Theory of spin-wave excitation in manganites

The role of the orbital degrees of freedom is studied theoretically for the spin dynamics of R(1-x)A(x)MnO(3). Based on the mean-field solution, a random-phase approximation calculation has been done and it is found that the d(x2-y2)-type orbital is essential for the double exchange (DE) interactions, i.e., the DE is basically a two-dimensional interaction. Based on this result compared with experiments, we propose that the orbital wave function is d(x2-y2)-type locally even in the metallic ferromagnetic state, which fluctuates quantum mechanically. Good agreement of the estimation with experiments suggests that the Jahn-Teller phonon has less importance on the spin dynamics.