Dielectric constant of insulating copper oxides including parent insulators of high-temperature superconductors

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The aim of the present project is to prepare single crystals of insulating copper oxides including parent insulators of high-temperature superconductors, to measure anisotropic dielectric constants as functions of temperature and frequency, and to clarify unique features to dielectric response in the parent insulators.Through this project, we have successful synthesized Bi_2Sr_<2-x>La_xCaCu_2O_8 as the least disordered parent insulator of the Bi-2212-type super conductor. We have also synthesized the Pb-substituted insulators (Bi,Pb)_2Sr_2ErCu_2O_8 with no superstructure in the BiO layer, which has been used for photoemission and scanning-tunneling-microscope(STM) experiments. These samples clealy exhibit the dielectric relaxation described by extended Debye-type relaxation, which suggest that the carrier density is spatially inhomogeneous in the CuO_2 plane. In fact, the STM experiment has shown that the pseudo-gap is inhomogenous in space.Among the insulating copper oxides, CaCu_3T_<14>O_<12> shows unusual substitution effects, where only 2% Mn can suppress the gigantic dielectric constant. This could be a clue to the elucidation of the dielectric response. We have further investigated the dielectric response of the conducting organic salts theta-(BEDT-TTF)_2MM'(SCN)_4(M=Rb and Cs, M'=Co and Zn), where charge order domains are inhomogeneously distributed. Most unexpectedly we have found gigantic nonlinear conduction for M=Cs, which would break a new ground to the condensed-matter physics.