不同组分厚度比的LaMnO3/SrTiO3异质界面电子结构和磁性的第一性原理研究

基于密度泛函理论的第一性原理计算, 研究了(LaMnO₃)_n/(SrTiO₃)_m(LMO/STO)异质界面的离子弛豫、电子结构和磁性质. 研究表明, 不同组分厚度比及界面类型时, 离子弛豫程度各不相同, 并且界面处的电子性质受此影响较大. 对于n型界面, 当LMO的厚度达到6个单胞层后, 电子会从LMO转移到STO, 转移的电子占据界面层Ti原子的3d电子轨道, 界面处出现二维电子气. 对于n型界面(LMO)_n/(STO)₂, 随着LMO厚度数n的增加, 由离子弛豫造成的结构畸变减小, 而界面处Ti原子周围电子的态密度和自旋极化却增大, 表明高厚度比的n型界面有利于产生高迁移率的二维电子气和自旋极化. 而对于p型(LMO)₂/(STO)₈界面, 在STO一侧基本没有结构畸变, 界面处无电子转移和自旋极化现象. 通过计算平均静电势发现n型和p型界面处的势差大小相差2 eV, 解释了p型界面不容易发生电荷转移的原因.

First-principles study of the electronic properties and magnetism of LaMnO3/SrTiO3 heterointerface with the different component thickness ratios

Using first-principles calculations based on density functional theory and projector augmented wave method, we investigate the thickness ratio dependences of the ionic relaxation, electronic structure, and magnetism of (LaMnO₃)_n/(SrTiO₃)_m heterostructure. Polar and nonpolar oxide interfaces have become a hot point of research in condensed matter physics; in this system, polar discontinuity at the interface may cause charge transfer to occur at interfaces between Mott and band insulating perovskites. Here, we consider two types of interfaces, namely n-type (LaO)⁺/(TiO₂)⁰ and p-type (MnO₂)^-/(SrO)⁰ interfaces. The results show that the different thickness ratios and interface-types lead to different degrees of ionic relaxation, inducing charges of different concentrations to transfer. The distortions of the oxygen octahedra are found to vary distinctly with the component thickness ratio (n:m), which is consistent with recent experimental results. Furthermore, both n and m are found to strongly affect the charge transfer. When the thickness of LaMnO₃ reaches a thickness of critical layers of 6 unit cells, the Mn-e_g electrons are transferred to the Ti-d_xy orbitals of SrTiO₃, which is caused by the interface polar discontinuity. Two-dimensional electron gas with high mobility is formed in an n-type (LaMnO₃)_n/(SrTiO₃)₂ interface region. Meanwhile, spin polarization of interface-layer Ti atoms becomes more obvious, which induces Ti magnetic moment to be close to 0.05μ_B. We find that Mn magnetic moment of 3.9μ_B is a larger value at the n-type interface than at the p-type interface. The above studied heterostructure favours ferromagnetic spin ordering rather than the A-type antiferromagnetic spin ordering of bulk LaMnO₃. Whether n-type or p-type (LaMnO₃)₂/(SrTiO₃)₈ interfaces consist of ultrathin LaMnO₃ layer and thicker SrTiO₃ layer, there is no structure distortion at the side of SrTiO₃ basically, which is in agreement with experimental results. Stronger interface-layer polar distortions for p-type interface prevent the electron transfer from occurring, and spin polarization of Ti cannot occur either. In addition, it is found that the two types of interfaces possess 2 eV potential difference by comparing the average electrostatic potential, thus charge transfer is more difficult to occur in the p-type interface than in the n-type interface.