Pseudogap of optimally doped La_1.85Sr_0.15CuO₄ observed by ultrahigh-resolution photoemission spectroscopy

We have performed an ultrahigh-resolution \(ΔE˜7 meV\) photoemission spectroscopy on La_1.85Sr_0.15CuO₄ \(T_c = 38 K\) at 16-250 K. We found a pseudogap at EF 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 \(Δ˜8 meV\) below T_c, suggesting a different origin from that in Bi₂Sr₂CaCu₂O₈.