新型稀磁半导体的先进谱学表征和性能研究

发布时间：2018-05-27 08:47

本文选题：稀磁半导体 + 铁磁序　；参考：《中国科学院大学(中国科学院物理研究所)》2017年博士论文

【摘要】：稀磁半导体材料(DMS)是可以同时调控电荷自由度和自旋自由度的功能性材料,在材料科学中展现出了丰富卓越的物理性能,引起了人们广泛的研究兴趣。在经典的Mn掺杂的III-V族稀磁半导体中,Mn对三价的Ga的替代会引入一个受主,同时也是磁矩的来源。Mn的这种双重作用复杂了相应的理论解释。最近发现的Mn掺杂的I-II-V半导体Li_(1+x)(Zn_(1-y)Mn_y)As和Mn掺杂的II-II-V semiconductor(Ba_(1-x)K_x)(Zn_(1-y)Mn_y)_2As_2是新型的稀磁半导体,在这些铁磁体中空穴和自旋的注入是可以分别调控的。在稀磁半导体中,人们已经在交换作用的控制中取得了一定的进展,然而对其在物理特性的微观起源中的理解仍然有限。具有元素和轨道壳层分辨的同步辐射谱学技术(发射、吸收和二色性)有着非常高的能量分辨率,非常适合探测特定电子态的结构,而高能X射线衍射实验能分辨出晶格的微小变化。将(Ba_(1-x)K_x)(Zn1-y Mny)_2As_2选为目标材料,研究了空穴掺杂下,导致其高温铁磁有序Tc的间接交换作用机制。除了X射线谱学技术,压力也做为另一个热力学维度引入,用来探究这些新型稀磁半导体中晶格、磁性和电子自由度之间的复杂相互关系。As的K边磁圆二色谱(XMCD)信号强度随着样品磁化强化的变化而变化,同时强烈依赖于样品Tc的大小。因此,As的K边磁圆二色信号可以用来代表块材样品的磁化强度。结合X射线发射谱(XES)、吸收谱(XAS)和磁圆二色谱的测量,实验结果表明空穴掺杂以减小Mn(3d)局域自旋电子态密度为代价,增强p-d轨道杂化作用强度(MnAs4四面体中电子的重排),从而增强Mn离子间的交换作用强度。空穴优掺杂和空穴欠掺杂的样品在压力作用下,都表现出带隙的减小,调控长程铁磁序的空穴媒介载流子变的弱局域,从而抑制样品的磁有序。这两个作用效果都是由于压力作用下As的4p态的强烈反应(能带展宽)所引起的。实际上,阴离子p态和阳离子d态的电子结构随压力的演变是和MnAs4四面体中几何构型的改变和层间As-As距离紧密相连的。这对理解结构和磁性之间的关系提供了重要的信息。使用金刚石压砧,结合同步辐射X射线衍射实验,研究了Li_(1+x)(Zn_(1-y)Mn_y)As压力作用下的结构演变。在11.6GPa左右,探测到了压力诱导的结构转变,高压下的新相被提出具有Pmca的空间群。结构转变诱导的元胞体积坍缩大概在8%。相变前的结构随压力的演变关系对将来的Li(Zn,Mn)As中的铁磁性研究具有重要的指导意义。通过金刚石压砧结合具有非常高的能量分辨率的同步辐射穆斯堡尔谱(SMS),研究了1111型铁基超导体SmFeAs O在良好静水压条件下的磁(Fe)相变。随着压力的增加,磁转变温度降低,最终在25GPa左右反铁磁序消失。结合给出超导转变温度的电输运实验结果,得到了SmFeAsO的P-T相图。这为理解高温超导体中超导性与反铁磁性之间的相互作用关系补充了重要的信息。
[Abstract]:Dilute magnetic semiconductor material (DMS) is a kind of functional material which can control the degree of freedom of charge and spin at the same time. It has shown a great deal of excellent physical properties in material science and aroused extensive research interest. The substitution of mn for trivalent Ga in the classical Mn-doped III-V family of diluted magnetic semiconductors will introduce a acceptor, which is also the source of magnetic moment. Recently, Mn-doped I-II-V semiconductor Li_(1 x)(Zn_(1-y)Mn_y)As and Mn-doped II-II-V semiconductor(Ba_(1-x)K_x)(Zn_(1-y)Mn_y)_2As_2 are new dilute magnetic semiconductors. The hole and spin injection in these ferromagnets can be controlled separately. In dilute magnetic semiconductors, some progress has been made in the control of exchange interactions. However, their understanding of the microcosmic origin of physical properties is still limited. Synchrotron radiation spectroscopy (emission, absorption and dichroism), which has elemental and orbital shell resolution, has a very high energy resolution and is well suited for detecting the structure of particular electronic states, The high energy X-ray diffraction can distinguish the small changes of lattice. The mechanism of indirect exchange interaction of Mny)_2As_2 with hole doping leading to ferromagnetic ordering Tc at high temperature is studied. In addition to X-ray spectroscopy, pressure was introduced as another thermodynamic dimension to explore the lattice in these new dilute magnetic semiconductors. The complex relationship between magnetic and electronic degrees of freedom. The intensity of the K-edge magnetic circular dichroism (XMCD) signal of as varies with the magnetization of the sample and strongly depends on the size of the sample Tc. Therefore, the K edge magnetic circular dichroism signal of as can be used to represent the magnetization of bulk samples. Combined with X-ray emission spectra, absorption spectra and magnetic circular dichroism, the experimental results show that hole doping is at the cost of decreasing the density of local spin electron states. The electron rearrangement in MnAs4 tetrahedron is enhanced to enhance the exchange intensity between mn ions. Under the pressure, both the sample with excellent hole doping and the sample with underdoped hole show the decrease of the band gap and the weak localization of the carrier of the hole which regulates the long range ferromagnetic order, thus restraining the magnetic order of the sample. Both of these effects are caused by the strong reaction (band broadening) of the 4p state of as under pressure. In fact, the evolution of the electronic structure of the anionic p-state and cationic d-state with the pressure is closely related to the change of the geometric configuration in the MnAs4 tetrahedron and the As-As distance between the layers. This provides important information for understanding the relationship between structure and magnetism. By using diamond anvil and synchrotron radiation X-ray diffraction experiment, the structure evolution under Li_(1 x)(Zn_(1-y)Mn_y)As pressure was studied. The pressure induced structural transition was detected around 11.6GPa, and the new phase at high pressure was proposed as a space group with Pmca. The cell volume collapse induced by structural transformation is about 8%. The relationship between the structure and the pressure before the phase transition is of great significance for the study of ferromagnetism in the future. By using diamond anvil and synchrotron radiation Mossbauer spectroscopy with very high energy resolution, the magnetic phase transition of 1111 iron based superconductor SmFeAs O under good hydrostatic pressure was studied. With the increase of pressure, the magnetic transition temperature decreases, and finally the antiferromagnetic order disappears around 25GPa. The P-T phase diagram of SmFeAsO is obtained by combining the experimental results of electrical transport at superconducting transition temperature. This provides important information for understanding the interaction between superconductivity and antiferromagnetism in high temperature superconductors.
【学位授予单位】：中国科学院大学(中国科学院物理研究所)
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TN304

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