Superconducting condensate residing on small Fermi pockets in underdoped cuprates

TL;DR

This study shows that in underdoped cuprates, superconductivity originates from small Fermi pockets, while quasiparticles are spread over larger Fermi surfaces, clarifying the pseudogap's non-superconducting nature.

Contribution

It provides direct experimental evidence that Cooper pairs are confined to small Fermi pockets, distinct from the quasiparticle distribution, advancing understanding of Fermi surface evolution in cuprates.

Findings

Supercurrent decreases rapidly with doping.

Quasiparticle resistance remains nearly unchanged with doping.

Cooper pairs are confined to small Fermi pockets.

Abstract

How does Fermi surface develop in cuprates upon doping of a parent Mott insulator, does it consist of large barrels or small pockets, which of them is responsible for superconductivity and what is a role of the pseudogap? Those are actively debated questions, important for understanding of high temperature superconductivity. Here we analyze doping dependence of interlayer tunneling in cuprates. We observe that with decreasing doping the supercurrent is rapidly decreasing, but the quasiparticle resistance at high bias remains almost unchanged. This indicates that Cooper pair and quasiparticle currents originate from different parts of Brillouin zone: Cooper pairs are residing only on small pockets, which are progressively shrinking with decreasing doping, but the quasiparticle current is integrated over the full length of barrels, which are only weakly doping dependent. The expanding pseudogap areas along the barrels do not contribute to pair current. This provides direct evidence for nonsuperconducting origin of the pseudogap.