Single-hole Dynamics And Superconducting Pairing Symmetry In Doped Spin-orbit Mott Insulator

Since the discovery of high-Tc superconductors,the study of doped Mott-insulators has been a central subject in the area of strongly correlated electron systems.Mott physics based on the Hubbard model,is at the root of various noble physical phenomena,like metal-insulator transition,magnetic spin order,high-Tc superconduc-tivity,colossal magnetoresistance and quantum criticality.In the past few decades,Mott physics has been used to explain electric and magnetic properties of various materi-als and great success has been achieved in 3d transition-metal oxides.Since 4d and 5d orbital electrons are less localized than 3d electrons,4d and 5d transition-metal materials were thought to be weak correlated metals with broad bands.However in recent experiments,4d material RuC13 and 5d material Na2IrO3 are found to be Mott insulators with strong spin-orbital coupling.The cooperation of spin-orbital coupling,crystal fields,and electronic correlation can induce many novel electronic and magnetic ground states in partially filled 4d.and 5d transition-metal oxides.Especially,the inter-play of these ingredients could lead to the so-called spin-orbital assisted Mott insulator,in which the electrons are described in terms of a half-filled Jeff=1/2 narrow band so that a small Hubbard interaction U is sufficient to open a Mott gap.Bond-related exchange interactions are found and the Kitaev interactions that underlying the cele-brated Kitaev honeycomb model can be realized in such insulators on the honeycomb lattice.Such materials are excellent candidate materials to search for Kitaev spin liq-uid.The interplay between spin exitations and charge dynamics is important in the theoretical understanding of the physics in these materials.In this thesis we investi-gate the dynamics of a single hole in various possible magnetically ordered phases of Heisenberg-Kitaev model.Another major subject in studying superconductors is the superconducting pairing symmetry.To investigate various particle-hole excitations and possible superconducting pairing symmetry in the doped systems,we ignore the effects of the eg orbitals and use the random-phase approximation.Our results show that the Jcff = 1/2 picture is robust in the doped systems,and the d-wave pairing is the most favourable superconducting pairing symmetry.This thesis is organized as follows:(1)We first introduce the properties of Mott insulators and present a review of the history of high-Tc superconductors.Then we introduce the spin-orbit assisted Mot-t insulators and its low-energy effective physics can be described by a pseudo-spin Jeff = 1/2 model.We demonstrate how to implement the Kitaev model in Mott insu-lators.Next we have a simple review of early works on the charge dynamics in doped Mott insulators.At last we introduce two methods we will use in this thsis,the self-consistent Born approximation and random phase approximation method.(2)Motivated by the progress in research of the 4d and 5d transition-metal com-pounds and the possible applications of the Heisenberg-Kitaev model in these materials,we investigate the dynamics of a single hole in various magnetically ordered phases of this model.We calculate the single particle spectrum by the self-consistent Born ap-proximation.Though the spectra are dominated by large incoherent spectral weight,there are low-energy coherent quasiparticle excitations in all of these phases.Inter-estingly,in the zigzag phase relevant to recent experiments,though the quasiparticle weights are largely suppressed in the physical spectra in the first Brillouin zone,we find that it recovers in the extended Brillouin zones.Moreover,our results reveal that the low-energy quasiparticle spectra are reduced with the increase of the Kitaev inter-action K.(3)We first introduce the band structure of a-RuCl3.Then we introduce two ex-periments,inelastic neutron scattering experiment and angle-resolved photoemission spectrum experiment of α-RuCl3.We derive a five-band model and the simplified three-band model.Considering the spin-orbital coupling and correlation effects in the three-band model,we investigate various particle-hole excitations and possible super-conducting pairing symmetry in the doped systems using the random-phase approxi-mation.We exhibit the static susceptibilities for different electron concentrations.By solving the gap function,we find that the d-wave pairing is the most favourable super-conducting pairing symmetry.In this thesis,we have investigated the single-hole dynamics and superconducting pairing symmetry in doped spin-orbit Mott insulator.We hope that our theoretical work may be used to explain some experimental phenomena,or bring some inspiration to future experimental research.