18
June 2024 ESRFnews
HIGH-PRESSURE SUPERCONDUCTIVITY
density of positive charge attracts another electron,
and the two electrons become connected as a so-called
Cooper pair. In this way, the electrons become immune
from lattice vibrations, as each electron can balance
any random kicks suffered by its partner. The result is
that they travel without resistance, and the ordinary
conductor becomes a “super” conductor.
Conventional superconductivity usually occurs
at very low temperatures. That is because at higher
temperatures, atomic lattices have enough energy to
overpower the distortions of positive charge required for
Cooper pairing. The maximum operating temperature
of conventional superconductors is not set in stone,
though, for if the atoms are less massive, it is easier for
the conducting electrons to attract them, and thereby
preserve the Cooper pairing when the lattice has more
energy. On that basis, a lattice made of solid metallic
hydrogen, the lightest element, theoretically ought to
superconduct easily at room temperature, but scientists
have been trying and failing to make that for decades
A turning point came in the late 2000s when
theorists Neil Ashcroft and Roald Hoffmann predicted
that hydrogen combined with other elements could
be a realistic alternative to metallic hydrogen Their
prediction was proved correct in 2015 when a group
led by Mikhail Eremets at the Max Planck Institute for
Chemistry in Mainz Germany synthesised hydrogen
sulfide H
3
S at a pressure of 90 GPa and claimed that it
turned superconducting at a record 203 K 83 C Then
came the synthesis of superhydrides starting with FeH
5
at the ESRF see figure 1 Another superhydride based
on lanthanum LaH
10
turned out to be very promising
Worst of all, its very nature interferes with the key tests
upon which claims of superconductivity can be made.
This is where the ESRF can help. Fed by the EBS
source, beamlines such as ID15B and ID27 are able
to extract crystallographic data at micron resolution,
so that the makeup of samples is clear, no matter how
heterogeneous they are. They can also do this repeatedly
at an array of temperatures and pressures, to expose how
complex changes in structure relate to the emergence
of superconductivity. Best of all, the ESRF has its
own specialist tools and expertise, so that even users
without them can get involved. “We want to have more
people working in this field,” says Gaston Garbarino,
the ESRF scientist in charge of ID15B. “And now we
have tools that are extremely powerful, to perform
full crystallographic analysis in all samples at extreme
conditions. We can solve the crystal structures and,
importantly, obtain results that are reproducible.”
Long journey
The history of superconductivity goes back to 1911
when the Dutch physicist Heike Kamerlingh Onnes
discovered that at just four degrees above absolute zero
the electrical resistance of mercury vanishes Over the
next few decades the same phenomenon was found
in several other metals but it was not until nearly half
a century after Kamerlingh Onness original discovery
that a trio of US physicists John Bardeen Leon Cooper
and John Robert Schrieffer figured out why it occurred
According to BCS theory which was named after
their initials an electron travelling through a conductor
attracts nearby positive atomic nuclei This higher
S C I E N C E 3 5 7 3 8 2
Fig. 1. In 2017, a group led by Paul Loubeyre at the CEA and the Université Paris-Saclay in France synthesised and studied FeH
5
, the first ever
superhydride, at the ESRF. Their XRD results allowed them to construct models of how increasing pressure allows more and more hydrogen to be
packed into an iron hydride’s structure, until above 130 GPa and the formation of FeH
5
, in which the hydrogen forms metallic “slabs” between quasi-
cubic FeH
3
units (Science 357 382). Though it is debated, and awaiting experimental investigation, some theoretical studies have suggested that
this buried metallic hydrogen within FeH
5
could make the material a superconductor.
67 86 130
pressure (GPa)
FeH
5
FeH
3
FeH
2
ɛʹ–FeH
3.5