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Resonant x-ray diffraction from chiral electric-polarization structures

DOI: 10.1103/PhysRevB.98.155410 DOI Help

Authors: Stephen Lovesey (Diamond Light Source) , Gerrit Van Der Laan (Diamond Light Source)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Physical Review B , VOL 98 , PAGES 155410

State: Published (Approved)
Published: October 2018

Abstract: Heterostructures of PbTiO3/SrTiO3 superlattices have shown the formation of “polar vortices,” in which a continuous rotation of ferroelectric polarization spontaneously forms. Recently, Shafer et al. [Proc. Natl. Acad. Sci. USA (PNAS) 115, 915 (2018)] reported strong nonmagnetic circular dichroism (CD) in resonant soft x-ray diffraction at the Ti L3 edge from such superlattices. The authors ascribe the CD to the chiral rotation of a polar vector. However, a polar vector is invisible to the parity-even electric-dipole transition which governs absorption in the soft x-ray region. A realistic, nonmagnetic explanation of the observed effect is found in Templeton-Templeton scattering. Following this route, the origin of the CD in Bragg diffraction is shown by us to be the chiral array of charge quadrupole moments that forms in these heterostructures. While there is no charge quadrupole moment in the spherically symmetric 3d0 valence state of Ti4+, the excited state 2p53d1(t2g) at the Ti L3 resonance is known to have a quadrupole moment. Our expressions for intensities of satellite Bragg spots in resonance-enhanced diffraction of circularly polarized x rays, including their harmonic content, account for all observations reported by Shafer et al. We predict both intensities of Bragg spots for the second harmonic of a chiral superlattice and circular polarization created from unpolarized x rays, in order that our successful explanation of existing diffraction data can be further scrutinized through renewed experimental investigations. The increased understanding of chiral dipole arrangements could open the door to switchable optical polarization.

Journal Keywords: chirality, handedness, dichroism, electric polarization, multilayers, diffraction, x-rays, quadrupole moment

Subject Areas: Physics, Materials, Technique Development


Technical Areas: Theoretical Physics