Study Of Electronic,magnetic,and Superconducting Properties Of Nano Materials Based On Boron And Silicon

From the experimentally successful realization of graphene in 2004,lowdimensional nanomaterials have aroused broad interests in the field of condensed matter physics and nanoscale science and technology.Following closely,the study of graphenelike materials such as silicene,germanene,phosphorene,borophene,monolayer boroncarbon,and monolayer boron-nitrogen etc.also drew researchers' attentions quickly.Experimentally,to realize various kinds of functions the electronic properties of these materials can be tuned by adatoms,tailoring,substitution doping etc.In this thesis,we have investigated the electronic,magnetic,and superconducting properties of the nanomaterials based on boron and silicon by density-functional calculations.Our results show that the adsorption energy of alkaline and alkaline earth atoms on BC₃ sheet is larger than the cohesive energy of the metal atoms themselves,and the adsorption systems are all nonmagnetic.Under a suitable external electric field,a considerable magnetism can be induced in these adsorption systems.Meanwhile,accompanying with the emergence of magnetism the electric dipole moments of the systems change suddenly.As a result,the alkaline and alkaline earth adatoms decorated BC₃ can be taken as potential materials to design magnetoelectric effect and switch devices.In addition,the Na inserted AA-staked BC₃ bilayer shows the properties of semimetal with Dirac-like fermions,and holds an excellent thermodynamic and kinetic stability.We have studied the vibrational and superconducting properties of the 2D boron allotropes which were searched by evolutionary algorithm combined with first-principles calculations.All the newly found 2D boron structures are bilayer or multilayers,and more stable than the so-called ?-sheet.Due to the hybridization of the ? and ? electrons on fermi surface,the newly found 2D structures show metallic features.Remarkably,conventional BCS superconductivity in the stable 2D boron structures is ubiquitous with the critical temperature above the liquid hydrogen temperature for certain configurations.Thus,2D boron structure may be a pure single-element material with the highest critical temperature on conditions without high pressure and external strain.At last,we have studied the electronic properties of the new types of silicon nanoribbons.Different from the cases of graphene nanoribbons?GNRs?,we found that the Christmas-tree silicene nanoribbons?CSiNRs?and tree-saw silicene nanoribbons?TSiNRs?are more stable than the zigzag silicene nanoribbons?ZSiNRs?.These new silicene nanoribbons show spin-semiconducting features with the ferromagnetic ground state of 2 ?B/cell distributed on both edges of ribbons.Under suitable external electric field or compressive strain,spin gapless semiconducting properties can be achieved in these new structures.Besides the spin polarization in energy space,the charge distributions of the edge states also separate in real space.These results show that CSiNRs and TSiNRs have potential applications in spintronics.