复杂过渡金属化合物的新奇物性的光电子谱研究

【作者】 杨乐仙；

【导师】 封东来；

【作者基本信息】 复旦大学 ， 凝聚态物理， 2011， 博士

【摘要】 过渡金属化合物体系中的强电子关联以及其中表现出来的丰富的相变、有序以及多自由度竞争,例如电荷,轨道,自旋以及晶格之间的相互作用,一直以来都是凝聚态物理研究领域的热点。随着各种新奇的材料不断被合成,新奇的物理性质被发现,引发了一场又一场凝聚态领域的研究热潮,推动了物理,化学,材料等交叉学科的发展。这些新奇的物理化学性质使得强关联电子材料有可能代替硅而成为新一代电子器件的基底材料,其潜在的应用价值也已经受到广泛关注。另一方面,超高真空、极低温以及超短脉冲激光等技术的迅速发展将我们不断地推进到更复杂、更微观以及超快的研究领域。由于材料尤其是单晶材料的物理化学等性质很大程度上决定于它的电子结构,因此对于材料电子结构的研究在理论和实验上都是非常重要的。而角分辨光电子能谱(Angle Resolved Photoemission Spectroscopy, ARPES)作为唯一一种可以直接在动量空间对材料的电子结构进行刻画的实验工具,被广泛的应用在物理、化学以及材料等领域。它在电子结构、相变以及各种有序现象等负责的研究领域发挥了非常重要的作用,为我们揭示了很多新奇物性的微观本质。而时间分辨的双光子光电子能谱(Time-Resolved Two Photon Photoemission, TR2PPE)更是对传统ARPES的一个强有力的补充。它能够直接在时间域和动量空间对电子的能态以及激发、驰豫等超快的动力学过程进行直接的研究。而载流子激发后的瞬态行为及非平衡态信息在表面、界面电子学,电热输运以及化学反应动力学等领域有重要的研究意义,对于这些动力学过程的理解为我们提供了研究多体体系内部基本相互作用的必要信息,本文首先简单介绍了ARPES技术,然后介绍了利用超高分辨的ARPES在新型铁基超导母体“122”以及“1111”体系中得到的一些结果,最后介绍了我们在实验室内搭建的TR2PPE系统以及在一些简单体系中取得的初步结果,主要的结果如下：1.利用高分辨的ARPES,我们对铁基超导“122”体系母体材料BaFe2As2的电子结构进行了详细的研究。我们率先给出了BaFe2As2正常态和低温自旋密度波(Spin Density Wave)态的电子结构。我们发现低温下体系发生能带劈裂,从而节省了体系的总能量,有利于SDW相变的发生。另外,我们在费米面上观察到可能的能隙打开,说明费米面嵌套机理在SDW相变中也许也起了一定的作用。我们的结果为铁基超导材料中SDW转变的奠定了基础。并为理解超导和SDW之间的关系提供了一定的参考意义。2.利用高分辨的ARPES,我们对“1111”体系的两种代表性的母体材料LaFeAsO和SmFeAsO的电子结构做了详细的研究。我们通过各种实验手段,在“1111”体系的两种代表性的母体材料中确认了一部分体带和表面带。我们发现低温下体带的重构节省了体系的总能量,有利于相变的发生。另外,“1111”体系中尖锐的准粒子峰和比较好的二维性有可能与为什么“1111”体系能够保持铁基超导体的超导转变温度的记录有关。我们的结果将有助于建立一个全局性的铁基超导体的物理图像。3.我们对1/2掺杂的La1-×Sr1+×M n04体系的电子结构做了研究。体系的谱线很宽且具有强耦合体系的非相干特性。有意思的是,这样一个强关联的绝缘体体系中的电荷有序原本应该是用局域的物理图像来描述,但是我们观察到了剩余费米面的嵌套。而且根据自相关函数计算的结果,剩余费米面嵌套在电荷有序中起到了非常重要的作用。我们的结果说明该类绝缘体材料中的电荷有序和传统的巡游的电荷密度波之间存在着某种相似性。也许不能用纯粹的局域化的理论来描述此类强关联绝缘体中的电荷有序。4.利用超短脉冲激光技术,我们在实验室内搭建了TR2PPE系统,用于未占据态、载流子寿命以及超快相变过程的研究。目前系统搭建基本完成,可以在不同的激发模式下切换。我们已经在单晶Cu等一些简单体系中得到一些初步的结果。更多还原

【Abstract】 The strong electronic correlation in the transitional metal compounds has attracted more and more attention due to the abundant novel phenomena in these systems. Varies materials with peculiar properties has been synthesized, triggering intensive research wave in condensed matter physics and promoting the development of the physics, chemistry and material science as well as their interdiscipline. These novel properties suggest that the strongly correlated systems might be a very good candidate for the fabrication of the next generation of electronic devices after Si. The potential application value of them has received attention from the industry. Moreover, the state-of-art technologies such as ultra-high vacuum, ultra-low temperature and ultra-short LASER system lead us to a world which is more complex, more microcosmic and ultra fast. As we known, the physical and chemical properties of the materials, especially the single crystals, are determined by their electronic structure to a great extent. Thus the research on the electronic structure is always the key problem both theoretically and experimentally. Angel Resolved Photoemission Spectroscopy (ARPES), as the unique instrument that could depict the electronic structure directly in the momentum space, is comprehensively used in the physics, chemistry and material science. It plays a key role in the research on the novel phenomena such as phase transitions, ordering and the competition between different degrees of freedom and reveals the microscopic nature of them. On the other hand, Time Resolved Two Photon Photoemission (TR2PPE) is a powerful compensation for the conventional ARPES. It could detect the electronic structure as well as the ultra-fast dynamical processes such as electron excitation, relaxation directly in the time domain and momentum space, while the transient behavior of the excited charge carriers and the non-equilibrium sates are crucial in the surface (interface) electronics, charge (thermal) transport and chemical reaction dynamics. The understanding of these dynamical processes provides us necessary information on the basic interaction in the strongly correlated system.In this dissertation, we report some interesting results in the electronic structure of the "122" and "1111" series of newly discovered iron based superconductors. Besides, the TR2PPE system in our group and some preliminary results are introduced. The corresponding results are as follows:1. The parent compound of "122" series of iron based compounds is studied by high resolution ARPES. The band splitting in the spin density wave (SDW) state is revealed. The system saves total energy by band splitting, which favors the SDW transition. On the other hand, we observed possible gap in the Fermi surface of certain band, suggesting the role of Fermi surface nesting mechanism in the phase transition. Our results set the foundation for the further study of the SDW transition in the iron based compounds. It would be helpful for the understanding of the relationship between superconductivity and SDW.2. We studied the electronic structure of the representative parent compounds of "1111" series in the iron based compounds. We identified the bulk states and the surface states induced by surface charge redistribution. We found that the system saves total energy by band reconstruction instead of gap opening. This is consistent with the results in the other series of iron based compounds. On the other hand, the sharp quasi-particle peaks and weak three dimensional nature of "1111" series might relate to the myth why it holds the record of the superconducting transition temperature of iron based superconductors. Our results would help to construct a global picture of the physics in the iron based compounds.3. We studied the electronic structure of half doped La1-xSr1+xMnO4. The spectra are quite broad and incoherent, which is consistent With the strongly correlated property of the system. The charge ordering in such a strongly correlated insulator should be described by a local picture. However, we observed the nesting of the remnant Fermi surface. Moreover, according to the analysis of auto-correlation, the nesting of the remnant Fermi surface play a key role in the charge ordering transition. Our results suggest that there is certain similarity between itinerant charge density wave in the metallic system and the charge ordering in such strongly correlated insulators. The pure local picture might not be enough to describe such charge ordering transition.4. Based on the ultra-short LASER system, we construct a TR2PPE system for the research on the unoccupied states, carrier lifetime and ultra fast phase transition. At present, the system has been completed. It has different excitation mode, which could be switched over easily. We have got some preliminary results on single crystalline Cu.更多还原