【作者】 刘豪；

【导师】 孟建桥；

【作者基本信息】 中南大学 ， 凝聚态物理， 2025， 博士

【摘要】 拓扑材料中的电荷载流子与磁性耦合可催生量子反常霍尔效应、轴子绝缘态等奇异量子现象,具有重要的物理意义和应用潜力。稀土元素铕（Eu）因其强自旋轨道耦合和局域f电子的磁相互作用,成为研究本征磁性拓扑材料的理想体系。Eu的4f电子磁性序与拓扑能带间的强关联效应,赋予材料丰富的输运特性和可调拓扑态,为探索新型量子物态提供了独特平台。然而,其磁性序与拓扑能带的相互作用机制仍不清晰,材料行为复杂且未知。深入研究Eu基拓扑材料,不仅有助于揭示其物理本质,还可推动拓扑量子现象的理论发展,为未来量子技术开辟新路径。本论文利用角分辨光电子能谱（Angle-Resolved Photoemission Spectroscopy,ARPES）和超快光谱技术,对轴子绝缘体EuIn₂As₂、拓扑半金属EuAgAs和EuAl₄进行了系统的探索,主要研究结果如下:（1）通过超快光谱揭示了轴子绝缘体EuIn₂As₂中拓扑电子结构和磁性耦合的特性。在反铁磁(（Antiferromagnetism,AFM）相变温度附近,准粒子弛豫寿命突然增大和振幅发生符号反转,反映了AFM序对拓扑电子结构的影响,例如能带劈裂和能隙打开。在AFM相,超快光谱观察到两个温度依赖的吉赫兹集体振动模式,其中一支源自磁振子激发。该自旋进动模式表明光激发载流子通过交换相互作用扰动了磁性结构。在高通量光激发下,EuIn₂As₂中存在光诱导的非热相变,如AFM磁矩的重定向。这些现象表明,巡游的拓扑能带电子和局域的磁性序之间具有密切的相互作用,脉冲激光能够有效地激发和探测这种作用。（2）通过ARPES实验和理论计算,确认了EuAgAs的AFM狄拉克半金属态。超快光谱实验揭示了在AFM相下存在光诱导的拓扑量子相变。ARPES实验探测到EuAgAs中狄拉克能带在不同温度下保持稳定。然而,超快光谱发现了在AFM相下明显的光通量阈值行为,表明与磁性序相关的电子结构受到了脉冲激光的调制,该过程涉及脉冲激光驱动的AFM狄拉克半金属态向FM外尔半金属态的转变。（3）通过ARPES实验发现EuAl₄的拓扑能带沿k_z方向存在费米面嵌套行为,其是电荷密度波（Charge Density Wave,CDW）转变的主要驱动力。进入CDW相后发生的能带折叠,使得费米能级附近的部分导带打开能隙,且能隙的温度依赖性与Peierls机制中平均场理论的预测一致,表明费米面嵌套对CDW转变的重要贡献。此外,实验观测到来源于Al的p轨道和Eu的d轨道电子的平带结构。这些电子的局域化会引入额外的磁矩和自旋激发,为理解中心对称材料中斯格明子晶格的形成提供了实验支持。通过ARPES和超快光谱实验发现Eu基材料中拓扑电子结构显著受到磁性序影响,揭示了潜在的激光诱导磁性结构变化所导致的拓扑相变现象。进一步研究发现,磁性序不仅影响电子结构,反之,拓扑能带的费米面嵌套行为同样促进了特殊自旋结构的产生。研究结果表明,Eu基拓扑材料凭借其丰富的磁性相变和清晰的拓扑特征,是研究磁性序与拓扑能带相互作用的理想平台。图67幅,表1个,参考文献299篇更多还原

【Abstract】 The coupling of charge carriers and magnetism in topological materials can give rise to exotic topological quantum phenomena,such as the quantum anomalous Hall effect and axion insulator states,with significant physical and application potential.The rare earth element europium（Eu）,with strong spin-orbit coupling and local f-electron magnetic interactions,serves as an ideal system for studying intrinsic magnetic topological materials.The correlation between Eu’s 4f magnetic order and topological bands enriches transport properties and enables tunable topological states,providing a unique platform for exploring novel quantum states.However,the interaction mechanism remains unclear,and material behavior is complex.In-depth research on Eu-based topological materials not only helps to reveal their physical essence but also promotes the theoretical development of topological quantum phenomena,opening up new paths for future quantum technologies.This thesis uses ARPES and ultrafast spectroscopy techniques to systematically explore the axion insulator EuIn₂As₂,topological semimetal EuAgAs,and EuAl₄.Key finding include:（1）The characteristics of the coupling between topological electronic structure and magnetism in the EuIn₂As₂ were revealed by ultrafast spectroscopy.Near the AFM order phase transition temperature,the sudden increase in quasiparticle relaxation lifetime and the sign reversal of the amplitude reflect the modification of the topological electronic structure by the AFM order,such as band splitting and gap opening.In the AFM phase,two temperature-dependent gigahertz collective vibration modes are observed in the spectrum,one of which originates from the magnon excitation.This spin precession mode indicates that photoexcited carriers perturb the magnetic structure through exchange interaction.High-fluence light excitation experiments suggest that there are light-induced non-thermal phase transitions in EuIn₂As₂,such as the reorientation of antiferromagnetic moments.These phenomena indicate that there is a close interaction between itinerant topological band electrons and localized magnetic order,and that pulsed lasers can effectively excite and detect this connection.（2）The AFM Dirac semimetal nature of EuAgAs was verified through ARPES experiments and theoretical calculations.Ultrafast pump-probe spectroscopy experiments have revealed the potential for a photoinduced topological quantum phase transition within the AFM phase.ARPES experiments detected that the Dirac bands in EuAgAs remained stable at different temperatures.However,ultrafast spectroscopy discovered a distinct photo-fluence threshold behavior in the AFM phase,indicating that the electronic structure related to the magnetic order was modulated by pulsed laser excitation.This process involves a pulsed-laser-driven transformation from the AFM Dirac semimetal state to the FM Weyl semimetal state.（3）ARPES experiments have revealed that the topological band of EuAl₄ exhibits Fermi surface nesting along the k_z direction,which is the main driving force for the CDW transition.Upon entering the CDW phase,the band folding occurs,causing a gap to open in the partial conduction band near the Fermi level,and the temperature dependence of the gap is consistent with the predictions of the mean-field theory in the Peierls mechanism,indicating the significant contribution of Fermi surface nesting to the CDW transition.Additionally,the flat band structure originated from the p orbitals of Al and the d orbitals of Euwas experimentally observed.The localization of these electrons introduces additional magnetic moments and spin excitations,providing experimental support for understanding the formation of skyrmion lattices in centrosymmetric materials.Through ARPES and ultrafast spectroscopy experiments,it was found that the topological electronic structure in Eu-based materials is significantly influenced by magnetic order,revealing the potential phenomenon of topological phase transition caused by photoinduced magnetic structure changes.Further research discovered that magnetic order not only affects the electronic structure,but conversely,the Fermi surface nesting behavior of topological bands also promotes the generation of special spin structures.The research results indicate that Eu-based topological materials,with their rich magnetic phase transitions and clear topological features,are an ideal platform for studying the interaction between magnetic order and topological bands.更多还原