金属氧化物功能材料奇异电子结构的高分辨角分辨光电子能谱研究

【作者】 欧宏炜；

【导师】 封东来；

【作者基本信息】 复旦大学 ， 凝聚态物理， 2009， 博士

【摘要】 强关联电子材料，特别是金属氧化物材料，因具有多自由度竞争（电荷、自旋、轨道、晶格等）和低维性，呈现出丰富多彩的物理性质和物质形态（例如，高温超导，巨热电势效应等），以及潜在的巨大应用价值，不仅成为凝聚态物理学界最热门的研究课题，更得到工业界的广泛关注。近年来，随着研究的不断深入，新型非常规高温超导体和更高热电系数材料的不断出现，金属氧化物功能材料引起越来越广泛的兴趣，促进了物理、化学、材料学等多个学科科学家间的紧密合作。目前，同步辐射技术与实验仪器的不断进步，为研究各种新型材料提供了强大的实验工具，并已有效地应用于研究新型电子材料的电子结构、相变和各种有序现象、小尺度结构和微量分析等方面。其中，角分辨光电子能谱作为目前唯一可以同时测量固体中费米能级附近电子能带结构、价电子的能量、运动方向和散射性质的实验手段，被广泛的应用于研究各种关联电子材料的电子结构、相变及其中的各种有序现象。本文介绍了利用高分辨角分辨光电子能谱技术对具有高热电系数的不匹配层状结构氧化物和三角晶格钴氧化合物，以及新发现的铁基高温超导体等材料的电子结构进行系统的研究，得到了以下结果：1．结合同步辐射技术，利用高分辨角分辨光电子能谱对具有巨热电系数的不匹配层状结构氧化物材料Bi₂Ba_1.3K_0.6Co_2.1O_7.94（BBKCO）的电子结构进行了细致的研究，揭示了在巨大应变存在下氧化物界面的电子结构。BBKCO的结构是由交替堆叠的岩盐结构[BiO／BaO]层和六角的[CoO₂]层构成，我们发现不同氧化物层的低能电子态被分别局限在界面两边的各自层内，但是仍然会受到另一边的非公度晶体场的散射作用。并且，由于岩盐层[BiO／BaO]巨大应力的存在，而导致产生大的电荷转移到邻近的[CoO₂]层。此外，我们还发现另一个奇异的性质，就是由于界面效应而被增强的电子-声子相互作用。我们的工作为理解氧化物界面上的物理问题提供了电子结构基础，并且也为将来设计氧化物电子学器件提供了重要的指导意义。2．利用高分辨角分辨光电子能谱技术对不同掺杂三角晶格钴氧化合物A_xCoO₂（A=Na，K）的电子结构进行了系统研究。这类材料不仅具有较高的热电系数，其中Na_xCoO₂还是非常规超导体Na_xCoO₂·yH₂O的母体材料，因此研究A_xCoO₂的电子结构及费米面的拓扑结构对解释其超导机理及热电性能有着重要的辅助作用。我们通过对不同掺杂样品的测量比较，得到了母体材料A_xCoO₂细致的能带结构、准粒子散射率及费米面拓扑结构等信息，加深了对这类钴氧化合物及其超导样品的理解。3．对新型铁基高温超导体SmO_1-xF_xFeAs的电子结构进行了角积分光电子能谱研究。我们通过研究1111系铁基超导体SmO_1-xF_xFeAs费米面附近的态密度随着温度变化而发生的演化，发现随着温度降低，在所有掺杂样品中都存在明显的态密度压制，并且观察到了两个特征能量尺度——10meV和80meV。但是，这两个能量尺度并不随着掺杂变化而发生变化，存在于所有掺杂样品中。对于10meV能量尺度的态密度压制，我们认为是由于反常的不随掺杂变化的正常态赝能隙导致的，当进入超导态时，它将演变为超导能隙；而对于80meV的压制，我们提醒指出，这可能是由于多晶样品中的杂质因素导致的。我们的工作在铁基超导体发现早期，首先对该类样品进行了系统细致的光电子能谱研究，为后续研究提供了很好的参考作用。4．利用高分辨角分辨光电子能谱技术对新型铁基超导体BaFe_2-xNi_xAs₂（x=0.1，0.16，0.2）单晶样品的电子结构进行了系统研究。我们的结果显示，作为122系铁基超导体母体材料的BaFe₂As₂的Fe位原子被同样带有磁性的Ni原子替代，这三种掺杂样中均不存在SDW而仅是超导现象，表现在能带结构上，就是随着Ni掺杂增加，能带仅表现出刚带模型的特点——费米能级的提高以及对应的能带向下推移，没有出现具有SDW转变特征的的能带劈裂及折叠现象，提供了一个简单清晰的能带结构图像，为研究其它铁基高温超导体的电子结构提供了参考意义。更多还原

【Abstract】 Strong correlated electronic materials, especially the metal oxidefunctional materials, which possess the characteristic of competition betweendifferent degrees of freedom（such as the charge, spin and orbital） and the lowdimentionality, present a variety of physical properties（e.g., high-temperaturesuperconductivity, giant thermoelectric potential effect） and enormous potentialvalue. Thus, they have not only become the hottest research topic incondensed matter physics and material science but also aroused extensiveconcern in the industrial community. In recent years, with the further advanceof research and the emergence of novel materials（such as the unconventionalsuperconductors and the materials with giant thermoelectric Seebackcoefficient）, the metal oxide functional materials have attracted tremendousinterests, and also promoted close cooperation among scientists in the field ofphysics, chemistry and material science. At present, with the progress of thesynchrotron radiation techniques and instruments, it offers a direct andeffective experimental probe to understand the fundamental problems in thesenovel materials. Among them, angle-resolved photoemission spectroscopy（ARPES）, which is the sole tool to simultaneously detect the electron’s energy,moving direction and scattering property near Fermi energy in solids, has beenwidely used in investigating the novel electronic structures, phase transitionsand various orderings in such strong correlated materials. In this dissertation,we report some progress in studying the electronic structures of themisfit-layered oxides, triangular-lattice cobalt oxides and Iron-basedsuperconductors by means of ARPES. Besides, we would like to reveal themicroscopic electronic structure of the formation of their different electronicground states. The corresponding results are listed as follows.1. The misfit-layered oxide with giant thermoelectric Seebeckcoefficient, Bi₂Ba_1.3K_0.6Co_2.1O_7.94, is studied by high resolution ARPES,which revealing the electronic structure of a highly strained oxide interface.We find that low-energy states are confined within alternatingrocksalt-structured[BiO/BaO] layers and hexagonal[CoO₂] layers on bothsides of the interface respectively, but still affected by the incommensurate crystal field scattering from the other side. Furthermore, the high strain onthe rocksalt layer induces large charge transfer to the[CoO₂] layer.Besides, a novel effect, the interfacial enhancement of electron-phononinteractions, is discovered. Our findings provide an electronic structurefoundation for understanding oxide interfaces and have significantguidance in designing oxide devices.2. The electronic structure of hexagonal structure cobalt oxidesA_xCoO₂（A=Na, K） is studied by means of high resolution ARPES. Thesetypes of materials not only possess a relatively high thermoelectric powerin the thermoelectric materials but are the parent compounds of theunconventional superconductors A_xCoO₂·H₂O as well. Therefore,revealing the general electronic structure and the Fermi surface topologyplays an auxiliary role in understanding the mechanism ofsuperconductivity and high thermoelectric power. The results of differentdopings of A_xCoO₂ are compared and the detailed electronic structure andthe information of Fermi surface are obtained, which help tocomprehensively understand the superconductivity and highthermoelectric power.3. The temperature dependence of the density-of-states in theiron-based superconductor SmO_1-xF_xFeAs（x=0, 0.12, 0.15, 0.2） isinvestigated by high resolution angle-integrated photoemissionspectroscopy. The density-of-states suppression is observed with thedecrease of temperature in all samples, revealing two characteristicenergy scales（10 meV and 80 meV）. However, no obvious dopingdependence is observed. We argue that the 10 meV suppression is due toan anomalously doping-independent normal state pseudogap, whichbecomes the superconducting gap once in the superconducting state; andalert the possibility that the 80 meV-scale suppression might be an artifactof the polycrystalline samples.4. The electronic structure of the Iron-based superconductorsBaFe_2-xNi_xAs₂（x=0.1, 0.16, 0.2） single crystals is studied systematically. Our results demonstrate that, with the partially substitution of Fe with Ni inthe parent compounds BaFe₂As₂, there is no SDW transition exiting inthese three dopings but only superconducting transition. With theincreasing doping of Ni, the electronic structure only shows the characterof rigid band model: there is no band splitting and folding, but Fermi levelraise and bands shift towards to higher binding energy. Our results providea simple and clear band picture, which is useful for the study of otheriron-based superconductors.更多还原