Metal -insulator -superconductor phase separation in the highly correlated organic conductors near the Mott transition.

在宏观尺度上对有机Mott体系κ-(BEDT-TTF)_2[N(CN)_2]Br的电子相分离进行了系统研究。采用同步辐射（SR）扫描微区红外光谱（SMIS）获得了相分离的真实空间成像。在有机Mott体系κ-[(h-BEDT-TTF)_<1-x>(d-BEDT-TTF)_x]_2Cu[N(CN)_2]Br的单晶表面获得了电子相分离的实验证据，其能带宽度由氢化BEDT-TTF分子（h-BEDT-TTF）与氘化BEDT-TTF分子（d-BEDT-TTF）之间的取代比x控制。使用SR的SMIS测量使我们能够显示绝缘和金属区域的畴结构的宏观尺寸。这一发现并不排除每个域或每个扫描点内部存在纳米级不均匀性的可能性，因为所获得的光谱可能导致被测点的纳米级不均匀性的平均值。但以往的核磁共振和磁化结果与本文的结果一致，表明相分离发生在宏观尺度上。微米级相分离的观察结果不同于最近在强相关过渡金属氧化物体系中发现的纳米级电子不均匀性，该体系是一个填充控制的Mott体系。因此，我们预计在没有热力学相变的填充控制Mott体系中会出现纳米尺度的内在非均匀性，如果出现宏观非均匀性，则应该伴随着带宽控制Mott体系中的相变和非平衡态等临界现象。本研究中发现的相分离的起源必须是反铁磁莫特绝缘体和超导相在一阶莫特跃迁时的双稳定性。

Systematic investigation of the electronic phase separation on macroscopic scale has been performed in the organic Mott system κ-(BEDT-TTF)_2[N(CN)_2]Br. Real space imaging of the phase separation is obtained by means of scanning micro-region infrared spectroscopy (SMIS) using the synchrotron radiation (SR). Experimental evidence of the electronic phase separation has been obtained on the single crystal surface of the organic Mott system κ-[(h-BEDT-TTF)_<1-x>(d-BEDT-TTF)_x]_2Cu[N(CN)_2]Br, of which band width is controlled by the substitution ratio x between the hydrogenated BEDT-TTF molecule (h-BEDT-TTF) and the deuterated one (d-BEDT-TTF). SMIS measurements using SR enable us to show the macroscopic size of the domain structure of the insulating and metallic regions. This finding does not exclude the possibility of the nano-scale inhomogeneity inside each domains or each scanning spot because the obtained spectrum may result in the average of nano-scale inhomogeneity in the measured spot. But close agreement between the previous results of NMR and magnetization and present results suggests that the phase separation occurs on macroscopic scale. The observation of the micro-meter scale phase separation is different from the recent findings of the nano-scale electronic inhomogeneity in the strongly correlated transition metal oxide system, which is a filling controlled Mott system. We therefore expect that the nano-scale intrinsic inhomogeneity appears in the filling controlled Mott system without thermodynamic phase transition, and the macroscopic inhomogeneity, if it comes, should accompany the critical phenomena like as the phase transition and non-equilibrium state in the band width controlled Mott system. The origin of the phase separation found in this study must be the bistability of the antiferromagnetic Mott insulator and superconducting phases just at the first order Mott transition.