Electric polarization in the macroscopic world and electrons moving at atomic scales

August 28, 2018

Femtosecond x-ray experiments in combination with a new theoretical approach establish a direct connection between electric properties in the macroscopic world and electron motions on the time and length scale of atoms. The results open a new route for understanding and tailoring the properties of ferroelectric materials.

Phenomena in the macroscopic world are described by classical physics while processes at atomic length and time scales are governed by the laws of quantum mechanics. The connection between microscopic and macroscopic physical quantities is far from being trivial and partly unexplained.

The electric polarization is a macroscopic quantity which describes the dipole moment of matter. The polarization originates from the peculiar electron distribution at the atomic scale in polar and ionic materials, among them the most interesting class of ferroelectrics. Their spontaneous electric polarization is widely applied in electronic sensors, memories, and switching devices. The link between polarizations, in particular time dependent ones, and microscopic electron densities is important for understanding and tailoring the properties of ferroelectrics.

Based on a new experimental and theoretical approach, scientists from the Max Born Institute have now established a direct quantitative connection between macroscopic electric polarizations and time-dependent microscopic electron densities. As they report in Physical Review B, atomic motions in ferroelectrics are launched by optical excitation and modulate the electron distribution on a femtosecond time scale (1 fs = 10 -15 seconds). The resulting dynamics of electron density are mapped by time-resolved x-ray powder diffraction. Such data allow for the generation of temporally and spatially resolved electron density maps from which the momentary macroscopic polarization is derived with the help of a new theoretical concept. The potential of the method is demonstrated with two prototype ferroelectric materials.

The theoretical work extends the existing quantum phase approach for calculating stationary macroscopic polarizations towards ultrafast nonequilibrium dynamics of electron charge and polarization. The theoretical key steps consist in deriving a microscopic current density from time-dependent electron density maps while minimizing the electron kinetic energy, and calculating the macroscopic polarization from the current density. This method is applied to the prototype ferroelectric material ammonium sulfate [(NH4)2SO4, Fig. 1] with the time dependent electron and current densities shown in the movie, https://www.mbi-berlin.de/en/current/index.html#2018_08_27. As a second prototype system, potassium dihydrogen phosphate [KH2PO4] was investigated. The analysis provides macroscopic polarizations and their absolute values as governed by microscopic vibrations.

The results establish ultrafast x-ray diffraction as a unique tool for grasping macroscopic electric properties of complex materials. The broad relevance of this new insight is underlined by the selection of the article as an "Editor's Suggestion".
-end-


Forschungsverbund Berlin

Related Polarization Articles from Brightsurf:

Highly sensitive detection of circularly polarized light without a filter
Japanese scientists developed a photodiode using a crystalline film composed of lead perovskite compounds with organic chiral molecules to detect circularly polarized light without a filter.

Anti-hacking based on the circular polarization direction of light
The Internet of Things (IoT) allowing smart phones, home appliances, drones and self-driving vehicles to exchange digital information in real time requires a powerful security solution, as it can have a direct impact on user safety and assets.

Germanium telluride's hidden properties at the nanoscale revealed
Germanium Telluride is an interesting candidate material for spintronic devices.

FAST reveals mystery of fast radio bursts from the universe
The Five-hundred-meter Aperture Spherical Radio Telescope (FAST) has revealed some mystery of the fast radio bursts, according to a study published in Nature on Oct.

Graphene detector reveals THz light's polarization
Physicists have created a broadband detector of terahertz radiation based on graphene.

Squaring the circle -- Breaking the symmetry of a sphere to control the polarization of light
Scientists at Tokyo Institute of Technology (Tokyo Tech, Japan) and Institute of Photonic Sciences (ICFO, Spain) develop a method to generate circularly polarized light from the ultimate symmetrical structure: the sphere.

Optical shaping of polarization anisotropy in a laterally-coupled-quantum-dot dimer
Coupled-quantum-dot (CQD) structures are considered to be an important building block in the development of scalable quantum devices.

Polarization of Br2 molecule in vanadium oxide cluster cavity and new alkane bromination
A hemispherical vanadium oxide cluster has a cavity that can accommodate a bromine molecule.

On-chip spin-Hall nanograting for simultaneously detecting phase and polarization singularities
A plasmonic spin-Hall nanograting structure that simultaneously detects both the polarization and phase singularities of the incident beam is reported.

A new theory about political polarization
A new model of opinion formation shows how the extent to which people like or dislike each other affects their political views -- and vice versa.

Read More: Polarization News and Polarization Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.