RUI: Conductivity, diffusion, and dispersion of photoexcited Dirac fermions in cadmium arsenide

非技术摘要：这项研究旨在研究最近发现的“狄拉克半金属”之一--砷化镉的关键电子和光学激发过程。它的电子表现得就像是无质量的，表现出非常高的迁移率和速度，所以这种材料可以被认为是石墨烯的整体类似物。它非常稳定，具有3D晶体结构，可以与现有电子产品集成。它也是实现磁性Weyl半金属的起始材料，在这种材料中，电子的自旋方向通常由它们的运动方向决定。本研究包括用化学气相沉积法在低温下合成和表征未掺杂和磁性掺杂的砷化镉大块晶体和薄膜。这项研究测量了电子的扩散，并使用了几种时间分辨的探测器，包括太赫兹光谱和光电发射。这项研究提供了对3-D狄拉克材料生长方法和电子性能的改进知识，这对于实现这种材料的技术前景非常重要。快速电子学、快速或宽带光学传感器或主动锁模激光等应用都依赖于这项研究中探索的超快和光学特性。这项工作支持研究生和本科生(后者在一所主要是本科生的机构工作)，他们从事砷化镉的生长和表征，操作激光实验，操作冷冻，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，学生们为各种各样的科学和技术职业做好了准备。技术摘要：狄拉克和韦尔材料具有手性反常、不寻常的量子磁阻和预测的巨大抗磁性等性质。它们几乎没有费米面，导致反常输运，其散射率、态密度和扩散系数强烈依赖于能量；电导率随频率线性上升。与石墨烯类似，这种材料的有限相空间表明，应该可以在亚皮秒时间尺度上控制它们的光学和输运性质，例如通过掺杂光激载流子。研究人员探索了这些光激发载流子的性质：它们的密度和温度；它们对扩散性和导电性的影响；以及它们的分散性。特别令人感兴趣的是确定光载体在什么条件下可以表现出与其宿主材料相同的独特的狄拉克行为。选择不同的泵浦光子能量，可以激发无质量的初始态或更高能量的大质量初始态。瞬变光栅法测量光载流子的扩散系数。太赫兹光谱测量它们的导电性，指示散射率、化学势和质量或无质量特征。时间分辨光电子能谱揭示了传统光电子能谱看不到的瞬时占据态。这项工作的另一部分改进了砷化镉晶体和薄膜的气相合成，探索了磁性原子掺杂的方法和效果，并有助于制备铁磁性Weyl半金属。

Non-technical Abstract:This study aims at investigating key electronic and optical excitation processes in cadmiumarsenide, one of the recently discovered "Dirac semimetals". Its electrons behave as though they are massless and exhibit very high mobilities and velocities, so the material may be considered the bulk analog of graphene. It is very stable, has a 3-D crystal structure, and can be integrated with existing electronics. It is also a starting material from which to realize a magnetic Weyl semimetal in which, unusually, the direction of the electrons' spin would be determined by the direction of their motion. This research includes the synthesis and characterization of cadmium arsenide bulk crystals and thin films, bothundoped and magnetically-doped, using chemical vapor deposition at reduced temperature. The researchmeasures electrons' diffusion, and uses several time-resolved probes including terahertz spectroscopy andphotoemission. Improved knowledge of growth methods and electronic properties of 3-D Dirac materials,as provided by this research, is important in realizing the materials' technological promise. Applicationssuch as fast electronics, fast or broadband optical sensors, or actively mode-locked lasers all rely onultrafast and optical properties explored in this research. This work supports graduate and undergraduateresearchers (the latter at a primarily-undergraduate institution), who engage with the growth andcharacterization of cadmium arsenide, operate laser experiments, handle cryogens, write computer code,and analyze complex sets of data. Because of the scientific and industrial relevance of condensed-matterphysics, and the rapid growth of ultrafast technology, the students become prepared for a wide variety ofscientific and technical careers.Technical Abstract:The Dirac and Weyl materials host properties including the chiral anomaly, unusual quantummagneto-resistance, and predicted giant diamagnetism. Their near-lack of a Fermi surface causesanomalous transport, with the scattering rate, density of states, and diffusivity strongly dependent onenergy; the conductivity rises linearly with frequency. The materials' restrictive phase-space suggests, inanalogy with graphene, that it should be possible to control their optical and transport properties on subpicosecond timescales, for instance by doping with photoexcited carriers. The investigators explore thenature of these photoexcited carriers: their density and temperature; their effect on diffusivity andconductivity; and their dispersion. Of particular interest is identifying the conditions under which photocarriers can exhibit the same distinctive Dirac behaviors as their host material. Selection of different pump-photon energies allow excitation of massless initial states or the higher-energy massive ones. Transient-grating spectroscopy measures photocarriers' diffusivity. Terrahertz spectroscopy measures their conductivity, indicative of scattering rate, chemical potential, and massive or massless character. Time-resolved photoemission reveals the transiently-occupied states invisible to traditional photoemission. Another part of this work improves vapor-based synthesis of cadmium arsenide crystals and films, exploring methods and effects of doping with magnetic atoms and contributing toward the effort to make a ferromagnetic Weyl semimetal.