Electron density changes at the crystal lattice sites upon superconducting phase transition

It is established that complex copper oxides exhibit a relationship between a change in the electron density at the metal crystal lattice site and the temperature (T _c) of a phase transition to the superconducting state. In crystals containing two structurally inequivalent positions of copper atoms, a change in the electron density upon the transition is different for these lattice sites. In particular, there is a limiting value of the electron density variation upon the superconducting transition, which is different for Cu(1) and Cu(2) lattice sites and corresponds to the two different minimum values of the correlation length. The electron density changes at the sites of a Kronig-Penney crystal lattice upon the superconducting transition are calculated. It is established that the transition from normal to superconducting phase is accompanied by an increase in the charge density at the center of the unit cell. This growth increases with T _c, which agrees with the Mössbauer spectroscopy data.