节点文献
基于同步辐射实验方法的低维系统物理性质研究
【作者】 杨科；
【导师】 封东来；
【作者基本信息】 复旦大学 ， 凝聚态物理， 2006， 博士
【摘要】 本论文介绍了我利用基于同步辐射的非弹性x光散射和角分辨光电子谱实验方法研究低维物理系统的一些工作。非弹性x光散射和角分辨光电子谱技术是两个互补的技术，非弹性x光散射探测的是电荷密度关联函数，告诉我们系统电子激发的信息；而角分辨光电子谱测到的是单粒子谱函数，告诉我们电子结构信息。依照维度的顺序，从低到高，本论文先从近似零维的有机小分子开始，而后是准一维的无机Mott绝缘体和有机链状导体，最后介绍我的关于准二维的高温超导体Bi2201系统的工作。我们首次将非弹性x光散射技术（IXS）运用于研究有机系统。证明了非弹性x光散射对有机系统是可见的。对于有机光学材料中的激子研究，之前电子能量损失谱（EELS）已有一些工作，这些工作得到了对于共轭有机分子来说，激子尺寸是分子尺寸而和分子具体结构无关的结论。但是对于EELS的数据解释，一直以来都存在争议。我们的工作提出了一个实验上的反例证明了之前对于EELS结果的解释是错误的。并且结合ZINDO／SCI的量子化学计算方法，我们发现对于Py-SO这样的有机复杂小分子系统，最低能量的激子尺度仅局限于一个分子中小的区域。推翻了之前EELS得到的结论。我们还观察到了最低能量激子的有限色散，约为130meV。说明分子间的作用在该系统中并不能完全忽略。我们的结果对于光学功能材料的设计有着指导意义，尤其是那些基于激子的转移和运动的材料。在成功将IXS应用于有机系统后，我们进一步研究了C60单晶系统。并得到了很漂亮的实验结果。对不同方向的能量损失谱比较，发现结果完全一致，说明我们得到的是C60分子中激发的信息，分子间的作用很弱。进一步的分析正在进行中。对于一维系统的研究，我主要利用共振x光非弹性散射研究了无机Mott绝缘体Sr2CuO3系统和利用角分辨光电子谱研究了有机导体TTF-TCNQ系统。对于一维的量子自旋系统Sr2CuO3的研究。和以前的工作不同，这个工作的实验是在Cu的1s到3d吸收边作的，得到和Cu 3d轨道相关的电子激发的更直接的信息。能量损失谱显示系统能量最低的激发约在2.0eV，比在1s到4p边得到的最低峰能量小了约0.6eV。且实验观察到的该特征峰的色散约有0.2eV，周期为π。这和在1s到4p观察到的结果都不一样，和光学测量结果比较，我们认为这个低能激发应该是d-d间的激发。对于一维有机链状导体TTF-TCNQ，通过对角分辨光电子谱结果的细致研究，我们发现存在一些反常的奇怪现象。我们观察到了约100meV的赝能隙存在，而且谱中有很宽的“准粒子”峰线型。另外，散射率和能量的关系表现出了类似费米液体的二次型行为，而且电阻率的测量也有类似现象。我们的工作，为理论提出了新的问题。对于铜氧化物超导体的研究，之前主要集中在欠掺杂区。而对过掺杂区，特别是超过掺杂区的研究较少。而要对高温超导机理有全面理解，对超过掺杂区的了解也是比不可少的。本工作利用角分辨光电子谱（ARPES）对单层的高温超导体Bi2201系统过掺杂，特别是超过掺杂区作了系统研究。超过掺杂区样品掺杂程度到超导消失。我们的得到的结果显示电—声相互作用对于节点方向的准粒子行为是占主导地位的。而在反节点区域，磁涨落的影响起了关键作用。我们的发现为目前存在的争论提供了又一个新的实验证据。另外，尽管在高掺杂的地方伴随着超导电性的逐渐消失，长程的磁涨落消失了。但是我们发现短程的磁涨落依然决定了准粒子的色散，这个区域的关联依然很强。而且，我们发现对于整个过掺杂区，系统依然表现“奇异金属”性质，而不像之前认为的那样，是“正常金属”。上述实验结果表明低维体系中存在着很多有趣的物理现象和未解决的关键问题。通过相关同步辐射技术的有效研究，我们能够在微观机理层面推进对它们的理解。同时在解决旧问题的同时，往往也会进一步提出新的更深入的问题。
【Abstract】 In this thesis, I have investigated the fundamental physical properties of several low dimensional systems by using inelastic x-ray sacattering (IXS) and angle resovled photoemission spectroscopy (ARPES) techniques. According to the dimensionality, I will first introduce the works about the small organic molecular system which can be considerd as zero dimensional system, and then I will talk about one dimensional system-inorganic chain SrCuO and orgnic chain TTF-TCNQ. In the end, I will introduce my work about the 2D cuprate superconductor Bi2201 system.We pioneered the study of exciton behavior in organic molecular systems with IXS. Combined with quantum chemical calculations based on ZINDO /SCI, which produce theoretical spetra in good agreement with experiments, we found that the lowest Frenkel exciton in a small molecule can be confined to only a fraction of the molecule, instead of extending over the entire molecule. The dispersion of this exciton is about 130 meV. Moreover, we present a clear experimental counterexample to prove that the momentum structure of the dynamic structure factor is NOT straightforwardly associated with the size of the exciton, providing strong experimental evidence for a critical theoretical claim.For the study of the quasi 1D Mott insulator SrCuO, unlike previous studies, we performed the resonant inelastic x-ray scattering (RIXS) experiments at the Cu 1s to 3d edge, obtaining more direct information on the electronic excitations involving the Cu 3d orbitals. The energy loss spectra indicate that the lowest peak is around 2．0 eV, which is 0．6 eV smaller than the lowest energy peak obtained at the 1s to 4p edge. The feature exhibits a total dispersion of about 0．2 eV with a periodicity ofπ. Comparison of the data with optical results supports the interpretation of the data as being from dd excitation.In the ARPES study of another one dimensional system-organic conductor TTF-TCNQ. I observed some anomalous phenomena. For example, there is a very deep pseudogap about 100meV and very broad "quasiparticle" peak in the spectra. Moreover, in such a 1D system, the temperature and energy dependence of the electron scattering rate exhibits Fermi liquid like behavior, in consistence with the resistivity measurements. These raised serious questions on the microscopic properties of such a system with both strong electron-electron interaction and electron-phonon interaction.Previous experimental studies on cuprate superconcutor have been mostly focusing on the anomalous, and certainly interesting, underdoped regime. On the other hand, although to be validated by experiments, the overdoped half of the phase diagram, particularly the heavily overdoped regime, is considered to be a "normal" metal regime where correlations are negligible. The electronic structure in this regime was seldom studied. To build a comprehensive picture of high temperature superconductivity, it is crucial to study the properties of the entire overdoped regime, particularly the heavily overdoped regime. We explore the electronic structure in the heavily overdoped regime of the single-layer cuprate superconductor BiPbSrCuO. We found that the nodal quasiparticle behavior is dominated mostly by phonons, while the antinodal quasiparticle line shape is dominated by spin fluctuations. Moreover, while long range spin fluctuations diminish at very high doping, the local magnetic fluctuations still dominate the quasiparticle dispersion, and the system exhibits a strange metal behavior in the entire overdoped regime.Our results indicate that there are plenty of interesting physical phenomena and unresolved problems in the low dimensional systems. By utilizing the powerful tools based on synchrotron radiation, we can understand the complications in the low dimensional system better. Moreover, it is often that we always can find new and more fundamental problems when we resolve old questions.