Investigation of quantum interference and quantum oscillations in 3D topological insulators

Abstract

The investigations on the topological phase of the matter first started with the experimental discovery of the quantum Hall effect (QHE) back in 1980. The experimental discovery of the quantum spin Hall effect (QSHE) was accomplished only in 2007 by Molenkamp's group in the HgTe/CdTe heterostructure, which was the discovery of a two-dimensional topological insulator (2D TI). The 2D TI concept was then generalized to the 3D, and the experimental verification of 3D TI was done a few years later in well-known thermo-electric material Bi2Se3. Here, the topological surface states were clearly observed in angle-resolved photoemission spectroscopy (ARPES) measurements. It has been about a decade since the field has started expanding rapidly, and numerous types of new topological phases are being discovered over the years. Nobel prize was awarded in 2016 for the discovery of topological phases. One of the exotic properties of a topological insulator is the spin momentum locking of the surface electrons. Due to this fact, the surface electrons become spin-polarized and don't mix. The surface electrons are backscattering prohibited, and thus TIs are very much lucrative for dissipationless electronic applications. The most critical application that has been speculated is that it can host the Majorana Fermion at the TI-superconductor interface, which is an elusive particle that is an antiparticle of its own. Majorana Fermion is believed to bring the revolution in the field of quantum computation. Thus the excitement for the research on the intriguing topological materials is still on the rise.

This thesis deals with the electrical transport measurements on thin films grown by pulsed laser deposition (PLD) and on nanoflakes exfoliated from four different single crystals of topological materials.

The thesis is organized as follows :

In chapter 1, we have provided the introduction of the quantum Hall effect which is the commencement of topology in experimental condensed matter physics, and the basic description of topological insulators. Then few classifications of TI relevant to our work and their properties are provided. We describe quantum interference and quantum oscillations' fundamental physics, which we have investigated and analyzed throughout the thesis.

In chapter 2, all the experimental techniques have been described starting from the thin films' growth by pulsed laser deposition (PLD) and the single crystals' growth by the modified Bridgman technique. After that, sample characterization methods and the device fabrications processes consisting of mechanical exfoliation from the bulk single crystal, electron beam lithography technique, metallization and packaging has been elaborately discussed. Next, we have provided the schematics of the cryostat where the samples have been measured.

Chapter 3 shows our first work, which is the magnetotransport measurement on the thin films of TI sample grown by PLD. The samples studied are Bi1Te1, and this material has been established to be a Dual TI that is a weak TI and a TCI at the same time. We have characterized the films and performed a thickness-dependent magnetotransport study. We concluded that with thickness, the electron-phonon coupling is emerging stronger in our samples.

In chapter 4, we have studied thoroughly the quantum oscillations, namely Shubnikov- de Haas oscillations (SdH) in nanoflake devices made from the single crystals Sb2Te2Se and Sn doped Sb2Te2Se. Analyzing the SdH oscillations we could extract the non-trivial value of Berry phase both for doped and undoped samples. Thus, we concluded that topological surface states (TSS) are robust against the impurity doping. We also derived other vital parameters like effective mass, Fermi wave vector, Fermi velocity, carrier concentration, mobility, Landau level broadening etc.

Chapter 5 probed the weak antilocalization (WAL) and universal conductance fluctuation (UCF) phenomena, which are the manifestations of quantum interference on the nanoflakes exfoliated from the strong 3D TI Bi1Sb1Te1.5Se1.5 single crystal. We then extracted the phase coherence length (Lφ) from WAL via Hikami-Larkin-Nagaoka fitting and from UCF via the autocorrelation function plot. The dependence of the Lφ on temperature shows surprising slow dephasing in our samples. Another surprise we found is that the RMS value of the conductance fluctuations' amplitude is higher than the universal value. We suggest the dephasing due to the presence of electron-hole charge puddles.

In chapter 6, we have discussed the conclusions that we have made from the studies, and then we have described the possible future works that can be done as an extension of the research, as presented in this thesis.

In the appendix, a 3D Dirac semimetal ZrTe5.1 grown by chemical vapor transport has been studied. The magnetoresistance has been shown to be as high as 1100 % at 2 K, and Lifshitz transition is also observed in the resistance vs. temperature plot. We have also demonstrated the thickness-dependent Raman study on exfoliated nanoflakes. This work is to be further investigated by parallel magnetic field measurements, Hall measurements, etc.