Physical properties of the double exchange and its relevance to the specific heat of double perovskite materials

In response to the experimental discovery of double perovskite Ru- base material, The magnetic and electronic properties of the double exchange mechanism are studied theoretically. The electron band structure and the spin wave spectrum response at zero temperature with the canting background are computed from the first principle Hamiltonian. The long range Coulomb interaction between charge carriers is found to be crucial in stabilizing the canting order. A Monte Carlo simulation is performed on de-Gennes's effective magnetic Hamiltonian with classical spin approximation. The peaks of the specific heat as a function of temperature, which correspond to FM(AF) and canting transitions, and its response to the external magnetic field are consistent with the observation. For certain region of parameters, a more complex canting order than layer type is found at sufficiently low temperature. The effective interaction between carriers via exchanging a magnon wave is found to be rather similar to that via exchanging an acoustic phonon. The pros and cons to a genuine superconducting order in this system is analyzed.