By Josh Perry, Editor
[email protected]

The study of superconductivity is frequently reaching new milestones, as researchers from the U.S. Department of Energy (DOE) Brookhaven National Laboratory (Upton, N.Y.) used novel tools, called OASIS, to create a phase diagram of high-temperature superconductors.

Brookhaven physicist Tonica Valla in the OASIS laboratory at Brookhaven National Laboratory. (Brookhaven Lab)

Researchers mapped the data that signals the point when superconductivity vanishes, which, as a report from the lab indicated, can also give insight into the origins of superconductive properties.

The superconductor being studied was made of layers of bismuth-oxide, strontium-oxide, calcium, and copper-oxide (BSCCO). Scientists created pure bismuth-oxide surfaces from crystals of the material and saw that superconductivity was triggered at a temperature of 94 Kelvin (-179°C), which was the highest temperature recorded.

“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),” the article noted.

In this case, researchers were interested in the charge vacancies that exposure to ozone caused. The article explained, “Holes facilitate the flow of current by giving the charges (electrons) somewhere to go.”

Increasing the number of holes, increases the superconductivity of the material. The material can become “over-doped” with holes and that changed the transition temperature from 94 Kelvin down to 50 Kelvin.

“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,” the article continued. “They discovered some interesting characteristics in the previously unexplored ‘far side’ of the phase diagram.”

On the over-doped side of the diagram, things became simpler. Interesting variations that have been previously discovered, such as pseudogaps in the electronic signature, particle spin, and charge densities, no longer remained on the far side.

The article added, “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).”

The research was recently published in Nature Communications. The abstract stated:

“In cuprate superconductors, the doping of carriers into the parent Mott insulator induces superconductivity and various other phases whose characteristic temperatures are typically plotted versus the doping level p. In most materials, p cannot be determined from the chemical composition, but it is derived from the superconducting transition temperature, T_c, using the assumption that the T_c dependence on doping is universal.

“Here, we present angle-resolved photoemission studies of Bi₂Sr₂CaCu₂O_8+δ, cleaved and annealed in vacuum or in ozone to reduce or increase the doping from the initial value corresponding to T_c = 91 K. We show that p can be determined from the underlying Fermi surfaces and that in-situ annealing allows mapping of a wide doping regime, covering the superconducting dome and the non-superconducting phase on the overdoped side.

“Our results show a surprisingly smooth dependence of the inferred Fermi surface with doping. In the highly overdoped regime, the superconducting gap approaches the value of 2Δ₀ = (4 ± 1)k_BT_c.”