Raman Scattering Evidence for Fluctuating Kagome-Plane Moments in $\mathrm{B} \parallel [111]$ in Pr$_2$Zr$_2$O$_7$ pyrochlore

Abstract

The exotic magnetic properties of the pyrochlore Pr$_2$Zr$_2$O$_7$ are determined by the coupling of spin, orbital, and lattice degrees of freedom. Magnetism in this material originates from the non-Kramers Pr$^{3+}$ ion, and the exotic behavior has been discussed both in the framework of dipole-quadrupole properties of magnetic moments and disorder splitting the non-Kramers doublet. We use magneto-Raman spectroscopy to probe the crystal electric field (CEF) excitations of Pr$^{3+}$ in magnetic fields up to 14 T applied along the [100] and [111] crystallographic directions at 2 K. For $\mathrm{B}\parallel[100]$, the field evolution of the Raman active crystal field modes is quantitatively described by conventional Zeeman splitting of the Pr$^{3+}$ ground state doublet, including the thermally populated upper Zeeman branch at elevated temperature. For $\mathrm{}{B}\parallel[111]$, the spectra separate into responses from the triangular and kagome sublattices, and the CEF excitations of Pr$^{3+}$ on the triangular lattice exhibit the expected Zeeman shift consistent with full moments polarization. In contrast, the CEF excitations of Pr$^{3+}$ on the kagome lattice do not split as expected and instead broaden strongly with increasing field. This behavior is inconsistent with a static polarized configuration and is captured phenomenologically by a motional-narrowing description in which the kagome plane moments fluctuate between Zeeman-split levels on a meV timescale. This result offers a natural explanation for the reduced magnetization observed in $\mathrm{}{B}\parallel[111]$ and provides further evidence of the dipole-quadrupole properties of magnetic moments. It highlights Raman scattering as a sensitive probe of exotic spin-orbital dynamics in frustrated magnets.