Quasiparticle mass enhancement approaching optimal doping in a high- <i>T</i> _c superconductor

TLDR

High‑Tc superconductivity may be enhanced by fluctuations of a broken‑symmetry phase near a quantum‑critical point, yet signatures of such quantum‑critical fluctuations in the electronic structure of cuprates have remained elusive. By applying magnetic fields above 90 T to YBa₂Cu₃O₆+δ across a wide doping range, the authors used magnetic quantum‑oscillation measurements to access the underlying metallic state and observed a pronounced increase in quasiparticle effective mass toward optimal doping. The observed mass enhancement signals a quantum‑critical point at p≈0.18, coinciding with the disappearance of the pseudogap, Kerr rotation, negative Hall coefficient, and charge order, indicating that the high‑Tc mechanism is strongest when these signatures converge.

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.