MRI: Acquisition of Helium Recovery Equipment For Time-Resolved ARPES at NSF-NeXUS

This MRI award funds the acquisition of an ultralow temperature refrigerator for cooling samples down to a temperature of 10 Kelvin (-440 degrees F, -260 degrees C) at the NSF-NeXUS (National Extreme Ultrafast Science) mid-scale facility, an open-access ultrafast laser user facility at the Ohio State University. Many of the most interesting phenomena in quantum materials occur at these very low temperatures, so this is a critical component for enabling a broad range of experiments. This project brings together a group of leading scientists in two-dimensional materials, chiral materials, magnetic materials, and topological materials, who are part of the first experiments and initial user base. Education and outreach activities highlight these advanced instruments, their scientific value, and societal impacts to audiences ranging from researchers in adjacent disciplines to graduate and undergraduate students. The goal of this research is to understand the dynamics of electron waves in quantum materials, which could lead to faster and more energy-efficient electronics and advanced quantum technologies. This is enabled by newly developed experimental techniques that directly image electron dynamics in time, space, energy, and momentum at the NeXUS mid-scale facility. The use of momentum microscopy (MM) and high-harmonic generation light sources produces a major advance in time-resolved, angle-resolved photoemission spectroscopy (TR-ARPES) by adding spatial resolution (~2 microns) and full Brillouin zone mapping. This enables pump-probe studies of carrier, exciton, and band structure dynamics in a variety of interesting quantum materials including two-dimensional materials (transition metal dichalcogenides, graphene, twisted moiré heterostructures, etc), topological materials with Dirac cones away from the gamma point, and chiral and magnetic materials for spintronics. Researchers on this project are utilizing the combined energy-, momentum-, spatial- and temporal-resolution of MM-based TR-ARPES to investigate and understand exciton relaxation processes, charge transfer in heterostructures, valley dynamics, and correlated ground states, as well as light-induced phase transitions and the dynamic manipulation of band structures and Berry curvature. The addition of the new closed-cycle cryocooler to the NeXUS TR-ARPES beamline enables the systematic variation of sample temperature down to 10 K, which is critical for studying exotic phases of quantum materials and also understanding the role of phonons on the dynamics. 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.