Delocalized fermions in underdoped cuprates

TL;DR

This study reveals that underdoped cuprates in a non-superconducting state host delocalized fermionic excitations, indicating a metallic ground state with a nodal spectrum, contrasting with insulating behavior in similar materials.

Contribution

It provides direct evidence of delocalized fermions in the ground state of underdoped cuprates, challenging previous notions of their insulating nature.

Findings

Delocalized fermionic excitations observed in non-superconducting state

Ground state of underdoped cuprates is metallic

Similar low-energy spectrum to d-wave superconductor

Abstract

Low temperature heat transport was used to investigate the ground state of high-purity single crystals of the lightly-doped cuprate YBa$_{2}$Cu$_{3}$O$_{6.33}$. Samples were measured on either side of the superconducting phase boundary, in both zero and applied magnetic field. We report the observation of delocalized fermionic excitations at zero energy in the non-superconducting state, which shows that the ground state of underdoped cuprates is metallic. Its low-energy spectrum appears to be similar to that of the d-wave superconductor, i.e. nodal. The insulating ground state observed in underdoped La$_{2-x}$Sr$_{x}$CuO$_4$ is attributed to the competing spin-density-wave order present in that system.