Gapped commensurate antiferromagnetic response in a strongly underdoped model cuprate superconductor

TL;DR

This study reveals that in a highly pure, underdoped cuprate superconductor, the antiferromagnetic response is gapped, commensurate, and lacks incommensurate or stripe order, providing a benchmark for theoretical models.

Contribution

It demonstrates a distinct magnetic response in HgBa2CuO4+δ, contrasting with other underdoped cuprates, highlighting the intrinsic behavior of CuO2 planes near the Mott-insulating state.

Findings

Antiferromagnetic response is gapped below ~6 meV.

Response is commensurate over a wide energy range.

No evidence of incommensurate or stripe magnetic order.

Abstract

It is a distinct possibility that spin fluctuations are the pairing interactions in a wide range of unconventional superconductors. In the case of the high-transition-temperature (high-$T_c$) cuprates, in which superconductivity emerges upon doping an antiferromagnetic Mott-insulating state, spin correlations might furthermore drive unusual pseudogap phenomena. Here we use polarized and unpolarized magnetic neutron scattering to study the simple tetragonal cuprate $\mathrm{HgBa}_{2}\mathrm{CuO}_{4+\delta}$ at very low doping ($T_c \approx 55$ K, hole concentration $p \approx 0.064$). In stark contrast to prior results for other underdoped cuprates, we find no evidence of incommensurate spin-density-wave, charge-spin stripe, or $q = 0$ magnetic order. Instead, the antiferromagnetic response in both the superconducting and pseudogap states is gapped below $\Delta_\mathrm{AF} \approx 6$ meV, commensurate over a wide energy range, and disperses above about 55 meV. Given the documented model nature of $\mathrm{HgBa}_{2}\mathrm{CuO}_{4+\delta}$, which exhibits high structural symmetry and minimal point disorder effects, we conclude that the observed behavior signifies the unmasked response of the quintessential $\mathrm{CuO}_{2}$ planes near the Mott-insulating state. These results for $\mathrm{HgBa}_{2}\mathrm{CuO}_{4+\delta}$ can therefore be expected to serve as a benchmark for a refined theoretical understanding of the cuprates.