Revealing Surface Soft Phonons in Incipient Ferroelectrics

Understanding surface dynamics in quantum materials is crucial for advancing quantum technology. However, the detection of surface vibrational modes at meV energies remains challenging due to the ultralow sampling volume and the needed spectroscopic resolution. Here, a surface-sensitive terahertz spectroscopy based on spin-polarized ultrafast electron transport is developed to uncover the softening of surface transverse optical phonons in two incipient ferroelectrics: KTaO₃ and SrTiO₃, providing unprecedented spectroscopic and time-domain details of surface phonons. For example, the softening of the transverse optical phonon in KTaO₃ deviates from the Curie-Weiss law and levels at finite energy, in agreement with quantum paraelectricity. In contrast, the transverse optical phonon in SrTiO₃ continues to soften and its frequency extrapolates to zero. However, the phonon peak broadens significantly below 35 K, indicating the fluctuation of soft phonon modes in the quantum paraelectric phase regime. Our findings underscore the ultralow-energy and surface sensitivity of this technique for revealing unique quantum states at surfaces and interfaces.