Spin-lattice Interaction Effects in Frustrated Antiferromagnets 韓 政勳, 賈 成龍 (成均館大)

Lattice-Coupled Antiferromagnetic Spin Model j ui uj Rij Spin and lattice are coupled through magneto-striction effect (Pytte, PRB 1974) Lattice displacement related to local spin-spin correlation by

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

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

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

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

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

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

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

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.

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

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