Transient translation symmetry breaking via quartic-order negative light-phonon coupling at the Brillouin zone boundary in KTaO${}_3$

Abstract

KTaO${}_3$ presents a rich hyper-Raman spectrum originating from two-phonon processes at the Brillouin zone boundary, indicating the possibility of driving these phonon modes using intense midinfrared laser sources. We obtained the coupling of light to the highest-frequency longitudinal optic phonon mode $Q_{\mathrm{HY}}$ at the $X$ $(0,0,\frac{1}{2})$ point by first principles calculations of the total energy as a function of the phonon coordinate $Q_{\mathrm{HY}}$ and electric field $E$. We find that the energy curve as a function of $Q_{\mathrm{HY}}$ softens for finite values of electric field, indicating the presence of $Q_{\mathrm{HY}}^2{E}^2$ nonlinearity with negative coupling coefficient. We studied the feasibility of utilizing this nonlinearity to transiently break the translation symmetry of the material by making the $Q_{\mathrm{HY}}$ mode unstable with an intense midinfrared pump pulse. We also considered the possibility that nonlinear phonon-phonon couplings can excite the lowest-frequency phonon coordinates $Q_{\mathrm{LZ}}$ and $Q_{\mathrm{LX}}$ at $X$ when the $Q_{\mathrm{HY}}$ mode is externally driven. The nonlinear phonon-phonon couplings were also obtained from first principles via total-energy calculations as a function of the phonon coordinates, and these were used to construct the coupled classical equations of motion for the phonon coordinates in the presence of an external pump term on $Q_{\mathrm{HY}}$. We numerically solved them for a range of pump frequencies and amplitudes and found three regimes where the translation symmetry is broken: i) rectification of the lowest-frequency coordinates due to large amplitude oscillation of the $Q_{\mathrm{HY}}$ coordinate about its equilibrium position, ii) rectification of only the $Q_{\mathrm{HY}}$ coordinate without displaced oscillations of the lowest-frequency coordinates, and iii) rectification of all three coordinates.

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Cited by in corpus (1)

• Tunable Spatiotemporal Orders in Driven Insulators