Macroscopic electric polarization and microscopic electron dynamics: Quantitative insight from femtosecond x-ray diffraction

Phenomena in the macroscopic world are described by classical physics while processes at the microscopic atomic scale are governed by the laws of quantum mechanics. The connection between microscopic and macroscopic physical quantities is far from being trivial and remains partly unexplained. This study presents a novel theoretical approach for calculating macroscopic polarizations from transient microscopic charge densities that are derived from ultrafast time-resolved x-ray diffraction experiments. This is important for understanding the properties of polar and ionic materials and, in particular, those of ferroelectrics with widespread technical applications.