published about iron-based superconductors since their discovery. During 2009 papers came at an average of
2. 5 per day. This flood of research has
revealed differences in how electrons
pair in the cuprates and in the iron-based compounds. But there are also
striking similarities in the families’ magnetic properties. All together, the findings leave physicists wondering whether
they have found a key to high-temperature superconductivity.
A string of super discoveries
The very first superconductor was found
in 1911, when Dutch physicist Heike
Kamerlingh Onnes discovered that
mercury lost its electrical resistance
when cooled to 4 kelvins. Over the next
50 years more superconductors were
uncovered, but all required temperatures below 25 kelvins.
In these old-school superconductors,
electrons travel through the material
in what are called Cooper pairs. A negatively charged electron passing through
a crystal lattice of positively charged ions
pulls nearby ions close, creating a region
of positive charge. This region attracts
another electron to come through, and it
pairs with the first. This pairing prevents
brian straughn, courtesy of J. paglione
After making iron-based compounds,
Johnpierre Paglione uses a refrigerator
of sorts (shown) to cool them down.
the electrons from bouncing around and
losing energy. But at “high” temperatures
(around 30 kelvins or above), the thinking went, heat energy would overwhelm
the Cooper pairs and break them apart.
Cuprates were the first high-temperature superconductors discovered. Physicists K. Alex Müller and J. Georg Bednorz
of the IBM Zürich Research Laboratory
in Switzerland found a brittle, ceramic
compound made of lanthanum, copper,
oxygen and barium that superconducted
at 35 kelvins, unprecedented at the time.
“The interesting thing about those
materials was that the mechanism of
superconductivity seemed new,” says
David Singh of Oak Ridge National Laboratory in Tennessee.
There was a flurry of excitement in the
late 1980s about cuprate superconductors. Scientists set about creating more
copper-oxide compounds that could
superconduct at high temperatures. But
after 20 years, scientists still couldn’t
agree on just how the cuprates worked.
“It was basically an unsolved prob-
lem,” Singh says. “People had ideas, but
there wasn’t a general consensus on
what were the right ideas.”
In the meantime, researchers looked
for other types of high-temperature
superconductors. A small breakthrough
came in 2001, when a well-known com-
pound called magnesium diboride was
discovered to superconduct at 40 kel-
vins. But it appeared to work through
traditional Cooper pairing and didn’t
shed much light on high-temperature
superconductivity more generally. After
years of little success, scientists began
thinking that the cuprates were some-
how a warm exception to the cold rule.
Then in 2008, Japanese scientists led
by Hideo Hosono of the Tokyo Institute
of Technology reported in the Journal
of the American Chemical Society that
they had found an iron-arsenic mix
that superconducted at 26 kelvins.
Before long, scientists found related
compounds working at temperatures up
to 55 kelvins.
The iron-based compounds could help
reveal how superconductivity worked at
relatively high temperatures, researchers