Science News - November 6, 2010

Superconducting
milestones
Finding materials that exhibit
resistance-free electric
flow at increasingly higher
temperatures has been a
continuing goal of physicists
since superconductivity was
discovered in 1911.

Mercury, 1911

First superconductor found

LaOFeAs, 2008

First known iron-based superconductor

0 5 10 15 20 25 30 35 40 45 50 55 60 65

Niobium-tin, 1954 operates at temperatures that allow the use of liquid helium as a coolant Temperature in kelvins

LaBaCuO, 1986 First recognized copper oxide with superconducting properties

thought. They immediately started creating more iron mixes and comparing the cuprates with the iron-based family in search of a common explanation.

Cuprate and contrast

The silvery black pebble that pops from Paglione’s oven is a crystal that contains alternating sheets of atoms. Iron-arsenic layers are stacked on top of films of barium the way noodles and tomato sauce are layered in lasagna.

Not all iron-based superconductors combine arsenic with the iron to make the noodle layer. Some use phosphorus, selenium or tellurium instead. And not all have barium; some use other elements such as lithium or mixes of lanthanum and oxygen, for example. And some dispense with the sauce altogether.

But all iron-based superconductors share the layered structure. And it’s always the layer with the iron that does the electron shuttling. The other layers provide some structural support and keep unneeded electrons out of the way.

Cuprates are also layered, except that instead of containing iron, the cuprates’ superconducting layers are made of copper and oxygen. The copper and oxygen bond so that the layers lie flat, while iron-containing layers, say iron and arsenic, are a bit more three-dimensional, with arsenic atoms embracing iron atoms from above and below.

But a good lasagna requires more than
noodles and sauce—it also needs
cheese. Superconductors often add
another ingredient too. In a process
called doping, some atoms are swapped
for others—a bit of cobalt replaces
some iron in one of the iron-based
superconductors, for instance.

All about the magnetism Some scientists believe that the breakdown of a property called antiferromagnetism may play a big role in high-temperature superconductivity. in an antiferromagnetic material, the magnetic fields of individual atoms line up in an alternating way (left). Various iron-based compounds transition (right) from an antiferromagnetic to a superconducting state.