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  • 2017-07-03 16:05:17
  • Article ID: 677397

Scientists Get First Direct Look at How Electrons 'Dance' with Vibrating Atoms

A precise new way to study materials shows this 'electron-phonon coupling' can be far stronger than predicted, and could potentially play a role in unconventional superconductivity.

  • Credit: Greg Stewart/SLAC National Accelerator Laboratory

    An animation shows how an infrared laser beam (orange) triggers atomic vibrations in a thin layer of iron selenide, which are then recorded by ultrafast X-ray laser pulses to create an ultrafast movie. The motion of the selenium atoms (red) changes the energy of the electron orbitals of the iron atoms (blue), and the resulting electron vibrations are recorded separately with a technique called ARPES (not shown).

  • Credit: Greg Stewart/SLAC National Accelerator Laboratory

    In this illustration, an infrared laser beam (orange) triggers atomic vibrations in a thin layer of iron selenide, which are then recorded by ultrafast X-ray laser pulses (white) to create an ultrafast movie. The motion of the selenium atoms (red) changes the energy of the electron orbitals of the iron atoms (blue), and the resulting electron vibrations are recorded separately with a technique called ARPES (not shown). The coupling of atomic positions and electronic energies is much stronger than previously thought and may significantly impact the material’s superconductivity.

  • Credit: Dawn Harmer/SLAC National Accelerator Laboratory

    SLAC/Stanford Professor Zhi-Xun Shen, left, and SLAC staff scientist Patrick Kirchmann with the ARPES instrument used to measure electron energy and momentum in an iron selenide film.

Menlo Park, Calif. — Scientists at the Department of Energy’s SLAC National Accelerator Laboratory and Stanford University have made the first direct measurements, and by far the most precise ones, of how electrons move in sync with atomic vibrations rippling through an exotic material, as if they were dancing to the same beat.

The vibrations are called phonons, and the electron-phonon coupling the researchers measured was 10 times stronger than theory had predicted – making it strong enough to potentially play a role in unconventional superconductivity, which allows materials to conduct electricity with no loss at unexpectedly high temperatures.

What’s more, the approach they developed gives scientists a completely new and direct way to study a wide range of “emergent” materials whose surprising properties emerge from the collective behavior of fundamental particles, such as electrons. The new approach investigates these materials through experiments alone, rather than relying on assumptions based on theory.

The experiments were carried out with SLAC’s Linac Coherent Light Source (LCLS) X-ray free-electron laser and with a technique called angle-resolved photoemission spectroscopy (ARPES) on the Stanford campus. The researchers described the study today in Science.

A ‘Breakthrough’ Approach

“I believe this result will have several impacts,” said Giulia Galli, a professor at the University of Chicago’s Institute for Molecular Engineering and senior scientist at the DOE’s Argonne National Laboratory who was not involved in the study.

“Of course they have applied the method to a very important material, one that everyone has been trying to figure out and understand, and this is great,” she said. “But the fact that they show they are able to measure the electron-phonon interaction, which is so important in so many materials and physical processes – this, I believe, is a breakthrough that will pave the way to many other experiments on many other materials.”

The ability to make this measurement, she added, will allow scientists to validate theories and computations that describe and predict the physics of these materials in a way they were never able to do before.

“These precision measurements will give us deep insights into how these materials behave,” said Zhi-Xun Shen, a professor at SLAC and Stanford and investigator with the Stanford Institute for Materials and Energy Sciences (SIMES) who led the study.

Extraordinarily Precise ‘Movies’

The team used SLAC’s LCLS to measure atomic vibrations and ARPES to measure the energy and momentum of electrons in a material called iron selenide. Combining the two techniques allowed them to observe electron-phonon coupling with extraordinary precision, on a timescale of femtoseconds – millionths of a billionth of a second – and within roughly a billionth of the width of a human hair.

“We were able to make a ‘movie,’ using the equivalent of two cameras to record the atomic vibrations and electron movements, and show that they wiggle at the same time, like two standing waves superimposed on each other,” said co-author Shuolong Yang, a postdoctoral researcher at Cornell University.

“It isn’t a movie in the ordinary sense of images you can watch on a screen,” he said. “But it does capture the phonon and electron movements in frames shot 100 trillion times per second, and we can string about 100 of them together just like movie frames to get a full picture of how they are linked.”

The iron selenide they studied is a curious material. It’s known to conduct electricity without loss, but only at extremely cold temperatures, and in a way that could not be entirely explained by established theories; that’s why it’s called an unconventional superconductor.

Pursuing an Intriguing Clue

But five years ago, a research group in China reported an intriguing observation: When an atomically thin layer of iron selenide is put on top of another material called STO -- named for its primary ingredients strontium, titanium and oxygen -- its maximum superconducting temperature jumps from 8 degrees to 60 degrees above absolute zero, or minus 213 degrees Celsius. Although that’s still really cold, it’s a much higher temperature than scientists expected, and it falls within the operating range of so-called “high-temperature superconductors,” whose discovery in 1986 set off a frenzy of research because of the revolutionary impact these perfectly efficient electrical transmitters could have on society.

Following up on this clue, Shen’s group examined the same combination of materials with ARPES. In a 2014 paper in Nature, they concluded that atomic vibrations in the STO travel up into the iron selenide and give electrons the additional energy they need to pair up and carry electricity with zero loss at higher temperatures than they would on their own.

This suggested that scientists might be able to achieve even higher maximum superconducting temperatures by changing a number of variables, such as the nature of the substrate beneath a superconducting film, all at the same time.

But could this coupling of atomic vibrations and collaborative electron behavior also take place in iron selenide alone, without a boost from a substrate? That’s what the current study aimed to find out.

Like Tapping a Bell With a Hammer

Shen’s team made a thicker, atomically uniform iron selenide film and hit it with infrared laser light to excite its 5-trillion-times-a-second atomic vibrations - like gently tapping a bell with a little hammer, SLAC staff scientist and co-author Patrick Kirchmann said. This got the vibrations oscillating in sync with each other throughout the film, so they could be more easily observed.

The team then measured the material’s atomic vibrations and electron behavior in two separate experiments. Yang, who was a Stanford graduate student at the time, led the ARPES measurement. Simon Gerber, a postdoctoral researcher in Shen’s group, led the LCLS measurements at SLAC; he has since joined the SwissFEL at the Paul Scherrer Institute in Switzerland as a staff scientist.  

The new study doesn’t prove that the coupling of atomic and electronic vibrations was responsible for boosting iron selenide’s superconducting temperature in the previous studies, Kirchmann said. But the combination of X-ray laser and ARPES observations should provide new and more sophisticated insights on the physics of material systems where several factors are at play at the same time, and hopefully move the field forward faster.

Other institutions involved in the study were the Advanced Light Source (ALS) at the DOE’s Lawrence Berkeley National Laboratory; the International Center for Quantum Materials at Peking University in Beijing; the University of California, Berkeley; and Chungbuk National University in Korea. LCLS and ALS are DOE Office of Science User Facilities.

SLAC is a multi-program laboratory exploring frontier questions in photon science, astrophysics, particle physics and accelerator research. Located in Menlo Park, California, SLAC is operated by Stanford University for the U.S. Department of Energy Office of Science. To learn more, please visit

SLAC National Accelerator Laboratory is supported by the Office of Science of the U.S. Department of Energy. The Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit

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Rutgers Scientists Discover 'Legos of Life'

Rutgers scientists have found the "Legos of life" - four core chemical structures that can be stacked together to build the myriad proteins inside every organism - after smashing and dissecting nearly 10,000 proteins to understand their component parts. The four building blocks make energy available for humans and all other living organisms, according to a study published online today in the Proceedings of the National Academy of Sciences.

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Theoretical Physicist Elena Belova Named to Editorial Board of Physics of Plasmas

Theoretical physicist Elena Belova named to editorial board of Physics of Plasmas

Superconducting X-Ray Laser Takes Shape in Silicon Valley

An area known for high-tech gadgets and innovation will soon be home to an advanced superconducting X-ray laser that stretches 3 miles in length, built by a collaboration of national laboratories. On January 19, the first section of the machine's new accelerator arrived by truck at SLAC National Accelerator Laboratory in Menlo Park after a cross-country journey that began in Batavia, Illinois, at Fermi National Accelerator Laboratory.

Kelsey Stoerzinger Earns Young Investigator Lectureship

Kelsey Stoerzinger, Pauling Fellow at Pacific Northwest National Laboratory, is one of the 2018 Caltech Young Investigator Lecturers in Engineering and Applied Physics.

North Dakota State University Joins Two National Distributed Computing Groups

The NDSU Center for Computationally Assisted Science and Technology (CCAST) joins OSG (Open Science Grid) and XSEDE (Extreme Science and Engineering Discovery Environment).

DOE Announces Funding for New HPC4Manufacturing Industry Projects

The Department of Energy's Advanced Manufacturing Office (AMO) today announced the funding of $1.87 million for seven new industry projects under an ongoing initiative designed to utilize DOE's high-performance computing (HPC) resources and expertise to advance U.S. manufacturing and clean energy technologies.

DOE Announces First Awardees for New HPC4Materials for Severe Environments Program

The Department of Energy's Office of Fossil Energy (FE) today announced the funding of $450,000 for the first two private-public partnerships under a brand-new initiative aimed at discovering, designing and scaling up production of novel materials for severe environments.

Two Argonne Scientists Recognized for a Decade of Breakthroughs

Two scientists with the U.S. Department of Energy's (DOE) Argonne National Laboratory have been named to the Web of Science's Highly Cited List of 2017, ranking in the top 1 percent of their peers by citations and subject area. Materials Scientist Khalil Amine and Energy and Environmental Policy Scientist David Streets say they are thrilled to see their work -- and the laboratory -- recognized in such a way.

Argonne Welcomes Department of Energy Secretary Perry

U.S. Department of Energy Secretary Rick Perry visited Argonne National Laboratory yesterday, getting a first-hand view of the multifaceted and interdisciplinary research program laboratory of the Department.

Argonne names John Quintana Deputy Laboratory Director for Operations and COO

John Quintana has been named Deputy Laboratory Director for Operations and Chief Operations Officer (COO) of the U.S. Department of Energy's (DOE) Argonne National Laboratory.

Developing Next-Generation Sensing Technologies

Recently, the Advanced Research Projects Agency-Energy (ARPA-E) announced $20 million in funding for 15 projects that will develop a new class of sensor systems to enable significant energy savings via reduced demand for heating and cooling in residential and commercial buildings.

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Exploring Past, Present, and Future Water Availability Regionally, Globally

New open-source software simulates river and runoff resources.

Arctic Photosynthetic Capacity and Carbon Dioxide Assimilation Underestimated by Terrestrial Biosphere Models

New measurements offer data vital to projecting plant response to environmental changes.

DRIFTing to Fast, Precise Data

Non-destructive technique identifies key variations in Alaskan soils, quickly providing insights into carbon levels.

Superconducting Tokamaks Are Standing Tall

Plasma physicists significantly improve the vertical stability of a Korean fusion device.

Graphene Flexes Its Muscle

Crumpling reduces rigidity in an otherwise stiff material, making it less prone to catastrophic failure.

Remotely Predicting Leaf Age in Tropical Forests

New approach offers data across species, sites, and canopies, providing insights into carbon uptake by forests.

What's the Noise Eating Quantum Bits?

The magnetic noise caused by adsorbed oxygen molecules is "eating at" the phase stability of quantum bits, mitigating the noise is vital for future quantum computers.

Rewritable Wires Could Mean No More Obsolete Circuitry

An electric field switches the conductivity on and off in atomic-scale channels, which could allow for upgrades at will.

Filtering Water Better than Nature

Water passes through human-made straws faster than the "gold standard" protein, allowing us to filter seawater.

Machine Learning Provides a Bridge to the Texture of the Quantum World

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