Investigation of the magneto-conductivity of topological materials
Creators: name: Kumar, Yogesh
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 510404
subject: Electronic and magnetic properties of condensed matter; superconductivity

description: Topological materials (TMs) have attracted wide attention from the scientific community due to their unique band structure. Applications exist for these materials in the fields of spintronics, memory devices and quantum computing. Among TMs, topological insulators (TIs) have arguably received the most attention and exhibit both insulating bulk states and conducting surface states (SS). The latter are protected by time-reversal symmetry (TRS). Dirac semimetals (DSMs) is another class that are characterized by the presence of Dirac cones with tilted dispersion relations and have been studied with potential applications in next generation electronic devices foreseen. The transport properties of these materials can reveal various physical parameters, such as charge carrier mobilities, residual resistance, conductivity, magnetoresistance (MR), as well as their temperature and magnetic field dependence. The SS can be probed by performing angle-resolved photoemission spectroscopy (ARPES) and scanning tunnelling microscopy (STM) measurements. However, an indirect way to probe these topological surface states (TSS) is to study the impact of the applied magnetic field and temperature on the magneto-transport properties of TMs using a physical property measuring system (PPMS). 

In this dissertation, measurements and analysis of TIs and DSMs are reported. Single crystals of these materials were grown by a solid-state reaction route using different heat treatments. They were subjected to structural and micro-structural characterization processes to confirm their phase purity, crystal structure, surface morphology and elemental composition. The magneto-transport properties were explored and measurements indicated significant weak antilocalization (WAL) and non-saturating MR. The overall magnetoconductivity (MC) was analysed by modified Hikami-Larkin-Nagaoka (HLN) model by considering the effect of quantum, classical and bulk contributions.

The pristine Bi single crystals exhibited Shubnikov-de Haas (SdH) oscillations confirmed the presence of the Fermi surface and Berry’s phase. Interestingly, a huge MR (17000%) is also observed. Moreover, angle-dependent MR measurements were performed for Bi0.95Sb0.05 and Bi0.95Sb0.05, which confirmed the dominant contribution to the transport mechanism from 2D surface charge carriers. In the subsequent chapters, a similar study was also carried out for both Bi2Se3 and Bi2Te3 TIs single crystals. 

To deeply understand the phenomena driving the transport mechanism in TIs, thin films with different thicknesses of 5 nm, 10 nm, 15 nm, and 20 nm, were deposited using the physical vapour deposition technique. The MC measurements suggests the dominance of SS in all the samples.

Lastly, the Type II DSM PdTe2 was investigated and HLN analysis of the magneto-transport measurements suggested the presence of TSS. The MR was linear and found to be non-saturating with an increase in the field. The Dirac point in these materials lies far below the Fermi level so the transport is dominated by non-relativistic carriers. Although, the MR was comparatively lower, the higher surface electron mobility may be beneficial for the development of future electronics.