Temperature dependent Raman and x-ray studies of spin-ice pyrochlore $Dy_2Ti_2O_7$ and non-magnetic pyrochlore $Lu_2Ti_2O_7$

摘要

We present here temperature-dependent Raman, x-ray diffraction and specificheat studies between room temperature and 12 K on single crystals of spin-icepyrochlore compound $Dy_2Ti_2O_7$ and its non-magnetic analogue $Lu_2Ti_2O_7$.Raman data show a "new" band not predicted by factor group analysis ofRaman-active modes for the pyrochlore structure in $Dy_2Ti_2O_7$, appearingbelow a temperature of $T_c=$110 K with a concomitant contraction of the cubicunit cell volume as determined from the powder x-ray diffraction analysis. Lowtemperature Raman experiments on O$^{18}$-isotope substituted $Dy_2Ti_2O_7$confirm the phonon origin of the "new" mode. These findings, absent in$Lu_2Ti_2O_7$, suggest that the room temperature cubic lattice of thepyrochlore $Dy_2Ti_2O_7$ undergoes a "subtle" structural transformation near$T_c$. We find anomalous \textit{red-shift} of some of the phonon modes in boththe $Dy_2Ti_2O_7$ and the $Lu_2Ti_2O_7$ as the temperature decreases, which isattributed to strong phonon-phonon anharmonic interactions.