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arXiv math-ph/0306016math-phmath.MP
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海森堡XXZ和XY模型的低温结果

Low Temperature Results for the Heisenberg XXZ and XY Models

J. Abbott

摘要:

本论文主要研究了量子自旋系统的低温行为。首先,我们提出了一个下界的自旋-1 XXZ链在有限体积的两个位置的哈密顿量的间隙。该估计是由Nachtergaele在(cond-mat/9410110)中开发的称为鞅方法的方法得出的。我们的界限依赖于一个假设,我们还不能在所有情况下分析验证。我们提出的数字证据,有力地表明我们的假设是有效的。第二个结果证明了自旋为1/2的d维XY模型在外加磁场的作用下,只要磁场强度足够大,在低温下不会发生相变。使用轮廓扩展肯尼迪的启发,我们表明,轮廓的权重满足条件的Kotecky和Preiss,这使我们能够表示系统的自由能作为一个集群扩展。作为设置的一部分,我们给出了一个简单的证明,当外部磁场的强度至少为2d时,全自旋上的状态是唯一的基态。

英文摘要:

This thesis contains two results for the low temperature behavior of quantum spin systems. First, we present a lower bound for the spin-1 XXZ chain in finite volumes in terms of the gap of the two-site Hamiltonian. The estimate is derived by a method developed by Nachtergaele in (cond-mat/9410110) called the Martingale Method. Our bound relies on an assumption which we have, as yet, been unable to verify analytically in all cases. We present numerical evidence that strongly indicates our assumption is valid. The second result is a proof that the spin-1/2, d-dimensional XY model in the presence of an external magnetic field does not undergo a phase transition at low temperature, provided that the strength of the field is great enough. Using a contour expansion inspired by Kennedy, we show that the weights of contours satisfy a condition of Kotecky and Preiss which allows us to express the free energy of the system as a cluster expansion. As part of the setup we give a simple proof that the all-spin-up state is the unique ground state when the external magnetic field has strength at least 2d.

机构、总结、中文摘要均由AI分析而来,可能存在错误,请以论文正文为准。

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