Coherent phonons in pyrochlore titanates A₂Ti₂O₇ (A= Dy, Gd, Tb): a phase transition in Dy₂Ti₂O₇ at 110 K

Abstract

We study the generation of coherent optical phonons in spin-frustrated pyrochlore single crystals Dy₂Ti₂O₇, Gd₂Ti₂O₇, and Tb₂Ti₂O₇ 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 Dy₂Ti₂O₇ (Gd₂Ti₂O₇), whereas three coherent phonons are generated at ~5.0, 8.6, and 9.7 THz for Tb₂Ti₂O₇. In the case of spin-ice Dy₂Ti₂O₇, 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 π 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.