New gap equation for a marginal Fermi liquid
Assuming a phenomenological self-energy $Im Σ(ω) \sim |ω|^{β\}, (β=1 $), which becomes gapped below $T_c$, we derived a new gap equation. The new gap equation contains the effect of the kinetic energy gain upon developing a superconducting order parameter. However, this new kinetic energy gain mechanism works only for a repulsive pairing potential leading to a s-wave state. In this case, compared to the usual potential energy gain in the superconducting state as in the BCS gap equation, the kinetic energy gain is more effective to easily achieve a high critical temperature $T_c$, since it is naturally Fermi energy scale. In view of the experimental evidences of the d-wave pairing state in the hole-doped copper-oxide high-$T_c$ superconductors, we discuss the implications of our results.