서강대학교

초록/요약

Bismuth selenide (Bi2Se3), which is one of the archetypal 3-dimensional topological insulators, has the metallic surface state protected by time-reversal symmetry inside the insulating bulk gap. Inducing magnetic order in the Bi2Se3 is an important theme for the realization of novel topological phases and the application of spintronics. One of the simple ways to establish the magnetism is doping magnetic impurities in Bi2Se3. The substituted magnetic impurity was mainly transition metal ions such as Cr, Mn and Fe, which have 3d electrons. Besides the 3d transition metal, rare-earth (RE) elements which have 4f electrons are good dopants in Bi2Se3 because the ionic radius of RE elements is similar with that of Bi ions, 4f electrons of RE elements are localized and RE elements have larger spin-orbit coupling and magnetic moments compared to the transition metal. In this thesis, rare-earth elements like Gd and Eu are substituted into Bi2Se3 and the substitution effects on their magnetic and electronic properties are investigated. In other words, the correlation between the magnetic order and the topological states in Bi2Se3 are studied from the various measurements. The topological nontrivial properties of RE-substituted Bi2Se3 are primarily explored by analyzing the Shubnikov-de Haas oscillations in magnetoresistance date, leading to gaining the information on the Berry phase. Although both substituted Gd and Eu ions induce the antiferromagnetic order in Bi2Se3 with the Néel temperature of about 6 K, the topological nontrivial state is disrupted by Gd substitution, while it remains after Eu substitution. These results suggest that Gd-substituted Bi2Se3 can be changed into the topological trivial phase due to the broken time-reversal symmetry and Eu-substituted Bi2Se3 can be transformed to new topological phases such as the antiferromagnetic topological insulator or Weyl semimetal.