Pseudogap of Optimally DopedLa1.85Sr0.15CuO4Observed by Ultrahigh-Resolution Photoemission Spectroscopy

We have performed an ultrahigh-resolution $\left(\Delta E\sim 7\mathrm{meV}\right)$ photoemission spectroscopy on ${\mathrm{La}}_{1.85}{\mathrm{Sr}}_{0.15}{\mathrm{CuO}}_4$ $(T_c=38\mathrm{}\mathrm{K})$ at 16–250 K. We found a pseudogap at $E_F$ with the size of 30–35 meV, which is gradually filled in by a transfer of states from the higher energy region with increasing temperature, but persists over 200 K. The gap energy is consistent with the characteristic temperatures of the magnetic susceptibility and the Hall coefficient. The temperature evolution is well scaled with that of the Hall coefficient. The pseudogap is not smoothly connected to the superconducting gap $\left(\Delta \sim 8\mathrm{meV}\right)$ below $T_c$, suggesting a different origin from that in ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_8$.