Spectroscopic Demarcation of Emergent Photons and Spinons in a Dipolar-Octupolar Quantum Spin Liquid

TL;DR

This study uses neutron scattering under magnetic fields to distinctly identify emergent photons and spinons in a dipolar-octupolar quantum spin liquid, confirming the $ ext{Ce}_2 ext{Zr}_2 ext{O}_7$ state and demonstrating a new field-tuning method.

Contribution

It introduces a high-field subtraction protocol to spectroscopically separate photon and spinon excitations in dipolar-octupolar QSLs, supported by theoretical calculations.

Findings

Weak magnetic fields suppress photon modes

Spinon continuum remains robust under field

Supports $ ext{Ce}_2 ext{Zr}_2 ext{O}_7$ as a $ ext{pi}$-flux QSI

Abstract

The identification of fractionalized excitations in quantum spin liquids (QSLs) remains a central challenge in condensed matter physics. In dipolar-octupolar (DO) pyrochlores, such as $\text{Ce}_2\text{Zr}_2\text{O}_7$, the candidate $\pi$-flux quantum spin ice (QSI) state is predicted to host both gapless emergent photons and a continuum of spinons. However, resolving these modes at zero field is complicated by their spectral overlap and the presence of nonmagnetic scattering near zero energy. Here, we report neutron scattering experiments on $\text{Ce}_2\text{Zr}_2\text{O}_7$ under a magnetic field along the $[1,1,1]$ direction. In contrast to previous unpolarized studies at zero-field that relied on high-temperature subtraction, we use a same-temperature high-field subtraction protocol to isolate the photon mode. Leveraging the selective coupling of the magnetic field to the dipolar degrees of freedom, we demonstrate the spectroscopic demarcation of these excitations. We observe that weak fields ($\approx 0.15$ T) suppress the low-energy photon weight while leaving the high-energy spinon continuum robust, albeit hardened. Our results, supported by gauge mean-field theory and exact diagonalization calculations, provide strong evidence for the $\pi$-flux QSI state and introduce a powerful field-tuning protocol for investigating DO-QSLs.