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We investigate low-temperature electronic states of the series of organic conductors β'' - [bis(ethylenedithio)tetrathiafulvalene] 4[(H3O)M(C2O4)3] G, where M and G represent trivalent metalions and guest organic molecules, respectively. Our structural analyses reveal that the replacement of M and G give rise to systematic change in the cell parameters, especially in the b-axis length, which has a positive correlation with the superconducting transition temperature Tc. Analysis of temperature and magnetic field dependences of the electrical resistance including the Shubnikov–de Haas oscillations elucidates that the variation of charge disproportionation, the effective mass, and the number of itinerant carriers can be systematically explained by the change of the b-axis length. The changes of the transfer integrals induced by stretching/compressing the b axis are confirmed by the band calculation. We discuss that electron correlations in quarter-filled electronic bands lead to charge disproportionation and the possibility of a novel pairing mechanism of superconductivity mediated by charge degrees of freedom.

Resistance and magnetic torque measurements are reported in a layered organic superconductor, $\ensuremath{\beta}''\ensuremath{-}{(\mathrm{BEDT}\ensuremath{-}\mathrm{TTF})}_{4}[({\mathrm{H}}_{3}\mathrm{O})\mathrm{Ga}{({\mathrm{C}}_{2}{\mathrm{O}}_{4})}_{3}]{\mathrm{C}}_{6}{\mathrm{H}}_{5}{\mathrm{NO}}_{2}$ with ${T}_{c}=4.8$ K, where BEDT-TTF stands for bis(ethylenedithio)tetrathiafulvalene. Because of the large anion between the BEDT-TTF conducting layers, the superconductivity of this salt is highly anisotropic. In magnetic fields parallel to the conducting layers for $T=0.4$ K, the magnetic torque shows a large diamagnetic signal associated with hysteresis up to $\ensuremath{\sim}21$ T, suggesting the upper critical field ${H}_{c2}\ensuremath{\gtrsim}21$ T at 0.4 K. The large reduction of the diamagnetic signal is observed above 16 T, which shows a Fulde and Ferrell and Larkin and Ovchinnikov (FFLO) phase transition. For $T=0.5$ K, the interlayer resistance has nonzero value in a wide field region up to ${H}_{c2}$, arising from the Josephson vortex dynamics. Successive dips in the second derivative curves of the resistance are observed between 16 T and ${H}_{c2}$, which are ascribed to the commensurability effect between the Josephson vortex lattice and the order parameter oscillation of the FFLO phase. The commensurability effect is observed only in nearly parallel fields, showing that the FFLO phase is stable in a very limited field angle region. The temperature-field phase diagram is determined.

The crystal of \ensuremath{\lambda}-${(\mathrm{BETS})}_{2}{(\mathrm{F}\mathrm{e}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}{)\mathrm{C}\mathrm{l}}_{4}$ with $x\ensuremath{\approx}0.45$ undergoes successive superconducting and superconductor-to-insulator transitions around liquid helium temperature. The large diamagnetic susceptibility (\ensuremath{\approx}-35 emu/mol at 4 K) observed in the superconducting phase of the system with $x\ensuremath{\approx}0.47,$ which exhibits a superconducting transition at 4.3 K and superconductor-to-insulator transition at 3.7 K, can be regarded as evidence for the bulk nature of the superconductor-to-insulator transition. The anisotropy of the susceptibility and spin-flop behavior indicate that the system takes a \ensuremath{\pi}-d coupled antiferromagnetic insulating ground state, which is essentially the same as that of \ensuremath{\lambda}-${(\mathrm{BETS})}_{2}{\mathrm{FeCl}}_{4}$ exhibiting a coupled metal-insulator and antiferromagnetic transition at 8.5 K. The easy axis is parallel to the c direction. Unlike the Ga-rich system $(xl0.35)$ with superconducting ground state, the Fe-rich system such as \ensuremath{\lambda}-${(\mathrm{BETS})}_{2}{(\mathrm{F}\mathrm{e}}_{0.7}{\mathrm{Ga}}_{0.3}{)\mathrm{C}\mathrm{l}}_{4}$ shows a metal-insulator transition at ambient pressure. At high pressure, however, the \ensuremath{\pi} and d electrons tend to be independent of each other and the system takes a superconducting ground state.

The electric and magnetic properties of a series of highly correlated two-dimensional organic \ensuremath{\pi} conductors incorporating magnetic ions (${\mathrm{Fe}}^{3+}$ with $S=\frac{5}{2}$), $\ensuremath{\lambda}\ensuremath{-}{\mathrm{BETS}}_{2}{\mathrm{FeBr}}_{x}{\mathrm{Cl}}_{4\ensuremath{-}x}$ [$\mathrm{B}\mathrm{E}\mathrm{T}\mathrm{S}=\mathrm{b}\mathrm{i}\mathrm{s}(\mathrm{ethylenedithio})\mathrm{tetraselenafulvalene},$ $x=0--0.8$] were examined by controlling the Br content (x). A broad resistivity maximum at 100 $(x\ensuremath{\approx}0)--50\mathrm{K}$ $(x\ensuremath{\approx}0.7)$ indicating the strong correlation of \ensuremath{\pi} conduction electrons becomes prominent with increasing x. At the same time the metal-insulator (MI) transition temperature ${(T}_{\mathrm{MI}})$ increases from 8.5 $(x=0)$ to 18 K $(x=0.7).$ The crystal with $x=0.8$ shows a semiconductor-insulator transition around 20 K. At $0lxl0.2,$ the MI transition and the antiferromagnetic (AF) transitions take place cooperatively around 8.5 K ${(T}_{N}\ensuremath{\approx}{T}_{\mathrm{MI}}).$ A large magnetization drop was observed at ${T}_{\mathrm{MI}}$ for the magnetic field parallel to the c axis $(H\ensuremath{\parallel}c),$ which indicates the appearance of localized \ensuremath{\pi} spins $(S=\frac{1}{2})$ and a strong AF coupling of \ensuremath{\pi} and d spin systems. For $0.3lxl0.5,$ two anomalies were observed in the magnetization-temperature $(M\ensuremath{-}T)$ curve. The high-temperature anomaly corresponds to a MI transition ${(T}_{\mathrm{MI}})$ and the low-temperature one corresponds to an AF ordering of the ${\mathrm{Fe}}^{3+}$ spins ${(T}_{N}).$ In this case, the relatively small magnetization drop observed at ${T}_{\mathrm{MI}}$ suggests a small coupling of \ensuremath{\pi} and d spin systems. For $xg0.6,$ the magnetization drop at ${T}_{\mathrm{MI}}$ disappeared, which shows that the \ensuremath{\pi} and d electron systems are decoupled and the \ensuremath{\pi} electron system undergoes MI transition independently of the d spin systems. The disappearance of the susceptibility anomaly at ${T}_{\mathrm{MI}}$ indicates that the \ensuremath{\pi} electron system transforms to a nonmagnetic insulating state below ${T}_{\mathrm{MI}}.$ On the other hand, the anisotropy of M showing the development of a AF spin structure of the Fe spins was observed independently of $x(0lxl0.8).$ Around $x=0.3\ensuremath{-}0.5,$ the direction of the easy axis of the AF spin structure changes from parallel to the c axis $(xl0.2)$ to perpendicular to it $(xg0.6).$ In other words, the direction of easy axis is varied according to the magnitude of $\ensuremath{\pi}\ensuremath{-}d$ coupling. Magnetoresistance measurements showed that ${T}_{\mathrm{MI}}$ of $\ensuremath{\lambda}\ensuremath{-}{\mathrm{BETS}}_{2}{\mathrm{FeBr}}_{0.7}{\mathrm{Cl}}_{3.3}$ is almost independent of the magnetic field below 1 kbar. But a field-restored highly conducting state similar to that of $\ensuremath{\lambda}\ensuremath{-}{\mathrm{BETS}}_{2}{\mathrm{FeCl}}_{4}$ was observed at high pressure. The Weiss temperature (\ensuremath{\theta}) estimated from the $M\ensuremath{-}T$ curve at ${T}_{\mathrm{MI}}lTl30\mathrm{K}$ decreases with increasing x.

The physical properties of a series of organic superconductors with fourfold quasistacking structures, $\ensuremath{\lambda}\ensuremath{-}{\mathrm{BETS}}_{2}\mathrm{Ga}{X}_{z}{Y}_{4\ensuremath{-}z}$ [BETS=bis(ethylenedithiotetraselenafulvalene; $X,Y=\mathrm{F},$ Cl, Br; $0lzl2)]$ are changed continuously by tuning the value of $z.$ The compound $\ensuremath{\lambda}\ensuremath{-}{\mathrm{BETS}}_{2}{\mathrm{GaBr}}_{z}{\mathrm{Cl}}_{4\ensuremath{-}z},$ has a superconducting ground state for $0lzl{z}_{c}$ (\ensuremath{\approx}0.8) and a semiconducting state for ${z}_{c}lzl2.0.$ The maximum ${T}_{c}$ is about 10 K, showing the existence of a 10-K class of non-ET organic superconductors [ET=bis(ethylenedithio)tetrathiafulvalene]. The temperature dependence of the resistivity closely resembles that for \ensuremath{\kappa}-type ET superconductors but the susceptibility behavior is different. The susceptibility of $\ensuremath{\lambda}\ensuremath{-}{\mathrm{BETS}}_{2}{\mathrm{GaBr}}_{1.5}{\mathrm{Cl}}_{2.5}$ is isotropic and decreases fairly sharply below 30 K. No indication of a spin-flop transition was observed at 10 K, suggesting that the insulating phase that neighbors the superconducting phase is not antiferromagnetic at least down to 10 K.

We have examined time-averaged charges of the molecular donors around the phase boundary between the insulating, superconducting, and metal phases for ${\ensuremath{\beta}}^{\ensuremath{''}}$-type ET [$\mathrm{ET}=\text{bis}$-ethelenedithio-tetrathiafulvalene] salts with one (two) hole(s) per two (three) molecules by using vibrational spectroscopy. Around the phase boundary, the site charges are neither those expected for a well-developed charge-ordered (CO) state nor a homogeneous distribution. The molecular charges exhibit an inhomogeneous distribution just above the insulator-superconductor transition temperature. We have analyzed the distribution of the site charges from the viewpoint of the alternation of intersite Coulomb interactions along the stacking direction. The closeness in the energy of several CO patterns mostly contributes to the inhomogeneous distribution, whereas a large difference (no difference) in the energy levels contributes to the CO (homogeneous) state. Our observations indicate that the instability due to the closeness in the energy of several distributions correlates with the insulator-superconductor transition in nondimerized or weakly dimerized molecular conductors.

Electron spin resonance reveals the spin behavior of conduction ($\ensuremath{\pi}$) and localized ($d$) electrons in $\ensuremath{\beta}\mathrm{\text{\ensuremath{-}}}(\mathrm{BDA}\mathrm{\text{\ensuremath{-}}}\mathrm{TTP}{)}_{2}M{\mathrm{Cl}}_{4}$ ($M=\mathrm{Fe}$, Ga). Both the ${\mathrm{Ga}}^{3+}(S=0)$ and ${\mathrm{Fe}}^{3+}(S=5/2)$ compounds exhibit a metal-insulator transition at 113 K with the simultaneous formation of a spin-singlet ground state in the $\ensuremath{\pi}$ electron system of the donor molecules. The behavior is consistent with charge ordering in $\ensuremath{\beta}\mathrm{\text{\ensuremath{-}}}(\mathrm{BDA}\mathrm{\text{\ensuremath{-}}}\mathrm{TTP}{)}_{2}M{\mathrm{Cl}}_{4}$ at the metal-insulator transition. At 5 K, the ${\mathrm{Fe}}^{3+}$ compound orders antiferromagnetically, even though the $\ensuremath{\pi}$ electrons, which normally would facilitate magnetic exchange, are localized nonmagnetic singlets.

We report magnetotransport measurements on the quasi-two-dimensional charge-transfer salts β″-(BEDT-TTF)4[(H3O)M(C2O4)3]Y, with Y=C6H5NO2 and C6H5CN using magnetic fields of up to 45 T and temperatures down to 0.5 K. A surprisingly robust superconducting state with an in-plane upper critical field Bc2 33T, comparable to the highest critical field of any BEDT-TTF superconductor, and critical temperature Tcâ 7K is observed when M=Ga and Y=C6H5NO2. The presence of magnetic M ions reduces the in-plane upper critical field to 18T for M=Cr and Y=C6H5NO2 and M=Fe and Y=C6H5CN. Prominent Shubnikov-de Haas oscillations are observed at low temperatures and high magnetic fields, showing that the superconducting salts possess Fermi surfaces with one or two small quasi-two-dimensional pockets, their total area comprising 6% of the room-temperature Brillouin zone; the quasiparticle effective masses were found to be enhanced when the ion M was magnetic (Fe or Cr). The low effective masses and quasiparticle densities, and the systematic variation of the properties of the β″-(BEDT-TTF)4[(H3O)M(C2O4)3]Y salts with unit-cell volume points to the possibility of a superconducting groundstate with a charge-fluctuation-mediated superconductivity mechanism such as that proposed by Merino and McKenzie [Phys. Rev. Lett. 87, 237002 (2001)], rather than the spin-fluctuation mechanism appropriate for the κ-(BEDT-TTF)2X salts. © 2005 The American Physical Society.

We have investigated pressure- and magnetic-field-dependent electrical transport properties in the charge transfer salts $\ensuremath{\beta}\text{\ensuremath{-}}{(\mathrm{BDA}\text{\ensuremath{-}}\mathrm{TTP})}_{2}M{\mathrm{Cl}}_{4}\phantom{\rule{0.3em}{0ex}}(M=\mathrm{Fe},\mathrm{Ga})$, both of which show a metal-insulator (MI) transition around $120\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ at ambient pressure. The zero field temperature-pressure phase diagrams of the two compounds are quite similar; the MI transition temperature decreases with pressure, and superconductivity is observed in both the magnetic and non-magnetic compounds above $\ensuremath{\sim}4.5\phantom{\rule{0.3em}{0ex}}\mathrm{kbar}$. Likewise, Shubnikov-de Haas effect measurements show nearly identical Fermi surfaces. These similarities suggest that the magnetic interaction $J$ between the conduction electrons and the magnetic moments in $\ensuremath{\beta}\text{\ensuremath{-}}{(\mathrm{BDA}\text{\ensuremath{-}}\mathrm{TTP})}_{2}{\mathrm{FeCl}}_{4}$ is small. Nevertheless, magnetoresistance measurements show remarkable differences and reveal that magnetic interactions with the conduction electrons are still effective in $M=\mathrm{Fe}$ compounds.

We report high-field magnetotransport measurements on ${\ensuremath{\beta}}^{\ensuremath{''}}\ensuremath{-}(\mathrm{BEDT}\ensuremath{-}\mathrm{TTF}{)}_{4}[({\mathrm{H}}_{3}\mathrm{O})M({\mathrm{C}}_{2}{\mathrm{O}}_{4}{)}_{3}]Y,$ where $M=\mathrm{Ga},$ Cr, and Fe and $Y={\mathrm{C}}_{5}{\mathrm{H}}_{5}\mathrm{N}.$ We observe similar Shubnikov--de Haas oscillations in all compounds, attributable to four quasi-two-dimensional Fermi-surface pockets, the largest of which corresponds to a cross-sectional area $\ensuremath{\approx}8.5%$ of the Brillouin zone. The cross-sectional areas of the pockets are in agreement with the expectations for a compensated semimetal, and the corresponding effective masses are $\ensuremath{\sim}{m}_{\mathrm{e}},$ rather small compared to those of other BEDT-TTF salts. Apart from the case of the smallest Fermi-surface pocket, varying the M ion seems to have little effect on the overall Fermi-surface topology or on the effective masses. Despite the fact that all samples show quantum oscillations at low temperatures, indicative of Fermi liquid behavior, the sample and temperature dependence of the interlayer resistivity suggest that these systems are intrinsically inhomogeneous. It is thought that intrinsic tendency to disorder in the anions and/or the ethylene groups of the BEDT-TTF molecules leads to the coexistence of insulating and metallic states at low temperatures. A notional phase diagram is given for the general family of ${\ensuremath{\beta}}^{\ensuremath{''}}\ensuremath{-}(\mathrm{BEDT}\ensuremath{-}\mathrm{TTF}{)}_{4}[({\mathrm{H}}_{3}\mathrm{O})M({\mathrm{C}}_{2}{\mathrm{O}}_{4}{)}_{3}]Y$ salts.