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More than 20 years have passed since physicists
discovered copper oxides with a high critical temperature
(Tc) for the onset of
superconductivity. The highest-performance ceramic
superconductors
have Tc > 77 K, the
temperature of liquid nitrogen, which
is a cheap and ubiquitous coolant in physics labs. The
underlying physical mechanism is still under debate,
because standard BCS theory (named for Nobel Laureates
John Bardeen, Leon Cooper, and John Robert Schrieffer)
cannot explain the microscopic phenomenon in terms of
electron pairs. Consequently interest in the cuprates
has declined. They are brittle materials; making
superconducting devices from them is hard because the
material cannot be fabricated into wires by any
mass-production process.
The Physics Hot Papers in this period show that
superconductivity research is now hotting up, thanks to the
unexpected discovery of a new class of iron-based
superconductors. Papers #2, #3, #7 and #9 capture the
tremendous interest stimulated by the recent discovery of
superconductivity at Tc = 26 K in
the iron-based oxypnictide
La(O1-xFx)FeAs (Y. Kamihari,
et al., J. Am. Chem. Soc., 130 [11]:
3296-7, 2008; currently #1 in the Chemistry Top Ten).
That research, by
Hideo Hosono and colleagues of the Tokyo Institute of
Technology, put high-temperature superconductivity back on
the agenda with a bang.
Paper #2 describes an experiment designed by Xian Hui Chen,
and conducted together with colleagues at the University of
Science and Technology, Hefei, China. They followed up on
the Japanese discovery paper by looking at
superconductivity in a related compound,
SmFeAsO1-x Fx, in which samarium is
substituted for lanthanum. They aimed to see how high they
could push Tc in
a layered rare-earth superconductor.
In doing so they broke the record for a
non-copper-oxide superconductor, by reaching
Tc = 43 K, comfortably above the
previous record of 39 K for magnesium diboride.
The Sm-doped material is intriguing: according to Chen, it
has Tc above that suggested by standard
BCS theory, which argues for the oxypnictides being
unconventional superconductors. Furthermore, the jump in
Tc from 26 K to 43 K just by
substituting Sm for La immediately suggested that further
research would produce higher Tc in
layered oxypnictides doped with F.
That’s where #3 takes us: in it Zhi-An Ren and
colleagues from Beijing, China, report
Tc = 55 K in the same F-doped compound.
In fact, related experiments by this group, in which they
also substituted Ce, Pr, and Nd, have shown that FeAs
superconductors constitute a new family with
Tc > 50 K. The high-citation rate of
#3 is partly driven by the comprehensive information it
gives on fabrication. The materials are grown using a
high-pressure technique similar to that used for turning
graphite to diamond.
Zhi-An Ren’s collaboration is also responsible for
#7, in which they point out that the compounds have a
simple structure of alternating FeAs and ReO layers (where
Re is a rare earth). Instead of doping with F to achieve
superconductivity, they created vacancies of oxygen atoms
in the lattice. That move creates more electron carriers,
which should be a more efficient approach to the
realization of superconductivity. And indeed, tuning the O
content leads to the occurrence of superconductivity in a
way that resembles the situation in cuprates. That’s
encouraging because the parallels between the two compounds
suggest that the arsenides with O vacancies rather than F
doping could be the more competitive choice for higher
Tc.
Newcomer #9 is a paper that neatly illustrates how research
on F-doped arsenides may contribute to fundamental physics.
The experiments described in this paper show how F doping
suppresses spin-density-wave (SDW) instabilities and leads
to superconductivity. SDW is a low-energy ordered state
that occurs at low temperatures. SDW inhibits the onset of
superconductivity.
Superconductivity is one of the most dramatic phenomena in
condensed matter physics. Part of the motivation for the
groups in China and Japan is the ultimate goal: the
realization of the phenomenon at room temperature. There
are plenty of physicists who will state informally that
room temperature operation is about as likely as cold
fusion, or hot fusion. But fast progress has energized
research. In 2008 there were at least seven international
symposia devoted to Fe-based superconductors, and those
events have no doubt propelled the citation rates. For
researchers it’s a matter of striking while the iron
is hot!
Dr. Simon Mitton is a Fellow of St. Edmund’s
College, Cambridge, U.K.