Quantum coherence and asymptotic freedom in spin-1/2 chains

Fractionalized excitations encode entangled states in quantum magnets such as XXZ quantum spin chains in 1D spin-1/2 systems. Our earlier work has shown that the effective spin-½ Heisenberg XXX chain material YbAlO3 retains its quantum excitations to temperatures much higher than its exchange couplings, pointing to an absence of collisional damping of fractional spinon quasiparticles. In fact, the Yb-chains appear to obey their low-temperature quantum Hamiltonian up to temperatures comparable to the crystal field splitting which stabilizes the Kramers doublet responsible for effective spin-½physics of Yb3+. Our recent measurements show that this high-temperature coherence is also present in the effective XXZ spin-chain analogue YbFeO3. This stability and relatively low interaction energy scales (0.25~0.5 meV) present a unique opportunity to experimentally study realizations of XXX and XXZ spin-½ chains in such Yb systems at high-temperature and a variety of conditions. Here we report experimentally determined quasiparticle velocities as functions of temperature and magnetic field in the short-time ballistic transport regimes accessible to inelastic neutron spectroscopy. We find that the velocities can be tuned by field and temperature, and asymptotically renormalize to values found for the non-interacting limit even in the interacting XXX model.