Xingye Lu

Magnetic fields can penetrate a superconductor in the form of quantum vortices, which consist of a core singularity with circulating currents. London’s quantization implies that there is one core singularity per quantum of magnetic flux in single-component superconductors. In this study, we report signatures of quantum vortex core fractionalization on the potassium-terminated surface of a multiband superconductor, KFe ₂ As ₂ . The observed splitting of single integer–flux vortices into several fractional vortices results in a disparity between the numbers of flux quanta and vortex cores. These fractional vortices often arrange in chains, which calculations show are characterized by a ℂP ² skyrmionic topological invariant; this constitutes a different type of topological defect: the chiral skyrmion. The disparate natures of integer and fractional vortices comprising skyrmions lead to distinct spectroscopic signatures.

Oxygen stoichiometry has been identified as a key parameter controlling superconductivity in the bilayer nickelate La ₃ Ni ₂ O _{7- δ} . Using resonant inelastic x-ray scattering, we systematically investigate the evolution of orbital and spin excitations in La ₃ Ni ₂ O _{7- δ} thin films with varying oxygen content. In vacuum-annealed samples, the suppression of the 1.6 eV dd excitation under π polarization reflects appreciable inner apical-oxygen vacancies, which locally disrupt the out-of-plane p-d hybridization and the corresponding interlayer superexchange pathways. Nevertheless, the oxygen-deficient films exhibit spin excitations originating from Q = (0.25, 0.25) with a similar dispersion to the as-grown sample, suggesting a mixed ground state of double spin stripe and spin-charge stripe orders with nearly identical spin correlations. By contrast, the magnon damping rate is slightly enhanced in the vacuum-annealed sample, reflecting a modest increase in electronic disorder associated with oxygen defects. Collectively, these findings reveal that short-range spin correlations in La ₃ Ni ₂ O _{7- δ} are insensitive to oxygen-vacancy-induced perturbations, including electron doping and local disruption of interlayer superexchange pathways, even in samples with appreciable oxygen deficiency, where the 1.6 eV dd excitation associated with Ni 3 d _{z ²} -O 2 p _z hybridization is suppressed by up to 58%.

CCaCo₂As₂是铁基超导“122”体系的钴基衍生物,其低温磁基态与塌缩四方结构、费米能级附近的Co 3d电子态以及Co缺位密切相关。本文以CaCo₂As₂单晶为研究对象,结合电输运、磁性测量和单晶中子衍射实验,系统研究其晶体结构和低温磁基态。实验结果表明,该样品表现出金属性输运特征,并在低温下发生反铁磁相变;磁化率和等温磁化曲线显示出明显的磁各向异性,说明其磁有序具有显著的单轴特征。单晶中子衍射结果进一步表明,样品保持ThCr₂Si₂型塌缩四方结构,并存在一定程度的Co缺位。低温磁衍射结果直接确认了CCaCo₂As₂的A型反铁磁结构,即Co磁矩在层内铁磁排列、相邻层间反铁磁排列,且磁矩主要沿c轴方向取向。进一步的变温中子衍射结果表明,该磁有序的相变奈尔温度为T_N ≈ 36 K。与已有报道相比，本文样品具有更高的Co缺位浓度、较低的T_N和类似的有序磁矩大小,支持了先前对CCaCo₂As₂体系中较低的磁相变温度可能与Co缺位对磁相互作用的削弱有关的认识。本研究为理解CCaCo₂As₂中塌缩四方结构、Co缺位与巡游磁性之间的耦合关系提供了实验依据。

在铁基超导的研究中，本征自旋激发谱是理解体系内磁关联、电子向列性和反铁磁涨落之间相互作用的实验基础，也是构建超导微观自旋模型的关键. 近年来，针对多个铁基超导体系退孪晶样品开展的非弹性中子散射实验在这一课题上取得了一系列进展. 在低能区，实验表明起源于带间散射的低能自旋激发和自旋共振峰具有两重旋转对称性，这一发现为具有 (d_yz) 轨道选择性的超导电性提供了进一步的实验证据. 在高能区，对退孪晶 BaFe_2As_2、NaFeAs、FeSe 体系的全谱磁激发进行的测量，为理解这些体系中磁激发的轨道选择性提供了新的实验证据. 本文将聚焦 BaFe_2As_2、NaFeAs、FeSe 三个典型体系，从多带洪特金属的轨道选择性角度出发，归纳并讨论铁基超导体中的向列自旋关联，以及自旋关联和激发的轨道选择性.

In the quest for topology- and correlation-driven quantum states, kagome lattice materials have garnered significant interest for their band structures, featuring flat bands (FBs) from the quantum destructive interference of the electronic wavefunction. Tuning an FB to the chemical potential could induce electronic instabilities and emergent orders. Despite extensive studies, direct evidence of FBs tuned to the chemical potential and their role in emergent orders in bulk materials remains lacking. Using angle-resolved photoemission spectroscopy, resonant inelastic X-ray scattering, and density functional theory, we show that the low-energy structure of the Cr-based kagome metal superconductor CsCr₃Sb₅ is dominated by FBs at the Fermi level. We also observe low-energy magnetic excitations evolving across the low-temperature transition, largely consistent with the FB shift. Our results suggest that the low-temperature order contains a magnetic origin and that the kagome FBs may play a role in the emergence of this order.