Applied Physics Seminar

**Refreshments in Watson Lobby at 2:45pm

Abstract:

Nonequilibrium dynamics in spin systems is a topic of great importance as it opens the door to exciting new fundamental scientific questions about thermalization, universality, and dynamical phase transitions beyond traditional condensed matter paradigms. Technologically, it might hold the key to achieve coherent spin transport as it directly relates to dissipationless information transport for low power and low loss spintronic devices. Inelastic neutron scattering (INS) is an indispensable tool to study spin excitations in complex magnetic materials. However, conventional INS spectrometers currently only perform steady-state measurements and probe averaged properties over many collision events between spin excitations in thermodynamic equilibrium, while the dynamics and thermalization processes of non-equilibrium states remain unknown. Recently, we designed and implemented a time-resolved laser-neutron pump-probe capability at HYSPEC at the Spallation Neutron Source. This capability allows us to excite out-of-equilibrium magnons with a nanosecond pulsed laser source and probe the resulting dynamics using INS. In this talk, I will discuss our recent progress in demonstrating and understanding laser-induced non-equilibrium magnon population in a gapped Heisenberg antiferromagnet, resolved by INS measurements. Using a Boltzmann approach, I will show the physical origin of these long-lived non-equilibrium magnon states and discuss some open questions that the current transport theory cannot resolve. This work is among the first attempts to perform nonequilibrium INS measurements and opens a new research direction to probe and understand time-resolved non-equilibrium processes of matter with microscopic momentum and energy resolution using neutrons.

More about the Speaker:

Chengyun Hua received her B.S. in Engineering Physics from University of Michigan in 2011, and her Ph.D. in Mechanical Engineering from California Institute of Technology in 2016. She is a R&D associate in Materials Science & Technology Division, Oak Ridge National Laboratory. Her research focuses on gaining a comprehensive picture of how energy is transported in solid at the scales of heat carriers, i.e. electrons, phonons, and spins. Using both theory and experiments, she is currently working on revealing time-reversal symmetry breaking in candidate topological superconductors using both squeezed and ultrafast time-resolved magneto-optical Kerr effect measurements, as well as understanding time-resolved out-of-equilibrium behaviors using neutrons.