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arXiv 2208.06910cond-mat.stat-mechcond-mat.dis-nn
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随机无序晶格中的自旋波理论:海森堡铁磁体

Spin-wave theory in a randomly disordered lattice: A Heisenberg ferromagnet

Z. J. Weiss, A. R. Massih

摘要:

本文从含有随机分布的杂质离子的海森堡铁磁体的哈密顿量出发,用二次量子化玻色算符的Dyson-Maleev变换表示自旋算符。然后利用量子统计场论的方法,导出了系统的配分函数和自由能。我们采用Matsubara热微扰方法来描述磁振子和磁振子定态场之间相互作用的部分哈密顿量。在对所有可能的杂质分布进行平均后,我们将系统的自由能表示为平均杂质浓度的函数。随后,我们在动量空间中用磁振子算符建立了温度T下的双时间单粒子格林函数,并由海森堡运动方程导出了格林函数的运动方程,然后求解得到的方程。由此,我们计算了系统的自能,然后计算了系统的谱密度。我们应用的形式主义的情况下,简单的立方晶格和计算的态密度,谱密度函数和寿命的磁振子作为能量的函数的几个值的平均浓度的晶格中的重离子。计算了磁振子能谱随平均杂质浓度c的变化,结果表明,在0.1 < c < 0.7的范围内,低激发态的激发能随c的增加而连续增加。我们使用谱密度函数来计算一些热物理量。我们已经获得了封闭形式的表达式的构型平均物理量的利益在一个统一的方式作为函数的c到任何顺序的c适用于低于临界渗滤浓度。

英文摘要:

Starting from the hamiltonian for the Heisenberg ferromagnet which comprise randomly distributed nonmagnetic ions as impurities in a Bravais lattice, we express the spin operators by means of the Dyson-Maleev transformation in terms of the Bose operators of the second quantization. Then by using methods of quantum statistical field theory, we derive the partition function and the free energy for the system. We adopt the Matsubara thermal perturbation method to a portion of the hamiltonian which describes the interaction between magnons and the stationary field of nonmagnetic ions. Upon averaging over all possible distributions of impurities, we express the free energy of the system as a function of the mean impurity concentration. Subsequently, we set up the double-time single particle Green function at temperature T in the momentum space in terms of magnon operators and derive the equation of motion for the Green function through the Heisenberg equation of motion and then solve the resulting equation. From this, we calculate the self-energy and then the spectral density for the system. We apply the formalism to the case of the simple cubic lattice and compute the density of states, the spectral density function and the lifetime of the magnons as a function of energy for several values of the mean concentration of nonmagnetic ions in the lattice. We calculate magnon energy spectrum as a function of average impurity concentration fraction c, which shows that for low lying states, the excitation energy increases continuously with c in the studied range 0.1 < c < 0.7. We use the spectral density function to compute some thermal quantities. We have obtained closed form expressions for the configurationally averaged physical quantities of interest in a unified fashion as functions of c to any order of c applicable below a critical percolation concentration.

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