Phonons and quasi-particle excitations in frustrated triangular lattice antiferromagnet Ca₃NiNb₂O₉: A proximate spin liquid

The frustrated model is a strong candidate for the highly entangled quantum spin-liquid (QSL) phase, hosting non-Abelian quasiparticle excitations [1-2] and the exotic quasiparticle (QP) excitations known as spinons [3]. The spin-1 triangular lattice antiferromagnet Ca₃NiNb₂O₉ and its sister compounds are proposed to support many-body quantum entangled states. Using Raman spectroscopy on a Ca₃NiNb₂O₉ single crystal, we find evidence of fractionalized QP excitations, indicating proximity to the QSL phase [4]. This aligns with the system's proposed higher-order fractional magnetization plateau, as these plateaus have an intricate relationship with the spin entanglement. We also observe an unconventional broad continuum with intensity following fermionic statistics. We observed strong phonon self-energy anomalies at ~ 50 K (T_SC - short-range spin-spin correlation transition temperature) well above the long-range magnetic ordering temperature (T_N ~ 4 K), which suggest, these anomalies do not arise from the conventional spin-phonon coupling as the spins are deep inside the thermal paramagnetic regime with i>=0. We attribute these anomalies in the phonons to the quantum spin and orbital fluctuations and possible coupling with the Majorana Fermions. We also observed changes in the phonons self-energy parameters in the vicinity of ~ 200 K (T*). As the temperature reaches T_SC, there is a significant redistribution of spectral weight at the demarcating Raman shift ω_F = 203 cm^-1 also reflected in the anomalous phonon-mode intensity evolution and show fermionic statistics in their intensity evolution. Additionally, phonon modes show Fano asymmetry, also conjectured as a fingerprint of the spin-liquid phase.

References:

[1] J. Wen, et al., Npj Quantum Materials 4,12 (2019).

[2] C. Nayak, et al., Rev. Mod. Phys. 80, 1083 (2008).

[3] X. G. Wen, Phys. Rev. B 65, 165113 (2002).

[4] S. Deswal et al., Phys. Rev. B 110, 024430 (2024).