Vibrating Atoms Switch the Electrical Polarization of Crystals

Ferroelectric crystals
Fig. 1: Top: Crystal lattice of the ferroelectric ammonium sulfate [(NH 4 ) 2 SO 4] with tilted ammonium (NH 4 + ) tetrahedra (nitrogen: blue, hydrogen: white) and sulfate (SO 4 2-) Tetrahedra (sulfur: yellow, oxygen: red). The green arrow shows the direction of macroscopic polarization P. Blue arrows: local dipoles between sulfur and oxygen atoms. The electron density in the gray plane is shown in the lower left, in Fig. 2 and in the film. Bottom left: Stationary electron density of sulfur and oxygen atoms with high values ​​in the sulfur atom (red) and smaller values ​​in the oxygen atoms (yellow). Bottom right: Change of the local dipoles with a delay time of 2.8 picoseconds (ps) after excitation of the ammonium sulfate crystallites. Anisotropic charge shift decreases the dipole pointing to the right and the other three dipoles.

Ferroelectric crystals have a macroscopic electrical polarization caused by the superposition of many dipoles on the atomic scale. Decisive here is the spatial separation of negatively charged electrons and positively charged atomic nuclei. A change in macroscopic polarization is expected as the atoms are set in motion, but the relationship between atomic motion and polarization is unknown. A time-resolved X-ray experiment now shows that atomic oscillations with a tiny deflection shift electrons over a 1000-fold greater distance between atoms and switch the macroscopic polarization on a time scale of one millionth of a millionth of a second.

Ferroelectric materials are of great importance for applications in electronic sensors, memories and switching elements. For their function, a controlled and rapid change in the electrical properties due to external mechanical forces or electrical voltages is important. This requires an understanding of the relationship between the atomic structure and the macroscopic electrical properties, including the physical mechanisms that determine the fastest possible dynamics of macroscopic polarization.

Scientists at the Max Born Institute in Berlin have now shown how atomic vibrations affect the macroscopic electrical polarization of the prototypical ferroelectric ammonium sulfate [Fig. 1] modulate less picoseconds on the timescale (1 picosecond (ps) = 1 millionth of a millionth of a second). In the latest issue of the journal Structural Dynamics [5, 024501 (2018)], they report on an ultra-short-time X-ray experiment that allows the quantitative recording of charge movements over distances in the range of atomic diameters (10 -10m = 100 picometers). In the measurements, an ultrashort optical excitation pulse vibrates atoms of the material, a powder of small crystallites. A delayed hard X-ray pulse is diffracted at the excited sample to detect the instantaneous atomic arrangement in the form of an X-ray diffraction pattern. The sequence of such snapshots results in a film of the so-called charge density map, from which the spatial distribution of the electrons and the atomic motions for each time point are determined. ([Fig. 2], [Movie]).

Ferroelectric crystals
Fig. 2: (a) Stationary electron density in the gray plane in Fig. 1. (b) Change in electron density with a delay time of 2.8 picoseconds (ps) after excitation of the ammonium sulfate crystallites. The circles mark the atomic positions, the black arrows indicate the shift of electronic charge between one of the oxygen atoms and the SO 3 unit of a sulfate ion over a length of about 100 picometers (pm). The vibration deflections of the atoms are smaller than the line width of the circles and therefore not recognizable. (c) The return transfer of the charge occurs with a delay time of 3.9 ps. Movie : The film shows the entire time evolution of the charge density map. Fig. 3: Upper sub-image: change in the SO bond length as a function of the delay time between excitation and X-ray pulses. The maximum change of 0.1 picometers (pm) is 1000 times smaller than the bond length itself. Middle sub-image: Displacement of electronic charge from one of the oxygen atoms to the SO 3 unit of the sulfate ion (left arrows in Fig. 2) as a function of the delay time. Lower panel: change of macroscopic polarization P along the c-axis of the crystallite. This corresponds predominantly to the sum of the changes of all local SO dipoles within the sulfate ions (red and blue arrows in Fig. 1 bottom right).

The electron density maps show an electron movement over lengths of 100 picometers (pm), more than 1000 times larger than the deflections of the atoms [Fig. 3]. This behavior determines the momentary local dipoles at the atomic scale [Fig. 1]. It is caused by the complex interaction of local electric fields and polarizable electron clouds of the atoms. Using a novel theoretical formalism, it is now possible to determine the electric polarization in the macroscopic world from the time-dependent charge distributions in the atomic world [Fig. 3]. The macroscopic polarization is strongly modulated by the atomic vibrations and even reverses its sign in time with the vibrations. The modulation frequency of 300 GHz is determined by the atomic oscillation frequency and corresponds to a reversal of polarization within 1.5 ps, much faster than in currently existing ferroelectric devices. At the surface of a crystallite occur electric fields of about 700 million volts per meter.
These results establish time-resolved ultra-short-time X-ray diffraction as a new method for combining atomic dynamics with macroscopic electrical properties. Thus, quantum theoretical predictions of electrical properties can be tested and new polar or ionic materials can be characterized with regard to their suitability for ultrahigh-frequency electronics.

Source : Forschungsverbund Berlin e. V