Sleuthing out Quantum Spin Liquidity in Cerium Based Pyrochlore Magnets

The search for quantum spin liquids – topological magnets with fractionalized excitations – has been a central theme in condensed matter and materials physics. While theories are no longer in short supply, tracking down materials has turned out to be remarkably tricky. Pyrochlore systems have proven particularly promising, hosting a classical Coulomb phase in the spin ices, with subsequent proposals of candidate quantum spin liquids in other pyrochlores. Here, we focus on the strongly spin-orbit coupled effective spin-1/2 pyrochlores cerium zirconate (Ce₂Zr₂O₇) and cerium stannate (Ce₂Sn₂O₇), analyzing recent thermodynamic and neutron scattering experiments, to identify a microscopic effective Hamiltonian through a combination of finite temperature Lanczos, Monte Carlo and analytical spin dynamics calculations. The parameter values we deduced for Ce₂Zr₂O₇ suggest a previously unobserved exotic phase, a pi-flux U(1) quantum spin liquid. Intriguingly, the octupolar nature of the magnetic moments makes them less prone to be affected by crystal imperfections or magnetic impurities, while also hiding some otherwise characteristic signatures from neutrons. The sister compound, Ce₂Sn₂O₇ appears to also lie in the quantum spin liquid phase, with a more pronounced octupolar signature of Ce moments. Recent neutron backscattering measurements on this material, with extremely high energy resolution, offer an unprecedented glimpse into the spectrum of the fractionalized excitations, spinons. The theoretical analysis corroborates the evidence of strong interaction between these particles and the emergent “photons” in the theory that mimics the quantum electrodynamics.