Distinct Charge Orders in the Planes and Chains of Ortho-III-Ordered YBa$_2$Cu$_3$O$_{6+δ}$ Superconductors Identified by Resonant Elastic X-ray Scattering (1207.3667v2)

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.

Distinct Charge Orders in Ortho-III-Ordered YBa$_2$Cu$_3$O$_{6+\delta}$ Superconductors

The paper "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" explores the complex charge density wave (CDW) phenomena within YBa$_2$Cu$_3$O$_{6+\delta}$ (YBCO) superconductors. Resonant elastic x-ray scattering (REXS) is employed to discern the distinct charge orders in the CuO$_2$ planes and the CuO chains characteristic of the Ortho-III ordered phase of these materials.

The work substantiates that CDW order in YBCO is not solely linked to the CuO$_2$ planes, where such phenomena have traditionally been observed, but is also inherently linked to the chain layers' oxygen ordering. The experimental method utilizes the tuning of x-ray energies around the Cu L$_3$ absorption edge, providing sensitivity to distinct electronic environments and allowing the investigation of modulation differences.

Key Findings

• Distinct Peaks: The authors identify two types of superlattice peaks: a commensurate peak at $Q = [0.33, 0, L]$ associated with the ortho-III oxygen ordering in the chain layers, and incommensurate peaks at $[0.30, 0, L]$ and $[0, 0.30, L]$ indicating potential CDW order intrinsic to the CuO$_2$ planes.
• Energy and Polarization Dependence: The ortho-III order displays sensitivity to both x-ray energy and polarization, which agrees well with theoretical models for Cu valence modulation in these systems. The plane-related CDW order conversely, reveals a distinct energy dependence, which is attributed to modulations in the Cu 3$d_{x^2-y^2}$ orbitals.
• Temperature Dependence: The CDW order in the planes emerges around 160 K and correlates with superconducting transitions, diminishing in strength below $T_c$. This temperature-dependent behavior offers further evidence of the interplay between density wave order and superconductivity.

Implications

The distinctness in the charge orders between the planes and chains emphasizes the role of orbital symmetries and the spatial energy modulations that possibly reflect underlying electronic inhomogeneities in the plane layers. The observed phenomena suggest that CDW in YBCO, typically characterized by low disorder and high $T_c$, could be a universal property among high-$T_c$ superconductors, potentially extended to the entire family of cuprates.

Future Directions

Further research into the nature of these charge orders could provide deeper insights into high-temperature superconductivity mechanisms. The distinction between chain layer induced order and plane-related CDW suggests unique interaction pathways that might be exploited to engineer or enhance superconducting properties via controlled oxygen ordering strategies. Additionally, investigating similar phenomena in other related cuprates could help elucidate the universality and possible critical conditions required for such CDW orders to manifest. This work continues to build toward a more comprehensive understanding of how structural and electronic phenomena interplay to affect superconductivity in cuprates, opening avenues for novel technological applications.