以掃描穿隧顯影及第一原理計算對三維狄拉克半金屬砷化鎘的表面性質探討

Abstract

被稱作「三維石墨烯材料」的狄拉克半金屬，在近年來因具有相對論性狄拉克費米態之特性，在新穎材料特性與元件應用領域的潛力上，吸引了學術圈的注意。若在狄拉克半金屬材料結構內，引入時間反演與空間反轉對稱的破壞，則結構內因對稱性下守恆的拓樸狄拉克粒子態被預測會分裂成一對具自旋極化的外耳費米子態，而此種外耳半金屬對於新穎傳輸現象的實踐上，可視為一大躍進。外耳費米子態被比喻為倒空間晶格內的「磁單極」，並因此在近年來受到科學家的矚目。目前被預測符合狄拉克半金屬特性的材料中，主要有砷化鎘和納化鉍，而我們針對前者較穩定的化性，進行了一系列的研究。砷化鎘材料目前已被證實具有許多別於傳統材料的特性，例如負巨磁阻、超高載子遷移率、應力導致之電性態相變，甚至近年來也有跟超導相關的討論。儘管以上諸多特點，很多都跟它本身的晶格排列有很大的關係，然而也正是那複雜的材料結構，讓科學家對於砷化鎘的原子模型，始終缺乏較為完整的理解。在我們的工作當中，透過掃描穿隧顯影以及密度泛函理論的第一原理計算，我們針對砷化鎘材料的(112)斷面進行徹底且縝密的研究，包含它表面的層狀結構和原子組態。除此之外，透過一個重新定義的單位晶格，我們成功詮釋一組首次觀測而得的表面晶格結構，並理解到此種結果跟原子模型、電性結構，以及它對於已發現之非傳統型傳輸現象間，有什麼樣的物理連結。總結來說，我們的成果給出了第一個針對砷化鎘材料表面的複雜結構，以及電子組態特性的完整詮釋，並且清楚指出這些特性在應用端上的可能性。

Dirac semimetals (DSM), three dimensional analogues of ultra-relativistic fermion systems like graphene, have been all the rage recently owing to their novel properties and potential in device application. By breaking time-reversal or spatial inversion symmetry, many believe the merged pair of symmetry-protected Dirac points in DSM would become spin-polarized Weyl nodes in Weyl semimetals (WSM), responsible for unconventional Fermi-arc state. Described as magnetic monopole in momentum-space, it has dragged much attention from scientists recently. Predicted DSM candidates include Cd₃As₂ and Na₃Bi, and we particularly focus on the former one for its chemical stability. Several features of Cd₃As₂ have been published already, such as giant magnetoresistance, ultrahigh mobility, pressure-induced phase transition, and even possible superconductivity recently. Most of these fascinating features are derived from the unique crystalline structure, though, few reports focus on it due to the complexity and lack of well-defined understanding of surface properties. Here, we perform a combined investigation of scanning tunneling microscopy (STM) and density functional theory (DFT) on Cd₃As₂ and characterize the cleaved (112) plane in detail, including atomic lattice and step-terrace morphology, and it agrees with previously proposed structure. In addition, a redefined unit cell is introduced to explain an unexpected surface morphology on (112) cleavage plane, revealing intrinsic bulk-like termination with defect-modified electronic configuration, and it shows direct evidence on some of the reported unconventional transport behavior. Therefore, our work comes with the very first detailed surface study on the sophisticated structure and non-trivial electronic properties of Cd₃As₂, and leads to a clear connection with device applications.