Order Parameter Segregation inCe0.7La0.3B6:4fOctopole and5dDipole Magnetic Order

Abstract

The intriguing thermophysical properties of ${\mathrm{CeB}}_6$ have been subject to investigation for more than 20 years. In particular, an exotic ground state, phase IV, emerges under doping with La. We report resonant x-ray scattering results on the order parameter symmetries in phase IV of ${\mathrm{Ce}}_{0.7}{\mathrm{La}}_{0.3}{\mathrm{B}}_6$, which condenses below $T_{\mathrm{IV}}=1.5\mathrm{K}$. The results reveal a degree of mesoscopic $5d$ dipole antiferromagnetic order, with propagation vector $Q_0=(\frac{1}{2}\frac{1}{2}\frac{1}{2})$, both below and above $T_{\mathrm{IV}}$. Below $T_{\mathrm{IV}}$, this polarization coexists with long-range $4f$ antiferro-octupole (AFO) order also at $Q_0$. The marked differences in temperature dependence and spatial correlation suggest a state of order parameter segregation at low temperature. A simple model of AFO order, consistent with the polarization dependent azimuth symmetries, the Bragg angle, and temperature dependence is given.