Publication date: 15th May 2025
Coherent energy propagation promises scatter-free, phase-preserving transport as a remedy to the resistive losses and Joule heating limiting conventional semiconductor technology. Here, stroboscopic scattering microscopy reveals coherent quasiparticle propagation in Nb3Se10Cl2 and NbOI2, two distinct low-dimensional niobium-based materials. Nb3Se10Cl2 is a newly synthesized, one-dimensional van der Waals material composed of alternating linear and chiral chains. Optical characterization indicates an indirect-gap semiconductor with low-lying, long-lived excited states at low temperatures. Imaging of ultrafast carrier dynamics captures ballistic carrier transport reaching up to 590 km/s within the first 450 fs. This early-time propagation is attributed to the formation of coherent excitons by J-aggregation within Nb2Se6Cl2 subunits. Conversely, NbOI2 is a well documented, two-dimensional van der Waals ferroelectric material, predicted to host coherent oscillations of its ferroelectric order (ferrons). Following above-gap excitation, these narrow-band terahertz oscillations spread along the polar axis at velocities exceeding 100 km/s for hundreds of picoseconds. Despite their distinct dimensionalities, both systems illustrate the link between structural anisotropy and directional, coherent transport. These observations highlight the versatility of stroboscopic scattering microscopy in tracking coherent quasiparticle flow and thereby help inform the design and synthesis of low-dimensional systems for excitonic circuitry and THz emitters.