Lattice effects in the colossal-magnetoresistance manganites

We study the combined influence of spin double exchange and Jahn-Teller lattice coupling to holes in the La(1-x)A(x)MnO(3) perovskites (A = Ca, Sr, Ba). Using a mean-field approximation for the double exchange and a variational Lang-Firsov approximation for the lattice degrees of freedom, we show that the lattice effects decrease the magnetic transition temperature, and also cause the maximal value of the transition temperature as a function of dopant concentration x to depend on the Jahn-Teller coupling strength. We find a continuous rapid crossover from a large polaronic state to a quasi-self-trapped small polaron state accompanying the magnetic transition.