pair. The frequency and temperature dependence of this dielectric response is similar to a glassy state of electrical dipoles, a relaxor ferroelectric (Figure 17, 18), which is at odds with the high quality single crystal nature of these systems. The onset of this anomalous dielectric response is found to scale with the charge gap indicating a relation with the conducting channel. Furthermore, the large variety of molecular conductive compounds which exhibit this effect and with only the dimer structure in common, would indicate that we are dealing with an electronic dipole state. The origin of this electronic dipole state is still unclear but the absence of magnetic field effect on the dielectric constant indicates that this is not a spin driven ferroelectricity but a charge driven one with nontrivial charge degrees of freedom. Further experiments should clarify the origin of this anomalous dielectric response.
Figure 17: Temperature and frequency dependence of the real part of the dielectric constant
in Me4P[Pd(dmit)2]2
Figure 18: Frequency dependence of the real and imaginary component of the AC resistivity
of Me4P[Pd(dmit)2]2 at 40K along the out of plane direction.
↑Top
