Spin and orbital dynamics in magneto-resistive manganites measured with femtosecond resonant soft x-ray scattering using FEL pulses

锰属一类材料，包括高温铜超导体和其他当前感兴趣的复杂固体。这些系统的特点是强烈的电子相互作用和显著的电子-晶格耦合，这反过来又导致了各种令人眼花缭乱的有趣现象，这些现象挑战了许多支撑我们对固态理解的概念。一些最有趣的现象发生在达到微观秩序时，其中包括电子、磁轨道和原子结构的周期性排列。这些有序态对外部刺激极其敏感，因此，磁场、压力、电场和光照射的应用可以使系统在宏观行为表现出巨大变化的相之间切换。这些固体不仅是我们对固态理解的前沿之一，而且还可能导致数据存储和处理以及新型传感器技术中的各种新应用。在我们的工作中，我们研究了光在锰中引发的绝缘体-金属转变。在光致绝缘体-金属跃迁中，每千个单位细胞有一个光子的辐照会导致整个晶体向金属态的协同跃迁，同时在超快时间尺度上发生许多微观参数的变化。由于这个过程非常快，因此可以将其用于高比特率数据存储和处理中的新应用。然而，理解和优化与这种转变相关的微观现象要求人们能够以相应的速度询问系统的结构参数。我们感兴趣的时间尺度是飞秒时间尺度，即原子移动和自旋模式重新排列的时间尺度。我们的工作旨在将使现代材料科学发生革命性变化的x射线技术扩展到飞秒时间尺度。在这里提出的研究计划中，我们寻求利用x射线自由电子激光来研究磁阻锰矿的微观动力学。我们将利用自由电子激光束前所未有的亮度、脉冲持续时间和空间相干性来研究锰中超快绝缘体-金属跃迁过程中轨道和磁性超结构的重排。我们将利用飞秒FEL脉冲的相干2纳米辐射进行时间分辨，共振x射线散射实验，这将获得在过渡到金属相期间轨道和自旋无序的信息。此外，利用这些脉冲的横向相干性将用于研究与bragg峰小角度偏差的散斑模式，从而获得远程（数十至数百nm）纹理的静态和动态重排信息。这也将是非常重要的，因为介观现象，成核和相分离被认为是这些系统动力学的微妙基础。

Manganites belong to a family of materials that include high-temperature cuprate superconductors and other complex solids of current interest. These systems are characterized by strongly interacting electrons and by significant electron-lattice coupling, which in turn result in a bewildering variety of interesting phenomena that defy many concepts underpinning our understanding of the solid state. Some of the most interesting phenomena occur when microscopic order is attained, which encompasses periodic arrangements in electronic, magnetic orbital and atomic structures. These ordered states are extremely sensitive to external stimulation, whereby the application of magnetic fields, pressure, electric fields and irradiation with light can switch the system among phases that exhibit colossal changes in their macroscopic behavior. These solids are then not only one of the frontiers of our understanding of the solid state, but may also lead to a variety of new applications in data storage and processing, as well as in novel sensor technologies. In our work, we study the insulator-metal transition triggered by light in the manganites. In the photo-induced insulator-metal transition irradiation with one photon every thousand unit cells results in a cooperative transition of the entire crystal toward a metallic state, with changes in many microscopic parameters at once occurring on the ultrafast timescale. Because this process is very fast, one may be able to exploit it for novel applications in high bit rate data storage and processing. Yet, understanding and optimizing the microscopic phenomena associated with such transition requires that one is able to interrogate the structural parameters of the system with commensurate speed. The timescale of interest is the femtosecond timescale, i.e. that where atoms move and spin patterns rearrange. Our work seeks to extend the same x-ray techniques that have revolutionized modern materials science to the femtosecond timescale. In the research program proposed here, we seek to exploit x-ray free electron lasers to investigate the microscopic dynamics of magneto-resistive manganites. We will use the unprecedented brillance, pulse duration and spatial coherence of Free Electron Laser beams to study the rearrangements of orbital and magnetic superstructures across the ultrafast insulator-metal transition in the manganites. We will exploit femtosecond FEL pulses of coherent 2-nm radiation to perform time-resolved, resonant x-ray scattering experiments, which will yield information on the disordering of orbitals and spins during the transition to the metallic phase. Also, the exploitation of the transverse coherence of these pulses will be used to study the speckle patterns at small angular deviations from the bragg peak, yielding information on static and dynamic rearrangements of the long-range (tens to hundreds of nm) texture. This will be also of great importance because mesoscopic phenomena, nucleation and phase separation are known to subtly underpin the dynamics of these systems.