Quantum fluctuations in the highly anisotropic S=1/2 triangular lattice antiferromagnet

The S = 1/2 Heisenberg triangular lattice antiferromagnet (TLAF) stands out as the paradigmatic example of frustrated quantum magnets where quantum fluctuations can play a dominant role. An ongoing challenge lies in understanding the influence of exchange anisotropy on the behavior of its collective modes. We present our inelastic neutron scattering (INS) study of Ba₂La₂CoTe₂O₁₂ (BLCTO), a Co²⁺-based J_eff=1/2 TLAF that exhibits 120^o order below T_N = 3.26 K. Our measurements of powder BLCTO revealed the full structure of the magnetic excitations both below and above T_N. The spin Hamiltonian was determined by a new state-of-the-art approach – fitting the energy-resolved paramagnetic excitations measured at T>T_N with classical simulations based on the Landau-Lifshitz equation. This procedure revealed an exceptionally strong easy-plane XXZ anisotropy (Δ ~0.54). The excitation spectrum below T_N exhibits a highly structured continuum having a larger spectral weight than that of single-magnon bands, demonstrating significant quantum fluctuations in BLCTO despite its strong easy-plane XXZ anisotropy. We further present a comparative analysis between the spectrum below T_N and advanced theoretical calculations based on magnons/spinons, shedding light on the nature of a S=1/2 TLAF in the intermediate regime between the Heisenberg (Δ = 1) and XY (Δ = 0) limits.