Ultrafast Imaging of Polarons

Arrangement of atoms in the crystal lattice (e.g., oxygen, shown in green) and the electron cloud shapes both shift to try to accommodate one another in a push-me, pull-you arrangement. This intermediate stage in response to the laser pulse energy kick is part of a two-step relaxation process that proves the existence of poltroons. (Source: BNL)

Arrangement of atoms in the crystal lattice (e.g., oxygen, shown in green) and the electron cloud shapes both shift to try to accommodate one another in a push-me, pull-you arrangement. This intermediate stage in response to the laser pulse energy kick is part of a two-step relaxation process that proves the existence of poltroons. (Source: BNL)

Many people picture electrical conduc­tivity as the flow of electrons without really thinking about the atomic structure of the material through which those charges are moving. But scientists who study strongly correlated electron materials such as high-tem­perature super­conductors and those with strong responses to magne­tism know that picture is far too sim­plistic. They know that the atoms play a crucial role in deter­mining a material’s properties. For example, electrical resistance is a mani­festation of electrons scattering off the atoms. Less evident is the concept that electrons and atoms can move coope­ratively to stop the flow of charge-or, in the other extreme, make electrons flow freely without resistance.

Now, a team led by physicist Yimei Zhu at the U.S. Department of Energy’s Brook­haven National Labo­ratory has produced definitive evidence that the movement of electrons has a direct effect on atomic arrangements, driving deformations in a material’s 3D crystalline lattice in ways that can drasti­cally alter the flow of current. Finding evidence for these strong electron-lattice inter­actions, poltroons, emphasizes the need to quantify their impact on complex phenomena such as super­conductivity and other promising properties. The team developed an ultrafast electron diffrac­tion system, a new laser-driven imaging technique and the first of its kind in the world-to capture the subtle atomic-scale lattice distortions. The method has wide­spread potential appli­cation for studying other dynamic processes.

“The technique is similar to using strobo­scopic photo­graphy to reveal the tra­jectory of a ball,” said Zhu. “Using different time delays between throwing the ball and snapping the photo, you can capture the dynamic action,” he said. But to image dynamics at the atomic scale, you need a much faster flash and a way to set sub­atomic scale objects in motion. The machine developed by the Brookhaven team uses a laser pulse to give electrons in a sample material a kick of energy. At the same time, a second laser split from the first generates very quick bursts of high-energy (2.8 MeV) electrons to probe the sample. The electrons that make up these 130-femto­second flashes and create dif­fraction patterns that reveal the positions of the atoms. By varying the time delay between the pulse and the probe, the scientists can capture the subtle shifts in atomic arrange­ments as the lattice responds to the kicked-up electrons.

“This is similar to x-ray dif­fraction, but by using electrons we get a much larger signal, and the high energy of the probe electrons gives us better access to measuring the precise motion of atoms,” Zhu said. Plus, his micro­scope can be built for a fraction of what it would cost to build an ultrafast x-ray light source.  The scientists used this technique to study the electron-lattice inter­actions in a manganese oxide, a material of long-standing interest because of how drama­tically its conduc­tivity can be affected by the presence of a magnetic field. They detected a telltale signa­ture of electrons inter­acting with and altering the shape of the atomic lattice-namely, a two-step relaxation exhi­bited by the kicked-up electrons and their sur­rounding atoms.

In a normal one-step relaxation, electrons kicked up by a burst of energy from one atomic location to another quickly adapt their shape to the new environment. “But in strongly corre­lated materials, the electrons are slowed down by inter­actions with other electrons and inter­actions with the lattice,” said Weiguo Yin, another Brook­haven physicist working on the study. “It’s like a traffic jam with lots of cars moving more slowly.”

In effect the negatively charged electrons and positively charged atomic nuclei respond to one another in a way that causes each to try to accom­modate the shape of the other. So an elongated electron cloud, when entering a symme­trical atomic space, begins to assume a more spherical shape, while at the same time, the atoms that make up the lattice, shift positions to try to accom­modate the elongated electron cloud. In the second step, this in-between, push-me, pull-you arrange­ment gradually relaxes to what would be expected in a one-step rela­xation.

“This two-step behavior, which we can see with our ultrafast electron dif­fraction, is the proof that the lattice vibra­tions are interacting with the electrons in a timely fashion. They are the proof that polarons exist,” Yin said. The finding yields insight into how the lattice response helps generate the huge decrease in electrical resistance the manga­nites experience in a magnetic field-an effect known as colossal magneto­resistance. “The electron cloud shapes are linked to the magnetic attri­butes of the electrons,” Yin explained. “When the magnetic moments of the electrons are aligned in a magnetic field, the electron cloud shape and the atomic arrange­ment become more symmetric and homogenous. Without the need to play the push-me, pull-you game, electric charges can flow more easily.”

This work shows that an ultrafast laser can quickly modify elec­tronic, magnetic, and lattice dynamics in strongly correlated electron materials-an approach that could result in promising new technical appli­cations, such as ultrafast memory or other high-speed electronic devices. “Our method can be used to better under­stand these dynamic inter­actions, and suggests that it will also be useful for studying other dynamic processes to discover hidden states and other exotic material behavior,” said Zhu. (Source: BNL)

Reference: J. Li et al.: Dichotomy in ultrafast atomic dynamics as direct evidence of polaron formation in manganites, NPJ Quant. Mat. 1, 16026 (2016); 10.1038/npjquantmats.2016.26

Link: Div. of Condensed Matter Physics and Materials Science, Brookhaven National Laboratory, Upton, USA

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