Two energy scales and close relationship between the pseudogap and superconductivity in underdoped cuprate superconductors

摘要

By measuring the low temperature specific heat, the low energy quasi-particleexcitation has been derived and analyzed in systematically dopedLa$_{2-x}$Sr$_{x}$CuO$_{4}$ single crystals. The Volovik's relation predictedfor a d-wave superconductor has been well demonstrated in wide doping regime,showing a robust evidence for the d-wave pairing symmetry. Furthermore thenodal gap slope $v_Delta$ of the superconducting gap is derived and is foundto follow the same doping dependence of the pseudogap obtained from ARPES andtunnelling measurement. This strongly suggests a close relationship between thepseudogap and superconductivity. Taking the entropy conservation into account,we argue that the ground state of the pseudogap phase should have Fermi arcswith finite density of states at zero K, and the transport data show that itbehaves like an insulator due to probably weak localization. A nodal metalpicture for the pseudogap phase cannot interpret the data. Based on the Fermiarc picture for the pseudogap phase it is found that the superconducting energyscale or $T_c$ in underdoped regime is governed by both the maximum gap and thespectral weight from the Fermi arcs. This suggests that there are two energyscales: superconducting energy scale and the pseudogap. The superconductivitymay be formed by the condensation of Fermi arc quasiparticles through pairingby exchanging virtue bosons.