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칼코젠 화합물의 원자구조와 위상절연상태 사이의 상관관계

칼코젠 화합물의 원자구조와 위상절연상태 사이의 상관관계
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Topological insulator is new class of matter which is classified under time-reversal symmetry. While bulk has insulating gap, conducting helical state arises at the boundary which are robust to perturbations and protected by time-reversal symmetry. Because of its anomalous features and potential applications such as spintronics and quantum computing, this research field is increasing rapidly. Topological insulator and trivial insulator cannot be transformed to each other unless its band gap is closed. Contrariwise, in order to induce topological nontrivial order to a trivial insulator, band inversion between valence and conduction bands is a necessary condition. In that sense, IV-VI compounds are good starting materials to find nontrivial order because band gap is narrow (0.2 ~ 0.3 eV) and band inversion is already observed under pressure or by mixing equivalent elements. In this thesis, first principles calculations are carried out to study structural variations as an origin of topological order transition. First, anisotropic strain is found to generally induce nontrivial order in IV-VI compounds and the materials are expected to host Majorana fermion. Next, the effect of phonon is considered within Born-Oppenheimer approximation. Specific phonon leads Z2 invariant to oscillate with the frequency of given phonon mode. Lastly, defect-induced topological order is found in GeTe. Migration of Ge atoms to tetrahedral site is a well known defect which commonly appears in phase change materials. The critical ratio of defect to induce band inversion is found by using tight-binding method within virtual crystal approximation. This thesis shows the correlation between structure and topological order of IV-VI compounds. This result would hopefully provide new insight of these materials.
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