Magnetic Excitations in the Ground State of $\mathrm{Yb_2Ti_2O_7}$

TL;DR

This study investigates the magnetic ground state of Yb2Ti2O7, revealing a phase transition at 280 mK, quasi-collinear ferromagnetic order, and complex magnetic excitations including a gapped mode and a broad continuum, challenging existing theoretical models.

Contribution

The paper provides detailed experimental insights into the magnetic excitations and ground state of Yb2Ti2O7, highlighting discrepancies with linear spin-wave theory predictions.

Findings

Phase transition at 280 mK with ferromagnetic order

Presence of a broad inelastic continuum up to 2.5 K

Exchange parameters from linear spin-wave theory do not match data

Abstract

We report an extensive study on the zero field ground state of a powder sample of the pyrochlore $\mathrm{Yb_2Ti_2O_7}$. A sharp heat capacity anomaly that labels a low temperature phase transition in this material is observed at 280 mK. Neutron diffraction shows that a \emph{quasi-collinear} ferromagnetic order develops below $T_\mathrm{c}$ with a magnetic moment of $0.87(2)\mu_\mathrm{B}$. High resolution inelastic neutron scattering measurements show, below the phase transition temperature, sharp gapped low-lying magnetic excitations coexisting with a remnant quasielastic contribution likely associated with persistent spin fluctuations. Moreover, a broad inelastic continuum of excitations at $\sim0.6$ meV is observed from the lowest measured temperature up to at least 2.5 K. At 10 K, the continuum has vanished and a broad quasielastic conventional paramagnetic scattering takes place at the observed energy range. Finally, we show that the exchange parameters obtained within the framework of linear spin-wave theory do not accurately describe the observed zero field inelastic neutron scattering data.