Dr. Alex McLeod, Columbia University

Bypassing the diffraction limit of conventional limit of light, near-field microscopy has
emerged as 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), a regime where quantum phase transitions such as the insulator-to-metal transition
(IMT) can emerge among so-called correlated electron materials. Using external stimuli including
temperature, strain, and even light, here I explore universal phenomenologies and the opportunities
for nano-scale control over the IMT among representative correlated electron materials. I first reveal
how spatial morphologies of insulator and metal domains provide clues to short-ranged interactions
between coupled order parameters in the rare-earth nickelate NdNiO3, selectively driving the IMT at
nanometer scales [1]. On the other hand, I demonstrate how long-ranged interactions with strain
produce nano-textured percolation and growth of striped insulator and metal domains in thin films of
the “canonical Mott insulator” V2O3 [2]. By tuning the energetic landscape controlling the transition,
I demonstrate active manipulation of these textures through nano-imaging of the IMT in layered
ruthenate single-crystals under in situ application of uniaxial strain and electrical current. Lastly,
nano-imaging of an epitaxial manganite LaCaMnO3 reveals how a unique combination of coupled
order parameters and strain susceptibility create conditions for a metastable IMT both “activated”
through optical excitation and “deactivated” through locally applied pressure [3]. These examples
highlight the singular capabilities of infrared nano-imaging deployed at cryogenic temperatures. In
summary, I will present my outlook for expansive opportunities to investigate quantum phase
transitions in correlated electron materials using nano-optics.

[1] 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, 1–6. (2018) doi.org/10.1038/s41567-018-0201-1 *These authors contributed
equally.
[2] 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. (2017) doi.org/10.1038/nphys3882
[3] 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. To
appear (2019) doi.org/10.1038/s41563-019-0533-y *These authors contributed equally.