二氧化铈基稀磁氧化物的第一性原理研究

基于第一性原理的计算方法研究了纯CeO_2、Co掺杂CeO_2和同时引入氧空位Vo和Co掺杂的CeO_2稀磁半导体体系.通过计算体系的能带结构和态密度,探讨了该体系磁性产生的机制.计算发现,纯CeO_2体系不具有磁性;没有氧空位Vo的Co掺杂CeO_2体系中,Co离子之间通过O原子发生超交换反铁磁耦合,体系无铁磁性;当氧空位Vo和Co离子同时存在于CeO_2体系中时,Co离子之间通过氧空位Vo发生铁磁耦合,该体系表现出铁磁性能.另外,由氧空位Vo诱导的Co离子之间的铁磁耦合不仅发生在紧邻的两个Co离子,而且可以扩展到几个原子距离的长度.计算结果证明了氧空位Vo诱导铁磁性耦合机制.本文工作将为CeO_2基稀磁半导体体系制备与磁学性质的研究提供支持.     

First-principles study on the magnetism in ZnS-based diluted magnetic semiconductors

The electronic and magnetic properties of wurtzite ZnS semiconductor doped with transition metal (Cr, Mn, Fe, Co, and Ni) atoms are studied by using the first-principle’s method in this paper. The ZnS bulk materials doped with Cr, Fe, and Ni are determined to be half-metallic, while those doped with Mn and Co impurities are found to be semiconducting. These doped transition metal ions have long range interactions mediated through the induced magnetic moments in anions and cations of host semiconductors. These doped ZnS-based diluted magnetic semiconductors seem to be good candidates for the future spintronic applications.     

用电化学沉积方法制备钴掺杂的氧化锌薄膜及其光学性质

通过选用乌洛托品作为络合剂,采用电化学沉积的方法成功地制备出钴掺杂的氧化锌薄膜。通过对样品的XRD表征,得出生长的样品为ZnO纤锌矿结构,并没有其他杂相峰,即没有出现分相;通过对样品XPS的分析显示Co离子在薄膜中以+2价的形式存在;为进一步验证Co²⁺离子进入ZnO的晶格,对掺杂不同Co²⁺浓度的样品进行PL谱的测量,从发光光谱上可以看出随着掺杂Co²⁺浓度的增加,带隙逐渐变窄,发光峰位红移,证明Co²⁺部分取代了Zn²⁺而进入了ZnO晶格中。     

Co和Mn共掺杂ZnO铁磁性的第一性原理

基于密度泛函理论(DFT)的总体能量平面波超软赝势方法,结合广义梯度近似(GGA),对Co单独掺杂ZnO和(Co,Mn)共掺杂ZnO的32原子超原胞体系进行了几何结构优化,计算了纤锌矿结构ZnO、Co-ZnO及(Co,Mn)共掺杂ZnO的能带结构、电子态密度,并对此进行了详细的分析。计算结果表明,ZnO中单独掺杂Co元素显示出铁磁性行为,Mn的引入减弱了Co-ZnO的铁磁性。     

不同价态Mn掺杂InN电子结构、磁学和光学性质的第一性原理研究

采用密度泛函理论体系下的广义梯度近似GGA+U平面波超软赝势方法,在构建了纤锌矿结构的InN超胞及三种不同有序占位Mn~(2+),Mn~(3+)价态分别掺杂InN超胞模型,并进行几何优化的基础上,计算了掺杂前后体系的电子结构、能量以及光学性质.计算结果表明:Mn掺杂后体系总能量和形成能降低,稳定性增加,并在费米能级附近引入自旋极化杂质带,体系具有明显的自旋极化现象.掺杂不同价态的Mn元素对体系电子结构和磁学性质产生了不同的影响.电子结构和磁性分析表明掺杂体系的磁性来源于p-d交换机制和双交换机制的共同作用,Mn~(3+)价态掺杂有利于掺杂体系的居里温度达到室温以上.与未掺杂InN相比,不同价态Mn元素掺杂后体系的静态介电函数显著增大,掺杂体系介电函数虚部和吸收光谱在低能区域出现了较强的新峰,分析认为这些新峰主要来自与费米能级附近自旋极化杂质带相关的跃迁.     

过渡金属掺杂的扶手椅型氮化硼纳米带的磁电子学特性及力-磁耦合效应

基于密度泛函理论的第一性原理计算方法,研究了多种过渡金属(TM)掺杂扶手椅型氮化硼纳米带(ABNNR-TM)的结构特点、磁电子特性及力-磁耦合效应.计算的结合能及分子动力学模拟表明ABNNRTM的几何结构是较稳定的,同时发现对于不同的TM掺杂,ABNNRs能表现出丰富的磁电子学特性,可以是双极化磁性半导体、一般磁性半导体、无磁半导体或无磁金属.双极化磁性半导体是一种重要的稀磁半导体材料,它在巨磁阻器件和自旋整流器件上有重要的应用.此外,力-磁偶合效应研究表明:ABNNR-TM的磁电子学特性对应力作用十分敏感,能实现无磁金属、无磁半导体、磁金属、磁半导体、双极化磁性半导体、半金属等之间的相变.特别是呈现的宽带隙半金属对于发展自旋电子器件有重要意义.这些结果表明:可以通过力学方法来调控ABNNR-TM的磁电子学特性.     

Nanosize induced effects in luminescent ZnS:Mn2+ quantum dots

Nanocrystals (NCs) of II–VI semiconductors of few nanometers average size, called quantum dots (QDs), are now intensely investigated as radiation detectors. Besides the expected quantum confinement and influence of surface states, our electron paramagnetic resonance investigations of cZnS QDs doped with Mn²⁺ ions, correlated with structural data, underline that other properties should be also taken into consideration in developing the II–VI semiconductor QDs as radiation detectors. Thus, the preferential localization of Mn²⁺ in the core of the cubic ZnS QDs at substitutional Zn²⁺ cation sites next to a stacking lattice defect is expected to lead, besides changes in the impurity energy levels, to specific aggregation properties. An outer shell of different composition can also influence the structural properties of the QDs core with effects on the optical properties as well.     

Induction of an atomically thin ferromagnetic semiconductor in 1T′ phase ReS2 by doping with transition metals

Two-dimensional 1T′ phase ReS₂, a transition metal dichalcogenide, has a unique structure and electronic properties that are independent of thickness. The pure phase is a nonmagnetic semiconductor. Using density functional theory calculations, we show that ReS₂ can be tuned to a magnetic semiconductor by doping with transition metal atoms. The magnetism mainly comes from the dopant transition metal and neighboring Re and S atoms as a result of competition between exchange splitting and crystal field splitting. ReS₂ doped with Co can be considered as a promising diluted magnetic semiconductor owing to its strong ferromagnetism with long-range ferromagnetic interaction, high Curie temperature (above room temperature) and good stability. These findings may stimulate experimental validation and facilitate the development of new atomically thin diluted magnetic semiconductors based on transition metal dichalcogenides.     

Extrinsic photonic band structure calculations of a doped semiconductor under an external magnetic field

Doped semiconductors are intrinsically homogeneous media. However, by applying an external magnetic field that has a spatially periodic variation, doped semiconductors can behave extrinsically like conventional photonic crystals. It has shown this possibility theoretically by calculating the dispersion relation, transmittance spectra, and the field distribution of a doped semiconductor under an external, spatially periodic magnetic field. The energy distribution for frequencies located in conduct band and value band are also clearly shown, which convinces the extrinsic photonic band structure characteristic.     

Mott transitions in heavily doped magnetic semiconductors

A generalization applicable to magnetic semiconductors is proposed for the Mott criterion for transitions of heavily doped semiconductors from an insulating into a highly conducting state. Based on this generalization, a study is made of insulator-metal transitions in a ferromagnetic semiconductor associated with temperature variations and of insulator-metal transitions in an antiferromagnetic semiconductor acted on by a magnetic field. These results are of independent interest for nondegenerate semiconductors as well, since they yield the temperature and field dependence for the impurity state radii and for the energies and magnetizations of unionized donors or acceptors. Fiz. Tverd. Tela (St. Petersburg) 40, 433–437 (March 1998)     

磁场中束缚极化子的有效质量

研究磁场中束缚极化子有效质量的性质,采用改进的线性组合算符和变分法讨论了磁场中强、弱耦合极化子的振动频率和有效质量与磁场B和库仑势的关系。以RbCl晶体为例进行了数值计算,结果表明:强耦合束缚磁极化子的振动频率λ和有效质量m^*随磁场B的增加而增大。弱耦合磁极化子的振动频率λ也随磁场的增加而增加,并且发现由于库仑势的存在,使得强耦合束缚磁极化子的振动频率λ和有效质量m^*有所增大。弱耦合磁极化子的有效质量仅和耦合强度α有关。     

Ferromagnetism in diluted magnetic semiconductor quantum dot arrays embedded in semiconductors

We present an Anderson-type model Hamiltonian with exchange coupling between the localized spins and the confined holes in the quantum dots to study the ferromagnetism in diluted magnetic semiconductor (DMS) quantum dot arrays embedded in semiconductors. The hybridization between the quantum-confined holes in the quantum dots and the itinerant holes in the semiconductor valence band makes possible hole transfer between the DMS quantum dots, which can induce the long range ferromagnetic order of the localized spins. In addition, it makes the carrier spins both in the quantum dots and in the semiconductors polarized. The spontaneous magnetization of the localized spins and the spin polarization of the holes are calculated using both the Weiss mean field approximation and the self-consistent spin wave approximation, which are developed for the present model.Received: 17 Mars 2003, Published online: 30 January 2004PACS: 75.75. + a Magnetic properties of nanostructures - 75.30.Ds Spin waves - 75.50.Dd Nonmetallic ferromagnetic materials - 75.50.Pp Magnetic semiconductors     

Spin dynamics in magnetic semiconductor nanostructures

The studies of the magnetic and electrical transport properties of ordered magnetic semiconductor nanostructures have been generalized. This new area lies at the intersection of nanotechnologies and fundamental problems of magnetism. The prospects for application of ferromagnetic semiconductors in spintronics have been discussed. A comparative analysis of the magnetic and electrical transport properties of nanowires, thin films, and bulk elemental semiconductors doped with transition metals has been performed. The influence of size effects on the spin dynamics, magnetization, and magnetoresistance of nanostructures has been considered.     

Ga vacancy induced ferromagnetism enhancement and electronic structures of RE-doped GaN

Because of their possible applications in spintronic and optoelectronic devices, GaN dilute magnetic semiconductors (DMSs) doped by rare-earth (RE) elements have attracted much attention since the high Curie temperature was obtained in RE-doped GaN DMSs and a colossal magnetic moment was observed in the Gd-doped GaN thin film. We have systemically studied the GaN DMSs doped by RE elements (La, Ce–Yb) using the full-potential linearized augmented plane wave method within the framework of density functional theory and adding the considerations of the electronic correlation and the spin-orbital coupling effects. We have studied the electronic structures of DMSs, especially for the contribution from f electrons. The origin of magnetism, magnetic interaction and the possible mechanism of the colossal magnetic moment were explored. We found that, for materials containing f electrons, electronic correlation was usually strong and the spin–orbital coupling was sometimes crucial in determining the magnetic ground state. It was found that GaN doped by La was non-magnetic. GaN doped by Ce, Nd, Pm, Eu, Gd, Tb and Tm are stabilized at antiferromagnetic phase, while GaN doped by other RE elements show strong ferromagnetism which is suitable materials for spintronic devices. Moreover, we have identified that the observed large enhancement of magnetic moment in GaN is mainly caused by Ga vacancies (3.0μB per Ga vacancy), instead of the spin polarization by magnetic ions or originating from N vacancies. Various defects, such as substitutional Mg for Ga, O for N under the RE doping were found to bring a reduction of ferromagnetism. In addition, intermediate bands were observed in some systems of GaN:RE and GaN with intrinsic defects, which possibly opens the potential application of RE-doped semiconductors in the third generation high efficiency photovoltaic devices.     

Ag-Cr共掺LiZnP新型稀磁半导体的光电性质

采用基于密度泛函理论的第一性原理平面波超软赝势法,对纯Li Zn P, Ag/Cr单掺和Ag-Cr共掺Li Zn P新型稀磁半导体进行了结构优化,计算并分析了掺杂体系的电子结构、磁性、形成能、差分电荷密度和光学性质.结果表明:非磁性元素Ag单掺后,材料表现为金属顺磁性;磁性元素Cr单掺后, sp-d杂化使态密度峰出现劈裂,体系变成金属铁磁性;而Ag-Cr共掺后,其性质与Ag和Cr单掺完全不同,变为半金属铁磁性,带隙值略微减小,导电能力增强,同时形成能降低,原子间的相互作用和键强度增强,晶胞的稳定性增强.通过比较光学性质发现,掺杂体系的介电函数虚部和光吸收谱在低能区均出现新的峰值,且当Ag-Cr共掺时介电峰峰值最高,同时复折射率函数在低能区发生明显变化,吸收边向低能方向延展,体系对低频电磁波吸收加强.     

Ab initio investigation of local magnetic structures around substitutional 3d transition metal impurities at cation sites in III-V and II-VI semiconductors

The local magnetic structures around substitutional 3d transition metal impurities at cation sites in zinc blende structures of III-V (GaN, GaAs) and II-VI (ZnTe) semiconductors are investigated by using a spin-polarized density functional theory. We find that Cr-, Co-, Cu-doped GaN, Cr-, Mn-doped GaAs and Cr-, Fe-, Ni-doped ZnTe are half metallic with 100% spin polarization. The magnetic moments due to these 3d transition metal (TM) ions are delocalized quite significantly on the surrounding ions of host semiconductors. These doped TM ions have long range interactions mediated through the induced magnetic moments in anions and cations of host semiconductors. For low impurity concentrations Mn in GaAs also has zero magnetic moment state due to Jahn-Teller structural distortions. Based upon half metallic character and delocalization of magnetic moments in the anions and cations of host semiconductors these above mentioned 3d TM-doped GaN, GaAs and ZnTe seem to be good candidates for spintronic applications.     

Excitonic properties of nanostructure semimagnetic semiconductors

Magneto-optical properties of semimagnetic semiconductors with nanometer-scale structures (nanostructures) are described. Superlattices of the CdTe/Cd_{1 − x}Mn_xTe and Cd_{1 − x}Mn_xTe/ZnTe systems and microcrystals of Cd_{1 − x}Mn_xSe were grown by epitaxy and sputtering methods. These semimagnetic semiconductor nanostructures show remarkably enhanced magneto-optical responses in the optical absorption, the luminescence and the dynamics of the confined excitons. The results are interpreted by the quantum-confined excitonic states interacting with the magnetic ion spins involved in the nanostructures.     

Magnetic field induced metal-insulator transition in semiconductors

Metal-insulator transition in doped semiconductors is investigated in the presence of intense magnetic fields. Using Thomas-Fermi screening function and a screened coulomb potential in the Hamiltonian, a two-parameter varitional calculation leads to metallization. The correlation effect among the electrons is considered through an effectivemass that depends on the spatial seperation between impurities. Results are provided for a many valley semiconductor (Si) and two single valley semiconductors (GaAs and CdTe). Our results show that in a magnetic field the critical concentration at which metallization occurs increases. The correlation effects bring out one order change in the critical concentration values, in intense magnetic fields.     

Ferromagnetism in Cu-doped ZnSe semiconducting quantum dots

The projection of integrating optical, magnetic and electronic functionalities into a single material have aggravated passionate attention in mounting wide band gap diluted magnetic semiconductor (DMS) in the midst of room temperature ferromagnetism. We report the evidence of ferromagnetism in Cu-doped ZnSe quantum dots (QDs) below room temperature, grown from a single source precursor by lyothermal method with the sizes of approximately 3.2–5.14 nm. QDs mainly exhibit paramagnetic behavior between 80 and 300 K, with a weak ferromagnetic/anti-ferromagnetic exchange at lower temperature as observed by superconducting quantum interference device (SQUID) magnetometer. From the Curie–Weiss behavior of the susceptibility, Curie temperature (T _c) of Cu-doped ZnSe sample has been evaluated. From EPR, we obtain the Lande-g factor in the Zeeman interaction term as 2.060. Photoluminescence and EPR measurements support and confirm the view that Cu²⁺ substitutes for Zn²⁺ in Cu-doped ZnSe quantum dots.     

Optical properties of semiconductor quantum dots in magnetic fields

Optical properties of semiconductor quantum dots in magnetic fields are reviewed. A theory is described based on a multi-band effective-mass approximation with a nonparabolic conduction electron dispersion, the direct Coulomb interaction, and the electron-hole exchange interaction taken into account. The transition from the quantum-confined Zeeman effect for a weak magnetic field to the quantum-confined Paschen-Back effect to a strong magnetic field is discussed in comparison with atomic spectra in magnetic fields. Experimental results of the optical properties of isolated CuCl, CdSSe, and Si quantum dots in magnetic fields are also discussed in conjunction with the theoretical results.