Phonon thermal Hall effect in non-magnetic Y$_2$Ti$_2$O$_7$

We report an investigation of the phonon thermal Hall effect in single crystal samples of Y$_2$Ti$_2$O$_7$, Dy$_2$Ti$_2$O$_7$, and DyYTi$_2$O$_7$. We measured the field-linear thermal Hall effect in all three samples. The temperature dependence of thermal Hall conductivities shows a peak around 15 K, which coincides with the peak positions of the longitudinal thermal conductivities. The temperature-dependent longitudinal thermal conductivities indicates that phonons dominate thermal transport in all three samples. However, the presence of Dy$^{3+}$ magnetic ions introduces significant effects on the field dependence of longitudinal thermal conductivities. The thermal Hall ratio is sizeable in all three samples and consistent with the values reported for other insulating materials exhibiting a phononic thermal Hall effect, though their exact underlying mechanism remains yet to be identified. The thermal Hall ratio is nearly the same for $\rm Y_2Ti_2O_7$ and DyYTi$_2$O$_7$, and slightly larger for Dy$_2$Ti$_2$O$_7$, suggesting that magnetic impurities are less significant in generating the phononic thermal Hall effect. Our observations of the phononic thermal Hall effect support an intrinsic origin in Y$_2$Ti$_2$O$_7$, but suggest a combination of intrinsic and extrinsic effects in Dy$_2$Ti$_2$O$_7$ and DyYTi$_2$O$_7$.
Comment: 5 pages, 3 figures