First-principles calculation of the spin-polarized current from transition-metal electrodes

本研究的目的是用密度泛函理论中的第一性原理方法计算过渡金属隧道结的自旋极化电流和金属电极的场发射电流。在我们的方法中，界面区域的格林函数是使用Inglesfield嵌入技术和线性化的增广平面波(LAPW)基集自洽计算的。具体内容如下：1.用全电势LAPW方法确定了半无限大电极内部三维晶体的电子结构。从单电子波函数的转移矩阵出发，计算了与电极表面密勒指数相对应的晶体的复杂能带结构。半无限大电极的嵌入电势是由步骤2.4中计算的瞬逝波和Bloch波构成的。用密度泛函理论自洽地计算了两个半无限大电极之间界面区的电子结构。用朗道公式计算了流过界面的弹道电流。我们用格林函数和嵌入势的虚部重新表述了公式。此外，我们还推导了一个新的公式，该公式也可以处理定域表面态的隧道电流。我们将这个公式应用于下列系统：1.我们研究了贵金属表面的场发射电流，并阐明了它与晶体取向的关系。结果表明，来自局域表面态的隧道电流可以起主导作用。我们研究了自旋极化的隧道电流通过隧道结，如铜/钴/铜，以及通过铁/镁/铁，这是已知的最佳候选磁性，随机存取存储器(MRAM)。

The purpose of the present study was to develop a method for calculating the spin-polarized current through transition-metal tunnel junctions and the field-emission current from metal electrodes by a first-principles approach within density-functional theory. In our method, the Green function in the interface region is calculated self-consistently Using the embedding technique of Inglesfield and the linearized augmented plane wave (LAPW) basis set. The details are as follows:1. The electronic structure of the 3-dimensional crystal in the interior of a semi-infinite electrode is determined by the full-potential LAPW method.2. The complex band structure of a crytal corresponding to the Miller index of the electrode surface is calculated from the transfer matrix for 1-electron wave functions.3. The embedding potential of a semi-infinite electrode is constructed from the evanescent and Bloch waves calculated in the step 2.4. The electronic structure of the interface region between two semi-infinite electrodes is computed self-consistently within density functional theory.5. The ballistic current flowing through the 'interface is calculated using the Landauer formula. We reformulated the formula in terms of the Green function and the imaginary part of the embedding potential. In addition, -we have derived a new formula that can treat also the tunnel current from localized surface states.We applied the present formalism to the following systems:1. We investigated field-emission currents from noble metal surfaces and clarified its dependence on the crystal orientation. It has turned out that the tunnel current from localized surface states can make a dominant contribution.2. We investigated the spin-polarized tunnel current through tunnel junctions such as Cu/Co/Cu and also that through Fe/MgO/Fe, which is known as a best candidate for magnetic, random access memory (MRAM).