Picosecond resolution of phonon dynamics with pulsed synchrotron radiation

在研究项目的第一部分，我们提出用超快时间分辨x射线散射来研究半导体晶体的晶格动力学，特别是多层纳米结构以及铁电晶格激发。提出了一种新的方法，它利用同步加速器ANKA的短脉冲X射线发射，并利用其500 MHz的重复率来建立泵浦-探测装置。在使用所谓的低阿尔法操作模式时，ANKA发射的辐射脉冲可能非常短(低至1ps)。安装工作已在过去一段时间内完成，现已投入使用。在这项提议中，我们想要在同位素纯的多层硅上进行ASOXS实验，分别是锗。声子的运动应该揭示这些被提议用作热电材料的多层系统中声子势垒的性质。利用ASOXS，我们还获得了钛酸钡铁电晶体中局域运动的动力学。局域运动和对称性给铁电相变的本质提供了一个独立的途径，并解决了描述弥散铁电有序的一个长期存在的问题。

In the first part of the research project we proposed to study the lattice dynamics of semicon-ductor crystals, in particular multilayered nanostructures as well as ferroelectric lattice excita-tions by using ultrafast time-resolved x-ray scattering. A novel approach was proposed which uses the short pulse x-ray emission at the synchrotron ANKA and using its 500 MHz repeti-tion rate for a pump-probe setup. Radiation pulses emitted at ANKA can be extremely short (down to 1 ps) when using the so-called low-alpha mode of operation. The setup has been completed within the past period and is operational. Within this proposal we want to conduct ASOXS experiments on isotopically pure multilayers of silicon, respectively germanium. The phonon motion should reveal information about the nature of the phonon barrier in these mul-tilayered systems that are proposed to serve as thermoelectric materials. With ASOXS we also get access to the dynamics of the local motion in barium titanate ferroelectric crystals. The local motion and symmetry should give an independent access to the nature of the ferroelectric phase transition and solve a long-standing problem on the description of dis-placive versus diffusive ferroelectric order.