The Fermi surface, or the highest occupied state in the electronic structure, allows direct determination of the doping level. This…
The Fermi surface, or the highest occupied state in the electronic structure, allows direct determination of the doping level. This picture shows the Fermi surface of the highly overdoped, non-superconducting BSCCO where the holes were added into the material by exposure to ozone. Credit: Brookhaven National Laboratory
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 called OASIS at the U.S. Department of Energy’s Brookhaven National Laboratory, they have uncovered previously unavailable details of the “phase diagram” or one of the most commonly studied high-temperature superconductors. De 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’s 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 could in turn give insight into what causes superconductivity in the first place.”
Unlocking the secrets of superconductivity holds great promise in addressing energy challenges. Materialer som kan føre strøm over lange afstande med ingen tab ville revolutionere strømtransmission, eliminere behovet for køling computer-pakkede datacentre og føre til nye former for energilagring, for eksempel. De hitch is dat, tegenwoordig, meest bekende superconductors, even de “high-temperature” varieties, moeten zichzelf superheld houden om hun huidige dragende magie te verrichten. Dus, wetenschappers hebben geprobeerd te begrijpen de belangrijkste kenmerken die leiden tot superconductiviteit in deze materialen met het doel van het ontdekken of het creëren van nieuwe materialen die kunnen werken bij temperaturen die meer praktisch zijn voor deze dagelijkse applicaties.
Het Brookhaven-team was studeren een bekende 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. Når de analyserede den elektroniske strukturen af den pristine kløftede overflade, så de samlede tegn på superkonduktivitet ved en overgangstemperatur (Tc) eller 94 Kelvin (-1
79 grader Celsius) – den højeste temperatur ved hvilke superkonduktivitetssett for dette godt studerte materiale.
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 forskellige betingelser (som du kan kartlegge temperaturen og trykkoordinatene ved hvilken væskende vann fryser når det er kjølt, eller forandrer seg til damp når oppvarmet). In deze case, de variabelen die wetenschappers waren geïnteresseerd in was hoeveel vacatures, of “gaten” werden toegevoegd, of “doped” in het materiaal door de blootstelling aan ozon. Holes facilitates 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 holes result 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 explains. “Then, with more holes, the material becomes over-doped, and Tc goes down for this material, to 50K.
This phase diagram for BSCCO plots the temperature at Kelvin on the y axis at which superconductivity sets in as more and more charge vacancies, or “holes,” are doped into the material (horizontal, x axis). On the underdoped page of the “dome” (left), as more holes are added, the transition temperature increases to a maximum of 94K, but if more holes are added, the transition temperature drops off. The red dashed line represents previously assumed dependence of superconductivity “dome,” while the black line represents the correct dependence, obtained from the new data (black dots). Dit was de eerste keer dat wetenschappers in staat waren om zeer overdreven monsters te maken, waardoor ze de fase van het fasechema kunnen schaduwen in geel waar superconductiviteit verdwijnt. Sporing av forsvinden kan hjelpe dem med at forstå hvad der forårsaker supraledning til at forekomme i første omgang. Credit: Brookhaven National Laboratory
“Until this study, nothing past that point was known because we could not get crystals doped above that level. But our new data takes us to a point of doping 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 overdosed 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 “pseudo-log” in the electronic signature, and variations in particle spin and charge densities-disappear on the overdoped far side of the dome.
Brookhaven Lab physicists Tonica Valla and Ilya Drozdov in the OASIS laboratory at Brookhaven National Laboratory. Credit: Brookhaven National Laboratory
“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 may not be that unconventional, “he added. “Vi kan fortsatt ikke kjenne sin opprinnelse, men på denne side af fasetidet ser det ud til noget, som teorien kan håndtere lettere, og det giver dig en enklere måte å se på problemet å prøve å forstå hva som skjer. “
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I. K. Drozdov et al., Phase diagram of Bi2Sr2CaCu2O8 + δ revisited, Nature Communications (2018). DOI: 10.1038 / s41467-018-07686-w