铁磁金属、稀磁半导体和半导体量子点中自旋电子学的光谱研究
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摘要
磁性材料中微波光电压的发现使得人们终于找到了利用电子自旋产生电能的途径，同时它给传统的有关静磁学与自旋动力学之间相互关系的问题开辟了新的研究方向。在这篇论文中，我们首先讨论在分子束外延生长的单晶Fe薄膜样品中利用微波光电压技术通过电学方法探测铁磁共振的可能性。为了描述实验结果，我们把自旋整流模型推广到存在磁晶各向异性的情形，拟合结果证明与已知的有关Fe薄膜中铁磁共振的理论符合得很好，并精确地重现了其中的磁晶各向异性，展现了该技术的拓展性和广泛适用性。进一步地，通过分析我们发现，有关磁矩进动的相位信息可以通过光电压的线型读取出来，这使得该技术有可能被用于探测相位相关的磁动力学过程，因而对于目前自旋电子学器件的设计和应用都具有重要的意义。接着我们把研究体系转换到铁磁半导体材料GaMnAs薄膜。通过微波光电压技术我们系统测量了其中铁磁共振的随温度的变化情况，为了把自旋整流模型推广到垂直薄膜表面磁化的情形，我们推导了不同磁化状态下光电压的一般公式，所得结果和实验数据很好地吻合。在p-d动态交换相互作用的理论框架下，我们讨论了实验得到的磁矩g因子、吉尔伯托衰减系数以及铁磁共振线型非均匀展宽的温度依赖关系，它们自洽地反映了GaMnAs体系铁磁相的Zener模型起源的本质。进一步地，实验发现低场范围的光电压显现出非同寻常的磁滞行为，通过Stoner-Wohlfarth模型，我们按照静磁学中的成核过程描述了这一现象，将其归结为磁化强度矢量本身的磁滞行为，同时磁输运测量结果的磁滞无关性与光电压之间的强烈对比揭示了磁矩进动相位的重要性。据此我们判断微波光电压技术不仅能够高效地探测自旋激发，还可对复杂的静磁学过程作深入的研究。
另一方面，以半导体量子点为代表的小量子结构为精确的自旋调控提供了良好的客体环境。利用显微荧光光谱技术，我们对半导体量子点体系中自旋相关的激子能态和相互作用做了研究。首先我们报道根据单个InAs／GaAs量子点的激子能级随磁场的变化情况得到的InAs量子点中激子的g因子同其能量的依赖关系，并由此探讨量子点工艺参数的重要性。接下来我们转换到CdSe／ZnSe量子点体系。对Mn掺杂的样品，为了在引入s-d交换相互作用导致的巨塞曼效应的同时不大幅牺牲量子点的荧光效率，我们设计了CdSe／(3 monolayers)ZnMnSe／ZnSe的三明治结构，实验证明其量子效率远高于CdSe／ZnMnSe结构。根据光谱特征，我们提出一种不同于之前报道的光激发载流子的弛豫途径。对于非掺杂样品，为了研究点间的耦合效应，我们制备了量子点分子(为双层量子点结构)样品，并通过激子基态的分裂证实了这种量子耦合效应的重要性。通过与Bell纠缠态的类比，我们用单自旋态的模型解释了实验结果。实验和计算结果都显示随着点间距离的减少，这种量子耦合效应随之增强，并导致荧光峰位的整体红移。进一步地，通过探测单个CdSe／ZnSe量子点的精细光谱结构，我们研究了各种小能量尺度的自旋有关的相互作用形式，包括带电激子态中的库仑相互作用、电子-空穴交换相互作用和激子自旋-核自旋间超精细相互作用。
The observation of microwave photovoltage in ferromagnetic materials isencouraging not only in that people finally find a way to generate electric power usingelectron spin but also in its enlightment on the physics about the inter-relationshipbetween magnetostatics and spin dynamics.First,we report electric detection offerromagnetic resonance (FMR) in epitaxially grown single crystal iron film throughmicrowave photovoltage (PV) technique.The experimental results agree well with theestablished theory about FMR in iron films,showing excellent extendability of such atechnique onto different ferromagnets as an effective way to study magnetocrystallineanisotropy and spin excitations.Furthermore,the information about the phase ofmagnetization precession is implicated in the lineshape of photovoltage,which makes itpossible to probe in details into magnetic phase dynamics that is of significance fordevising spintronic devices.Second,ferromagnetic resonance in Ga_(0.98)Mn_(0.02)As has beenelectrically detected through microwave PV technique as well.To extend the spinrectification theory to this promising material,general formalism of photovoltage undermagnetocrystalline anisotropy and arbitrary magnetization direction has been derived.The model turns out to agree with the data quite well,and the results are discussed in thespirit of a kinetic exchange picture,which reflects the Zener model nature of the carrier-mediated ferromagnetism in GaMnAs systems.Moreover,the observed unusualhysteresis of PV is understood in certain nucleation processes describable by the Stoner-Wohlfarth model.The sharp contrast provided by the hysteresis free magneto-transportmeasurement results uncovers the important role of microwave phase for PV generation.Based on this work,it is expected that the detection of microwave PV is not only apowerful way to study spin excitations but can also be applied to probe in detail intocomplicate magnetostatic processes.
On the other hand,semiconductor in microscopic structures on a nanometer scale,like semiconductor quantum dots for instance,provides people with an excellent systemfor realizing accurate spin operation.By taking advantage ofμ-photoluminescence (μ-PL)spectroscopy,we first report the evolution of exciton states' emission in single InAs/GaAsquantum dots,a representative ofⅢ-Ⅴsystems,as a function of external magnetic field,which enables us to obtain the excitonic Land(?) g factor and its dependence on the excitonenergy.Then we switch toⅡ-Ⅵsystems,represented by CdSe/ZnSe quantum dots.For the Mn doped sample with the Giant Zeeman Effect introduced by s-d exchangeinteraction,we devise a tri-layer structure which proves to effectively enhance thequantum efficiency of luminescence,and implies a different mechanism accounting forthe suppression of radiative recombination with the presence of Mn impurities.For the undoped samples,we study the inter-dot coupling effect in a series of single quantum dot molecules through the splitting of ground exciton state's emission.The experimental data are well explained in iso-spin language by making an analogy of Bell entangled states.Asthe inter-dot separation distance decreases,the coupling strength increases gradually andbrings forth the red-shift of the emission peaks.Moreover,by investigating the finestructures of PL spectrum of single CdSe/ZnSe quantum dots,we probe into the spinrelated interactions on a quite small energy scale,such as the Coulomb interaction incharged exctions,the electron-hole exchange interaction and the exciton spin-nucleusspin hyper-fine interaction.
另一方面，以半导体量子点为代表的小量子结构为精确的自旋调控提供了良好的客体环境。利用显微荧光光谱技术，我们对半导体量子点体系中自旋相关的激子能态和相互作用做了研究。首先我们报道根据单个InAs／GaAs量子点的激子能级随磁场的变化情况得到的InAs量子点中激子的g因子同其能量的依赖关系，并由此探讨量子点工艺参数的重要性。接下来我们转换到CdSe／ZnSe量子点体系。对Mn掺杂的样品，为了在引入s-d交换相互作用导致的巨塞曼效应的同时不大幅牺牲量子点的荧光效率，我们设计了CdSe／(3 monolayers)ZnMnSe／ZnSe的三明治结构，实验证明其量子效率远高于CdSe／ZnMnSe结构。根据光谱特征，我们提出一种不同于之前报道的光激发载流子的弛豫途径。对于非掺杂样品，为了研究点间的耦合效应，我们制备了量子点分子(为双层量子点结构)样品，并通过激子基态的分裂证实了这种量子耦合效应的重要性。通过与Bell纠缠态的类比，我们用单自旋态的模型解释了实验结果。实验和计算结果都显示随着点间距离的减少，这种量子耦合效应随之增强，并导致荧光峰位的整体红移。进一步地，通过探测单个CdSe／ZnSe量子点的精细光谱结构，我们研究了各种小能量尺度的自旋有关的相互作用形式，包括带电激子态中的库仑相互作用、电子-空穴交换相互作用和激子自旋-核自旋间超精细相互作用。
The observation of microwave photovoltage in ferromagnetic materials isencouraging not only in that people finally find a way to generate electric power usingelectron spin but also in its enlightment on the physics about the inter-relationshipbetween magnetostatics and spin dynamics.First,we report electric detection offerromagnetic resonance (FMR) in epitaxially grown single crystal iron film throughmicrowave photovoltage (PV) technique.The experimental results agree well with theestablished theory about FMR in iron films,showing excellent extendability of such atechnique onto different ferromagnets as an effective way to study magnetocrystallineanisotropy and spin excitations.Furthermore,the information about the phase ofmagnetization precession is implicated in the lineshape of photovoltage,which makes itpossible to probe in details into magnetic phase dynamics that is of significance fordevising spintronic devices.Second,ferromagnetic resonance in Ga_(0.98)Mn_(0.02)As has beenelectrically detected through microwave PV technique as well.To extend the spinrectification theory to this promising material,general formalism of photovoltage undermagnetocrystalline anisotropy and arbitrary magnetization direction has been derived.The model turns out to agree with the data quite well,and the results are discussed in thespirit of a kinetic exchange picture,which reflects the Zener model nature of the carrier-mediated ferromagnetism in GaMnAs systems.Moreover,the observed unusualhysteresis of PV is understood in certain nucleation processes describable by the Stoner-Wohlfarth model.The sharp contrast provided by the hysteresis free magneto-transportmeasurement results uncovers the important role of microwave phase for PV generation.Based on this work,it is expected that the detection of microwave PV is not only apowerful way to study spin excitations but can also be applied to probe in detail intocomplicate magnetostatic processes.
On the other hand,semiconductor in microscopic structures on a nanometer scale,like semiconductor quantum dots for instance,provides people with an excellent systemfor realizing accurate spin operation.By taking advantage ofμ-photoluminescence (μ-PL)spectroscopy,we first report the evolution of exciton states' emission in single InAs/GaAsquantum dots,a representative ofⅢ-Ⅴsystems,as a function of external magnetic field,which enables us to obtain the excitonic Land(?) g factor and its dependence on the excitonenergy.Then we switch toⅡ-Ⅵsystems,represented by CdSe/ZnSe quantum dots.For the Mn doped sample with the Giant Zeeman Effect introduced by s-d exchangeinteraction,we devise a tri-layer structure which proves to effectively enhance thequantum efficiency of luminescence,and implies a different mechanism accounting forthe suppression of radiative recombination with the presence of Mn impurities.For the undoped samples,we study the inter-dot coupling effect in a series of single quantum dot molecules through the splitting of ground exciton state's emission.The experimental data are well explained in iso-spin language by making an analogy of Bell entangled states.Asthe inter-dot separation distance decreases,the coupling strength increases gradually andbrings forth the red-shift of the emission peaks.Moreover,by investigating the finestructures of PL spectrum of single CdSe/ZnSe quantum dots,we probe into the spinrelated interactions on a quite small energy scale,such as the Coulomb interaction incharged exctions,the electron-hole exchange interaction and the exciton spin-nucleusspin hyper-fine interaction.
引文
1.王充,《论衡》中国东汉。
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