时间：10月21日（周三）13:30-14:30

地点：勤园20-518

报告摘要：

Iron-based superconductors (FeSCs) are a pivotal frontier for exploring unconventional superconductivity, particularly regarding the role of time-reversal symmetry breaking (TRSB) in exotic electronic states. While bulk TRSB has been detected via μSR, atomic-scale evidence of its interplay with competing orders remains scarce. Here, we present atomic-scale STM studies of the iron arsenide superconductor Ba_1–xK_xFe₂As₂ (BKFA) [1] and KFe₂As₂ [2]. First, we investigate the charge density wave (CDW) order on the non-superconducting As-terminated surface of optimally doped BKFA (x ≈ 0.77), where bulk TRSB occurs. We observe a 2 × 2 order pattern driven by saddle-point nesting, facilitated by surface hole doping that shifts Fermi-level saddle points. Second, we report the anomalous splitting of superconducting vortices in surface-doped KFe2As2. Due to surface electron doping, the K-terminated surface achieves a superconducting critical temperature significantly higher than the bulk, reaching a doping level near optimal. Under an external magnetic field at low temperatures, we observe that a fraction of vortices splits into sub-vortices carrying fractional flux. These "fractional vortices" exhibit distinct zero-bias conductance features. Our results, supported by theoretical modeling, suggest that vortex splitting occurs along phase boundaries of the multi-component order parameter, providing direct real-space evidence of the complex interplay between TRSB, CDWs, and superconductivity in multiband systems.

References

[1] Nat Commun, 16(1):253, January 2025.

[2] Science, 393(6806):80–84, July 2026.

报告人简介：

严志明（Chi Ming Yim），上海交通大学李政道研究所副教授。聚焦于强关联电子材料的基态性质和非传统（非常规）超导，主要利用低温扫描隧道显微镜与谱学技术（STM/STS）开展研究。于2012年在英国伦敦大学学院（University College London, UCL）获得博士学位，曾先后在伦敦大学学院（2012–2016年）和圣安德鲁斯大学（University of St Andrews，2016–2020年）从事博士后研究。于2020年加入李政道研究所。迄今，在Science、Nature Communications、Science Advances、Physical Review Letters等期刊上发表论文44篇，承担国家自然科学基金、国家重点研发计划课题等项目。