Ultrafast terahertz scanning tunneling microscopy of Weyl materials
Abstract
Weyl semimetals are an exciting new class of topological matter with wide-ranging potential applications. Such materials feature Weyl cones in their bulk that are characterized by both linear quasiparticle dispersion and topological protection. This union opens the door to new possibilities, including combining high-speed electronics with topological quantum computing. However, this ambitious vision hinges on experimental tools that can both probe topology and capture ultrafast dynamics on the technologically relevant, nanometer length scale. Angle-resolved photoemission spectroscopy (ARPES) has been an essential, pioneering technique for the discovery of Weyl materials but has limited spatial resolution. Conversely, scanning tunneling microscopy (STM) achieves exquisite, atomic-scale spatial resolution and has been used to visualize quasiparticle scattering involving Fermi arc surface states, which are a defining property of Weyl materials. Meanwhile, optical experiments incorporating femtosecond pulses have demonstrated photoinduced shift currents and structural dynamics with the potential to control Weyl topology on ultrafast timescales. Here, the advantages of STM and ultrafast optics will be combined by introducing lightwave-driven terahertz STM (THz-STM) as a new tool for probing Weyl materials. The project will explore ultrafast dynamics and coherent control on the surfaces of Weyl materials with atomically resolved, low-temperature THz-STM. The project will establish THz-STM of Weyl semimetals, develop new THz-STM modalities for probing Weyl topology, and explore ultrafast dynamics and coherent topological control concepts in an unprecedented regime. The project is divided into three parallel phases: (i) In the first phase, the atomic scale motion of electron densities that lead to femtosecond shift currents in the type-I Weyl material TaAs will be resolved. THz-STM snapshot imaging will directly visualize in-plane electron motion, while out-of-plane motion will be revealed by cross-correlating the THz field generated by the shift current with the THz-STM probe field. The effects of pump-pulse frequency and local defect scattering on the dynamics will be studied to address key open questions. (ii) In the second phase, the impulsively excited THz shear mode in the type-II Weyl material WTe2 will be investigated. A novel THz scanning tunneling spectroscopy (THz-STS) inversion algorithm will be used to extract the transient differential conductivity as a function of time after photoexcitation, thereby revealing the temporal evolution of the local density of states. Building on this approach, an ultrafast analog of STM quasiparticle interference will be developed to probe the time-dependence of the Fermi arc surface states via scattering near defects. These measurements will be a key test of how ultrafast shear motion impacts the Weyl topology of WTe2. (iii) In the third phase, novel ways to coherently control Weyl topology with atomic scale precision will be explored using tip-coupled THz pulses. Control schemes will be demonstrated based on extreme THz fields and extreme THz-induced tunnel currents at the tip apex. These concepts will be tested for TaAs and WTe2 and then extended to new and emerging nanostructured Weyl materials, with the goal of establishing a foundation for future technological applications.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Jun 25, 2021
- Source ID
- W911NF2110153
Entities
People
- Tyler L. Cocker
Organizations
- Army Contracting Command
- Michigan State University
- United States Army