Phonon-Engineered Topological Conductors

Nontechnical description The global demand for computing and communications is soaring. The corresponding need for energy efficient and high speed electronics outpaces what can be made from conventional materials. Quantum materials offer a path forward. This project investigates a class of quantum materials whose interior behaves as an electrical insulator while the surface is conducting. Such topological materials have unique advantages. Unlike ordinary metals and semiconductors, they are inherently immune to disorder, defects, and imperfections. Two obstacles have blocked their use in practical devices. First, the protected electronic channels typically coexist with ordinary, unprotected electrons. Second, thermal vibrations of atoms at room temperature and above collide with and scatter electrons. These effects combine to destroy the advantages provided by topological protection. This project directly addresses both obstacles. The research team selects topological conductors in which the protected features span as much of the electronic structure as possible. They then embed them in engineered multilayer structures that mechanically filter out the vibrations that scatter electrons. This combination can achieve robust conduction at room temperature, closing a persistent gap between laboratory discovery and real world needs. This interdisciplinary project trains materials scientists and engineers at the intersection of nanoscience and quantum materials. K-12 students and educators are engaged through outreach efforts to ensure that the scientific advances of this research reach broadly into the community and help build a next generation STEM workforce. Technical description Topological semimetals and insulators resist elastic backscattering because of the symmetry and topology of their quantum mechanical band structure. Still, this protection does not extend to inelastic phonon scattering, and many topological materials also suffer from ordinary carrier pockets that dilute both electronic transport and photonic responses. This project advances electronic and photonic functionality in topological conductors through a co-design strategy in which the phonon spectrum is treated as an independent design variable alongside the electronic band structure, an approach not previously applied to topological systems. The principal investigator and co-principal investigator select topological conductors whose protected states extend across the full electronic momentum space of the crystal, thereby maximizing the role of topology in measurable properties. These conductors are then embedded in heterostructures with acoustically hard cladding layers that spectrally filter the low-energy, long-wavelength phonons most responsible for inelastic scattering at device-relevant temperatures. The research integrates predictive modeling of acoustic phonon dispersions in layered structures; fabrication of nanoscale heterostructures with systematically varied cladding thickness and interface quality; and a correlated measurement campaign that includes electrical transport, quantum-coherence interferometry, and polarization-resolved photocurrent microscopy. The latter technique quantifies electrical currents generated solely by light, without any applied voltage, which serve as direct fingerprints of the topological character of the electronic states. Phonon spectra are measured before and after acoustic cladding using Brillouin-Mandelstam spectroscopy, an inelastic light-scattering technique sensitive to acoustic phonons, and Raman spectroscopy, enabling direct correlation between engineered changes in the phonon spectrum and improvements in both transport and optical response. The anticipated outcomes are a set of design principles linking acoustic boundary conditions to electronic and photonic performance in topological conductors, and a demonstrated pathway to room-temperature, high-mobility topological transport and robust light-driven currents relevant to low-dissipation interconnects and wide-band photodetectors. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.