Direct time domain sampling of sub-THz coherent acoustic phonon spectra in SrTiO$_3$ using ultrafast X-ray diffraction

TL;DR

This paper demonstrates a method to directly measure sub-THz coherent acoustic phonon spectra in SrTiO$_3$ using ultrafast X-ray diffraction, revealing frequency-dependent phonon lifetimes and nonlinear effects at high amplitudes.

Contribution

It introduces a novel approach for time-domain sampling of sub-THz phonons in SrTiO$_3$ with ultrafast X-ray diffraction, providing new insights into phonon lifetimes and nonlinear effects.

Findings

Phonon lifetime follows 1/ω^2 law at sub-THz frequencies

Spectrum shift attributed to nonlinear effects from high amplitude phonons

Method enables direct measurement of phonon spectra in the time domain

Abstract

We synthesize sub-THz longitudinal quasi-monochromatic acoustic phonons in a SrTiO$_3$ single crystal using a SrRuO$_3$/SrTiO$_3$ superlattice as an optical-acoustic transducer. The generated acoustic phonon spectrum is determined using ultrafast X-ray diffraction. The analysis of the generated phonon spectrum in the time domain reveals a k-vector dependent phonon lifetime. It is observed that even at sub-THz frequencies the phonon lifetime agrees with the 1/$\omega^2$ power law known from Akhiezer's model for hyper sound attenuation. The observed shift of the synthesized spectrum to the higher $q$ is discussed in the framework of non-linear effects appearing due to the high amplitude of the synthesized phonons.