Electron pockets in the Fermi surface of hole-doped high-Tc superconductors

Abstract: High-temperature superconductivity occurs as copper oxides are chemically tuned to have a carrier concentration intermediate between their metallic state at high doping and their insulating state at zero doping. The underlying evolution of the electron system in the absence of superconductivity is still unclear and a question of central importance is whether it involves any intermediate phase with broken symmetry. The Fermi surface of underdoped YBa2Cu3Oy and YBa2Cu4O8 was recently shown to include small pockets in contrast with the large cylinder characteristic of the overdoped regime1, pointing to a topological change in the Fermi surface. Here we report the observation of a negative Hall resistance in the magnetic field-induced normal state of YBa2Cu3Oy and YBa2Cu4O8, which reveals that these pockets are electron-like. We propose that electron pockets arise most likely from a reconstruction of the Fermi surface caused by the onset of a density-wave phase, as is thought to occur in the electron-doped materials near the onset of antiferromagnetic order Comparison with materials of the La2CuO4 family that exhibit spin/charge density-wave order suggests that a Fermi surface reconstruction also occurs in those materials, pointing to a generic property of high-Tc superconductors.

• The paper demonstrates that electron pockets exist in underdoped YBCO, established through precise Hall effect measurements indicating Fermi surface reconstruction.
• It identifies Shubnikov-de Haas oscillations that confirm high electron mobility, reconciling earlier discrepancies with the Luttinger sum rule.
• The comparative analysis suggests that density-wave formation underlies the Fermi surface modification, a universal feature in hole-doped high-Tc superconductors.

Evaluation of Electron Pockets in the Fermi Surface of Hole-Doped High-Temperature Superconductors

The paper presents a comprehensive investigation into the electronic structure of high-temperature superconductors, specifically underdoped YBa2Cu3O7−δ (YBCO) and YBa2Cu4O8 (Y124) cuprates. Utilizing the Hall effect as a diagnostic tool, the authors provide compelling evidence for the existence of electron pockets in the Fermi surface, identifying Fermi surface reconstruction as a ubiquitous feature in hole-doped high-temperature superconductors.

Key Findings

1. Negative Hall Coefficient: The study highlights measurements of the Hall resistance in underdoped Y123 and Y124 samples, revealing a negative Hall coefficient in the normal state at lower temperatures. This signifies the presence of electron-like pockets in the Fermi surface of YBCO cuprates.
2. Fermi Surface Reconstruction: The emergence of electron pockets implies a Fermi surface reconstruction linked to density-wave formation. This paper associates such changes with transitions seen in other La2−xBaxCuO4 (LBCO) compatible cuprates, suggesting a density-wave phase that affects the Fermi surface topology by inducing both electron and hole-like components.
3. Electron Mobility Hypergraphy: Observations of Shubnikov-de Haas oscillations in the ortho-II phase of Y123 show that these oscillations originate from a high-mobility electron pocket. This finding reconciles previous contradictions with the Luttinger sum rule, which faces apparent violations when considering only hole pockets.
4. Comparative Analysis: By comparing with electron-doped cuprates, which exhibit antiferromagnetic order, the authors suggest that a similar mechanism might be responsible for the Fermi surface modification in hole-doped counterparts. However, no direct evidence is yet available for long-range density-wave order in YBCO.

Implications and Speculative Ideas

1. Understanding High-Tc Superconductors: This study underpins the critical relevance of Fermi surface topology modifications in understanding the exotic properties of high-Tc superconductors. It sets a foundation for further exploration into the microscopic mechanisms driving superconductivity in these complex oxides.
2. Experimental Directions: The paper underscores the necessity for diffraction investigations to verify hypothetical scenarios suggesting fluctuating or short-range density-wave orders. Theoretical models may need to include unconventional order parameters to elucidate the observed phenomena fully.
3. Impact on Material Engineering: The findings implicate impurities and disorder as significant factors influencing electron pocket visibility, impacting the design and synthesis of superconducting materials with optimized properties.
4. Future Studies: This work encourages deeper studies into the role of CuO chains in such materials and their contribution to anisotropic electrical properties, especially in clean YBCO variants, compared to counterparts like LSCO and BSLCO.

In summary, the paper significantly enriches the understanding of the electronic landscape of hole-doped high-temperature superconductors, pointing to a universal phenomenon of Fermi surface reconstruction with additional implications for condensed matter physics and applied material science. Further experimental validation and theoretical elaboration could yield transformative insights into the fundamental mechanisms steering high-temperature superconductivity.