Rice University Isolates Photon, Spinon Modes with Field Subtraction

Researchers at Rice University and the University of Toronto, Oak Ridge National Laboratory, and Rutgers University have successfully isolated and identified both gapless emergent photons and a continuum of spinons, rare quantum excitations, within the material Ce₂Zr₂O₇, a dipolar-octupolar pyrochlore theorized to host this unusual combination. Resolving these excitations at zero field has long been challenging due to spectral overlap and interfering nonmagnetic scattering; however, the team employed a novel “same-temperature high-field subtraction protocol” to cleanly separate the signals. Unlike previous studies relying on high-temperature subtraction, this technique leverages the selective coupling of a magnetic field to the material’s dipolar degrees of freedom. The findings, supported by theoretical calculations, provide strong evidence for a π-flux quantum spin ice state and a new method for investigating these complex quantum systems, as Bin Gao, a researcher at Rice University, and colleagues report their findings in Phys. 136.

Dipolar-Octupolar Pyrochlores and Emergent Excitations

Researchers have long sought to definitively identify these excitations at zero magnetic field, but spectral overlap and nonmagnetic scattering near zero energy have presented significant challenges to observation. A team led by Pengcheng Dai at Rice University, Yong Baek Kim at the University of Toronto, Andrey Podlesnyak at Oak Ridge National Laboratory, and Sang-Wook Cheong at Rutgers University has now employed a novel technique to overcome these hurdles, offering compelling evidence for the existence of both photon and spinon modes in Ce₂Zr₂O₇. The team reports observing that weak magnetic fields, approximately 0.15 T, effectively suppress the low-energy photon contribution while leaving the higher-energy spinon continuum largely unaffected, though with a slight increase in energy. These findings, bolstered by gauge mean-field theory and exact diagonalization calculations, strongly support the hypothesis of a π-flux QSI state within Ce₂Zr₂O₇.

Previous investigations of this dipolar-octupolar pyrochlore relied on high-temperature subtraction methods to disentangle these signals, a technique prone to inaccuracies and limitations in resolving the subtle energy differences between the excitations.