拓樸絕緣體Bi2Se3的核磁共振研究

Abstract

本論文主要是利用核磁共振（NMR）技術來研究五種不同方法製備的Bi2Se3單晶樣品，探討其晶格缺陷、電子能帶結構、與拓樸絕緣體的物理特性。我們量測樣品中的209Bi NMR訊號，發現每個樣品的頻譜並不盡相同，經譜線分析，我們認為這些樣品皆有不同程度的晶格缺陷，但因材料是半導體，微小缺陷即可造成導電載子濃度巨大的改變，所以造成了NMR譜線明顯的不同。從NMR譜線的偏移及原子自旋鬆弛時間發現，晶格缺陷較少的樣品，有較小的譜線偏移及較長的原子自旋鬆弛時間。另外，樣品的209Bi NMR頻譜出現了9支譜線，其譜線強度看似不像由原子核的quadrupolar effect所造成，但是藉由改變樣品與外加磁場的量測角度，我們驗證了這9支譜線確實還是由quadrupolar effect所造成，並且確定了所觀察到的訊號都是從樣品內部的Bi原子核所貢獻，至於Bi2Se3的特殊拓樸絕緣體表面態則無法由NMR技術來觀測到。

Topological insulators are novel electric materials that have a bulk band gap like an ordinary insulator but have protected conducting states on the edge or surface. This thesis reports a nuclear magnetic resonance (NMR) study on the topological insulator Bi2Se3. We measured 209Bi NMR in five different Bi2Se3 single crystals prepared by different methods. All samples show different NMR spectra. We found that these crystals have different levels of defects. Though these defects are too small to be detected by x-ray diffraction, they can make significant difference in the charge carrier density, owing to the semiconductor nature of Bi2Se3. We were able to extract the charge carrier density information from the NMR frequency shift and nuclear spin lattice relaxation rate, and compare with resistivity measurement. There are 9 peaks in the 209Bi spectrum and their intensities do not look like the ones for the nuclear quadrupolar spectrum. However, by measuring spectra at different crystal orientation, we confirm that the 9 peaks are indeed from quadrupolar splitting and there is only one Bi site seen by NMR. Most NMR signals are from the Bi nuclei in the bulk states and the topological surface states could not be probed by NMR technique.