Spin-Peierls Effect on Frustrated Spin System s

1. uj Rij j ui i Spin-Peierls Effect on Frustrated Spin Systems Cheng Long Jia and Jung Hoon Han Department of Physics, Bk21 Physics Research Division, and Institute for Basic Science, Sungkyunkwan University, Suwon 440-746, Korea ----Lattice-Coupled AFM Model---- Hexagon organization of Spins in Pyrochlore Spin and lattice are coupled through magneto-striction effect Lattice displacement related to local spin-spin correlation by Arrangement of 4 types of hexagons Arrangement of 3 types of spin directions ----Partial Valence-Bond-Solid---- ----Application to Frustrated Lattices---- Experiments on YMnO3 Triangular and Kagome Lattice NO LATTICE DEFORMATION! S Lee et al. PRB(R) 2005 Expansion of Mn triangles (red) at the onset of magnetic order IDEA: Spin-Lattice Coupling –induced VBS ? Pyrochlore Lattice Magnetic order doesn’t work, but valence-bond order might. Structure Partial VBS ORDER Tetrahedron as a building block Ground state condition At most 4 VB’s can form for coordination = 6 Some links must lack a VB and expand. for each tetrahedron Macroscopic ground state degeneracy in pristine Heisenberg model Ground State Experiments on ZnCr2O4 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 Spins on Cr3+(S=3/2) order antiferromagnetically accompanied by cubic-to-tetragonal distortion (S.H. Lee, PRL, 2000) Below Tc Neutron scattering structure factor consistent with hexagon spin cluster, i.e. neutrons scatter off a hexagon cluster of spins (S.H. Lee, Nature, 2002) Above Tc A New Excitation Lifting of Degeneracy through Spin-Lattice Coupling Tetrahedron Unit (Tschernyshyov et al. PRL 2002) Hexagon Unit (Jia et al. PRB 2005) PVBS Spin Wave Comparison