The Unusual Superconducting State of Underdoped Cuprates

TL;DR

This paper investigates the unusual superconducting state in underdoped cuprates, showing that the critical temperature scales with doping and gap, and predicting the upper critical field's dependence on doping, based on quasiparticle excitations.

Contribution

It provides a theoretical explanation for the doping dependence of $T_c$ and $H_{c2}$ in underdoped cuprates, linking experimental observations with quasiparticle models within the t-J framework.

Findings

$T_c$ is proportional to $x\Delta_0$

$H_{c2}$ is proportional to $x^2$

Part of the gap remains in the normal state

Abstract

There is increasing experimental evidence that the superconducting energy gap $\Delta_0$ in the underdoped cuprates is independent of doping concentration $x$ while the superfluid density is linear in $x$. We show that under these conditions, thermal excitation of the quasiparticles is very effective in destroying the superconducting state, so that $T_c$ is proportional to $x\Delta_0$ and part of the gap structure remains in the normal state. We then estimate $H_{c2}$ and predict it to be proportional to $x^2$. We also discuss to what extent the assumptions that go into the quasiparticle description can be derived in the U(1) and SU(2) formulations of the t-J model.