Bi2-xPbxSrzCo2Oy和(Bi0.25Sb0.75)2 (Se2.35Te0.65)的核磁共振研究

Abstract

本論文主要是利用核磁共振(NMR)的技術來探討磁性材料Bi2-xPbxSrzCo2Oy(x=0-0.61, y=8.47-8.62, z=1.78-1.99)在不同的Pb濃度中的磁性變化以及拓譜絕緣體(Bi0.25Sb0.75)2 (Se2.35Te0.65)的特殊電子結構。首先，我們發現Bi2-xPbxSrzCo2Oy的磁性會隨Pb的濃度而改變，且室溫的磁化率隨Pb 的加入而急速上升。我們量測樣品中的59Co的NMR頻率位移在不同溫度下的變化，發現此磁化率上升與含鉛樣品中Co4+的軌道磁化率增加有關。 拓譜絕緣體是一種具有特殊表面態的材料，根據理論，這種表面態並不會因為晶格缺陷或排列錯亂而被破壞。樣品(Bi0.25Sb0.75)2(Se2.35Te0.65)是已知的拓譜絕緣體材料Bi2Se3摻雜了Sb和Te兩個元素所產生的，且仍具有拓譜表面態。我們發現這些摻雜是均勻分布在Bi2Se3的結構中，確認了所出現的表面態的確是樣品整體所出現的結果，而不是由局部Bi2Se3所造成的。

The electronic band structure of topological insulator and layered cobalt oxides structure all have attracted much attention, because their unusual physical properties. The topological insulator is a material which has an insulating bulk state and gapless surface states. The electronic structure of Co ion in the layered cobalt oxides structure which have two splitted t2g and eg orbitals originated from 5-degenerate 3d-orbital. This thesis reports Nuclear Magnetic Resonance (NMR) study of the topological insulator (Bi0.25Sb0.75)2 (Se2.35Te0.65) and the layered cobalt oxides compounds Bi2-xPbxSrzCo2Oy (x=0-0.61, y=8.47-8.62, z=1.78-1.99). We measured the 209Bi and 121Sb NMR spectra in (Bi0.25Sb0.75)2 (Se2.35Te0.65) single crystal. We found that the 209Bi NMR spectra of (Bi0.25Sb0.75)2(Se2.35Te0.65) is extraordinary broader than the 209Bi NMR spectra of Bi2Se3, suggesting that the Bi atoms are randomly distribution in the crystal. The spin-spin relaxation time (T2) of 209Bi and 121Sb are all show two T2 components, it means that the both atoms are bonding with Se and Te two atoms. According to our experimental results, the (Bi0.25Sb0.75)2 (Se2.35Te0.65) has strong lattice distortion than Bi2Se3. We measured the 59Co NMR spectra in three different chemical compositions Bi2-xPbxSrzCo2Oy single crystal with different temperatures. The two different charge states of Co3+ and Co4+ peak are clearly showed in 59Co NMR spectra, and the Pb doping increases the Co4+ peak intensity. We also measured Co3+ NMR frequency shift as a function of temperature for the three different samples. We found that the Co3+ NMR frequency shift at high temperature range (T>100K) is similarity in three different samples, it means that the local susceptibility of Co3+ ions is same at three samples.