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    Title: 以第一原理(LDA+U+GW)研究多鐵性材料電子結構
    Other Titles: Electronic Structure of Multiferroic Materials: First-Princples LDA+U+GW Study
    Authors: 薛宏中
    Contributors: 淡江大學物理學系
    Keywords: LDA+U 方法;限制RPA 方法;哈柏形式之局域庫倫位勢U;遮蔽庫倫位勢W;準粒子;GW 計算;多鐵性材料;LDA+U;Constrained random phase approximation;Hubbard Coulomb potential U;Screened Coulomb interaction W;Quasiparticle;GW calculation;Multiferroics
    Date: 2010
    Issue Date: 2011-07-05 23:41:14 (UTC+8)
    Abstract: 多鐵性材料的多項新奇物理性質,主要與其特殊的電子結構有關。拜最近發展的 LDA+U 方法之賜,我們才得以一窺多鐵性材料中,同時具備鐵電性與鐵磁(或反鐵磁) 特性之奧妙。LDA+U 計算方法,主要是在靜態密度泛函理論(static DFT)的基礎上,加 入哈柏(Hubbard)形式之局域庫倫位勢U。雖然,過去研究顯示,透過LDA+U 計算方 法,能清楚地說明多鐵性材料中的鐵電性,可能由晶體結構效應或更複雜的電子相關 作用(例如:電子軌道有序、電子電荷有序及電子自旋有序)所引起。然而,一般計算中, 局域庫倫位勢U 與交換強度J,皆被定位為可調整之參數;而設定不同的參數值,常 會導致不同的計算結果。如此缺乏嚴格的定義,使得LDA+U 計算在多鐵性材料所得 之結果,引起相當爭議。為了解決這個難題,近年來新發展的限制RPA 方法,提供直 接由第一原理方法計算參數U 與J。事實上,這些參數皆與多電子遮蔽庫倫位勢W 有 關。因此,在本計畫中,我們將首先利用限制RPA 方法,由第一原理計算包含d 電子 的多鐵性材料(YMnO3 及其掺雜系統)中,U 與J 的參數值。其中,因為在多鐵性材料 中,局域分佈之d 電子,扮演關鍵性的角色。所以,我們採用最大局域化之Wannier 函數作為基底,以反應d 電子的局域性;進一步而言,為了瞭解多鐵性材料中電子軌 道再耦合現象,我們必須考慮多電子遮蔽效應,以計算更準確的電子結構。因此,進 行多鐵性材料的準粒子GW 計算,遂成為不可或缺的工作。我們相信此計畫之研究成 果,對於未來多鐵性材料之第一原理研究,將產生相當重要之影響。
    The fascinating physical properties of multiferroic materials are originated from their unique electronic structures. Thanks to the state-of-the-art LDA+U method which including on-site Hubbard Coulomb potential U in the framework of static DFT, the coexistence of ferroelectric and ferro-/antiferro- magnetic characters in multiferroics can be understood comprehensively. Indeed, first-principles LDA+U calculations elucidate the origin of ferroelectricity in multiferroic materials can be either structural effect or more complicated correlation effects, such as orbital ordering, charge ordering , and spin ordering. However, the key empirical parameters, on-site Coulomb interaction U and exchange parameter J, obtained from different approaches sometimes yield to very different results. Actually, the lack of well-defined parameters provokes the ambiguity of LDA+U results in multiferroic materials. According to the recent development of constrained random phase approximation (cRPA) method, parameters U and J, mainly related to the screened Coulomb interaction W, can be calculated from first-principles. Therefore, in this project, we will firstly calculate the U and J in a typical multiferroics, YMnO3 and its doping system, by performing cRPA calculation. Taking into account the localized character of the d electron which plays an essential role in multiferroic compounds, maximally localized Wannier functions are used as the basis set to evaluate the parameters U and J for d-electrons. Furthermore, in order to understand the insight of the vital rehybridization effects in multiferroic structure, the accurate electronic structure including many-body screening effects is necessary. Thus, based on the appropriate LDA+U results, we will carry out the quasiparticle GW calculations of YMnO3 and related doping systems. A significant impact of this work on the first-principles study on multiferroics is promising.
    Appears in Collections:[物理學系暨研究所] 研究報告

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