Dynamics, Entanglement, and the Classical Point in the Transverse-Field XXZ Chain Cs₂CoCl₄

We investigate the quantum phase transition in the S=1/2 transverse-field XXZ spin chain using multiple entanglement measures and witnesses. As the field strength increases, this system passes through two quantum critical points of different universality classes, as well as a classical point. Consequently, the entanglement can be tuned by simply changing the magnetic field. Using DMRG calculations we study the entanglement entropy, one-tangle, two-tangle, and quantum Fisher information. We also report high-resolution inelastic neutron scattering data on Cs₂CoCl₄, and the experimental extraction of entanglement witnesses from the neutron data. We find that quantum Fisher information values extracted from experiment and simulation show good agreement, demonstrating it is an experimentally viable probe of entanglement.