Time-resolved diffraction of photo-induced phase transitions in 1D metal wires on semiconductor surfaces

半导体表面一维金属线光致相变的时间分辨衍射

• 批准号: 313472472
• 负责人: Professor Dr. Michael Horn-von Hoegen
• 金额: --
• 依托单位: Dekanat Fakultät für Physik
• 依托单位国家: 德国
• 项目类别: Research Units
• 财政年份: 2016
• 资助国家: 德国
• 起止时间: 2015-12-31 至 2020-12-31
• 项目状态: 已结题
• 来源: https://gepris.dfg.de/gepris/projekt/313472472?language=en
• 关键词: Time; resolved; diffraction; photo; induced

The project E7 aims at a fundamental understanding of the non-equilibrium structural dynamics of quasi 1D-metal wires on semiconductor surfaces upon optical excitation. With a fs-laser pulse a non-equilibrium situation in the electron system is induced. Subsequently also the lattice system responds to the excitation. The 1D-metal wires at the surface may undergo a non-thermally driven phase transition, characteristic phonon modes may be excited, or the surface may simply become heated. Employing the transient response of the system as an additional dimension for the analysis, the interaction strength of electron-phonon coupling, phonon-phonon coupling or electron-spin coupling can be deduced. Thus hidden parameters become available, which are not accessible from experiments under equilibrium conditions. Here we employ ultra-fast time resolved reflection high energy electron diffraction (tr-RHEED) at surfaces as the method of choice to follow changes of symmetry, size of unit cell, position of atoms, and vibrational excitations on the relevant sub-ps timescale. With our studies we focus on two different prototypical atomic wire systems: the first exhibiting an ordered spin chain and the second a Peierls instability. The Si(553)-Au adsorbate system is known for the formation of an anti-ferromagnetic ordered spin chain of Si step edge atoms along the wires which is apparent in diffraction through additional spots or streaks. The destruction, recovery, or reorganisation of this spin chain upon fs-laser excitation will be studied through a transient spot profile analysis in tr-RHEED. No mass transport or bond-breaking is associated with the order-disorder phase transition. In contrast to this the Si(111)-In adsorbate system exhibits a Peierls like instability giving rise to a first order phase transition between the (8×2) reconstructed ground state and a (4×1) reconstructed high temperature state. The (8×2) ground state is non-thermally driven by a fs-laser pulse into a super-cooled metastable (4×1)-phase. The recovery dynamics towards the ground state should be studied and manipulated through changes of temperature and the controlled introduction of adsorbates. These studies will be complemented with the time resolved electron spectroscopy (tr-ARPES) employed in E5 for the investigation of the electron dynamics in identical systems.

E7项目旨在从根本上了解半导体表面上准一维金属线在光激发下的非平衡结构动力学。在飞秒激光脉冲作用下，电子系统会出现非平衡态。随后，晶格系统也响应于激励。表面处的1D金属线可以经历非热驱动的相变，特征声子模式可以被激发，或者表面可以简单地被加热。利用系统的瞬态响应作为分析的附加维度，可以推导出电子-声子耦合、声子-声子耦合或电子-自旋耦合的相互作用强度。因此，隐藏的参数变得可用，这是无法从平衡条件下的实验。在这里，我们采用超快时间分辨反射高能电子衍射（TR-RHEED）在表面作为选择的方法，以遵循对称性的变化，晶胞的大小，原子的位置，和振动激发相关的亚ps时间尺度。在我们的研究中，我们专注于两种不同的原型原子线系统：第一个表现出有序的自旋链和第二个Peierls不稳定性。已知Si（553）-Au吸附物系统形成Si台阶边缘原子的反铁磁有序自旋链，该自旋链沿线沿着，这在通过附加斑点或条纹的衍射中是明显的。在飞秒激光激发下，该自旋链的破坏、恢复或重组将通过tr-RHEED中的瞬态光斑轮廓分析来研究。没有质量传输或键断裂与有序-无序相变。与此相反，Si（111）-In吸附质体系表现出Peierls不稳定性，在（8×2）重构基态和（4×1）重构高温态之间发生一级相变。（8×2）基态被飞秒激光脉冲非热驱动到过冷亚稳（4×1）相。应通过改变温度和有控制地引入吸附物来研究和操纵向基态的恢复动力学。这些研究将与E5中采用的时间分辨电子能谱（tr-ARPES）进行补充，用于研究相同系统中的电子动力学。