Condensed Matter Seminar "Anharmonic effects on phonon eigenvectors and S(Q,E) in quantum paraelectric SrTiO3"

The quantum paraelectric behavior and strongly anharmonic lattice dynamics of SrTiO3 have attracted interest for decades. Reflecting the incipient ferroelectric (FE) instability near the quantum critical point and couplings between transverse acoustic (TA) and optic (TO) phonons, anomalous phonon intensities were observed from inelastic neutron scattering (INS) experiments on SrTiO3 using the state-of-art instruments at Oak Ridge National Laboratory. The S(Q,E) data reveal a strongly anomalous evolution of TA intensity as TO softens. The experimental trends are confirmed and rationalized using DFT simulations including anharmonic renormalization. By analyzing the temperature-dependent force constants (FC) and eigenvectors, it is found that the structure factors of phonon modes change dramatically with temperature, as a direct consequence of the anharmonicity in this system. Moreover, we identify that the changes of Ti and O eigenvectors are responsible for these striking observations, originating from FC changes in the Ti-O bonds. These results establish how temperature-dependent phonon intensities from INS can provide direct insights into the behavior of phonon eigenvectors, and also show how first-principles simulations can rationalize such anharmonic effects.