As industrial computing needs grow, the size and energy consumption of the relevant hardware must keep up with those demands. A solution to this dilemma could lie in superconducting materials, which reduce that energy consumption exponentially. Imagine cooling a giant data center - full of constantly running servers - down to nearly absolute zero, enabling large-scale computation with incredible energy efficiency.
Researchers led by Jiun-Haw Chu, a University of Washington associate professor of physics and Clean Energy Institute researcher, and Philip Ryan, a physicist at the U.S. Department of Energy's Argonne National Laboratory, have made a discovery that could enable this more efficient future. In a paper published Nov. 24 in Science Advances, the team reports finding a superconducting material that is uniquely sensitive to outside stimuli, enabling the superconducting properties to be enhanced or suppressed at will. This discovery could enable new opportunities for switchable, energy-efficient superconducting circuits.
Superconductivity is a quantum mechanical phase of matter in which an electrical current can flow through a material with zero resistance. This leads to perfect electronic transport efficiency. Superconductors are used in the most powerful electromagnets for advanced technologies such as magnetic resonance imaging, particle accelerators, fusion reactors and even levitating trains. Superconductors are also used in quantum computing.
Today's electronics use semiconducting transistors to switch electric currents on and off quickly, creating the binary ones and zeroes used in information processing. Since these currents must flow through materials with finite electrical resistance, some of the energy is wasted as heat. This is why your computer heats up over time. The low temperatures needed for superconductivity - usually more than 200 degrees Fahrenheit below freezing - makes those materials impractical for hand-held devices. However, they could conceivably be useful on an industrial scale.
The research team, under the direction of Shua Sanchez - then a UW doctoral student in physics and a fellow at the UW Clean Energy Institute - examined an unusual superconducting material with exceptional tunability. This crystal is made of flat sheets of ferromagnetic europium atoms sandwiched between superconducting layers of iron, cobalt and arsenic atoms. Finding ferromagnetism and superconductivity together in nature is extremely rare, according to Sanchez, as one phase usually overpowers the other.
"It is actually a very uncomfortable situation for the superconducting layers, as they are pierced by the magnetic fields from the surrounding europium atoms," said Sanchez, who is now a postdoctoral researcher at the Massachusetts Institute of Technology. "This weakens the superconductivity and results in a finite electrical resistance."
To understand the interaction of these phases, Sanchez spent a year as a resident at one of the nation's leading X-ray light sources, the Advanced Photon Source, a DOE Office of Science user facility at Argonne. Working with physicists at the APS, Sanchez developed a comprehensive characterization platform capable of probing microscopic details of complex materials.
Using a combination of X-ray techniques, Sanchez and his collaborators showed that applying a magnetic field to the crystal can reorient the europium magnetic field lines to run parallel to the superconducting layers. This removes their antagonistic effects and allows a zero-resistance state to emerge. Using electrical measurements and X-ray scattering techniques, the researchers confirmed that they could control the behavior of the material.
"The nature of independent parameters controlling superconductivity is quite fascinating, as one could map out a complete method of controlling this effect," said Ryan. "This potential posits several fascinating ideas including the ability to regulate field sensitivity for quantum devices."
The team then applied stresses to the crystal with interesting results. They found the superconductivity could be either boosted enough to overcome the magnetism - even without re-orienting the field - or weakened enough that the magnetic reorientation could no longer produce the zero-resistance state. This additional parameter allows for the material's sensitivity to magnetism to be controlled and customized.
"This material is exciting because you have a close competition between multiple phases, and by applying a small stress or magnetic field, you can boost one phase over the other to turn the superconductivity on and off," said Sanchez. "The vast majority of superconductors aren't nearly as easily switchable."
Additional co-authors are Gilberto Fabbris, Yongseong Choi and Jong-Woo Kim with Argonne's APS; Jonathan DeStefano, Elliott Rosenberg and Yue Shi of the UW Department of Physics; Paul Malinowski, a postdoctoral researcher at Cornell University; Yina Huang of the Zhejiang University of Science and Technology in China; and Igor Mazin of George Mason University. The research was funded by the National Science Foundation, the David and Lucile Packard Foundation, the U.S. Department of Energy, the National Science Foundation of China, the Chinese Ministry of Public Security and the Air Force Office of Scientific Research.
Adapted from a press release by Argonne National Laboratory.
