Sleuthing out exotic quantum spin liquidity in the spin-orbit coupled pyrochlore magnet Ce₂Zr₂O₇

The search for quantum spin liquids (QSLs) -- topological magnets with fractionalized excitations -- has been a central theme in condensed matter and materials physics. While theories are plentiful, tracking them down in materials has turned out to be tricky because of the difficulty to diagnose experimentally a state with only topological, rather than conventional, forms of order. Materials with pyrochlore geometries have proven particularly promising, hosting a classical Coulomb phase in the spin ices Dy/Ho₂Ti₂O₇, with subsequent proposals of candidate QSLs in other pyrochlores (e.g. NaCaNi₂F₇). On this front, connecting experiments with detailed theory to demonstrate the existence of a robust QSL has remained a central challenge. I will present work motivated by recent thermodynamic and neutron scattering experiments on the pseudo spin-1/2 pyrochlore Ce₂Zr₂O₇ where we identified its microscopic effective Hamiltonian through a combination of finite temperature Lanczos, Monte Carlo and spin dynamics calculations [1]. The magnetic properties of Ce₂Zr₂O₇ emerge from interactions between cerium (Ce³⁺) ions, whose ground state doublet (with J = 5/2,m_J = ±3/2), with a dipole-octupole character, arises from strong spin orbit coupling and crystal field effects. The Hamiltonian parameter values suggest a previously theorized but unobserved exotic phase, a π-flux U(1) QSL, and it allows us to predict its response to an applied magnetic field. The continuum seen in the dynamical structure factor, consistent with the existence of a gapless QSL, is largely suppressed on the introduction of a magnetic field, giving way to Bragg peaks. However, the absence of any dispersive modes is strongly reflective of the octupolar nature of the low energy modes, a finding that can be directly tested in neutron experiments. We suggest that the octupolar nature of the moments makes them less prone to be affected by crystal imperfections or magnetic impurities, while also hiding some otherwise characteristic signatures from neutrons, making this QSL arguably more stable than its more conventional counterparts.

[1] A. Bhardwaj, S. Zhang, H. Yan, R. Moessner, A. Nevidomskyy, H.J. Changlani, npj Quantum Materials, 7, 1, 51 (2022)