19
June 2024 ESRFnews
HIGH-PRESSURE SUPERCONDUCTIVITY
In 2019, a group led by Russell Hemley at the University
of Illinois Chicago in the US claimed that it turned into
a superconductor at a pressure of around 200 GPa and a
temperature of 260 K (–13 °C).
The attainment of room-temperature superconduc-
tivity looked to be just a matter of time – and so it was,
apparently, in two extraordinary claims by a group led
by Ranga Dias at the University of Rochester in New
York, US. The first claim was for a compound made of
carbon, sulphur and hydrogen (CSH), which was said
to superconduct at a pressure of 267 GPa and a tem-
perature of about 287 K (14 °C), while the second was
for lutetium hydride doped with nitrogen, which was
said to work at just 1 GPa and 294 K (21 °C). However,
both papers were soon retracted due to concerns over
data integrity, and this year an internal investigation at
Rochester reportedly found Dias guilty of misconduct.
While the Dias case has been exceptional, his
claims are not the only ones to have come under close
scrutiny. The trouble is that the two major signatures of
superconductivity – plummets in electrical resistivity
and magnetic susceptibility – are both very difficult
to observe in high-pressure experiments. If it follows
a strong theoretical prediction, an experimental claim
is much more persuasive, but that relies on a positive
match between theoretical and experimental crystal
structures. In the past, due to limitations in beam
focusing, synchrotron X-ray diffraction in extreme
conditions has not had the resolution to discern
different structures within highly heterogeneous
samples.
A new brilliance
That has changed with the ESRFEBS Thanks to
the extremely low emittance and high brilliance of
the upgraded light source beamlines such as ID15B
and the newly refurbished ID27 are able to focus
Xray beams down to less than a micrometre making
it possible to record highquality diffraction patterns
from even the smallest individual crystals at extreme
pressures and temperatures The power of the single
crystal Xray diffraction SCXRD technique was
demonstrated earlier this year when researchers at the
University of Bayreuth came to the ESRF to study a
Using ID27, ID15B and
ID11, the very high flux and
small beam sizes provided
by the EBS allowed us to
identify and solve the
crystal structure of even
the tiniest crystallites
promising system, yttrium and hydrogen, at pressures
from 87 to 171 GPa, and discovered five previously
unknown phases (Sci. Adv. 10 eadl5416). According
to one of the team members, Dominique Laniel at the
University of Edinburgh in the UK, the new phases
were unanticipated by theory, and could very well not
have ever been discovered at all – had the researchers
not come to the ESRF. “Using ID27, ID15B and ID11,
the very high flux and small beam sizes allowed us to
identify and solve the crystal structure of even the
tiniest crystallites,” he says.
Such beam properties have other benefits too, Laniel
adds. When measuring electrical resistivity, a fine map
of the sample cavity can expose whether there is actually
a path from electrode to electrode comprised of the same
phase, or whether there are heterogeneities. Moreover,
the high flux opens up the unprecedented possibility – so
long as the electron count of the other elements is not too
high – of directly determining the position of hydrogen
atoms in the structural model, which is usually very
difficult with synchrotron data.
ESRF SCXRD will help to clarify the structures
of other complex hydride systems. Whatever the
structure of a sample, however, superconductivity must
still be established. Difficulties with the usual direct
measurements of electrical resistivity, usually noise,
can to some extent be assuaged with the use of indirect,
alternating-current techniques. But measurements
of magnetic susceptibility are inherently tough, as
the diamond anvil cells (DACs) used to generate
extreme pressures will always themselves have a
residual magnetic response Back in 2016 Eremets
and colleagues designed a novel way to circumvent
this problem in which they immersed a foil enriched
with the tin isotope
119
Sn in a sample of H
3
S before
placing the lot in a DAC Once at the correct pressure
and temperature the researchers subject the device to
synchrotron Mössbauer spectroscopy at the ESRFs
former ID18 beamline
Specific probe
Mössbauer spectroscopy is unique in that it can measure
the nuclear resonance from a specific isotope in
this case the
119
Sn while ignoring any other source
E S R F
A recent study
by the long-term
ESRF user Leonid
Dubrovinsky
(above) and
colleagues has
shown just how
complex high-
pressure samples
can be.