Raman spectroscopy of bimagnon and trimagnon excitations and rotonlike points in a distorted triangular lattice antiferromagnet

We investigate the experimental signatures of Raman spectroscopy of bimagnon and trimagnon excitations in the distorted triangular lattice antiferromagnets α-LCr2O4 (L=Sr, Ca). Motivated by Raman scattering experiments, we utilize spin wave theory to analyze the nearly 120° spin-3/2 spiral-ordered antiferromagnetic ground state to compute the single-magnon density of states, single-magnon dispersion, and bimagnon and trimagnon Raman spectra (polarized and unpolarized). We perform calculations of the Heisenberg antiferromagnetic Hamiltonian that incorporates magnetic interactions (exchange, anisotropy, and interlayer coupling) and lattice distortion within a four-sublattice unit cell. We investigate the Hamiltonian for both model parameter sets and experimentally proposed magnetic interactions for α-LCr2O4 (L=Sr, Ca). It is found that Raman scattering is capable of capturing the effect of the rotonlike M and M′ points on the bimagnon Raman spectrum. Our calculation confirms the connection between single-magnon rotonlike excitation energy and the bimagnon Raman excitation spectrum observed experimentally. The roton energy minimum in momentum space is half of the energy of a bimagnon excitation signal. The experimental magnetic Raman scattering result displays two peaks which have Raman shifts of 15 and 40 meV. Theoretical modeling and analysis of the experimental spectrum of α-SrCr2O4 within our distorted Heisenberg Hamiltonian lattice suggest that the low-energy peak at 15 meV is associated with the bimagnon excitation, whereas the high-energy peak around 40 meV is primarily a trimagnon excitation. Based on our fitting procedure we propose a set of magnetic interaction parameters for α-SrCr2O4. These parameters reproduce not only the experimental Raman spectrum but also the inelastic neutron scattering response (including capturing high-energy magnon branches). We also compute the unpolarized bimagnon and trimagnon Raman spectra for α-CaCr2O4. In contrast to its Sr cousin the Ca-based material has an enhanced bimagnon response, with the high-energy peak still dominated by the trimagnon excitation. Furthermore, the polarization sensitivity of the Raman spectrum can be utilized to distinguish the bimagnon and trimagnon excitation channels.