The role of oxygen for expressing superconductivity in La-Ca-Cu-O compounds

Two La1.82Ca1.18Cu2O6+delta (2126) compounds exhibited different properties depending upon how they were synthesized. The compound prepared under high oxygen pressure showed superconductivity. But the compound prepared under low oxygen pressure did not exhibit superconductivity, and showed a metal-i nsulator transition. Our study on these compounds shows that a small amount of additional oxygen intercalated into the superconducting phase plays an important role for superconductivity. The Fermi surface of non-superconduct ing 2126 compound possesses nesting phenomena, which is the reason for the M-I transition. On the other hand, the superconducting 2126 compound does n ot show Fermi surface nesting. This is because the additional oxygen remove s some electrons from Cu d-orbitals, thereby bradking the Fermi surface nes ting.