  Based on the magnetoresistance, magnetization, ultrasound, and
crystallographic data, we studied the role of the cooperative Jahn-Teller
effect in the charge ordered (CO) state for La1-xCaxMnO3. We found that, with
increasing the fraction of Q3 mode of Jahn-Teller distortion and decreasing
that of Q2 mode in the CO state, the magnetic structure evolves from CE-type to
C-type and the orbital ordering changes from 3d(x2-r2)/3d(y2-r2)-type to
3d(x2-z2)-type, with the strength of ferromagnetism and the phase separation
tendency being suppressed. At the same time, the stability of the CO state and
the cooperative Jahn-Teller lattice distortion increase. These effects imply
that the cooperative Jahn-Teller effect with different vibration modes is the
key ingredient in understanding the essential physics of the CO state.
