Quantum interference of impurity bound states in Bi_2Sr_2Ca(Cu_(1-x)Zn_x)_2O_(8+δ) probed by scanning tunneling spectroscopy

摘要

In conventional superconductors, magnetic impurities form an impurity band due to quantum interference of the impurity bound states, leading to suppression of the superconducting transition temperature. Such quantum interference effects can also be expected in c/-wave superconductors. Here, we use scanning tunneling microscopy to investigate the effect of multiple nonmagnetic impurities on the local electronic structure of the high-temperature superconductor Bi_2Sr_2Ca(Cu_(1-x)Zn_x)_2O_(8+δ). We find several fingerprints of quantum interference of the impurity bound states, including (i) a two-dimensional modulation of local density of states with a period of approximately 5.4 A along the a and b axes, which is indicative of the J-wave superconducting nature of the cuprates; (ii) abrupt spatial variations of the impurity bound-state energy; (iii) an appearance of positive energy states; and (iv) a split of the impurity bound state. All of these findings provide important insight into how the impurity band in d-wave superconductors is formed.