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.
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