Quasiparticle mass enhancement approaching optimal doping in a high-Tc superconductor

TL;DR

This study reveals a significant increase in quasiparticle effective mass near optimal doping in a high-Tc superconductor, indicating a quantum-critical point that correlates with multiple experimental signatures and may be key to understanding high-temperature superconductivity.

Contribution

It provides direct evidence of quantum-criticality in cuprates through mass enhancement and identifies a specific doping level where multiple phenomena converge, advancing the understanding of high-Tc mechanisms.

Findings

Mass enhancement observed near optimal doping

Quantum-critical point identified at p_crit ≈ 0.18

Convergence of pseudogap, Kerr rotation, Hall coefficient, and charge order signals

Abstract

In the quest for superconductors with high transition temperatures (T$_\mathrm{c}$s), one emerging motif is that unconventional superconductivity is enhanced by fluctuations of a broken-symmetry phase near a quantum-critical point. While recent experiments have suggested the existence of the requisite broken symmetry phase in the high-T$_\mathrm{c}$ cuprates, the signature of quantum-critical fluctuations in the electronic structure has thus far remained elusive, leaving their importance for high-T$_\mathrm{c}$ superconductivity in question. We use magnetic fields exceeding 90 tesla to access the underlying metallic state of the cuprate YBa2Cu3O6+$_\delta$ over an unprecedented range of doping, and magnetic quantum oscillations reveal a strong enhancement in the quasiparticle effective mass toward optimal doping. This mass enhancement is a characteristic signature of quantum criticality, and identifies a quantum-critical point at p$_{crit}$ $\approx$ 0.18. This point also represents the juncture of the vanishing pseudogap energy scale and the disappearance of Kerr rotation, the negative Hall coefficient, and the recently observed charge order, suggesting a mechanism of high-T$_\mathrm{c}$ that is strongest when these definitive experimental signatures of the underdoped cuprates converge at a quantum critical point.