A study at the Department of Energy’s SLAC National Accelerator Laboratory suggests for the first time how scientists might deliberately engineer superconductors that work at higher temperatures.
In their report, a team led by SLAC and Stanford University researchers explains why a thin layer of iron selenide superconducts — carries electricity with 100 percent efficiency — at much higher temperatures when placed atop another material, which is called STO for its main ingredients strontium, titanium and oxygen.
In the new study, the scientists concluded that natural trillion-times-per-second vibrations in the STO travel up into the iron selenide film in distinct packets, like volleys of water droplets shaken off by a wet dog. These vibrations give electrons the energy they need to pair up and superconduct at higher temperatures than they would on their own.
“Our simulations indicate that this approach – using natural vibrations in one material to boost superconductivity in another – could be used to raise the operating temperature of iron-based superconductors by at least 50 percent,” said Zhi-Xun Shen, a professor at SLAC and Stanford University and senior author of the study.
This work could be leveraged to raise the operating temperature of iron based superconductors above 77 degrees which can be reached with liquid nitrogen. Liquid nitrogen cooling is far cheaper and can be used for more practical applications. Other advances have enabled iron based superconductors to achieve current density above 100,000 amps per square centimeter which is also needed for practical applications.
While that’s still nowhere close to room temperature, he added, “We now have the first example of a mechanism that could be used to engineer high-temperature superconductors with atom-by-atom control and make them better.”
The new results “point to a new direction that people have not considered before,” Moore said. “They have the potential to really break records in high-temperature superconductivity and give us a new understanding of things we’ve been struggling with for years.”
He added that SLAC is developing a new X-ray beamline at SSRL with a more advanced ARPES system to create and study these and other exotic materials. “This paper predicts a new pathway to engineering superconductivity in these materials,” Moore said, “and we’re building the tools to do just that.”
Caption: In this illustration, a single layer of superconducting iron selenide (balls and sticks) has been placed stop another material known as STO for its main ingredients selenium, titanium and oxygen. The STO is shown as blue pyramids, which represent the arrangement of its atoms. A study at SLAC found that when natural vibrations (green glow) from the STO move up into the iron selenide film, electrons in the film (white spheres) can pair up and conduct electricity with 100 percent efficiency at much higher temperatures than before. The results suggest a way to deliberately engineer superconductors that work at even higher temperatures. Credit: SLAC National Accelerator Laboratory
In 2013, the critical current density of the new iron-based (FeSe0.5Te0.5-) superconductor reached more than 1 million amperes (amps) per square centimeter, which is several hundred times more than regular copper wires can carry over the same area. Under an intense 30-tesla magnetic field, the film carried a record-high 200,000 amperes per square centimeter.
The scientists used x-rays from the Stanford Synchrotron Radiation Lightsource to eject electrons from iron selenide films, and study their properties including energy and angular momentum. They found that some electrons had less energy than expected, and that the difference was exactly the energy of the vibrations in the selenium titanate substrate. An individual quantum packet of vibration – known as a phonon – couples to each electron pair, bringing them together to achieve superconductivity, Shen explains. When the electrons are ejected by x-rays, they lose energy to excite these phonons.
‘We show unambiguously how the substrate can play a role in enhancing superconductivity,’ Shen says. He adds that these phonons should even help when electrons are otherwise paired up by phenomena other than lattice ripples. He now plans to attempt to use this approach in other materials, and see whether sandwiching a superconductor between two substrates could provide greater enhancement.
Xingjiang Zhou from the National Lab for Superconductivity in Beijing, China, who helped bring iron selenide superconductivity up to 65K, calls this finding ‘important’. ‘If the interpretation is right, it provides a new way of searching for new high temperature superconductors. However, to pin down whether the enhancement is caused by such a peculiar electron–phonon coupling, more experimental work needs to be done.’
Films of iron selenide (FeSe) one unit cell thick grown on strontium titanate (SrTiO3 or STO) substrates have recently shown superconducting energy gaps opening at temperatures close to the boiling point of liquid nitrogen (77 kelvin), which is a record for the iron-based superconductors. The gap opening temperature usually sets the superconducting transition temperature Tc, as the gap signals the formation of Cooper pairs, the bound electron states responsible for superconductivity. To understand why Cooper pairs form at such high temperatures, we examine the role of the SrTiO3 substrate. Here we report high-resolution angle-resolved photoemission spectroscopy results that reveal an unexpected characteristic of the single-unit-cell FeSe/SrTiO3 system: shake-off bands suggesting the presence of bosonic modes, most probably oxygen optical phonons in SrTiO3 which couple to the FeSe electrons with only a small momentum transfer. Such interfacial coupling assists superconductivity in most channels, including those mediated by spin fluctuations. Our calculations suggest that this coupling is responsible for raising the superconducting gap opening temperature in single-unit-cell FeSe/SrTiO3.
SOURCES – Chemistry World, Eurekalert, Brookfield National Lab, Nature
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