Fig. 2

Phonon dispersions around two Bragg peaks (0, 0, −15) and (1, 0, −5) measured at T = 17 K. a, b phonon dispersing from (0, 0, −15) along the [100] and [001] directions, respectively. c, d, phonon dispersing from (1, 0, −5) along the [100] and [001] directions, respectively. The data plotted in a, b were collected on a time-of-flight (TOF) spectrometer AMATERAS, and those in c, d were collected on a triple-axis spectrometer (TAS) PANDA. For the TOF data plotted in a, b, which are the phonon dispersions along one direction, one needs to sum over intensities for a certain thickness along the other two directions to improve the statistics. The data plotted against the H direction in a were obtained by integrating the intensities in (q, 0 + K, −15 + L) with a thickness of K and L ranging from −0.1 to 0.1 rlu, and −16 to −14 rlu, respectively. Those in b were integrated with H = K = [−0.05, 0.05] rlu. E and q represent the phonon energy and momentum, respectively. TA, LA, and TO represent transverse acoustic, longitudinal acoustic and transverse optic modes, respectively. Solid lines are the dispersions obtained by fitting the energy scans at a series of qs as shown in Fig. 3. The white streaks along the diagonal direction in b were due to the lack of detector coverage wherein