Experimental observation of Bose condensation in high-temperature superconductors

Emission Mossbauer spectroscopy on ⁶⁷Cu (⁶⁷Zn) and ⁶⁷Ga (⁶⁷Zn) isotopes was used to show that for the superconductors Nd_1.85Ce_0.15CuO₄, La_1.85Sr_0.15CuO₄, and Tl₂Ba₂Ca₂CuO₈ 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 non- uniformity produced in the electron density by a Bose condensate of Cooper pairs was observed for La_1.85Sr_0.15CuO₄.