Fractionalized excitations in quantum magnets such as 1D Heisenberg S=1/2 systems (quantum spin chains) present evidence of quantum entangled states in these systems. Conventionally, systems with larger angular momenta (S≧3/2) are closer to the classical limit and quantum entanglement is less important. Rare earth systems can break this rule to exhibit quantum behaviors by the selection of ground-state Kramers doublets which can act as spins-effective ½. The 4f electrons involved in these doublets are meanwhile protected from coherence-destroying fluctuations by outer 5s shells and by spin-preserving angular momentum conservation laws.
I will present a series of neutron scattering measurements on quantum magnets YbAlO3 and Yb3O5O12 to evaluate how these single-ionic properties affect the degree of information loss in a spin-system. At low temperatures, we evaluate entanglement witnesses such as the quantum Fisher information to establish lower bounds on multipartite entanglement. The results show that the dispersive quasiparticle band-structure is preserved to temperatures 20-times greater than the strength of nearest-neighbor couplings, despite an increased thermal population of phonons and crystal-field modes. A comparison to theory confirms that the behavior of the spin-system is approaching its infinite-temperature limit and allows us to extract a timescale of quasiparticle decoherence due to the heat bath.
I will also discuss our ongoing work to measure the magnetic response of these systems to NIR laser photoexcitation. This is possible using our newly developed fiberoptic setup for low-temperature neutron studies under optical pumping, which allows us to perform time-resolved measurements of millisecond-timescale dynamics.
References:
[1] L. S. Wu, S. E. Nikitin, Z. Wang, W. Zhu, C. D. Batista, A. M. Tsvelik, A. M. Samarakoon, D. A. Tennant, M. Brando, L. Vasylechko, M. Frontzek, A. T. Savici, G. Sala, G. Ehlers, A. D. Christianson, M. D. Lumsden, A. Podlesnyak, Tomonaga–Luttinger liquid behavior and spinon confinement in YbAlO3, Nature Comm., 10, 698 (2019).
[2] A. Scheie, Pontus Laurell, A. M. Samarakoon, B. Lake, S. E. Nagler, G. E. Granroth, S. Okamoto, G. Alvarez, D. A. Tennant. Witnessing entanglement in quantum magnets using neutron scattering. Phys. Rev. B 103, 224434 (2021).
[3] L L. Kish, et al., High-temperature quantum coherence of spinons in a rare-earth spin chain. arXiv:2406.16753 [cond-mat.str-el]