Chirality and magnetic quadrupole order in Pb(TiO)Cu4(PO4)4 probed by interference scattering in resonant x-ray diffraction

Circular dichroism observed by resonant x-ray diffraction at the $\mathrm{Cu}\phantom{\rule{0.16em}{0ex}}{L}_{3}$ edge was investigated in a chiral antiferromagnet, $\mathrm{Pb}(\mathrm{TiO}){\mathrm{Cu}}_{4}{({\mathrm{PO}}_{4})}_{4}$, which shows magnetic quadrupole order $({T}_{\mathrm{N}}=7\phantom{\rule{0.16em}{0ex}}\mathrm{K})$. At temperatures above ${T}_{\mathrm{N}}$, space-group-forbidden reflection 100 is observed due to the anisotropic tensor of susceptibility (ATS) scattering. With decreasing temperature, the reflection intensity shows substantial circular dichroism below ${T}_{\mathrm{N}}$. We found one-to-one correspondence between the sign of the circular dichroism and that of the sample's crystallographic chirality. In addition, the reflection intensity depends on the product of poling magnetic and electric fields. The circular dichroism observed in this study is interpreted as the interference between the ATS and the magnetic scatterings. This finding shows that the interference scattering probes both the chirality and the order parameter of the magnetic quadrupole order in this chiral antiferromagnet.