Correlation between Superconducting Gap and Pseudogap in High-T _c Cuprates

Abstract

We studied the low-temperature energy gap 2Δ ₀ on Bi ₂ Sr ₂ CaCu ₂ o _8+δ (Bi2212) and La _2−x Sr _x CuCO ₄ (La214) systematically over a wide range of doping level p using STS, break junction tunneling spectroscopy, Raman scattering and low-T electronic specific heat data. We have also studied the electronic specific heat of La214 in the normal state at T > T _c , and confirmed that pseudogap behavior appears at around T*, below which the in-plane resistivity ρ and magnetic susceptibility χ tend to be slightly suppressed. Similar suppression appears in ρ and χ of Bi2212 below the onset temperature of pseudogap T*. It is pointed out in the present study that 2Δ ₀ is closely related to T* in both Bi2212 and La214 systems; T* ∼2Δ ₀ /4.3k _B . It is also pointed out that 2Δ ₀ is in almost linear proportion to k _B T _max (≪ T*), where T _max is the temperature exhibiting a broad peak in χ−T curves and k _B T _max can be considered to give a measure of the effective antiferromagnetic exchange energy J _eff. The α factors in 2Δ ₀ ∼ αk _B T _max ∼ αJ _eff are ∼1 for La214 and ∼2 for Bi2212, respectively. We also report that in both Bi22l2 and La214 systems T _c roughly scales with pΔ ₀ except in highly doped samples, where T _c ∝2Δ ₀ .