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

当前x射线和超快科学的前沿之一涉及到这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，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它们的奇异物质状态的关键，它们都可以用x射线在接近平衡的情况下以越来越多的细节被探测到。除了对静态结构的研究之外，时间无关的非弹性散射技术为研究这些系统的动态特性打开了一个新的窗口。现在可以用x射线测量初等激发，这与中子散射、拉曼或电子能量损失技术所提供的方法是互补的。然而，绝大多数的非弹性研究只能提供一种接近平衡的动力学观点。在平行发展中，飞秒技术的快速改进使得对全新现象的测量成为可能。超短激发和频闪时间相关探测的使用为化学过渡态的物理学以及物理和生物系统的基本动力学打开了一扇窗。迄今为止，这一领域的技术主要是在光学和红外领域发展起来的，直到现在，它才成熟到可以扩展到x射线波长。由于与x射线相比，近可见波长的探测能力有限，所以人们常说，超快光学科学可以探测到事物发生的速度，尽管人们永远无法确定发生了什么。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途径，为研究速度和正在发生的事情提供新的线索。在过去的十年里，仪器的发展跨越了高阶激光谐波的应用，激光产生的等离子体x射线，同步加速器环上的束辐射，基于直线加速器的源和其他基于激光操纵存储电子的方案。凝聚态物理领域在开发这些资源方面可以说是最活跃的，一些早期的应用已经包含了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射线源的建设将以前所未有的辉煌的100微米波长激光器为主导。目前，唯一能正常工作的x射线自由电子激光器是位于汉堡的FLASH设备，它的激光波长为13.5 nm，在第二纵向模式下，激光波长为4.5 nm，为波动器的三次谐波。2007年夏天，计划进行一次升级，使激光波长达到6纳米，产生2纳米的相干x射线。目前正在实施的设计将提供大约1012个光子/脉冲在6纳米和1010个光子/脉冲在2纳米，工作频率为10赫兹。这些软x射线自由电子激光脉冲的早期应用已经得到证明，导致了纳米长度尺度上图案的相干成像。在本文提出的实验中，我们计划首次使用飞秒自由电子激光脉冲进行时间分辨衍射。然而，与其检测原子晶体位置的重排，我们将试图重建磁性和电子模式的几何排列，它们在这些化合物中形成具有不同周期性的超晶格，而不是原子晶格。预计这些系统中的非平衡相变动力学将导致重大的重排，并可能导致这种电子秩序的熔化。

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.