Spinon freedom in quantum square ice

Abstract

Recent theoretical and experimental efforts have been focused on the identification of excitations in quantum spin ice. Due to their relation to the magnetic monopoles of classical spin ice, their quantum counterparts, called spinons, are a highly sought-after manifestation of fractionalization in frustrated quantum magnets like Yb2Ti2O7. Of particular current interest is the quantum dynamics of spinons, namely, their modes of propagation and interaction with the strongly correlated spin background. To investigate this dynamics, we study excited quantum square ice, as captured by the spin-1/2 checkerboard-lattice XXZ model. We formulate effective free-spinon theories in the strong Ising coupling limit, with spinons either deconfined or artificially confined to nearest-neighbor distance, and calculate the corresponding approximate dynamic spin-structure factors (DSFs). We then evaluate the DSF of the fully interacting model exactly for clusters of up to 72 sites. The resulting spectra allow us to identify dispersive fingerprints of coherent spinon propagation in the correlated ``vacuum'' of quantum square ice within an extended low-energy regime. We thus provide unbiased evidence for the formation of coherent quasiparticles in quantum spin ice above the Ising gap.