Gapped commensurate antiferromagnetic response in a strongly underdoped model cuprate superconductor

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 ${\mathrm{HgBa}}_2{\mathrm{CuO}}_{4+\hat{I}´}$ 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 $\hat{I}{”}_{\mathrm{AF}}\approx 6$ meV, commensurate over a wide energy range, and disperses above about 55 meV. Given the documented model nature of ${\mathrm{HgBa}}_2{\mathrm{CuO}}_{4+\hat{I}´}$, 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 ${\mathrm{HgBa}}_2{\mathrm{CuO}}_{4+\hat{I}´}$ can therefore be expected to serve as a benchmark for a refined theoretical understanding of the cuprates.

Main text: 18 pages, 6 figures. Supplement: 17 pages, 10 figures