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

摘要：本研究旨在研究新发现的“狄拉克半金属”之一的砷化镉的关键电子和光学激发过程。它的电子表现得好像是无质量的，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大块模拟物。它非常稳定，具有三维晶体结构，并且可以与现有的电子设备集成。它也是实现磁性Weyl半金属的起始材料，在这种半金属中，不同寻常的是，电子的自旋方向将由它们的运动方向决定。本研究包括使用化学气相沉积在低温下合成和表征砷化镉体晶体和薄膜，包括掺杂和磁掺杂。这项研究测量了电子的扩散，并使用了几种时间分辨探针，包括太赫兹光谱和光电发射。本研究提高了对三维狄拉克材料生长方法和电子性能的认识，对于实现该材料的技术前景至关重要。诸如快速电子，快速或宽带光学传感器，或主动锁模激光器等应用都依赖于本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研究人员（后者在一个主要的本科机构），他们从事砷化镉的生长和表征，操作激光实验，处理低温，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：Dirac和Weyl材料的性质包括手性异常、异常量子磁电阻和预测的巨抗磁性。它们几乎缺乏费米表面导致了异常输运，散射率、态密度和扩散率强烈依赖于能量；电导率随频率线性上升。与石墨烯不同，这种材料的限制性相空间表明，在亚皮秒时间尺度上控制其光学和输运特性是可能的，例如通过掺杂光激发载流子。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；它们对扩散率和电导率的影响；以及它们的分散。特别感兴趣的是确定光载流子可以表现出与其宿主材料相同的独特狄拉克行为的条件。选择不同的泵浦光子能量可以激发无质量初始态或高能量的有质量初始态。瞬态光栅光谱测量光载流子的扩散系数。太赫兹光谱学测量它们的电导率、散射率、化学势和质量或无质量特性。时间分辨光发射揭示了传统光发射不可见的瞬态占据态。这项工作的另一部分改进了砷化镉晶体和薄膜的汽基合成，探索了磁性原子掺杂的方法和效果，并为制造铁磁Weyl半金属做出了贡献。

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.