p，n型掺杂剂与Mn共掺杂GaN的电磁性质

采用基于密度泛函理论的第一性原理平面波赝势法计算Mg,Zn,Si,O和Mn共掺GaN,分析比较共掺杂后的电子结构和磁学性质,并分别用平均场近似的海森伯模型和Zener理论估算共掺杂后体系的居里温度（T_C）.计算表明：共掺杂后体系均在能隙深处产生自旋极化杂质带,具有半金属性,能产生自旋注入.p型共掺杂（GaN：Mn-Mg＼Zn）后体系具有较GaN：Mn更稳定的FM态且能使T_C升高;而n型共掺杂（GaN：Mn-Si＼O）后体系FM态稳定性 关键词： Mg Zn Si O和Mn共掺GaN 第一性原理 T_C）')" href="#">居里温度（T_C）     

极化诱导的内建电场对Mn δ掺杂的GaN/AlGaN量子阱居里温度的调制

采用传输矩阵方法分析极化诱导的内建电场对Mn δ掺杂的GaN/Al_xGa_1-xN量子阱居里温度(T_C)的调制作用.通过解薛定谔方程计算出在不同的内建电场条件下半导体量子阱局域态内的基态空穴能级和波函数分布情况，并在此基础上确定量子阱内Mn δ掺杂情况下T_C随内建电场的变化趋势，分析了不同量子阱结构引起的内建电场分布变化及其对T_C的影响.在耦合双量子阱中通过调节左右阱的不对称性可以得到T_C近3倍的增长. 关键词： GaN 量子阱 内建电场 居里温度     

金属有机化学气相淀积技术制备GaMnN薄膜材料光学性质研究

研究由MOCVD 技术制备的 GaMnN 外延薄膜光吸收谱.实验发现Mn掺杂后较未掺杂GaN吸收系数在近紫外区增加,在吸收谱低能区144 eV附近观察到吸收峰,吸收系数随Mn浓度的增加而增大.实验结果与基于密度泛函理论的第一性原理计算结果一致,结合理论计算分析认为144 eV附近的吸收峰源于Mn³⁺离子e态与t₂态间的带内跃迁⁵T₂→⁵E. 关键词： GaMnN MOCVD 密度泛函理论 光学性质     

多晶Si0.9654Mn0.0346:B薄膜的磁性研究

利用磁控溅射和快速热处理的方法制备了Mn,B共掺的多晶硅薄膜(Si_0.9654Mn_0.0346:B).磁性和结构研究发现薄膜有两个铁磁相.低温铁磁相来源于杂相Mn₄Si₇,高温铁磁相(居里温度T_C～250K)是由Mn原子掺杂进入Si晶格导致.晶化后的薄膜利用射频等离子体增强化学气相沉积系统(PECVD)进行短暂(4min)的氢化处理后发现,薄膜的微结构没有发生变化而饱和磁化强度却随着 关键词： 磁性半导体 硅 氢化     

空位缺陷及Mg替位对纤锌矿(Ga,Mn)N电子结构和磁光性能的影响

采用自旋密度泛函理论框架下的广义梯度近似(GGA+U)平面波超软赝势方法,构建了未掺杂纤锌矿GaN超胞、三种不同有序占位Mn双掺GaN,(Mn,Mg)共掺杂GaN以及存在空位缺陷的Mn掺杂GaN超胞模型,分别对所有模型的能带结构、电子态密度、能量以及光学性质进行了计算.计算结果表明:与纯的GaN相比,Mn掺杂GaN体系的体积略有增大,掺杂体系居里温度能够达到室温以上;随着双掺杂Mn-Mn间距的增大,体系总能量和形成能升高、稳定性下降、掺杂越难;(Mn,Mg)共掺杂并不能有效增大掺杂体系磁矩,也不能达到提高掺杂体系居里温度的作用;Ga空位缺陷和N空位缺陷的存在不利于Mn掺杂GaN形成稳定的铁磁有序.此外,Mn离子的掺入在费米能级附近引入自旋极化杂质带,正是由于费米能级附近自旋极化杂质带中不同电子态间的跃迁,介电函数虚部在0.6868eV附近、光吸收谱在1.25eV附近分别出现了一个较强的新峰.     

Mn掺杂GaN电子结构和光学性质研究

采用基于密度泛函理论的第一性原理平面波赝势法计算不同Mn浓度掺杂GaN晶体的电子结构和光学性质.计算结果表明Mn掺杂GaN使得Mn 3d与N 2p轨道杂化,产生自旋极化杂质带,材料表现为半金属性,非常适于自旋注入,说明该种材料是实现自旋电子器件的理想材料,折射率在带隙处出现峰值,紫外区光吸收系数随Mn浓度的增加而增大. 关键词： Mn掺杂GaN 第一性原理 电子结构 光学性质     

新型稀磁半导体Mn掺杂LiZnAs的第一性原理研究

采用基于密度泛函理论的第一性原理平面波超软赝势方法, 对纯LiZnAs, Mn掺杂的LiZnAs, Li过量和不足下Mn掺杂的LiZnAs体系进行几何结构优化, 计算并对比分析了体系的电子结构、半金属性、光学性质及形成能.结果表明新型稀磁半导体Li (Zn_0.875Mn_0.125) As, Li_1.1 (Zn_0.875Mn_0.125) As和Li_0.9 (Zn_0.875Mn_0.125) As均表现为100%自旋注入, 材料均具有半金属性, Li过量和不足下体系的半金属性明显增强. Li过量可以提高体系的居里温度, 改善材料的导电性, 使体系的形成能降低. 说明LiZnAs半导体可以实现自旋和电荷注入机理的分离, 磁性和电性可以分别通过Mn的掺入和Li的含量进行调控. 进一步对比分析光学性质发现, 低能区的介电函数虚部和复折射率函数明显受到Li的化学计量数的影响. 关键词： Mn掺杂LiZnAs 电子结构 光学性质 第一性原理     

13C及29Si核磁共振研究苯乙烯-二甲基硅氧烷嵌段共聚物的弛豫及分子运动

用¹³C及²⁹Si核磁共振研究了苯乙烯(S)及二甲基硅氧烷(Si)嵌段共聚物中硅氧烷软段的固体及溶液谱的自旋-晶格弛豫时间T₁。固态嵌段共聚物主链²⁹Si及侧甲基¹³C的T₁都与均聚物的T₁相近,但在CdCl₃溶液中各种嵌段共聚物的T₁与均聚硅氧烷相差颇大。用偶极-偶极相互作用来解释高聚物的自旋-晶格弛豫。苯乙烯-二甲基硅氧烷嵌段共聚物具两相结构,所以嵌段共聚物中软段及硬段微区中链段的运动与在均聚物分子中链段的运动模式基本相同。而CdCl₃对聚苯乙烯或聚硅氧烷都是良溶剂,软段硬段之间有相互影响。所以其链段运动与均聚物不同,从而导致链段运动的相关时间τ_c变短和T₁的增长。     

嵌入La和Gd原子的Si24笼团簇的稳定性

用梯度修正自旋极化密度泛函(DFT)电子结构计算,研究了具有T_h和D_2d对称性包裹La和Gd原子的Si₂₄富勒烯的稳定性.结果表明Gd@Si₂₄具有很高的磁性而La@Si₂₄的磁性完全猝灭.这些结果有可能导致Si基富勒烯团簇新的结构类型. 关键词： 24和Gd@Si₂₄包裹团簇')" href="#">La@Si₂₄和Gd@Si₂₄包裹团簇 自旋极化密度泛函 稳定性 电磁性质     

Mn-Mg共掺杂AlON荧光粉制备及荧光性质

采用固相反应法成功合成了一系列Mn-Mg共掺的Al₂₃O₂₇N₅荧光粉。结果显示煅烧条件和Mn的掺杂浓度对Mn-Mg共掺的AlON荧光粉的制备和发光性质都有重要影响。获得纯相AlON的温度从未掺杂的1 900 ℃下降到10% Mn和10% Mg(摩尔分数)共掺杂的1 800 ℃。晶格参数随着Mn掺杂浓度升高而增加,说明Mn进入了AlON晶格中。未掺杂和Mg单掺以及10% Mn和10% Mg共掺杂样品的发射光谱都含有一个350～600 nm之间的宽带发射。绿光发射归属于Mn的3d电子之间的传输。蓝光发射可能与杂质或Al空位相关。     

Optical properties and radiation effects in layered crystals of (CnH2n+nNH3)2MCl4: n = 1, 2, 3; M = Cd,Cu, Mn

Abstract

A new absorption band has been found at 5.10 eV in (C _n H_{2n + 1}NH₃)₂CdCl₄: n = 1, 2, 3 in addition to the absorption bands of CdCl₂ whose electronic structure resembles the former crystals. The energy of the additional peak shifts with temperature by as much as 0.38 eV from 5.10eV at room temperature (RT) to 5.48 eV at liquid nitrogen temperature. This large peak shift is attributed to a structural phase transition between these two temperatures. A new type of electron center has been found in these crystals (M = Cd, Mn; n = 1, 2, 3) irradiated with X-rays at 15 K in addition to the Cl₂ ^?. This shows optical absorption bands (IR bands) in the infrared region of 10 ～ 20 kcm ^?1. The IR bands are assigned to an electron center where an electron is trapped at an ammonium site in the neighborhood of a Cl^? vacancy.     

Amorphous germanium III. Optical properties

The optical properties of thick sputtered films (～30μ) of amorphous Ge, grown with different substrate temperatures (0ˇ-T _sˇ-350°C), were obtained between 0·05 and 4·5 eV by a combination of reflectance, transmittance and ellipsometric measurements. The refractive index at 0·15 eV decreases monotonically with increasing T _s, or equivalently, with increasing density, and is 4·13±0·05 eV in the highest density films. The absorption edge is approximately exponential (10²?α?10⁴ cm^?1) but shifts monotonically to higher energy and increases in slope with increasing T _s. Similarly, the peak in ε₂ grows by about 10% and shifts by about 0·15 eV to higher energies, reaching a maximum of about 23 at 2·90±0·05 eV in the high density films. The peak in the transition strength ω²ε₂ occurs at 4·2±0·2 eV in all films, but increases in magnitude with increasing T _s. The sum rules for n _eff(ω) and ε_0,eff(ω) are evaluated for ▄ω?5 eV and vary monotonically with T _s. These trends are neither compatible with Galeener's void resonance theory nor with changes in the oxygen content of the films, determined by the examination of absorption peaks at 0·053 eV and 0·09 eV. An explanation, suggested here and expanded in I, is based on the observed changes in the structure of the network and voids.     

Light-induced absorption change in Fe-doped GaN

The properties of a light-induced absorption (LIA) change were investigated in a semi-insulating Fe-doped GaN crystal. The temporal profile and a spectrum of the LIA were measured in the temperature range from 15 K to 300 K. The LIA was almost proportional to the light intensity and showed slight saturation behavior. The relaxation time constant, in the range of milliseconds, was equal to the lifetime of infrared photoluminescence related to the Fe³⁺ internal transition Fe³⁺(⁴T₁)→Fe³⁺(⁶A₁). The LIA was attributed to an excited-state absorption from Fe³⁺(⁴T₁). Some of the peak energies observed in the LIA spectrum showed good agreement with transition energies expected from an energy diagram of Fe-doped GaN crystal.     

The electronic structure of KMnO4 investigated by photoemission and electron-energy-loss spectroscopy

From photoemission and electron-energy-loss data the following picture of KMnO₄, with Mn^VII (with a formal charge state Mn⁷⁺ (3d ⁰)) tetrahedrally surrounded by four O^2–-ions, is deduced: strong covalent bonding between Mn^VII and O^2– leads to a considerable occupation of the Mn-3 d shell. The ground state of the (MnO₄)^–1 molecule is an orbital and spin singlet as seen by the absence of any multiplet splitting in the Mn core levels. The valence band shows a four peak structure extending form 4 eV to 8 eV below the Fermi energy. The first peak at 4.2 eV has mainly O-2p character. The remaining peaks are of strongly mixed Mn-3d/O-2p character due to the covalent bonding. This mixing decreases with increasing binding energy. The electron energy loss data show a variety of structures between 2 eV and 10 eV independent of the primary electron energy which defines them as dipole allowed charge-transfer transitions. An additional excitation at 1.8 eV decreases quickly in intensity with increasing electron energy which classifies it as a dipole or spin forbidden transition in the compound. This energy is close to the value of 1.6 eV reported for the activation energy observed in electrical transport data. The results are compared to quantum chemical molecular orbital calculations of the (MnO₄)^–1 molecule.Physics Department, Allahabad University Allahabad 211002, India     

Hebel-Slichter peak and superconducting energy gap in Rb3C60 observed by muon spin relaxation

Muon spin relaxation has been observed in both the normal and superconducting states of Rb₃C₆₀ (T _c=29.3K). The field dependence of theT ₁ spin relaxation rate is due to muonium undergoing spin-exchange scattering with conduction electrons, making this the first observation of muonium in a metal. The temperature dependence ofT ₁ ^–1 shows a Hebel-Slichter coherence peak just belowT _c which is not seen in¹³C spin relaxation. The peak can be fit assuming spin relaxation due to interaction with the quasiparticle excitations of a BCS superconductor provided the density of states is broadened relative to that of BCS. Such fits yield a value for the zero temperature energy gap, ₀/k _B , of 53(4)K, consistent with weak-coupling BCS.     

F-Zentren und Kolloide in der amorphen,hexagonalen und kubischen Phase von LiJ

By simultaneous evaporation of LiI and Li onto a cooled substrate F centers can be produced in the hexagonal (78 K<T _K <200 K) and amorphous (T _K <78 K) phase of one and the same salt. In both modifications there exist two types of centers F and F^*. The F^* center differs from the cubic F center (T _d -symmetry) by a nearby Frenkel defect. In hexagonal films the normal F band peaks at 2.58 eV, whereas the transitions of the F^* center appear at 2.92 and 2.58 eV too. Polarized irradiation at 20 K causes a dichroic behaviour of the F^* centers. Both types of centers can be transformed into one another photochemically. In the amorphous phase all transitions are shifted to lower energies by about 0.1 eV. After the phase change amorphous→hexagonal the absorption bands shift back by the same amount of energy. AboveT _K =230 K the excess metal forms colloids. The absorption bands are due to colloidal centers embedded in the crystalline material (2.25 eV) and films adsorbed to the crystallites (3.1 eV), respectively. By annealing a particle growth can be observed. After electrolytic colouration cubic single crystals of LiI exhibit an absorption band peaking at 2.36 eV. However, it is not yet sure, if this band is allowed to be ascribed to F centers.     

Resonant two-photon absorption in the Cr2O3 antiferromagnet

Two-photon absorption (TPA) spectra of the Cr₂O₃ model antiferromagnet have been studied for two polarization configurations at energies ranging from 2.5 to 3.55 eV. Several strong TPA peaks with the maximum coefficient β ～ 0.08 cm/MW are observed in the 2.7–3.1-eV range characterized by the ⁴ A ₂(F) → ⁴ T ₁(F) d-d one-photon transition of the Cr³⁺ ion. An analysis of the results obtained suggests that the general form of the TPA spectrum in this range is primarily determined by the resonance at the ² E(G) and ² T ₁(G) intermediate levels. At energies above 3.44 eV, the TPA coefficient sharply increases up to β ～ 0.1 cm/MW due to transitions between the valence and conduction bands.     

Possible manifestation of spin fluctuations in the temperature behavior of the resistivity of Sm<Subscript>1.85</Subscript>Ce<Subscript>0.15</Subscript>CuO<Subscript>4</Subscript> thin films

A pronounced step-like (kink) behavior in the temperature dependence of resistivity ρ(T) is observed in the optimally doped Sm_1.85Ce_0.15CuO₄ thin films around T _sf = 87 K and attributed to the manifestation of strong-spin fluctuations induced by Sm³⁺ moments with the energy ħω_sf = k _B T _sf ≃ 7 meV. The experimental data are found to be well fitted by the residual (zero-temperature) ρ_res, electron-phonon ρ_e-ph(T) = AT, and electron-electron ρ_e-e(T) = BT ² contributions in addition to the fluctuation-induced contribution ρ_sf(T) due to thermal broadening effects (of the width ω_sf). According to the best fit, the plasmon frequency, impurity scattering rate, electron-phonon coupling constant, and Fermi energy are estimated as ω_p = 2.1 meV, τ ₀ ⁻¹ = 9.5 × 10⁻¹⁴ s⁻¹, λ = 1.2, and E _F = 0.2 eV, respectively. The text was submitted by the authors in English.