Researchers used the Advanced Photon Source to confirm the rare properties of this material, which could lead to more efficient large-scale computing.
Physicists at the University of Washington and the U.S. Department of Energy’s (DOE) Argonne National Laboratory have made a discovery that could help enable this more efficient future. Researchers have found a superconducting material that is uniquely sensitive to outside stimuli, enabling the superconducting properties to be enhanced or suppressed at will.
Today’s electronics use semiconducting transistors to quickly switch electric currents on and off, creating the binary ones and zeroes used in information processing. As 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, led by Shua Sanchez of the University of Washington (now at the Massachusetts Institute of Technology), 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.
Shua Sanchez spent a year as a resident at one of the nation’s leading X-ray light sources, the Advanced Photon Source (APS), a DOE Office of Science user facility at Argonne. While there he was supported by DOE’s Science Graduate Student Research program. Working with physicists at APS beamlines 4-ID and 6-ID, 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 were able to show 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 causes a zero-resistance state to emerge. Using electrical measurements and X-ray scattering techniques, scientists were able to confirm 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 Argonne’s Philip Ryan, a co-author on the paper. “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,” Sanchez said. “The vast majority of superconductors aren’t nearly as easily switchable.”
Strain-switchable field-induced superconductivity
Science Advances- Field-induced superconductivity is a rare phenomenon where an applied magnetic field enhances or induces superconductivity. Here, we use applied stress as a control switch between a field-tunable superconducting state and a robust non–field-tunable state. This marks the first demonstration of a strain-tunable superconducting spin valve with infinite magnetoresistance. We combine tunable uniaxial stress and applied magnetic field on the ferromagnetic superconductor Eu(Fe0.88Co0.12)2As2 to shift the field-induced zero-resistance temperature between 4 K and a record-high value of 10 K. We use x-ray diffraction and spectroscopy measurements under stress and field to reveal that strain tuning of the nematic order and field tuning of the ferromagnetism act as independent control parameters of the superconductivity. Combining comprehensive measurements with DFT calculations, we propose that field-induced superconductivity arises from a novel mechanism, namely, the uniquely dominant effect of the Eu dipolar field when the exchange field splitting is nearly zero.
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