Conical spin order with chiral quadrupole helix in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>CsCuCl</mml:mi><mml:mn>3</mml:mn></mml:msub></mml:math>
Hiroki Ueda, Elizabeth Skoropata, Max Burian, Victor Ukleev, Gérard Sylvester Perren, Ludmila Leroy, Julien Zaccaro, U. Staub
Abstract
Here we report a resonant x-ray diffraction (RXD) study at the Cu ${L}_{3}$ edge on the multichiral system ${\mathrm{CsCuCl}}_{3}$, exhibiting helical magnetic order in a chiral crystal structure. RXD is a powerful technique to disentangle electronic degrees of freedom due to its sensitivity to electric monopoles (charge), magnetic dipoles (spin), and electric quadrupoles (orbital). We characterize electric quadrupole moments around Cu ascribed to the unoccupied Cu $3d$ orbital, whose quantization axis is off the basal plane. Detailed investigation of magnetic reflections reveals additional sinusoidal modulations along the principal axis superimposed on the reported helical structure, i.e., a longitudinal conical (helical-butterfly) structure. The out-of-plane modulations imply significant spin-orbit interaction despite $S=1/2$ of ${\mathrm{Cu}}^{2+}$.