Inelastic neutron scattering on Ce pyrochlores: Signatures of electric monopoles

TL;DR

This paper analyzes inelastic neutron scattering data on Ce pyrochlores, identifying signatures of electric monopoles and their overlap with gauge photons, providing new insights into the low-energy excitations of dipolar U(1) spin liquids.

Contribution

It offers a detailed interpretation of neutron scattering spectra, highlighting the role of electric monopoles and their spectral features in dipolar U(1) spin liquids, and predicts experimental signatures.

Findings

Electric monopole continuum overlaps with gauge photons at low energies.

Background dual π flux enhances the density of states at lower energies.

Electric monopoles likely dominate the low-energy spectrum in experiments.

Abstract

We revisit the pyrochlore spin liquid materials Ce$_2$Sn$_2$O$_7$ and Ce$_2$Zr$_2$O$_7$ by examining the existing experiments. We continue to rely on the special properties of the dipole-octupole nature of the Ce$^{3+}$ moment. The inelastic neutron scattering (INS) measurement in the octupolar U(1) spin liquid selects the (gapped) spinon continuum, and thus has suppressed spectral weights below the energy threshold of two spinon gaps. This measurement, however, includes all other emergent excitations at lower energies in the dipolar U(1) spin liquid, in particular, the gapless photon and the continuum of the electric monopoles. Although the electric monopole continuum is weakly gapped (compared to the larger spinon gap), the energy scale is actually close to the gauge photons, and the spectrum largely overlaps with the photons. Due to the background dual $\pi$ flux for the electric monopoles, the density of states is enhanced at lower energies, creating peak structures. This can be contrasted with the linearly suppressed spectral weight of the gauge photons at low energies. We propose that the electric monopole continuum should be mostly responsible for the low-energy spectrum in the INS measurement in the dipolar U(1) spin liquid. With these understanding and calculation, we discuss the available experimental results and predict further experiments for Ce$_2$Sn$_2$O$_7$ and Ce$_2$Zr$_2$O$_7$.