转角双层石墨烯中莫特转变附近的涌现重费米子与超导电性
Emergent heavy fermion and superconductivity near Mott transition in twisted bilayer graphene
AI总结：
该研究提出转角双层石墨烯整数填充下的涌现重费米子统一框架，揭示其莫特转变的近藤屏蔽本质及反洪特耦合诱导的超导穹顶等现象。
AI中文摘要：
在带宽调控的莫特转变附近，金属的费米速度$v_F$和准粒子留数$Z$通常会趋于零。本文研究表明，类似现象在整数填充的转角双层石墨烯（TBG）中涌现，且可通过投影有源带极限下的涌现重费米子框架进行描述。与纳入远程能带的模型不同，我们的有效重费米子描述源于通过解耦的电荷与局域磁矩扇区实现的“混合价莫特”物理。在电荷扇区中，有源带$c(\boldsymbol{k})$与涌现的“正交费米子”$ψ(\boldsymbol{k})$发生杂化，在$|\boldsymbol{k}| > k_*$（$k_*$确定动量斑的大小）处打开大的莫特能隙，并在电中性（$ν=0$）下于$\boldsymbol{k}=0$附近形成二次能带接触半金属。正交费米子是双空穴和空穴激发的线性组合，可表示为$ψ_i \sim (δn^f_i+\frac{1}{2})^{-1} f_i$。$ψ$与局域磁矩$ψ'$之间的涌现近藤耦合$J_K \sim U$（$U$为局域哈伯德相互作用）将莫特转变构建为一种近藤屏蔽转变，由转角$θ$调控。在魔角之外，近藤屏蔽的重半金属在$T_K$（近藤温度）以下形成，且$Z$趋于零。引入反洪特耦合$J_A$后，即使在$ν=0$时，莫特边界附近也会产生s波全能隙或向列型、有节点能隙的超导穹顶。在其他整数填充$ν= \pm 1, \pm 2$下，增加带宽首先将小能隙莫特态驱动为中间态的二次能带接触半金属，随后进入具有大费米面的重费米液体。我们的结果为包含来自f轨道的巡游载流子和局域磁矩的涌现重费米子物理建立了统一框架。
英文摘要：
Near a bandwidth-tuned Mott transition, the Fermi velocity $v_F$ and quasiparticle residue $Z$ of a metal often vanish. Here, we show that analogous phenomena emerge in twisted bilayer graphene (TBG) at integer fillings and can be captured by an emergent heavy-fermion framework within a projective active-band limit. Unlike models incorporating remote bands, our effective heavy-fermion description arises from \textit{mixed-valence Mott} physics via decoupled charge and local-moment sectors. In the charge sector, active bands $c(\mathbf{k})$ hybridize with an emergent \emph{orthogonal fermion} $ψ(\mathbf{k})$ to open a large Mott gap at $|\mathbf{k}| > k_*$ ($k_*$ sets the momentum-patch size) and a quadratic band-touching semimetal near $\mathbf{k}=0$ at neutrality ($ν=0$). The orthogonal fermion is a linear combination of the doublon and holon excitations and may be written as $ψ_i \sim (δn^f_i+\frac{1}{2})^{-1} f_i$. An emergent Kondo coupling $J_K \sim U$ ($U$ is the local Hubbard interaction) between $ψ$ and local moments $ψ'$ frames the Mott transition as a Kondo screening transition, tuned by the twist angle $θ$. Away from the magic angle, a Kondo-screened heavy semimetal develops below $T_K$ (the Kondo temperature) with vanishing $Z$. Introducing anti-Hund's coupling $J_A$ generates an s-wave fully gapped or nematic, nodally gapped superconducting dome near the Mott boundary even at $ν=0$. At other integer fillings $ν= \pm 1, \pm 2$, increasing bandwidth first drives the small-gap Mott state into an intermediate quadratic band-touching semimetal before entering a heavy Fermi liquid with large Fermi surfaces. Our results establish a unified framework for emergent heavy fermion physics with both itinerant carriers and local moments from the $f$ orbital.
