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

Figure 1.
Two sets of Cu spin orientations on a hexagon from the honeycomb lattice; either the "green" or "blue" set of spins describe the magnetic arrangement at a given time, illustrating that there is no preferred spin orientation; i.e the spins are "frustrated". Credit: Satoru Nakatsuji, University of Tokyo

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

  1. "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.

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Frank (Bud) Bridges, University of California Santa Cruz

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