BN纳米管中硼空位诱导铁磁性可能性的从头算研究外文翻译资料

本科毕业设计（论文）

外文翻译

Ab Initio Investigation about the Possibility of Ferromag- netism Induced by Boron Vacancy in BN Nanotubes

外文正文：

We study structural deformation, electronic states, and intrinsic magnetism induced by the cation vacancy(VB) in boron nitride nanotubes (BNNTs), in compa- rison with the BN sheet, using spin-polarized density functional theory. Two types of vacancy configurations are observed in tubes, depending on the nature of the vacancy and the local stress. The underlying formation mechanisms are discussed from the viewpoint of the electronic and geometrical (or stress-induced) effects. Under addi- tional stress, the VB prefers the open configuration to the closed 5-1DB confi- gura- tion, showing different properties and potential applications. Due to the strong localiz- ation, the magnetic interaction between the VB-ind- uced moments is short- range along either the axis or the circumference, meaning the ferromagnetism is difficult to be present. The simulation of negative charge injection implies VB- defective BNNTs might be promising candidates for spin-transport devices.In addition, the effects of surro- unding H and F atoms on the spin-polarized states and magnetism of defective BNNTs are also explored.

I.Introduction

In recent years, the intrinsic magnetism induced by native defects in sp-electron systems has attracted increasing attention for potential applications in spintronics,because the conventionally magnetic impur- ity doping can be completely skipped in the fabrication procedure. It was theoretically shown that cation vacancies in bulk wide-gap nitrides result in the appearance of local moments, and may induce the ferrom- agnetism in some cases.In contrast to the bulk, low-dimensional nano structures (e.g., nanotube and nanoribbon) are more appropriate candidates for microelectronic devices, catering to the ongoing miniatu- rization. Boron nitride nanotubes (BNNTs) have high thermal and chemical stabilities and thus are expected to be used as high-temperature nanod- evices or protective shields of nanodevices in specific circumstances.

It is meaningful to study what kind of conditions can induce ferromagnetism of BNNTs all the while. Similar to dopants5 or adatoms,6,7 the vacancies in BNNTs, native8 or created by electron irradiation,9 can also generate new spin-polarized states in the gap10 and induce local moments.8 This is not unexpected because any localized sp states in solids have the possibility to form local moments in essence.5-7,11,12 This, however, does not ensure the existence of ferromagnetism in systems, which is crucial for spintronic devices.7 To the best of our knowledge,this critical issue whether local moments induced by the defect states can lead to a collective magnetism by the long-range coupling, as well as its implications for practical spintronic applications, was not basically involved in previous reports on vacancy-induced intrinsic ferromagnetism of BNNTs. Consequently it is not explicitly addressed whether BNNTs containing vacancies or what kind of defective BNNTs could be used as nanospintronic devices.

Noting the theoretical prediction of cation vacancies-induced ferromagnetism in wide-gap nitrides3 and experimental observations of B monovacancies in the BN sheet,13 in this article, we studied the dependencies of geometry and stability of the cation vacancy (VB), as well as the spin-polarized electronic structure, in BNNTs on ambient conditions using spin-polarized density functional theory. First, we confirmed that the vacancy configuration, as well as the spin-polarized states, is related to the nature of the vacancy and the local stress. Successively, we discussed the formation mechanisms of different configurations from the interaction between the nearest-neighbor (NN) atoms of the vacancy (i.e., electronic effects) and the induced stress distribution (i.e., geometrical effects). In particular, we focused on the open configuration of the VB, including formation or existence conditions, spin-polarized electronic structures, chemical activity and magnetism, and application prospects. Finally, for applications in spintronics, we proposed to inject negative charges into such sp-electron systems and studied the properties.

II.Calculation Method and Model

Since most of grown BNNTs prefer a zigzag orientation,14 here we focused on zigzag BNNTs [i.e., the (10,0) tube was considered]. All calculations were performed using the Vienna ab initio simulation package (VASP)15 within the framework of spin-polarized density functional theory (DFT). The generalized gradient approxim- ation of PBE exchange correlation functional16 and the projector augmented wave potentials describing the electron-ion interaction17 were employed. A cutoff energy of 400 eV was used for the plane-wave basis set, and further increase in cutoff showed little difference in results. A tetragonal 1 times; 1 times; 3 supercell was adopted, with a vacuum region of 14 Aring; between tubes. Integration over the Brillouin zone was done using the Monkhorst-Pack scheme18 with 1 times;1 times; 3 k-points for structural optimization and 1 times; 1 times; 7 k-points for electronic structure calculations. The vacancy was created by the removal of a B or N atom from the tube. The NN atoms of the vacancy were defined as 1, 2, and 3 as shown by imag.(a)in Figure 1. All atoms were fully relaxed until the Hellmann-Feynman force on each atom was less than 10 meV/Aring;. Furthermore, the climbing image nudged elastic band (NEB)method19 was used to calculate the activation barrier for structural transition.

Figure 1. Minimum-energy path (MEP) of the trans

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本科毕业设计（论文）

外文翻译

学 院：理学院

专 业：应用物理学

班 级：应物181

学 　 号：2018210216028

学生姓名：王志雄

指导教师：杨旭昕

二○一九年六月

Ab Initio Investigation about the Possibility of Ferromag- netism Induced by Boron Vacancy in BN Nanotubes

外文正文：

We study structural deformation, electronic states, and intrinsic magnetism induced by the cation vacancy(VB) in boron nitride nanotubes (BNNTs), in compa- rison with the BN sheet, using spin-polarized density functional theory. Two types of vacancy configurations are observed in tubes, depending on the nature of the vacancy and the local stress. The underlying formation mechanisms are discussed from the viewpoint of the electronic and geometrical (or stress-induced) effects. Under addi- tional stress, the VB prefers the open configuration to the closed 5-1DB confi- gura- tion, showing different properties and potential applications. Due to the strong localiz- ation, the magnetic interaction between the VB-ind- uced moments is short- range along either the axis or the circumference, meaning the ferromagnetism is difficult to be present. The simulation of negative charge injection implies VB- defective BNNTs might be promising candidates for spin-transport devices.In addition, the effects of surro- unding H and F atoms on the spin-polarized states and magnetism of defective BNNTs are also explored.

I.Introduction

In recent years, the intrinsic magnetism induced by native defects in sp-electron systems has attracted increasing attention for potential applications in spintronics,because the conventionally magnetic impur- ity doping can be completely skipped in the fabrication procedure. It was theoretically shown that cation vacancies in bulk wide-gap nitrides result in the appearance of local moments, and may induce the ferrom- agnetism in some cases.In contrast to the bulk, low-dimensional nano structures (e.g., nanotube and nanoribbon) are more appropriate candidates for microelectronic devices, catering to the ongoing miniatu- rization. Boron nitride nanotubes (BNNTs) have high thermal and chemical stabilities and thus are expected to be used as high-temperature nanod- evices or protective shields of nanodevices in specific circumstances.

It is meaningful to study what kind of conditions can induce ferromagnetism of BNNTs all the while. Similar to dopants5 or adatoms,6,7 the vacancies in BNNTs, native8 or created by electron irradiation,9 can also generate new spin-polarized states in the gap10 and induce local moments.8 This is not unexpected because any localized sp states in solids have the possibility to form local moments in essence.5-7,11,12 This, however, does not ensure the existence of ferromagnetism in systems, which is crucial for spintronic devices.7 To the best of our knowledge,this critical issue whether local moments induced by the defect states can lead to a collective magnetism by the long-range coupling, as well as its implications for practical spintronic applications, was not basically involved in previous reports on vacancy-induced intrinsic ferromagnetism of BNNTs. Consequently it is not explicitly addressed whether BNNTs containing vacancies or what kind of defective BNNTs could be used as nanospintronic devices.

Noting the theoretical prediction of cation vacancies-induced ferromagnetism in wide-gap nitrides3 and experimental observations of B monovacancies in the BN sheet,13 in this article, we studied the dependencies of geometry and stability of the cation vacancy (VB), as well as the spin-polarized electronic structure, in BNNTs on ambient conditions using spin-polarized density functional theory. First, we confirmed that the vacancy configuration, as well as the spin-polarized states, is related to the nature of the vacancy and the local stress. Successively, we discussed the formation mechanisms of different configurations from the interaction between the nearest-neighbor (NN) atoms of the vacancy (i.e., electronic effects) and the induced stress distribution (i.e., geometrical effects). In particular, we focused on the open configuration of the VB, including formation or existence conditions, spin-polarized electronic structures, chemical activity and magnetism, and application prospects. Finally, for applications in spintronics, we proposed to inject negative charges into such sp-electron systems and studied the properties.

II.Calculation Method and Model

Since most of grown BNNTs prefer a zigzag orientation,14 here we focused on zigzag BNNTs [i.e., the (10,0) tube was considered]. All calculations were performed using the Vienna ab initio simulation package (VASP)15 within the framework of spin-polarized density functional theory (DFT). The generalized gradient approxim- ation of PBE exchange correlation functional16 and the projector augmented wave potentials describing the electron-ion interaction17 were employed. A cutoff energy of 400 eV was used for the plane-wave basis set, and further increase in cutoff showed little difference in results. A tetragonal 1 times; 1 times; 3 supercell was adopted, with a vacuum region of 14 Aring; between tubes. Integration over the Brillouin zone was done using the Monkhorst-Pack scheme18 with 1 times;1 times; 3 k-points for structural optimization and 1 times; 1 times; 7 k-points for electronic structure calculations. The vacancy was created by the removal of a B or N atom from the tube. The NN atoms of the vacancy were defined as 1, 2, and 3 as shown by imag.(a)in Figure 1. All atoms were fully relaxed until the Hellmann-Feynman force on each atom was less than 10 meV/Aring;. Furthermore, the climbing image nudged elastic band (NEB)method19 was used to calculate the activation barrier for structural transition.

Figure 1. Minimum-energy path (MEP) of the trans

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