Ubiquitous Magnetic Excitations in the Ytterbium Pyrochlores

The ytterbium pyrochlores, Yb$_{\mathrm{2}}$B$_{\mathrm{2}}$O$_{\mathrm{7}}$ (B $=$ Sn, Ti, Ge) are well described in terms of S$_{\mathrm{eff}}=$1/2 quantum spins with local XY anisotropy, decorating the cubic pyrochlore lattice and interacting via anisotropic exchange. While structurally only the non-magnetic B-site cation, and hence, primarily the lattice parameter, is changing across the series Yb$_{\mathrm{2}}$B$_{\mathrm{2}}$O$_{\mathrm{7}}$ (B $=$ Sn, Ti, Ge), a range of magnetic behavior is observed. The low temperature magnetism in Yb$_{\mathrm{2}}$Ti$_{\mathrm{2}}$O$_{\mathrm{7}}$ and Yb$_{\mathrm{2}}$Sn$_{\mathrm{2}}$O$_{\mathrm{7}}$ has ferromagnetic character. Conversely, Yb$_{\mathrm{2}}$Ge$_{\mathrm{2}}$O$_{\mathrm{7}}$ displays an antiferromagnetically ordered Neel state at low temperatures. We present a comparative analysis of the spin dynamic properties of these three systems using inelastic neutron scattering. While the static properties of the ytterbium pyrochlores are distinct from one another, we find a ubiquitous character to the spin dynamics. The inelastic scattering for each of these ytterbium pyrochlores show a gapless continuum of spin excitations, that tends to resemble over-damped ferromagnetic spin waves at low Q. Furthermore, the specific heat for each of these materials follows a common form with a broad, high-temperature anomaly followed by a sharp low-temperature anomaly. We find that the dynamic properties correlate strongly with the broad specific heat anomaly but remain unchanged across the sharp, low temperature specific heat anomaly.