Scientific American - USA (2019-10)

October 2019, ScientificAmerican.com 51

is “a sort of statistical limit,” meaning that such materials are

simply less likely to exist. Phonon-mediated pairing tends to be

stronger in wobblier atomic lattices (a perfectly rigid lattice

could not support conventional superconductivity, which

requires the lattice to pull toward an electron). Therefore, the

exceptionally robust pairing needed for high-temperature con-

ventional superconductivity seems to demand a special type of

crystal structure, analogous to the elaborate designs engineers

employ in modern bridges to keep them sturdy despite their flex-

ing with the wind.

So room-temperature superconductors, if they exist, are

undoubtedly rare. Yet hope springs from the immensity of the

searchable landscape: the approximately 100 stable elements in

the periodic table could yield 4,950 combinations of two,

161,700 of three, and so on. Factor in choices of stoichiometry

(the ratios of elements in a compound) and lattice structure,

and the possibilities are endless. So how do scientists find the

exceptional materials in that chemical haystack?

THE SUPERCONDUCTOR DREAM

one morning in November 2017, Somayazulu was driving to work

and racking his brain. The test to confirm LaH 10 ’s superconduc-

tivity was not going well. It required replacement of a metal gas-

ket in the diamond anvil cell with an insulating material to pre-

vent a short circuit during measurement of the resistance. But

for months the hydrogen gas had been leaking out of every

design the team tried. “Every day we’d come in and discuss, and

we’d try once more,” Somayazulu says. “It was very frustrating.”

Then, sitting in traffic on the D.C. Capital Beltway, he had an

idea: “Why don’t we use a source of hydrogen that is solid?”

Somayazulu thought that ammonia borane, a hydrogen-rich

substance he knew of from earlier research, just might release

hydrogen in the right way. After several months of refinement,

the design worked. He saw LaH 10 ’s resistance plummet at 265

kelvins. He quickly snapped a picture with his phone, and then

the team’s computer program crashed and the cell’s diamonds

disintegrated. The photograph was all that was left of their feat,

and it would be another six months before they could repeat it.

Somayazulu had spent nearly a quarter of a century trying to

compress hydrogen into a superconductor. This was a dream

Hemley had been chasing for decades, based on a prediction first

made by physicist Neil Ashcroft of Cornell University in 1968. It

could take as much as 10  million atmospheres of pressure to

achieve such a material, Ashcroft acknowledged in 1983, but he

theorized that a second element added to hydrogen might reduce

that requirement by acting like a wedge to break up the H 2 mole-

cules that hydrogen is prone to form. Thus freed, the hydrogen

atoms could vibrate in ways conducive to high-temperature

superconductivity: the pliable bonds between them would pro-

mote strong phonon coupling between electrons, and their low

atomic mass would foster phonons that vibrated at an unusually

high frequency (and therefore high energy), which would attract

electrons in large numbers to the condensate.

For years after arriving from India in 1994 to work with

Hemley as a postdoctoral fellow at the Carnegie Institution,

Somayazulu dutifully crushed and heated myriad hydrogen

mixtures in various ways, finding plenty of interesting physics

but no superconductivity. “Here I am trying to dope hydrogen

systematically with all kinds of things,” he says. “I’m squeezing

Ion Electron

Cooper pair

Superconductor Primer

Within a superconductor, complex quantum-mechanical

effects allow electricity to flow without resistance. A theory

known as BCS (after its three inventors’ initials) describes

a basic picture of how it works, although physicists think

the details for many superconducting materials are more

complicated. The BCS process goes like this:

Illustration by Jen Christiansen

As a negatively

charged electron

travels through

a lattice of posi­

tively charged

ions, the ions

pull toward it,

scrunching up

the lattice.

The resulting

bunching of

positive charge

pulls another

electron toward

the first.

The two electrons

form a connection

that links them

into a united

entity, called

a Cooper pair.

A large number

of Cooper pairs

synchronize and

combine to

create a giant

wave, known as

a Bose­Einstein

condensate,

that is so large

it can pass

through the lat­

tice unimpeded.

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