Revealing subterahertz atomic vibrations in quantum paraelectrics by surface-sensitive spintronic terahertz spectroscopy

TL;DR

This paper introduces a surface-sensitive spintronic terahertz spectroscopy technique capable of detecting subterahertz atomic vibrations in quantum paraelectrics, revealing surface phonon dynamics relevant for quantum materials and interfaces.

Contribution

The study develops a novel surface-sensitive THz spectroscopy method that can probe nanometer-scale surface phonon modes in quantum paraelectrics, providing new insights into surface dynamics.

Findings

Surface soft TO1 phonon mode in KTaO3 softens and sharpens with temperature.

In SrTiO3, the TO1 mode broadens below the quantum paraelectric crossover.

The technique detects surface phonon behaviors deviating from bulk properties.

Abstract

Understanding surface collective dynamics in quantum materials is crucial for advancing quantum technologies. For example, surface phonon modes in quantum paraelectrics are thought to play an essential role in facilitating interfacial superconductivity. However, detecting these modes, especially below 1 terahertz (THz), is challenging due to limited sampling volumes and the need for high spectroscopic resolution. Here, we report surface soft transverse optical (TO1) phonon dynamics in KTaO3 and SrTiO3 by developing surface-sensitive spintronic THz spectroscopy that can sense the collective modes only a few nanometers deep from the surface. In KTaO3, the TO1 mode softens and sharpens with decreasing temperature, leveling off at 0.7 THz. In contrast, this mode in SrTiO3 broadens significantly below the quantum paraelectric crossover and coincides with the hardening of a sub-meV phonon mode related to the antiferrodistortive transition. These observations that deviate from their bulk properties may have implications for interfacial superconductivity and ferroelectricity. The developed technique opens opportunities for sensing low-energy surface excitations.