Optical Properties of Two-Dimensional Organic Conductors: Signatures of Charge Ordering and Correlation Effects
Optical Properties of Two-Dimensional Organic Conductors: Signatures of Charge Ordering and Correlation Effects
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二维有机导体的光学性质：电荷排序和相关效应的特征
DOI：
10.1002/chin.200505259
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发表时间：
2005
期刊：
影响因子：
--
通讯作者：
N. Drichko
中科院分区：
文献类型：
--
作者：
M. Dressel;N. Drichko
从广义上讲，光学研究是表征新材料和研究固体性质的最强大的技术之一，因为电磁波覆盖了很大的能量范围，并且对不同类型的激发敏感。我们不仅可以获得关于电子和磁性行为的信息，还可以获得关于振动自由度的信息，包括晶格性质以及分子内振动。[1]在射频范围及以下，如果晶格变软，可以观察到宽的弛豫特征，如铁电相变所知，但在无序或某种玻璃态行为的情况下，该光谱区域也具有上级重要性。与密度波和其他对称性破缺基态相关的集体激发位于低频，通常远低于等离子体频率（戈德斯通模式）。这些电子和磁性长程有序的基态在低维金属中具有特别的相关性，在低维金属中电子气表现出不稳定的趋势。2在超导的情况下，单粒子态密度的能隙预计在与超导转变温度相当的能量范围内，即在微波频率和远红外光谱范围之间。特别重要的是由于电子-电子和电子-声子相互作用的影响，这可能会导致一定的散射率的频率依赖性和增强的有效质量的载流子。1有机金属的等离子体边缘通常位于红外线，只有少数高导电材料例外，即使在更高的能量下。如果涉及不同的电子能带，在常规材料中对传导和价电子的光学响应的明确分配已经是困难的; 1，3-5一般来说，在合成金属的情况下不可能有明确的区别。这一事实使得这些化合物的光学数据的分析不是一个简单的任务，这也是自分子导体早期以来许多讨论的原因。正如从经典半导体中所知道的，带间跃迁导致光谱中的显著特征，并且可以进行非常详细的分析。6
Optical studies, in a broad sense, are one of the most powerful techniques to characterize new materials and to investigate the properties of solids because electromagnetic waves cover a large range of energy and are sensitive to different kinds of excitation. We can obtain information not only on the electronic and the magnetic behavior, but also on the vibrational degrees of freedom, which include lattice properties as well as intramolecular vibrations. 1 In the radio frequency range and below, broad relaxational features can be observed if the lattice becomes soft, as known from ferroelectric phase transitions, but this spectral region is also of superior importance in cases of disorder or some sort of glassy behavior. Collective excitations which are relevant for density waves and other broken-symmetry ground states are located at low frequencies often well below the plasma frequency (Goldstone modes). These ground states of electronic and magnetic long-range order are of particular relevance in low-dimensional metals where the electron gas exhibits the tendency toward instabilities. 2 In the case of superconductivity, the energy gap in the single-particle density of states is expected in an energy range comparable to the superconducting transition temperature, ie, between microwave frequencies and the far-infrared spectral range. Of particular importance are effects due to electron-electron and electron-phonon interaction, which may lead to a certain frequency dependence of the scattering rate and an enhanced effective mass of the charge carriers. 1 The plasma edge for organic metals is commonly located in the infrared and only for a few exceptions of highly conducting materials even at higher energies. The unequivocal assignment of the optical response to conduction and valence electrons is already difficult in conventional materials if different electronic bands are involved; 1, 3-5 in general, no clear-cut distinction is possible in the case of synthetic metals. This fact makes the analysis of optical data of these compounds not a straightforward task, and it is also the reason for many discussions since the early days of molecular conductors. As known from classical semiconductors, interband transitions lead to pronounced signatures in the optical spectrum and can be analyzed in great detail. 6