Snapshots of cooperative atomic motions in the optical suppression of charge density waves
Snapshots of cooperative atomic motions in the optical suppression of charge density waves
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DOI：
10.1038/nature09539
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发表时间：
2010-12-09
期刊：
影响因子：
64.8
通讯作者：
Miller, R. J. Dwayne
中科院分区：
文献类型：
--
作者：
Eichberger, Maximilian;Schaefer, Hanjo;Miller, R. J. Dwayne
宏观量子现象，如高温超导性、巨磁电阻、铁磁性和铁磁性，源于纳米尺度上电子、声子和自旋之间不同相互作用的微妙平衡(1)。因此，研究强耦合电子-晶格系统中这些不同自由度之间的相互作用对于理解它们并优化它们的性质至关重要。电荷密度波（CDW）材料(2)具有固有的电子密度调制和相关的周期性晶格畸变，是研究这种高度协同现象的理想模型系统。利用飞秒时间分辨技术，可以通过突然扰动电子分布来直接观察这些相互作用，同时跟踪不同子系统之间的能量松弛途径和耦合强度（3-7）。在cdw上进行了许多时间分辨实验（8-13），探测电子子系统的动力学。然而，周期性晶格畸变的动力学只是间接推断出来的（14）。本文利用飞秒电子衍射技术（15）对准二维CDW体系1T-TaS2进行了研究，提供了直接的原子水平的结构动力学信息。实际上，我们已经直接观察到由光诱导的电子空间分布变化引起的原子运动。周期性晶格畸变的振幅约为0.1埃，在相当于相应集体模式周期的一半的时间尺度上（约250飞秒）被抑制约20%。这些高度合作的电子驱动原子运动伴随着快速的电子-声子能量转移（类似于350飞秒），随后是CDW的快速恢复（类似于4皮秒）。观察到的结构动力学中的协同程度是显著的，并说明了获得指导强相关系统物理过程的原子水平视角的重要性。
Macroscopic quantum phenomena such as high-temperature superconductivity, colossal magnetoresistance, ferrimagnetism and ferromagnetism arise from a delicate balance of different interactions among electrons, phonons and spins on the nanoscale(1). The study of the interplay among these various degrees of freedom in strongly coupled electron-lattice systems is thus crucial to their understanding and for optimizing their properties. Charge-density-wave (CDW) materials(2), with their inherent modulation of the electron density and associated periodic lattice distortion, represent ideal model systems for the study of such highly cooperative phenomena. With femtosecond time-resolved techniques, it is possible to observe these interactions directly by abruptly perturbing the electronic distribution while keeping track of energy relaxation pathways and coupling strengths among the different subsystems(3-7). Numerous time-resolved experiments have been performed on CDWs(8-13), probing the dynamics of the electronic subsystem. However, the dynamics of the periodic lattice distortion have been only indirectly inferred(14). Here we provide direct atomic-level information on the structural dynamics by using femtosecond electron diffraction(15) to study the quasi two-dimensional CDW system 1T-TaS2. Effectively, we have directly observed the atomic motions that result from the optically induced change in the electronic spatial distribution. The periodic lattice distortion, which has an amplitude of similar to 0.1 angstrom, is suppressed by about 20% on a timescale (similar to 250 femtoseconds) comparable to half the period of the corresponding collective mode. These highly cooperative, electronically driven atomic motions are accompanied by a rapid electron-phonon energy transfer (similar to 350 femtoseconds) and are followed by fast recovery of the CDW (similar to 4 picoseconds). The degree of cooperativity in the observed structural dynamics is remarkable and illustrates the importance of obtaining atomic-level perspectives of the processes directing the physics of strongly correlated systems.