Total angular momentum conservation in laser-induced femtosecond magnetism

Spin momentum is not a classical quantity [1,2]. It is unclear how the conservation law affects spin momentum change in laser-induced femtosecond magnetization [3]. In solids, the rotational symmetry is lifted by the translational symmetry, and the spin and orbital momenta components of different total angular momenta mix to some extent. This mixing is the origin of the time-dependent total angular momentum in experiments. The remaining unmixed portion accounts for an extra spin change in three independent circularly-polarized laser experiments [4]. \\[0pt] [1] G. P. Zhang, Y. Bai, W. H\"ubner, G. Lefkidis, and T. F. George, J. Appl. Phys. {\bf 103}, 07B113 (2008). [2] G. P. Zhang and W. H\"ubner, Phys. Rev. Lett. {\bf 85}, 3025 (2000). [3] G. P. Zhang, Phys. Rev. Lett. {\bf 101}, 187203 (2008). [4] G. P. Zhang and T. F. George, Phys. Rev. B {\bf 78}, 052407 (2008).