二维材料中的Fröhlich双极化子
Fröhlich Bipolarons in Two-Dimensional Materials
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- University of Iceland（冰岛大学）
- ITMO University（伊托莫大学）
- United Arab Emirates University（阿联酋大学）
- Universidad del País Vasco（巴斯克大学）
- Donostia International Physics Center (DIPC)（圣塞巴斯蒂安国际物理中心）
- IKERBASQUE, Basque Foundation for Science（伊克尔巴斯基奎巴斯克科学基金会）
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中文总结 AI 辅助
本文通过Feynman路径积分变分方法研究二维极性单层中的双极化子形成，发现非局域介电屏蔽导致稳定区域强烈依赖于极化率比值，且强耦合下无稳定双极化子，表明极性单层不利于其存在。
中文摘要 AI 辅助
受二维材料物理学进展以及最近Fröhlich模型二维推广的启发，我们研究了极性单层中双极化子的形成。由于二维中非常特殊的非局域介电屏蔽，这一情形与传统的Fröhlich模型有质的区别。在单层中，(i)长波长LO声子获得非平凡的色散关系；(ii)Fröhlich电子-声子耦合顶点变得依赖于动量，并在小动量处是规则的；(iii)载流子之间的直接排斥采取Keldysh-Rytova形式。利用Feynman路径积分变分方法，我们表明，与具有无色散声子的通常准二维模型相比，稳定双极化子存在的参数空间区域被强烈修改。具体而言，在最有利的极限下，当静态极化率与高频极化率之比$\sigma_0$趋于无穷大时，足以形成双极化子的下临界耦合可以变得任意小。更令人惊讶的是，我们证明在强耦合极限下不存在稳定的双极化子：在孤立单层中，稳定区域总是局限于有限的耦合常数范围。总的来说，稳定区域强烈地向极化率比值$\sigma_0$的大值方向移动，远超代表性晶体中的参数范围，这表明极性单层不利于双极化子的形成。
英文摘要
Motivated by the progress in the physics of two-dimensional materials and the recent two-dimensional generalization of the Fröhlich model, we study the formation of bipolarons in polar monolayers. Because of very special nonlocal dielectric screening in two dimensions, this setting differs qualitatively from the conventional Fröhlich model. In monolayers, (i) long wavelength LO phonons acquire a nontrivial dispersion; (ii) the Fröhlich electron-phonon vertex becomes momentum-dependent and regular at small momenta; and (iii) direct repulsion between charge carriers takes the Keldysh-Rytova form. Using the Feynman path-integral variational approach, we show that the region in the parameter space where stable bipolarons exist is strongly modified compared to the usual quasi-two-dimensional model with dispersionless phonons. Specifically, in the most favorable limit, when the ratio $σ_0$ of the static polarizability to the high-frequency polarizability tends to infinity, the lower critical coupling, sufficient for the formation of bipolarons, can be made arbitrarily small. More surprisingly, we demonstrate that no stable bipolarons can exist in the strong coupling limit: in the isolated monolayer the stability region is always confined to a finite range of coupling constants. In general, the stability region is strongly shifted toward large values of the polarizability ratio $σ_0$, well beyond the parameter regimes in representative crystals, which indicates that polar monolayers do not favor bipolarons.
