In the August issue of Physics Today, there is an article entitled “Iron-based superconductors” by the magazine’s senior editor, Charles Day. The magazine summarizes it this way: “For 22 years ceramic oxides of copper seemed to offer the only way to reach high-temperature superconductivity. Now, a new and unexpected route is being charted: through semimetal compounds of iron.”
It makes me wonder whether we are about to see a repeat of the mania of the late 1980s with the discovery of “high-temperature” superconductivity in a family of cuprate ceramics with a particular crystal structure that gave them the designation of perovskites.
The article begins with this paragraph:
Iron, the archetypal ferromagnet, is not supposed to be compatible with superconductivity. Iron’s locally polarized spins, all pointing in the same direction, create a magnetic field that would wring apart any Cooper pairs that tried to form. It therefore came as a surprise when, in February of last year, Hideo Hosono of the Tokyo Institute of Technology published the discovery of a superconductor that contains iron:1 fluorine-doped LaFeAsO (see PHYSICS TODAY, May 2008, page 11).
After another paragraph discussing the physics of the new materials, Day notes that a flurry of publishing activity has begun.
The new superconductors are captivating theorists and experimenters alike. Since Hosono’s discovery, preprints about them have appeared on the arXiv server at a near-steady rate of 2.5 a day. Those papers document a remarkable explosion of knowledge. Within just two months, physicists, mostly in China, had substituted other rare-earth elements for lanthanum, found optimal electron and hole dopants, and doubled Tc to 55 K. Within six months, LaFeAsO was joined by three other families of Fe-based superconductors that share the first family’s iron-containing layers but with different, or no, interlayers.
Nearly a quarter-century has passed since 1986, when J. Georg Bednorz and Alexander Mueller (Sorry, I don’t know how to get a u-umlaut for Mu(e)ller) discovered an anomalously high superconducting critical temperature in a perovskite, 35K. The next year, they had won the Nobel Prize for Physics, and the Tc value had climbed past liquid nitrogen temperatures (77K) on its way to about 125K.
Unfortunately, the new high-Tc superconductors are difficult to work with, so their applications are somewhat limited. But they are finding a few applications. Equally unfortunate is the difficulty of developing a full theoretical understanding of the mechanism of their superconductivity. No one has done for the cuprates what John Bardeen, Leon Cooper, and Robert Schrieffer did in 1957, describing the Cooper-pair mechanism behind superconductivity in metals, which was first noted by Heike Kamerlingh-Onnes in 1911.
Who is to say how far materials scientists will be able to push Tc with the new iron-based materials? No one is uttering the words “room-temperature superconductivity” yet. That remains “the holy grail” of the field.
But perhaps this discovery will provide clues for theoreticians as well as materials scientists. Perhaps there will be a new BCS theory and a surge to a new record value of Tc.
Perhaps Hosono and some clever theoreticians will join soon the century-long parade of Nobel laureates in superconductivity: Kamerlingh-Onnes (1913); Bardeen, Cooper, and Schrieffer (1972); and Bednorz and Mueller (1987).
Charles Day concludes his article with this assessment
Whether or not the Fe-based superconductors prove technologically useful, they have certainly rejuvenated the field of high-Tc superconductivity. Funding agencies in Germany and Japan have inaugurated new streams of money for research into the new superconductors. The excitement may not be as high as when the cuprates made their debut in 1986, but progress has been faster.
So stay tuned! This is a field that has given us a century’s worth of surprises.
Fred Bortz
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