分子與金屬介面的組態與磁性之第一原理研究

Abstract

這篇論文使用基於泛含密度理論的第一原理計算分子與金屬材料介面或金屬與金屬介面間的物理性質，特別著重在不同磁性組態的行為，旨在了解一些特殊物理性質背後的物理原理。文章研究的系統有三種：1. 由兩層磁性金屬夾著一層非磁性金屬構成的巨磁阻系統；2. 將銅沉積在鉍表面形成的合金；3. 具有手性的胜肽（peptide）分子在帶磁性的鈷鍍金表面的吸附力研究。 早期的巨磁阻系統研究發現，該系統兩側金屬的磁性方向可能不是整齊的相互平行或相互反平行，而是帶有部分垂直分量。為此，我們計算了不同厚度的中間非磁性金屬層的層間交換耦合（interlayer exchange coupling），以了解兩側磁性金屬磁矩方向選擇的特性。此外，在兩側磁矩相互垂直的系統中，我們發現了成對出現的特殊能帶，這些能帶在實空間的磁矩沿垂直介面方向呈現順時針方向和逆時針方向旋轉。 實驗發現，銅在鉍表面沉積後形成的二銅一鉍（BiCu$_2$）合金，在施加特定偏壓时，该合金的電導會突然大幅上升。我們嘗試通過第一原理計算此系統的能帶結構及態密度，以材料的物理性質從理論上理解電導大幅上升的原因。 實驗測量了具有手性的胜肽分子在磁性表面的吸附力，發現不同磁性方向（進表面或出表面）之間存在相當強度的差距。利用第一原理計算，我們計算此系統的能量，試圖探討吸附力差距的來源及其物理機制。

This dissertation employed first-principles calculations to study molecule-metal and metal-metal interfaces, with a particular focus on their configurations and magnetic properties. The objective was to elucidate the physical mechanisms underlying unusual phenomena. Three systems are studied in this research: 1. a giant magnetoresistive system consisting of two layers of magnetic metal sandwiching a non-magnetic metal spacer; 2. an alloy formed by depositing Cu on a Bi surface; and 3. the adsorption of a chiral peptide molecule on Au/Co surfaces with magnetic moment. Initial studies of the giant magnetoresistive system indicated that the direction of metallic magnetism in both side layers may not be perfectly parallel or antiparallel, but may have a partially perpendicular component. To address this, we calculate the interlayer exchange coupling (IEC) of the system with different thicknesses of the non-magnetic spacer to improve our understanding of its properties concerning the orientations of the magnetic moments on both sides. Furthermore, in configurations where the magnetic moments are perpendicular to each other, we observed a pair of stripes of magnetic moments rotating clockwise and counterclockwise in real space along a direction perpendicular to the interface. The deposition of Cu on the Bi surface results in the formation of the BiCu$_2$ alloy, which exhibits a sudden and significant increase in conductivity at a specific bias voltage. To comprehend the underlying physical mechanism responsible for this dramatic increase in conductivity, we have calculated the band structure and density of states of this system from first principles. The adsorption of chiral molecules on magnetic surfaces was measured and the results showed significant differences in magnetic orientation attraction (in-plane vs. out-of-plane). To understand the physics behind this difference, we performed ab initio calculations to determine the energy of the system.