Nanoscale Phenomenology from Visualizing Pair Formation Experiment

TL;DR

This paper analyzes nanoscale regions in high-T_c superconductors, revealing how their charge and percolation properties relate to the emergence and disappearance of superconductivity at specific doping levels.

Contribution

It introduces a charge and percolation analysis of nanoscale regions, linking their properties to critical doping levels and the presence of bosonic or fermionic charge carriers.

Findings

NRs carry charge one at the onset of superconductivity, indicating hole bosons.

At the disappearance of superconductivity, NRs carry charge two, involving normal phase hole fermions.

Percolation of bosons breaks down at the second critical doping, explaining the loss of superconductivity.

Abstract

Recently, Gomes et al. [1] have visualized the gap formation in nanoscale regions (NRs) above the critical temperature T_c in the high-T_c superconductor Bi_2Sr_2CaCu_2O_{8+\delta}. It has been found that, as the temperature lowers, the NRs expand in the bulk superconducting state consisted of inhomogeneities. The fact that the size of the inhomogeneity [2] is close to the minimal size of the NR [1] leads to a conclusion that the superconducting phase is a result of these overlapped NRs. In the present paper we perform the charge and percolation regime analysis of NRs and show that at the first critical doping x_{c1}, when the superconductivity starts on, each NR carries the positive electric charge one in units of electron charge, thus we attribute the NR to a single hole boson, and the percolation lines connecting these bosons emerge. At the second critical doping x_{c2}, when the superconductivity disappears, our analysis demonstrates that the charge of each NR equals two. The origin of x_{c2} can be understood by introducing additional normal phase hole fermions in NRs, whose concentration appearing above x_{c1} increases smoothly with the doping and breaks the percolation lines of bosons at x_{c2}. The last one results in disappearing the bulk bosonic property of the pseudogap (PG) region, which explains the upper bound for existence of vortices in Nernst effect [3]. Since [1] has demonstrated the absence of NRs at the PG boundary one can conclude that along this boundary, as well as in x_{c2}, all bosons disappear.