第一原理計算在 AA 堆疊多層石墨烯中的電子聲子耦合

Abstract

近年研究發現，在數種展現超導性的扭曲石墨烯系統中，低能電子的電荷密度集中於 AA 堆疊區域，因此這些區域的電子—聲子耦合（electron-phonon coupling, EPC）備受關注。本研究利用第一原理計算，探討了 AA 堆疊多層石墨烯的振動特性與電子—聲子交互作用。 在 AA 堆疊雙層石墨烯中，位於布里淵區（Brillouin-zone）中心與邊角的面內光學聲子，會分裂成具相反層間對稱性（layer parities）的兩個分支。由於 AA 堆疊低能電子態具有成鍵與反鍵（bonding–antibonding）特性，層間同相模式（in-phase mode）的電子—聲子耦合會消失，而層間反相模式（out-of-phase mode）則保有有限的耦合強度，此行為可由緊束縛模型（tight-binding model）準確描述，且符合基於對稱性的選擇定則。 儘管耦合矩陣元素的整體數值小於單層石墨烯，但 AA 堆疊雙層中近乎完美的谷內與谷間費米環嵌套（Fermi-ring nesting），導致層反對稱模式的聲子線寬（phonon linewidth）顯著增強。 隨著層數增加，面內聲子進一步分裂成多個具有特定鏡像對稱性（mirror parity）的模式，且聲子線寬的貢獻透過受鏡像對稱選擇定則限制的狄拉克錐內與錐間錐散射通道重新分布。特別是，層交替反對稱模式（Layer-Alternating Antisymmetric Modes）的線寬佔據主導地位，並隨層數增加而大幅增長，這是由於受手徵對稱性（chiral symmetry）保護的狄拉克錐配對散射過程增強所致。 本研究結果確定了 AA 堆疊石墨烯是探索層狀石墨烯系統中強電子—聲子耦合的平台，並為未來研究對齊式與莫爾（moiré）超晶格提供了理論基礎。

It has been discovered that in several twisted graphene systems exhibiting superconductivity, the charge density of low-energy electrons is localized in the AA-stacked regions. Therefore, the electron-phonon coupling (EPC) in these regions is of particular interest. In this work, we investigate the EPC of AA-stacked multilayers using first-principles calculations. In the bilayer limit, in-plane optical phonons split into branches with opposite layer parities. We demonstrate that symmetry selection rules dictate the EPC: layer-symmetric modes decouple due to the bonding–antibonding nature of electronic states, while layer-antisymmetric modes retain finite coupling. Despite smaller coupling matrix elements compared to monolayer graphene, the antisymmetric modes exhibit pronounced linewidth broadening driven by nearly perfect Fermi-ring nesting. In multilayers, mirror-parity selection rules further constrain scattering channels. Notably, the linewidths of layer-alternating antisymmetric modes dominate and scale with layer number, enhanced by Dirac-cone pair-scattering processes protected by chiral symmetry. These results identify AA-stacked graphene as a distinct platform for exploring strong EPC, providing a theoretical basis for understanding moiré superlattices.