Magnons and Spinons in ${\mathrm{B}\mathrm{a}}_2{\mathrm{C}\mathrm{o}\mathrm{T}\mathrm{e}\mathrm{O}}_6$: A Composite System of Isolated Spin-$1\mathrm{/}2$ Triangular Heisenberg-like and Frustrated Honeycomb Ising-like Antiferromagnets

We report the neutron scattering results on magnetic orderings and excitations in ${\mathrm{B}\mathrm{a}}_2{\mathrm{C}\mathrm{o}\mathrm{T}\mathrm{e}\mathrm{O}}_6$ composed of two almost isolated subsystems A and B, which are described as an $S=1\mathrm{/}2$ triangular Heisenberg-like antiferromagnet and a frustrated honeycomb Ising-like antiferromagnet, respectively. Stripy ordering of subsystem B was confirmed below $T_{\mathrm{N}1}=12.0$ K, whereas sharp streaks were observed along $(1\mathrm{/}3,1\mathrm{/}3,L)$ and $(2\mathrm{/}3,2\mathrm{/}3,L)$ at 0.3 K ($\ll T_{\mathrm{N}2}=3.0$ 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 ${\mathrm{B}\mathrm{a}}_3{\mathrm{C}\mathrm{o}\mathrm{S}\mathrm{b}}_2{\mathrm{O}}_9$, 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.