New theoretical work is binding the strands of superconductivity together
and it appears plausible that we may finally master this field of physics in
the same way we finally mastered laser technology.
At least everyone is now in a fever pitch generating new results and
making real discernable progress. For
far too long work in this area has been tortuous. I literally checked it out every few years. This is the first time folks are really
excited.
All of which means that we should soon be able to produce an advanced
skin for making a magnetic field exclusion vessel or MFEV also known better as
a UFO of the flying saucer type.
05.08.14 |
The microscopic structure of high-temperature superconductors has
long puzzled scientists seeking to harness their virtually limitless
technological potential. Now at last researchers have deciphered the cryptic
structure of one class of the superconductors, providing a basis for theories
about how they manage to transport electricity with perfect efficiency when
cooled, and how scientists might raise their operating temperature closer
to the climes of everyday life.
This goal, if realized, could
make an array of fantastical-sounding technologies commercially viable, from
power grids that never lose energy and cheap water purification systems to magnetically levitating vehicles. Scientists believe
room-temperature superconductivity would have an impact on a par with that of
the laser, a 1960 invention that now plays an important role in an estimated $7.5 trillion in economic activity.
“In the same way that a laser
is a hell of a lot more powerful than a light bulb, room-temperature
superconductivity would completely change how you transport electricity and
enable new ways of using electricity,” said Louis Taillefer, a professor of physics at the University of Sherbrooke in
Quebec.
Materials that superconduct under much warmer conditions than
their ultra-cooled predecessors were discovered in 1986, winning IBM
researchers Georg Bednorz and K. Alex Müller the Nobel Prize in physics soon
after. But 28 years later, these “high-temperature” superconductors still fall
short of room temperature by more than 100 degrees Celsius. The materials’
complexity has so far frustrated the dream of dialing up their operating
temperatures. Researchers say the new findings are firmly setting them on the
right track.
“If you want to cure a disease,
first you have to discover that microbes exist. This is like discovering which
microbes exist,” said J. C. Seamus Davis, a professor of physics at Cornell University and St. Andrews
University and director of the Department of Energy’s Center for Emergent
Superconductivity at Brookhaven National Laboratory.
The “microbes” in this case are
ripples of electrons inside the superconductors that are called charge density
waves. The fine-grained structure of the waves, reported in two new papers by
independent groups of researchers, suggests that they may be driven by the same
force as superconductivity. Davis and his colleagues directly visualized
the waves in a study posted online in April,
corroborating indirect evidence reported in February by a
team led by Riccardo Comin, a postdoctoral fellow at the University of Toronto.
“It’s a beautiful paper,”
said Dirk Morr, a professor of physics at the
University of Illinois, Chicago, speaking of the work of Davis and his
colleagues. “One can really trust this result and build our theories from it.”
Subir Sachdev, a professor of physics at Harvard University who helped devise
Davis’ study, correctly predicted the form of the charge density waves in a paper last year, which detailed
a possible mechanism behind both the waves and high-temperature
superconductivity. Though further tests are needed, Sachdev’s theory is
garnering support from many experts, who say it succinctly captures key
features of the materials.
Taken together, the various
findings are at last starting to build a comprehensive picture of the physics
behind high-temperature superconductivity. “This is the first time I feel like
we’re making real progress,” said Andrea Damascelli, a professor of physics at the
University of British Columbia who led two recent studies on charge density
waves. “A lot of different observations which have been made over decades
did not make sense with each other, and now they do.”
The rate of progress in recent months has been “almost
overwhelming for us,” Comin said. With better experimental tools at their
disposal, he and other researchers described rushing to publish one interesting
new result after another as fascinating papers by their competitors piled up on
their desks.
“It’s been a roller coaster ride,” said Davis. “It’s been like 24
hours a day for weeks.”
The Face of the Enemy
High-temperature superconductivity
seems like a miracle of quantum mechanics, one that could be harnessed to great
effect if only it could be understood.
The property is exhibited primarily by cuprates, brittle ceramic
materials composed of two-dimensional sheets of copper and oxygen separated by
more complicated layers of atoms. When cuprates are cooled below a certain
temperature, electrons in the
copper-oxygen sheets suddenly overcome their mutual repulsion and pair up. With
their powers combined, they behave like a different type of particle
altogether, a boson, which has the unique ability to join with other bosons
into a coherent swarm that moves as one. This bosonic swarm perfectly
conducts electricity. A current flowing through a loop of cuprate wire will
persist forever — or as long as the liquid-nitrogen fridge stays on.
“The biggest question in the field is, what force binds the
electrons together?” Taillefer said. “Because if you can understand the force,
you can strengthen the force.”
In “conventional”
superconductivity, the kind exhibited by many metals when they are cooled near
absolute zero (zero degrees on the Kelvin scale, or minus 273.15 degrees
Celsius), electron pairing is caused by gentle pressure waves that breeze
through the metals. When an electron gets swept along by one of these waves,
another follows in its wake, attracted by the positively charged metal atoms
that shift toward the passing electron. But this light breeze cannot possibly
explain pairing in cuprates, which survives at up to 160 kelvins (minus 113 C).
Many competing forces seem to influence the electrons simultaneously, and the
force that binds them together over such a broad temperature range must be
strong enough to overcome others that strive to keep them apart. The devil is
in disentangling the forces. In the words of Pegor Aynajian, an assistant professor of
physics at Binghamton University in New York, “It feels like we’re in a battlefield
and we don’t know who’s our ally and who’s our enemy.”
The first sign of what looks
increasingly like the enemy — charge density waves, also known as “charge
order” — came in 2002. Using a new kind of microscope that could map currents
on the surface of cuprates with nanometer resolution, Davis, then a professor
at the University of California, Berkeley, and Jennifer Hoffman, his graduate student at the time, discovered a minute pattern of denser and less-dense ripples of electrons that appeared
wherever they blasted the cuprate with a powerful magnetic field, an effect
that suppressed superconductivity. Soon, other labs reported more actions that
both killed superconductivity and produced the waves, such as raising the
temperature or lowering the cuprates’ oxygen concentration.
“You start to build this picture in which charge density waves are
lurking, waiting to take over when anything unfriendly to superconductivity
happens,” said Hoffman, who is now an associate professor at Harvard.
It seemed possible that if the force shaping electrons into charge
density waves could be suppressed, its rival, the force that forms superconducting
pairs, would flourish. But some researchers argued that the ripples of
electrons were merely a surface anomaly and irrelevant to superconductivity.
The community remained divided
until 2012, when two groups using a technique called resonant X-ray scattering
managed to detect charge density waves deep inside cuprates, cementing the
importance of the waves. As the groups published their findings in Science and Nature Physics, two new collaborations
formed, one led by Damascelli and the other by Ali Yazdani of Princeton University, with plans to characterize the
waves even more thoroughly. Finishing in a dead heat, the rival groups’ independent studies appeared together in Science in January 2014. They
confirmed that charge density waves are a ubiquitous phenomenon in cuprates and
that they strenuously oppose superconductivity, prevailing as the temperature
rises.
“Now we know this superconducting state has to fight for survival
against this other state,” said Taillefer. “I don’t know how much that charge
order hurts you. But boy, it’s time to find out.”
A D-Wave Pattern
To defeat the enemy on the superconductor battleground, scientists
first needed to understand it. That required a closer look at the underlying
structure of charge density waves. How do electrons, which are confined to the
orbits of atoms, give rise to the waves that ripple through cuprates’
copper-oxygen layers?
Davis and his Cornell group have been steadily improving their
microscope’s resolution over the years, and in 2007 they managed to detect
variations in the distribution of electrons within cuprates’ smallest nooks and
crannies: the “unit cells” that tile the materials’ copper-oxygen layers. Each
cell consists of a central copper atom bonded to oxygen atoms at its northern
and eastern edges. The scientists discovered that electrons are more likely to
be found along the northern bond in some unit cells and the eastern bond in
others. Davis suspected that this uneven distribution of electrons inside the
cells, a form known as “d-wave,” was the root source of the charge density
waves that appear to undulate across multiple unit cells. “But we just couldn’t
close the deal,” he said.
Meanwhile at Harvard, Sachdev also wondered whether the d-wave
arrangement of electrons observed in Davis’ 2007 work was the true microscopic
structure of the charge density waves. On a Saturday afternoon in March of this
year, Sachdev emailed Davis asking if he had been able to infer anything about
the waves from his electron distribution data. Davis replied that he had long
suspected that the two phenomena were connected, but that he couldn’t come up
with the right algorithm for gleaning one from the other. Within an hour,
Sachdev devised a procedure that he thought would do the trick and sent it
over. Sure enough, by applying Sachdev’s algorithm to a new round of data,
Davis and his group mapped out the structure of the charge density waves,
showing that the d-wave distribution of electrons was, indeed, their source.
“The paper establishes that the two patterns are the same,”
Sachdev said. “It just works beautifully.”
The results fully confirmed the earlier report by Damascelli,
Comin and their co-authors, which used X-ray data to reveal the same d-wave
form of the charge density waves. Although Damascelli’s group reached the
milestone first, Davis says his team went further. “Basically they published
indirect evidence for the same state as we have visualized directly,” he said.
The waves’ structure is particularly suggestive, researchers say,
because superconducting pairs of electrons also have a d-wave configuration.
It’s as if both arrangements of electrons were cast from the same mold. “Until
a few months ago my thought was, OK, you have charge density waves, who cares?
What’s the relevance to the high-temperature superconductivity?” Damascelli
said. “This tells me these phenomena feed off the same interaction.”
Conjoined Twins
Many theorists believe both phenomena are caused by a quantum
mechanical effect called antiferromagnetism, a tendency in some materials for
neighboring electrons to spin in opposite directions. The effect sets up a
chessboard pattern of electrons spinning upward and downward. Just as squares
along diagonals on a chessboard have the same color, electrons positioned along
45-degree angles spin the same way.
Antiferromagnetism has long been considered one of the likeliest
agents to be responsible for high-temperature superconductivity. Proponents of
the idea argue that the force coupling electrons is essentially an attraction
between oppositely spinning neighbors. This explains why the electron pairs
always form along the cardinal directions in the crystal lattice but never
along the diagonals — another d-wave arrangement. This well-known d-wave nature
of superconductivity is slightly different from that of charge density waves.
But in a theory that Sachdev and his collaborators have developed, “we find
that the two d-waves are indeed linked to each other.”
In 2010, Sachdev and his
student Max Metlitski showed mathematically that antiferromagnetism
could cause pairing not only between electrons, but also between electrons and
“holes,” or places in the orbits of atoms where electrons could exist but are
missing. Electron-hole pairs are widely regarded as the basic building blocks
of charge density waves, just as pairs of electrons are the building blocks of
superconductivity. Furthermore, in July 2013, Sachdev and another student,
Rolando La Placa, showed that the resulting
electron-hole pairs would arrange themselves in a d-wave form — in this case,
the kind observed in Davis’ recent experiment.
In short, antiferromagnetism could generate the d-wave patterns of
both superconductivity and its rival, charge density waves.
“It makes sense that
antiferromagnetism is the parent state of both charge density waves and
superconductivity,” said Suchitra Sebastian, a physicist at Cambridge
University whose own new work on charge density waves will appear soon in
Nature. “Many key aspects we know so far are consistent with that, and it seems
very likely to be the explanation of what is happening.”
Hoffman called Sachdev’s new framework a “beautiful
descriptive theory.” But she pointed out that it is not yet refined enough to
predict how the balance of charge density waves and superconductivity vary with
temperature, magnetic field or type of cuprate. “The ultimate goal is of course
a predictive theory which will allow new materials and new technology,” she
said.
Other theories tying together
antiferromagnetism, charge density waves and superconductivity remain in play,
said Steve Kivelson, a theoretical physicist at
Stanford University who has been arguing for 20 years that the three phenomena
might be intertwined. The correct theory of their relationship is far from
settled, Kivelson said, and the biggest advance has been on the experimental
side: “I think probably the focus should be more on the experiment.”
Even if Sachdev’s theory (or some other) turns out to be correct,
it remains to be seen whether materials scientists can find a way to
significantly turn up the heat on superconductivity. It might simply be
impossible. But in recent years, these scientists have proved remarkably
successful at tweaking the knobs on nature’s raw materials. “They are usually
miraculous at doing this,” Davis said.
Sachdev’s theory makes a suggestion. It indicates that the two
types of pairs, electron-electron and electron-hole, are equally likely
consequences of antiferromagnetism, so changing the strength of that
interaction won’t help superconductivity dominate over charge density waves.
But there are other differences between the pairs — for instance, electron-hole
pairs move more sluggishly through the cuprate lattice — and altering a certain
property of the material would kill off these slow movers. How to tweak this
property is, Sachdev said, “obviously the question we are thinking about.”
Other researchers have their own ideas about how to increase the
temperature range across which superconductivity dominates over charge density
waves. Some refused to divulge their approaches. “The prize is so big,” said
Hoffman, explaining the competitiveness of the field. “If somebody finds a
room-temperature superconductor, that’s huge, in terms of personal fame, in
terms of gifts to humanity, in terms of prestige and legacy.”
The biggest advantage of
elevating superconductivity to room temperature would be accessibility. Just as
the laser and computers unexpectedly yielded the Internet, many uses of
superconductivity are probably still unknown. The point is to make the
technology available and see what happens. “Right now there are a bunch of
highly specialized guys in the lab fooling around with superconductivity,”
Taillefer said. “That’s not what you want. You want the whole planet fooling
around with superconductivity.”
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