发表机构
Indian Institute of Technology, Madras; Nanyang Technological University; SRM Institute of Science and Technology（印度马德拉斯理工学院; 南洋理工大学; SRM科学与技术学院）
机构由 AI 辅助整理，请以论文原文为准。AI 中文总结
本文研究二维 Lieb 晶格吸引性 Hubbard–Kondo 模型中的多带超导体，发现 Kondo 与配对竞争导致磁性超导序，并识别出两种 Lifshitz 型相变，其中螺旋序产生无能隙磁性超导态。
AI 中文摘要
Lifshitz 相变是一类不破坏对称性的相变，通常与金属系统相关，当化学势扫过能带极值点或鞍点时，费米面的连通性发生改变。这一现象学自然地适用于超导态，其中发生重构的对象是准粒子能量最小的流形，而非费米面。本文关注多带超导体中的 Lifshitz 型相变。为此，我们研究二维 Lieb 晶格上的吸引性 Hubbard–Kondo 模型，其中经典核心自旋与巡游电子耦合，采用实空间 Bogoliubov–de Gennes 平均场理论和解析格林函数计算。Kondo 通道与配对通道之间的竞争导致整个体系中呈现磁性超导电性，在接近半填充时出现 $(\pi,\pi)$ 序，随后转变为 $(0,\pi)$ 序，并在更强的 Kondo 耦合下转变为非公度螺旋序 $(0,q)$。在这种包含平带的多带超导体中，我们识别出两种不等价的 Lifshitz 型相变。当核心自旋退耦合（Kondo 耦合设为零）时，最小能量等值线随化学势变形，在 $\mu=-2t$ 处，围绕 $M$ 点的空穴口袋打开形成完美嵌套的正方形，并重新连接成围绕 $\Gamma$ 点的口袋；这种嵌套决定了 $(\pi,\pi)$ 序出现的位置。在有限 Kondo 耦合下，对于特定化学势，流形本身的维度会降低。我们针对每种磁序解析地得到了这一结果，并表明只有螺旋序能使最小能量穿过零，从而产生节点，留下无能隙的磁性超导体，在态密度和谱函数中具有清晰的信号。
英文摘要
A Lifshitz transition is a class of non-symmetry-breaking transitions typically associated with metallic systems wherein the Fermi surface changes its connectivity as the chemical potential sweeps through a band extremum or a saddle point. The phenomenology naturally adapts itself to the superconducting state, where the object that reconstructs is the manifold of minimum quasiparticle energy rather than the Fermi surface. This paper concerns Lifshitz-type transitions in multi-band superconductors. Thus, we focus on the attractive Hubbard--Kondo model on the two-dimensional Lieb lattice, with classical core spins coupled to the itinerant electrons, by laying recourse to a real-space Bogoliubov--de Gennes mean-field theory and an analytic Green's function calculation. The competition between the Kondo and pairing channels yields a magnetic superconductor throughout, with $(π,π)$ order near half filling giving way to $(0,π)$ and then, at stronger Kondo coupling, to an incommensurate spiral $(0,q)$. In such a multiband superconductor hosting a flat band, we identify two inequivalent Lifshitz-type transitions. With the core spins decoupled (the Kondo coupling set to zero), the minimum energy contour deforms with chemical potential, a hole pocket about $M$ opening into a perfectly nested square at $μ=-2t$ and reconnecting into pockets about $Γ$; this nesting fixes where $(π,π)$ order sets in. At finite Kondo coupling, for certain chemical potential, the dimension of the manifold itself drops. We obtain it analytically for each magnetic order and show that only the spiral carries the minimum energy through zero, nucleating nodes and leaving a gapless magnetic superconductor, with clear signatures in the density of states and the spectral function.
Comments21 pages including references and appendix
