Compensated electron and hole pockets in an underdoped high Tc superconductor

TL;DR

This study maps the Fermi surface topology of underdoped high-Tc YBCO using quantum oscillations, revealing multiple pockets with contrasting properties and suggesting a possible secondary Fermi surface instability related to the metal-insulator transition.

Contribution

The paper provides high-resolution measurements of quantum oscillations in underdoped YBCO, resolving multiple Fermi surface pockets and analyzing their properties in relation to translational symmetry breaking.

Findings

Identified three distinct quantum oscillation frequencies indicating multiple Fermi pockets.

Found a nearly two-dimensional  pocket and a deeply corrugated  pocket with contrasting properties.

Suggested a possible secondary Fermi surface instability related to the metal-insulator quantum critical point.

Abstract

We report quantum oscillations in the underdoped high Tc YBCO over a wide range in magnetic field 28<B<85 T corresponding to ~12 oscillations, enabling the Fermi surface topology to be mapped to high resolution. As earlier reported by Sebastian et al., we find a Fermi surface comprising multiple pockets, as revealed by the additional distinct quantum oscillation frequencies and harmonics reported in this work. We find the originally reported broad low frequency Fourier peak at 535 T to be clearly resolved into three separate peaks at 460 T, 532 T and 602 T. Our increased resolution and angle-resolved measurements identify these frequencies to originate from two similarly sized pockets with greatly contrasting degrees of interlayer corrugation. The spectrally dominant frequency originates from a pocket (\alpha) that is almost ideally two-dimensional in form. In contrast, the newly resolved weaker adjacent spectral features originate from a deeply corrugated pocket (\gamma). On comparison with band structure, the d-wave symmetry of the interlayer dispersion locates the minimally corrugated \alpha pocket at the 'nodal' point where holes are located in a translational symmetry-broken scenario, and the significantly corrugated \gamma pocket at the 'antinodal' point in the Brillouin zone, where electrons are located in a translational symmetry-broken scenario. Translational symmetry breaking by an SDW is suggested from the strong suppression of Zeeman splitting for the spectrally dominant pocket, additional evidence for which is provided from the harmonics we resolve in the present experiments. Given the similarity in \alpha and \gamma pocket sizes, their opposite carrier type and the previous report of a diverging effective mass, we discuss the possibility of a secondary Fermi surface instability at low dopings of the excitonic insulator type, associated with the metal-insulator QCP.