田島 裕之

The title compound (ET=bis(ethylene)dithiotetrathiafulvalene TCE=1,1,2-trichloroethane) is a new molecular conductor obtained by the standard electrochemical oxidation. It has a beta " -donor arrangement without any crystallographic disorder or non-integral stoichiometry. The electrical behavior, which was sensitive to pressure, was semiconducting (35 Scm(-1) at RT) at ambient pressure with a clear hump around 60 K accompanied by a large hysterisis. No corresponding anomalies, however, were observed in its temperature-dependent magnetic susceptibility. The calculated band structure is two-dimensional, which agrees well with the observed physical properties.

Magnetoresistance, magnetization, and reflectance spectra were measured for τ-(EDO-S,S-DMEDT-TTF)2(AuBr2)1+y, (y∼0.75) (EDO-S,S-DMEDT-TTF = ethylenedioxy-S,S-dimethylethylenedithio-tetrathiafulvalene) and τ-(P-S,S-DMEDT-TTF)2(AuBr2)1+y, (y∼0.75) (P-S,S-DMEDT-TTF = pyrazino-S,S-dimethylethylenedithio-tetrathiafulvalene). The saturation of magnetization curve and Drude dispersion imply the weak ferromagnetic and metallic nature of former compound at low temperature. For the comparison with this compound, magnetoresistance of latter salt was measured up to 24 T.

Temperature dependence of the optical conductivity spectra have been studied for single crystals of θ-ET2MM'(SCN)4 (MM'=RbCo, RbZn, CsZn). The spectra of the CsZn salt are Drude-like down to 20 K. For the RbCo and RbZn salts, the mid-infrared (MIR) band exhibits single-peaked feature in the conductive state above 190 K and in the quenched state below 190 K, while it exhibits triple-peaked feature in the relaxed state below 190 K. Charge-ordering patterns on these salts are deduced from the spectral analysis using the meanfield calculations.

We report two studies for (DMe-DCNQI)(2)Li1-xCux recently carried out by our group. In the first study, we report the measurements of electrical resistivity, magnetic susceptibility, reflectance spectra for (DMe-DCNQI)(2)Li1-xCux. The point of this study is that the x-T phase diagram of the system can be divided into three categories, i. e., 0 less than or equal to x less than or equal to 0.14, 0.17 less than or equal to x less than or equal to 0.3, 0.3 less than or equal to x less than or equal to 1.0. In the second study, we report the results of the electrical resistivity measurements for (DMe-DCNQI)(2)Cu under the uni-axial pressure applied parallel and perpendicular to the stacking axis of DMe-DCNQI (c-axis).

We found the giant negative magnetoresistance in TPP[Fe(Pc)(CN)(2)](2) below 50K, The magnetic susceptibility shows a highly anisotropic Curie behavior. Under the field along the a-axis (the c-axis), we observed the large (small) negative magnetoresistance and the large (small) magnetic susceptibility. This experimental results suggest the pi -d interaction between the one-dimensional conduction electrons and the local magnetic moments. We proposed the field-induced reduction of the spin scattering of the conduction electrons by the local moments as the origin of the negative magnetoresistance. In the isostructural compound TPP[Co(Pc)(CN)(2)](2) without magnetic moments, we observed large positive magnetoresistance. The experimental result of the Go-compound suggests that the local moments are essential for the mechanism of the negative magnetoresistance of the Fe-compound.

An electrolysis of bis(ethylenedithio)tetrathiafulvalene (ET) and a Mn-cluster in a 1,1,2-trichloroethane solution containing 10%(vol/vol) of ethanol yields black lustrous single crystals, β″-(ET)3(MnCl4)(1,1,2-C2H3Cl3) based on the X-ray structural study. The crystal structure can be characterized as alternating two-dimensional donor sheets and insulating sheets made of isolated [MnCl4]2- ions and the 1,1,2-C2H3Cl3 molecules. Every pair of the neighbouring donor molecules has a large displacement along both short and long molecular axes. A tight-binding band calculation suggests that this arrangement should lead to a weak but isotropic intermolecular interaction in the donor sheets, and this in turn should lead to a marginally metallic or semiconducting electronic structure.Although the polarized reflectance spectra and the temperature-dependent spin susceptibility derived from the EPR spectra on the single crystal indicate metallic nature, the electrical behaviour under atmospheric pressure is semiconductive with room temperature conductivity of 35 S cm-1 and apparent activation energy of 0.023 eV. It exhibits a resistive hump with hysteresis at around 60 K, which could be associated with a structural transition demonstrated by a series of low temperature X-ray oscillation photographs. The magnetic susceptibility at 4.5-300 K does not exhibit any anomaly and is well reproduced by the Curie-Weiss model with Curie constant C/emu K mol-1 = 3.89 and Weiss temperature θ/K = -0.10. This magnetic behaviour can be quantitatively understood as the sum of Pauli paramagnetism of the π-electronsand the contribution from the local spins on the Mn(II) ions with d5-configuration (S = 5/2). High pressure easily suppresses the increase in resistivity at low temperature, and the electrical behaviour is particularly sensitive to the first 3-5 kbar. © The Royal Society of Chemistry.

An electrolysis of bis(ethylenedithio)tetrathiafulvalene (ET) and a Mn-cluster in a 1,1,2-trichloroethane solution containing 10% (vol/vol) of ethanol yields black lustrous single crystals, β″-(ET)3(MnCl4)(1,1,2-C2H 3Cl3) based on the X-ray structural study. The crystal structure can be characterized as alternating two-dimensional donor sheets and insulating sheets made of isolated [MnCl4]2- ions and the 1,1,2-C2H3Cl3 molecules. Every pair of the neighbouring donor molecules has a large displacement along both short and long molecular axes. A tight-binding band calculation suggests that this arrangement should lead to a weak but isotropic intermolecular interaction in the donor sheets, and this in turn should lead to a marginally metallic or semiconducting electronic structure. Although the polarized reflectance spectra and the temperature-dependent spin susceptibility derived from the EPR spectra on the single crystal indicate metallic nature, the electrical behaviour under atmospheric pressure is semiconductive with room temperature conductivity of 35 S cm-1 and apparent activation energy of 0.023 eV. It exhibits a resistive hump with hysteresis at around 60 K, which could be associated with a structural transition demonstrated by a series of low temperature X-ray oscillation photographs. The magnetic susceptibility at 4.5-300 K does not exhibit any anomaly and is well reproduced by the Curie-Weiss model with Curie constant C/emu K mol-1 = 3.89 and Weiss temperature θ/K = -0.10. This magnetic behaviour can be quantitatively understood as the sum of Pauli paramagnetism of the π-electrons and the contribution from the local spins on the Mn(II) ions with d5-configuration (S = 5/2). High pressure easily suppresses the increase in resistivity at low temperature, and the electrical behaviour is particularly sensitive to the first 3-5 kbar.

Electrochemical oxidation of PTMA[MIII(Pc)(CN)2] (PTMA = phenyltrimethylammonium, Pc = phthalocyaninato, and M = Co or Fe) gives partially oxidized salts (PTMA)x[MIII(Pc)(CN)2]·y(solvent) (solvent = acetonitrile or acetone). The obtained crystals in which the solvent molecules partially occupy the cationic sites are isomorphous regardless of M and solvent. They are highly conducting and metallic behavior is observed in the conductivity and thermoelectric power measurements. In contrast to the Pauli-like Behavior of the CoIIIsalt, Curie-like behavior with an anomaly due to antiferromagnetic interactions was observed for the FeIIIsalt in magnetic susceptibility measurements. The existence of local magnetic moments in the FeIIIsalt leads to a significant influence on the transport properties, suggesting the existence of π-d interactions in this system.

(TTM-TTP)AuI_2 (TTM-TTP: 2,5-bis[4,5-bis(methylthio)-1,3-dithiol-2-ylidene]-1,3,4,6-tetra-thiapentalene) has a dimerized structure along the donor stacking direction, and shows semi-conducting behavior below room temperature. ESR, static magnetic susceptibility, and optical reflectance of this salt have been measured to investigate the spin state and the electronic correlation. The ESR signal has been observed from room temperature to 3 K, and the spin susceptibility shows paramagnetic behavior with a rapid decrease below 10 K. The static magnetic susceptibility is paramagnetic and has an anomaly around 10 K in agreement with the ESR result. The chain axis optical reflectance spectra show clear optical gap in the mid-infrared region. An attempt is undertaken to analyze the optical spectrum by means of the one-dimensional dimerized Hubbard model, which suggests that the on-site Coulomb repulsion, U, is small and the spin polarization is located at the marginal paramagnetic boundary. These results indicate that this compound is not a band-insulator but the Mott insulator with a small spin gap.

Temperature dependence of the polarized reflectance spectra have been measured on single crystals of θ-ET2MM′ (SCN)4 (abbreviated as θ-MM′; MM′ - RbCo, RbZn, and CsZn). The reflectance spectra of the θ-RbCo and θ-RbZn salts exhibit drastic change around 190 K for the samples cooled slowly (dT/dt∼-0.1 K/min). This spectral change disappears for the samples cooled rapidly (dT/dt∼-1 K/min). The reflectance spectra of the θ-CsZn salt exhibit Drude-like features down to 20 K. Charge-ordering patterns on these salts are deduced from the spectral analysis using the mean-field calculations.

Temperature dependence of the polarized reflectance spectra have been measured on single crystals of theta-ET2M M' (SCN)(4) (abbreviated as theta-M M' ; M M' = RbCo, RbZn, and CsZn). The reflectance spectra of the theta-RbCo and theta-RhZn salts exhibit drastic change around 190 K for the samples cooled slowly (dT/dt similar to -0.1 K/min). This spectral change disappears for the samples cooled rapidly (dT/dt similar to -1 K/min). The reflectance spectra of the theta-CsZn salt exhibit Drude-like features down to 20 K. Charge-ordering patterns on these salts are deduced from the spectral analysis using the mean-field calculations.

We have found giant negative magnetoresistance in the one-dimensional conductor TPP[Fe(Pc)(CN)2]2 below 50 K. The reduction of the resistance is larger in the field perpendicular to the one-dimensional axis than parallel to the axis. The magnetic susceptibility shows anisotropic Curie-Weiss behavior. The experimental results suggest the interaction between the one-dimensional electron system and the local moments. The reduction of the spin scattering of the itinerant electrons by the local moments under the field is proposed as the origin of the giant negative magnetoresistance.

We have found giant negative magnetoresistance in the one-dimensional conductor TPP[Fe(Pc)(CN)(2)](2) below 50 K. The reduction of the resistance is larger in the field perpendicular to the one-dimensional axis than parallel to the axis. The magnetic susceptibility shows anisotropic Curie-Weiss behavior. The experimental results suggest the interaction between the one-dimensional electron system and the local moments. The reduction of the spin scattering of the itinerant electrons by the local moments under the field is proposed as the origin of the giant negative magnetoresistance.

We measured the static spin susceptibility of the quasi-one-dimensional molecular conductors, (DMe-DCNQI)(2)Li1-xCux (0 less than or equal to x less than or equal to 1). We separated the susceptibility component due to itinerant electrons from that due to local magnetic moments by assuming that the temperature dependence of the latter component obeys the Curie-Weiss law. From the temperature dependence of the former component, we conclude that the ground state of this system is nonmagnetic for x less than or equal to 0.14 and paramagnetic For 0.17 less than or equal to x. The phase boundary thus determined is consistent with the phase diagram already determined by resistivity measurements. (C) 2000 Elsevier Science Ltd. All rights reserved.

A conducting partially oxidized salt of TPP[Fe(Pc)(CN)2]2, which contains paramagnetic iron(III), has been obtained by electrocrystallization of TPP[Fe(Pc)(CN)2]. The crystal is composed of one-dimensional TPP arrays surrounded by slipped-stacked one-dimensional Fe(Pc)(CN)2 chains. This structure is isomorphous with TPP[Co(Pc)(CN)2]2, in which the central metal is non-magnetic cobalt(III). The electrical conductivity at room temperature, ca. 10 Ω-1 cm-1, is about one order of magnitude lower than that of the Co analogue. On lowering the temperature, the conductivity shows a much steeper decline compared with the temperature dependence of the conductivity of TPP[Co(Pc)(CN)2]2. The band width, estimated from both thermoelectric power measurements (which show apparent metallic behavior at high temperatures) and an extended Huckel calculation, is found to be the same as that obtained for TPP[Co(Pc)(CN)2]2. This is consistent with observations from the single-crystal reflectance spectra, in which both partially oxidized salts show almost the same plasma edge. On the other hand, the magnetic susceptibility of TPP[Fe(PC)(CN)2]2 is quite different from that of TPP[Co(Pc)(CN)2]2; both of the magnetic moments originated from Fe111 and the π-radical seem to be localized in TPP[Fe(Pc)(CN)2]2, while the moments originated from the π-radical in TPP[Co(Pc)(CN)2]2 are interacting with each other to lead to temperature-independent Paul-like susceptibility. At low temperatures a decrease in the susceptibility has been observed, suggesting the existence of antiferromagnetic interactions.

We found the giant negative magnetoresistance in the one-dimensional π-d system TPP[Fe(Pc)(CN)2]2 below 50K. The large negative magnetoresistance was observed for the field along the a-axis, while the small one along the c-axis. We also observed the anisotropic Curie behavior of the magnetic susceptibility. The large (small) magnetoresistance was observed in the field orientation for the large (small) magnetic susceptibility. The reduction of the spin-flip scattering of the itinerant electrons by the local moments under the magnetic field was proposed as the origin of the negative magnetoresistance. In the isostructural compound TPP[Co(Pc)(CN)2]2 we observed the large positive magnetoresistance. This positive magnetoresistance cannot be explained in terms of the orbital effects under the field.

We found the giant negative magnetoresistance in the one-dimensional pi-d system TPP[Fe(Pc)(CN)(2)](2) below 50K. The large negative magnetoresistance was observed for the field along the a-axis, while the small one along the c-axis. We also observed the anisotropic Curie behavior of the magnetic susceptibility. The large (small) magnetoresistance was observed in the field orientation for the large (small) magnetic susceptibility. The reduction of the spin-flip scattering of the itinerant electrons by the local moments under the magnetic field was proposed as the origin of the negative magnetoresistance. In the isostructural compound TPP[Co(Pc)(CN)(2)](2) we observed the large positive magnetoresistance. This positive magnetoresistance cannot be explained in terms of the orbital effects under the field.

We report the optical properties of molecular conductor (IEDT)[Pd(dmit)(2)]. We observed a striking peak in the spectral range around 11000 cm(-1) for the Light polarized along the dimeric direction. This peak suggests the strong dimerization of Pd(dmit)(2) molecules, leading the inversion between HOMO and LUMO energy levels of Pd(dmit)(2). This peak intensity enables to estimate the charge transfer rho similar to 0.35 from IEDT to Pd(dmit)(2).

Organic superconductor (TMET-STF)(2)BF4 (Tc=4.1K) exhibits resistivity anomalies around 13K and 100K. In order to clarify the electronic state, we performed the optical and transport measurement. The reflectance spectra are well explained by the molecular orbital calculations. The interlayer resistance below 100K depends on the cooling rate around 100K, In the superconducting state under slow cooling process, the interlayer coherence length is comparative with the interlayer distance, leading to two-dimensional character of the superconducting state.

We systematically study transport and optical properties of charge-transfer salts, (DMe-DCNQI)(2)Li1-xCux (0 less than or equal to x less than or equal to 1). In the optical studies, we found the two absorption bands, one attributable to collective excitations associated with 4k(F) CDW (A-band) and the other to single-particle excitations across the Peierls gap (B-band). The intensity of the former band is almost constant for x less than or equal to 0.29 and steeply increases for x greater than or equal to 0.39. The latter band appears around 3000 cm(-1) for 0 less than or equal to x less than or equal to 0.14, and shifts downward for x greater than or equal to 0.17.

Although DMTSA-BF4 has it half-filled band, the electrical resistivity and thermopower shows that this compound is metallic and undergoes metal-insulator transition at ca. 150 K, By polarized reflection spectrum, this compound is characterized as a pseudo-one dimentional metal with a bandwidth of 0.8 eV. The band calculation suggests a one-dimentional nature of the Fermi surfavce. The optical transition from the bottom of the conduction band to the top of the folded conduction band is observed in the infrared region. The: temperature dependence of the reflection spectrum suggests a strong fluctuation of the lattice dimerization above the phase transition temperature.

Isostructural anion radical salts Me4X(CPDT-TCNQ)(2)(X=N,P,As) have one-dimensional electronic structure, and show several type of metal-insulator transitions depending on X. The result of the polarized reflectance spectra shows that the compounds are probably purely one-dimensional electronic system. The results of X-ray photographs and crystal structure analyses indicate that the origins of the transitions are 4k(F) CDW accompanying a cation ordering in the Me4As, Me4P salts and by 2k(F) CDW in the Me4N salt.

XX'e systetmatically study trarasport and optical properties of' charge-transfer salts, (lVIe3-DCINQI)3Li) .Ctr. (0< x< l). The tertaperatture dependence of electrical resistivity exhibitssernicondtrcting behsxvior at x : 0, a metal-sernicondtrctor transitiota for 0.08< x< 0.29, andtmetallic behavior doxvn to 4.2 K f'or I >0.39. A senaicondtrctor-senaaicondtrctor transitiora isobserxxed arotrnd 60K for x< 0.14. The reflectance spectra at [00111 tetnperattrre reveal txvooptical excitations in the iraf'rared region. The first excitatioru appears in salts witlu I / O f'ar1)CIIOXX' 650 crn ' . Time second excitation is tlae so-called mid-inJ'rar'ed bartd, whicla appears around3000 Clll " for' 0< I< 0.14, arnd shifts doxvnxv?'trd for' x >0.17. We propose a hypotlaesis whichprovides an explarnatiota lootl? f'or the raaetallic condtrction and f'or the mid-infrared band in aone-ditnensiotaal condtrctor haxzitag a qtrarter-filled bauad.

The compounds (TTM-TTP)(I3)5/3 and (TSM-TTP)(I3)5/3 which have polyiodide chains along the donor stacking direction and show characteristic “copper” luster, exhibit metallic behavior above TMI≈20 K. High-pressure resistivity, the Raman spectra, and optical reflectance of these salts have been measured to investigate the origin of the low-temperature insulating state and the iodine species. For (TSM-TTP)(I3)5/3 two kinds of conducting behavior have been observed below 20 K some low-conducting samples have shown an increase of the resistivity of more than 103 times, but other high-conducting samples have shown an increase of less than 10 times. The increase of resistivity is almost suppressed under a pressure of 11.5 kbar in the high-conducting phase of (TSM-TTP)(I3)5/3. The Raman spectra provide clear evidence that the polyiodide chain is composed of I- 3 in these compounds. The Raman spectra and the x-ray photographs indicate that the increase of resistivity originates in the disorder. The chain axis optical reflectance spectra show plasma edges appearing in the infrared region and three peaks from the infrared to the visible range the latter originate from the intramolecular transition and polyiodide ions. Temperature dependence of optical conductivity is metallic even below TMI. From these results, the origin of the low-temperature insulating state is attributed to disorder driven localization. © 1999 The American Physical Society.

Low-energy absorption bands in the polarized reflection spectrum of metal-like DMTSA-BF4 (2,3-dimethyltetraselenoanthracene-BF4) were interpreted based on a one-dimensional band theory using a point charge approximation, Drude-like dispersion observed in the E parallel to c reflection spectrum was assigned to the intra-band transition along the conducting axis. A broad absorption band at ca. 6900 cm(-1) in the E perpendicular to c reflection spectrum was assigned to the inter-branch transition from the lower to the upper branch produced by folding the HOMO band at the zone boundary. The transition probability of this electronic transition originated from the zigzag stacking structure of DMTSA molecules in this crystal, The calculation of the conductivity spectra of this transition according to this proposal agrees well with the observed conductivity spectrum. (C) 1999 Elsevier Science Ltd. All rights reserved.

We report the measurement of the upper critical field Bc2 in the recently discovered molecular superconductor β′-Et2Me2P[Pd(dmit)2]2. The large anisotropic ratio Bc2∥l Bc2⊥ ≈ 25 at low temperature indicates a strong two-dimensional character superconductivity. Temperature dependences of Bc2∥ (intra-plane direction) and Bc2⊥ (inter-plane direction) exhibit unusual non-linear behaviors. Anomalous magnetoresistance is observed around the superconducting transition temperature in the inter-plane direction. Both anomalies are ascribed to the effect of the Josephson-coupled superconducting layers. © 1999 Elsevier Science Ltd. All rights reserved.

The optical conductivity spectra of one-dimensional quarter-filled molecular conductors are calculated at T = 0 on the basis of the mean-field approximation applied to the extended Hubbard Hamiltonian. The calculated spectra exhibit a remarkable absorption band in the mid-infrared range. The spectral intensity obtained in this approximation is roughly consistent with that evaluated by Mila through the sum rule, except when the exciton formation due to the nearest neighbor Coulomb interaction (V) is dominant. The results are qualitatively consistent with experimental results. (C) 1999 Elsevier Science Ltd. All rights reserved.

We report the anisotropy of the upper critical magnetic field and the pressure-temperature phase diagram for a molecular superconductor, α-EDT-TTF[Ni(dmit)2], which is a charge-transfer salt formed between EDT-TTF and Ni(dmit)2. The critical field was found to be almost isotropic within the conductive ab-plane. The pressure-temperature (P-T) phase diagram shows decreasing of a superconducting transition temperature by increasing an applied pressure. These results are discussed in connection with the electrical-anisotropy studies reported for the normal-conducting state of this salts. [molecular superconductor, low-dimensional conductor, superconductivity, pressure-temperature phase diagram, upper critical magnetic field]. © 1998, The Japan Society of High Pressure Science and Technology. All rights reserved.

The anion radical salt β′-Et2Me2P[Pd(dmit)2]2 is non-metallic at ambient pressure. An application of pressure induces a metallic behavior and the system exhibits a superconducting transition in the pressure region of 6.9-10.4 kbar with Tc = 4.0-1.8 K (onset). Under higher pressure, however, non-metallic behavior appears in the low-temperature region. The isostructural salt β′-Et2Me2Sb[Pd(dmit)2]2 also shows pressure-induced metallic behavior. However, not a superconducting transition above 1.6 K, nor a high-pressure non-metallic behavior is observed at pressures up to 16.4 kbar. These Pd(dmit)2 salts can be characterized by an existence of two different types of bands near the Fermi level. The counter cation dependence of the electronic state under pressure suggests that dimensionality of the electronic structure plays an important role. © 1998 Elsevier Science Ltd.

Under ambient pressure, the cation radical salt (TMET-STF)(2)BF4, where the tight-binding band calculation suggests coexistence of one- and two-dimensional Fermi surfaces associated with two different conduction layers within the crystal, shows two types of resistivity anomalies around 115 and 13 K, followed by a superconducting transition at 4.1 K(onset).

The anisotropy of the upper critical field, Meissner and diamagnetic shielding effect, temperature dependence of magnetic susceptibility were examined for an ambient pressure superconductor, α-EDT-TTF[Ni(dmit)2]. The upper critical field was found to be almost isotropic within conductive ab-plane. The Meissner and diamagnetic shielding effect revealed that this salt is a bulk superconductor that has a diamagnetic transition of 1.39 K. The magnetic susceptibility does not exhibit any drop suggesting CDW formation.

Shubnikov-de Haas (SdH) effect and angular-dependent magnetoresistance oscillation (ADMRO) were observed for an organic conductor, κ-(BETS)2GaC14. The data of SdH oscillation revealed two kinds of closed orbits (α- and β-orbits), having cross-sections of 7.79 × 1014cm-2 and 4.152 × 1015 cm-2, respectively. The cyclotron masses for both orbits were estimated to be 1.2me and 2.4me, respectively. In addition to these SdH oscillations an oscillation attributable to Stark quantum interference (SQI) was observed. The data of ADMRO revealed that the shape of Fermi surface is consistent with the extended Hückel calculation.

Temperature dependence of polarized infrared reflectance spectra were measured for single crystals of(Me(2)DCNQI-d(7))(2)Cu under the stress reduced as weak as possible. The reflectance spectra exhibit drastic change suggesting a crossover from a three-dimensional conductor into a one-dimensional insulator, while this feature smears in the stressed sample. Based on the reflectivity data, it is concluded that the metal-insulator transition is accompanied by discontinuous formation of CDW.

Synthesis, structure and physical properties of novel LnCl(x), [Ln=Ce, Nd, Sm, Er, Yb x similar or equal to 5] salts based on several organic pi-donors are reported as the first example of the molecular conductors containing a magnetic ''Lanthanide'' ion that contain 4f localized electrons in counter anion.

High-resolution thermal-expansion measurements on high-quality single crystals of the title compound reveal thermodynamic evidence for a second-order phase transition at TA = 7.9K. According to the Ehrenfest relation the negative signs of the discontinuities in the coefficient of thermal expansion for directions parallel and perpendicular to the conducting planes correspond to negative uniaxial-pressure dependences of TA with a predominant effect along the intralayer direction. At a temperature slightly above TA, i.e. around 12K, measurements perpendicular to the planes reveal indications for a second anomaly which is field-independent for B ≤ 9.5T but shows a peculiar dependence on the thermal history.

This paper presents a theoretical model applicable to molecular conductors, such as TTP and M(dmit)2 salts [M = Ni, Pd, Pt], where charge carriers originate from two kinds of molecular orbitals. The model Hamiltonian consists of two independent Hubbard chains. The degree of charge transfer between the two chains is calculated. The results qualitatively agree with experimental data. © 1997 Elsevier Science Ltd.

The ambient-pressure superconductor α-(EDT-TTF)[Ni(dmit)2], which is a charge transfer salt formed between EDT-TTF and Ni(dmit)2, exhibits a superconducting transition at 1.3 K and a phase transition around 20 K featuring a characteristic resistance hump. We estimated that the room-temperature electrical conductivity was 140-370 and 14-36 S cm-1 for the current parallel and perpendicular to the conducting plane respectively. We measured the upper critical magnetic field (Hc2) in the superconducting phase. The temperature dependence of Hc2 was estimated |dHc2/dT| T=Tc≈-0.27 T K-1 (S||cx-axis); |dHc2/ dT| T=TC≈-3.5 T K-1 (B||ab-plane). The anisotropy of Hc2 within the ab-plane was found to be quite small, indicating a two-dimensionally isotropic character within the ab-plane in the superconducting state of the salt.

Meissner and diamagnetic shielding effects were investigated for an organic superconductor, α-EDT-TTF[Ni(dmit)2] (dmit = 4,5-dimer-capto-1,3-dithiole-2-thione, EDT-TTF = ethylenedithio-tetrathiafulvalene), under ambient pressure. The onset of the diamagnetic transition occurred at 1.39 K. From the measurements of the Meissner effect, the volume fraction of the superconducting phase was estimated to be more than 62%. From the measurements of the diamagnetic shielding effect, this fraction was estimated to be about 93%. This high level of volume fraction shows that the superconducting phase is the main phase in α-EDT-TTF[Ni(dmit)2].

Electrical resistivity under a high pressure and the upper critical magnetic field (H_c2) in the superconducting state were investigated on the ambient-pressure superconductor &alpha; -(EDT-TTF)[Ni(dmit)₂], which exhibits a phase transition around 20K and the superconducting transition at 1.3K under ambient pressure. Above 9.9, kbar, the superconducting transition was not observed down to 0.5K, while the former transition survived up to 13.7kbar. The temperature dependence of H_c2 was estimated: |dH_c2/dT|_{T=T_c} &asymp; -0.27T/K (Bparallel c^*-axis); |dH_c2/dT|_{T=T_c} &asymp; -3.5T/K (B// ab-plane). The anisotropy of H_c2 within the ab-plane was found to be quite small indicating the two-dimensionally isotropic character within the ab-plane in the superconducting state of the salt.

We investigated transport properties and optical reflectivity of incommensurate misfit layer compounds (LaS)1.16VS2 and (CeS)1.19(TiS2)n (stage index n = 1,2). Electrical conduction measurements revealed that (CeS)1.19(TiS2)n is metallic while (LaS)1.16VS2 is non-metallic. The existence of a short-range scattering process of the order of 10 Å was indicated for both electrons and phonons in (CeS)1.19(TiS2)n, suggesting that the local lattice modulation induced by the incommensurate structure has an important contribution to the conduction of electrons and phonons. Optical reflectance spectra for (LaS)1.16VS2 exhibited the enhancement of the carrier scattering, indicating a different character of the conduction band of (LaS)1.16VS2 in comparison with metallic (CeS)1.19(TiS2)n. We propose the possibility of carrier localization in (LaS)1.16VS2 in relation to the incommensurate structure of these compounds.

Electrical resistivity, thermoelectric power, and magnetic susceptibility were investigated under ambient pressure for Me(4)N[Ni(dmit&gt;(2)](2) which exhibits superconductivity under applied pressure. The metal-insulator transition of this salt was reexamined, and then the transition temperature was determined to be about 40 K from the temperature dependence of the thermoelectric power and electrical resistivity. The magnetic susceptibility revealed another phase transition at 11 K from paramagnetic insulator to diamagnetic insulator. This newly-found transition was attributed to the spin-Peierls transition.

The Shubnikov-de Haas (SdH) effect and angular-dependent magnetoresistance oscillation (ADMRO) were observed in magnetoresistance measurements at 0.5 K for an organic conductor κ-(BETS)2GaCl4. From a frequency of a SdH oscillation, a Fermi surface cross-section perpendicular to the crystallographic b-axis is estimated to be 4.20 × 1015cm-2, which corresponds to 105% of the first Brillouin zone. From the results of ADMRO, a Fermi wave number along the crystallographic c-axis is evaluated to be 0.35 Å-1. Both results are consistent with a band structure calculated on the basis of the extended Hückel method.

The optical reflectance spectra of incommensurate misfit layer compounds (MS)(x)TaS2 (M=rare-earth metal, Pb, Sn) were measured in the range of 3.1 to 0.093 eV and were analyzed by the Drude-Lorentz model. The spectra show clear Drude edges whose energies were significantly lowered from that of the host material 2H-TaS2. The charge transfer from MS to TaS2 was estimated on the basis of the rigid-band model. We found that the effective mass of carriers in the TaS2 conduction band depended on the degree of charge transfer from MS to TaS2, which was explained by the modification of the electronic structure from a two-carrier to a one-carrier system with respect to the degree of charge transfer. The degree of charge transfer depending on rare-earth elements is related to the modification of the band structure of MS due to the lattice incommensurability between the MS and TaS, sublattices, which is the most peculiar nature of (MS)(x)TaS2.

The temperature dependence of polarized infrared reflectance spectra was investigated on the single crystal of (Me₂DCNQI-d₇)₂Cu whose electrical resistivity was continuously monitored by the four-probe method. Above 77, K, the resistivity decreases on lowering the temperature, and the reflectance spectra exhibit Drude-like dispersion for both polarizations parallel and perpendicular to the crystallographic c-axis. Between 77, K and 75, K, the resistivity increases more than 10⁶ times that of 77, K, and the reflectance spectra exhibit drastic change suggesting dimensionality crossover from a three-dimensional conductor into a one-dimensional insulator. Based on the reflectivity data it was concluded that the metal-insulator transition in this salt is accompanied by discontinuous formation of CDW.

Polarized reflectance spectra were measured on bis-fused TTF radical-cation salts. The conductivity spectra of (TTM-TTP)I-3 exhibit an optical gap around 1000 cm(-1) at room temperature, despite the metallic behavior in DC conductivity down to 160 K. The effective onsite Coulomb repulsion in the Hubbard model, U, is evaluated to be 0.5 eV, by use of the one-dimensional regular-stack model. The reflectance spectra of (TMET-TTP)(PF6)(0.27) exhibit two-dimensional weak dispersions in the infrared region, being consistent with semiconductive behavior of this salt.

Magnetoresistance of organic conductor, α-Me2Me2N[Ni(dmit)2]2, was measured for the magnetic field along a*-axis. We found four different types of closed orbits (α-, β-, γ-, σ) through Shubnikov-de Haas (SdH) oscillation. The areas of these four closed orbits are estimated to be 0.23%, 4.6%, 87.4%, 11.3% of first Brillouin zone, respectively. The results are consistent with the band structure recently derived from the crystal structure at 11K. Contrary to α-Me2Et2N[Ni(dmit)2]2, α-C7H16N[Ni(dmit)2]2 shows neither angular dependent magnetoresistance oscillation nor SdH oscillation for the magnetic field up to 8 T. © 1995.

The structure and physical properties of TPP(tetraphenylghosphonium)(1/3)[Ni(dmit)(2)] complex were examined. The crystal has a layered structure where the conducting sheets of the dmit complex are separated by the layers of the large counter cation. Within the layer, two of the three Ni(dmit)(2) species form a dimer which stacks in [010] direction. The remaining Ni(dmit)(2) unit is disposed almost perpendicular to the stack and separates adjacent columns of Ni(dmit)(2) dimers. The crystal shows a room-temperature conductivity of ca. 10 S/cm along the long axis (column direction) and a semiconducting temperature dependence below room temperature. The in-plane anisotropy of conductivity was of the order of 1:100. The magnetic susceptibility measurement showed Curie-like behavior below room temperature.

We present experimental evidences showing coexistence of superconductivity with CDW (or SDW) in α-EDT-TTF[Ni(dmit)2], a new ambient-pressure superconductor with donor-acceptor structure. The electrical resistance of this salt exhibits a hump around 10∼14 K and drops at 1.3 K due to the superconducting transition. Based on the electrical anisotropy derived from reflectance spectra and weak-field magnetoresistance, this resistance hump is assigned to the CDW (or SDW) formation within EDT-TTF chain. Above 20 K both EDT-TTF and Ni(dmit)2 chain contribute to the electrical conduction, while below 10 K electrical conduction is mainly through Ni(dmit)2 chain. Temperature-pressure phase diagram determined by resistance measurements under pressure is presented. © 1995.