Detection of multipolar orders in the spin-orbit-coupled 5d Mott insulator Ba_{2}MgReO_{6}

Abstract

Synchrotron x-ray-diffraction measurements on single crystals of the double perovskite Ba_{2}MgReO_{6} with 5d^{1} electronic configurations reveal the successive phase transitions from a disordered state to a quadrupolar order at T_{q}=33K, and then, to a dipolar ordered state at T_{m}=18K. The magnetic reflections observed below T_{m} are indexed to a propagation vector with k=[001], indicating a canted antiferromagnetic order. The very small deformation of ReO_{6} octahedra observed below T_{q} demonstrates that the quadrupolar order is composed of antiferroically arranged Q_{x^{2}–y^{2}} and ferroically arranged Q_{3z^{2}–r^{2}} moments. These observations are consistent with the mean-field theory for spin-orbit-entangled electrons. Our findings are a significant step towards understanding the intriguing physics of multipoles realized by spin-orbit-entangled 5d electrons.