Title: Optical control of 4f orbital state in rare-earth metals

Abstract: Rare-earth (RE) 4f elements play a key role for technological applications. The strong magneto-crystalline anisotropy (MCA) and large magnetic moment of the 4f electron shell makes these materials important ingredients for magnetic devices. Being able to control magnetism in 4f materials on ultrafast timescale is one of the major challenges in ultrafast spin physics. Required for an ultrafast, non-thermal control of magnetic alignment are pathways to directly act on the magnetic moment. With the spin angular momentum being a conserved quantity, it is the orbital degrees of freedom that provides an ultrafast handle on the atomic magnetic moment, but mechanisms for light-induced changes of 4f orbital angular momenta have not been discussed, yet. Here we show for Tb metal that after near-infrared laser excitation, 5d-4f electron-electron scattering leads to selective excitations in the 4f shell, which modify the orbital state and locally flip the MCA within femtoseconds. Our study provides a new level of understanding of optically controlled magnetism in 4f metals and shows a path towards ultrafast, non-thermal modification of the 4f-atomic magnetic moments.