Magnetic-field induced quantum phase transitions in spin-1/2 XXZ chain materials

Low-dimensional quantum spin systems offer an ideal playground to study the generic behavior close to magnetic-field induced quantum phase transitions. Of particular interest is the XXZ spin-1/2 chain model with anisotropy parameter Δ=J_z/J_xy of the exchange couplings acting either on the z- or the x(y)-spin components. For Δ=0, 1 and ∞, respectively, the exactly solvable XY, Heisenberg and Ising chain models are covered, but real materials are typically located in between these models and there are additional intra- and/or inter-chain couplings. In this contribution, our recent experimental studies of model materials with field-induced quantum phase transitions will be presented. Cu(C₄H₄N₂)(NO₃)₂, is an almost ideal realization of the Heisenberg chain with weak intra-chain coupling, such that we could study its field induced quantum phase transition in great detail. The experimental data are almost perfectly reproduced by Bethe-Ansatz calculations over a wide temperature and field range and on approaching the quantum critical field the systematic evolution of the generic quantum critical behavior is clearly observed in the experimental data [1]. In BaCo₂V₂O₈, the Co²⁺ ions realize effective spin-1/2 chains with pronounced Ising anisotropy. Here, sizable inter-chain couplings cause long-range, 3D antiferromagnetic order with complex magnetic-field temperature phase diagrams [2]. For a particular transverse-field direction, however, the field-induced suppression of the 3D magnetic order is well separated from the region where we observe the characteristic quantum critical behavior expected for the one-dimensional Ising spin chain in transverse magnetic field [3].
[1] O. Breunig, T.L., et al. Science Advances 3, eaao3773 (2017)
[2] S. Niesen, T.L., et al. Phys. Rev. B 87, 224413 (2013)
[3] Zhe Wang, T.L., et al. Phys. Rev. Lett. 120, 207205 (2018).