RUI: Using Coherent Phonons for Ultrafast Control of the Dirac Node of SrMnSb2

非技术：在最近发现的“狄拉克半金属”中，电子表现得好像没有质量，并表现出非常高的迁移率。这项研究旨在研究一种这样的半金属，SrMnSb2。从理论上预测，暴露在短脉冲激光下可能会从根本上、迅速地、可逆地改变材料的电子特性。该项目将SrMnSb2暴露在激光脉冲下，随后进行了几次先进的光学和x射线测量，以了解脉冲对材料原子位置和电子特性的影响。本项目支持本科院校的本科生和博士后进行激光实验操作，参与自由电子激光器和其他主要设施的实验，编写计算机代码，分析复杂数据集。由于凝聚态物理的科学和工业相关性，以及超快技术的快速发展，学生和博士后为各种各样的科学和技术事业做好了准备。技术：新发现的Dirac和Weyl半金属具有线性分散的电子带，它们在一个节点上交叉，但这个节点有时可能是间隙的。在亚皮秒时间尺度上打开和关闭间隙的能力，如果实现，可以为许多材料的奇异光学效应提供超快的开关；它可以让研究人员根据需要创造狄拉克费米子，并探索和调整静态材料中可用的电子状态。在一种有间隙的狄拉克材料SrMnSb2中，电子结构计算已经确定了一种特殊的声子模式，如果激发到足够高的振幅，当声子振荡时，原子的位置会发生足够的移动，从而短暂而周期性地关闭间隙。该项目的目的是光学激发声子，相干和高振幅，并观察随后的动力学，从而实现亚皮秒光学控制这种狄拉克半金属的间隙，并阐明控制间隙关闭的物理。该研究由几个光学实验组成，每个实验都激发相干声子，然后以超快分辨率探测其效应。瞬态光栅光谱学探索使用多个激光脉冲来控制和放大声子。超快x射线衍射测量声子振荡期间原子从平衡状态的绝对位移随时间的变化。最后，光泵、中红外探测光谱测量了由此产生的节点间隙的振荡，可能显示了在高振荡幅度下节点间隙的闭合。总之，这些测量可以建立原子位置、节点间隙和驱动振荡的脉冲之间的定量关系。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Non-technical:In the recently discovered 'Dirac semimetals', electrons behave as though massless and exhibit very high mobilities. This study aims to investigate one such semimetal, SrMnSb2. It has been theoretically predicted that exposure to a short pulse of laser light might change the material's electronic properties, radically, rapidly and reversibly. This project exposes SrMnSb2 to laser pulses, following them up with several advanced optical and X-ray measurements to learn the pulses' influence on the material's atomic positions and electronic properties. This project supports undergraduate and post-doctoral researchers at a primarily-undergraduate institution, who operate laser experiments, participate in experiments at free-electron lasers and other major facilities, write computer code, and analyze complex sets of data. Because of the scientific and industrial relevance of condensed-matter physics, and the rapid growth of ultrafast technology, the students and post-doc become prepared for a wide variety of scientific and technical careers. Technical:The newly discovered Dirac and Weyl semimetals have linearly-dispersing electronic bands, which cross at a node, but this node may sometimes be gapped. The ability to open and close a gap on sub-picosecond timescales could, if achieved, provide an ultrafast on-off switch for many of the materials' exotic optical effects; it could allow researchers to create Dirac fermions on demand and to explore and tune electronic states beyond those available in static materials. In one gapped Dirac material, SrMnSb2, electronic-structure calculations have identified a particular phonon mode which, if excited to sufficiently high amplitude, would shift the atoms' positions enough to close the gap, briefly and periodically, as the phonon oscillates. The purpose of this project is to optically excite the phonon, coherently and to high amplitude, and to observe the subsequent dynamics, thereby enabling sub-picosecond optical control of the gap of this Dirac semimetal and elucidating the physics that governs the gap's closing. The research consists of several optical experiments, each exciting the coherent phonon and then probing its effect with ultrafast resolution. Transient-grating spectroscopy explores the use of multiple laser pulses to control and amplify the phonon. Ultrafast X-ray diffraction measures the time-dependent, absolute displacement of atoms from equilibrium during the phonon's oscillation. Finally, optical-pump, mid-infrared-probe spectroscopy measures the resulting oscillation of the nodal gap, possibly showing the gap's closure at high oscillation amplitude. Together, these measurements can establish the quantitative relations between atomic position, nodal gap, and the pulses that drive the oscillation.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.