Experimental Quantum Seminar - Alex McLeod (Columbia University) “Quantum phase transitions through the nano-lens: exploration and control with low-temperature nano-optics”

“Quantum phase transitions through the nano-lens: exploration and control with low-temperature nano-optics”
Interactions between electronic, structural, and magnetic degrees of freedom in so-called quantum materials can give rise to competing for 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 a 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.
Host: Bonnie Fleming