纤锌矿MgxZn1-xO/Mg0.3Zn0.7O抛物量子阱中极化子能级

采用改进的Lee-Low-Pines(LLP)中间耦合方法研究纤锌矿Mg x Zn1-x O/Mg0.3Zn0.7O抛物量子阱材料中的极化子能级,给出极化子基态能量、跃迁能量(第一激发态到基态)和不同支长波光学声子对电子态能级的贡献随量子阱宽度d的变化规律。理论计算中考虑了纤锌矿Mg x Zn1-x O/Mg0.3Zn0.7O抛物量子阱材料中声子模的各向异性和介电常数、声子(类LO和类TO)频率等随空间坐标Z变化(SD)效应对极化子能量的影响。结果表明,Mg x Zn1-x O/Mg0.3Zn0.7O抛物量子阱中电子与长波光学声子相互作用对极化子能级的移动很大,使得极化子能量明显降低。阱宽较小时,半空间长波光学声子对极化子能量的贡献较大,而定域长波光学声子的贡献较小;阱宽较大时,情况则正好相反。在d的变化范围内,电子与长波光学声子相互作用对极化子能级的移动(约67~79 meV)比Al x Ga1-x As/Al0.3Ga0.7As抛物量子阱中的相应值(约1.8~3.2 meV)大得多。因此,讨论ZnO基量子阱中电子态问题时要考虑电子与长波光学声子的相互作用。     

对称量子阱中极化子的有效质量和自陷能

采用变分微扰法,对量子阱中极化子的有效质量和自陷能进行了研究.考虑到阱对于电子波函数的量子限制效应和界面声子的影响,计算了各支声子与电子相互作用对自陷能的贡献,讨论了极化子的有效质量和自陷能随量子阱宽度的变化关系,进一步说明了量子阱的尺度效应和界面声子作用的重要性. 关键词：     

极化子效应对ZnS/CdSe量子点三阶极化率的影响

在有效质量近似下,利用量子力学的密度矩阵理论,采用无限深势阱模型,从理论上计算了考虑极化子效应后在导带子带间跃迁时ZnS/CdSe柱型核壳结构量子点二次电光效应(QEOE)和电吸收过程(EA)的三阶极化率。通过数值计算,分析了电子-LO声子和电子-IO声子相互作用对ZnS/CdSe柱型核壳结构量子点二次电光效应和电吸收过程的三阶极化率的影响。结果表明,极化子效应对二次电光效应的三阶极化率χ⁽³⁾_QEDE和电吸收过程的三阶极化率χ⁽³⁾_EA都有很大影响,并且影响的大小与量子点的尺寸大小有关。     

导带弯曲对有限深GaN/Ga1-xAlxN球形量子点中束缚极化子的影响及其压力效应

采用三角势近似界面导带弯曲,研究了有限高势垒GaN/Ga1-x Al x N球形量子点中束缚极化子的结合能及其压力效应。数值计算了杂质态与声子之间相互作用对结合能的影响,同时与方形势垒进行了比较。结果表明,随着电子面密度的增加,导带弯曲效应增强,束缚极化子结合能逐渐下降。当电子面密度n s=(6.0,8.0)×1011/cm2且量子点半径R>10 nm时,束缚极化子的结合能趋近于一个相同且较小的值。结合能的极化效应主要来自杂质与光学声子相互作用的贡献。     

Cd+离子5s2S1/2→5p2P3/2电子碰撞激发截面和退激辐射光子极化度的理论研究

本文采用全相对论扭曲波方法计算了Cd⁺离子5s²S_1/2 → 5p²P_3/2电子碰撞激发总截面、磁能级的激发截面以及退激辐射光子极化度. 详细讨论了电子关联效应对激发截面以及退激辐射光子极化度的影响. 我们发现, 在低能碰撞部分(<10 eV), Core-价关联对电子碰撞激发截面的贡献非常重要, 与不考虑Core-价关联的结果相比, Core-价关联的计算结果使得激发截面降低了1/2到2/3; 在高能碰撞部分(>80 eV), Core-价关联的贡献不是非常明显, 但与不考虑Core-价关联的结果相比, 其激发截面也降低了15%. 然而, 对于退激辐射光子极化度, Core-价关联的贡献非常小, 其影响是可以忽略的.     

极化子效应对ZnS/CdSe柱型核壳量子点简并四波混频过程的影响

理论计算了带有极化子效应的ZnS/CdSe柱型核壳结构量子点简并四波混频的三阶极化率.在有效质量近似下,采用无限深势阱模型,导出了带有极化子效应的柱型量子点的三阶非线性光学极化率的解析表达式,解三维薛定谔方程得到电子的本征能量和波函数.分析了电子-LO声子和电子-IO声子相互作用对ZnS/CdSe核壳结构量子点简并四波混频的三阶极化率的影响.结果表明,当量子点尺寸一定时,带有极化子效应的χ(3)DFWM比没有极化子时的三阶极化率提高了近3个数量级,并且电子-LO声子的影响比电子-IO声子的影响大得多;当固定核壳量子点的阱宽而内外半径发生变化时,χ(3)DFWM的峰值随之变化,而且极化子效应影响的大小也随之发生变化.     

极性晶体膜中的束缚极化子的有效质量

采用格林函数的方法,研究极性晶体膜中束缚极化子的有效质量随膜厚d的变化关系。得出电子-体LO声子相互作用以及电子-SO声子相互作用都对束缚极化子的有效质量有贡献。通过对KCl半导体膜的数值计算表明,束缚极化子的有效质量随膜厚d的增加而减少;当膜厚小于5 nm时电子-SO声子相互作用对束缚极化子的有效质量起主要贡献,但是当膜厚大于10 nm时,电子-体LO声子相互作用对束缚极化子的有效质量起主要贡献,当膜厚大于5 nm而小于10 nm时,二者共同影响束缚极化子的有效质量;另外,由于束缚势的存在,使束缚极化子的有效质量增大,这主要是由于束缚势的存在,使电子-声子间的相互作用增强,极化子效应增大而引起的。     

In0.53Ga0.47As/In0.52Al0.48As量子阱中的正磁电阻效应

在低温强磁场条件下,对In_0.53Ga_0.47As/In_0.52Al_0.48As量子阱中的二维电子气进行了磁输运测试.在低磁场范围内观察到正磁电阻效应,在高磁场下这一正磁电阻趋于饱和,分析表明这一现象与二维电子气中的电子占据两个子带有关.在考虑了两个子带之间的散射效应后,通过分析低磁场下的正磁电阻,得到了每个子带电子的迁移率,结果表明第二子带电子的迁移率高于第一子带电子的迁移率.进一步分析表明,这主要是由两个子带之间的 关键词： 二维电子气 正磁电阻 子带散射     

La0.67Sr0.08Na0.25MnO3的奇特输运性质及CMR效应

用固相反应法制备了La_0.67Sr_0.08Na_0.25MnO₃样品.通过磁化强度-温度(M-T)曲线、电阻率-温度(ρ-T)曲线以及ρ-T拟合曲线研究了样品的输运性质及庞磁电阻(colossal magnetoresistance，CMR)效应.结果表明，ρ-T曲线和磁电阻-温度(MR-T)曲线均出现双峰现象；高温峰是伴随顺磁-铁磁(PM-FM)相变出现绝缘体-金属(I-M)相变，低温峰是颗粒界面效应；两个绝缘相输运机理不同：较低温度下(248K<T<274K)，ρ(T)符合极化子的可变程跃迁模型，而在更高温区(330K<T<374K)，ρ(T)符合极化子近邻跃迁模型；两个类金属相输运机理也不同：在低温区(67K<T<186K)，满足ρ-T^2.5关系，输运机理是自旋波散射和电-磁子散射作用，而在高温区(292K<T<304K)，满足ρ-T²关系，输运机理是单磁子散射作用. 关键词： 庞磁电阻 金属-绝缘体转变 晶界效应 输运行为     

抛物量子线中弱耦合极化子的有效质量和光学声子平均数

讨论电子与体纵光学(LO)声子弱耦合时对抛物量子线中极化子性质的影响.采用Tokuda改进的线性组合算符法、Lagrange乘子和变分法,导出了抛物量子线中弱耦合极化子的有效质量和光学声子平均数随拉格朗日乘子变化的规律及极化子振动频率随量子线约束强度的变化规律.并以ZnS量子线为例进行了数值计算,结果表明:抛物量子线中弱耦合极化子的有效质量m^*和光学声子平均数N随着拉格朗日乘子u的增加而增大;该结论与体材料中结论基本一致,但量子线中的效应比体材料更明显,表明量子线对电子约束的增强,使极化子效应更明显.同时,极化子振动频率λ随约束强度ω₀的增强而增大.     

Electrical investigations in the normal and incommensurate phases of [N(CH3)4]2ZnCl4 single crystals

The dielectric permittivity (?) of TMA-ZC single crystals was measured along the mean crystallographic axes a, b and c, in a temperature range from 273 to 340 K. The ?-T relationship exhibited peak values at T_i=296 K and T_c1=279 K for the three axes. These peaks are attributed to the contribution of discommensurations. The d.c. and a.c. electrical conductivity showed anomalous variation at the same transition temperatures, with a remarkable change in the value of the activation energy around the transition temperatures. The j-E characteristic indicates different types of electrical conduction. The mechanism of the phase transition and the electrical process were discussed on the basis of Shottky and Frenkel conduction mechanisms.     

Strong orbital exchange scattering in (La1−xNdx)Sn3

In the dilute alloys (La_1-xNd_x)Sn₃ a large anomalous contribution to the zero field Hall constant has been found, which is proportional to the magnetic susceptibility of the Nd³⁺ ions. On the other hand no corresponding effect appears in (La_1-xGd_x)Sn₃. We conclude that the observed anomalous Hall effect is due to the orbital exchange scattering of the conduction electrons by the Nd-impurities. The coupling energy of this interaction is found to be of the same order of magnitude or even larger than that of the isotropic spin exchange interaction     

Heat capacity and thermal expansion of CdCr2Se4 and CdCr2S4

The thermodynamic properties of the spinel ferromagnetic compounds CdCr₂Se₄ and CdCr₂S₄ have been investigated by making heat capacity and thermal expansion measurements on single crystals. For both compounds, the ferromagnetic transition is marked by λ-type thermal anomalies, and the results provide a pressure dependence of the transition temperatures that is in agreement with direct measurements. Below the transition, CdCr₂S₄ shows an anomalous heat-capacity contribution and negative thermal expansion, which are in contrast to the conventional behavior found in CdCr₂Se₄.     

Spin-dependent thermoelectric effect in Co2Fe0.4Mn0.6Si thin film with perpendicular magnetic anisotropy

We report the anomalous Nernst effect in trilayers containing a thin film of the half-metallic ferromagnetic Heusler alloy Co₂Fe_0.4Mn_0.6Si with perpendicular magnetic anisotropy. The structure is MgO/CFMS/Pd and we have studied the variation of anomalous Hall and Nernst effects as a function of CFMS and Pd thickness. The anomalous Nernst coefficient reaches 0.5 μV/K at room temperature and we have observed a strong dependence of the anomalous Nernst coefficient on the thickness of both layers. Our results indicate that inducing perpendicular magnetic anisotropy in a strongly spin-polarising Heusler alloy such as CFMS is very promising for new thermoelectric devices based on exploiting the anomalous Nernst effect.     

Anomalous temperature dependence of elastic constant c44 in V, Nb, Ta, Pd, and Pt

The anomalous temperature dependence of elastic constant c₄₄ for elements V, Nb, Ta, Pd, and Pt, has been calculated using first-principles theory. It is shown that the variation of elastic constant for simple elements can be approximated as the sum of thermal expansion and electronic components. The thermal expansion contributes the normal linearly decreasing effect to the elastic constant with temperature, while electronic contribution is determined by the unique character of electronic structure of elements and leads to the anomalous effect to the elastic constant with temperature.     

Temperature dependence of the work function of Bi2Sr2CaCu2O8 single crystal cleaved at low temperature

We measured the anomalous change in the work function of Bi₂Sr₂CaCu₂O₈ around a critical temperature (T_c ≈ 85 K). The work function becomes a minimum at T_c; the work function decreases in a normal-conductive state and then increases in a superconductive state as the temperature decreases. An increase in the work function for a transition from a normal-conductive state to a superconductive state at 0 K is about 9 meV. The contribution of the chemical potential and the surface dipole barrier to the work function are discussed.     

Nature of magnetic impurity induced superparamagnetism and anomalous Hall effect in β-FeSi2 single crystals

The magnetic properties of β-FeSi₂ single crystals were investigated using magnetization and anomalous Hall effect measurements. These measurements clearly show the appearance of superparamagnetism, the origin of which is attributed to defect-induced magnetic aggregates. Unlike previous reports, based on our combined analysis using magnetization and the anomalous Hall effect, we found that two kinds of aggregates, which are magnetically distinct, exist in our samples. Among the two types found, one gives an anomalous Hall term, which follows a side-jump mechanism while the other is a completely new kind, suggesting that magnetic properties of β-FeSi₂ are very unconventional. The sizes of the aggregates were estimated from our experimental data.     

Temperature dependence of infrared and Raman modes in polymeric RbC60

Temperature dependence of the intra-molecular vibrational modes of C₆₀ in the quasi-1D polymeric RbC₆₀, across the low temperature transition at ∼50 K, has been probed through infrared (IR) and Raman spectroscopies. With the lowering of temperature, the split IR modes of RbC₆₀ are seen to harden but below 50 K a small but definitive signature of an anomalous softening is observed. In addition, the background IR transmission shows an increase below 50 K with the opening of a well defined gap in the electronic spectrum. The implications of these results, along with those of Raman measurements, are discussed in terms of the interaction of intra-molecular phonons with electrons and spin excitations in the system.     

Magnetocaloric effect: Overcoming the magnetic limit

We have studied anomalous peaks observed in magnetocaloric −ΔS(T) curves for systems that undergo first-order magnetostructural transitions. The origin of those peaks, which can exceed the conventional magnetic limit, R ln(2J+1), is discussed on thermodynamic bases by introducing an additional-exchange contribution (due to exchange constant variation arising from magnetostructural transition). We also applied a semiphenomenological model to include this additional-exchange contribution in Gd₅Si₂Ge₂- and MnAs-based systems, obtaining excellent results for the observed magnetocaloric effect.     

Factors affecting coercivity in (Y,Sm,Tm)3(FeGa)5O12 and (Y,Sm,Lu,Ca)3(FeGe)5O12 LPE films

Factors which influence coercivity, H_c, in (Y,Sm,Lu,Ca)₃(FeGe)₅O₁₂ films and in (Y,Sm,Tm)₃(FeGa)₅O₁₂ films grown by LPE have been identified. An anomalous layer at the film-substrate interface exhibits coercivity values different from that of the middle, bulk, portion of the film. The contribution of the transient layer at the substrate interface could be reduced by increasing the rotation rate while immersing the substrate into the melt. Films containing Ga show lower coercivities than films containing CaGe possibly because films with Ga are more uniform in composition. Films with Ga show increased coercivities with increasing growth rates and with increasing Sm content throughout the film.