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

E7项目旨在对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有一个基本的了解。用fs激光脉冲诱导电子系统中的非平衡状态。随后，晶格系统也对激励作出响应。表面的1d金属线可能经历非热驱动的相变，特征声子模式可能被激发，或者表面可能只是被加热。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。这样就可以得到在平衡条件下无法从实验中得到的隐藏参数。本文采用超快速时间分辨反射高能电子衍射（tr-RHEED）方法，在相关的次ps时间尺度上跟踪对称、晶胞大小、原子位置和振动激发的变化。在我们的研究中，我们主要关注两种不同的原子线系统原型：第一种表现出有序的自旋链，第二种表现出佩尔斯不稳定性。Si(553)-Au吸附体系以沿金属丝形成反铁磁性有序的Si阶边原子自旋链而闻名，这在衍射中通过附加的斑点或条纹是明显的。该自旋链在fs激光激发下的破坏、恢复或重组将通过tr-RHEED的瞬态光斑分布分析来研究。没有质量输运或键断裂与有序-无序相变有关。与此相反，Si(111)-In吸附体系表现出类似Peierls的不稳定性，导致（8×2）重建基态和（4×1）重建高温态之间的一阶相变。（8×2）基态由fs激光脉冲非热驱动进入过冷亚稳态（4×1）相。对基态的恢复动力学应进行研究，并通过改变温度和控制吸附剂的引入来控制。这些研究将与E5中使用的时间分辨电子能谱（tr-ARPES）进行补充，以研究相同系统中的电子动力学。

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.