Tool Reveals Mechanism Behind High-Temperature Superconductivity

The atomic vibrations in a material and its electrons are much closely bound than previously thought

3 min read

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
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.
Greg Stewart/SLAC National Accelerator Laboratory

Scientists at the Department of Energy’s SLAC National Accelerator Laboratory and Stanford University have combined two microscopy techniques to peer into the interactions that occur between electrons and the atomic vibrations of a material. They found that the coupling between electrons and atomic vibrations is ten times stronger than anyone had previously believed.

This new insight could lead to superconductivity at much higher temperatures than previously thought possible, leading to a large ripple effect on applications including improved energy transmission in cables and faster electronics and communication.

In research described in the journal Science, the scientists combined an X-ray free-electron laser together with a technique called angle-resolved photoemission spectroscopy (ARPES) to image the atomic vibrations of a material and to see how those vibrations affect the electrons in the same material.

SLAC’s Linac Coherent Light Source (LCLS) X-ray free-electron laser provided the measurements of the atomic vibrations known as phonons while the ARPES was able to measure the momentum and energy of electrons in iron selenide.

Iron selenide is a material that has garnered increased interest of late in the world of superconductivity. A team of researchers in China observed five years ago that when you place an atomically thin layer of it over an alloy of strontium, titanium and oxygen (STO), the temperature for achieving superconductivity rose from 8 degrees to 60 degrees Celsius above absolute zero. That is still pretty cold, but in the world of superconductivity it represents a huge difference.

While room-temperature superconductivity remains a distant prospect, this kind of research seems to put it squarely within the realm of possibility.

“Higher temperature superconductivity itself with other good properties (such as critical field and current) would already be very impactful,” explained 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, in an e-mail interview with IEEE Spectrum. “We used to have a dogma that is impossible.  Now we know the old dogma (30-40 [degrees] Kelvin being the upper limit) is not correct, room-temperature superconductivity is extremely hard, but there is no known reason to believe it is impossible.”

Once these initial observations of iron selenide were reported, Shen started to investigate this material combination with the ARPES tools available at the SLAC labs. In a 2014 paper published in Nature, Shen and his colleagues sorted out what was causing the effect. It turns out that the 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.

The implications of this suggested that if one were to play around with the substrate material, it might be possible to raise the temperature for superconductivity even higher. But Shen wanted to see if this coupling between the atomic vibrations and the electrons in iron selenide would occur without any substrate, forming the basis of this most recent research.

By using a slighter thicker version of the iron selenide that was atomically uniform in its structure, the scientists triggered 5-trillion-times-a-second atomic vibrations in the material by hitting it with infrared laser light. With the X-ray free-electron laser they could see and measure these vibrations and then, with the ARPES, image how the electrons behaved.

At this point, the scientists are not prepared to say that there is a direct connection between this strong coupling between phonons and electrons is what causes the higher-temperature superconductivity, but the combined microscopy techniques should help lead to an answer.

There are several possibilities for the higher-temperature superconductivity, according to Shen. For example, electron-electron and electron-phonon interaction can both contribute, or electron-electron acts through electron-phonon. 

“What this experiment has shown is that the fact previous simple theory of electron-phonon interaction cannot explain the superconductivity in this compound does not mean that we need to throw out the phonon,” said Shen  “It could be that all players are active, and we need to look at the problem in a more holistic way.”

While iron selenide has led to this new technique and the resulting observations, this research has broader implications.

Shen added: “There are multiple players in action, atoms, electrons, etc. We used to think of them in isolation and in most simple terms. This work shows that their interplay can make the whole far more powerful than the individuals in isolation, dramatically. This is likely a route to a broader range of materials with interesting and extreme properties.”

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