Thesis Title: "Probing quantum transport of weak topological insulators, engineered heterostructures of 
strange metals with strong topological insulators and 3D Dirac semimetals."

Name of the Student: Mr. Nirmal K Sebastian

Degree Registered: Ph.D. Engineering 

Advisor: Prof. Anil Kumar P.S (Physics)  and  Dr. Shivakumara. C (SSCU)

Date: 21st April 2026, (Tuesday), 10:00 AM

Venue:  CeNSE Seminar Hall 

Abstract:

Topological materials are known for hosting unusual electronic states that come mainly from strong 
spin-orbit coupling(SOC) and band topology. In this work, we study three different yet, closely related 
topological phases to understand how weak antilocalization (WAL), weak localization (WL), and quantum interference 
appear in them, along with the nature of quantum oscillations in surface states with high mobility.


In the first part of this thesis, we find that thin films of Bi₂Te₃ grown in the R3m phase behave like a weak 
topological insulator. When the magnetic field is applied perpendicular to the side surface, we clearly observe 
Shubnikov-de Haas quantum oscillations in electrical resistivity at very high magnetic fields. Interestingly, 
the quantum oscillations only appear for this geometry, indicating that the side surface states play the dominant 
role here and has much larger charge carrier mobility compared to the bulk and the other surfaces. The extracted 
oscillatory part after a background subtraction is fitted well with Lifshitz Kosevic equations that validates the 
very large mobility and associated physical properties of the side surface. This highlights how the side surfaces of 
weak TIs are quite crucial if we want to use them in devices later. Accessing the side surfaces of weak topological 
insulators is very essential in harnessing their true potential. We also observe WAL feature that fits nicely to the 
2D Hikami-Larkin-Nagaoka (HLN) expression. This further confirms the existence of strong SOC for the side surfaces. 
This work experimentally realizes the emergence of a weak TI phase in R3m phase of Bi₂Te₃.

In the next part of this thesis, we look at thin films of YBa₂Cu₃O₇−δ/Bi₂Se₃ heterostructures, where YBa₂Cu₃O₇−δ is 
in the strange metal phase. Here, we see a crossover from WAL to WL with increasing magnetic field and temperature. 
By fitting the magnetoconductance to a two-component HLN model, we find competing length scales such as dephasing length,
spin-orbit scattering length, and mean free path. At low temperatures, the conductance changes logarithmically with 
temperature, which suggests that electron-electron interaction effects are also important. This shows that when a correlated 
phase like strange-metal YBCO is placed in close proximity to a topological insulator, the quantum interference properties 
of the TI surface states can be tuned quite strongly, and strong correlations can be induced upon proximity of a TI surface 
to a strongly and non-locally entangled electronic species like a strange metal.

Finally, we study the electrical transport properties of Bi₄Se₃, which is expected to be a 3D Dirac semimetal. Previous reports 
have shown it to be a 3-D Dirac semimetal via DFT calculations, but the electrical transport is still lacking. Here, we report 
electrical transport studies of Bi₄Se₃ for the first time. The variation of resistivity with temperature shows a semimetallic nature.
Furthermore, upon applying various strengths of magnetic fields, the Dirac cone doesn’t gap out in bulk, which is indicative of true 
3-D nature of the Dirac fermions. We also observe a clear WAL signal that is best fit using a 3D version of the HLN model, 
supporting that bulk Dirac fermions with strong spin-orbit coupling and a Berry phase of π are present. 
The structure consists of alternating Bi₂Se₃ layers and Bi bilayers, and it belongs in the R-3m space group, 
similar to some layered telluride systems. Also, the variation of the characteristic length scales follow a power 
law decaying behaviour, where the exponent describes dephasing via 3-D electron-electron interactions. All these results 
confirm that Bi₄Se₃ is a 3-D Dirac semimetal.

Together, these results give a more complete picture of how strong SdH oscillations can arise from side surfaces 
of weak TIs, the elusive nature of competition between WAL and WL for the surface states of strong TIs in proximity 
to a strange metal, and nature of quantum diffusion and 3-D WAL in 3D Dirac semimetals. Understanding how these 
topological effects evolve is useful for designing hybrid systems for topological quantum computing and low-dissipation 
electronic/spintronic applications.