Study of surface to bulk coupling and Coulomb interaction in Topological Insulator thin films

Singh, Sourabh (2018) Study of surface to bulk coupling and Coulomb interaction in Topological Insulator thin films. PhD thesis, Indian Institute of Science Education and Research Kolkata.

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Abstract

The quest for technological advances is intrinsically connected to the discovery of novel materials with interesting properties. The advent of a new class of material known as Topological Insulators (TI) ushered a new promise in this endeavor. The newest member in the insulator family was theoretically predicted and experimentally observed and subsequently confirmed by ARPES in the last decade. TI hosts a set of states on the surface connecting the conduction and valence band. Thus making it an unconventional class of material which conducts only on the surface. These surface states are helical spin polarized and protected by Time reversal invariance symmetry. A linear Dirac dispersion relation classifies these surface states spanning the bulk bands. The non-trivial topology of the bulk gives rises to a fascinating set of surface states. These unique properties make them an ideal candidate for providing a platform for the construction of low power spintronic devices and fault tolerant quantum computation. One of the biggest obstacles in making topological materials readily available for technological purposes is the unwanted contribution and participation of the trivial bulk carriers in transport mechanism. This thesis has a twofold motivation: one to obtain better bulk insulating TI samples and the other to understand the quantum transport behavior in the low temperature regime specifically the intricate relationship between the surface and bulk states. A series of Bismuth based bulk insulating TI thin films have been prepared using Pulsed Laser Deposition (PLD) technique and their magnetotransport properties have been studied. Chapter 1 provides a brief introduction on TIs. In chapter 2 the thin film growth techniques, basic characterization tools and measurement system has been discussed. The chapter begins with a discussion on PLD. Unique deposition techniques such as two-step deposition and shadow mask has been incorporated to prepare best quality thin films. Sample fabrication is followed by a brief review on characterization tools and measurement set-up. Second generation TI materials such as Bi2Se3 (BS) and Bi2Te (BT), inherit defects in the bulk resulting in undesirable contribution to conduction. This undesirable bulk contribution in turn masks the surface conduction. It is essential to quench the bulk conductance which is achieved by simultaneous reduction of the defect density and compensation of the defect sites. Chapter 3 covers the transport and magnetoresistance (MR) results of BiSbTeSe1.6 (BSTS) thin films. The bulk insulating behavior of these BSTS thin films is manifested in the resistance vs. temperature (R-T) behavior. In the perpendicular field orientation the low field MR exhibits a logarithmic cusp around the zero magnetic field. This phenomenon is known as Weak Antilocalization (WAL) and is one of the key features of surface states in TI. Standard Hikami Larkin Nagaoka (HLN) equation was used to extract the two important parameters phase coherence length and ‘α’ a universal number which counts the number of independent coherent conducting channels participating in the conduction process. In the case of a disordered two-dimensional system the presence of both localization and electron-electron interaction (EEI) results in a logarithmic anomaly correction to the Drude conductivity. WAL incorporates a positive correction to the conductance while EEI lowers the conductance due to the depletion of density of states. BSTS thin films of varying thicknesses are systematically studied to understand the role of Coulombic interactions in dictating the ground state of our samples. EEI and WAL effects play essential role in understanding the electron conduction in TI thin films. At low temperatures and in the absence of magnetic field both effects exhibit a perceivable temperature dependent correction in the classical resistance. Parallel MR exhibits a logarithmic field dependence for higher fields (> 4T), which is consistent with Lee and Ramakrishnan theory. The parallel field MR is due to the presence of EEI and Zeeman splitting. The contribution of EEI to the transport properties were investigated and the Coulomb screening parameter ‘F’ was determined from magnetic field dependent R-T measurements, parallel field MR measurements and Hall measurements. The screening parameter characterizes the strength of the Coulomb interaction between the carriers due to the diffusive nature of the system. Theoretically the value of F should vary from 0-1 but a negative F value obtained from all three measurement techniques signifies the role of spin-orbit coupled carriers of the surface states. Chapter 4 deals with the concept of surface to bulk coupling in TI thin films. It is an imperative for an ideal TI to have a minimum bulk contribution in the conduction process. Practical applications also demand that the communication between the surface and the bulk states should be absent. The bulk-boundary correspondence intrinsically connects the bulk and the surface states. The extent of surface to bulk coupling drastically changes the properties of TI materials. R-T curve depicts a new understanding in this regard as the resistance model varies from series to parallel. We investigate the surface to bulk coupling in a set of Bi2Se2Te (BST) thin films by careful analysis of the R-T behavior and magnetoconductance measurements performed in the perpendicular magnetic field orientation. A coherent and comprehensive investigation on surface to bulk coupling is achieved by tuning the temperature of the sample and performing perpendicular MR measurements. The variation of α with temperature is used to explain the nature of surface to bulk coupling and ways to control it. Temperature dependent phase coherence length exhibits an interesting cross-over from electron-phonon to electron-electron scattering dominated dephasing mechanism. The subject matter of Chapter 5 is finding a new tool to quantify the surface to bulk coupling in TI thin films. Thin films of Bi2Se2Te (BST) and BiSbTeSe1.6 (BSTS) were prepared by PLD technique and then subjected to MR measurements. A linear and non-saturating magnetoresistance (LMR) was observed for both BST and BSTS thin films in the high field regime. Currently a great deal of attention is being paid to the large and non-saturating LMR of TIs. This is motivated by potential applications in magnetic recording and by the challenge of understanding the physical origin of the magnetoresistance, which is very different from that of conventional metals. Systematic temperature dependent MR studies provide deeper insights into the origin of LMR and the role of surface to bulk coupling. The existing models (Abrikosov’s quantum model & Parish-Littlewood classical model) failed to capture the essence of LMR in our samples. The presence of both the bulk and surface states thus motivated us to comprehend that this LMR is due to the competition between these two states. A modified Hikami Larkin Nagaoka equation was used to fit the data and the excellent agreement between the theory and experiment thus justifies this endeavor of ours. The cross-over field is the field at which the MR changes its nature from logarithmic to linear. We argue that the value of cross over field is a measure of the extent of surface to bulk coupling in a sample. To the best of our knowledge this is the first time such a measure of surface to bulk coupling that has been reported in literature. It is essential to understand the role of surface to bulk coupling and the factors which control it. The issue of surface to bulk coupling is not yet settled and is an important feature that needs to be taken into account when one is planning to fabricate devices. Chapter 6 presents a slight digression from the earlier chapters as we explore the properties of bulk in compensated TI thin films in this chapter. A complete understanding of the surface to bulk coupling is incomplete unless we understand the properties of the bulk in TI thin films. BiSbTeSe1.6 (BSTS) thin films were prepared on Si (100) substrates and by varying the growth parameters epitaxial polycrystalline bulk insulating thin films were obtained. A series of resistance vs temperature and magnetotransport measurements were performed on these BSTS thin films. This chapter focusses on the transport mechanism of the bulk. Compensation doping which leads to bulk insulating nature of the thin films also brings few new features in the bulk properties. Formation of charge puddles results in Efros-Shklovskii (ES) hopping type transport behavior in the bulk at the intermediate temperature regime (60 K – 150 K). It also explains the small activation energy value as compared to an ideal compensated TI where its value should be ~ 150 meV. The last chapter ends with summarizing the results of this thesis and possible directions of future study in these TI samples.

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