T-EXAFS研究过渡金属二硼化物的晶格动力学行为及其同位素效应

【作者】 储旺盛；

【导师】 吴自玉； 刘文汉；

【作者基本信息】 中国科学技术大学 ， 凝聚态物理， 2007， 博士

【摘要】 同步辐射光源具有能谱连续、高通亮和亮度、高相干性等优异特性，因此人们利用同步辐射光源发展了一系列的实验方法来研究丰富多彩的物理、化学和生物世界。其中，X射线吸收谱学（XAS）是研究原子周围局域配位环境的有力工具之一。目前XAFS已经在材料科学、纳米科学、环境科学和生命科学等研究领域发挥着巨大的作用。据统计，在世界各地的同步辐射装置上，各种形式的XAFS实验站曾占到全部实验站总数的1／4～1／3。本论文首先详细调研和总结了XAS的原理、实验方法、实验装置和数据处理方法。首次详尽地总结了目前多重散射理论在XAS领域的应用，给出了基于此理论的XANES谱学的理论推导，并将此理论推广到EXAFS谱学。其次，本论文阐述了如何利用依赖温度的EXAFS谱（T-EXAFS）去探测材料的局域振动信息，并将此方法应用于研究近年来发现的二硼化镁超导体的超导机制。2001年人们发现二硼化镁具有很高的超导转变温度（高达39K），颠覆了传统的思想观念，即常规的BCS超导体不可能突破T_c=30K。这个奇异超导体的发现也刺激着人们在其它的硼化物中，特别是与二硼化镁具有同样晶体结构的二元硼化物中，寻找新的具有高超导转变温度的超导体。然而，令人惊奇的是，到目前为止，还没有一个二元硼化物的超导转变温度超过10K。此外，二硼化镁超导机制的研究还给停滞了十几年的超导理论研究注入了新的动力。二硼化镁材料作为超导理论研究的理想模型，具有下面两个很明显的优势：1)同高温超导材料铜氧化物相比，二硼化镁具有非常简单的晶体结构，这对其微观电子结构、声子结构、电声相互作用等机制的研究极为有利；2)二硼化镁具有的超导转变温度介于低温超导体和高温超导体之间，通过对其超导机理的研究，将极大地促进高温超导理论的研究。虽然自2001年以来，关于MgB₂超导电性的研究已有成百上千篇高质量的论文发表出来。今人遗憾的是，至今人们对二硼化镁具有高的超导转变温度的真正原因还是不清楚。由于过渡金属二硼化物和二硼化镁在超导电性上的差异和晶体结构上的相似性，使得过渡金属二硼化物成为研究二硼化镁超导电性的理想的参考模型。过渡金属与二硼化镁相比，具有以下的不同：1)过渡金属二硼化物具有d电子结构；2)过渡金属二硼化物中的金属离子的质量相对较大。d电子的数目影响着这些化合物的电子结构性质，例如在这些过渡金属二硼化物中，只有二硼化钪（ScB₂）利二硼化钇（YB₂）和二硼化镁一样，硼的p_xy带在A点位于费米能级之上。金属二硼化物中，金属原子质量的差异预计将在两个方面调节这些化合物的晶格振动特性：1)质量很轻的金属将增强体系的声子频率；2)由于B原子的质量非常得小，重金属原子的振动将很难与硼原子的振动耦合。也许正是金属二硼化物中金属的质量差异调控着晶格的动力学性质，从而影响着这些体系的电声耦合，导致各自不同的超导性质。然而，到目前为止，研究这些体系动力学结构的方法还主要局限于非弹性中子散射方法。这个方法能够给出体系的广义声子态密度，但是很难给出体系的局域振动模。因此，迄今我们还不是很清楚在这些具有相同结构的金属二硼化物体系中，金属是如何调节晶格振动特性，从而影响其超导电性的。EXAFS由于其元素选择性，能够探测选定中心的局域结构信息。通常我们利用EXAFS探测的局域结构信息包含吸收中心周围的配位原子类型、数目、距离和无序度。其中，无序度包含静态无序和热无序，热无序包含有局域结构动力学信息。由于受X射线的激发，吸收原子的芯态电子成为光电子，光电子在吸收原子和配位原子之间的单次散射导致了EXAFS信号的产生。因此，EXAFS信号携带的结构信息是二体相关的，与XRD实验所获得独立原子的动力学信息具有显著的不同。通过测量EXAFS振荡随温度的变化（T-EXAFS），原子对分布函数宽度σ的温度依赖关系能够被提取出来，分离出决定宏观物性的不同贡献，从而将宏观观测的物性同微观原子局域振动相互作用联系起来。本论文我们利用XANES谱并结合从头计算研究了3d过渡金属二硼化物的电子结构。然后，利用T-EXAFS精确测量了4d和IVB族过渡金属二硼化物的Debye-Waller（DW）因子对温度的依赖行为。对于过渡金属二硼化物的阳离子和阴离子配位层，我们首次观察到DW vs．T函数展现出奇异的依赖行为，这种行为我们不能够简单地利用传统的Einstein关联模型进行描述了。为此，我们发展了一个含有两个Einstein特征温度的扩展Einstein关联模型，利用这个模型我们成功地描述了这种依赖行为。通过扩展Einstein模型，我们获得了两个决定阳离子和阴离子配位层局域振动的振动模式，其中一个振动模式具有很高的频率，而另一个振动模式的本征频率则很低，这两个振动模式分别对应于B原子相对振动的光学声子和对应于金属原子集体振动的声学声子。通过对比研究4d过渡金属二硼化物的局域振动，我们发现由于过渡金属4d电子数目的不同导致成键强度的不同，从而影响了局域振动频率的大小。也就是说，过渡金属原子通过调制其组成二硼化物的电子结构而影响着其晶格振动动力学行为。通过对比研究IVB族过渡金属二硼化物的局域振动，我们发现过渡金属的质量影响着阳离子和阴离子配位层局域振动的退耦合程度，这个退耦合程度我们可以通过扩展Einstein模型中的参数a来表征。过渡金属质量越大，退耦合效应就越强。利用我们发展的扩展Einstein模型，我们还第一次直接测量了与MgB₂具有相同晶体结构和相似电子能带结构的YB₂的局域振动动力学行为中的B同位素效应。在¹⁰B和¹¹B的两个YB₂中，其局域振动频率存在一个能量漂移，这个漂移对于光学声子振动显得尤为明显～4.3meV，对于声学声子振动这个漂移为1～2 meV。通过YB₂局域振动的同位素效应研究，我们从实验上直接将B-B的振动和此类非强关联体系的超导电性关联起来。简而言之，通过这些过渡金属二硼化物的T-EXAFS研究以及和MgB₂的对比，我们获得了下面两个至关重要的结论：1)在过渡金属二硼化物中，由于其特殊的层状结构以及金属和硼原子之间巨大的质量差异，使得金属原子和硼原子之间发生了退耦合效应，这种退耦合效应随着金属质量的增加越发明显。正是这个振动退耦合效应使得过渡金属二硼化物无法向MgB₂那样获得令人惊奇的超导转变温度；2)通过研究YB₂的局域振动的B同位素效应研究，我们从实验上直接证实了B-B的振动对于金属二硼化物超导电性起着直观重要的作用。我们研究得到的这两个结论对于理解到底什么因素使得MgB₂具有如此不同的超导电性起到积极的作用。在金属二硼化物中，B-B的振动是超导电声耦合的关键，但是单纯的B-B振动使得二硼化物的超到电性趋于平庸。在MgB₂中，正是Mg金属原子和B原子在质量上的接近，使得MgB₂中B原子的振动和Mg原子的振动发生了关键性的耦合，导致了其超导转变温度的增加。更多还原

【Abstract】 Synchrotron radiation （SR） sources can provide a powerful x-ray beam in a wide energy range, extremely intense and with a high polarization rate. Nowadays, there are 55 SR facilities operational in the world that provide a great deal of x-ray experimental opportunities. Among the different SR techniques the x-ray absorption spectroscopy （XAS） is one of the most powerful to investigate the local electronic and geometrical structure around a photoabsorber with sub-Angstrom resolution. Up to date, XAS has been extensively applied to material science, nano-science, enviromental science, life science, etc. According to the latest statistics, the XAS station represent about 1/4, probably -1/3 of all experimental stations in the SR facilities existing all over the world.In this thesis, we first introduced XAS principles, than the experimental methods, the BSRF facility and finally a detailed discussion of the data analysis will be presented. For the first time, we discuss the applications of the multiple-scattering theory to XANES data and the possible extention to the treatment of EXAFS. In addition, we will introduce a special EXAFS method: the temperature-dependent EXAFS （T-EXAFS）, which may provide unique information about the lattice dynamics. The superconductive mechanism of MgB₂, a new exotic superconductor was investigated with such technique.Magnesium diboride is superconducting at 39 K and its discovery dates back at the beginning of this century, characterized by the highest known transition temperature for a non-copper-oxide bulk material. Its discovery is a relevant breakthrough because is the first conventional superconductor system that overcome the theoretical value of the BCS theory set to T_C = 30 K. The MgB₂ superconductivity triggered the research towards other diboride compounds, in particular transition-metal diborides, systems that all crystallize in the well known AlB₂ structure, although none of them showed superconductivity above 10 K. Moreover, great interest both from the experimental as well as the theoretical point of view is the understanding of the superconductive mechanism in this binary compound characterized by:1) a simple lattice structure and the lack of both magnetic contributions and a strong electron-electron correlation;2) a critical temperature （i.e., for MgB₂） very close to those of the high-Tc superconducting cuprates （HTSC）.Theoretical researches regarding HTSC superconductive mechanism did not produced relevant discoveries in the last two decades and the discovery the MgB₂ superconductor represent a new opportunity. Many teams are focusing their interest to MgB₂ and its related binaries, and since 2001 hundreds of investigations are now available. Still, the superconducting mechanism of this system is an open problem.Due to their identical geometrical structure associated to different （and lower） critical temperature, transition-metal diborides （TMB₂） represent ideal reference models of the superconductive mechanism in MgB₂. Compared with this latter system, transition-metal diborides differ by:1) the occupancy of the d state;2) the large mass of the metal atoms.Clearly, the number of d electrons affect the electronic structure. As an example, among all transition-metal diborides, only ScB₂ and YB₂ share similar band structures with MgB₂, and in fact, in these systems the p_xy bands lie at the A point above the Fermi level. Energy band calculations of MgB₂ shows that Bσband may significantly affect the superconductive mechanism. The mass difference of metal atom should modulate the lattice vibrational property because:1) the light mass of metal enhances the phonon frequency;2) the vibrations of an heavy transition metal do not couple with the vibration of boron due to their large mass difference.Nevertheless, theoretical studies predict a coupling dynamic that plays a key role in order to increase the MgB₂ critical temperature not yet confirmed by experiments. Inelastic neutron scattering technique represents a common tool to investigate the lattice dynamics of these systems, but this method cannot return local information of lattice vibration or correlation information of the atomic pair. However, it is well known that a significant coupling between the local structure and other properties of the system such as superconductivity occurs. Due to its element selectivity EXAFS can give local structure information such as the type of coordination atom, the coordination number, the coordination distance and the disorder degree around the absorbing atom. Moreover, the Debye-Waller factor has two components: the static disorder and the thermal disorder. This latter is correlated to the lattice dynamics and these information may be extracted by EXAFS, e.g., by the pair distribution function because the EXAFS signal is associated to the photoelectron scattering between the absorbing atom and its first neighbors. The same correlation can not be obtained by the XRD technique. Moreover, a detailed temperature-dependent study （T-EXAFS） of the width of the Pair Distribution Function （PDF）,σ, may allow to separate the different contributions and to provide a connection between macroscopic observations and the underlying atomic interactions.In this thesis, we present first XANES data at the metal K-edge of 3d transition-metal diborides and ab initio full multiple-scattering calculations. Moreover, we measured the temperature dependence of the Debye-Waller factors in 4d and IVB serial transition-metal diborides have been characterized with this technique. An anomalous behavior of the Debye-Waller factor of the TM-B pair has been detected for the first time. The anomalous behavior is not described by the Einstein model. Therefore, a modified Einstein model taking in consideration two frequencies was introduced to describe the observed behavior. Using this model, the observed behavior is interpreted as the superposition of an optical mode corresponding to phonon vibrations induced by the boron sublattice and an acoustic mode corresponding to the metal sublattice vibration.By comparison of the XANES spectra of 4d transition-diborides which have similar metal mass but different number of 4d electron, because electronic structure affects the strength between metal and boron atoms we found that the vibrational frequency is affected by the number of 4d electrons.By comparison of the XANES spectra of IVB serial transition-diborides which have similar electronic structures but different metal masses we observed an effective decoupling between metal and boron vibrations. The phenomenon may be characterized by the parameter a in the modulated mode and the decoupling is a function of the metal mass.For the first time we combined T-EXAFS and the modulated Einstein model, we investigated also the boron isotope effect and the lattice dynamics of YB₂, a system that has the same crystal structure and a similar electronic structure of MgB₂. An evident energy shift in the lattice vibration （- 4.3 meV） for the optical mode and about 1.2 meV for the phonon mode, was observed for both boron isotopic compounds. Data also confirm the hypothesis that B-B vibrational mode plays an role in the superconductive mechanism in the transition-metal diborides and in particular in MgB₂.To summarize, two are the relevant results obtained by the lattice dynamics study of transition-metal diborides presented in this thesis:a) in agreement with theoretical calculations, the B-B vibrational mode plays a key role in the electron-phonon coupling, an important parameter of the BCS theory.b) experimental data support the model in which the vibrational decoupling between metal and boron atoms is correlated to the mechanism that significantly reduce the superconducting critical temperatures of TMB₂ respect to the MgB₂ compound;These results are important to improve the knowledge of the MgB₂ system and its superconducting properties. In metal diborides, the B-B vibration is a relevant item for the electron-phonon interaction, but a pure the B-B vibration makes the superconductive property of transition-metal diboride go to commonness. Moreover in MgB₂ system, a strong coupling between boron and metal vibrations occurs due to the large difference of mass, then introducing a high critical temperature.更多还原