[论文解读] Suppression of Spectral Gap and Flat Bands on a Cuprate Superconductor Side-Surface

Gabriele Domaine, Mihir Date+28 more|arXiv (Cornell University)|Mar 3, 2026

Physics of Superconductivity and Magnetism被引用 0

一句话总结

该论文报道在过掺 LSCO (x=0.22) 的 CuO 侧表面进行动量分辨的 ARPES，暴露出被抑制的超导隙和预测的零能量平带峰的缺失，归因于表面粗糙和通过自洽 BdG 模型的体积 Anderson 器件。

ABSTRACT

Side surfaces of cuprate superconductors are expected to display a suppressed $d$-wave order parameter and zero-energy topological flat bands with a large density of states, making them susceptible to symmetry broken orders. Yet such surfaces have never been investigated with momentum-resolved, surface-sensitive probes, because high-temperature superconductors rarely cleave along them. Using focused-ion-beam milling to define a controlled breaking point, we expose pristine (110) side surfaces of overdoped La$_{2-x}$Sr$_x$CuO$_4$ ($x=0.22$) suitable for angle-resolved photoemission. We observe the suppression of the superconducting spectral gap within our energy resolution ($\sim 4~\mathrm{meV}$), and surprisingly, the expected zero-energy flat band peak is also suppressed, despite the high topographic quality of the surface. Self-consistent Bogoliubov--de~Gennes calculations show that the measured geometric roughness of the cleaved surface is too weak to eliminate these modes. The calculations further demonstrate that bulk inhomogeneities characteristic of high-temperature superconductors, modelled as moderate Anderson-type disorder, can broaden the flat-band states beyond detectability. Our results provide the first momentum-resolved view of the electronic structure on a cuprate side surface and reveal disorder as the key factor currently preventing appearance of flat bands and their associated correlated orders.

研究动机与目标

• 在铜酸盐 d 波超导体中探索侧表面作为拓扑性和强相关性平台的动机与意义。
• 使用聚焦离子束铣削暴露出干净的 (110) LSCO 侧表面以实现 ARPES 测量。
• 研究拓扑平带表态及其零能量特征在表面破坏下是否仍然存在。
• 评估几何边缘粗糙度和体无序在抑制或掩盖平带态中的作用。

提出的方法

• 通过聚焦离子束铣削暴露 LSCO (x=0.22) 的 (110) 侧表面以实现 ARPES 访问。
• 在 6 K 下以光子能量 79 eV 和 95 eV 进行高分辨率 ARPES，以绘制节点和反节点分散。
• 在 kz 宽化考虑下，将实验光谱转写为沿 [11] 和 [1-10] 方向的动量分辨数据。
• 构建带有自洽最近邻 d 波配对场的一波带紧束缚模型来模拟边缘态。
• 从 AFM 数据引入几何边缘粗糙度和 Anderson 型体无序，研究它们对平带态的影响。
• 在带边界条件的圆柱状薄板上计算光谱函数，并包括 kz 宽化以与 ARPES 进行比较。

Figure 1: Edge states in clean $d_{x^{2}-y^{2}}$ -wave superconductors. a ) Illustration of the realization of novel phenomena at the intersection of topology, strong correlations and superconductivity, ranging from topological superconductivity (TSC), unconventional superconductivity (USC) and corr

实验结果

研究问题

• RQ1 铜酸盐的侧表面是否如对 d 波超导体所预测的那样容纳零能量的拓扑平带态？
• RQ2 表面粗糙度和体无序如何影响边缘态在动量分辨测量中的可见性及超导隙？
• RQ3 自洽 BdG 计算能否调和观测到的隙抑制与平带特征的存在或缺失？

主要发现

• 在 (110) LSCO 侧表面的光谱隙在仪器分辨率 (~4 meV) 范围内被抑制。
• 尽管表面质量较高，在 ARPES 数据中未观测到零能量平带峰。
• BdG 计算显示当前测得的表面粗糙度不足以摧毁平带态。
• 引入中等强度的 Anderson 型体无序会使平带态变宽，从而抑制其可检测性。
• 无序驱动的碎裂和表面敏感性解释了在 ARPES 中未出现清晰的平带信号，而在 STM 中的 ZBCP 可能来自纳米尺度的无序变异。

Figure 2: Sample preparation a ) Illustration showing the different stages of the sample preparation. b ) Temperature dependence of the magnetic susceptibility measured by SQUID magnetometry for a piece of the original rod (red) and of a small part of the milled region of the pillar (blue). Measurem