Phonon squeezed state in high-T_c superconductors

We study the phonon states in high T_c superconducting materials within a weak-coupling polaron model. It is confirmed that the squeezed state of phonons gives lower ground state energy of the whole system than the two-phonon coherent state and the displaced state proposed previously. We also examine a condition that the squeezed state occurs. It follows that the squeezed state can exist when the interband electron transfer is allowed in many-band system.