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The idea to probe, change and control functional materials properties with the help of light has long intrigued researchers in materials science. Using femtosecond laser pulses it is now possible to control the magnetic order or even enhance superconductivity. Femtosecond laser excitation of solid-state systems creates out-of-equilibrium hot electrons that cool down by transferring their energy to other degrees of freedom and ultimately to lattice vibrations of the solid. The understanding of this strongly non-equilibrium dynamics in solids is still very limited, in spite of its emerging importance from a fundamental and applied science viewpoint. The three-temperature model, commonly used for magnetic metals, assumes that the electronic, spin and lattice subsystems are each in separate equilibrium at all times and reach global equilibrium by exchanging heat. The equilibrium concept of “temperature” is so powerful that is is often ignored that it takes time before non-equilibrium dynamics can be described by it. In this lecture I will give an overview how modern ultrafast spectroscopy and scattering techniques allow us to determine in a momentum-resolved way the electron, spin and lattice excitations in the laser-heated state. These results demonstrate that we need to view non-equilibrium dynamics by its associated flow of energy between the various degrees of freedom.

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Hermann Dürr, Uppsala University, Sweden
Seminar Room of the Institute of Physics II
Contact: Paul van Loosdrecht