快猫短视频

SUPERCONDUCTIVITY is creating a buzz again with the announcement of a new
material that is said to have zero electrical resistance at room temperature.
The claim, from researchers in Croatia, comes just a few weeks after the
discovery that the simple chemical magnesium diboride superconducts at temperatures
up to almost twice those needed for other metallic superconductors to work
(快猫短视频, 3 March, p 6).

The Croatian scientists say that current will flow effortlessly through their
material, a mixture of lead carbonate and lead and silver oxides, at up to about
30 掳C. 鈥淭hese results are suggestive of a transition to a superconducting
state,鈥� says Georg Bednorz of the IBM Zurich Research Laboratory, who shared the
1987 Nobel physics prize for discovering cuprate superconductors. But because of
numerous false alarms in this field, researchers are treating the announcement
with caution, especially as no one has yet managed to reproduce the results.

Danijel Djurek, a physicist at A. Volta Applied Ceramics in Zagreb, Croatia,
claims that he discovered his superconducting ceramic mixture in the late 1980s.
But he was unable to pin down the structure and formula of the material, and his
research was interrupted by years of war, following Croatia鈥檚 split from
Yugoslavia. Now Djurek and his team say they have finally hit on a formula that
works reliably and reproducibly at room temperature.

Some telltale signs of superconductivity are easy to spot. For example, a
graph of resistance plotted against temperature shows a characteristic drop at
the temperature at which the material becomes superconducting. Physicists
usually require other evidence too, such as the ability to expel all magnetic
fields. Djurek鈥檚 material seems to do this too.

Archie Campbell, director of Cambridge University鈥檚 Interdisciplinary
Research Centre in Superconductivity, says the data clearly shows the hallmark
of a superconductor. 鈥淭his is not a small effect. There鈥檚 no room for
misinterpretation,鈥� he says, adding it鈥檚 either superconductivity or it鈥檚 a
mistake. Nevertheless, all the researchers contacted by 快猫短视频 were
extremely reluctant to start popping corks. 鈥淚 have some concerns which keep my
enthusiasm on a moderate level,鈥� says Bednorz.

The biggest question mark hangs over the failure of other groups to replicate
the results. Paul Chu, director of the Texas Center for Superconductivity at the
University of Houston, has been following Djurek鈥檚 work for some time. 鈥淲e
recently tried to use his new formula but failed to reproduce his results,鈥� says
Chu. 鈥淚 think we will try a little more. It鈥檚 too important to ignore.鈥� Djurek
has raised some doubts by not yet supplying other labs with samples prepared by
his team. He says he will have them ready in two or three weeks.

Robert Cava of Princeton University is highly sceptical, however. He points
out that dozens of groups around the world reproduced magnesium diboride鈥檚
superconductivity within a few weeks of the discovery being announced. The world
has had plenty of time to reproduce Djurek鈥檚 results but found nothing, Cava
says.