Coherent phonons in pyrochlore titanatesA2Ti2O7(A=Dy, Gd, Tb): A phase transition in Dy2Ti2O7at 110 K

Abstract

We study the generation of coherent optical phonons in spin-frustrated pyrochlore single crystals Dy${}_{2}$Ti${}_{2}$O${}_{7}$, Gd${}_{2}$Ti${}_{2}$O${}_{7}$, and Tb${}_{2}$Ti${}_{2}$O${}_{7}$ using femtosecond laser pulses (65 fs, 1.57 eV) in degenerate time-resolved transmission experiments as a function of temperature from 4 to 296 K. At 4 K, two coherent phonons are observed at $~$5.3 THz (5.0 THz) and $~$9.3 THz (9.4 THz) for Dy${}_{2}$Ti${}_{2}$O${}_{7}$ (Gd${}_{2}$Ti${}_{2}$O${}_{7}$), whereas three coherent phonons are generated at $~$5.0, 8.6, and 9.7 THz for Tb${}_{2}$Ti${}_{2}$O${}_{7}$. In the case of spin-ice Dy${}_{2}$Ti${}_{2}$O${}_{7}$, a clear discontinuity is observed in the linewidths of both the coherent phonons as well as in the phase of lower-energy coherent phonon mode, indicating a subtle structural change at 110 K. Another important observation is a phase difference of $\ensuremath{\pi}$ between the modes in all the samples, thus suggesting that the driving forces behind the generation of these modes could be different in nature, unlike a purely impulsive or displacive mechanism.