Investigation of Nonequilibrium States and Photoinduced Transitions in Topological Superlattices

University Of California-Berkeley, Berkeley CA

Investigators

Abstract

Nontechnical description: This project investigates new ways of manipulating quantum materials with ultrashort laser pulses, which can have a significant impact on developing energy-efficient future technologies. The material of interest is a thin film made up of alternating layers of two types of complex oxides that exhibit different electric polarization properties but are structurally similar. The resulting thin film has a highly periodic pattern of polarization domains with typical sizes in the range of 10 nanometers. By shining laser pulses on the material, the investigators dynamically modify the spatial order of these patterns. Studying the temporal evolution of these patterns by stroboscopic measurements with a temporal resolution fast enough to observe electron movements provides an understanding of the underlying mechanisms of pattern formation. Finding routes to optically control polarization patterns could have far-reaching applications. These patterns can encode information that can be rapidly changed by light. This reduces the amount of energy used compared to conventional electronics while enabling many orders of magnitude faster switching in non-volatile memories or computation devices. This project includes educational activities aimed at training the next generation of scientists and engineers at the interface between physics, chemistry, and materials science. These activities will engage with K-12 students, undergraduate and graduate students, and other members of the community to promote diversity in STEM fields and provide valuable training opportunities. Technical description: This project investigates nonequilibrium states and photoinduced transitions in topological superlattices. The research focuses on a ferroelectric/paraelectric heterostructure composed of alternating strontium titanate and lead titanate layers (STO-PTO). These heterostructures exhibit a variety of topological arrangement of local electric polarization forming long-range orders of polar vortices and skyrmions near equilibrium. While traditional tuning knobs like composition, strain, and temperature only affect near-equilibrium properties, rapid improvements in ultrafast laser technology presents a new opportunity to dynamically manipulate these spatially-complex polarization properties. Ultrafast pump-probe schemes specific to parts of the intrinsically complex STO-PTO system is used to investigate the temporal evolution of out-of-equilibrium phases and local topology. Novel experimental tools, including ultrafast broadband extreme-ultraviolet absorption spectroscopy (UBXAS) and extreme ultraviolet second harmonic generation (XUV-SHG), are used to measure superlattice order parameters, trace energetic relaxation processes following photoexcitation, and detect proximitized broken-symmetry states at the interface in real-time. Ultrafast electron microscopy (UEM) is applied to unravel real-space dynamics of the polar skyrmion order and the impact of topological defects on long-range order. The project provides novel insights into interface and defect-mediated correlations in mesoscale systems. 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.