First-Principles Design Of Superconductive And Multiferroic Two-Dimensional Materials

Graphene has attracted enormous interests since it was successfully exfoliated from the bulk crystal.Graphene is a single-atom-thick carbon film that exhibits various unprecedented properties,such as ultrahigh carrier mobility at room temperature,quantum hall effect,large theoretical specific surface area,excellent optical transparency,high Young's modulus,and excellent thermal conductivity.The outstanding properties of Graphene inspire researchers'great enthusiasm.Then it also raised the question whether superconductivity would exist in those 2D sheets.Following this way,searching for exotic 2D superconductors with higher superconducting transition temperature?T_C?is highly desired,not only for revealing interesting physics but also for potential electronic applications.On the other hand,2D materials with ferromagnetic/ferroelectric/multiferroic properties have attracted great interests,promising more functional choices for devices based on 2D materials.While most reports are theoretical analysis,a few experimental confirmations are also available for some systems.Until now,however,predicted 2D multiferroics are with coexisting yet only loosely coupled ferroelectricity and magnetism?so called type-I?.In this thesis,the DFT calculations are employed to study the superconductivity and multiferroic properties in 2D materials.The main conclusions are summarized as following,1)Superconductivity of 2D materials with metal intercalation.The electronic structure and lattice dynamics of 2D materials with monolayer metal intercalated have been calculated via first-principles density functional theory and density functional perturbation theory.According to the electron-phonon interaction,it is predicted that these 2D materials can be transformed from semiconductor to superconductor by metal intercalation.More interestingly,the biaxial tensile strain can significantly enhance the superconducting temperature up to 10K in Na-intercalated MoS₂.We also predicted the superconductivity in blue phosphorus bilayer with high translation temperature.In addition,the phonon mean free path at room temperature is also greatly improved in Na-intercalated MoSe₂,which is advantageous for related applications.2)Superconductivity of monolayer Mo₂C:the key role of functional groups.Monolayer Mo₂C is a new member of two-dimensional materials.Here the electronic structure and lattice dynamics of monolayer Mo₂C are calculated.According to the electron-phonon interaction,it is predicted that monolayer Mo₂C could be a quasi-two-dimensional superconductor and the effects of functional-groups are crucially important considering its unsaturated surface.Despite the suppressed superconductivity by chalcogen adsorption,our most interesting prediction is that the electron-phonon interaction of monolayer Mo₂C can be greatly enhanced by bromine absorption,suggesting that Mo₂CBr₂ is a good candidate for a nanoscale superconductor.3)Type-II multiferroic Hf₂VC₂F₂ MXene monolayer with high transition temperature.Achieving multiferroic two-dimensional?2D?materials enable numerous functionalities in nanoscale devices.Until now,however,predicted 2D multiferroics are very few and with coexisting yet only loosely coupled?type-I?ferroelectricity and magnetism.Here,a type-II multiferroic MXene Hf₂VC₂F₂ monolayer is identified,where ferroelectricity directly originates from its magnetism.The noncollinear 120°Y-type spin order generates a polarization,which is perpendicular to the spin helical plane.Remarkably,the multiferroic transition is estimated to occur above room temperature.Our investigation will open the door to a new branch of 2D materials in the pursuit of intrinsically strong magnetoelectricity.