Singlet-triplet excitations and long range entanglement in the spin-orbital liquid candidate FeSc2S4
N. J. Laurita, J. Deisenhofer, LiDong Pan, C. M. Morris, M. Schmidt, M. Johnsson, V. Tsurkan, A. Loidl, and N. P. Armitage
Theoretical models of the spin-orbital liquid (SOL) candidate FeSc2S4 had predicted this material to be in close proximity to a quantum critical point separating a spin-orbital liquid phase from a long-range ordered magnetic phase. In our research, we examined the magnetic excitations of FeSc2S4 through time-domain terahertz spectroscopy under an applied magnetic field. At low temperatures an excitation emerged that we attributed to a singlet-triplet excitation from the SOL ground state. A three-fold splitting of this excitation was observed as a function of applied magnetic field. As singlet-triplet excitations are typically not allowed in pure spin systems, our results demonstrated the entangled spin and orbital character of the singlet ground and triplet excited states. Using experimentally obtained parameters we compared to existing theoretical models to determine FeSc2S4’s proximity to the quantum critical point. In the context of these models, we estimated that the characteristic length of the singlet correlations to be 8.2 in units of nearest neighbor lattice constants which established FeSc2S4 as a SOL with long-range entanglement.
An asymmetric splitting of an antiferromagnetic resonance through quartic exchange interactions in multiferroic h-HoMnO3
N. J. Laurita, Yi Luo, Rongwei Hu, Meixia Wu, S. W. Cheong, O. Tchernyshyov, and N. P. Armitage
The symmetric splitting of two spin-wave branches in an antiferromagnetic resonance (AFR) experiment has been an essential measurement of antiferromagnets for over half a century. In this research, we performed time-domain THz spectroscopy experiments with circularly polarized light on the low symmetry multiferroic h-HoMnO3 (HMO) to reveal an AFR of the Mn sublattice that splits asymmetrically in magnetic field, with a 50% difference in g-factors between the high and low energy branches of this excitation. The temperature dependence revealed this asymmetry to be related to the Ho sublattice magnetization. Furthermore, we uncovered a drastic renormalization of the g-factors of the Mn AFR at the Ho spin ordering temperature, confirming strong Ho-Mn coupling. Theoretical calculations demonstrated these results are not explained by conventional exchange mechanisms alone and are only reproduced if quartic spin interactions between Ho-Mn moments are also considered. Our results provided a paradigm for the optical study of such novel spin interactions in an AFR experiment.
Low energy magnon dynamics and magneto-optics of the skyrmionic Mott insulator Cu2OSeO3
N. J. Laurita, G. G. Marcus, B. A. Trump, J. Kindervater, M. B. Stone, T. M. McQueen, C. L. Broholm, and N. P. Armitage
In this work, we presented a comprehensive study of the low energy optical magnetic response of the skyrmionic Mott insulator Cu2OSeO3 via high resolution time-domain THz spectroscopy. In zero field, we found a new magnetic excitation which had not been predicted by spin-wave theory. We showed, with accompanying time-of-flight neutron scattering experiments, that this excitation is a zone folded magnon from the R to Γ points of the Brillouin zone. Highly sensitive polarimetry experiments performed in weak magnetic fields observed Faraday and Kerr rotations proportional to the sample magnetization, allowing for optical detection of the skyrmion phase and construction of a magnetic phase diagram. In large magnetic fields, we observed the magnetically active uniform mode of the ferrimagnetic field polarized phase whose dynamics as a function of field and temperature were studied. We found the uniform mode to decay through a non-Gilbert damping mechanism and to possesses a finite spontaneous decay rate in the zero temperature limit. Our observations were attributed to Dzyaloshinkii-Moriya interactions, which had been proposed to be exceptionally strong in Cu2OSeO3 and are expected to impact the low energy magnetic response of such chiral magnets.