An international team of scientists, including several
at Johns Hopkins, has detected a hidden magnetic "quantum
order" that extends over chains of nearly 100 atoms in a
material that is otherwise magnetically disordered.
The findings, published online July 26 in the journal
Science, may have implications for the design of
devices and materials for quantum information processing,
including large-scale quantum computers capable of tackling
problems exponentially faster than can conventional
computers.
The team's results are important because they
demonstrate that the magnetic moments (the measure of the
strength of a magnetic source) of a large number of atoms
can band together to form quantum states much like those of
a very large molecule. Though on the surface these atomic
"compass needles" seem to be disorganized and disordered,
the team was able to discern "a beautiful, underlying
quantum order," said team member Collin Broholm, a
professor in the Henry
A. Rowland Department of Physics and Astronomy in Johns
Hopkins' Krieger School of Arts and Sciences.
"Quantum mechanics is normally appreciated only on the
atomic scale. However, here we present evidence for a very
long and very quantum mechanical magnetic molecule,"
Broholm said. "While disordered to a classical observer,
the magnetic moments of almost 100 nickel atoms arranged in
a row within a solid were shown to display an underlying
quantum coherence limited only by chemical and thermal
impurities. The progress we made is really a demonstration
of quantum coherence among a larger number of atoms in a
magnet than ever before."
In addition, the team has established the factors that
affect the distance over which the hidden "quantum order"
can be maintained.
That distance, as well as how it changes as a result
of heating and chemical impurities in the material, may
well prove to be essential in determining whether the
material will have practical applications.
The team studied a ceramic material consisting of
chains of nickel-centered oxygen octahedra laid end to end.
The chains are not ordinary magnets such as those people
use to post reminders onto refrigerator doors; instead,
they are an exotic, quantum spin liquid in which electron
spins (analogous to tiny bar magnets) point in random
directions with no particular order, even at very low
temperatures.
To measure the quantum order through this classically
disordered liquid, scientists used neutrons to image the
magnetic excitations — also called "flips" —
and the distances over which they could propagate. The
experiments were performed at the National Institute of
Standards and Technology Center for Neutron Research in the
United States and at the ISIS particle accelerator of the
Rutherford Appleton Laboratory in the United Kingdom.
The team found that, despite the apparent classical
disorder, magnetic excitations could propagate over long
distances — up to 30 nanometers — at low
temperature. (A nanometer is a billionth of a meter.)
The team members also discovered that they could limit
coherence or make it disappear by introducing defects into
the material through chemical impurities or heating. These
defects "break the chains" into independent subchains, each
with its own hidden order. This part of the research is the
first step toward engineered spin-based quantum states in
ceramics.
"Apart from the sheer beauty and mystique of quantum
order beyond the atomic scale, there are very exciting
prospects for applications in quantum computing to
dramatically speed a wide range of computing that our
society relies upon," Broholm said.
Collaborators on this research include Guangyong Xu,
of Johns Hopkins and the U.S. Department of Energy's
Brookhaven National Laboratory; Broholm, Ying Chen and
Michel Kenzelmann, all of Johns Hopkins and the NIST Center
for Neutron Research; Yeong-Ah Soh, of Dartmouth College;
Gabriel Aeppli, of the London Centre for Nanotechnology and
University College London; John F. DiTusa, of Louisiana
State University; Christopher D. Frost, of the ISIS
Facility, Rutherford Appleton Laboratory, U.K.; Toshimitsu
Ito and Kunihiko Oka, of the National Institute of Advanced
Industrial Science and Technology, Japan; and Hidenori
Takagi, from AIST and the University of Tokyo.
The work was funded by the Office of Basic Energy
Sciences within the U.S. Department of Energy's Office of
Science, the National Science Foundation, the Wolfson-Royal
Society (U.K.) and the Basic Technologies program of the
U.K. Research Councils.
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