[Paper Review] Revealing the ortho-II Band Folding in YBa2Cu3O7-delta Films

Yasmine Sassa, M. Radović+11 more|arXiv (Cornell University)|Jan 28, 2011

Physics of Superconductivity and Magnetism12 citations

TL;DR

This study uses in-situ pulsed laser deposition to grow oxygen-ordered YBa2Cu3O7-δ films, enabling high-resolution angle-resolved photoemission spectroscopy (ARPES) that directly reveals ortho-II band folding in the Fermi surface. The results confirm theoretical predictions of band folding due to 1D CuO chain ordering, demonstrating that both oxygen-vacancy order and surface cleanliness are essential for observing bulk electronic properties in underdoped cuprates.

ABSTRACT

We present an angle-resolved photoelectron spectroscopy study of YBa2Cu3O7-delta films in situ grown by pulsed laser deposition. We have successfully produced underdoped surfaces with ordered oxygen vacancies within the CuO chains resulting in a clear ortho-II band folding of the Fermi surface. This indicates that order within the CuO chains affects the electronic properties of the CuO2 planes. Our results highlight the importance of having not only the correct surface carrier concentration, but also a very well ordered and clean surface in order that photoemission data on this compound be representative of the bulk.

Motivation & Objective

• To overcome the longstanding challenge of observing ortho-II band folding in YBa2Cu3O7-δ via ARPES due to surface disorder and doping issues.
• To establish that surface oxygen-vacancy ordering in CuO chains is essential for observing bulk-like electronic structure in underdoped YBCO.
• To demonstrate that both correct carrier concentration and long-range order are required for ARPES to yield representative data on YBCO.
• To provide experimental validation for theoretical models predicting band folding in ortho-II YBCO, which were previously unconfirmed.

Proposed method

• In-situ pulsed laser deposition (PLD) was used to grow high-quality, oxygen-ordered YBa2Cu3O7-δ films with controlled stoichiometry and surface termination.
• Angle-resolved photoemission spectroscopy (ARPES) was performed at the SIS X09LA beamline of the Swiss Light Source using 70 eV circularly polarized photons and high energy resolution (15–25 meV).
• Momentum distribution curves (MDCs) were extracted and fitted with Lorentzian functions to determine the Fermi momentum (kF) at the Fermi level.
• A tight-binding model with parameters t = 558 ± 50 meV, t′/t = 0.49 ± 0.03, t′′/t′ = 0.5 ± 0.03, μ = -469 ± 90 meV, and V = 75 meV was used to simulate the Fermi surface.
• Theoretical Fermi surface maps were calculated for twinned ortho-II samples, including unit-cell doubling along the a-axis due to (2×1) chain ordering.
• Simulated intensity maps were compared with experimental data to validate the presence of band folding and match the observed kF positions.

Experimental results

Research questions

• RQ1 Can ortho-II band folding be experimentally observed in YBa2Cu3O7-δ films using ARPES despite surface challenges?
• RQ2 What role does oxygen-vacancy ordering in CuO chains play in stabilizing the surface and influencing the electronic structure of the CuO2 planes?
• RQ3 Why were previous ARPES studies on YBCO, including K-doped samples, unable to detect ortho-II band folding?
• RQ4 To what extent does surface termination and carrier concentration affect the representativeness of ARPES data for the bulk electronic structure?
• RQ5 How does the presence of twinning and (2×1) chain superstructure influence the observed Fermi surface topology?

Key findings

• ARPES measurements on in-situ grown, oxygen-ordered YBCO films clearly reveal ortho-II band folding in the Fermi surface, confirming theoretical predictions.
• The observed Fermi surface exhibits a folded structure due to a (2×1) superstructure from ordered oxygen vacancies in the CuO chains, leading to a reduced Brillouin zone.
• The experimental kF positions at the Fermi level show excellent agreement with simulated intensity maps based on a tight-binding model with t = 558 ± 50 meV and V = 75 meV.
• The study demonstrates that surface oxygen-vacancy ordering is essential for observing bulk-like electronic properties, explaining prior failures in ARPES studies.
• The results validate theoretical models that include ortho-II band folding and suggest its relevance for explaining quantum oscillations and broken four-fold symmetry in underdoped cuprates.
• The work establishes that both correct doping and long-range order are necessary for ARPES to yield representative data, resolving inconsistencies in earlier studies.