For superconductors, the discovery comes from the disorder

Discovered over 100 years ago, superconductivity continues to captivate scientists looking to develop components that deliver highly efficient energy transmission, high-speed electronics, or quantum bits for next-generation computing. However, determining what makes substances become – or stop being – superconductors remains a central issue in finding new candidates for this special class of materials.

In potential superconductors, electrons can be organized in different ways. Some enhance the superconducting effect, others inhibit it. In a new study, scientists from the Argonne National Laboratory of the US Department of Energy (DOE) explained how two such arrangements compete with one another and ultimately affect the temperature at which a material becomes superconducting .

In the superconducting state, the electrons meet in so-called Cooper pairs, in which the movement of the electrons is correlated; at each moment, the velocities of the electrons participating in a given pair are opposite. In the end, the movement of all the electrons is coupled – no single electron can do the right thing – which leads to the lossless flow of electricity: superconductivity.

Generally, the more the pair is strongly coupled and the larger the number of participating electrons, the higher the superconducting transition temperature will be.

Potentially superconducting materials at high temperatures are not simple elements, but complex compounds containing many elements. It turns out that, besides superconductivity, electrons can exhibit different properties at low temperatures, including magnetism or the order of charge density waves. In a charge density wave, the electrons form a periodic pattern of high and low concentration inside the material. The bound electrons in the charge density wave do not participate in the superconductivity and the two phenomena compete with each other.

"If you remove electrons to place them in a charge density wave, the strength of your superconducting effect will diminish," said Ulrich Welp, materials scientist in Argonne, author of the study.

The work of the Argonne team is based on the realization that the order of charge density waves and superconductivity are affected differently by the imperfections of the material. By introducing the disorder, the researchers removed a charge density wave, disrupting the periodic structure of the charge density while having little effect on superconductivity. This opens the way to the balance between the order of competing charge density waves and superconductivity.

To introduce the disorder so as to alter the state of the charge density wave, while leaving the superconducting state largely intact, the researchers used the particle irradiation. By striking the material with a proton beam, the researchers removed a few atoms, thus altering the overall electronic structure while preserving the chemical composition of the material.

To get an idea of ​​the fate of charge density waves, the researchers used state-of-the-art X-ray scattering at Argonne's Advanced Photon Source (APS), an office user facility. DOE Science and the Cornell High Energy Synchrotron Source. . "X-ray scattering was essential for observing the intricacies of this electronic order in the material," said Argonne physicist and study author Zahir Islam. "We discovered that a dilute concentration of disordered atoms actually decreased the charge density wave in order to improve superconductivity."

According to Islam, if the current genius of the APS allowed systematic studies of charge density waves of tiny samples of single crystals despite a relatively weak diffusion force, the next planned modernization of the The facility will provide researchers with the greatest sensitivity to observe these phenomena. In addition, scientists will benefit from studying these materials in extreme environments, especially under high magnetic fields, to tip the balance in favor of charge density waves to better understand high temperature superconductivity. .

As part of their research, scientists have studied a material called copper oxide, lanthanum, barium (LBCO). In this material, the superconducting temperature nearly dropped to reach absolute zero (-273 degrees Celsius) when the material reached a certain chemical composition. However, for closely related compositions, the transition temperature remained relatively high. Scientists believe that this effect of superconductivity by cooling is due to the presence of charge density waves and that the suppression of the charge density wave could induce even higher transition temperatures.

Wai-Kwong Kwok, a distinguished member of the Argonne and study author, says Wai-Kwong Kwok, distinguished researcher from Argonne and author of the study. "From the point of view of the superconductor, the enemy of my enemy is really my friend," he said.

An article based on the study, "Disorder increases the critical temperature of a superconductor in cuprate", was published in the May 13 issue Proceedings of the National Academy of Sciences.