Perturbation Calculation for the 16 Lowest-Energy States in Spin Doughnut Mo_<75>V_<20>(Condensed Matter Physics)

概要

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A possible explanation for the low-temperature Curie law observed in the frustrated magnetic cluster Mo_<75>V_<20> has been presented in [J. Phys. Soc. Jpn. 76 (2007), 083709], where the proposed antiferromagnetic Heisenberg model contains one ten-spin ring with coupling constant J=388K and two five-spin rings with J"=81K, which are coupled to each other by another coupling J'=163K so as to form isoceles triangles of one J-bond and two J'-bonds. Surprisingly enough, although J'>J", the five-spin rings behave as free spins at low temperatures, which leads to the low-temperature Curie law. We develop a perturbation theory to determine why the doublet behavior of the five-spin rings formed by J"-bonds is not smeared out by the stronger coupling J'. To be precise, we derive the second-order effective Hamiltonian for the 16-fold degenerate ground state at J'=0, and obtain the splitting width of the 16 lowest-energy states as cJ'^2/J, where c≃0.072 for the 20-site model of Mo_<75>V_<20> and c=54(5√<13>-18)/13≃0.115 for the 12-site counterpart. We also study the dependence of the specific heat on small magnetic fields (up to 10T) and find that the low-temperature specific heat below 10K shows unique behavior originating from the 16 lowest-energy states, which contain four triplet states.
理論物理学刊行会の論文
2009-11-25