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Direct evidence of intercalation in a topological insulator turned superconductor

Somnath Bhattacharyya

5 February 2013

Retrieved phase of exit face wave of an intercalated region of uniform thickness within Cu doped Bi2Se3 single crystal reveals three distinct types of atomic columns, the position of Cu atoms are shown in the schematic

Topological insulators are one of the most interesting candidates in contemporary research because of their remarkably different surface and bulk states. Due to strong spin-orbit coupling these materials show relatively longer range quantum entanglement which makes them suitable for building next generation quantum computational devices. Amongst other such materials, Bi2Se3 is of particular importance due to its large bulk band gap and surface states with a single Dirac fermionic mode which shows a linear, lightlike dispersion. Recently Bi2Se3 came into the limelight by showing superconductivity when doped with Cu. While an intercalated CuxBi2Se3 (0.1 xBi2-xSe3 does not exhibit any trace of superconductivity. This made researchers to believe that Cu intercalation into Bi2Se3 structure is crucial to make it superconducting. It was also reported that any long range ordering in Cu intercalation was absent in the bulk material so, in other words Cu intercalated regions of nanometer size range are distributed within bulk Bi2Se3 matrix. Therefore, the presence of intercalation can only be proved if the structure of the material is being investigated at the atomic scale. High Resolution Transmission Electron Microscopy (HRTEM) is the most effective technique to reveal the presence of intercalation though no thorough investigation of this kind on high quality single crystals of CuxBi2Se3 is reported yet. Motivation of the present study is to investigate a high-quality single crystalline sample of superconducting Cu doped Bi2Se3 at atomic scale using HRTEM to verify the existence of intercalation which was postulated as the prime reason for superconductivity in this class of materials.

For the first time, this study reveals the presence of intercalated structure at nanoscale within the bulk material which is supposed to be the prime reason for supercounductivity in doped topological insulator. A careful study exhibits the presence of two intercalation sites in this material when projected along the [-1-1 0] direction. Stoichiometry of the intercalated unit cell was also calculated from the projected mean inner potential.

Direct evidence of intercalation in a topological insulator turned superconductor Tanmay Das, Somnath Bhattacharyya, Bhanu Prakash Joshi, Arumugum Thamizhavel, Srinivasan Ramakrishnan, Materials Letters, 98, 330-373, 2013.

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