  Orbital dynamics in time-reversal-symmetric centrosymmetric systems is
examined theoretically. Contrary to common belief, we demonstrate that many
aspects of orbital dynamics are qualitatively different from spin dynamics
because the algebraic properties of the orbital and spin angular momentum
operators are different. This difference generates interesting orbital
responses, which do not have spin counterparts. For instance, the orbital
angular momentum expectation values may oscillate even without breaking neither
the time-reversal nor the inversion symmetry. Our quantum Boltzmann approach
reproduces the previous result on the orbital Hall effect and reveals
additional orbital dynamics phenomena, whose detection schemes are discussed
briefly. Our work will be useful for the experimental differentiation of the
orbital dynamics from the spin dynamics.
