1Max-Planck-Institut fur Chemie,
Hahn-Meitner-Weg 1, 55128 Mainz,
Germany.
2Institut fur Anorganische Chemie
und Analytische Chemie, Johannes
Gutenberg-Universtat Mainz,
Staudingerweg 9, 55099 Mainz,
Germany.
Nature 525 (3 September 2015)
73-76.

RESEARCH NEWS & VIEWS

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SUPER C O N DU CT I V I T Y

Extraordinarily
conventional
Attitudes to high-temperature superconductivity have swung from disbelief
to a conviction that it occurs only ‘unconventionally’. But conventional
superconductivity is now reported at record high temperatures. See Letter p.73
IGOR I. MAZINIn 1911, the physicist Heike Kamelingh
Onnes was puzzled to observe 1 thatmercury became an ideal conductor below
4.2 kelvin. How could all the electrons in a
metal cooperate so as to carry electric current without resistance? Common wisdom© 2015 Macmillan Publishers Limited. All rights reserved

dictates that there is nothing ideal in thisworld. Nobody’s perfect! No crystals without
defects can be created, no wheel can roll without friction, no glass can be 100% transparent. Yet subsequent experiments confirmedthat the resistivity of many metals suddenly
drops to exactly zero at a sufficiently lowtemperature. Chaotic motion introduced byNEWS & VIEWS RESEARCH
heat destroys electronic cooperation, so formany years it was believed that this phenomenon, now known as superconductivity, islimited to ultra-low temperatures. But on
page 73 of this issue, Drozdov and colleagues2
report a superconductor that works at about
200 K — a temperature that actually exists on
Earth’s surface.
For decades, physicists were in the dark
about the origin of superconductivity. The
discovery in 1938 of another cooperative
phenomenon3 — superfluidity in helium at
2 K — offered the first clue4. This complete
lack of viscosity turned out to be a direct consequence of quantum mechanics. All quantum
particles are characterized by a ‘spin’ number;
if this is an integer (as for helium-4 atoms),
the particles can combine into a single object
so large that it cannot be disturbed by suchnuisances as friction or viscosity. This effectis called Bose–Einstein condensation.
But electrons, which conduct electricity,
have a spin of ±½, and so are not subject toBose–Einstein condensation. In 1957, John
Bardeen, Leon Cooper and Bob Schrieffer
therefore proposed that the interaction ofelectrons with metal ions creates an attractive interaction that forces the electrons
to combine in pairs5. These ‘Cooper pairs’
have a net spin of zero, and can form a Bose–
Einstein condensate. This theory also allows
the transition temperature, Tc, below which
superconductivity occurs for a given metal,
to be estimated.
Most elemental superconductors had beendiscovered by 1957, and all had Tc values ofless than 10 K. For the next two decades, scientists worked with various compounds, butfailed to increase Tc by even a factor of three.
Not surprisingly, most physicists began tobelieve that nature imposes a fundamental,
but as-yet unexplained, Tc limit of 25–30 K.
The problem was succinctly formulated by
the materials scientist Bernd Matthias in 1964:“Why has it been relatively easy, within the last
10 years, to reach transition temperatures of
17 to 18 K in many intermetallic systems andimpossible to raise this value even by as little
as half a degree?”6 Eight years later, Marvin
Cohen and Phillip Anderson pointed out that
if electrons interact too strongly with the ionsin a metal, they can break the lattice apart7.
On this basis, they estimated that the highest
Tc for conventional superconductors (thosedriven by the electron–ion interaction) isapproximately 30 K.
Although the argument seemed convincing,some physicists remained hesitant. In theearly 1970s, Vitaly Ginzburg — one of the top
theorists of the time — organized a group inMoscow to explore routes to high-temperature
superconductivity. One of his team’s principal
results was that a key assumption by Cohenand Anderson was flawed, and that Tc could,
in principle, be arbitrarily high even in aconventional superconductor8.

Another prominent physicist who did
not subscribe to the idea of a universal limit
was Neil Ashcroft. In the late 1960s, he9 andGinzburg10 proposed that, if hydrogen could
become metallic, the energy of its ionic vibrations would be so high that even a moderately
strong electron–ion coupling could result in arather high Tc. Unfortunately, metallization ofhydrogen has proved to be extremely difficult.
It was then pointed out that hydrogen-richcompounds might be better targets11,12, but it
is only now that this idea has been realized, asreported by Drozdov and colleagues.
In the meantime, three major breakthroughs
occurred in superconductivity. First, cuprate
superconductors were discovered13 in 1986;
within seven years, the Tc for these compounds
reached 133 K (ref. 14). These have beenrecognized as ‘unconventional’ superconductors, driven by interactions among electrons,
rather than by electron–ion interactions.
The second was the discovery15, in 2001, ofmagnesium diboride — a conventional superconductor whose Tc is 40 K. This relatively highnumber is due to the low mass of boron, andto the fact that strong electron–ion couplingis ensured becauset he conduc t ing
A previously
ele c t rons comeunknown material from the boron,predicted to be aand the boron ionshigh-temperature form a rigid sub­superconductor
lattice. The physics
has beenof superconductivity
experimentallyin magnesium
confirmed todiboride turned out
be one.
to be considerablymore complex thanfor other conventional superconductors knownat the time, but was understood within a yearof the original discovery. At last, theorists could
accurately calculate the critical temperature of arather complicated material. This encouraged
scientists to seek quantitative predictions for
new superconducting materials.
The third breakthrough was the discovery16of iron-based superconductors in 2008.
These materials seem to be unconventional
and, although of great interest, have neversurpassed the Tc of the cuprates.
Drozdov and co-workers report a fourth
breakthrough: superconductivity at approximately 200 K in a hydrogen-rich compound,sulfur hydride, at about 90 gigapascals — apressure hardly achievable just a few years ago.
Not only is this a 50% increase over the previous record for Tc, but the authors convincingly
argue that the observed superconductivity isconventional, vindicating the ideas of Ashcroft
and Ginzburg.
Moreover, this is the first time that apreviously unknown material predicted to bea high-temperature superconductor has been
experimentally confirmed to be one. A computational study17 of hydrogen-rich materials
under pressure had reported that the sulfur

hydride H3S would be a superconductor withTc in the range 190–200 K at 200 giga­pascals —very close to the now-reported experimentalvalue. Drozdov et al. studied H2S, but it seems
that at high pressure this decomposes intoelemental sulfur and hydrogen-rich H3S.
It is therefore highly likely that the super­
conducting material is H3S. More-accurate
calculations18–20 yielded a Tc value approximately 20% higher than in the earlier computational study17. There is some disagreement
about which small effects, not accounted forin standard computations, are responsible
for this overestimate, but it is amazing thattheorists quibble about a 20% inaccuracy infirst-principles calculations when even an orderof-magnitude estimation was considered
practically impossible only 40 years ago.
In 1796, the philosopher Wilhelm Hegel
introduced the concept of spiral progress: anintellectual proposition is superseded by itsnegation, but later the negation itself is negated;the original thesis is then reinstated, but at ahigher level of development. The generality ofthis concept can be philosophically disputed,
but Hegel’s idea seems to be confirmed by thefact that the holy grail of superconductors has
been discovered in the same group of materials as the first known superconductors, after atiresome quest along exotic routes. ■Igor I. Mazin is at the Center for
Computational Materials Science, Naval
Research Laboratory, Washington,
DC 20375-5320, USA.
e-mail: mazin@dave.nrl.navy.mil
1. Kamerlingh Onnes, H. Commun. Phys. Lab. Univ.
Leiden No. 120 (1911).
2. Drozdov, A. P., Eremets, M. I., Troyan, I. A.,
Ksenofontov, V. & Shylin, S. I. Nature 525, 73–76(2015).
3. Kapitza, P. Nature 141, 74 (1938).
4. London, F. Nature 141, 643–644 (1938).
5. Bardeen, J., Cooper, L. N. & Schrieffer, J. R. Phys.
Rev. 108, 1175 (1957).
6. Anderson, P. W. & Matthias, B. T. Science 144,373–381 (1964).
7. Cohen, M. L. & Anderson, P. W. in Superconductivity
in d- and f-Band Metals (ed. Douglass, D. H.) 17–27(American Inst. Physics, 1972).
8. Dolgov, O. V., Kirzhnits, D. A. & Maksimov, E. G. Rev.
Mod. Phys. 53, 81 (1981).
9. Ashcroft, N. W. Phys. Rev. Lett. 21, 1748 (1968).
10. Ginzburg, V. L. J. Stat. Phys. 1, 3–24 (1969).
11. Ginzburg, V. L. & Kirzhnits, D. A. (eds) HighTemperature Superconductivity Chapter 1(Consultants Bureau, 1982) [transl. from Russian].
12. Ashcroft, N. W. Phys. Rev. Lett. 92, 187002 (2004).
13. Bednorz, J. G. & Müller, K. A. Z. Physik B 64,
189–193 (1986).
14. Schilling, A., Cantoni, M., Guo, J. D. & Ott, H. R.
Nature 363, 56–58 (1993).
15. Nagamatsu, J., Nakagawa, N., Muranaka, T.,
Zenitani, Y. & Akimitsu, J. Nature 410, 63–64 (2001).
16. Kamihara, K. Y., Watanabe, T., Hirano, M. & Hosono, H.
J. Am. Chem. Soc. 130, 3296–3297 (2008).
17. Duan, D. et al. Sci. Rep. 4, 6968 (2014).
18. Errea, I. et al. Phys. Rev. Lett. 114, 157004 (2015).
19. Flores-Livas, J. A., Sanna, A. & Gross, E. K. U. Preprint
at http://arxiv.org/abs/1501.06336 (2015).
20. Akashi, R., Kawamura, M., Tsuneyuki, S., Nomura, Y.
& Arita, R. Phys. Rev. B 91, 224513 (2015).
This article was published online on 17 August 2015.

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