To elucidate the underlying nature of the hidden order (HO) state in heavy-fermion compound URu(2)Si(2), we measure electrical transport properties of ultraclean crystals in a high field, low temperature regime. Unlike previous studies, the present system with much less impurity scattering resolves a distinct anomaly of the Hall resistivity at H;{*} = 22.5 T, well below the destruction field of the HO phase = or approximately 36 T. In addition, a novel quantum oscillation appears above a magnetic field slightly below H;{*}. These results indicate an abrupt reconstruction of the Fermi surface, which implies a possible phase transition well within the HO phase caused by a band-dependent destruction of the HO parameter.

We report X-ray irradiation-induced carrier doping effects on the electrical conductivity in the organic dimer-Mott insulators κ-(ET)2X with X = Cu[N(CN)2]Cl and Cu2(CN)3. For κ-(ET)2Cu[N(CN)2]Cl, we have observed a large decrease of the resistivity by 40 % with the irradiation at 300 K and the metal-like temperature dependence down to about 50 K. The irradiation-induced defects expected at the donor molecule sites might cause a local imbalance of the charge transfer in the crystal. Such molecular defects result in the effective doping of carriers into the half-filled dimer-Mott insulators.KEYWORDS: Mott insulator, organic conductor, BEDT-TTF, carrier doping, X-ray irradiation A metal -insulator (MI) transition has been one of the central issues in strongly correlated electron system. Among the various types of the MI transitions, the Mott transition has been the most attractive phenomenon arising from the electron-electron interactions in a wide range of materials.1) A Mott insulator derives from the large Coulomb repulsion energy U in comparison with the band width W . Two different types of the Mott transitions exist: one can change the strength of the interaction U/W with keeping the carrier-filling its commensurate value, referred to as a band-width controlled Mott transition, and one can introduce carriers to the commensurate density with keeping the strength of the interaction constant, referred to as a band-filling controlled one. The former case, for example, can be seen in vanadium oxides and molecular conductors and the latter in high-T c copper oxides.Organic charge transfer salts based on the donor molecule BEDT-TTF, bis(ethylenedithio)-tetrathiafulvalene abbreviated ET, have been recognized as one of the highly correlated electron system.2) Among them, κ-(ET) 2 X with X = Cu(NCS) 2 , Cu[N(CN) 2 ]Y (Y = Br and Cl), Cu 2 (CN) 3 , etc. has attracted considerable attention from the point of view of a strongly correlated quasi-two-dimensional electron system because the strong dimer structure consisting of two ET molecules makes the conduction band close to half-filled with the effective Coulomb energy U eff on the dimer.2-4) Furthermore softness of the lattice with high compressibility enables us to modulate the band width easily by small physical and chemical pressures with keeping the band-filling unchanged.5-7) Thus the κ-(ET) 2 X system has been considered to be one of the typical band-width controlled Mott systems.The modulation of the band-filling has been actively examined for the molecular materials because the carrier doping into the insulators and also into the conductors must be important to tune the electronic functions. A carrier injection by the field effect technique has been extensively studied in the organic crystals for aiming at the future electronic devices.8) Another way to control *

The suppression of superconductivity by nonmagnetic disorder is investigated systematically in the organic superconductor κ-(BEDT-TTF)2Cu(NCS)2. We introduce a nonmagnetic disorder arising from molecule substitution in part with deuterated BEDT-TTF or BMDT-TTF for BEDT-TTF molecules and molecular defects introduced by X-ray irradiation. A quantitative evaluation of the scattering time τ dHvA is carried out by de Haas-van Alphen (dHvA) effect measurement. A large reduction in Tc with a linear dependence on 1/τ dHvA is found in the small-disorder region below 1/τ dHvA ≃ 1 × 10 12 s −1 in both the BMDT-TTF moleculesubstituted and X-ray-irradiated samples. The observed linear relation between Tc and 1/τ dHvA is in agreement with the Abrikosov-Gorkov (AG) formula, at least in the small-disorder region. This observation is reasonably consistent with the unconventional superconductivity proposed thus far for the present organic superconductor. A deviation from the AG formula, however, is observed in the large-disorder region above 1/τ dHvA ≃ 1 × 10 12 s −1 , which reproduces the previous transport study (J. G. Analytis et al.: Phys. Rev. Lett. 96 (2006) 177002). We present some interpretations of this deviation from the viewpoints of superconductivity and the inherent difficulties in the evaluation of scattering time.

We report an impurity effect in the organic superconductor κ-(ET)2Cu(NCS)2 by substitution of the ET molecule with an analogue, bis(methyleneditio)tetrathiafulvalene (MT). The superconducting transition temperature decreases with increasing substitution. The in-plane magnetic penetration depth is enhanced with substitution, which is quantitatively attributed to the decrease in the in-plane mean free path. The enhancement of the penetration depth can also explain the reduction of the effective pinning in terms of the condensation energy.

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