Unveiling Topology-Induced Phonons Modes Using High-Spatial-Resolution Polarization-Selective Electron Energy-Loss Spectroscopy

Off-axis electron energy-loss spectroscopy (EELS) in the monochromated scanning transmission electron microscope (STEM) possesses sufficiently high spatial and energy resolution to directly measure localized phonon modes, extending all the way throughout the mid-infrared spectral regime. Here, ‘off-axis’ refers to an experimental geometry where the EEL signal is collected at an angle deflected away from the optic axis of the STEM. The technique was pioneered with the goal of improving spatial resolution by suppressing the measurement of delocalized dipole-like excitations that are localized in angular space directly to the optic axis.

However, recently a new benefit to the off-axis geometry has been uncovered, which is that the angle to which the collection solid angle is deflected away from the optic axis, preferentially selects phonon modes polarized in that direction. As a result, not only does off-axis EELS enable atomic-/near-atomic-resolution measurement of localized phonon modes, it simultaneously possesses the electron microscopy equivalent of linear polarization selectivity.

Here, I will introduce and explain the power of high-spatial-resolution polarization-selective vibrational EELS and demonstrate its application to heterostructures with a strong dependence on localized polarization. The mixed real-space/reciprocal-space measurements of off-axis EELS enable us to probe the non-equivalence of polar interfaces in nitride superlattices and unveil chirality driven phonon localization in polarization vortices in complex oxides.