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arXiv 1204.0622cond-mat.stat-mechmath-phmath.MP
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Kagome-lattice Potts模型的临界流形

Critical manifold of the kagome-lattice Potts model

Jesper Lykke Jacobsen, Christian R. Scullard

摘要:

任何二维无限正则格G都可以通过用G的有限子图B平铺平面来产生;我们称B为G的基。我们引入了依赖于B的双参数图多项式P_B(q,v)及其在G中的嵌入。代数曲线P_B(q,v)= 0是q态Potts模型临界流形的一个近似,其中v = exp(K)-1定义在G上.该曲线预测了铁磁(v>0)和反铁磁(v<0)区域的相图。对于更大的基B,近似变得越来越精确,我们猜想P_B(q,v)= 0提供了无限B极限中的精确临界流形。此外,对于某些格G,或对于任意G上的伊辛模型(q=2),P_B(q,v)对B的任意选择都分解:递归因子的零集提供了精确的临界流形。在这个意义上,P_B(q,v)的计算可以用来检验G上Potts模型的精确可解性.我们说明的方法,为正方形晶格,波茨模型已被完全解决,和戈薇格,它没有。对于正方形晶格,我们正确地再现了已知的相图,包括反铁磁过渡和奇异的Berker-Kadanoff相。对于kagome格,采取最小的基础与六个边缘,我们恢复一个众所周知的(但现在反驳)猜想的F.Y.吴更大的基数提供了对这个公式的连续改进,给出了吴方法的自然扩展。多项式预测与数值计算非常吻合。当v>0时,预测的临界耦合v_c的精度对于6边基为10^{-4}或10^{-5}量级,对于所研究的最大基(具有36边)提高到10^{-6}或10^{-7}量级。

英文摘要:

Any two-dimensional infinite regular lattice G can be produced by tiling the plane with a finite subgraph B of G; we call B a basis of G. We introduce a two-parameter graph polynomial P_B(q,v) that depends on B and its embedding in G. The algebraic curve P_B(q,v) = 0 is shown to provide an approximation to the critical manifold of the q-state Potts model, with coupling v = exp(K)-1, defined on G. This curve predicts the phase diagram both in the ferromagnetic (v>0) and antiferromagnetic (v<0) regions. For larger bases B the approximations become increasingly accurate, and we conjecture that P_B(q,v) = 0 provides the exact critical manifold in the limit of infinite B. Furthermore, for some lattices G, or for the Ising model (q=2) on any G, P_B(q,v) factorises for any choice of B: the zero set of the recurrent factor then provides the exact critical manifold. In this sense, the computation of P_B(q,v) can be used to detect exact solvability of the Potts model on G. We illustrate the method for the square lattice, where the Potts model has been exactly solved, and the kagome lattice, where it has not. For the square lattice we correctly reproduce the known phase diagram, including the antiferromagnetic transition and the singularities in the Berker-Kadanoff phase. For the kagome lattice, taking the smallest basis with six edges we recover a well-known (but now refuted) conjecture of F.Y. Wu. Larger bases provide successive improvements on this formula, giving a natural extension of Wu's approach. The polynomial predictions are in excellent agreement with numerical computations. For v>0 the accuracy of the predicted critical coupling v_c is of the order 10^{-4} or 10^{-5} for the 6-edge basis, and improves to 10^{-6} or 10^{-7} for the largest basis studied (with 36 edges).

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