Role of LO phonon on pseudo-gap phase in high-Tc cuprate superconductors

Abstract

In analyzing the role of optical phonon on transition temperature Tc, the simplified extended BCS-like theory was tried in success and why Coulomb interaction must be large was understood. Oxygen (O) -isotope effect on Tc was explained in good agreement with more than 120 experimental data. The superconductivity appears in itself due to Bose-condensation of bipolarons coupled with surrounding local distortion and the change is discussed basing on the relative movement of light O-atoms in high-Tc oxides. From the present theoretical formula, LO phonon modes are made mainly at O-sites, and the mode couple to electrons changes from Z.B. type (under-doped region) to Z.C. type (over-doped region) at a doped point (x = 1/8 in La2-xBaxCuO4). At optimally doped point, the vibration type of O-atoms exchanges through zero vibration, and therefore, the concentration takes the form of dip point of Tc. In over-doped region, LO-Z.C. mode with negative pressure dependence plays and condenses simultaneously at Tc, whereas in under-doped region, LO-Z.B. mode with positive pressure dependence couples to electrons to make charged boson at pseudo-gap phase, and condensed for T under Tc. By introducing the strong electron correlation in addition to the conventional mechanism of superconductivity, Tc-equation would be derived for d-wave superconductors.