Sr掺杂对La_(1-x)Sr_xMnO_3/LaAlO_3/SrTiO_3界面电子结构的影响

采用基于密度泛函理论的第一性原理计算方法,系统地研究了La_(1-x)Sr_xMnO_3层中Sr的掺杂方式和掺杂量对4La_(1-x)Sr_xMnO_3/3LaAlO_3/4SrTiO_3(LSMO/LAO/STO)异质结构原子和电子结构的影响.结果表明:对于相同的Sr掺杂量,掺杂方式的差异对体系电子结构的影响微弱,不会导致体系发生金属-绝缘体转变;掺杂量的不同对体系电子结构有着显著的影响,当Sr的掺杂量较少时,LAO/STO界面处存在着准二维电子气,当Sr的掺杂量高于1/3时,LAO/STO界面处准二维电子气消失.我们相信,Sr的引入以及通过Sr掺杂量的改变可以对LSMO覆盖层极化进行调控,这也是导致体系LAO/STO界面处金属-绝缘体转变的可能原因,进一步为极化灾变机制导致的界面处电子重构提供了证据.

Effect of Sr doping on electronic structure of La1-xSrxMnO3/LaAlO3/SrTiO3 heterointerface

In the past decades, the interface between two oxides LaAlO₃ (LAO) and SrTiO₃ (STO) has attracted much attention since a quasi-two-dimensional electron gas (q2DEG) at the interface was observed. It is generally believed that polar discontinuity at polar/non-polar oxide interface is responsible for the emergence of q2DEG at the interface. Recently, how to modulate the q2DEG at the interface is becoming a new research focus. Capping other oxide thin layer on LAO layer is one of alternative approaches to controlling the generation of q2DEG at interface. However the mechanism or origin for tuning q2DEG at capped LAO/STO interface has not yet completely understood. Using the first-principles calculations within the density functional theory, the electronic properties of La_1-xSr_xMnO₃-capped LaAlO₃/SrTiO₃ heterointerfaces with different doping concentrations of Sr atoms are investigated. The system is composed of four layers of La_1-xSr_xMnO₃ (LSMO), three layers of LAO and four layers of STO, denoted as 4LSMO/3LAO/4STO. The interface is normal to the001] direction of cubic phase, namely (La_1-xSr_xO) layer and (MnO₂) layer appear alternately at LSMO, and (LaO) layer and (AlO₂) layer appear alternately at LAO. In the absence of LSMO layers, q2DEG does not appear at the LAO/STO interface. It is found that the electronic structure of 4LSMO/3LAO/4STO can be tuned significantly by capping LSMO layers. For concentration of doped Sr atoms less than 1/3, a q2DEG at LAO/STO interface is observed. In this case, a strong polarization existing in LSMO, together with the polarization in LAO, forces the electrons to be redistributed, thus inducing the q2DEG at LAO/STO interface. With the increase of the concentration of Sr atoms, the polarization in LSMO becomes weaker and weaker. When the concentration is higher than 1/3, the polaried electric field fails to make the electrons redistributed, thus the q2DEG disappears from interface.#br#Another interesting feature of the present work relates to the distribution of Sr atoms in LSMO. It is found that the electronic structure of 4LSMO/3LAO/4STO changes little with respect to the distribution of Sr atoms in LSMO. The system does not undergo the conductor-to-insulator transition for Sr atoms doping at different sites as long as the concentration of Sr does not change. The reason could be understood as follows. The LSMO layer is in a metallic state, the extra electrons, which are generated due to substituting La with Sr, will be delocalized rather than localized at each doped Sr atom. It is reasonable to expect that the electronic structure of the system should be less sensitive to the specific doping site of Sr in LSMO.