Fluctuations in quantum transport in topological insulators

Abstract

Topological insulators (TIs) belong to a new class of materials characterized by linear surface states, which emerge in the bulk band gap due to non-trivial topology of the band structure. These surface states have various interesting properties like spin-momentum locking, protection against time reversal symmetry breaking from non-magnetic impurities, Berry phase, and exhibit a plethora of fundamentally important phenomenon like Majorana Fermions, magneto-electric effect etc. TIs are also promising candidates for several applications such as quantum computation, low power electronics, spintronics, and photo-detectors. Quantum interference in these systems also leads to weak anti-localization because of Berry phase. However, despite extensive transport and spectroscopic investigation in these systems, the understanding of conductance fluctuations in the quantum transport regime is still in its incipient stages, and the sensitivity of these surface states to different dephasing mechanisms is still a matter of debate. Understanding of these phenomena are imperative for applications of TIs in various quantum technologies. Opto-electronic response in these materials also has been hard to achieve, due to phonon mediated inter and intra-band transitions, which lead to a high recombination rate of the photo-generated charge carriers. The major goal of this thesis has been to explore the role of quantum interference, and external potential fluctuations in topologically protected materials, and also enhancing the opto-electronic sensitivity of these materials by forming binary hybrids. The thesis can be broadly divided into two parts. In the first part, we have probed the fundamental aspects of these materials by performing time and gate voltage-dependent conductance fluctuations, and magnetoresistance. Our investigation of time-dependent conductance fluctuations proves that, even in thin TIs, the intrinsic bulk defects are the dominant source of noise, unlike other two-dimensional materials, where the noise arises due to trapping-detrapping at the channel-oxide interface, or at the contacts. Conductance fluctuations, measured by xi scanning the Fermi energy, provide an unambiguous proof of the intrinsic preservation of time reversal symmetry in topological insulators. The magnetic-field dependence of the fluctuations clearly indicate that they originate from universal conductance fluctuations in mesoscopic samples, and also shows a transition from the ground state symplectic to unitary symmetry class. Quantum transport measurements in a variety of device configurations, in both exfoliated and epitaxially grown samples, have revealed the saturation of phase coherence length at low temperature, which may originate due to electron-magnetic fluctuations in the bulk of the TIs. In the second part, we focus on opto-electronic investigations, and demonstrate that TIs, when integrated with graphene, can lead to extremely high detectivity, and can be used as sensors in the near infra-red regime with a resolution of 10 photons.