טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentAmit Ribak
SubjectThe Effect of Electrochemical Intercalation of Copper
and Bulk Doping of Iron Impurities on the
Topological Insulator Bi25e3
DepartmentDepartment of Physics
Supervisor Professor Kanigel Amit
Full Thesis textFull thesis text - English Version


Abstract

Topological insulators (TIs) are a class of materials which have an insulating bulk but the surface has conducting gapless states. This class of materials was first predicted theoretically, and soon after such materials were found and measured experimentally. Among the different materials, Bi2Se3 became the most studied 3D topological insulator. Although it's not a perfect bulk insulator, Bi2Se3 has a simple band structure with a single Dirac point at momentum 0 and a relatively large spin-orbit gap. Its layered structure allows intercalation as well as Bi substitution, hence allowing to change its properties substantially.


The following experimental work has two main parts: In the first part we focus on electrochemical intercalation of copper between the Bi2Se3   layers in order to turn it into a superconductor. Starting from a non-trivial insulator, the intercalation may result in a non-trivial superconductor - a topological superconductor. Our goal is to synthesize high quality crystals with high superconducting volume fraction. By doing so, we believe we'll be able to perform future measurements which will shed some light on the topological properties of this crystal.

We describe a successful method to synthesize such a material and show experimental evidence of superconductivity without ruining the unique surface states. We examine the effect of the intercalation process on the crystal structure using ARPES and XRD and notice that on some samples a major change in the band structure occurs. This change looks like the change in the band structure of pure Bi2Se3 which turns into a superconductor under pressure.

In the second part of the work we look at the effect of magnetic impurities on the surface states. Theory predicts that the conducting surface states of a topological insulator are protected from backscattering by any disorder as long as time reversal symmetry is maintained. Magnetic impurities of Fe break time reversal symmetry, hence they can open a gap in the surface states, turning the massless Dirac fermions into massive.

We look at different Fe concentration in the bulk of Bi2Se3 and look for such a gap opening. We find that only at high concentration of Fe a gap opens, and associate this behavior to the orientation of the impurities magnetic moments, as measured using X-ray magnetic circular dichroism. Our goal is to understand why a gap opens only at high Fe concentration and learn about the nature of magnetic ordering in Bi2Se3.