Angular Resolved Photoemission on Pb-bi2201: Doping- Dependent Evolution of the Pseudogap in the Underdoped Case

In search for the pairing mechanism of the hole doped high temperature superconductors there is still lively debate about the nature of the pseudogap [1] [5]. Some groups report a vanishing pseudogap around optimum doping [2] leading to the idea of a quantum critical point. Other measurements show a smooth convergence of the pseudogap temperature with the superconducting critical temperature in the overdoped region. This suggests to treat the pseudogap state as a normal state precursor of the superconducting gap due to local, dynamic pairing correlations in a state without long range phase coherence [3]. Another striking fact revealed in the single-layer cuprate Bi2Sr2CuO6 (Bi2201) and double-layer cuprate Bi2Sr2CaCu2O8 (Bi2212) a nearly independent pseudogap temperature T with the number of CuO2 layers [4] while the magnitude of the pseudogap, Δ itself is not. We focused our work on angular resolved photoemission (ARPES) measurements of the Lead and Lanthanum doped one-layer single crystal of Bi2201. Here we present temperature dependent ARPES data near the antinodal direction of four high-quality single crystals from the optimum doped Pb0.4-Bi2201 (Tc=32K) down to the very underdoped (Tc~0K) case. The analysis of the data reveals the evolution of the pseudogap temperature T and magnitude Δ with respect to doping. For the three superconducting samples we found a nearly constant ratio between the zero-temperature gap and the pseudogap-temperature of Δ(0)~ 0.49kB T*.