A team of international physicists that includes researchers from the
National Institute of Standards and Technology (NIST) has found
experimental evidence of a highly sought-after type of arrangement of
atomic magnetic moments, or spins, in a series of materials. Their work,
one of the very few studies of this particular spin state, which has
been postulated as a possible underlying mechanism for high-temperature
superconductivity, may eventually serve as a test of current and future
theoretical models of exotic spin states.
At the NIST Center for Neutron Research (NCNR) and the Hahn-Meitner
Institute in Berlin, Germany, the scientists used intense beams of
neutrons to probe a series of antiferromagnets, materials in which
each spin - an intrinsic property of an atom
that produces a tiny magnetic field called a magnetic �moment�
- cancels another, giving the material a net magnetic field of
zero. The results, described in the Aug. 26 (2007)
online edition of Nature Materials, revealed evidence of a rare and
pporly understood �quantum paramagnetic� spin state, in which
neighboring spins pair up to form �entangled spin singlets� that have
an ordered pattern and that allow the material to weakly respond to an
outside magnetic field - i.e., become
paramagnetic.
The antiferromagnets used in this work are composed mainly of zinc and
copper, and are distinguished by their proportions of each, with the
number of copper ions determined by the number of zinc ions. At the
atomic level, the material is formed of many repeating layers. The
atoms of each layer are arranged into a structure known as a �kagome
lattice,� a pattern of triangles laid point-to-point whose basic unit
resembles a six-point star.
Physicists have been studying antiferromagnets with kagome structures
over the last 20 years because they suspected these materials harbored
interesting spin structures. But good model systems, like the zinc/copper
compounds used by this group, had not been identified.
At the NCNR, the researchers determined how varying concentrations of
zinc and copper and varying temperatures affected fluctuations in the
way the spins are arranged in these materials. Using a neutron
spectrometer at the Hahn-Meitner Institute, they also investigated the
effect of external magnetic fields of varying strengths. The group
uncovered several magnetic phases in addition to the quantum
paramagnetic state and were able to construct a complete phase diagram
as a function of the zinc concentration and temperature. They are
planning further experimental and theoretical studies to learn more
about the kagome system.
Further Information and Source:
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S.-H. Lee, H. Kikuchi, Y. Qiu, B. Lake, Q. Huang, K. Habicht and K.
Kiefer: Quantum-spin-liquid states in the two-dimensional kagome
antiferromagnets ZnxCu4-x(OD)6Cl2.
Nature Materials advance online publication;
DOI:10.1038/nmat1986
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This work was led by S.-H. Lee at the University
of Virginia. The other participating institutions are the University
of Fukui in Japan, and the Hahn-Meitner Institute and the Technical
University of Berlin, both in Germany.