Scientists enter unexplored territory in superconductivity search

Scientists mapping out the quantum characteristics of superconductors–materials that conduct electricity with no energy loss–have entered a new regime.

Using newly connected tools named OASIS at the U.S. Department of Energy's Brookhaven National Laboratory, they've uncovered previously inaccessible details of the “phase diagram” of one of the most commonly studied “high-temperature” superconductors. The newly mapped data includes signals of what happens when superconductivity vanishes.

“In terms of superconductivity, this may sound bad, but if you study some phenomenon, it is always good to be able to approach it from its origin,” said Brookhaven physicist Tonica Valla, who led the study just published in the journal Nature Communications. “If you have a chance to see how superconductivity disappears, that in turn might give insight into what causes superconductivity in the first place.”

Unlocking the secrets of superconductivity holds great promise in addressing energy challenges. Materials able to carry current over long distances with no loss would revolutionize power transmission, eliminate the need for cooling computer-packed data centers, and lead to new forms of energy storage, for example.

The hitch is that, at present, most known superconductors, even the “high-temperature” varieties, must themselves be kept super cold to perform their current-carrying magic. So, scientists have been trying to understand the key characteristics that cause superconductivity in these materials with the goal of discovering or creating new materials that can operate at temperatures more practical for these everyday applications.

The Brookhaven team was studying a well-known high-temperature superconductor made of layers that include bismuth-oxide, strontium-oxide, calcium, and copper-oxide (abbreviated as BSCCO). Cleaving crystals of this material creates pristine bismuth-oxide surfaces.

When they analyzed the electronic structure of the pristine cleaved surface, they saw telltale signs of superconductivity at a transition temperature (Tc) of 94 Kelvin (-179 degrees Celsius)–the highest temperature at which superconductivity sets in for this well-studied material.

The team then heated samples in ozone (O3) and found that they could achieve high doping levels and explore previously unexplored portions of this material's phase diagram, which is a map-like graph showing how the material changes its properties at different temperatures under different conditions (similar to the way you can map out the temperature and pressure coordinates at which liquid water freezes when it is cooled, or changes to steam when heated).

In this case, the variable the scientists were interested in was how many charge vacancies, or “holes,” were added, or “doped” into the material by the exposure to ozone. Holes facilitate the flow of current by giving the charges (electrons) somewhere to go.

“For this material, if you start with the crystal of 'parent' compound, which is an insulator (meaning no conductivity), the introduction of holes results in superconductivity,” Valla said.

As more holes are added, the superconductivity gets stronger and at higher temperatures up to a maximum at 94 Kelvin, he explained. “Then, with more holes, the material becomes 'over-doped,' and Tc goes down–for this material, to 50 K.

“Until this study, nothing past that point was known because we couldn't get crystals doped above that level. But our new data takes us to a point of doping way beyond the previous limit, to a point where Tc is not measurable.”

Said Valla, “That means we can now explore the entire dome-shaped curve of superconductivity in this material, which is something that nobody has been able to do before.”

The team created samples heated in a vacuum (to produce underdoped material) and in ozone (to make overdoped samples) and plotted points along the entire superconducting dome. They discovered some interesting characteristics in the previously unexplored “far side” of the phase diagram.

“What we saw is that things become much simpler,” Valla said. Some of the quirkier characteristics that exist on the well-explored side of the map and complicate scientists' understanding of high-temperature superconductivity–things like a “pseudogap” in the electronic signature, and variations in particle spin and charge densities–disappear on the overdoped far side of the dome.

“This side of the phase diagram is somewhat like what we expect to see in more conventional superconductivity,” Valla said, referring to the oldest known metal-based superconductors.

“When superconductivity is free of these other things that complicate the picture, then what is left is superconductivity that perhaps is not that unconventional,” he added. “We still might not know its origin, but on this side of the phase diagram, it looks like something that theory can handle more easily, and it gives you a simpler way of looking at the problem to try to understand what is going on.”

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SIDEBAR: Combination of Uniquely Connected Tools

The tools scientists used in this study are part of a suite of three that Brookhaven Lab has built named OASIS to explore materials such as high-temperature superconductors. The idea is to connect the tools with ultra-high vacuum sample-transfer lines so scientists can create and study samples using multiple techniques without ever exposing the experimental materials to the atmosphere (and all its potentially “contaminating” substances, including oxygen). OASIS is a tool that connects sample preparation capabilities of oxide molecular beam epitaxy (OMBE) synthesis with electronic structure characterization tools: angle resolved photoemission spectroscopy (ARPES) and spectroscopic imaging-scanning tunneling microscopy (SI-STM).

In this case, the scientists used ARPES to examine the samples' electronic structure. ARPES uses light to measure “electronic excitations” in the sample. These measurements provide a sort of electronic fingerprint that describes the energy and movement of electrons and how they interact with other types of excitations–say, distortions or vibrations in the crystal lattice, variations in temperature, or imperfections or impurities.

After studying pristine samples, the scientists transported them via vacuum tube to an OMBE machine where they could anneal (heat) the crystals under a steady stream of ozone.

The connected tools allow the scientists to transfer samples back and forth to study the material both before and after heating in both a vacuum and ozone to create both the underdoped and overdoped samples needed to map out the phase diagram.

In this paper, the spectroscopic imaging-scanning tunneling microscope (SI-STM) connected to the previously mentioned ARPES and OMBE modules was not employed. A complementary SI-STM study of the BSCCO samples is currently ongoing.

Brookhaven National Laboratory is supported by the Office of Science of the U.S. Department of Energy. The Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.

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One of ten national laboratories overseen and primarily funded by the Office of Science of the U.S. Department of Energy (DOE), Brookhaven National Laboratory conducts research in the physical, biomedical, and environmental sciences, as well as in energy technologies and national security. Brookhaven Lab also builds and operates major scientific facilities available to university, industry and government researchers. Brookhaven is operated and managed for DOE's Office of Science by Brookhaven Science Associates, a limited-liability company founded by the Research Foundation for the State University of New York on behalf of Stony Brook University, the largest academic user of Laboratory facilities, and Battelle, a nonprofit applied science and technology organization.

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