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

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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.