Insulator-Metal Transition and Elastic anomalies of ET organic conductors.

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The purpose of this project is to investigate the origin of Insulator-to-Metal (IM) crossover in BEDT-TTF (ET) organic conductors by means of ultrasonic measurements. Through this project, we developed a new ultrasonic apparatus with GM-type cryocooler, which enables us to measure sound velocity, ultrasonic attenuation and resistivity automatically in the temperature range between 5 K and room temperature. In this project, we have performed the investigations on IM transition of manganites and skutterudite compounds as relating materials, together with ET organic conductors. The results are summarized as follows.(1) Elastic anomalies of k-(ET)_2Cu[N(CN)_2]Br We measured longitudinal elastic sonstants C_<11> and C_<33> of κ-(ET)_2Cu[N(CN)_2]Br, which shows the IM cross over behavior at around 100K. Both C_<11> and C_<44> show remarkable elastic softening at around 40 K. These anomalies are not affected by the application of the magnetic field. It has been predicted by the theory that th … More is IM behavior is related to the superconductivity of this system. The superconductivity is much affected by the magnetic field. These results suggest weak correlation between IM crossover behavior and superconductivity. We have investigated the coupling between vortices and elastic strain.(2) Elastic anomalies associated with Insulator-Metal transition in Manganites We have investigate layered Manganite La_<2-2%>Sr_<1+2%>Mn_2O_7 as a function of Sr concentration. We focused our attention on the 3d electronic state of Mn, when we change hole concentration. We have estimated it by using band width, crystalline electric field scheme and overlap integral, which are evaluated based on a suitable model. In addition, we have carried out the elastic constant measurements of (La_<1-Z>Pr_Z)_<1.2>Sr_<1.8>Mn_2O_7. This system shows Insulator-to-metal transition at 4.2 K in the magnetic field of 7 T. We found that the longitudinal elastic constant s***s a remarkable softening acoompanied with IM transition, and shows a hardening in higher fields. This behavior is quite different from ET system. These systematic investigation on various systems is expected to give important information on the origin of IM transition of ET system.(3) Elastic anomalies in Filled skutterudite compounds Filled skutterudite compounds, which are described by ReTr_4X_<12> (Re : Rare earth element, Tr : Transition metal, X : Pnictgen) have been investigated by ultrasonic methods. The elastic constants of LaFe_4P_<12>, PrFe_4P_<12>, NdFe_4P_<12>, LaRu_4Sb_<12>, CeRu_4Sb_<12> and PrRu_4Sb_<12> have been measured, so far. The elastic softening of (C_<11>-C_<12>)/2 has been found in PrFe_4P_<12>. This confirms that the 4f ground state of this system is G_3. The peculiar field dependence of the elastic constant clearly shows the evidence of quadrupolar Kondo effect, ant that this effect is much affected by the magnetic field. Less