When it comes to superconductivity, physicists usually require three separate strands of evidence to confirm the claim. First, a material must have zero resistance. Second, the material must show the Meisner effect by reflecting an external magnetic field. And finally, these effects must switch on at a specific critical temperature.
In most superconducting materials, the transition to zero resistance and the Meisner effect occur at the same critical temperature. But in recent years, some physicists have found some cuprates in which the transition to zero resistance occurs at a lower temperature than the Meisner effect.
So at low temperatures, the cuprate acts like a normal superconductor. As the temperature rises, it goes through a first transition and loses its zero resistance while maintaining the Meisner effect. Then as the temperature rises further, it goes through a second transition in which the Meisner effect disappears and the material becomes an ordinary conductor. In underdoped yttrium barium copper oxide (YBCO), the first transition occurs at 85K while the second at over 200K.
But since both effects are manifestations of superconductivity, how can this be?
Today, Vladimir Kresin at the Lawrence Berkeley National Laboratory and Stuart Wolf at the University of Virginia put forward a theory. They think that these cuprates consist of two components with different transition temperatures: the component with the higher transition temperature forms islands in a matrix with a lower transition temperature.
That explains why the material has two transition temperatures, they say. Below 85K, both components are superconductors. But as the temperature rises above 85K, the matrix becomes a conventional conductor introducing finite resistance. However, the island component maintains its superconductivity.
That’s why measurements on the bulk material show finite resistance but also the Meisner effect.
What’s interesting about the island component is that it must be a superconductor at temperatures above 200K, possibly as high as 250K. That’s room temperature.
That raises an obvious question: what’s the difference between the island component and the matrix component? Kresin and Wolf don’t know but they make a suggestion. Superconductors are extraordinarily sensitive to the mix of atoms they are composed of. Their idea is that the high temperature islands form where atomic isotopes subtly change the material properties.
Exactly how an isotope can do this isn’t clear. But Kresin and Wolf say that one experiment has shown that the substitution of O-18 for O-16 in another cuprate dramatically increases the second transition temperature.
That’s potentially exciting. In effect, these guys say they’ve discovered a room temperature superconductor, albeit one that works inside a lower temperature superconductor. Whether this materials can be isolated so that the effect appears in a standalone bulk material will be an important question to investigate.
However, these guys will have to a do a little more work to convince everyone else. The field of superconductivity is littered with reports of high temperature superconductors that have later turned out to be difficult or impossible to reproduce. Researchers even have a name for these findings: USOs–unidentified superconducting objects.
A few years ago, we looked at Kresin’s claim to have found aluminium nanoclusters that superconduct at 200K. We’ve seen nothing since.
Kresin and Wolf say they plan to do further investigations. If we hear more from them or others who have repeated their work, we’ll know there’s something to these claims. If not, we’ll have to chalk it up as another USO.
Ref: arxiv.org/abs/1109.0341: Inhomogeneous Superconducting State and Intrinsic Tc : Near Room Temperature Superconductivity in the Cuprates