Sleuthing out exotic quantum spin liquidity in the pyrochlore magnet Ce$_2$Zr$_2$O$_7$

TL;DR

This paper identifies a novel quantum spin liquid phase in the pyrochlore Ce$_2$Zr$_2$O$_7$ by analyzing experimental data and proposing an effective Hamiltonian, revealing a potentially more stable exotic topological state.

Contribution

It introduces a microscopic model for Ce$_2$Zr$_2$O$_7$ that suggests a stable $rac{ ext{pi}}{1}$-flux U(1) quantum spin liquid phase, connecting experiments with detailed theoretical analysis.

Findings

Identification of a $rac{ ext{pi}}{1}$-flux U(1) QSL in Ce$_2$Zr$_2$O$_7$

Effective Hamiltonian consistent with thermodynamic and neutron data

Octupolar moments enhance stability and obscure some neutron signatures

Abstract

The search for quantum spin liquids (QSL) -- 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, in large part because of the difficulty to diagnose experimentally a state with only topological, rather than conventional, forms of order. Pyrochlore systems have proven particularly promising, hosting a classical Coulomb phase in the spin ices Dy/Ho$_2$Ti$_2$O$_7$, with subsequent proposals of candidate QSLs in other pyrochlores. Connecting experiment with detailed theory exhibiting a robust QSL has remained a central challenge. Here, focusing on the strongly spin-orbit coupled effective $S=1/2$ pyrochlore Ce$_2$Zr$_2$O$_7$, we analyse 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. Its parameter values suggest a previously unobserved exotic phase, a $\pi$-flux U(1) QSL. Intriguingly, 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.