Magnetic and Electronic Properties of Topological Superconductors and 2D Magnetic Materials

Abstract

Quantum materials that present strong electronic correlations and non-trivial topological orders are one focus of current research. The surge of interests in quantum materials is driven not only by their potential applications in disruptive quantum technologies but also by their own 'exotic' nature of physics. In this thesis, we report the study of superconducting and magnetic properties of two representative novel quantum materials: topological superconductors and two-dimensional (2D) magnets. In2Bi is a topological non-symmorphic superconductor, the non-trivial topology of which is derived from the non-symmorphic crystalline symmetries. In the superconducting phase of In2Bi, the topological surface states are proximity-coupled to the superconducting bulk and exhibit a Meissner-like screening to the ac field. The topological surface diamagnetic response was found robust against dc fields and disappeared as a new phase transition below the critical field (Hc3) of standard surface superconductivity. For another layered Z2 topological superconductor beta-PdBi2, an anomalously reversible and linear magnetisation was observed over a large field range within the superconducting phase. The ac susceptibility exhibits a large diamagnetic screening until a field just below Hc2, and sees an abrupt, first-order-like transition to the normal state. In the tunnelling spectra, the superconducting gap of beta-PdBi2 was found to exhibit an unusual two-trend temperature and parallel-field dependence. The anomalies seen in both magnetic and tunnelling measurements point out a possible 'bulk-surface' composite-phase in beta-PdBi2. To facilitate the study of magnetism in 2D materials, we employed a ballistic graphene Hall magnetometry, which enabled us to directly measure the magnetic field and domain wall structures of atomically thin chromium tribromide (CrBr3), a prototype 2D magnetic material used in this work. Out-of-plane ferromagnetism and a weak layer dependence of Curie temperature were found in CrBr3, suggesting its inherent 2D magnetism. As an application extension of 2D magnetic materials, we synthesised water suspensions of 2D cobalt-doped titanium oxide (CTO). The remarkable magnetic anisotropy and optical birefringence of the suspended CTO crystals endows a giant magneto-birefringence effect, which enables an excellent field-tuneable colouration, a goal sought-after for many years.

Date of Award	31 Dec 2020
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Andre Geim (Supervisor) & Irina Grigorieva (Supervisor)