Stanford Report, March 6, 2002
'Quantum
magnets' help scientists understand physical limits of matter In superconductors, electrons zip around with virtually no resistance
or energy loss. In insulators, however, they barely move, lacking the
energy to overcome high resistance. Strangely, scientists can turn certain
unusual insulators into high-temperature superconductors by adding the
right impurities. Conventional theories predict that these insulators
should actually be ordinary conductors. Researchers are still working
to understand the exotic magnetic properties of these materials, which
are strongly influenced by their quantum nature.
"These materials have extremely exotic properties that really have eluded
physical description," says Martin Greven, an assistant professor in Stanford's
Department of Applied Physics and at the Stanford Synchrotron Radiation
Laboratory. "We are interested in arriving at a quantitative understanding
of their magnetic properties."
With Stanford graduate students Owen Vajk and Patrick Mang and physicists
Peter Gehring and Jeffrey Lynn at the National Institute of Standards
and Technology (NIST), Greven has been trying to understand the complex
magnetic behavior of the insulators as magnetic atoms are randomly replaced
with nonmagnetic impurity atoms. Instead of yielding a superconductor,
the introduction of nonmagnetic impurities creates a novel model magnet.
From left, physicists
Owen Vajk, Martin Greven and Patrick Mang display exotic crystals they've
made in the Geballe Laboratory for Advanced Materials on campus. Layered
in the crystals are 'quantum magnets.' Photo: L.A. Cicero
The scientists used a powerful technique called neutron
scattering to take "snapshots" of the magnetic behavior within crystals
of the insulator. In the March 1 issue of the journal Science,
they report that they have been able to reach the point at which the random
impurities disrupt the long-range magnetic order of the crystals. While
tomorrow's quantum magnets may find applications in technology, today's
materials provide models with which scientists can test theoretical predictions,
as well as the physical limits of matter.
The art of growing exotic crystals
Making materials to study quantum magnetism is no easy task. It takes
manual dexterity and long hours to monitor the crystal growth. While the
United States lags behind Japan in this field, Greven's group has built
a world-class crystal growth facility with funding provided by the U.S.
Department of Energy, the National Science Foundation and the Alfred P.
Sloan Foundation. Says Greven: "Clearly, if the students can grow their
own samples, then they can go on and do their own science. If you have
to ask others, perhaps far away, for samples, they may or may not be able
to give them to you. They may have different interests. So growing your
own crystals helps you define your own research. It helps you to be at
the forefront of new discoveries."
Vajk, a fifth-year graduate student, led the crystal-growing effort
at Stanford's new Geballe Laboratory for Advanced Materials. With help
from Mang, a fourth-year graduate student, he was the first to succeed
at something other researchers have been attempting for more than a decade.
Starting with the building blocks of a classic high-temperature superconductor
-- lanthanum copper oxide -- he added zinc and magnesium and melted the
material in a special furnace using focused, high-powered light. "If you
don't get it right, then the crystal isn't stable," Vajk says, "and you
end up with a crystal that will disintegrate."
It took more than a week to grow each of the 2-inch crystals used in
their experiments. The crystals are formed from alternating magnetic copper-oxide
and nonmagnetic lanthanum-oxide layers, making the individual magnetic
layers act as model two-dimensional systems. The nonmagnetic zinc and
magnesium atoms are randomly interspersed in the magnetic layers, replacing
magnetic copper atoms.
Electrons in atoms have electronic properties as a consequence of their
charge, and magnetic properties as a consequence of their orientation,
or "spin." In a magnetic sheet of copper and oxygen atoms, the electron
spins tend to alternate between "up" and "down" in a pattern like the
black and red squares on a checkerboard. Matter in such a configuration
is referred to as an antiferromagnet. This arrangement of billions of
atoms on the checkerboard, or two-dimensional lattice, does not simply
result from the individual copper atoms themselves, but from the interaction
of neighboring electron spins with each other. Quantum mechanical effects
play a much stronger role in antiferromagnets than in ferromagnets, such
as refrigerator magnets, where spins tend to align parallel to each other.
Neutron scattering detects quantum fluctuations
After the crystals were grown, the researchers took them to Maryland
for analysis. At the NIST Center for Neutron Research, the physicists
probed the magnetic structure of the crystals with a technique -- neutron
scattering -- that is not available on the West Coast. "The neutron beams
we use are produced in a research nuclear reactor," explains Vajk. "Electric
charge doesn't matter to neutrons, but neutrons interact with the magnetic
moments of electrons. So neutrons can 'see' magnetic structure and magnetic
fluctuations."
Says Greven: "What we have been measuring with this technique is a snapshot
of the spins on these checkerboards." NIST physicist Lynn adds, "We could
not have attacked this problem without neutrons." The important role of
neutron scattering in modern research was recognized with the 1994 Nobel
Prize in physics, which was awarded to Clifford G. Shull and Bertram N.
Brockhouse for their pioneering work in developing this technique.
The distance over which magnetic spins "talk" to each other depends
on the temperature, Greven says. "At room temperature, in the crystals
we were looking at, they don't talk over a distance larger than four or
five neighbors. But when you cool these crystals, the distance over which
the magnetic moments can exchange information -- talk to each other --
increases in a nontrivial fashion due to both their quantum nature as
well as the presence of the impurities, and eventually exceeds hundreds
of neighbors."
The NIST measurements also enabled the physicists to track the low-temperature
long-range magnetic order as magnetic atoms were replaced with nonmagnetic
ones. They found that quantum fluctuations lead to an erosion of magnetic
order with increased dilution, and that when approximately 40 percent
of the magnetic atoms were replaced with nonmagnetic atoms, the spins
of neighboring atoms were no longer connected, or ordered, throughout
the system.
Back at Stanford, Vajk uses computer simulations to perform so-called
"Monte Carlo" calculations of these randomly diluted quantum magnets.
This technique lets him simulate properties of a two-dimensional model
without having to solve the problem exactly, and compare the results to
the experimental data. The numerical results confirm the scientists' understanding
of the properties of these crystals. The combined experimental and numerical
data should help guide physicists in developing a theoretical description
of these quantum magnets.
"Once we truly understand these and related materials and their magnetic
properties as a function of impurity content, we can hope to design better
materials in the lab that then have better properties from a technological
point of view," says Greven.
Physicists
create unusual insulator that 'eludes physical description,' serves as
model magnet for experiments
BY DAWN LEVY
Each two-inch crystal
takes more than a week to grow. To study quantum magnetism, scientists
replace magnetic with nonmagnetic atoms. Photo: L.A. Cicero
