Polar magnetism and chemical bond in α-RuCl3


 The micaceous black allotrope of ruthenium trichloride is the subject of many recent experimental and theoretical studies. Johnson et al. infer a candidate for the magnetic structure (Cc2/m) from results gathered in an extensive set of experiments on an untwined sample of α-RuCl3 [Phys. Rev. B 92, 235119 (2015)]. The proposed zigzag antiferromagnetic ground state of Ru ions does not respond to bulk magnetic probes, with optical rotation and all forms of dichroism prohibited by symmetry. Experimental techniques exploited by Johnson et al. included x-ray and magnetic neutron diffraction. Properties of the candidate magnetic structure not previously explored include polar magnetism. In a general case, anapoles are capable of generating interactions between magnetic ions, as in an electrical Dzyaloshinskii-Moryia interaction [T. A. Kaplan and S. D. Mahanti, Phys. Rev. B 83, 174432 (2011) and H. J. Zhao et al., Nat. Mat. 20, 341 (2021)]. From symmetry-informed calculations we show that, the magnetic candidate permits Bragg spots that arise solely from Dirac multipoles. Stringent tests of Cc2/m can also be accomplished by performing resonant x-ray diffraction with signal enhancement from the chlorine K-edge. X-ray absorption spectra published for α-RuCl3 possess a significant low-energy feature [K. W. Plumb et al., Phys. Rev. B 90, 041112(R) (2014)]. Many experimental studies of other Cl-metal compounds concluded that identical features hallmark the chemical bond. Using a monoclinic Cc2/m structure, we predict the contribution to Bragg diffraction at the Cl K-edge absorption. Specifically, the variation of intensity of Bragg spots with rotation of the sample about the reflection vector. The two principal topics of our studies, polar magnetism and the chemical bond in α-RuCl3, are brought together in a minimal model of magnetic Ru ions in Cc2/m.