Ultrafast Lattice Vibrations’ Interaction with Electrons

2024-03-21

Researchers at Uppsala University have in a collaboration with researchers in Germany exploited the fundamental conservation laws of energy and momentum to track the scattering of spins after laser excitation in time-momentum space.

Understanding elementary scattering processes due to the interactions between electrons and lattice vibrations is of importance in a wide range of condensed matter phenomena such as superconductivity, electronic and spin transport, ultrafast demagnetization and laser induced phase transitions.

In a pioneering experiment in 1996, it was shown that femtosecond-optical pulses can be used to demagnetize nickel1, i.e. the ferromagnetic alignment of the Nickel electron spins, on the femtosecond timescale. However, it took more than 25 years before experiments with femtosecond x-ray and electron pulses could detect the missing spin angular momentum as chiral lattice vibrations. The mechanism for the loss of ultrafast angular momentum remains unclear to this day.

The scattering of electrons from one plane to another (Brillouin zone) in a laser excited system defined by the exchange momentum vector (yellow arrow) in reciprocal space. The Brillouin zone (red polyhedron) is the space encompassing all the momentums of electrons in a solid. Image: Vishal Shokeen.

The new observation is the first that directly detects electrons and lattice vibrations involved in ultrafast demagnetization. The researchers used free electron lasers, a short pulse light source, to observe electrons in nickel, moving on their natural timescale (femtoseconds) as they interact with lattice vibrations. This results in electrons that disappear in some states and reappear in others while the change in electron energy and momentum corresponds to transiently occupied lattice vibrations. These results provide the first direct visualization of electron – phonon scattering processes. Moreover, during these scattering processes, a net flow of spins results in the redistribution of magnetic moments breaking the equilibrium picture of equivalence of magnetism and exchange splitting in nickel.

Article reference

Vishal Shokeen et al.​ Real-time observation of non-equilibrium phonon-electron energy and angular momentum flow in laser-heated nickel. Sci. Adv.10, eadj 2407 (2024). DOI:10.1126/sciadv.adj2407https://www.science.org/doi/10.1126/sciadv.adj2407

Read more

1E. Beaurepaire, J.-C. Merle, A. Daunois, J.-Y. Bigot, Ultrafast Spin Dynamics in Ferromagnetic Nickel. Phys. Rev. Lett. 76, 4250–4253 (1996).

Contact

Vishal Shokeen, researcher at the division of FREIA, Department of Physics and Astronomy

Last modified: 2023-08-04