Spin-lattice Interaction Effects in Frustrated Antiferromagnets

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Spin-lattice Interaction Effectsin Frustrated
Antiferromagnets
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Lattice-Coupled Antiferromagnetic Spin Model

• Spin and lattice are coupled
• through magneto-striction
• effect (Pytte, PRB 1974)

Lattice displacement related to local spin-spin
correlation by
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Application to Frustrated Lattice Triangular,
Kagome
NO LATTICE DEFORMATION!
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3D Frustrated Lattice - Pyrochlore
Tetrahedron as a building block Ground state
condition for each tetrahedron
Macroscopic ground state degeneracy in pristine
Heisenberg model
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Experiments on Pyrochlore ZnCr2O4
Spins on Cr3(S3/2) order antiferromagnetically a
cccompanied by cubic-to-tetragonal distortion
(SHLee, PRL, 2000)
Below Tc
Neutron scattering structure factor consistent
with hexagon spin cluster, i.e. neutrons scatter
off a hexagon cluster of spins (SHLee, Nature,
2002)
Above Tc
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Lifting of degeneracy through spin-lattice
coupling
Tetrahedron Unit (Tschernyshyov et al. PRL 2002)
Hexagon Unit (Jia et al. PRB 2005)
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Energy Comparison
In comparing energies for both distortion modes,
Tetrahedron-based distortion has a lower energy,
but hexagon shrinkage is a viable, metastable
state
SHLee et al. Nature 2002
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Hexagon Organization of Spins in Pyrochlore
(a)
(b)
Arrangement of 4 types of hexagons
Arrangement of 3 types of spin directions
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Experiments on Triangular S2 AFM YMnO3
S Lee et al. PRB 2005
Expansion of Mn triangles (red) at the onset of
magnetic order
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IDEA Spin-Lattice Coupling-induced VBS ?
Magnetic order doesnt work, but valence-bond
order might.
PARTIAL VBS ORDER
At most 4 VBs can form for coordination 6
Some links must lack a VB and expand.
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Calculations
Schwinger boson mean-field theory calculation
(cond-mat/0505573) supports a ground state with
lattice distortion and modulated bond amplitudes,
consistent with our picture of partial VBS
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A New Excitation
We calculate excitation spectra for the VBS
within SMA
Comparison
Spin wave
VBS