For the First Time,
Researchers See Individual Atoms Keep Away From Each Other or Bunch up as Pairs
September 15, 2016
Observations
of atomic interactions could help pave way to room-temperature superconductors.
Jennifer Chu | MIT News Office September 15, 2016
If you bottle up a gas and try to
image its atoms using today’s most powerful microscopes, you will see little
more than a shadowy blur. Atoms zip around at lightning speeds and are
difficult to pin down at ambient temperatures.
If, however, these atoms are
plunged to ultracold temperatures, they slow to a crawl, and scientists can
start to study how they can form exotic states of matter, such as superfluids,
superconductors, and quantum magnets.
Physicists at MIT have now cooled a
gas of potassium atoms to several nanokelvins — just a hair above absolute zero
— and trapped the atoms within a two-dimensional sheet of an optical lattice
created by crisscrossing lasers. Using a high-resolution microscope, the
researchers took images of the cooled atoms residing in the lattice.
By looking at correlations between
the atoms’ positions in hundreds of such images, the team observed individual
atoms interacting in some rather peculiar ways, based on their position in the
lattice. Some atoms exhibited “antisocial” behavior and kept away from each
other, while some bunched together with alternating magnetic orientations.
Others appeared to piggyback on each other, creating pairs of atoms next to
empty spaces, or holes.
The team believes that these
spatial correlations may shed light on the origins of superconducting behavior.
Superconductors are remarkable materials in which electrons pair up and travel
without friction, meaning that no energy is lost in the journey. If
superconductors can be designed to exist at room temperature, they could
initiate an entirely new, incredibly efficient era for anything that relies on
electrical power.
Martin Zwierlein, professor of
physics and principal investigator at MIT’s NSF Center
“Learning from this atomic model,
we can understand what’s really going on in these superconductors, and what one
should do to make higher-temperature superconductors, approaching hopefully
room temperature,” Zwierlein says.
Zwierlein and his colleagues’
results appear in the Sept. 16 issue of the journal Science.
Co-authors include experimentalists from the MIT-Harvard
Center for Ultracold Atoms, MIT’s
Research Laboratory of Electronics, and two theory groups from San
Jose State University , Ohio State
University, the University of Rio de Janeiro , and Penn State University
“Atoms as stand-ins for
electrons”
Today, it is impossible to model
the behavior of high‐temperature
superconductors, even using the most powerful computers in the world, as the
interactions between electrons are very strong. Zwierlein and his team sought
instead to design a “quantum simulator,” using atoms in a gas as stand-ins for
electrons in a superconducting solid.
The group based its rationale on
several historical lines of reasoning: First, in 1925 Austrian physicist
Wolfgang Pauli formulated what is now called the Pauli exclusion principle,
which states that no two electrons may occupy the same quantum state — such as
spin, or position — at the same time. Pauli also postulated that electrons
maintain a certain sphere of personal space, known as the “Pauli hole.”
His theory turned out to explain
the periodic table of elements: Different configurations of electrons give rise
to specific elements, making carbon atoms, for instance, distinct from hydrogen
atoms.
The Italian physicist Enrico Fermi
soon realized that this same principle could be applied not just to electrons,
but also to atoms in a gas: The extent to which atoms like to keep to
themselves can define the properties, such as compressibility, of a gas.
“He also realized these gases at
low temperatures would behave in peculiar ways,” Zwierlein says.
British physicist John Hubbard then
incorporated Pauli’s principle in a theory that is now known as the
Fermi-Hubbard model, which is the simplest model of interacting atoms, hopping
across a lattice. Today, the model is thought to explain the basis for
superconductivity. And while theorists have been able to use the model to
calculate the behavior of superconducting electrons, they have only been able
to do so in situations where the electrons interact weakly with each other.
“That’s a big reason why we don’t
understand high-temperature superconductors, where the electrons are very
strongly interacting,” Zwierlein says. “There’s no classical computer in the
world that can calculate what will happen at very low temperatures to
interacting [electrons]. Their spatial correlations have also never been
observed in situ, because no one has a microscope to look at every single
electron.”
Carving out personal space
Zwierlein’s team sought to design
an experiment to realize the Fermi-Hubbard model with atoms, in hopes of seeing
behavior of ultracold atoms analogous to that of electrons in high-temperature
superconductors.
The group had previously designed
an experimental protocol to first cool a gas of atoms to near absolute zero,
then trap them in a two-dimensional plane of a laser-generated lattice. At such
ultracold temperatures, the atoms slowed down enough for researchers to capture
them in images for the first time, as they interacted across the lattice.
At the edges of the lattice, where
the gas was more dilute, the researchers observed atoms forming Pauli holes,
maintaining a certain amount of personal space within the lattice.
“They carve out a little space for
themselves where it’s very unlikely to find a second guy inside that space,”
Zwierlein says.
Where the gas was more compressed,
the team observed something unexpected: Atoms were more amenable to having
close neighbors, and were in fact very tightly bunched. These atoms exhibited
alternating magnetic orientations.
“These are beautiful,
antiferromagnetic correlations, with a checkerboard pattern — up, down, up,
down,” Zwierlein describes.
At the same time, these atoms were
found to often hop on top of one another, creating a pair of atoms next to an
empty lattice square. This, Zwierlein says, is reminiscent of a mechanism
proposed for high-temperature superconductivity, in which electron pairs
resonating between adjacent lattice sites can zip through the material without
friction if there is just the right amount of empty space to let them through.
Ultimately, he says the team’s
experiments in gases can help scientists identify ideal conditions for
superconductivity to arise in solids.
Zwierlein explains: “For us, these
effects occur at nanokelvin because we are working with dilute atomic gases. If
you have a dense piece of matter, these same effects may well happen at room
temperature.”
Currently, the team has been able
to achieve ultracold temperatures in gases that are equivalent to hundreds of
kelvins in solids. To induce superconductivity, Zwierlein says the group will
have to cool their gases by another factor of five or so.
“We haven’t played all of our
tricks yet, so we think we can get colder,” he says.
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