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arXiv 1802.09152cond-mat.stat-mech
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有序和无序量子自旋系统中的非静态效应I:横场中铁磁伊辛模型的理论表述、能隙和基态能量的非广延项

Non-static effects in ordered and disordered quantum spin systems I: theoretical formulation, energy gap and non-extensive terms of ground-state energy of the ferromagnetic Ising model in a transverse field

Yang Wei Koh

摘要:

在量子自旋模型的配分函数的路径积分公式中,大多数当前的处理采用所谓的静态近似来简化对所有可能路径求和的过程。虽然静态近似足以研究系统的热力学方面,但忽略了路径积分的路径中的时间依赖性或非静态波动所作的贡献。这个非静态分量相对于静态部分是非常小的,并且对于计算小的非广延量(例如路径积分框架内的能隙)来说,必须对其进行仔细处理。我们提出了一种形式主义,将非静态的影响到一类自旋模型的路径积分计算,其配分函数是可约化的跟踪一个单一的自旋(通常被称为有效的哈密顿量)。我们首先证明了单自旋轨迹的时间依赖行为是由泡利方程。利用含时微扰理论得到了泡利方程解的微扰展开式,进而得到了单自旋迹的微扰展开式。这给了我们一个微扰展开的路径积分,可以集成系统使用标准的技术。在本文中,我们详细地发展了上述的理论框架,并将其应用到一个简单的有序自旋模型,无限程铁磁伊辛模型在横向场。我们用这种非静态方法计算了两个非广延量子:基态能量的$N^0$和$N^{-1}$项($N$ =自旋数)以及基态和第一激发态之间的能隙。通过与Holstein-Primakoff变换和数值对角化哈密顿量的结果进行比较,验证了本文的结果。简要讨论了该方法在量子自旋玻璃中的应用。

英文摘要:

In the path integral formulation of the partition function of quantum spin models, most current treatments employ the so-called static approximation to simplify the process of summing over all possible paths. Although sufficient for studying the thermodynamic aspects of the system, static approximation ignores the contributions made by time-dependent, or non-static, fluctuations in the paths of the path integral. This non-static component is very small relative to the static part, and its careful treatment is necessary for the calculation of small non-extensive quantities such as the energy gap within the path integral framework. We propose a formalism for incorporating non-static effects into the path integral calculation of a class of spin models whose partition functions are reducible to the trace of a single spin (often known as the effective Hamiltonian). We first show that the time-dependent behavior of the single spin trace is governed by the Pauli equation. Time-dependent perturbation theory is used to obtain a perturbative expansion of the solution of the Pauli equation, and then for the single spin trace. This gives us a perturbative expansion of the path integral which can be integrated systematically using standard techniques. In this paper, we develop the theoretical framework outlined above in detail and apply it to a simple ordered spin model, the infinite-range ferromagnetic Ising model in a transverse field. We calculated two non-extensive quantites with this non-static approach: the $N^0$ and $N^{-1}$ terms of the ground-state energy ($N$=number of spins) and the energy gap between the ground and first-excited states. We checked our results by comparing with those of Holstein-Primakoff transform and numerical diagonalization of the Hamiltonian. The application of the method to quantum spin-glasses is briefly discussed.

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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