Anomalous continuum scattering and higher-order van Hove singularity in the strongly anisotropic S = 1/2 triangular lattice antiferromagnet

Abstract

The S = 1/2 triangular lattice antiferromagnet (TLAF) is a paradigmatic exampleof frustrated quantum magnetism. An ongoing challenge involves under-standing the influence of exchange anisotropy on the collective behaviorwithin such systems. Using inelastic neutron scattering (INS) and advancedcalculation techniques, we have studied the low and high-temperature spindynamics of Ba2La 2CoTe2O 12 (BLCTO): a Co2+-based Jeff = 1/2 TLAF that exhibits120° order below T_ N = 3.26 K. We determined the spin Hamiltonian by fittingthe energy-resolved paramagnetic excitations measured at T > T_N, revealingexceptionally strong easy-plane XXZ anisotropy. Below T_N , the excitationspectrum exhibits a high energy continuum having a larger spectral weightthan the single-magnon modes, suggesting a scenario characterized by a spi-non confinement length that markedly exceeds the lattice spacing. We con-jecture that this phenomenon arises from the proximity to a quantum meltingpoint, even under strong easy-plane XXZ anisotropy. Finally, we highlightcharacteristic flat features in the excitation spectrum, which are connected tohigher-order van Hove singularities in the magnon dispersion directly inducedby easy-plane XXZ anisotropy. Our results provide a rare experimental insightinto the nature of highly anisotropic S = 1/2 TLAFs between the Heisenberg andXY limits.