Tracking the ultrafast nonequilibrium energy flow between electronic and lattice degrees of freedom in crystalline nickel

Femtosecond laser excitation of ferromagnetic Ni opened up the field of ultrafast magnetism in the mid-1990's. However, it is only today that it has become possible to track the ultrafast energy flow between laser-heated electrons and lattice vibrations. Ultrafast electron scattering and first-principles calculations establish that the phonon system remains in a nonequilibrium state for many picoseconds and that it can even transfer energy back to the electrons.