Dec. 20, 2012 — Following
up on earlier theoretical predictions, MIT researchers have now demonstrated
experimentally the existence of a fundamentally new kind of magnetic behavior,
adding to the two previously known states of magnetism.
Ferromagnetism -- the simple magnetism of a bar magnet
or compass needle -- has been known for centuries. In a second type of
magnetism, antiferromagnetism, the magnetic fields of the ions within a metal
or alloy cancel each other out. In both cases, the materials become magnetic
only when cooled below a certain critical temperature. The prediction and
discovery of antiferromagnetism -- the basis for the read heads in today's
computer hard disks -- won Nobel Prizes in physics for Louis Neel in 1970 and
for MIT professor emeritus Clifford Shull in 1994.
"We're showing that there is a third fundamental
state for magnetism," says MIT professor of physics Young Lee. The
experimental work showing the existence of this new state, called a quantum
spin liquid (QSL), is reported this week in the journalNature, with Lee
as the senior author and Tianheng Han, who earned his PhD in physics at MIT
earlier this year, as lead author.
The QSL is a solid crystal, but its magnetic state is
described as liquid: Unlike the other two kinds of magnetism, the magnetic
orientations of the individual particles within it fluctuate constantly,
resembling the constant motion of molecules within a true liquid.
Finding the evidence
There is no static order to the magnetic orientations,
known as magnetic moments, within the material, Lee explains. "But there
is a strong interaction between them, and due to quantum effects, they don't
lock in place," he says.
Although it is extremely difficult to measure, or
prove the existence, of this exotic state, Lee says, "this is one of the
strongest experimental data sets out there that [does] this. What used to just
be in theorists' models is a real physical system."
Philip Anderson, a leading theorist, first proposed
the concept in 1987, saying that this state could be relevant to
high-temperature superconductors, Lee says. "Ever since then, physicists
have wanted to make such a state," he adds. "It's only in the past
few years that we've made progress."
The material itself is a crystal of a mineral called
herbertsmithite. Lee and his colleagues first succeeded in making a large, pure
crystal of this material last year -- a process that took 10 months -- and have
since been studying its properties in detail.
"This was a multidisciplinary collaboration, with
physicists and chemists," Lee explains. "You need both … to
synthesize the material and study it with advanced physics techniques.
Theorists were also crucial to this."
Through its experiments, the team made a significant
discovery, Lee says: They found a state with fractionalized excitations, which
had been predicted by some theorists but was a highly controversial idea. While
most matter has discrete quantum states whose changes are expressed as whole
numbers, this QSL material exhibits fractional quantum states. In fact, the
researchers found that these excited states, called spinons, form a continuum.
This observation, they say in their Nature paper, is "a remarkable
first."
Scattering neutrons
To measure this state, the team used a technique
called neutron scattering, which is Lee's specialty. To actually carry out the
measurements, they used a neutron spectrometer at the National Institute of
Standards and Technology (NIST) in Gaithersburg, Md.
The results, Lee says, are "really strong
evidence of this fractionalization" of the spin states. "That's a
fundamental theoretical prediction for spin liquids that we are seeing in a
clear and detailed way for the first time."
It may take a long time to translate this "very
fundamental research" into practical applications, Lee says. The work
could possibly lead to advances in data storage or communications, he says --
perhaps using an exotic quantum phenomenon called long-range entanglement, in
which two widely separated particles can instantaneously influence each other's
states. The findings could also bear on research into high-temperature
superconductors, and could ultimately lead to new developments in that field,
he says.
"We have to get a more comprehensive
understanding of the big picture," Lee says. "There is no theory that
describes everything that we're seeing."
Subir Sachdev, a professor of physics at Harvard
University who was not connected with this work, says that these findings,
which have been anticipated for decades, "are very significant and open a
new chapter in the study of quantum entanglement in many-body systems."
The detection of such states, he says, was an "exceptionally difficult
task. Young Lee and his group brilliantly overcame these challenges in their
beautiful experiment."
In addition to Lee and Han, the work was carried out
by J.S. Helton of NIST, research scientist Shaoyan Chu of MIT's Center for
Materials Science and Engineering, MIT chemistry professor Daniel Nocera, Jose Rodriguez-Rivera
of NIST and the University of Maryland, and Colin Broholm of Johns Hopkins
University. The work was supported by the U.S. Department of Energy and the
National Science Foundation.
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