Modelling of quantum phase transitions in Dirac materials.

Abstract

In this thesis, first-principles computations of the electronic ground state are used to investigate the underlying nature of the quantum phase transitions in selected Dirac materials and their associated hybrid materials. Various methods are used to break the symmetry of the ground state electronic structure and to tune the emergent phases. These involve the application of an external electric, magnetic or electromagnetic fields, and the manipulation of the internal intrinsic fields of materials, such as the introduction of strong spin orbit coupling, electron (or hole) doping, including a strong short-ranged disorder potential. The findings show that applying a perpendicular electric or magnetic field with a staggered potential in the underlying lattice allows for a tunable electronic transition between trivial and nontrivial quantum states. Signatures of the near-field electrodynamics of carriers in nanoclusters reveal the appearance of a quantum fluid phase at the distinct energies where topological quantum phase transitions occur. Emergence of the field-induced carrier density wave phase shows that the collective excitation mode is a distinct potential for carrier transmission in spintronic, optoelectronic, and photonic technologies. Furthermore, two types of insulating Dirac material are used as the tunnel barrier region of the perpendicular tunnel junction architecture. The resulting heterostructure is an artificially assembled metal-insulator-metal multilayer, and this serves as a generic platform for characterizing spin transport properties in spintronic devices. The dependence of the emergent phenomenon of proximity induced magneto-electronic coupling on the tunnel barrier material is unraveled. By analyzing the effect of the changes in the electronic structure on the spin transmission properties, it is found that the metal-insulator-metal platform exhibits a quantum phase transition by responding sensitively to both the tunnel barrier material and the applied perpendicular electric field during in-service operation of a spintronic device. The results show that when the electric field approaches its critical amplitude, the spin density of states exhibits a discontinuous change from half-metallic to metallic transport character in the presence of monolayer hexagonal boron nitride as a tunnel barrier material, contrary to when the monolayer molybdenum disulphide is inserted in the tunnel barrier region. The role of the applied electric field in the phase transition is characterized in terms of a spin-flip transition and an induced interfacial charge transfer. It is also found that the abrupt discontinuity in the changes in the spin-flip energy with increase in applied electric field provides the necessary and sufficient evidence of a first-order quantum phase transition in the spin transport phase. These findings show that the material of the tunnel barrier layer creates a non-trivial function in defining the magnetoelectric couplings that occur dynamically during spin tunneling.