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A.E.Myasnikova and E.N.Myasnikov
We have shown earlier that the strong-coupling large radius polarons (i.e. polarons that occur at strong electron-phonon interaction in a medium where the conduction band is not narrow) contain phonon condensate. (In other words, the polarization field in such polarons is in quantum-coherent state.) At the polaron photodissociation the phonon condensate decays into different number of phonons in each act the probability of appearance of some number of phonons is determined by Poisson distribution as it is ordinary for quantum-coherent states. Average number of phonons is n=2Ep/Ephon (where Ep and Ephon are the polaron binding energy and the phonon energy, respectively), and non-zero probabilities occur for radiation of phonons from 0 up to 2n. Therefore, the band in ARPES caused by photodissociation of strong-coupling large-radius polarons is a broad band with the shape determined by Poisson distribution. It can be structured or unstructured depending on the phonon dispersion since a distance between neighbouring lines comprising the band is the phonon energy. Half-width of the band is in the interval 1.3 - 1.7Ep, depending on the phonon energy. The band maximum is situated approximately at the electron energy Ephot-W-3.2Ep (where Ephot is the photon energy, W is work function), and its position does not depend on the electron wave vector direction. For example, we can calculate the polaron binding energies in YBa2Cu3O6+y, Nd2CuO4-y, La2CuO4+y and La2-xSrxCuO4+y from the position of maximum of the mid-infrared band in their optical conductivity spectra. Then the maximum of the bands in ARPES of these substances caused by photodissociation of the polarons will be approximately at the electron energy (with respect to the energy Ephot-W) -0.48eV, -0.52eV, -0.44eV and -0.4eV, respectively. |
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