Spin fluctuations associated with the collapse of the pseudogap in a cuprate superconductor

TL;DR

This study reveals that low-energy spin fluctuations intensify near the pseudogap collapse in cuprate superconductors, suggesting a link between magnetic excitations and the strange metal behavior in the normal state.

Contribution

The paper provides direct neutron scattering evidence of spin fluctuations associated with the pseudogap collapse, connecting magnetic excitations to thermodynamic anomalies in cuprates.

Findings

Low-energy spin excitations peak near the pseudogap critical doping.

Magnetic response is enhanced under an applied magnetic field.

Spin fluctuations are present throughout the superconducting phase diagram.

Abstract

Theories of the origin of superconductivity in cuprates are dependent on an understanding of their normal state which exhibits various competing orders. Transport and thermodynamic measurements on La$_{2-x}$Sr$_x$CuO$_4$ show signatures of a quantum critical point, including a peak in the electronic specific heat $C$ versus doping $p$, near the doping $p^{\star}$ where the pseudogap collapses. The fundamental nature of the fluctuations associated with this peak is unclear. Here we use inelastic neutron scattering to show that close to $T_c$ and near $p^{\star}$, there are very-low-energy collective spin excitations with characteristic energies $\hbar \Gamma \approx$~5 meV. Cooling and applying a 8.8~T magnetic field creates a mixed state with a stronger magnetic response below 10~meV. We conclude that the low-energy spin-fluctuations are due to the collapse of the pseudogap combined with an underlying tendency to magnetic order. We show that the large specific heat near $p^{\star}$ can be understood in terms of collective spin fluctuations. The spin fluctuations we measure exist across the superconducting phase diagram and may be related to the strange metal behaviour observed in overdoped cuprates.