Toshihiro Takahashi

We have carried out a series of measurements of angular dependence of solid-state NMR spectrum using single crystal samples on various organic molecular conductors, in order to investigate the natures of the electronic states at low temperatures. We confirmed a charge ordered insulating state in alpha-(BEDT-TTF)(2)I-3 and large charge disproportionation in the metallic state of this salt. In another charge ordered system, theta-(BEDT-TTF)(2)RbZn(SCN)(4), we observed unusual NMR line broadening, proportional to resonance shift, in the metallic state above the transition. We found that this broadening is due to charge disproportionation, or more correctly, due to the inhomogeneity of local susceptibility at nuclear sites and analyzed its dynamics. We observed similar broadening in various organic molecular conductors as well, such as theta-(BEDT-TTF)(2)CsZn(SCN)(4), an exotic Bechgaad salt, (TMTSF)(2)FSO3, and lambda-type BETS salts, lambda-(BETS)(2)(Fe,Ga)Cl-4. We found the mechanism of CD in each system is different, respectively. (c) 2012 Elsevier B.V. All rights reserved.

LiNbO3-type MnMO3 (M = Ti, Sn) were synthesized under high pressure and temperature; their structures and magnetic, dielectric, and thermal properties were investigated; and their relationships were discussed. Optical second harmonic generation and synchrotron powder X-ray diffraction measurements revealed that both of the compounds possess a polar LiNbO3-type structure at room temperature. Weak ferromagnetism due to canted antiferromagnetic interaction was observed at 25 and 50 K for MnTiO3 and MnSnO3, respectively. Anomalies in the dielectric permittivity were observed at the weak ferromagnetic transition temperature for both the compounds, indicating the correlation between magnetic and dielectric properties. These results indicate that LiNbO3-type compounds with magnetic cations are new candidates for multiferroic materials.

An A-site ordered perovskite oxide SrCu3Ti4O12 was synthesized under high pressure and temperature conditions. The sample was characterized by the synchrotron X-ray diffraction, dielectric and magnetic susceptibility measurements. The synchrotron X-ray Rietveld refinement revealed that the strontium ion fully occupied on the A-site, while the copper ion at the A'-site was slightly deficient. The dielectric constant was about 160 at 1 kHz at room temperature. The anti-ferromagnetic (AFM) order with a Neel-temperature T-N of at 24.5 K was observed. The ionic radii of the A-site ion had little influence on AFM indirect exchange interaction between Cu2+ ions in A Cu3Ti4O12.

We performed Se-77 NMR mesurements to investigate the electronic properties of the two dimensional BETS layers of the organic superconductor, lambda-(BETS)(2)GaCl4, which is isostructural with the field induced superconductor, lambda-(BETS)(2)FeCl4. The broadening of NMR spectrum at low temperatures was confirmed to be due to charge disproportionation in the two dimensional BETS conducting layer in the Ga compound as it was in the Fe compound [J. Phys. Soc. Jpn. 76 (2007) 124708]. From the analysis of the anisotropy of central shift and linewidth, we estimated that degree of charge disproportionation in the Ga salt is about half as large as that observed in the Fe salt. The present result clearly confirms that the magnetic Fe ions are not responsible for the broadening and that a charge disproportionated matallic state exists in the close vicinity of the superconducting phase of the Ga salt.

Magnetic properties of organic conductors beta'-Et2Me2Sb[Pd(dmit)(2)](2) (Et = C2H5, Me = CH3), P2(1)/m crystal of EtMe3P[Pd(dmit)(2)](2) and alpha-(BETS)(2)I-3 were investigated by muon spin relaxation (mu SR) method. It has been suggested that Et2Me2Sb[Pd(dmit)(2)](2) exhibits a charge separation phase below similar to 70 K and that EtMe3P[Pd(dmit)(2)](2) exhibits a spin-Peierlslike state below similar to 25 K although it has a quasi two-dimensional structure. Our zero-field mu SR results indicate that these Pd(dmit)(2) salts show quite similar behavior to an organic spin-Peierls system although changes in the mu SR time spectra of the EtMe3P salt was ambiguous. For alpha-(BETS)(2)I-3, a metal-insulator transition is expected to occur at T-MI similar to 50 K. We observed a slight enhancement of the muon spin relaxation below T-MI and it gradually became remarkable at lower temperatures. The present result may reveal the existence of a spin-singlet ground state showing similar temperature dependence of the muon spin relaxation rate to that in the Pd(dmit)(2) salts. However, the possibility that the relaxation in this system originates from magnetic fluctuations is also still conceivable.

We performed Se-77 NMR on a single crystal sample of the field-induced superconductor lambda-(BETS)(2)FeCl4. Our result obtained in the paramagnetic state provide a microscopic insight on the exchange interaction J between the spins s of the BETS pi conduction electrons and the Fe localized d spins S. The absolute value of the Knight shift K decreases when the polarization of the Fe spins increases. This reflects the "negative" spin polarization of the pi electrons through the exchange interaction J. The value of J has been estimated from the temperature and the magnetic field dependence of K and found to be in good agreement with that deduced from transport measurements [Balicas et al.: Phys. Rev. Lett. 87 (2001) 067002]. This provides direct microscopic evidence that the field-induced superconductivity is due to the compensation effect predicted by Jaccarino and Peter [Phys. Rev. Lett. 9 (1962) 290]. Furthermore, an anomalous broadening of the NMR line has been observed at low temperatures, which suggests the existence of charge disproportionation in the metallic state neighboring the superconducting phase.

The spatial charge arrangement of a typical quasi-two-dimensional organic conductor alpha-(BEDT-TTF)(2)I-3 is revealed by single crystal structure analysis using synchrotron radiation. The results show that the horizontal-stripe-type structure, which was suggested by the mean field theory, is established. We also find the charge disproportionation above the metal-insulator transition temperature and a significant change in the transfer integrals caused by the phase transition. Our result elucidates the insulating phase of this material as the 2k(F) charge density localization.

We experimentally determined the spin density distribution to investigate the electronic states of the (DMe-DCNQI)(2)M systems, where M is Li, Ag, and Cu. The Knight shift is measured with solid-state high-resolution NMR of H-1 and C-13 and is analyzed together with the reported N-15 data. The Knight shift data at each atom of the DMe-DCNQI molecule give the pi-electron spin distribution of the Li salt, which might correspond to the charge distribution in the insulating and localized spin system of the Li salt. The average fraction of the spin susceptibility chi(DCNQI)/chi(0) in the DMe-DCNQI molecule of the Ag and Cu salts relative to that of the Li salt is obtained to be 0.82 and 0.71, respectively. Thus, if the valence of the Ag ion is unity, the DMe-DCNQI molecule of the Ag salt has the missing spin fraction by 0.18. One possible origin of this missing spin is a modification of the lowest unoccupied molecular orbital of the DMe-DCNQI molecule due to the pi-d hybridization with the Ag d(xy) orbitals, which could be one of the origins for the difference of the physical properties from the Li salt, as in the enhancement of the charge hopping rate between the neighboring DMe-DCNQI columns in the Ag salt. In the Cu salt case, the missing spin fraction of 0.29 would correspond to the d-spin hole number of 0.28 of the Cu d(xy) orbital. Finally, it is stressed that all the present results are consistent with both the first-principles calculation and the X-ray photoelectron spectroscopy study.

Present status of experiments on charge ordering (CO) phenomena observed in organic molecular conductors is reviewed. Experimental techniques to detect and analyze the CO, i.e., NMR, X-ray and Infrared/Raman spectroscopy, are described and the experimental results in typical materials exhibiting CO are summarized. (DI-DCNQI)(2)Ag, (TMTTF)(2)X, alpha-(BEDT-TTF)(2)I-3, theta-(BEDT-TTF)(2)RbZn(SCN)(4) and related materials are discussed. Charge disproportionation without long-range order observed in numbers of materials are also included.

Se-77 NMR measurements have been performed in the normal state of the organic, magnetic field induced superconductor lambda(BETS)(2)FeCl4, to examine the interaction between Fe local spins and;T conduction electrons. Large, negative Knight shift values were observed at fields lower than the one corresponding to onset of field induced superconductivity. This evidences that the conduction electrons feel a negative internal field coming from the Fe spins through the exchange interaction (J) over right arrows (.) (S) over right arrow. This, suggests that the superconductivity induced by high magnetic fields has its origin in the Jaccarino-Peter compensation mechanism. The estimated value of the exchange interaction J, is 30% smaller than that predicted by the resistivity measurements.

We carried out Se-77 NMR measurements on BETS2(Cl(2)TCNQ) under pressure in order to investigate the magnetic properties of the insulating state which appears above 0.6 GPa. The relaxation rate 1/T-1 at 0.7 GPa shows small peak-like anomaly at 20 K, indicating a spin density wave transition as observed in BETS2(Br(2)TCNQ).

The quarter-filled pi band systems, (DMc-DCNQI)(2)M(M = Li1-xCux(x <= 0.14), Ag) were systematically studied with electron paramagnetic resonance (EPR). The intercolumn spin hopping rate D-perpendicular to in Li1-xCux-salt was obtained from the EPR linewidth. The temperature dependence of D, can be understood with the hole soliton model which also explains the DC conductivity. The pi - d mixing of the Cu-salt enhances both D-perpendicular to and sigma(perpendicular to) by 10(3) times more than the Li-salt, which is consistent with the fact that the only Cu-salt has three-dimensional Fermi surface, but that Ag-salt is one-dimensional in spite of the mixing enhancement of D-perpendicular to by 10 times more than the Li-salt.

Solid state high-resolution NMR of H-1 and C-13 along with N-15 is analyzed to investigate the electronic states of the charge transfer salts, (DMe-DCNQI)(2)M, (M=Li, Ag, and Cu). We determined the spin/charge distribution in a DMe-DCNQI molecule of the Li-salt from the Knight shifts at each atom on the molecule. It is found that the obtained charge distribution is similar to the theoretical prediction. The charge density on the DCNQI molecules of the Ag-salt is found to be smaller by 20% than the Li-salt, which could be an origin of differences from the Li-salt. This result is consistent with the first principle calculations (T. Miyazaki and K. Terakura, Phys. Rev. B 54, 10452 (1996)).

We have investigated the sliding motion of the SDW in the organic compound (TMTTF)(2)Br under the pressure with the measurement of the non-linear conductivity. Below the SDW transition temperature T-SDW, we observed the sharp increase of conductivity with the clear threshold electric field E-T, associated with the depinning of the SDW. For the typical sample, the temperature dependence of E-T shows a peak around 0.3(SDW) in each pressure. In addition, the field hysteresis with the switching appears in the dc I-V characteristics around 03T(SDW). The excess conductivity, dominated by the kinetic friction, shows a sharp increase below 0.3T(SDW). These behaviors indicate that the sliding mechanism of SDW varies across 0.3(SDW), which is almost equal to the sub-phase transition temperature. This fact suggests that the transition at 0.37(SDW) is associated with both the magnetic and charge degrees of freedom.

Charge ordering phenomena, which are of keen interest in the physics of cot-related electrons, appear in various molecular conductors. It accompanies a symmetry breaking in the lattice. In some types of lattices, however, their topology is against the symmetry breaking. We discuss possible NMR signatures of the frustration effect on the charge ordering.

The electronic properties of a novel 3:1 BEDT-TTF salt were investigated by 13C-NMR measurements on a single crystal sample based on selectively C-13-enriched BEDT-TTF molecule. Static susceptibility, Knight shift and relaxation rate were measured and a Korringa. relation was confirmed. The correlation enhancement of Korringa co-efficient was found very small in comparison to the usual 2:1 BEDT-TTF salts.

The electronic properties of (TMTSF)(2)FSO3 have been investigated under pressure of 0.65 GPa by measuring Se-77-NMR lineshape, spin-lattice relaxation rate (T-1(-1)) and spin-spin relaxation rate (T-2(-1)) measurements on a single crystal sample. The NMR signal was found abnormally broadened below 90 K. Below 40 K, a narrow component appears and coexists with the broader one. The temperature dependence of T-1(-1) suggests that the broader component comes from a metallic phase and the narrow one from an insulating phase. The activation energy determined for the insulating component was found three times lower than the value found at ambient pressure. From the T-2(-1) measurements, the broadening of the metallic component was confirmed as inhomogeneous. An unusual double peak anomaly of T-2(-1) suggests the existence of extremely slow fluctuations of the inhomogeneous local fields below 90 K at this pressure.

The spin-density wave (SDW) phase with an incommensurate wave vector is induced as a ground state in the organic compound (TMTTF)(2)Br under the pressure above 0.5 GPa. We investigated the collective sliding motion of the SDW condensate with measuring the non-linear conductivity in this SDW phase. The measurement was done with dc and pulse methods. Below the SDW transition temperature T-SDW, the sharp increase of conductivity from the ohmic one was observed with the clearly defined threshold electric field E-T. This indicates the depinning of the SDW and the excess conductivity above E-T is associated with the sliding motion of the SDW. The absolute value of E-T is fairly larger than that observed in (TMTSF)(2)PF6, whose ground state is the incommensurate SDW at ambient pressure. With decreasing the temperature from T-SDW, E-T increases gradually and shows a relatively sharp drop after a peak around 0.3T(SDW). This temperature is supposed to be connected with the sub-phase transition proposed from the temperature dependence of NMR relaxation rate. In addition, the excess conductivity normalized to the ohmic one, which corresponds to the current carried by the SDW condensate, shows a dip at 0.3T(SDW) with decreasing the temperature. In the dc I-V characteristics, the hysteretic behavior near E-T appears only around 0.3T(SDW). These behaviors suggest that the sliding mechanism of the SDW condensate varies across 0.3T(SDW). We discuss the dynamics of the SDW in (TMTTF)(2)Br in comparison with that in (TMTSF)(2)X, considering the sub-phase transition.

We showed how the charge order in (DI-DCNQI)(2)Ag is melted by pressure, by investigations of the NMR spectra and the resistivity parallel and perpendicular to the conducting c-axis. When the system is pressurized, the second-order-transition temperature of the charge ordering is lowered and the disproportionation ratio is depressed. Under higher pressures, the transition changes into first-order one and eventually it vanishes at 21.6 kbar. At a low-pressure and high-temperature region, temperature dependence of rho(parallel to) and rho(perpendicular to) are contrasting. The melting mechanism of the charge ordering is very likely due to the "dimensional crossover". Furthermore, in the high-pressure metallic region, curious T-3 dependence of the resistivity is observed, implying unprecedented electron-electron scattering mechanism.

The nature of the electron liquid phase in a quasi-one-dimensional charge-ordering system composed of uniform chains is investigated by means of comparison between the parallel and perpendicular resistivity under pressures. At lower pressures, the liquid phase is the one-dimensional liquid where electrons are confined in the conducting chains. At higher pressures where the charge-ordering transition vanishes, however, it acquires the three-dimensional Fermi-liquid nature at low temperatures away from the one-dimensional liquid. This result shows that the collapse of the charge order is due to the "dimensional crossover".

A charge-ordered insulator, (DI-DCNQI)(2)Ag, with a quasi-one-dimensional quarter-filled band is metallized by pressure. It was found that the charge order melts into a curious metallic state with cubic-temperature dependence of the resistivity, which implies the unprecedented mechanism of the electron-electron scattering. We constructed the pressure-temperature phase diagram, where the melting line has a tricritical point dividing the second-order line at low pressures and the first-order line at high pressures just before it vanishes.

Cubic pyrochlore Pb2Ru2O6.50+/-0.02 single crystals were grown from a PbO flux by a slow-cooling method under ambient atmosphere. The obtained crystals were black and formed quasi-cubes and cuboctahedra. The largest crystal with a size up to 9 x 7 x 4 mm(3) was grown from the mixture of RuO2 and PbO with a molar ratio RuO2/PbO = 1/9, which was heated to 1250 degreesC, held for 10 h, and then cooled at a constant rate of 5.0 degreesC/h to 950 degreesC. It was found that the vaporization of PbO is important for crystal growth. The electronic resistivities for the crystal were 2.3 x 10(-4) Omegacm at 300 K and 2.0 x 10(-6) Omegacm at 2K, respectively. The temperature dependence of the resistivity indicates that this compound is metallic and the DC magnetic susceptibility reflects the delocalized character for electrons of Ru. (C) 2004 Elsevier B.V. All rights reserved.

We report the results of measurements of the conductance and the low-frequency dielectric permittivity of the quasi-one-dimensional compound (DI-DCNQI)(2)Ag. We analyse the physical nature of the phase transition occurring in this compound into a charge-ordered state and the formation of the appropriate energy gap. We compare the dielectric properties of (DI-DCNQI)(2)Ag with those resulting from charge-ordered states recently reported in (TMTTF)(2)X salts.

(DMe-DCNQI)(2)M (M = Li, Ag) is a 1/4-filled spin-Peierls system. We study the spin/charge dynamics in the insulating state by EPR. The linewidth shows exponential dependence at T much greater than T-sp. Further, pressure enhances mainly prefactor of the exponential dependences. These results are successfully understood in terms of the relaxation accompanied with the intercolumn hopping caused by the spin-orbit interaction both for the Li and Ag salts. This is consistent with the scenario that the hole and spin solitons help the intercolumn charge and spin transports. (C) 2003 Elsevier B.V. All rights reserved.

C-13 NMR measurements were carried out to investigate the electronic state of Mixed-stack organic charge-transfer complex (BEDO-TTF)(Cl(2)TCNQ). From the remarkable temperature dependence of the central shift and relaxation rate, we have confirmed that the transition at 110 K is from paramagnetic to nonmagnetic spin-singlet phase.

In order to characterise the electronic properties in sub-phases of (TMTSF)(2)FSO3 under pressure. Se-77, and F-19 NMR Knight shift (K) and relaxation rate (1/T-1) measurements were performed at ambient and 0.82 GPa pressure. The insulating phase below the metal-insulator transition (MIT) at 89 K at ambient pressure (AP) was confirmed as a nonmagnetic state with a gap of Delta/k(B)similar to420 K. This MIT is strongly coupled with anion ordering. The MIT temperature decreases under pressure, and many electronic anomalies (or transitions) have been observed from transport measurements. The reduction of density of states in the metallic state by applying pressure was confirmed by the analysis of K and Korringa constant (1/T1T). A nonmagnetic insulating transition corresponding to the MIT at AP at 89 K, was observed at similar to45K at 0.82 GPa. A spin (charge) gap in the insulating state at 0.82 GPa is smaller (Delta/k(B)similar to90 K) than that at AP.

High field ESR measurements have been performed on (DI-DCNQI)(2)Ag single crystals at 1.9 K under 1 bar and 2.4 kbar in the frequency region from 70 to 160 GHz using the pulsed magnetic field up to 8 T. This compound undergoes the antiferromagnetic transition at T-N = 5.5 K and the anti ferromagnetic resonance (AFMR) was observed under 1 bar. The AFMR was also observed under 2.4 kbar and the zero-field antiferromagnetic gap is found to be reduced under pressure. The origin of this result is discussed.

Single crystal C-13 NMR measurements were carried out at pressures of 0.7 and 1.4 GPa to investigate the unusual high-pressure state of alpha-(BEDT-TTF)(2)I-3. Large temperature dependences of susceptibility was observed at 0.7 GPa. However, the temperature dependence of relaxation rate seems to contradict with the rapid drop of carrier density determined by Hall effect measurements at low temperatures. This may require us a serious reconsideration of the nature of the high-pressure phase

Large enhancement of antiferromagnetic (AF) transition temperature T-N by pressure was observed in a quasi two-dimensional organic conductor, beta'-(Me4P)[Pd(dmit)(2)](2). Similar results are obtained in Et2Me2P salt at 5 kbar. It is indicated that pressure tends to reduce magnetic frustrations leading to the stabilization on AF state. A single crystal of beta'-(Et2Me2P)[Pd(dmit)(2)](2) was also examined at ambient pressure. A clear evidence of AF transition was obtained at around 16 K by C-13 NMR relaxation rate 1/T-1. This is consistent with ESR study, but different from our polycrystalline experiments.

To clarify the role of charge ordering at the MI transitions in theta-phase compounds, we performed C-13-NMR measurements in B-(BEDT-TTF)(2)CsZn(SCN)(4) and B-(BEDT-TTF)(2)RbZn(SCN)(4) under pressure. We found that charge ordering is strongly affected by pressure. No long-range static charge ordering. is stabilized in Cs salt. Moreover, we found a region where charge disproportionation already develops but no long-range static charge ordering is stabilized.

The roles of donor layers and acceptor stacks in the electronic properties of the charge transfer complex, (BETS)(2)(Br(2)TCNQ) were investigated by means of site-selective NMR measurements. Line broadening of Se-77 NMR indicating a magnetic ordering on the BETS site was observed at the metal-insulator transition temperature 12 K. This transition is caused by the spin-density-wave formation on the BETS layers. C-13 NMR on the cyano carbon in the Br(2)TCNQ molecule confirms that a spin singlet formation starts to develop on the one dimensional Br(2)TCNQ columns below 170 K and its three dimensional ordering appears below 80 K. From the analysis of the NMR line shape and shift, magnetic susceptibility was decomposed into the contributions of the BETS and Br(2)TCNQ sites.