Quantum oscillations and the Fermi surface in an underdoped high-Tc superconductor (0801.1281v1)

Abstract: Despite twenty years of research, the phase diagram of high transition- temperature superconductors remains enigmatic. A central issue is the origin of the differences in the physical properties of these copper oxides doped to opposite sides of the superconducting region. In the overdoped regime, the material behaves as a reasonably conventional metal, with a large Fermi surface. The underdoped regime, however, is highly anomalous and appears to have no coherent Fermi surface, but only disconnected "Fermi arcs". The fundamental question, then, is whether underdoped copper oxides have a Fermi surface, and if so, whether it is topologically different from that seen in the overdoped regime. Here we report the observation of quantum oscillations in the electrical resistance of the oxygen-ordered copper oxide YBa2Cu3O6.5, establishing the existence of a well-defined Fermi surface in the ground state of underdoped copper oxides, once superconductivity is suppressed by a magnetic field. The low oscillation frequency reveals a Fermi surface made of small pockets, in contrast to the large cylinder characteristic of the overdoped regime. Two possible interpretations are discussed: either a small pocket is part of the band structure specific to YBa2Cu3O6.5 or small pockets arise from a topological change at a critical point in the phase diagram. Our understanding of high-transition temperature (high-Tc) superconductors will depend critically on which of these two interpretations proves to be correct.

• The paper demonstrates that quantum oscillations under high magnetic fields reveal coherent, small Fermi pockets in underdoped YBa₂Cu₃O₆.₅.
• The study employs resistance measurements with a low oscillation frequency of 530 T to deduce the Fermi surface geometry.
• The findings challenge conventional Fermi liquid models and suggest the need for theories that incorporate electron correlations and symmetry-breaking phases.

Quantum Oscillations and Fermi Surface in Underdoped High-Temperature Superconductors

The paper "Quantum oscillations and Fermi surface in an underdoped high temperature superconductor" addresses a pivotal question in the field of condensed matter physics: the nature of the Fermi surface in underdoped cuprate superconductors. The authors utilize quantum oscillations in the electrical resistance of YBa₂Cu₃O₆.₅ (YBCO) to elucidate the Fermi surface characteristics in the underdoped regime.

High-temperature superconductors (HTSCs), particularly the cuprates, exhibit enigmatic phase diagrams, where properties vary dramatically between the underdoped and overdoped regimes. In the overdoped regime, the Fermi surface presents as large and coherent, typical of a conventional metal. Conversely, the underdoped region suggests a fragmented Fermi surface, comprised of disconnected "Fermi arcs."

Key Findings

1. Quantum Oscillations Observed: By suppressing superconductivity through the application of a high magnetic field, this paper provides unambiguous evidence of quantum oscillations, thereby confirming a well-defined Fermi surface even in the underdoped state. The oscillatory behavior in resistance serves as a robust indicator of a coherent Fermi surface.
2. Fermi Surface Geometry: The quantum oscillation data reveal a Fermi surface that consists of small pockets, starkly contrasting with the large cylindrical surfaces observed in overdoped cuprates. This is deduced from a low oscillation frequency of 530 T, indicative of small k-space areas enclosed by the Fermi surface.
3. Potential Scenarios:
   • Band Structure Scenario: The small pocket might be an intrinsic element of the band structure unique to YBCO. This scenario aligns with early band structure calculations showing FS sheets associated with the CuO chains and planes.
   • Topological Change Scenario: Alternatively, the small Fermi pockets could result from a topological transformation at a critical point in the phase diagram, which distinguishes the underdoped from the overdoped regime. This transformation might involve electron correlations and potential symmetry-breaking phases that modify the FS topology.

Discussion and Implications

The observed difference in the Fermi surface topology between the underdoped and overdoped regimes is a fundamental aspect of high-Tc superconductivity, potentially linked to the enigmatic pseudogap phase. The paper's results have profound implications on theoretical models of HTSCs, suggesting a need to consider electronic correlations and possibly symmetry-breaking phases when modeling the underdoped state. In particular, the experimental deviation from the large Fermi surface scenario—described by the Luttinger sum rule—raises questions about the nature of charge carriers in the underdoped regime.

The larger theoretical framework might compare the reconstructed FS to that seen in electron-doped cuprates, where density-wave orders and accompanying nodal FS pockets are proposed. This comparison gives a lens to reconcile the observed quantum oscillations with ARPES findings of "Fermi arcs." However, the possibility of Luttinger sum rule violation, if confirmed, would mark a significant departure from conventional metallic behavior, posing a new challenge for theoretical physics.

Future Directions

Further research may focus on:

• Verifying FS Topology: Additional experimental techniques, such as ARPES and scanning tunneling spectroscopy, should aim to validate the small-pocket FS topology detected by quantum oscillations.
• Exploring Critical Points: Establishing the critical doping point separating the underdoped and overdoped regimes could clarify electronic phase transitions within cuprates.
• Advanced Theoretical Models: Developing theories that incorporate strong electron interactions and possible broken symmetries might provide a comprehensive model of the underdoped state's electronic properties.

This work pushes the boundaries of our understanding of HTSCs by supplying crucial evidence regarding the FS features in the underdoped state, prompting a re-evaluation of prevailing theories and models.