Yingying Peng, Jianqiao Meng, Daixiang Mou, Junfeng He, Lin Zhao, Yue Wu, Guodong Liu, Xiaoli Dong, Shaolong He, Jun Zhang, Xiaoyang Wang, Qinjun Peng, Zhimin Wang, Shenjin Zhang, Feng Yang, Chuangtian Chen, Zuyan Xu, T. K. Lee, X. J. Zhou
The parent compound of the copper-oxide high temperature superconductors is a Mott insulator. Superconductivity is realized by doping an appropriate amount of charge carriers. How a Mott insulator transforms into a superconductor is crucial in understanding the unusual physical properties of high temperature superconductors and the superconductivity mechanism. Systematic investigations of the electronic structure in the lightly-doped region, especially across the insulator-superconductor transition are necessary but so far have not reached a consistent picture. In this paper, we report high resolution angle-resolved photoemission measurement on heavily underdoped Bi2Sr2-xLax}CuO6+d system. The electronic structure of the lightly-doped samples exhibit a number of characteristics: existence of an energy gap along the (0,0)-(pi,pi) nodal direction, d-wave-like anisotropic energy gap along the underlying Fermi surface, and coexistence of coherence peak and a broad hump in the photoemission spectra. Our results reveal a clear insulator-superconductor transition at a critical doping level of ~0.10 where the nodal energy gap approaches zero, the three-dimensional antiferromagnetic order disappears, and superconductivity starts to emerge. These observations clearly signal a close connection between the nodal gap, antiferromagnetic insulating phase, and high temperature superconductivity. They point to the importance of combining the electron correlation, antiferromagnetism and strong electron-phonon coupling in understanding high temperature superconductors in the very underdoped region.
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http://arxiv.org/abs/1302.3017
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