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

Christoph Hauf,Antonio-Andres Hernandez Salvador,Thomas Elsaesser,Marcel Holtz,Michael Woerner

doi:10.1063/1.5026494

Abstract

Quantum theory has linked microscopic currents and macroscopic polarizations of ferroelectrics, but the interplay of lattice excitations and charge dynamics on atomic length and time scales is an open problem. Upon phonon excitation in the prototypical ferroelectric ammonium sulfate [(NH4)2SO4], we determine transient charge density maps by femtosecond x-ray diffraction. A newly discovered low frequency-mode with a 3 ps period and sub-picometer amplitudes induces periodic charge relocations over some 100 pm, a hallmark of soft-mode behavior. The transient charge density allows for deriving the macroscopic polarization, showing a periodic reversal of polarity.

Highlights

Ferroelectrics display a macroscopic electric polarization which originates from their peculiar electronic structure
The elegant formalism presented in Refs. 4–7 expresses the polarization difference in terms of a quantum (Berry) phase calculated from the cell-periodic part of the electronic wavefunction
Our results demonstrate the potential of ultrafast x-ray diffraction for unraveling the microscopic mechanisms behind ferroelectricity and for mapping the intrinsically ultrafast dynamics of electric polarizations upon phonon excitations

Summary

Introduction

Ferroelectrics display a macroscopic electric polarization which originates from their peculiar electronic structure. Crystalline ferroelectric materials show a rich variety of lattice geometries and microscopic distributions of electronic charge within their unit cell.. Extensive theoretical work has shown that, in contrast to early simplistic concepts, the macroscopic polarization P(r) cannot be derived solely from the time-independent microscopic electron density q(r).. One has to calculate polarization differences between different states of the crystal from the respective electronic wavefunction. 4–7 expresses the polarization difference in terms of a quantum (Berry) phase calculated from the cell-periodic part of the electronic wavefunction.. 4–7 expresses the polarization difference in terms of a quantum (Berry) phase calculated from the cell-periodic part of the electronic wavefunction.8 This method has been applied to calculate the stationary macroscopic polarization of a number of prototype ferroelectrics The elegant formalism presented in Refs. 4–7 expresses the polarization difference in terms of a quantum (Berry) phase calculated from the cell-periodic part of the electronic wavefunction. This method has been applied to calculate the stationary macroscopic polarization of a number of prototype ferroelectrics

Methods

Results

Discussion

Conclusion