Soft-mode driven polarity reversal in ferroelectrics mapped by ultrafast x-ray diffraction

Area of Science:

• Condensed Matter Physics
• Materials Science
• Crystallography

Background:

• Quantum theory connects microscopic currents and macroscopic polarizations in ferroelectrics.
• The interplay between lattice excitations and charge dynamics at atomic scales remains an open research problem.

Purpose of the Study:

• To investigate the atomic-scale dynamics of charge and lattice excitations in ferroelectrics.
• To understand the relationship between phonon excitation and charge dynamics in ammonium sulfate.

Main Methods:

• Femtosecond X-ray diffraction was employed to probe transient charge density.
• Phonon excitation was induced in ammonium sulfate [(NH4)2SO4].

Main Results:

• A novel low-frequency mode with a 3 ps period and sub-picometer amplitudes was discovered.
• This mode induces periodic charge relocations of approximately 100 pm, characteristic of soft-mode behavior.
• Transient charge density maps allowed for the derivation of macroscopic polarization, revealing periodic polarity reversal.

Conclusions:

• The study reveals a direct link between lattice dynamics and charge redistribution in ferroelectrics.
• Soft-mode behavior plays a crucial role in driving polarization dynamics at the nanoscale.
• Femtosecond X-ray diffraction is a powerful tool for observing ultrafast phenomena in materials.