Comparison of Quantum and Classical Relaxation in Spin Dynamics

The classical Landau-Lifshitz equation with a damping term has been derived from the time evolution of a quantum mechanical wave function under the assumption of a non-Hermitian Hamilton operator. Further, the trajectory of a classical spin (S) has been compared with the expectation value of the spin operator ((S) over cap). A good agreement between classical and quantum mechanical trajectories can be found for Hamiltonians linear in (S) over cap or S, respectively. Quadratic or higher order terms in the Hamiltonian result in a disagreement. DOI: 10.1103/PhysRevLett.110.147201