A many-body quantum system on the verge of instability between two competing ground states may exhibit quantumcritical phenomena 1,2 , as has been intensively studied for magnetic systems. The Mott metal-insulator transition 3 , a phenomenon that is central to many investigations of strongly correlated electrons, is also supposed to be quantum critical, although this has so far not been demonstrated experimentally. Here, we report experimental evidence for the quantum-critical nature of the Mott instability, obtained by investigating the electron transport of three organic systems with di erent ground states under continuously controlled pressure. The resistivity obeys the material-independent quantum-critical scaling relation bifurcating into a Fermi liquid or Mott insulator, irrespective of the ground states. Electrons on the verge of becoming delocalized behave like a strange quantum-critical fluid before becoming a Fermi liquid.Mutually interacting electrons with sufficiently strong Coulomb repulsion U fall into the Mott insulating state when the carrier density corresponds to an electron per site (a half-filled band) 3 . As the bandwidth W is increased by pressure or chemical substitution, the electrons gain kinetic energy and become itinerant at a critical value of W/U . The Mott transition, a marked phase transition between a metal and an insulator, is a collective manifestation of imbalance in the particle-wave duality of electrons. As one of the main issues in the quantum physics of condensed matter, the quantumcritical nature of the Mott transition awaits clarification. In contrast to intensive theoretical studies 4-6 , however, this issue has not yet been addressed experimentally because most Mott transitions in real systems have critical points at finite temperatures 7-11 ; thus, they are not genuine quantum phase transitions.In general, quantum criticality is observed at the temperature T sufficiently lower than the competing energy scales underlying the phase transition 1,2 , which are the bandwidth W and on-site Coulomb energy U in the case of the Mott transition. Thus, even if the system's critical point, T c , is finite, unlike the genuine quantum phase transition, in the case that T c is orders of magnitude lower than W and U , there is a vast temperature region of T c < T U , W , where the system can experience quantum criticality (Fig. 1a). Indeed, using dynamical mean field theory (DMFT), which can properly describe the Mott transition 12 , the authors of refs 4,13 have suggested the scaling of transport for quantum criticality in an intermediate temperature range well above T c .To explore the possible Mott quantum criticality from the experimental side, we performed pressure studies of the electron transport for three different quasi-two-dimensional organic Mott insulators with anisotropic triangular lattices, κ-(ET) 2 Cu 2 (CN) 3 , κ-(ET) 2 Cu[N(CN) 2 ]Cl and EtMe 3 Sb[Pd(dmit) 2 ] 2 (hereafter abbreviated to κ-Cu 2 (CN) 3 , κ-Cl and EtMe 3 Sb-dmit, respectively), where

We reinvestigated the two C=C stretching modes of the five-membered rings of ET (ET = bis(ethylenedithio)tetrathiafulvalene), namely, nu(2) (in-phase mode) and nu(27) (out-of-phase mode). The frequency of the nu(27) mode of ET(+) was corrected to be approximately 1400 cm(-1), which was identified from the polarized infrared reflectance spectra of (ET)(ClO(4)), (ET)(AuBr(2)Cl(2)), and the deuterium- or (13)C-substituted compounds of (ET)(AuBr(2)Cl(2)). It was clarified from DFT calculations that the frequency of the nu(27) mode of the flat ET(0) molecule was significantly different from that of the boat-shaped ET(0) molecule. We obtained the linear relationship between the frequency and the charge on the molecule, rho, for the flat ET molecule, which was shown to be nu(27)(rho) = 1398 + 140(1 - rho) cm(-1). The frequency shift due to oxidation is remarkably larger than that reported in previous studies. The fractional charges of several ET salts in a charge-ordered state can be successfully estimated by applying this relationship. Therefore, the nu(27) mode is an efficient probe to detect rho in the charge-transfer salts of ET. Similarly, a linear relationship for the nu(2) mode was obtained as nu(2)(rho) = 1447 + 120(1 - rho). This relationship was successfully applied to the charge-poor molecule of theta-type ET salts in the charge-ordered state but could not be applied to the charge-rich molecule. This discrepancy was semiquantitatively explained by the hybridization between the nu(2) and nu(3) modes.

The thermally induced insulator-metal transition ͑IMT͒ has been investigated for single crystals of undoped and lightly doped LaCoO 3 by measurements of the optical-conductivity spectra, the resistivity, and the Hall coefficient up to 800 K. The IMT around 500 K, where the spin-state transition from the nonmagnetic (S ϭ0) ground state to the intermediate-spin (Sϭ1) state is nearly completed, is characterized by a large energy-scale change of the electronic structure as well as by a steep increase of carrier number up to a typical metallic value due to closing of the charge gap. These behaviors are strikingly parallel to those characteristic of the Mott transition. ͓S0163-1829͑98͒50628-5͔PRB 58

We demonstrate that an organic Mott insulator, 0 -(BEDT-TTF) 2 ICl 2 , is metallized by application of extremely high pressure by means of the cubic anvil press. When the metallic state is stabilized, superconductivity with the highest transition-temperature (T c ) among the organic systems appears. The lower limit of the pressure at which the superconducting transition manifests itself is approximately 7.0 GPa and T c reaches a maximum value, 14.2 K (onset), at 8.2 GPa. The pressure-temperature phase diagram is constructed and discussed in terms of the comparison with those of other organic systems.

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We have investigated the dielectric response in an antiferromagnetic dimer-Mott insulator β'-(BEDT-TTF)2ICl2 with square dimer lattice, compared to a spin liquid candidate κ-(BEDT-TTF)2Cu2(CN)3. Temperature dependence of the dielectric constant shows a peak structure obeying Curie-Weiss law with a strong frequency dependence. We found an anisotropic glassy ferroelectricity by pyrocurrent measurements, which suggests the charge disproportionation resulting in an electric dipole in a dimer. Each ferroelectric and antiferromagnetic transition temperatures is closely related to the antiferromagnetic interaction energy and a freezing temperature of dipole dynamics in a dimer, respectively. These correspondences suggest the possible charge-spin coupled degrees of freedom in the system.