Experiments
demonstrate ‘quantum spin liquid,’ which could have applications in new
computer memory storage.
MIT News, David L.
Chandler, MIT News Office, December 2012
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MIT physicists grew this pure crystal of herbertsmithite in their
laboratory. This sample, which took 10 months to grow, is 7 mm long
(just over a quarter-inch) and weighs 0.2 grams.Image: Tianheng Han
|
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 journal Nature, 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.
"Recalibration of Knowledge" – Jan 14, 2012 (Kryon channelled by Lee Carroll) - (Subjects: Channelling, God-Creator, Benevolent Design, New Energy, Shift of Human Consciousness, (Old) Souls, Reincarnation, Gaia, Old Energies (Africa,Terrorists, Cuba, Iran, North Korea, Venezuela ... ), Weather, Rejuvenation, Akash, Nicolas Tesla / Einstein, Cold Fusion, Magnetics, Lemuria, Atomic Structure (Electrons, Particles, Polarity, Self Balancing, Magnetism, Higgs Boson), Entanglement, "Life is necessary for a Universe to exist and not the other way around", DNA, Humans (Baby getting ready, First Breath, Stem Cells, Embryonic Stem Cells, Rejuvenation), Global Unity, ... etc.) - (Text Version)
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