SSRL X-rays Illuminate Frustrated Materials
SSRL Science
Summary - August 2012

The electronic, spin, and ionic structures of closely packed atoms in solids
are strongly co-dependent and interactions of these three lattices, whether
innate or due to subtle manipulation, can cause exotic properties to emerge.
The strong coupling among these lattices can also suppress a physical property
through "frustration," the term for an incompatibility of symmetries. A
long-theorized state of matter arising from frustration is the quantum spin
liquid, in which the orientations of the atomic magnets are continually
fluctuating - as in the continuous motions of atoms in a liquid. This novel
state of matter was first observed in
Ba3CuSb2O9 by an international team of
investigators, including a group using SSRL.
Experiments the team performed at SSRL showed that, although there is a strong
local magnetic moment, ionic frustration prevents the spins from freezing even
at the lowest measured temperature - 100 mK. This is in direct contrast to what
usually happens to magnetic ions located within a crystal lattice. Strong local
magnetic and electric forces between the spins usually lead to a preferred
alignment of the atomic moments at low temperatures. For example, the atomic
magnets in ferromagnets such as iron are aligned parallel to each other while
in anti-ferromagnets they are anti-parallel. Even other frustrated systems tend
to freeze the individual atomic moment alignments when cooled sufficiently,
forming what are called quantum spin glasses. Not so for
Ba3CuSb2O9.
Nakatsuji and Bridges, et al., used several characterization techniques
to investigate Ba3CuSb2O9 in an effort to
understand why perfectly stoichiometric
Ba3CuSb2O9 resists spin alignment, while
slightly nonstoichiometric material gives rises to magnetic order. Extended
X-ray absorption fine structure (EXAFS) analysis performed on Beam Line 10-2
provides part of the answer: It revealed that even in a perfectly
stoichiometric material the copper-antimony pairs are ordered to a very high
degree, forming highly anisotropic "dumbbells" which are aligned along the ±
c-axis, forming a honeycomb lattice with no net electric polarization. EXAFS
measurements also showed that the oxygen shell around the copper ion is
distorted due to the Jahn-Teller (J-T) effect, as would be expected for a
magnetic Cu+2 species. Such a high degree of dumbbell order and J-T
distortion makes the structure quite anisotropic locally. Diffraction
measurements on slightly non-stoichiometric material shows that this local
anisotropy can propagate over a long range, making the crystal lattice
orthorhombic and the material magnetically ordered. However, diffraction
measurements on perfectly stoichiometric material show no orthorhombic
distortion of the hexagonal honey-comb like arrangement of Cu-Sb dumbbells. In
this 3-D honeycomb arrangement there are triangles of copper atoms, on which
the magnetic interactions J(1) and J(2), which tend to align spins
anti-parallel ,are frustrated. The authors were, therefore, able to conclude
that the average, perfect hexagonal symmetrical ordering of the Cu-Sb dumbbells
did not lead to any preferred spin orientation. The resulting frustration
quenches any bulk electrical or magnetic polarizability in
Ba3CuSb2O9, giving rise to the quantum spin
liquid state.
The EXAFS experiments, which were key in identifying
Ba3CuSb2O9 as a quantum
spin liquid, raised as many questions as they answered. A static J-T distortion
of the oxygen lattice should lead to a preferred local orientation and hence
destroy the quantum spin state.. However, EXAFS measurements probe the lattice
at a very short time scale. Does the observation of a J-T distortion
co-existing with the spin liquid state indicate that the J-T distortion is
fluctuating too fast for the spins to follow? Clearly, this first instance of a
quantum spin liquid has not given up all its secrets.
Primary Citation
- "Spin-Orbital Short-Range Order on a Honeycomb-Based Lattice," S. Nakatsuji, K. Kuga, K. Kimura, R. Satake, N. Katayama, E. Nishibori, H. Sawa, R. Ishii, M. Hagiwara, F. Bridges, T. U. Ito, W. Higemoto, Y. Karaki, M. Halim, A. A. Nugroho, J. A. Rodriguez-Rivera, M. A. Green, C. Broholm, Science, Vol. 336 no. 6081 pp. 559-563.
Related Links
- Science Perspective
- Supplementary Materials
- Nakatsuji Lab Highlight
- Todai Research Highlight
- NIST Center for Neutron Research (NCNR) Highlight
Contact
Frank (Bud) Bridges, University of California Santa Cruz
Full Science Highlight
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