Magnons and Spinons in ${\mathrm{Ba}}_2{\mathrm{CoTeO}}_6$: A Composite System of Isolated Spin-$1/2$ Triangular Heisenberg-like and Frustrated Honeycomb Ising-like Antiferromagnets

Abstract

We report the neutron scattering results on magnetic orderings and excitations in ${\mathrm{Ba}}_2{\mathrm{CoTeO}}_6$ composed of two almost isolated subsystems A and B, which are described as an $S=1/2$ triangular Heisenberg-like antiferromagnet and a frustrated honeycomb Ising-like antiferromagnet, respectively. Stripy ordering of subsystem B was confirmed below $T_{N1}=12.0$ K, whereas sharp streaks were observed along $(1/3,1/3,L)$ and $(2/3,2/3,L)$ at 0.3 K ($\ll T_{N2}=3.0$ K). This indicates the two-dimensional nature of ordering in subsystem A. It was found that the excitation spectra of both subsystems are well separated and independent of each other. The excitation spectrum of subsystem A is composed of two single-magnon branches with roton-like minima at the M point and a clearly structured intense continuum, as similarly observed in ${\mathrm{Ba}}_3{\mathrm{CoSb}}_2{\mathrm{O}}_9$, which is strongly indicative of spinon excitations. Dispersion curves for subsystem B can be described by linear spin wave theory within the third-neighbor exchange interaction.

6 pages, 4 figures

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