First-Principles Study Of Magnetic Topological Material And Topological Superconductor

Topological quantum states have emerged as one of the frontier research fields in condensed matter physics.The interplay with topology,magnetism and superconductivity provides a promising platform for emergent quantum phenomena,which is thus of crucial importance to both fundamen-tal research and practical applications.In magnetic topological materials,the competing mechanism between spin-orbit coupling and magnetic order can be used to manipulate topological states that are advantageous for spintronic devices.The Majorana zero modes in the vortex of topological super-conductors can be used to construct topological qubits,which are the promising material platform for topological quantum computation.In this thesis,based on the first-principles calculations of novel quantum states and materials,we study the properties of magnetic topological states and topological superconductivity.The main research content are summarized as follows:1.The topological properties in checkerboard antiferromagnetic monolayer Fe Se are systemati-cally studied.It is found that monolayer Fe Se contains generous magnetic topological phases:（1）The isolated fragile topological flat band protected by S_4zsymmetry presents just below the Fermi level.（2）The fragile topology gives rise to a second-order topological insulator,which would induce bound states with fractional charge e/2 at the corners of the sample.（3）The system is very close to an anti-ferromagnetic topological insulator that exhibits gapless edge states.These results point out the first S_4z-protected fragile topological material,and well explain the previous experimental observations of topological bound states and edge states in monolayer Fe Se,which provides a new platform to study the intriguing properties of the magnetic fragile topology.2.A computational program that characterizes the superconducting energy spectrum and super-conducting topological invariants from first-principles calculations is introduced.Based on this pro-gram,the topological superconducting phases in superconductor/topological insulator/ferromagnetic insulator heterostructures are studied.Taking the Bi₂Se₃as an example,the competition between mag-netic exchange coupling and superconducting pairing in topological surface states is simulated,and the general rule to get the superconducting Chern number is obtained,which verifies the validity of the program.The computational program can also be extended to study other topological superconduct-ing systems,such as magnetic topological insulator/superconductor heterostructure and superconduc-tor/ferromagnetic insulator heterostructure,which will greatly promote the development of topological superconductors.3.A new strategy to design superconducting topological metal by intercalating the superconduct-ing units into topological insulators is introduced,which enables simultaneous superconducting pairing and topological surface states in a single material.Following this strategy,Pd Bi₂Te₄and Pd Bi₂Te₅are predicted to be experimentally synthesizable and ideal superconducting topological metals.First-principles calculations verified their stability,superconductivity,and topological properties.Further calculated results based on our developed program identify that the chiral topological superconducting phase could be realized by introducing Zeeman effect on the surface of Pd Bi₂Te₄or Pd Bi₂Te₅thin films.Therefore,they are highly possible to be one of the promising candidates for topological super-conductors.