Distinct Charge Orders in the Planes and Chains of Ortho-III-Ordered YBa$_2$Cu$_3$O$_{6+\delta}$ Superconductors Identified by Resonant Elastic X-ray Scattering

TL;DR

This study uses resonant elastic x-ray scattering to distinguish charge density wave orders in the CuO$_2$ planes and chain layers of YBa$_2$Cu$_3$O$_{6+ extdelta}$, revealing their distinct nature and origin.

Contribution

It demonstrates that charge orders in the chains and planes are separate phenomena with different energy, polarization, and temperature dependencies, advancing understanding of charge order in cuprates.

Findings

Ortho-III CDW order in chains is distinct from plane CDW order.

Superlattice reflection at Q=[0.33 0 L] is due to oxygen ordering in chains.

Plane CDW peaks are incommensurate and unrelated to chain structure.

Abstract

Recently, charge density wave (CDW) order in the CuO$_2$ planes of underdoped YBa$_2$Cu$_3$O$_{6+\delta}$ was detected using resonant soft x-ray scattering. An important question remains: is the chain layer responsible for this charge ordering? Here, we explore the energy and polarization dependence of the resonant scattering intensity in a detwinned sample of YBa$_2$Cu$_3$O$_{6.75}$ with ortho-III oxygen ordering in the chain layer. We show that the ortho-III CDW order in the chains is distinct from the CDW order in the planes. The ortho-III structure gives rise to a commensurate superlattice reflection at $Q$=[0.33 0 $L$] whose energy and polarization dependence agrees with expectations for oxygen ordering and a spatial modulation of the Cu valence in the chains. Incommensurate peaks at [0.30 0 $L$] and [0 0.30 $L$] from the CDW order in the planes are shown to be distinct in $Q$ as well as their temperature, energy, and polarization dependence, and are thus unrelated to the structure of the chain layer. Moreover, the energy dependence of the CDW order in the planes is shown to result from a spatial modulation of energies of the Cu 2$p$ to 3$d_{x^2-y^2}$ transition, similar to stripe-ordered 214 cuprates.