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

本项目的目的是制备含高温超导体母绝缘体的绝缘氧化铜单晶，测量各向异性介电常数随温度和频率的变化，并阐明母绝缘体介电响应的独特特征。通过本项目，我们成功合成了Bi_2Sr_<2-x>La_xCaCu_2O_8作为bi -2212型超导体中无序度最小的母绝缘体。我们还合成了在BiO层中没有超结构的Pb取代绝缘体（Bi,Pb）_2Sr_2ErCu_2O_8，并将其用于光电发射和扫描隧道显微镜（STM）实验。这些样品明显表现出扩展debye型弛豫所描述的介电弛豫，表明载流子密度在CuO_2平面上是空间不均匀的。实际上，STM实验表明赝隙在空间上是不均匀的。在绝缘铜氧化物中，CaCu_3T_<14>O_<12>表现出不同寻常的取代效应，其中只有2%的Mn可以抑制巨大的介电常数。这可能是解释介电响应的一个线索。我们进一步研究了导电有机盐theta-(BEDT-TTF)_2MM'(SCN)_4（M=Rb和Cs， M'=Co和Zn）的介电响应，其中电荷序域分布不均匀。最令人意想不到的是，我们发现了M=Cs的巨大非线性传导，这将为凝聚态物理学开辟新的领域。

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.