Exploiting Femtosecond X-ray Pulses from a Free Electron Laser to Study Ultrafast Spin and Orbital Dynamics in Manganites

利用自由电子激光器的飞秒 X 射线脉冲研究锰酸盐中的超快自旋和轨道动力学

• 批准号: EP/F028857/1
• 负责人: Andrea Cavalleri
• 金额: $ 3.14万
• 依托单位: University of Oxford
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2007
• 资助国家: 英国
• 起止时间: 2007 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=EP%2FF028857%2F1
• 关键词: Exploiting; Femtosecond; ray; Pulses; Free

One of the current frontiers in both x-ray and ultrafast science involves the convergence of the two underlying technologies. On the one side, x-ray radiation at synchrotron storage rings has revolutionized modern science by providing insight into the static, microscopic structure of matter. In complex condensed matter systems of interests to us, the arrangements of atoms, spins, electrons and orbitals, key to the understanding their exotic states of matter, can all be detected with ever increasing detail near equilibrium using x-rays. Beyond the study of static structures, time independent inelastic scattering techniques have opened a new window on the dynamic properties of these systems. Elementary excitations can now be measured with x-rays in complementary ways to what afforded by neutron scattering, Raman or electron-energy loss techniques. Yet, the great majority of inelastic studies can only provide a near-equilibrium view of the dynamics. In a parallel development, the rapid improvement of femtosecond technology has allowed for measurements of entirely novel phenomena. The use of ultrashort excitation and of stroboscopic time-dependent detection has opened a window on the physics of chemical transition states, as well as on that of elementary dynamics of physical, and biological systems. The technology in this area has been hitherto developed largely in the optical and infrared regime, and only now is it becoming ripe for extension to the x-ray wavelengths. Because of the limited probing power of near-visible wavelengths compared to x-rays, it is often said that ultrafast optical science can probe how fast things are happening, although one is never sure not what is happening. Our work is focused at the measurement of the non-equilibrium pathways that regulate non-equilibrium phase transitions with femtosecond x-rays, shedding new light into how fast as well as into what is happening. Instrumentation development over the last decade has spanned the application of high-order laser harmonics, laser-produced plasma x-rays, bunched radiation at synchrotron rings, linac-based sources and other schemes that are based on the laser manipulation of stored electrons. The field of condensed matter physics has been arguably the most active in exploiting these sources and some of the early applications have encompassed the direct measurement of atomic dynamics, rearranging on the femtosecond timescale. The construction of next generation x-ray sources will lead by the end of the decade in +ngstrom-wavelength lasers of unprecedented brilliance. Currently, the only functioning x-ray free electron laser is the FLASH facility in Hamburg, which lases at 13.5 nm and, on a second longitudinal mode, at the 4.5-nm, third harmonic of the undulator. In the summer of 2007, an upgrade is planned that will bring the lasing wavelength to 6 nm, producing coherent x-rays at 2 nm. The design that is currently being implemented will provide approximately 1012 photons/pulse at 6 nm and 1010 photons/pulse at 2 nm, operating at 10 Hz. Early applications of these soft X-ray free electron laser pulses have already been demonstrated, resulting in coherent imaging of patterns on nanometer length scales.In the experiments proposed here, we plan to use time resolved diffraction with femtosecond Free Electron Laser Pulses for the first time. However, rather than detecting rearrangements in the crystallographic positions of the atoms, we will seek to reconstruct the geometric arrangements of magnetic and electronic patterns, which in these compounds form super-lattices with different periodicity than the atomic lattice. It is expected that the non-equilibrium phase-transition dynamics in these systems will result in significant rearrangements and likely in melting of such electronic order.

X 射线和超快科学当前的前沿之一涉及两种基础技术的融合。一方面，同步加速器存储环的 X 射线辐射通过提供对物质静态微观结构的洞察而彻底改变了现代科学。在我们感兴趣的复杂凝聚态物质系统中，原子、自旋、电子和轨道的排列是理解其奇异物质状态的关键，都可以使用 X 射线在接近平衡状态下以不断增加的细节进行检测。除了静态结构的研究之外，时间无关的非弹性散射技术为了解这些系统的动态特性打开了一个新的窗口。现在可以用 X 射线测量基本激发，其方式与中子散射、拉曼或电子能量损失技术提供的方法互补。然而，绝大多数非弹性研究只能提供动力学的近平衡观点。在并行发展中，飞秒技术的快速改进使得能够测量全新的现象。超短激发和频闪时间相关检测的使用打开了化学过渡态物理学以及物理和生物系统基本动力学的窗口。迄今为止，该领域的技术主要在光学和红外领域发展，直到现在扩展到 X 射线波长才变得成熟。由于与 X 射线相比，近可见光波长的探测能力有限，人们常说超快光学科学可以探测事物发生的速度有多快，尽管人们永远无法确定正在发生什么。我们的工作重点是用飞秒 X 射线测量调节非平衡相变的非平衡路径，为了解相变的速度以及正在发生的情况提供新的线索。过去十年的仪器发展涵盖了高阶激光谐波、激光产生的等离子体 X 射线、同步加速器环处的聚束辐射、直线加速器源以及其他基于激光操纵存储电子的方案的应用。凝聚态物理领域可以说是利用这些来源最活跃的领域，一些早期应用包括原子动力学的直接测量，在飞秒时间尺度上重新排列。到本世纪末，下一代 X 射线源的建设将引领具有前所未有的辉煌的+ngstrom 波长激光器。目前，唯一运行的无 X 射线电子激光器是位于汉堡的 FLASH 设施，其激光波长为 13.5 nm，在第二纵模上，激光波长为 4.5 nm，即波荡器的三次谐波。 2007 年夏天，计划进行升级，将激光波长提高到 6 nm，产生 2 nm 的相干 X 射线。目前正在实施的设计将在 6 nm 处提供大约 1012 个光子/脉冲，在 2 nm 处提供大约 1010 个光子/脉冲，工作频率为 10 Hz。这些软 X 射线自由电子激光脉冲的早期应用已经得到证实，可在纳米长度尺度上实现图案的相干成像。在这里提出的实验中，我们计划首次使用飞秒自由电子激光脉冲的时间分辨衍射时间。然而，我们不是检测原子晶体位置的重排，而是寻求重建磁性和电子图案的几何排列，这些图案在这些化合物中形成具有与原子晶格不同的周期性的超晶格。预计这些系统中的非平衡相变动力学将导致显着的重排，并可能导致这种电子顺序的融化。