利用第一原理計算鈉二钒四氧化物磁性和能帶結構

Abstract

過渡金屬氧化物電子在特定溫度下會因為強庫倫作用效應，使電子只能填入簡併後的軌道，造成電子軌道形成一維結構材料。大部分一維過渡金屬材料在低溫時會因為磁性上的擾動使金屬態轉變成絕緣態。但是在2007年發現鈉二钒四氧化物在低溫時金屬態會與磁性擾動共存。在低溫時沿著鏈方向可以探測到金屬的行為而有很大的磁性異向能。 我們利用密度泛函原理的平面波虛位勢方法去計算準一維鈉二钒四氧化物的電子結構和磁性自旋交換作用。從電子能帶結構計算上，t2g能帶在低溫的orthohombic會部份佔據在費米能級上。我們可以找出7種反鐵磁態和鐵磁態去計算總能量高低去決定可能的基態。能量最低的基態發生在鈉二钒四氧化物結構的中間層和上下兩層钒原子為反鐵磁態的行為，我們並定義此基態為AFM1態。藉由計算能帶結構，態密度，自旋交換海森堡模型，和傳輸與費米面性質的討論。在沿著ΓX和ΓYTZ方向的AFM1價帶上的能量會比鐵磁態高，可以知道有強的钒氧鍵結。從態密度計算，我們可以發現在費米能級上存在半金屬性質。從自旋交換作用的計算，只討論鄰近和次鄰近的耦合效應。在ac平面上，最近鄰近的八面體钒原子有很強的直接鐵磁交換作用。八面體和四面體钒原子間有弱的鐵磁耦合。次鄰近八面體钒原子間會藉由中間的氧原子形成反鐵磁交換作用而形成明顯的磁性異向能。所以沿鏈方向之間為鐵磁態耦合，鍊和鏈之間為反鐵磁態耦合。從費米面傳輸性質分析，也可以看出在ac平面的多數電洞平面會對應到反鐵磁超級耦合(super-super antiferromagnetic spin exchange interaction)。實驗上延著鏈方向的金屬態傳輸特性和沿著垂直鏈方向明顯的磁性異向能效應也是可以從費米面的計算得出。

The electronic structure and spin exchange interactions of quasi-one dimensional oxide NaV2O4 have been studied with the density functional theory (DFT) using the plane wave (PW) pseudopotential method. From the electronic band structure calculations, the t2g bands are partially occupied at the Fermi level in the low temperature orthorhombic structure. We estimated the AFM1 spin configuration is NaV2O4 ground state, and we further discussed the band structure, density of states, Heisenberg model spin-spin exchange interaction, Fermi surface and transport properties. The AFM1 band structure showed that the dyz orbitals contributed the great conductivity in the chain direction at Fermi level. The AFM1 valence band energy along ΓX and ΓYTZ path is higher than FM electronic band structure and the result showed a strong V(3d)-O(2p) hybridization. From the DOS calculations, we also found the metallic property at the Fermi level in AFM1 ground state. In order to estimate the anisotropy of an effective spin-spin exchange interaction, the total energy of different eight Neel magnetic configurations are considered as the sum of the nearest-neighbor spin-spin configuration in spin Heisenberg model. The nearest octahedral site vanadiums show strong direct exchange interactions on diagonal direction in ac plane. The octahedral and tetrahedral site vanadiums show indirect super-exchange interactions through the intermediate oxygen 2p orbitals. It is found that a remarkably anisotropy exist perpendicular to the chains. The intrachain spin configurations are FM direct coupling. The density of states, band structure and exchange interaction calculations show half-metallic quasi-1D property. The strong FM intrachain interactions coexist with the weak AFM interchain spin configurations. The results are in good agreement with the positive Curie-Weiss and the AFM transition shape in experiments. The ground state AFM1 band structure showed a strong V (3d)-O(2p) hybridization effects along ΓX and ΓYTZ path. The majority large hole surface spreaded in the ac plane corresponded to the super-super antiferromagnetic spin exchange interaction coupling of strong V (3d)-O(2p) hybridization effects in AFM1 Fermi surface. The metallic transport property along the chain axis and the large anisotropy conclusions in AFM1 type Fermi surface satisfy the experiment results.