Hubbard-Peierls模型的Monte-Carlo phase diagram研究$π$共轭聚合物的自旋交叉跃迁
Monte-Carlo phase diagram of a Hubbard-Peierls model in the search for spin crossover transition in $π$-conjugated polymers
摘要：
本文用Monte Carlo方法研究了一维$π$共轭聚合物的扩展Hubbard-Peierls模型的有限温度性质。该模型结合了电子-声子和超精细相互作用，并在平均场水平下求解半填充。特别是，我们探索的模型作为电子-电子和电子-声子相互作用的强度的函数。在低温下，随着电子-电子相互作用强度的增加，系统呈现出抗磁性到反铁磁性的转变。同时，通过增加电子-声子耦合，存在从均匀到Peierls二聚几何的转变。正如预期的那样，这种Peierls二聚相在有限温度下由于热振动而消失。更有趣的是在有限温度下电子-声子和电子-电子相互作用之间的相互作用。特别是，我们证明了在一定区域的参数空间有一个自旋交叉，系统从低自旋过渡到高自旋状态的温度增加。与二价磁性分子中的标准自旋交叉非常相似，这种跃迁是熵驱动的。最后讨论了超精细相互作用在相图中所起的作用。
英文摘要：
We present a Monte Carlo study of the finite temperature properties of an extended Hubbard-Peierls model describing one dimensional $π$-conjugated polymers. The model incorporates electron-phonon and hyperfine interaction and it is solved at the mean field level for half filling. In particular we explore the model as a function of the strength of electron-electron and electron-phonon interactions. At low temperature the system presents a diamagnetic to antiferromagnetic transition as the electron-electron interaction strength increases. At the same time by increasing the electron-phonon coupling there is a transition from a homogeneous to a Peierls dimerized geometry. As expected such a Peierls dimerized phase disappears at finite temperature as a result of thermal vibrations. More intriguing is the interplay between the electron-phonon and the electron-electron interactions at finite temperature. In particular we demonstrate that for a certain region of the parameter space there is a spin-crossover, where the system transits from a low-spin to a high-spin state as the temperature increases. In close analogy to standard spin-crossover in divalent magnetic molecules such a transition is entropy driven. Finally we discuss the role played by the hyperfine interaction over the phase diagram.
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