Light-induced translation symmetry breaking via nonlinear phononics

Abstract

Light has a wavelength that is usually longer than the size of the unit cell of crystals. Hence, even intense light pulses are not expected to break the translation symmetry of materials. However, certain materials, including KTaO$_3$, exhibit peaks in their Raman spectra corresponding to their Brillouin zone boundary phonons due to second-order Raman processes, which provide a mechanism to drive these phonons using intense midinfrared lasers. We investigated the possibility of breaking the translation symmetry of KTaO$_3$ by driving its highest-frequency transverse optic mode $Q_{\textrm{HX}}$ at the $X$ $(0,\frac{1}{2},0)$ point. Our first principles calculations show that the energy curve of the transverse acoustic mode $Q_{\textrm{LZ}}$ at $X$ softens and develops a double-well shape as the value of the $Q_{\textrm{HX}}$ coordinate is increased, while that of the other transverse acoustic component $Q_{\textrm{LX}}$ hardens when the value of the $Q_{\textrm{HX}}$ coordinate is similarly varied. We performed similar total energy calculations as a function of the $Q_{\textrm{HX}}$ coordinate and electric field to extract the nonlinear coupling between them. These were then used to construct the coupled equations of motion for the three phonon coordinates in the presence of an external pump term on the $Q_{\textrm{HX}}$ mode, which we numerically solved for a range of pump frequencies and amplitudes. We find that 465 MV/cm is the smallest pump amplitude that leads to an oscillation of the $Q_{\textrm{LZ}}$ mode at a displaced position, hence, breaking the translation symmetry of the material. Such highly intense light pulses cannot be generate by currently available laser sources, and they have the possibility to damage the material. Nevertheless, our work shows that light can in principle be used to break the translation symmetry of a material via nonlinear phononics.