Two-electronic Moessbauer the centers with negative correlation energy as the tool of research of Bose-condensation in superconductors

Two-electronic centers with negative correlation energy; crystal probe; Bose condensation of Cooper pairs; changes of electronic density in the lattice sites; isomer shift; gravity center of a spectrum; superconducting phase transition; temperature of transition; minimal size of a Cooper pair. Objects of research: solid solutions of lead chalcogenides Pb_1-x-ySn_xNa_yS, Pb_1-x-ySn_xNa_ySe, Pb_1-x-yGe_xNa_yS, Pb_1-x-yGe_xNa_ySe, (Pb_1-xSn_x)_1-zIn_zTe; classical super-conductor Nb₃Al; high-temperature superconductors La_2-x(Sr,Ba)_xCuO₄, YBa₂Cu₃O_7-x, YBa₂Cu₄O₈, Tl₂Ba₂Ca_n-1Cu_nO_2n+4, Bi₂Sr₂Ca_n-1Cu_nO_2n+4, HgBa₂Ca_n-1Cu_nO_2n+2, where n=1,2,3; Objective of study: development and application of methodology of the Moessbauer spectroscopy of systems with negative correlation energy in order to identify the impurity U- centers in semiconductors and Bose condensation of Cooper pairs in classical and high-temperature superconductors. Method of research: emission Moessbauer spectroscopy on ^119mmSn (^119mSn), ⁷³As(⁷³Ge), ⁶⁷Cu(⁶⁷Zn), ⁶⁷Ga(⁶⁷Zn), ⁵⁷Co(^57mFe), ^119mTe(^119mSn) isotopes; absorption Moessbauer spectroscopy on ¹¹⁹Sn, ¹⁵¹Eu, ⁵⁷Fe isotopes; temperature dependence of the conductivity; Hall effect. Obtained results and their novelty. The research of the crystal lead chalcogenides has shown that tin and germanium impurity atoms are U^- centers of a donor type, neutral and twice ionized states of the U^- centers are in cation sites of a lattice. Temperature dependence of the chemical potential, the first and the second ionization energy of U^- centers, correlation energy and energy of activation of own conductivity as well as their dependence on concentration of acceptors and tin atoms have been determined for tin impurity atoms. Impurity tin and germanium atoms specially entered into antistructural positions of lead chalcogenides lattices (a method of radiation doping) are electrically inactive. The two-electronic exchange between neutral and ionized impurity U^- tin (and germanium) centers in PbSe lattice and the effect of stabilization of a unitary ionized state of a U^- tin center in PbS lattice have been determined experimentally. The two-electronic exchange between neutral and ionized U^- tin centers in Ag_1-ySn_1+ySe₂ solid solutions and one-electronic exchange between Sn⁴⁺ centers and zone states in Ag_1-ySn_1+yTe₂ solid solutions have been determined. The europium impurity atoms in cation PbS sublattice are electrically active and in the case of partly compensated material the process of fast electronic exchange between neutral and ionized centers have been observed. It is shown that the superconducting phase transition in semiconductors, classical and high-temperature superconductors is accompanied by changes in the electronic density within cation sites of a crystal lattice. The specified changes in the electronic density depend on a position of a Moessbauer probe in a lattice, as well as on temperature of crystal transition into a superconducting phase. In case of compounds containing nonequivalent positions for copper atoms Cu (1) and Cu (2) in a crystal lattice a correlation between the greatest possible changes in the electronic density and minimal possible values of the standard correlation length, differing in Cu(1) and Cu(2) sublattices, have been determined. According to calculations of changes in the electronic density within the sites of an one-dimensional crystal lattice, the transition from normal to a superconducting phase is accompanied by increase of the electronic density at the center of an elementary cell, and this change is bigger the higher is the transition temperature into a superconducting state. The practical importance: the experimental technique of the emission Moessbauer spectroscopy on ⁶⁷Zn and ⁷³Ge probes have been realized, which allows to investigate of electronic structure of superconductors. A Doppler modulation system for registration of the Moessbauer spectra with record-breaking narrow lines, and the system of maintenance for of various temperatures a source and an absorber have been created for an establishment of correlation between isomer of a shift Moessbauer spectrum and of critical temperature superconductivity of transition. The express-technique of allocation of non-carrying radioactive isotopes have been realized. The results of determination of changes in the electronic density in crystals through their transition into a superconducting state and detection of spatial heterogeneity of the Cooper pairs Bose condensate can have great value for development of the theory of high-temperature superconductivity. Area of application: solid-state physics, physics of semiconductors, the theory of superconductivity. (author)