Hauf Christoph, Hernandez Salvador Antonio-Andres, Holtz Marcel, Woerner Michael, Elsaesser Thomas
Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie, 12489 Berlin, Germany.
Struct Dyn. 2018 Apr 6;5(2):024501. doi: 10.1063/1.5026494. eCollection 2018 Mar.
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 [(NH)SO], 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.
量子理论已将铁电体的微观电流与宏观极化联系起来，但晶格激发与原子长度和时间尺度上的电荷动力学之间的相互作用仍是一个悬而未决的问题。在典型铁电体硫酸铵[(NH)SO]中进行声子激发时，我们通过飞秒X射线衍射确定了瞬态电荷密度图。一种新发现的周期为3皮秒、振幅小于皮米的低频模式会在约100皮米的范围内引起周期性电荷重新分布，这是软模行为的一个标志。瞬态电荷密度有助于推导宏观极化，显示出周期性的极性反转。
