Doping induced spinless collective excitations of charge 2e in doped Mott insulators

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Based on the Hubbard model description of the doped Mott insulator, we prove that there must exist a spinless collective excitation of charge 2e and of energy U-2μ peaked at momentum (π/a,π/a) in the non-superconducting state due to doping. Such a collective excitation arises from the electron pair correlation. Its existence is related to the breakdown of an approximate SU(2) particle-hole symmetry, from which an effective field theory of the new excitations is constructed.