题目：Calculated Exchange Interactions and Competing S=0 and S=1 States in Doped NdNiO₂

报告人：万贤纲 教授,南京大学物理学院

时间：2020年10月8日下午16:00

地点：腾讯会议

报告摘要：

Using density functional based LDA+U method and linear-response theory, we study the magnetic exchange interactions of the superconductor Nd_1-xSr_xNiO₂. Our calculated nearest-neighbor exchange constant J1=82 meV is large, weakly affected by doping and is only slightly smaller than that found in the sister compound CaCuO₂. We however find that the hole doping significantly enhances the inter-layer exchange coupling as it affects the magnetic moment of the Ni-3d{3z2-r2} orbital. This can be understood in terms of small hybridization of Ni-3d{3z2-r2} within the NiO2 plane which results in a flat band near the Fermi level, and its large overlap along z direction. We also demonstrate that the Nd-5d states appearing at the Fermi level, do not affect the magnetic exchange interactions, and thus may not participate in the superconductivity of this compound. Whereas many previous works emphasize the importance of the Ni-3d{x2-y2} and Nd-5d orbitals, we instead propose that the material can be described by a Ni-3d{x2-y2}/Ni-3d{3z2-r2} two-band model. Its solution is studied here on the level of Dynamical Mean Field Theory and reveals an underlying Mott insulating state which, depending on precise values of the intra-atomic Hunds coupling, produces upon doping competing S=0 and S=1 two-hole states at low energies that lead to very different quasiparticle band structures. We propose that trends upon doping in spin excitational spectrum and quasiparticle density of state can be a way to probe Ni 3d8 configuration.

主要参考文献：

1.Wan, XG ; Turner, AM; Vishwanath, A; Savrasov, SY. Topological semimetal and Fermi-arc surface states in the electronic structure of pyrochlore iridates . PHYSICAL REVIEW B 83, 205101(2011).

2.Wan, XG ; Ding, HC ; Savrasov, SY ; Duan, CG. Electron-phonon superconductivity near charge-density-wave instability in LaO0.5F0.5BiS2: Density-functional calculations . Phys. Rev. B 87, 115124(2013).

3.Wan, XG; Vishwanath, A; Savrasov, SY. Computational Design of Axion Insulators Based on 5d Spinel Compounds. Phys. Rev. Lett. 108, 146601(2012).

报告人简介：

万贤纲，南京大学物理学院教授、博士生导师。于2000年在南京大学获得理论物理博士学位，2001年起在南京大学工作，历任讲师、副教授、教授。主要研究方向包括计算凝聚态物理研究和材料设计、关联电子系统数值计算、磁性材料研究、Weyl半金属研究、新型超导研究等，已在Nature Communications、PRL等期刊发表论文数十篇，入选教育部新世纪优秀人才，先后获得2014年度香港大学Daniel Tsui Fellowship（崔琦物理学奖）、2015年国家杰出青年科学基金、2020年第二届全国创新争先奖。

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