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

非技术性：在最近发现的“狄拉克半金属”中，电子的行为似乎是无质量的，并表现出非常高的迁移率。本研究旨在研究这种半金属材料中的一种--SrMnSb2。理论上预测，暴露在短脉冲激光下可能会从根本上、迅速和可逆地改变材料的电子性质。该项目将SrMnSb2暴露在激光脉冲下，随后进行几项先进的光学和X射线测量，以了解脉冲对材料原子位置和电子性质的影响。该项目支持本科生和博士后研究人员，他们操作激光实验，参与自由电子激光和其他主要设施的实验，编写计算机代码，并分析复杂的数据集。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职业做好了准备。技术：新发现的狄拉克和韦尔半金属具有线性分散的电子能带，这些电子带在一个结点交叉，但这个结点有时可能是有间隙的。如果实现了在亚皮秒时间尺度上打开和关闭间隙的能力，就可以为这种材料的许多奇异光学效应提供一个超快的开关；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态。在一种有缝隙的狄拉克材料SrMnSb2中，电子结构计算发现了一种特殊的声子模式，如果激发到足够高的幅度，随着声子的振荡，这种模式将使原子的位置发生足够的变化，从而短暂而周期性地关闭能隙。这个项目的目的是光学激发声子，相干和高幅度，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能隙，并阐明支配能隙关闭的物理学。这项研究包括几个光学实验，每个实验都激发相干声子，然后以超快分辨率探测其影响。瞬变光栅光谱学探索了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过程中原子偏离平衡的随时间变化的绝对位移。最后，光泵，中红外探测光谱学测量由此产生的节点间隙的振荡，可能在高振荡幅度下显示间隙的闭合。总而言之，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。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.