Co-substitution effect on electronic structure of high-<i>T</i>_c cuprate superconductor, Bi₂Sr₂Ca(Cu_1-<i>x</i>Co<i>_x</i>)₂O₈

Conventional superconductivity occurs when electron pairs are formed. However, the pairing mechanism of high-T_c superconductivity is still controversial. One may expect that experimental clues to this problem can be obtained from the behavior of superconducting gap, which represents the binding energy of an electron pair. It has previously been reported that the decrease in critical temperature, T_c, of Bi₂Sr₂Ca(Cu_1-xCo_x)₂O₈ is proportional to Co substitution, x. Thus, it gives us a good opportunity to investigate the relation between T_c and the energy gap. Here, we report a high-resolution angel-resolved photoemission study of Bi₂Sr₂Ca(Cu_1-xCo_x)₂O₈ by using ultraviolet laser and synchrotron radiation. Even though T_c decreases from 91 K to 55 K with 4 % substitution of Co for Cu, almost no decrease in the superconducting gap has been observed. We also measured the residual intensity within the gap as a function of temperature. We have found that the residual intensity for Co 4% samples is larger than that for pristine samples. These results suggest that the Co substitution results in reduction of the density of the electron pairs rather than the change in the binding energy of the electron pairs.