Emergent Optical Phononic Modes upon Nanoscale Mesogenic Phase Transitions
Emergent Optical Phononic Modes upon Nanoscale Mesogenic Phase Transitions
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DOI：
10.1021/acs.nanolett.7b01324
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发表时间：
2017-06-01
期刊：
影响因子：
10.8
通讯作者：
Cunsolo, Alessandro
中科院分区：
文献类型：
--
作者：
Bolmatov, Dima;Zhernenkov, Mikhail;Cunsolo, Alessandro
由于解析低能模式的实验技术的局限性，在分子尺度上对软物质系统中的声子集体激发的研究一直具有挑战性。非弹性 X 射线散射 (IXS) 的最新进展使得对此类系统的研究能够在 meV 激发能量下具有前所未有的光谱对比度。特别是，有可能揭示其形态和相行为易于操纵的材料（例如介晶系统）中的低能集体运动。理解具有 Q 依赖性的集体模式行为是实现基于样本结构控制的热管理的关键。后者具有大量能源创新的巨大潜力。作为实现这一目标的第一步，我们对液晶样品 D7AOB 进行了高对比度 IXS 测量，该样品表现出类似固体的动态特征，例如纵向和横向声子模式的共存。我们第一次发现这些太赫兹声子激发持续存在于晶体相、近晶A相和各向同性相中。此外，中间近晶 A 相显示出支持范德华介导的非流体动力模式，具有类似光学的声子行为。通过选择样品介晶相来实现集体太赫兹尺度的可调性代表了操纵软超材料特性的新机会。
The investigation of phononic collective excitations in soft matter systems at the molecular scale has always been challenging due to limitations of experimental techniques in resolving low-energy modes. Recent advances in inelastic X-ray scattering (IXS) enabled the study of such systems with unprecedented spectral contrast at meV excitation energies. In particular, it has become possible to shed light on the low-energy collective motions in materials whose morphology and phase behavior can easily be manipulated, such as mesogenic systems. The understanding of collective mode behavior with a Q-dependence is the key to implement heat management based on the control of a sample structure. The latter has great potential for a large number of energy-inspired innovations. As a first step toward this goal, we carried out high contrast IXS measurements on a liquid crystal sample, D7AOB, which exhibits solid-like dynamic features, such as the coexistence of longitudinal and transverse phononic modes. For the first time, we found that these terahertz phononic excitations persist-in the crystal, smectic A, and isotropic phases. Furthermore, the intermediate smectic A phase is shown to support a van der Waals-mediated nonhydrodynamic mode with an optical-like phononic behavior. The tunability of the collective terahertz scales via selection of the sample mesogenic phase represents a new opportunity to manipulate properties of soft metamaterials.