Structure of the magnetic excitations in the spin-1/2 triangular-lattice Heisenberg antiferromagnet BaCoSbO

Ito, Saya*; Kurita, Nubuyuki*; Tanaka, Hidekazu*; Kawamura, Seiko   ; Nakajima, Kenji   ; Ito, Shinichi*; Kuwahara, Keitaro*; Kakurai, Kazuhisa*

A spin-1/2 triangular-lattice Heisenberg antiferromagnet (TLHAF) is a prototypical frustrated quantum magnet, which exhibits remarkable quantum many-body effects that arise from the synergy between spin frustration and quantum fluctuation. The ground-state properties of a spin-1/2 TLHAF are theoretically well understood. However, magnetic excitations are less well understood and the theoretical consensus is limited. The experimental study of the magnetic excitations in spin-1/2 TLHAFs has also been limited. Here we show the whole picture of magnetic excitations in the spin-1/2 TLHAF BaCoSbO investigated by inelastic neutron scattering. Significantly different from the theory, the excitation spectra have a three-stage energy structure. The lowest first stage is composed of dispersion branches of single-magnon excitations. The second and third stages are dispersive continua accompanied by columnar continuum extending above 10 meV, which is six times larger than the exchange interaction J=1.67 meV. Our results indicate the necessity of a new theoretical framework.