Solved? Cracking the code of metallic hydrogen

[Garrison L. Hilliard]

A 40-year quest to create a legendary superconductor may be over, say
Harvard researchers.

Charlie Wood
Staff | @walkingthedot

JANUARY 27, 2017 —With just one proton and one electron, you’d think
there isn’t much to learn about the simplest atom in the universe. Yet
hydrogen is full of secrets, and a Harvard team says they’ve cracked
one more.

Scientists have been chasing hydrogen’s elusive metallic form for over
80 years, foiled by the confoundingly high pressures required to
create it. Now, researchers have caught the tell-tale glint of metal
in their apparatus, according to a paper published Thursday in
Science. Experts consider this novel state of hydrogen a “holy grail”
of solid state physics for its potentially world-changing
applications, from big bang-for-your-buck rocket fuel to ideal
electric wires.

It starts with the universe's most common atom. “We all love
hydrogen,” physicist Gilbert Collins of Lawrence Livermore National
Laboratory, who was not involved in the research, told Science News in
August. “It has the essence of being simple, so that we think we can
calculate something and understand it, while at the same time it has
such a devious nature that it’s perhaps the least understandable
material there is.”

In any environment humans could survive, hydrogen exists as a gas. But
just as water can transform into ice at low temperatures, hydrogen
gas, like most substances, can become liquid or even solid if you cool
it or squeeze it enough. Physicists call these transformations “state
changes,” and they go far beyond the solid, liquid, and gas phases
many study in school. Even good old water has been shown to have at
least a dozen forms beyond the familiar three.

The key to the alchemy of hydrogen is pressure. By definition, metals
share electrons that can flow freely throughout the substance, which
is why metal spoons get hot while wooden spoons don’t. This property
also makes them good at carrying electricity, as well as heat. To get
hydrogen to perform this trick, scientists had to squeeze the atoms so
hard that electrons got knocked loose from their original atoms,
becoming free to move around.

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But achieving such high pressures isn’t easy. Early estimates
predicted that hydrogen would become metal at 250,000 times the
atmospheric pressure of sea level, but as scientists devised ever
craftier experiments, the goal kept receding just out of reach.

"There have been dozens of theoretical papers, and they all have
different critical pressures for when it becomes metallic," paper
co-author Isaac Silvera told Live Science.

It's a question that can only be answered experimentally, and
scientists around the world have been racing to create the exotic form
of hydrogen in the lab.

“If you look at the literature for the last 30 years,” Eugene
Gregoryanz, a physicist at the University of Edinburgh, previously
told Science News, “I think every five years there is a claim that we
finally metallized hydrogen.” Skepticism surrounding this latest
announcement is high, too. At least five experts already told Nature
they don’t believe the claim.

But even mistaken claims move the field forward. In the last 40 years,
physicists have learned of at least four forms of solid hydrogen, one
as recently as 2011, and a 2015 experiment led to a record-breaking
new superconductor.

This time, however, Silvera thinks his team has really done it. "No
one has ever encountered metallic hydrogen because it's never existed
on Earth before," he told Live Science.

"It was really exciting," he said in a press release. "[Post doctoral
fellow] Ranga [Dias] was running the experiment, and we thought we
might get there, but when he called me and said, 'The sample is
shining,' I went running down there, and it was metallic hydrogen."

The magic number seems to be almost 5 million times atmospheric
pressure, 20 times early estimates. Far surpassing the pressure at the
center of Earth’s core, you’d have to take a deep dive into Jupiter,
which some scientists believe harbors oceans of liquid metallic
hydrogen, to experience such a harsh environment.

To achieve such conditions on Earth, Silva’s team took solid hydrogen
at minus 452 degrees Fahrenheit (about 20 degrees below its freezing
point, and about 8 degrees above absolute zero) and crushed it between
two diamond anvils. Keeping even the hardest substance on Earth from
shattering at millions of times the atmospheric pressure is devilishly
difficult, so the team resorted to a number of tricks, such as
applying a special coating to keep the hydrogen from seeping into the
diamond, and polishing the surface with a chemical process rather than
the typical diamond powder, to avoid leaving atom-sized divots in the
surface.

"We designed the system so that all the things that can lead to the
breaking of a diamond were not there," Silvera told Live Science.

The substance currently awaits further testing in the diamond vice
that created it, but one of the more tantalizing predictions will be
tested when the team removes the pressure. Previous work suggests that
metallic hydrogen could survive at normal temperatures and pressures,
which would have immense consequences for the fields of electronics,
magnetics, space propulsion, and more.

"One prediction that's very important is metallic hydrogen is
predicted to be meta-stable," Silvera said in a press release. "That
means if you take the pressure off, it will stay metallic, similar to
the way diamonds form from graphite under intense heat and pressure,
but remain a diamond when that pressure and heat is removed."

If true, it means the metal could serve as a superconductor, a
material that carries electricity with near-perfect efficiency. Found
in a variety of advanced electronics including Magnetic Resonance
Imaging (MRI) machines and futuristic Maglev trains, superconductors
have been limited in their utility by the requirement that they
operate only in very cold conditions.

The current record holder works in environments as warm as -94 degrees
Fahrenheit, which is excellent news for residents of Antarctica.

For the rest of us, a room temperature superconductor could mean an
end to losing the 5 to 10 percent of energy wasted by resistance in
power lines each year. That might sound low, but could be enough to
offset all the gasoline the US burns in a year, according to the
collaborative journalism initiative Inside Energy.

Another potential application of the new material is rocket fuel,
because its concentrated energy could pack more than three times the
punch of standard propellants.

Metallic hydrogen “would easily allow you to explore the outer
planets,” Silvera said. “We would be able to put rockets into orbit
with only one stage, versus two, and could send up larger payloads, so
it could be very important.”

But for now, it’s back to the lab to test the sample’s stability and
conductivity. After more than four decades of work on metallic
hydrogen, Silva said he’s thrilled to see the results.

http://www.csmonitor.com/Science/2017/0127/Solved-Cracking-the-code-of-metallic-hydrogen