Second Superconducting Transition Driven by Charge Density Wave

Instability of charge density wave can drive successive superconducting transitions. The transition at a lower temperature can be detected by the softening of phonons, whose symmetry is compatible with the symmetry of the product of the two superconducting order parameters. It is argued that the second transition is responsible for tiny anomalies around T c2 ≈35 K in high-temperature superconductors YBa 2 Cu 3 O 7 with superconducting critical temperature T c ≈90 K. It is also argued that the anisotropic Cooper pairing is responsible for high-temperature superconductivity not only in YBa 2 Cu 3 O 7 but also in the other cuprate oxides.