Antiferromagnetic excitations in very underdoped HgBa₂CuO_4+δ measured by neutron scattering

It is a distinct possibility that spin fluctuations are the pairing interactions in a wide range of unconventional superconductors [1]. In the case of the cuprates, in which superconductivity emerges upon doping an antiferromagnetic Mott-insulating state, spin correlations might furthermore drive unusual pseudogap phenomena. HgBa₂Cu­O_4+δ (Hg1201) is a model experimental system, as it exhibits a particularly simple crystal structure and the highest superconducting transition temperature (nearly 100 K) among all single-layer cuprates. However, sizable single crystals with low hole concentrations have been unavailable. We have succeeded in preparing a large Hg1201 sample (mass ~ 1 g) with T_c ≈ 30 K and a hole concentration of just above 5%. Here we report both time-of-flight and triple-axis inelastic neutron scattering measurements of the antiferromagnetic response close to the lower edge of the superconducting dome. In contrast to results for other cuprates for this part of the phase diagram, but consistent with prior results for Hg1201 at higher doping [2,3], the data reveal gapped excitations and no evidence for stripe or spin-density-wave correlations.

[1] D. J. Scalapino, Rev. Mod. Phys. 84, 1383 (2012)

[2] M. K. Chan et al., Phys. Rev. Lett. 117, 1 (2016)

[3] M. K. Chan et al., Nat. Commun. 7, 1 (2016)