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

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.

Monitoring molecular nonadiabatic dynamics with femtosecond X-ray diffraction

“…The molecular charge density need not be the same in these two cases, and our comparison has not accounted for this possible difference (i.e., we use the same molecular charge density to calculate S2 and S1). Moreover, intermolecular coupling in the condensed phase can lead to electron relocations (62,63). These interesting effects go beyond the scope of this work, but can be treated with the same formalism by dropping the assumption that all molecules have identical charge densities.…”

Phonon driven charge dynamics in polycrystalline acetylsalicylic acid mapped by ultrafast x-ray diffraction

“…The emitted x-ray pulses are collected, monochromatized, and focused to an ∼100 μ m spot size at the sample position by a Montel multilayer mirror (Incoatec) providing a flux of ∼5 × 10 6 photons/s. Further details of this table-top femtosecond hard x-ray source and the entire experimental setup have been described earlier 9,16,18 …”

Soft-mode driven dynamics in ferroelectrics -new insight from ultrafast terahertz and x-ray experiments

“…X-ray powder diffraction was also applied to determine transient charge density maps of ferroelectric ammonium sulfate [(NH 4 ) 2 SO 4 , Fig. 1], a prototypical ferroelectric which undergoes a phase transition to the paraelectric phase at a temperature T C = 223 K [6]. Upon 3-photon absorption by 70-fs pulses at 400 nm, we observe unchanged angular positions of the Debye Scherrer rings (Fig.…”

“…Nonlinear THz spectroscopy maps the vibrational response but provides only indirect information on the electronic charge motions connected with nonequilibrium soft-mode excitations. The latter issue is addressed by femtosecond x-ray diffraction which allows for deriving the spatially and temporally resolved electron density from the transient structure factors of the different x-ray Bragg reflections [3,5,6]. First x-ray powder diffraction experiments with a 100-fs time resolution on aspirin suggest pronounced relocations of electrons on a 100 pm length scale which are induced by coherent few picometer displacements of the 1.1 THz soft mode.…”