Quantum oscillations and the Fermi surface in an underdoped high-Tc superconductor

TL;DR

This study provides evidence of a well-defined Fermi surface in underdoped high-Tc superconductor YBa2Cu3O6.5 through quantum oscillations, revealing small Fermi pockets and advancing understanding of its electronic structure.

Contribution

It reports the first observation of quantum oscillations in underdoped high-Tc superconductor, demonstrating the existence of small Fermi surface pockets in the ground state.

Findings

Quantum oscillations observed in underdoped YBa2Cu3O6.5.

Fermi surface consists of small pockets, not large cylinders.

Two interpretations: band structure feature or topological change.

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.