Elucidating the mechanism of millimeter-long transport of photogenerated carriers in topological insulators

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NON-TECHNICAL SUMMARY:When light is absorbed by a material such as a semiconductor, the number of free electrons and holes increases, resulting in increased electrical conductivity. This phenomenon is known as photoconductivity. Understanding how photocarriers transport charge is key to many important applications such as solar cells and light detectors. A new category of fascinating materials, called topological insulators, has been discovered over the past decade. These materials have unique electronic properties at the surface and may find technological applications. The principal investigator’s research group has recently demonstrated unusual long-distance transport of photogenerated charge carriers in topological insulators at low temperatures. This suggests the formation of a quantum state known as an exciton condensate. If confirmed, this quantum state has potential uses in quantum computing. The aim of this project is to study this phenomena by a variety of experimental techniques and understand the physical mechanism behind it. This project also educates and trains undergraduate and graduate students in the rapidly advancing research area of quantum and spintronic devices and offers outreach activities targeting K-12 students.TECHNICAL SUMMARY:The PI’s recent experimental studies of three-dimensional topological insulators (TIs) have revealed unusual non-local photocurrent. Remarkably, the locally photogenerated carriers can transport over a millimeter along the intrinsic Sb-doped Bi2Se3 nanoribbons before recombination at up to 40 K, accompanied by an internal quantum efficiency as high as 60%. The observation of highly dissipationless transport of photogenerated carriers implies the formation of superfluidic exciton condensate. The goal of this project is to elucidate the physical mechanism of this highly nonlocal photocurrent generation. A suite of innovative spatially, temporally, and spectrally resolved techniques are applied to achieve this goal. Spatially resolved helicity-dependent photocurrent and the surface magneto-optical Kerr effect is investigated to understand the spin polarization and spin-momentum coupling of the photogenerated carriers at the TI surface. Infrared photocurrent spectroscopy, angle-resolved photoemission spectroscopy (ARPES), and scanning tunneling spectroscopy (STS) will be performed to provide evidence for excitonic gap opening in the surface Dirac cone. Thinner and more intrinsic samples and topological materials beyond Bi2Se3 are studied to achieve higher critical temperatures. If confirmed, topological exciton condensation is expected to provide a new and exciting paradigm exploiting chiral superfluidic excitons towards high temperature quantum logic and novel spintronic devices.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.