Interactions between electronic, structural, and magnetic degrees of freedom in so-called quantum materials can give rise to competing ground states with vastly distinct properties. Quantum phase transitions between these states afford opportunities for on-demand functionality and, when resolved at characteristic nanometer scales, reveal the physics underlying these interactions. Bypassing the diffraction limit of conventional limit of light, infrared nano-imaging has already become an invaluable tool for nanometer-resolved optical investigations of inhomogeneous materials. Recent instrumental developments have brought this technique to cryogenic temperatures (down to T=20K) [1], a regime where exotic insulator- to-metal transitions (IMTs) can emerge in quantum materials. Using external stimuli including temperature, strain, and even light, here I explore universal phenomenologies and opportunities for nano-scale control over the IMT among representative complex oxides. First, a nano-imaging comparison of two prototypic quantum materials demonstrates how insulator/metal domain textures reveal interactions of fundamentally different order parameters governing the IMTs in NdNiO3 and V2O3 thin films [1,2]. Next, by tuning the energetic landscape controlling the transition, I demonstrate active manipulation of these domain textures by nano-imaging the IMT in calcium ruthenate single-crystals under in situ uniaxial strain and electrical current, promising a route to nano-susceptibility measurements [3]. Lastly, nano-imaging of an epitaxial manganite reveals how a unique coupling of metallicity, magnetism, and strain create conditions for a metastable IMT “activated” through optical excitation and “deactivated” through locally applied pressure [4]. These examples highlight the singular capabilities of infrared nano-imaging deployed at cryogenic temperatures, promising expansive opportunities for future investigations of phase transitions in quantum materials.

[1] A. S. McLeod, E. Van Heumen, J.G. Ramirez, S. Wang, T. Saerbeck, S. Guenon, … I.K. Schuller, & D.N. Basov. Nanotextured phase coexistence in the correlated insulator V2O3. Nature Physics 13, 80-86 (2017) https://doi.org/10.1038/nphys3882. [2] K.W. Post*, A. S. McLeod*, M. Hepting, M. Bluschke, Y. Wang, G. Cristiani, … & D.N. Basov. Coexisting first- and second-order electronic phase transitions in a correlated oxide. Nature Physics 14, 1056 (2018) https://doi.org/10.1038/s41567-018-0201-1. *These authors contributed equally. [3] J. Zhang*, A. S. McLeod*, Q. Han*, X. Chen, H. A. Bechtel, ... D. N. Basov, A. J. Millis, Y. Maeno, & M. K. Liu. Nano-resolved current-induced metal-insulator transition in the Mott insulator Ca2RuO4. Phys. Rev. X 9, 011032 (2019) https://doi.org/10.1103/PhysRevX.9.011032. *These authors contributed equally. [4] A. S. McLeod*, J.D. Zhang*, M.Q. Gu*, … W.B. Wu, J. Rondinelli, R.D. Averitt, & D.N. Basov. Multi-messenger nano-probes of hidden magnetism in a strained manganite. Nature Materials (2019) https://doi.org/10.1038/s41563-019-0533-y. *These authors contributed equally