Abstract:

　　Bypassing the diffraction limit of conventional limit of light, near-field microscopy has emerged as an invaluable tool for nanometer-resolved optical investigations of functional materials. This technique has been conventionally restricted to ambient conditions, impeding the low temperature investigation of quantum materials using nano-optics. Here I present several key discoveries from a novel infrared near-field microscope suitable for operation at ultra-high vacuum conditions and liquid helium temperatures (25-450K) [1]: First, graphene is widely considered a promising platform for infrared plasmonics owing to its high optical confinement and gate-tunability. However, the fundamental limits to plasmon propagation within ultra-high mobility graphene devices have remained largely unexplored. Enabled by real-space“scanning plasmon interferometry," our low-temperature near-field microscope resolves the effect of phonon scattering and dielectric loss on ballistic propagation of graphene plasmons at T<70K [2]. Second, I demonstrate the technique's ability to resolve coexisting electronic phases among phase-changing correlated electron materials. Among these, the "canonical correlated insulator" V₂O₃ is found to exhibit rich textured coexistence of insulator and metal phases through its Mott transition at 170K that can be rationalized through detailed analysis of nano-resolved infrared images, thus demonstrating the ubiquitous influence of strain in the physics of correlated oxides. Lastly, the rare-earth nickelates represent a comparatively unique class of oxides that exhibit an abrupt charge order-driven insulator-metal transition (IMT). I present the first nano-optical imaging of this phenomenon observed at 150K in NdNiO₃, providing rich clues to the coupled orders driving the IMT [3]. These examples highlight the singular capabilities of nano-infrared imaging deployed at cryogenic temperatures, inspiring promising outlooks for the future development and application of this technique.

[1] McLeod, A. S., Van Heumen, E., Ramirez, J. G., Wang, S., Saerbeck, T., Guenon, S., … Basov, D. N. (2017). Nanotextured phase coexistence in the correlated insulator V2O3. Nature Physics. https://doi.org/10.1038/nphys3882

[2] Ni, G. X., McLeod, A. S., Sun, Z., Wang, L., Xiong, L., Post, K. W., … Basov, D. N. (2018). Fundamental limits to graphene plasmonics. Nature, 557(7706), 530–533. https://doi.org/10.1038/s41586-018-0136-9

[3] Post, K. W., McLeod, A. S., Hepting, M., Bluschke, M., Wang, Y., Cristiani, G., … Basov, D. N. (2018). Coexisting first- and second-order electronic phase transitions in a correlated oxide. Nature Physics, 1–6. https://doi.org/10.1038/s41567-018-0201-1

Biography：

　　Dr. Alexander S. McLeod is an eminent authority on the technique of near-field optical microscopy and spectroscopy of oxides and quantum materials, especially at cryogenic temperatures.  In early 2017, Dr. McLeod received his PhD from the University of California San Diego under the supervision of Prof. Dimitri Basov.  As a U.S. Dept. of Energy Office of Science Graduate Fellow, there he developed a theoretical framework for the interpretation of near-field nano-spectroscopy, applying the technique to investigations of Li-ion battery technologies, extraterrestrial materials, and graphene.  He constructed the first operational infrared near-field microscope for nano-optical studies of quantum materials at cryogenic temperatures and ultra-high vacuum conditions. This cryo-nano-IR technique enabled first nano-resolved imaging of the insulator-metal transition in correlated electron oxides such as V₂O₃ and NdNiO₃, and in charge density wave materials including TaS₂ and superconducting organic salts.  As a Director’s Fellow at the Columbia Nano Initiative in New York City, his present work leverages cryo-nano-IR to study propagation and loss mechanisms of polaritons in two-dimensional materials, including graphene and semiconducting transition metal dichalcogenides, and to investigate the nano-scale electronic properties of phase-changing correlated electron oxides.