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Spin-lattice Interaction Effects in Frustrated Antiferromagnets

Spin-lattice Interaction Effects in Frustrated Antiferromagnets. 韓 政勳 , 賈 成龍 ( 成均館大 ). u j. R ij. j. u i. i. L attice- C oupled Antiferromagnetic S pin M odel. Spin and lattice are coupled through magneto-striction effect ( Pytte, PRB 1974). Lattice displacement related to

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Spin-lattice Interaction Effects in Frustrated Antiferromagnets

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  1. Spin-lattice Interaction Effectsin Frustrated Antiferromagnets 韓 政勳, 賈 成龍 (成均館大)

  2. uj Rij j ui i 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

  3. Application to Frustrated Lattice – Triangular, Kagome NO LATTICE DEFORMATION!

  4. 3D Frustrated Lattice - Pyrochlore Tetrahedron as a building block Ground state condition for each tetrahedron Macroscopic ground state degeneracy in pristine Heisenberg model

  5. Experiments on Pyrochlore – ZnCr2O4 Spins on Cr3+(S=3/2) order antiferromagnetically acccompanied 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

  6. Lifting of degeneracy through spin-lattice coupling Tetrahedron Unit (Tschernyshyov et al. PRL 2002) Hexagon Unit (Jia et al. PRB 2005)

  7. 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

  8. Hexagon Organization of Spins in Pyrochlore (a) (b) Arrangement of 4 types of hexagons Arrangement of 3 types of spin directions

  9. Experiments on Triangular S=2 AFM – YMnO3 S Lee et al. PRB 2005 Expansion of Mn triangles (red) at the onset of magnetic order

  10. IDEA: Spin-Lattice Coupling-induced VBS ? Magnetic order doesn’t work, but valence-bond order might. PARTIAL VBS ORDER At most 4 VB’s can form for coordination = 6 Some links must lack a VB and expand.

  11. 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

  12. A New Excitation We calculate excitation spectra for the VBS within SMA Comparison Spin wave VBS

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