arXiv 2407.144092024-07-19cond-mat.str-elcond-mat.supr-con

基于第一性原理评估La$_{3}$Ni$_{2}$O$_{7}$中自旋与电荷条纹的形成

Assessing the formation of spin and charge stripes in La$_{3}$Ni$_{2}$O$_{7}$ from first-principles

Harrison LaBollita, Victor Pardo, Michael R. Norman, Antia S. Botana

备注：: 8 pages, 4 figures
发表信息：: Phys. Rev. Materials 8, L111801 2024
DOI：: 10.1103/PhysRevMaterials.8.L111801

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AI总结：

本研究用DFT+U方法发现高压超导双层镍酸盐La₃Ni₂O₇基态为单自旋-电荷条纹相，揭示其与铜氧化物的磁性差异及压力下金属化机制，为理解其超导起源提供关键线索。

摘要：

我们采用关联密度泛函理论方法（DFT + Hubbard $U$），研究了在压力下会进入超导态的双层Ruddlesden-Popper（RP，拉德尔斯登-波珀）镍酸盐La$_{3}$Ni$_{2}$O$_{7}$的自旋密度波态。我们预测，该双层RP材料的基态是单自旋-电荷条纹相，具有面内up$^\nearrow$/up/down$^\nearrow$/down对角条纹，其中up$^\nearrow$/down$^\nearrow$为低自旋（形式上为Ni$^{3+}$：$d^7$），up/down为高自旋（形式上为Ni$^{2+}$：$d^8$）。该解（即使在$U=0$时也呈绝缘性）的主要特征是费米能级附近$d_{x^{2}-y^{2}}$能带占据主导地位，通过氧空位引入电子时会对这些能带进行掺杂。尽管在$d_{x^{2}-y^{2}}$能带的主导作用方面与铜氧化物相似，但La$_{3}$Ni$_{2}$O$_{7}$的磁性基态存在明显差异：双层内的面外反铁磁耦合（与形成类自旋单态构型的$d_{z^2}$轨道相关）被发现是主导耦合，而面内相互作用因基态的条纹有序性而减弱。在压力作用下，这种条纹状磁性基态的本质保持相似，但带宽的增加会使La$_{3}$Ni$_{2}$O$_{7}$迅速转变为金属态，此时费米能级附近的所有电子活动在很大程度上都涉及$d_{x^2-y^2}$轨道。这与铜氧化物的情况相似，可能为理解该材料在压力下超导性的产生机制提供关键见解。

英文摘要：

We employ correlated density-functional theory methods (DFT + Hubbard $U$) to investigate the spin-density wave state of the bilayer Ruddlesden-Popper (RP) nickelate La$_{3}$Ni$_{2}$O$_{7}$ which becomes superconducting under pressure. We predict that the ground state of this bilayer RP material is a single spin-charge stripe phase with in-plane up$^\prime$/up/down$^\prime$/down diagonal stripes with up$^\prime$/down$^\prime$ being low spin (formally Ni$^{3+}$: $d^7$) and up/down being high spin (formally Ni$^{2+}$: $d^8$). The main feature of this solution (that is insulating even at $U=0$) is the dominant role of $d_{x^{2}-y^{2}}$ bands around the Fermi level, which would become doped with the introduction of electrons via oxygen vacancies. In spite of the similarity with cuprates in terms of the dominant role of $d_{x^{2}-y^{2}}$ bands, some differences are apparent in the magnetic ground state of La$_{3}$Ni$_{2}$O$_{7}$: the antiferromagnetic out-of-plane coupling within the bilayer (linked to the $d_{z^2}$ orbitals forming a spin-singlet-like configuration) is found to be the dominant one while in-plane interactions are reduced due to the stripe order of the ground state. With pressure, this striped magnetic ground state remains similar in nature but the increase in bandwidth quickly transitions La$_{3}$Ni$_{2}$O$_{7}$ into a metallic state with all the activity close to the Fermi level involving, to a large extent, $d_{x^2-y^2}$ orbitals. This is reminiscent of the cuprates and may provide key insights into how superconductivity arises in this material under pressure.

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