52 Scientific American, October 2019
Hydrogen atoms
Lanthanum atomHydrogen source
LanthanumGasketElectrical leadDiamondNot drawn to scale InsulatorDiamond Anvil Cell Lanthanum Hydride Structure
Extreme pressureSOURCE: “SYNTHESIS AND STABILITY OF LANTHANUM SUPERHYDRIDES,” BY ZACHARY M. GEBALLE ET AL.,
IN ANGEWANDTE CHEMIE INTERNATIONAL EDITION, VOL. 57, NO. 3; JANUARY 15, 2018 (lanthanum hydride structure)Illustration by Jen Christiansenit to higher and higher pressures, and nothing is happening,
and I’m kind of thinking, ‘Was Ashcroft wrong?’ ”
Ashcroft, in fact, was right, but it took the help of a new class
of “structure search” computer programs to prove it. The pro-
grams seek viable compounds by virtually moving atoms around
in search of a stable crystal structure, which, by the second law
of thermodynamics, is that with the lowest capacity to lose ener-
gy as heat. Some programs use an evolutionary search approach
that starts with a group of crystal structures, mashes them up,
selects the fittest of the offspring to breed, then repeats the pro-
cess until the best of the bunch is found. Scientists then apply
BCS to evaluate that structure’s potential for superconductivity
and to estimate its critical temperature.
In 2012 a group in China led by Yanming Ma used one such
program to predict, in line with Ashcroft’s ideas, that calcium
hydride (CaH 6 ) could be made at pressures created by diamond
anvil cells and would superconduct at a high temperature. Hem-
ley and his team were soon crushing calcium into hydrogen, and
they were not alone.
In 2014 a group led by Mikhail Eremets in Germany, following
up on another of Ma’s predictions—that hydrogen sulfide (H 2 S),
the noxious gas that rotten eggs emit, would superconduct at 80
kelvins under sufficient pressure—squeezed the smelly gas in a
diamond anvil cell and saw, to the team members’ surprise, that it
superconducted at 203 kelvins instead. Eremets had chanced on
another superconducting compound, H 3 S, which held the high-
temperature record before the synthesis of LaH 10.
Hemley’s quest had become a race. In 2017, with help from a
postdoc named Hanyu Liu from Ma’s group, he used a structure-
search algorithm to predict LaH 10 and gave his group the march-
ing orders that led to that compound’s synthesis. Eremets soon
made it, too; he confirmed the telltale resistivity drop, and, most
recently, put it through a more comprehensive battery of tests to
confirm its compatibility with BCS theory. It passed.
These discoveries combine elements of design with surprise.
LaH 10 , for example, grew out of Hemley’s suggestion that Liu focus
on compounds with the most hydrogen possible, to best approxi-
mate Ashcroft’s original idea. On the other hand, LaH 10 is believed
to derive its high-temperature performance in part from the vibra-
tional modes of its special clathrate structure, in which hydrogen
atoms enclose a lanthanum atom in a “cage”—a configuration that
theorists “would have never guessed,” says Eva Zurek, a chemist
who carries out structure searches at the University of Buffalo. But
whether by design or surprise, the new programs have made theo-
rists such as Ma and Zurek suddenly more relevant to the super-
conductor search. “I think experimentalists are taking us a lot
more seriously than in the past,” Zurek^ says.DESIGN PRINCIPLES
that theoriStS expedited the discovery of H 3 S and LaH 10 , con-
ventional superconductors to which BCS theory applies, is one
thing. What is more surprising is that they might do the same
for unconventional superconductors, for which physicists have
no working theory at all.
LaH 10 , in fact, was not the only big superconductivity story of- The other was the discovery of the phenomenon in twisted
bilayer graphene. Graphene is a single-atom-thick sheet of car-
bon atoms arranged in a hexagonal lattice. Twisted bilayer gra-
phene consists of two such sheets, one on top of the other, with
their lattices rotated by an angle. Despite its low critical temper-
ature of 1.7 kelvins, this material has uncommonly strong Coo-
per pair bonds. Its simple structure involving a single element
has inspired hope that it can be understood theoretically and
that it might elucidate unconventional superconductivity in
general. The discovery straddles the line between serendipity
and computer foresight—“It’s half and half,” says Pablo Jarillo-
Herrero, head of the group at the Massachusetts Institute of
Technology behind the finding. The material superconducts only
at a specific “magic” twist angle of 1.1 degrees, a value that first
popped out of a computer model. Yet although theorists correct-
ly predicted that this angle would produce a spike in electron–
electron interactions, they did not guess that it would lead to
superconductivity. That surprise was uncovered in the lab.
Still, the find highlights the potential of what Norman calls
design principles: calculable qualities that can help predict super-
conductivity even in the absence of a comprehensive theory. Mat-
thias’s first five rules were such principles, but exceptions to each
ultimately arose in work with unconventional superconductors.
Norman, however, pointed out in a 2016 paper that even uncon-
ventional superconductors of different classes display suggestive
similarities, including many features of their phase diagrams,
which are plots that show how their properties change with vari-
Lanthanum Hydride
Lanthanum hydride, or LaH 10 , the highest-
temperature superconductor known, can
function at a surprisingly warm 17 degrees
Fahrenheit and possibly warmer, albeit at
excruciatingly high pressure. Scientists created
LaH 10 in 2017, using a so-called diamond anvil
cell to compress hydrogen and lanthanum. The
resulting material contains a lattice of hydrogen
atoms enclosing a single lanthanum atom
(pink) in a cagelike structure, which seems to
be particularly con ducive to superconductivity.