Emission Mossbauer spectroscopy on ~(67)Cu (~(67)Zn) and ~(67)Ga (~(67)Zn) isotopes was used to show that for the superconductors Nd_(1.85)Ce_(0.15)CuO_4, La_(1.85)Sr_(0.15)CuO_4, and Tl_2Ba_2CaCuO_8 in the temperature range T > T_c the temperature dependence of the center of gravity S of the Mossbauer spectrum is determined by the second-order Doppler shift, while in the range T < T_c the Bose condensation of Cooper pairs influences the value of S (here T_c is the superconducting transition temperature). The spatial nonuniformity produced in the electron density by a Bose condensate of Cooper pairs was observed for La_(1.85)Sr_(0.15)CuO_4.
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