Non-Technical Abstract: Many materials can make the transition from nonconducting (insulator) to conducting (metal) through applied pressure, temperature, or photoexcitation. In certain quantum material systems, this insulator to metal transition can be achieved using very modest electric current or voltage, making potential applications to fast electrical switch, energy-efficient memory or transistor devices possible. To understand the mechanism of current or voltage induced switching, however, is nontrivial. Among many practical obstacles, one major setback is the inability to distinguish between the current-driven nonthermal state and a 'trivial' heat-induced high-temperature phase. Moreover, inhomogeneous strain, defects, and chemical doping add complications, especially when samples approach submicron scale, which is common in modern nano-electronic devices. This research project aims to develop new techniques and establish a coordinated microscopic mapping of the optical and thermal properties of quantum materials during conductivity switching induced by electric current. More specifically, this research provides a quantitative description of the current-driven local variations in optical dielectric function, temperature, and thermal conductivity at characteristic length scales ranging from nanometers to microns, using state-of-the-art scanning probe microscopy and spectroscopy. Based on systematic studies of representative quantum materials such as correlated transition metal oxides with 3d or 4d orbitals, the Principle Investigator will establish an integrated research and education program to explore nonequilibrium states at the nanoscale over a broad infrared to terahertz frequency range. This research also provides sophisticated training to young researchers and a future generation of scientists in a broad range of knowledge. This includes various types of microscopy, spectroscopy, and nanofabrication techniques.
Quantum materials host a surprisingly diverse set of phase changes when their electrons or lattice are perturbed by various stimuli. By combining the scattering-type scanning near-field optical microscope and scanning thermal microscope into one integrated apparatus, this research studies the current-induced nonequilibrium insulator-to-metal phase transitions in representative quantum materials (e.g. Ca2-xSrxRuO4). The research team will create an experimental routine to distinguish between current-driven and thermal or strain induced phases in bulk single crystals or epitaxial thin films. Dielectric constant, critical current density, nanoscale electrical and heat current transport can be systematically investigated with a spatial resolution smaller than 50 nm. Near-field experiments can be aided with theoretical effort (Boltzman theory) to study nonlocal phenoemon in the proximity of the electrodes at the nanoscale. This work establishes a rigorous procedure to scrutinize the thermal and nonthermal current-driven phase transition in quantum materials. The nanoscale optical and thermal imaging investigation in controlled experimental environments provides a unique platform to systematically study current induced mesoscopic phase transition, separation, electron-lattice correlations and nonlocal effects. The methodology is widely applicable, and the dielectric constant extraction program can be extremely beneficial for the optics community. Research in this area will not only profoundly broaden the fundamental knowledge of topics including Mott physics in transition metal oxides and nonlocal heat transport in nanodevices but will also open new routes to control the intrinsic electronic properties with electric means at the nanoscale.
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.
