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超导体的磁性杂质效应以及其中存在的束缚态(即Yu-Shiba-Rusinov 态)一直受到较多的关注. 最近, 在实验室中, 成功发现了石墨烯类超导材料中Yu-Shiba-Rusinov 态的存在. 本文在实空间建立描述石墨烯材料超导态的有效哈密顿量, 考虑单个磁性杂质, 构造Bogoliubov-de Gennes (BdG)方程, 并对超导序参量做自洽计算, 在此基础上, 理论研究了石墨烯类超导体的磁性杂质效应. 计算结果显示, 仅当超导的配对对称性是传统的s波配对时, 能隙内会出现Yu-Shiba-Rusinov 束缚态, 束缚态的位置以及强度和杂质的磁矩有关, 且强度显示出了明显的正负非对称性, 但对于p+ip和d+id配对对称性, 则不存在能隙内的束缚态. 本文的理论计算结果一方面对实验现象做了合理解释, 另一方面指出了石墨烯和传统超导组成的异质结系统, 石墨烯层由于临近效应诱导出来的超导配对项仍然是s波配对.The magnetic impurity effects and the existence of bound states (i.e. Yu-Shiba-Rusinov states) in superconductors have been a topic of great interest. Recently, the existence of Yu-Shiba-Rusinov states in graphene-based superconducting materials has been successfully observed in the laboratory. In this work, an effective Hamiltonian in real space is established to describe the superconducting state of graphene materials by considering a single magnetic impurity. Thus the Bogoliubov-de Gennes (BdG) equation is constructed and the self-consistency calculations of the superconducting order parameter are conducted. On this basis, the effects of magnetic impurities on graphene-like superconductors are investigated theoretically. The numerical results show that the Yu-Shiba-Rusinov state can only appear in the symmetry of the superconducting pair of the traditional s-wave coupling. The position and strength of the bound state are related to the magnetic moment of the impurity, showing a notable electron-hole asymmetry. There are no bound states in the energy gap for other pairing symmetries. This theoretical calculation not only provides a reasonable explanation for experimental phenomena, but also demonstrates that the heterojunction system composed of graphene and traditional superconductors has an s-wave superconducting pairing induced by the proximity effect in the graphene layer.
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Keywords:
- graphene/
- superconductor/
- magnetic impurity/
- bound state
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