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本文通过高分辨电子能量损失谱(EELS)的实验分析与密度泛函计算相结合的方法,研究了NaxCoO2.yH2O系统的电子结构特性和电子关联效应。实验结果表明,金属性Na0.3CoO2材料和电荷有序绝缘体Na0.5CoO2的O-K吸收边存在明显的结构差别,在Na0.5CoO2的EELS中第一个峰发生明显分裂,密度泛函计算证实这种分裂现象和电子关联效应直接相关。通过理论模拟谱线与实验谱线的比较确定了其电子关联强度为U=3.0 eV。在Na0.3CoO2.yH2O(y=0,0.6,1.3)超导体系中,实验发现其能损谱的低能部分随着水含量的增加发生系统的变化,损失峰向低能量方向逐步移动。电子结构计算表明水分子的插入可以引起费米面附近能态原子轨道杂化情况的改变,从而导致EELS的变化。
In this paper, the electronic structure and electronic correlation of NaxCoO2.yH2O system have been studied by means of the experimental analysis of high resolution electron energy loss spectroscopy (EELS) and the density functional theory. The experimental results show that there is a clear structural difference between the OK absorption edge of the metallic Na0.3CoO2 material and the charged ordered insulator Na0.5CoO2. The first peak of the EELS of Na0.5CoO2 is obviously split, and the density functional theory confirms this The phenomenon of division is directly related to the electronic correlation effect. The electron correlation intensity of U = 3.0 eV was determined by comparison between theoretical simulated spectrum and experimental spectrum. In the Na0.3CoO2.yH2O (y = 0,0.6,1.3) superconducting system, we found that the low energy part of energy loss spectrum changes systematically with the increase of water content, and the peak of the loss gradually moves toward the low energy direction. The calculation of electronic structure shows that the insertion of water molecules can cause the change of atomic orbital hybridization near the Fermi surface, resulting in the change of EELS.