Superconducting Condensation Energy and Pseudogap Formation in La_2-xSr_xCuO₄: New Energy scale for Superconductivity

Abstract

The electronic specific heat C_el was measured for La_2-xSr_xCuO₄ to examine the superconducting condensation energy at T=0, U₀. We reconfirmed that U₀ is markedly reduced even in slightly underdoped samples whose T_c's are still more than 30 K. The reduction of U₀ results from the suppression of the effective superconducting gap, in addition to that of the density of states at the Fermi level E_F, caused by the pseudogap formation in the normal state. The doping-level ( p) dependence of U₀ as well as T_c can be explained over a wide p range by introducing an effective energy gap Δ_eff=β pΔ₀ (β=4.5) instead of Δ₀, where Δ₀ is the maximum gap at T<< T_c.