High-$T_c$ mechanism through analysis of diverging effective mass for YaBa$_2$Cu$_3$O$_{6+x}$ and pairing symmetry in cuprate superconductors

Abstract

In order to clarify the high-$T_c$ mechanism in inhomogeneous cuprate layer superconductors, we deduce and find the correlation strength not revealed before, contributing to the formation of the Cooper pair and the 2-D density of state, and demonstrate the pairing symmetry in the superconductors still controversial. To the open questions, the fitting and analysis of the diverging effective mass with decreasing doping, extracted from the acquired quantum-oscillation data in underdoped YBCOO$_{6+x}$ superconductors, can provide solutions. Here, the results of the fitting using the extended Brinkman-Rice(BR) picture reveal the nodal constant Fermi energy with the maximum carrier density, a constant Coulomb correlation strength $k_{BR}$=$U/U_c$>0.90, and a growing Fermi arc from the nodal Fermi point to the isotropic Fermi surface with an increasing $x$. The growing of the Fermi arc indicates that a superconducting gap develops with $x$ from the node to the anti-node. The large $k_{BR}$ results from the $d$-wave MIT for the pseudogap phase in lightly doped superconductors, which can be direct evidence for high-$T_c$ superconductivity. The quantum critical point is regarded as the nodal Fermi point satisfied with the BR picture. The experimentally-measured mass diverging behavior is an average effect and the true effective mass is constant. As an application of the nodal constant carrier density, to find a superconducting node gap, the ARPES data and tunneling data are analyzed. The superconducting node gap is a precursor of $s$-wave symmetry in underdoped cuprates. The half-flux quantum, induced by the circulation of $d$-wave supercurrent and observed by the phase sensitive Josephson-pi junction experiments, is not shown due to anisotropic or asymmetric effect appearing in superconductors with trapped flux. The absence of $d$-wave superconducting pairing symmetry is also revealed.