Sb. Nam, TRANSITION-TEMPERATURE OF SUPERCONDUCTORS - LOWER AND UPPER-BOUNDS OFTHE DENSITY-OF-STATES N(0) FOR SUPERCONDUCTIVITY, Journal of the Korean Physical Society, 28(1), 1995, pp. 106-108
For superconductivity, a large dynamical screening factor F in the Cou
lomb interaction is found to be necessary. Using an analytical express
ion for F, it is shown that there are lower and upper bounds of the de
nsity of states at the Fermi level N(0) [doping x] for superconductivi
ty. A critical point in the x - T(c) plane is found in the case of the
Coulomb interaction greater than the electron phonon interaction at l
arge x. An algebraic expression for the isotope effect applicable to l
ow T(c) and high T(c) superconductors is given.