Protection of spin excitations in Yb quantum magnets

Fractionalized excitations present evidence of entangled states in quantum magnets such as 1D Heisenberg S=1/2 systems (quantum spin chains). 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 by exhibiting quantum behaviors by acting as S_eff=½ systems due to the selection of ground-state Kramers doublets by crystal electric fields and by virtue of strong spin-orbit coupling. Furthermore, the 4f electronic subshells responsible for magnetism in the rare-earths experience protection from coherence-destroying fluctuations by the outer 5s shells and by spin-preserving angular momentum conservation laws.



Here we present neutron scattering measurements on quantum magnets YbAlO3 and Yb3O5O12 to evaluate how the protection of 4f electrons might help preserve information in a spin-system despite its coupling to a heat-bath at finite temperature. At low temperatures, we evaluate quantum entanglement witnesses such as quantum Fisher information to establish lower bounds on multipartite entanglement in the system. We then show that the dispersive quasiparticle band-structure can be preserved to temperatures twenty-times greater than the strength of nearest-neighbor couplings in the system, despite an increased thermal population of phonons and crystal-field modes. Analytical calculations and finite-temperature DMRG confirm that the behavior of the spin-system approaches the infinite-temperature limit in the absence of these couplings, allowing us to extract a time-scale of quasiparticle decoherence due to the heat bath.