以第一原理研究雙鈦鈣及二維蜂巢式結構中的新型磁性半導體

Abstract

在此篇論文中我們研究兩種可能的磁性半導體，第一種為La為主的雙鈦鈣結構，第二種為二維的蜂巢式結構。我們是根據密度泛涵理論做結構最佳化計算利用ＶＡＳＰ為計算軟體。由四種磁相態出發：鐵磁態(FM)、亞鐵磁態(FiM)、反鐵磁態(AFM)、無磁性(NM)，使用廣義梯度近似(GGA)以及考慮庫倫排斥效應(GGA+U)。 第一章，我們簡單介紹磁性半導體的歷史以及研究發展，以及我們可能找到的磁性半導體候選者。 第二章，我們簡單介紹密度泛涵理論以及計算方法，包括Born-Oppenheimer 近似Hohenberg-Kohn 理論, Kohn-Sham 方程式以及交換關聯效應、局域密度近似與廣義梯度近似。使用的計算軟體為VASP，其使用平面波展開方琺來計算，並介紹庫倫電子關聯效應。 第三章我們介紹磁性半導體的一些特徵以及特性。我們也簡介了第一個發現為磁性半導體材料EuO。另外，在本文中也介紹，傳統半導體材料中添加過度金屬所形成的diluted magnetic semiconductors (DMSs).之後，我們提到了雙鈦鈣結構中所形成的相變溫度接近室溫的磁性半導體材料。 第四章我們簡介了兩類可能的磁性半導體材料，第一類為La2MM’O6 (M, M’= transitionmetal ions)，第二類為La2FeCoO6,、La2FeIrO6、La2FeRhO6。這兩類的磁性半導體材料是由於superexchange機制所形成。在考慮庫倫排斥力的效應下，這兩類的磁性半導體性質仍然維持。 第五章介紹了二維蜂巢式的結構以及所找到的磁性半導體，在考慮庫倫排斥力的效應下依然存在。 第六章給予總結，並希望我們找到的磁性半導體材料對未來實驗合成有幫助。 關鍵字:密度泛涵理論，第一原理計算，磁性半導體，雙鈦鈣結構，二維蜂巢式結構

In this thesis, we investigated two possible candidates series of magnetic semiconductor (MS) in the La-based double perovskites structure and 2D honeycomb structure. The calculation is based on the density functional theory (DFT) with full structure optimization by generalized gradient approximation (GGA) and plus the onsite Coulomb interaction (GGA+U) scheme. The projector augmented wave (PAW) method within conjugate-gradient method implemented in VASP package. In chapter 1, we briefly introduced the previous researches of MS compounds and their important applications in spintronic field. In chapter 2, we introduced the density functional theory (DFT) and computational method. Firstly, we introduced the Born-Oppenheimer approximation. Based on the Born-Oppenheimer approximation, the DFT introduce later. The DFT divide into three part, the Hohenberg-Kohn theorems, the Kohn-Sham equations, the Exchange correlation functional, local (spin) density approximation and the GGA, as well as computational method we used. Sencondly , we introduced the PAW method within VASP package and the GGA+U method. In chapter 3, we introduced the characteristics and properties of MS materials. Also, we review the MS compounds of first discovery in EuO compound and the diluted magnetic semiconductors (DMSs). Latter, we give a detail discussion about double perovskites structure and the initial magnetic states configurations. Moreover, we introduce two-dimension transition metal dichalcogneides structure with their recently experimental and theoretical research results. Finally, the calculation procedure with a diagram and explain the calculation detailed and setting parameters. In chapter 4, for La-based double perovskites La2MM’O6 (M, M’= transitionmetal ions), we classified the possible MS compound into 2 groups according to the electronic configuration of the MM’ ion pairs. In Group1 Ferromagnetic Insulator (FM-IS): La2NiM’O6 (M’= Mn, Tc, Re, Ti, Zr, and Hf). In Group2 Ferromagnetic Semiconductor (FM-SE): La2FeB’O6 (M’= Co, Rh, and Ir). We calculated energy difference for four magnetic states which are ferromagnetic, ferrimagnetic, antiferromagnetic, and non-magnetic states in both GGA and GGA+U schemes. The FM-IS observed in La2NiMO6 (M = Mn, Tc, and Re) was most likely a mixture of high spin (HS) and low spin (LS) states; the electrons transferred from the filled LS Ni eg states to the half-filled HS Mn (Tc). On the other hand, the FM-IS in La2NiMO6 (M = Ti, Zr, and Hf) was caused by the electron transfer from the half-filled LS Ti (Zr and Hf) eg orbital of HS Ni to the empty eg orbital of LS Ti (Zr, Hf). The FM insulating state of La2NiMO6 (M = Mn. Tc, Ti, Zr, and Hf) remained the same, whereas it changed from metal to insulator for La2NiReO6 based on the generalized gradient approximation +U calculation. The La2FeCoO6 is a potential candidate for FM-SE whereas La2FeIrO6 and La2FeRhO6 show energy difference about 9.0 meV, which is not large, but still indicates a substantial preference toward an antiferromagnetic (AFM) state. For the GGA+U scheme, La2FeCoO6 remains a stable FM semiconductor, whereas La2FeRhO6 and La2FeIrO6 show the FM and AFM states of which are degenerate with each other.

In chapter 5, ｗe discussed ferromagnetic property and compared with the experimental result. By suppression of the GGA+U, the energy gap is expected to be large and approach to 0.65 eV of VSe2 monolayer. In chapter 6, we will make a summary of our work. It includes the research method, the main results and the mechanism of causing MS compounds. We hope that this thesis on the searching MS compounds in double perovskites and 2D honeycomb structures are useful for experimental research and bring up the research upsurge of MS materials.