Probing phonon softening in ferroelectrics by the scanning probe microwave spectroscopy

TL;DR

This paper explores how scanning probe microwave spectroscopy can detect soft phonon dynamics in ferroelectrics near phase transitions, offering a nanoscale method to study ferroelectric properties.

Contribution

It provides analytical expressions linking microwave measurable quantities to soft phonon behavior in ferroelectrics, highlighting their changes near the Curie temperature.

Findings

Strong influence of soft phonon dispersion on surface potential and impedance near Curie point

Quantified how impedance and dielectric loss spectra respond to phonon dynamics

Sets groundwork for nanoscale ferroelectric phase transition characterization

Abstract

Microwave measurements have recently been successfully applied to measure ferroelectric materials on the nanoscale, including detection of polarization switching and ferroelectric domain walls. Here we discuss the question whether scanning probe microscopy (SPM) operating at microwave frequency can identify the changes associated with the soft phonon dynamics in a ferroic. The analytical expressions for the electric potential, complex impedance and dielectric losses are derived and analyzed, since these physical quantities are linked to experimentally-measurable properties of the ferroic. As a ferroic we consider virtual or proper ferroelectric with an optic phonon mode that softens at a Curie point. We also consider a decay mechanism linked to the conductance of the ferroic, and thus manifesting itself as the dielectric loss in the material. Our key finding is that the influence of the soft phonon dispersion on the surface potential distribution, complex impedance and dielectric losses are evidently strong in the vicinity (10-30 K) of the Curie temperature. Furthermore, we quantified how the spatial distribution and frequency spectra of the complex impedance and the dielectric losses react on the dynamics of the soft phonons near the Curie point. These results set the stage for characterization of polar phase transitions with nanoscale microwave measurements, providing a complementary approach to well established electromechanical measurements for fundamental understanding of ferroelectric properties as well as their applications in telecommunication and computing.